Processing, nanoindentation and scratch testing of alumina-coated YTZP | Boletín de la Sociedad Española de Cerámica y Vidrio

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B) Imagen MEB-BSE de la muestra MO-99 con los puntos en los que se han realizado los microanálisis. C) Fragmento de vidrio (MO-40) de forma semicircular con restos de metal adheridos. D) Fragmento de vidrio (MO-68) con restos de metal adherido. E) Fragmento de vidrio (MO-98) con restos de metal adherido. F) Imagen MEB-BSE de la muestra MO-40 con una capa con altas concentraciones de hierro." ] ] ] "autores" => array:1 [ 0 => array:2 [ "autoresLista" => "Miquel Àngel Capellà Galmés, Daniel Albero Santacreu" "autores" => array:2 [ 0 => array:2 [ "nombre" => "Miquel Àngel" "apellidos" => "Capellà Galmés" ] 1 => array:2 [ "nombre" => "Daniel" "apellidos" => "Albero Santacreu" ] ] ] ] ] "idiomaDefecto" => "es" "EPUB" => "https://multimedia.elsevier.es/PublicationsMultimediaV1/item/epub/S0366317515000564?idApp=UINPBA00004N" "url" => "/03663175/0000005400000004/v1_201508140009/S0366317515000564/v1_201508140009/es/main.assets" ] "itemAnterior" => array:18 [ "pii" => "S0366317515000680" "issn" => "03663175" "doi" => "10.1016/S0366-3175(15)00068-0" "estado" => "S300" "fechaPublicacion" => "2015-07-01" "aid" => "80000045" "documento" => "simple-article" "crossmark" => 0 "licencia" => "http://creativecommons.org/licenses/by-nc-nd/4.0/" "subdocumento" => "mis" "cita" => "Bol Soc Esp Ceram Vidr. 2015;54:v-vi" "abierto" => array:3 [ "ES" => true "ES2" => true "LATM" => true ] "gratuito" => true "lecturas" => array:2 [ "total" => 2349 "formatos" => array:3 [ "EPUB" => 16 "HTML" => 1137 "PDF" => 1196 ] ] "en" => array:6 [ "idiomaDefecto" => true "titulo" => "Editorial" "tienePdf" => "en" "tieneTextoCompleto" => 0 "paginas" => array:1 [ 0 => array:2 [ "paginaInicial" => "v" "paginaFinal" => "vi" ] ] "contienePdf" => array:1 [ "en" => true ] ] "idiomaDefecto" => "en" "EPUB" => "https://multimedia.elsevier.es/PublicationsMultimediaV1/item/epub/S0366317515000680?idApp=UINPBA00004N" "url" => "/03663175/0000005400000004/v1_201508140009/S0366317515000680/v1_201508140009/en/main.assets" ] "en" => array:19 [ "idiomaDefecto" => true "titulo" => "Processing, nanoindentation and scratch testing of alumina-coated YTZP" "tieneTextoCompleto" => true "paginas" => array:1 [ 0 => array:2 [ "paginaInicial" => "133" "paginaFinal" => "141" ] ] "autores" => array:1 [ 0 => array:4 [ "autoresLista" => "Jorge Valle, Marc Anglada, Begoña Ferrari, Carmen Baudín" "autores" => array:4 [ 0 => array:3 [ "nombre" => "Jorge" "apellidos" => "Valle" "referencia" => array:2 [ 0 => array:2 [ "etiqueta" => "a" "identificador" => "aff0005" ] 1 => array:2 [ "etiqueta" => "b" "identificador" => "aff0010" ] ] ] 1 => array:3 [ "nombre" => "Marc" "apellidos" => "Anglada" "referencia" => array:1 [ 0 => array:2 [ "etiqueta" => "b" "identificador" => "aff0010" ] ] ] 2 => array:3 [ "nombre" => "Begoña" "apellidos" => "Ferrari" "referencia" => array:1 [ 0 => array:2 [ "etiqueta" => "a" "identificador" => "aff0005" ] ] ] 3 => array:4 [ "nombre" => "Carmen" "apellidos" => "Baudín" "email" => array:1 [ 0 => "cbaudin@icv.csic.es" ] "referencia" => array:2 [ 0 => array:2 [ "etiqueta" => "a" "identificador" => "aff0005" ] 1 => array:2 [ "etiqueta" => "*" "identificador" => "cor0005" ] ] ] ] "afiliaciones" => array:2 [ 0 => array:3 [ "entidad" => "Instituto de Cerámica y Vidrio, CSIC, Madrid, Spain" "etiqueta" => "a" "identificador" => "aff0005" ] 1 => array:3 [ "entidad" => "Department of Materials Science and Engineering/ETSEIB, Barcelona, Spain" "etiqueta" => "b" "identificador" => "aff0010" ] ] "correspondencia" => array:1 [ 0 => array:3 [ "identificador" => "cor0005" "etiqueta" => "⁎" "correspondencia" => "Corresponding author." ] ] ] ] "titulosAlternativos" => array:1 [ "es" => array:1 [ "titulo" => "Procesamiento, nanoindentación y rayado de materiales de YTZP recubiertos con alúmina" ] ] "resumenGrafico" => array:2 [ "original" => 0 "multimedia" => array:7 [ "identificador" => "fig0035" "etiqueta" => "Fig. 7" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr7.jpeg" "Alto" => 3200 "Ancho" => 2847 "Tamanyo" => 801669 ] ] "descripcion" => array:1 [ "en" => "Scanning electron microscopy micrographs of scratch tested specimens. (a–c) Secondary electrons. (d–f) Back scattered electrons: the white phase is the zirconia substrate. (a and d) Solid loading 1vol% Al2O3. (b and e) Solid loading 3vol% Al2O3. (c and f) Solid loading 5vol% Al2O3." ] ] ] "textoCompleto" => "IntroductionMaterials constituted by tetragonal zirconia polycrystals doped with 3mol % yttria (YTZP) combine high fracture strength (>1000MPa), fracture toughness (∼5MPam1/2) and high reliability (Weibull modulus>10) at room temperature <a class="elsevierStyleCrossRefs" href="#bib0160">[1,2]</a>. However, these materials present relatively moderate hardness (HV≈12–13GPa) <a class="elsevierStyleCrossRef" href="#bib0165">[2]</a> which may limit their use in some structural applications. Moreover, they present limitations such as environmentally assisted slow crack growth and metastable transformation of the tetragonal phase to the monoclinic phase under wet environments from room temperature up to 400°C. Transformation results in surface roughening and microcracking, which leads to lower wear resistance <a class="elsevierStyleCrossRefs" href="#bib0165">[2–6]</a>. On the other hand, alumina offers a unique combination of high hardness (HV≈18–20GPa) <a class="elsevierStyleCrossRefs" href="#bib0165">[2,7]</a>, corrosion resistance, thermal stability and high wear resistance <a class="elsevierStyleCrossRef" href="#bib0195">[8]</a>. However, it has relatively low fracture toughness (≈2.5–4MPam1/2) and low and variable strength <a class="elsevierStyleCrossRef" href="#bib0200">[9]</a>.One approach to solve limitations of both alumina and YTZP ceramics is the development of alumina-zirconia composites to combine the advantageous characteristics of both phases. In particular, zirconia-toughened alumina (ZTA) with low zirconia content as second phase (<15vol%) shows higher fracture toughness and hardness than alumina and YTZP, respectively <a class="elsevierStyleCrossRefs" href="#bib0165">[2,10–14]</a>. However, YTZP as second phase in ZTA composites is under tension <a class="elsevierStyleCrossRef" href="#bib0225">[14]</a>, so it is still liable to ageing in contact with wet environments <a class="elsevierStyleCrossRefs" href="#bib0165">[2,10,13]</a>, although significant resistance to ageing may be achieved <a class="elsevierStyleCrossRef" href="#bib0165">[2]</a>. Another design strategy, for improving the surface hardness of YTZP materials while avoiding the problem related to phase transformation, is to produce laminated structures designed to induce compressive residual stresses at the surface by combining ceramics with different thermal expansion coefficients. For instance, alumina-YTZP symmetrical laminated structures in which surface alumina layers are subjected to compressive residual stresses show hardness values similar to those of single phase alumina combined with improved apparent surface fracture toughness and fracture strength <a class="elsevierStyleCrossRefs" href="#bib0230">[15–17]</a>. These laminated structures have a great potential for applications involving wear processes <a class="elsevierStyleCrossRef" href="#bib0245">[18]</a>.In a previous work <a class="elsevierStyleCrossRef" href="#bib0250">[19]</a>, alumina-coated YTZP materials were proposed as means to improve the wear resistance of YTZP. YTZP specimens coated by homogeneous alumina layers of about 250–200μm were obtained by dipping pre-sintered YTZP substrates with open porosity in alumina suspensions and subsequent sintering. The wear resistance of the coated specimens was much higher than that of monolithic YTZP. However, the coatings presented some porosity resulting in Young's modulus (Berkovich nanoindentation: E≈330GPa) and hardness (Vickers Hardness≈16.6GPa; Berkovich nanoindentation: H≈22GPa) <a class="elsevierStyleCrossRef" href="#bib0255">[20]</a> lower than those of a reference dense alumina (Berkovich nanoindentation: E=422±10GPa, H=26±1GPa; Vickers Hardness=19.6±0.6GPa) <a class="elsevierStyleCrossRefs" href="#bib0260">[21,22]</a>. The fact that coating was done only on one face of relatively large thickness specimens was identified as responsible for such poor mechanical performance.In this work, the processing parameters have been changed focusing coatings with higher density. In order to facilitate the compatibility of shrinkages during sintering and thermal expansion strains on cooling, symmetric specimens have been prepared using thinner discs and suspensions with lower solid contents. As terms of comparison, specimens using the suspension with high solid loading previously used have been processed and characterised.ExperimentalHigh purity commercial zirconia powders (TZ-3YS, Tosoh, Japan) were isostatically pressed (200MPa) in the shape of cylinders. After a thermal treatment at 1150°C for 2h in air, the cylinders were cut into discs of approximately 10mm in diameter and 4mm in thickness and rectified for surface conditioning. Open porosity of these pre-sintered zirconia discs was approximately 43%.Suspensions of alumina (Condea, HPA05, USA) were prepared in ethanol to solid loadings of 2, 3, 5 and 10vol% adding 0.5wt. %, on a dry solids basis, of PEI (Polyethylenimine, Mw 2000, Aldrich) as dispersant, and sonicated for 2min (Ultrasonication Probe, UP 400S, Hielscher, Germany). The coatings were deposited on both sides of the YTZP discs, using a lift to dip and withdraw the sample in the alumina suspension. The system is provided with a clam which allows grabbing the disc letting free both sides of the disc and masking edges. The YTZP substrates were dipped and maintained in the alumina suspensions for 20s, after which they were withdrawn at 7mm/s.After drying in air at room temperature, the coated compacts were sintered at 1280°C during 4h and 1500°C for 2h, using 2°Cmin−1 as heating and cooling rates. Pre-sintering and sintering temperatures were selected from sintering studies to reach full density of reference single-phase monolithic YTZP and alumina compacts while avoiding undesirable grain growth <a class="elsevierStyleCrossRef" href="#bib0265">[22]</a>. Final dimensions of the sintered discs were diameter φ≈9.5mm and thickness, t≈3mm.The three series of specimens corresponding to the three different solids loading in the suspensions (2, 3 and 5vol%) were fabricated in four different batches, named S0, S1, S2 and S3, to investigate reproducibility of the shaping process.Diamond machined cross sections of the sintered coated specimens were characterised by scanning electron microscopy (Tabletop Microscope TM-1000, Hitachi, Japan) after polishing with diamond past of sizes down to 3μm. The thickness of the coatings was measured directly from the micrographs.Young Modulus and hardness determinations were carried out in the cross sections. Berkovich nanoindentations of 500nm depth were done (MTS Nanoindenter XP, USA) and the load-penetration depth curves during the tests were recorded. Data were taken in continuous stiffness measurement (CSM) mode, in which a simultaneous oscillating force is applied which is several orders of magnitude lower than the nominal load, allowing measuring the properties in the whole range of penetration. Hardness values and Young's modulus given are those corresponding to maximum penetration depth and they were calculated using the method of Oliver and Pharr <a class="elsevierStyleCrossRef" href="#bib0270">[23]</a>. For the zirconia substrates, the results presented are the average of 9 values determined in an arrangement of 3x3 indentations separated 25μm. For each alumina coating, the values reported are the average and standard deviation of 6 randomly located indentations, avoiding defects like pores and grain pullout induced by polishing.Scratch tests were performed in a CSM Revetest scratch tester (CSM Instruments SA, Switzerland), using a Rockwell indenter (120° diamond cone). The length of each scratch was 5mm, and the load was increased linearly from 0 to 150N. The chipping load was taken from observations of the scratch track in the optical microscope (Olympus LEXT, Japan), identifying the point where chipping started. For each processing condition, reported values are the average of three tests and errors are the standard deviations.ResultsMicrostructure and mechanical properties<a class="elsevierStyleCrossRef" href="#fig0005">Fig. 1</a> shows characteristic cross sections of the specimens prepared using the highest solid loading (10vol%). In general, the coatings presented high density and its thickness was around 250μm. However, delamination was observed at the coating-substrate interfaces. Additionally, transverse cracks that even entered the substrates and cracks parallel to the interfaces were found in the coatings (<a class="elsevierStyleCrossRef" href="#fig0005">Fig. 1</a>c). These later two kinds of crack were very open and presented non flat faces.<elsevierMultimedia ident="fig0005"></elsevierMultimedia><a class="elsevierStyleCrossRef" href="#fig0010">Fig. 2</a> shows characteristic cross sections of the sintered specimens prepared using solid loadings of 1, 3 and 5vol%, and the measured thickness values are plotted in <a class="elsevierStyleCrossRef" href="#fig0015">Fig. 3</a>. Thickness of the coatings was coincident at both sides of the specimens and reproducibility was found for different batches. Thickness increased smoothly with solids loading of the suspension up to 5vol%. From 5 to 10vol% a sharp increase of thickness was found. The associated increase of the dispersion of data reveals the decrease of the reliability of the shaping process.<elsevierMultimedia ident="fig0010"></elsevierMultimedia><elsevierMultimedia ident="fig0015"></elsevierMultimedia>In general, defect free and dense coatings and good joining with the substrate were obtained (<a class="elsevierStyleCrossRef" href="#fig0010">Fig. 2</a>). The most frequent defects were material volumes protruding even detached from the coatings as those shown in <a class="elsevierStyleCrossRef" href="#fig0020">Fig. 4</a>.<elsevierMultimedia ident="fig0020"></elsevierMultimedia>Young's modulus and hardness of the substrates and the coatings as a function of the solid loading are plotted in <a class="elsevierStyleCrossRef" href="#fig0025">Fig. 5</a>. There are no significant differences between the values for substrates. Results for the coatings present higher relative variability (>2×) than for the substrates. Both properties are significantly lower for the coatings prepared with the suspension with the lowest and the largest solid loadings (1 and 10vol%) and similar for 3 and 5vol%. The small increase from 3 to 5vol% stays inside the variability of the results.<elsevierMultimedia ident="fig0025"></elsevierMultimedia>Scratch testsCharacteristic results of scratch tests are shown in <a class="elsevierStyleCrossRef" href="#fig0030">Fig. 6</a>. Optical microscopy micrographs of the residual scratch tracks and the corresponding plots of the evolution of different quantitative parameters as a function of the increasing normal load are provided. The localisation of the first chipping together with the corresponding value of the normal load is signalled in each micrograph. Typical values of the plotted parameters for the different shaping conditions are summarised in <a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>.<elsevierMultimedia ident="fig0030"></elsevierMultimedia><elsevierMultimedia ident="tbl0005"></elsevierMultimedia>In all specimens, scratch grooves could be optically observed from the beginning of the scratch test where the smaller loads were applied. For 1vol% specimens the whole residual groove presented bright contrast and limited lateral cracking was revealed by the dark zones at the edges of the track from loads about 110N (<a class="elsevierStyleCrossRef" href="#fig0030">Fig. 6</a>a). Extensive lateral cracking was observed from loads about 60 and 95N for 3 and 5vol% specimens, respectively. In these later cases, the bright contrast eventually disappeared under the extensive lateral cracking (<a class="elsevierStyleCrossRef" href="#fig0030">Fig. 6</a>b and c).The penetration depths increased along the scratch lengths. For 3 and 5vol%, the low-load portions of the penetration depth profiles were smooth, with negligible fluctuations up to loads about 7N where a sudden valley appeared in the profiles (<a class="elsevierStyleCrossRef" href="#fig0030">Fig. 6</a>b and c, <a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>). For these specimens, numerous clearly defined valleys appeared for applied loads higher than 30N.For specimens shaped with 1vol% solid loading (<a class="elsevierStyleCrossRef" href="#fig0030">Fig. 6</a>a, <a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>), the penetration depth curves showed defined valleys practically from the beginning of loading and especially from 15N. It has to be remarked that the actual residual depths of the grooves are smaller than the measured maximum penetration depths due to elastic recovery of the material when unloading.The frictional force is a tangential force induced by the applied normal load and the displacement of the contact between the conical tip and the sample surface when the tip is moving forward. The coefficient of friction (COF) is the ratio of this force to the applied normal load. Both parameters presented typical uncertainties in the low load region, due to the instability of the contact when the conical pointer is beginning to plow into the specimen <a class="elsevierStyleCrossRefs" href="#bib0275">[24,25]</a>. After this initial transient interval, they increased continuously with the applied force presenting some oscillations until a force of around 30N was reached. Then, the frictional force increased continuously with the applied force and presented small singular peaks and valleys practically through the whole test. Maximum values reached at the end of the test were similar for 3 and 5vol% and higher than those for 1vol%. The COF behaved accordingly and the final COF values were also similar and the highest for 3 and 5vol%.<a class="elsevierStyleCrossRef" href="#fig0035">Fig. 7</a> shows characteristic scanning electron micrographs of scratch portions about 1.2mm from the end of the scratch track of tested specimens. The characteristics of the damage are revealed by the images obtained in back scattered mode (<a class="elsevierStyleCrossRef" href="#fig0035">Fig. 7</a>d–f). All the specimens presented extensive chipping at both sides of the scratch grooves at high loads. For specimens shaped using the lowest solid contents (1 and 3vol%), the zirconia substrates are revealed while for the ones shaped using 5vol% the substrate is practically not detected meaning that chipping took place in the coating without hardly reaching the substrate.<elsevierMultimedia ident="fig0035"></elsevierMultimedia>DiscussionMicrostructure and mechanical propertiesResults described above show that it has been possible to process specimens with crack-free alumina coatings using suspensions with 1–5vol% solid loading (<a class="elsevierStyleCrossRefs" href="#fig0005">Figs. 1–4</a>) whereas extensive cracking appeared for 10vol% suspensions (<a class="elsevierStyleCrossRef" href="#fig0005">Fig. 1</a>). The obtained coatings are reproducible in thickness and density for concentrations within the range 1–5vol% of dispersed alumina in ethanol suspension. Thickness increases from values below 80μm for 5vol% suspensions (<a class="elsevierStyleCrossRefs" href="#fig0010">Figs. 2 and 3</a>) up to 250μm for a 10vol% suspension. Coating growth has not a linear evolution with the concentration of the suspension.As discussed in the introduction, residual stresses would be expected to develop during cooling from the sintering temperature in the substrate as well as in the coating due to thermal expansion mismatch between YTZP and alumina.Expected values for residual stresses can be obtained using the simple model of a symmetric laminate the thermal expansion coefficient of alumina, αA, and YTZP, αZ, (average from 25 to 1000°C: αA≈8.2×10−6K−1 and αZ≈1 0.6×10−6K−1) <a class="elsevierStyleCrossRefs" href="#bib0285">[26–28]</a> the experimental Young's modulus of the YTZP substrates and the coatings, and the mean thicknesses for the substrate, ts≈3mm, and the coatings (<a class="elsevierStyleCrossRef" href="#tbl0010">Table 2</a>, <a class="elsevierStyleCrossRef" href="#fig0025">Fig. 5</a>). 1200°C can be selected as limit temperature for accommodation of stresses because no stress relaxation by diffusional creep occurs in fine-grained alumina at temperatures lower than 1200°C <a class="elsevierStyleCrossRef" href="#bib0300">[29]</a>.<elsevierMultimedia ident="tbl0010"></elsevierMultimedia>High compressive stresses will be developed in the coatings while substrates will be subjected to very low tensile stresses (<a class="elsevierStyleCrossRef" href="#tbl0010">Table 2</a>). In principle, compressive stresses would not lead to cracking of the coatings and the expected tensile stresses are not sufficient to originate cracking of the YTZP substrates. The observed delamination in the specimens with the thickest coatings (<a class="elsevierStyleCrossRef" href="#fig0005">Fig. 1</a>) can be attributed to the presence of high shear stresses at the interfaces. The non flat faces of the additional transverse and parallel cracks with large openings reveal that they were probably formed during sintering by a combination of stresses due to thermal expansion and sintering rate mismatches.Cracks were not observed in the specimens with thinnest coatings (<a class="elsevierStyleCrossRef" href="#fig0010">Fig. 2</a>). Even though the necessary condition for delamination should be fulfilled due to the high shear stresses expected at the interfaces, the critical relationship to produce cracking between thickness and residual stresses proposed by Evans <a class="elsevierStyleCrossRef" href="#bib0300">[29]</a> is not reached in the thinner coatings.Young's modulus, E, and hardness, H, of the substrates were of the same level as those determined for dense YTZP materials by nanoindentation (E=246±12GPa, H=16.9±0.9GPa <a class="elsevierStyleCrossRef" href="#bib0275">[24]</a>.E and H values for the coating shaped from the lowest solid content suspension were the lowest and similar to those previously obtained for non-optimised coatings discussed in the introduction <a class="elsevierStyleCrossRef" href="#bib0255">[20]</a>. Both properties were higher and similar for 3 and 5vol% These later values were slightly lower than those obtained using the same experimental procedure for the reference alumina with 99% of theoretical density (E≈422GPa, H≈26GPa) <a class="elsevierStyleCrossRef" href="#bib0260">[21]</a>.The differences between the values of E and H of the coatings and those of the reference alumina should be related to the higher porosity of the former. Even though it has been reported that tensile residual stresses can affect the measured E and H<a class="elsevierStyleCrossRefs" href="#bib0305">[30,31]</a>, this effect should necessarily be very small for the coatings studied here due to the absence of pile up, as observed in other ceramics <a class="elsevierStyleCrossRef" href="#bib0315">[32]</a>. Porosity, P, in the coatings can be evaluated from the experimental Young's modulus values using the well known exponential relation between porosity and Young's modulus:E=E0exp(−bP)with E0 the Young's of the fully dense material, b, a parameter which depends on the spatial configuration of the pores assembly and the material, and P, the volume fraction of porosity.This relationship was originally obtained empirically and derived latter from MSA (Minimum Solid Area) models. These MSA models were proved to be valid up to levels of porosity of more than 20% in the case of spherical pores in cubic stacking <a class="elsevierStyleCrossRefs" href="#bib0320">[33,34]</a>.From the Young's modulus value obtained for the reference alumina using the same experimental method (<a class="elsevierStyleCrossRef" href="#bib0260">[21]</a>, 422GPa) and its porosity (1%) and the value b=3 determined for alumina with spherical pores <a class="elsevierStyleCrossRefs" href="#bib0330">[35,36]</a> it can be inferred the Young's modulus for the totally dense alumina, E0≈435GPa. From the same relationship and the experimental E values, the porosity of the coatings fabricated using the lowest solid content should be the highest and decrease for 3 and 5vol% (<a class="elsevierStyleCrossRef" href="#tbl0010">Table 2</a>).Summarising, increasing solid content in the suspension used to shape the coatings leads to increasing coating density. The sharpest increase occurs when the solid content increases from 1 to 3vol%, further raise of the solid content level to 5vol% leads to slight density improvements. Moreover, a decrease of the density and cracking of the coatings occur when more concentrated suspensions (10vol%) are used. Young's modulus and hardness increase with increasing density of the coatings.Scratch testsInduced scratch damage is a complicate phenomenon that includes a combination of processes, elastic and plastic deformation and brittle fracture. Grain dislodgement, microcracks, radial cracks and lateral cracks with associated chipping contribute to brittle fracture damage. Due to the limited slip systems in ceramics, the main deformation mechanism is brittle fracture, leading to the formation of chips along the scratch grooves, as it has been reported for bulk alumina <a class="elsevierStyleCrossRefs" href="#bib0275">[24,25,37]</a>. Plastic deformation in alumina is limited to the initial stages of the process. The behaviour of YTZP under scratch is different from that typical of ceramics as extensive plastic deformation and phase transformation might occur under localised loads <a class="elsevierStyleCrossRefs" href="#bib0345">[38,39]</a>.For the three alumina coatings studied in the present work, the characteristic features of the scratch grooves and the dependence of the quantitative parameters with increasing loads are similar to those reported for different bulk alumina materials <a class="elsevierStyleCrossRefs" href="#bib0275">[24,25,37]</a>. An increasing residual scratch groove width with load is observed in all tests. The central part of the groove is plastically deformed and surrounded by brittle damage whose extension depends on the scratch load. Brittle damage is often associated to lateral cracking and chipping.Alumina deformation can be purely elastic-plastic at the initiation of the process, which gives smooth penetration depth profiles for low depths. The appearance of a sudden valley in the profiles indicates the presence of brittle fracture. Further brittle damage is revealed by the irregular oscillation of the profile under continuously increasing load. Fluctuations are also observed in the frictional force, which increases with the applied load. Oscillations in the penetration depth and force profiles reflect the intermittent nature of material removal during scratching. Large fluctuations usually reflect the removal of large chips <a class="elsevierStyleCrossRef" href="#bib0340">[37]</a>.For the lowest density coating (1vol%, <a class="elsevierStyleCrossRef" href="#fig0030">Fig. 6</a>a, <a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>) brittle damage revealed by valleys in the penetration occurred from the beginning of the tests. It was more significant from lower loads than for the denser coatings. Maximum penetration depth was the lowest and the load for chipping was the highest. The zirconia substrate could be observed in some zones (<a class="elsevierStyleCrossRef" href="#fig0035">Fig. 7</a>a and d) indicating total detachment of the coating.Brittle damage in the high density coatings (3 and 5vol%, <a class="elsevierStyleCrossRef" href="#fig0030">Fig. 6</a>b and c, <a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>) started at higher loads and initially the severity of the damage, measured by the penetration depth, remained of the same order as for the low density coating. However, severe chipping occurred for higher loads, especially for the 5vol%, for which the zirconia substrate was clearly revealed (<a class="elsevierStyleCrossRef" href="#fig0035">Fig. 7</a>b and e).The above discussed observations can be related to scratch damage processes described as follows. In the low density coating, sinking of the porous material under the localised stresses as well as fracture initiation from pores subjected to the shear stresses originated by the tangential loads occurred from the beginning. Both processes can take place from relatively low loads and will originate material detachment from the coating. As load increases, the depth of the groove increases reaching values of the order of the coating thickness (≈12μm, <a class="elsevierStyleCrossRef" href="#fig0025">Fig. 5</a>). At this point the YTZP substrate is in direct contact with the indenter and will experience plastic deformation which accommodation would require lateral cracking (<a class="elsevierStyleCrossRefs" href="#fig0030">Figs. 6 and 7</a>a and d).The highest density coatings fabricated from suspensions with 3 and 5vol% solid loading (porosities=4 and 2.5vol%, <a class="elsevierStyleCrossRef" href="#tbl0010">Table 2</a>) did not experience sinking under the applied loads and their resistance to tangential stresses was higher. Consequently, the specimens presented relatively small maximum penetration depths (≈50–64μm, <a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>) which for the densest material (5vol%) were even smaller than the coating widths (≈60–65μm). Extensive chipping in these cases comes from deformation of the coatings and not of the substrates (<a class="elsevierStyleCrossRefs" href="#fig0030">Figs. 6b, c and 7b, f</a>). For the coatings with the lowest porosity (2.5%, <a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>), substrates remained coated after testing.ConclusionsIt is possible to obtain reliable alumina coatings on zirconia specimens by dipping porous zirconia specimens into stable alumina suspensions in such a way that a symmetric tri-layer structure is obtained.Optimum processing can be manipulated in terms of the solid content of the alumina suspension which determines the density of the coating.The optimised specimens present high resistance to scratch up to loads of 150N." "textoCompletoSecciones" => array:1 [ "secciones" => array:11 [ 0 => array:3 [ "identificador" => "xres541039" "titulo" => "Abstract" "secciones" => array:1 [ 0 => array:1 [ "identificador" => "abst0005" ] ] ] 1 => array:2 [ "identificador" => "xpalclavsec560549" "titulo" => "Keywords" ] 2 => array:3 [ "identificador" => "xres541040" "titulo" => "Resumen" "secciones" => array:1 [ 0 => array:1 [ "identificador" => "abst0010" ] ] ] 3 => array:2 [ "identificador" => "xpalclavsec560548" "titulo" => "Palabras clave" ] 4 => array:2 [ "identificador" => "sec0005" "titulo" => "Introduction" ] 5 => array:2 [ "identificador" => "sec0010" "titulo" => "Experimental" ] 6 => array:3 [ "identificador" => "sec0015" "titulo" => "Results" "secciones" => array:2 [ 0 => array:2 [ "identificador" => "sec0020" "titulo" => "Microstructure and mechanical properties" ] 1 => array:2 [ "identificador" => "sec0025" "titulo" => "Scratch tests" ] ] ] 7 => array:3 [ "identificador" => "sec0030" "titulo" => "Discussion" "secciones" => array:2 [ 0 => array:2 [ "identificador" => "sec0035" "titulo" => "Microstructure and mechanical properties" ] 1 => array:2 [ "identificador" => "sec0040" "titulo" => "Scratch tests" ] ] ] 8 => array:2 [ "identificador" => "sec0045" "titulo" => "Conclusions" ] 9 => array:2 [ "identificador" => "xack183081" "titulo" => "Acknowledgements" ] 10 => array:1 [ "titulo" => "References" ] ] ] "pdfFichero" => "main.pdf" "tienePdf" => true "fechaRecibido" => "2015-05-18" "fechaAceptado" => "2015-07-21" "PalabrasClave" => array:2 [ "en" => array:1 [ 0 => array:4 [ "clase" => "keyword" "titulo" => "Keywords" "identificador" => "xpalclavsec560549" "palabras" => array:5 [ 0 => "Alumina" 1 => "YTZP" 2 => "Coatings" 3 => "Nanoindentation" 4 => "Scratch" ] ] ] "es" => array:1 [ 0 => array:4 [ "clase" => "keyword" "titulo" => "Palabras clave" "identificador" => "xpalclavsec560548" "palabras" => array:5 [ 0 => "Alúmina" 1 => "YTZP" 2 => "Recubrimientos" 3 => "Nanoindentación" 4 => "Rayado" ] ] ] ] "tieneResumen" => true "resumen" => array:2 [ "en" => array:2 [ "titulo" => "Abstract" "resumen" => "In this work, alumina-coated YTZP materials are proposed as means to combine the mechanical reliability of YTZP with the stiffness and hardness of alumina. Additionally, compressive stresses are developed in the alumina coating when cooling from the sintering temperature due to the thermal expansion mismatch between alumina and YTZP.The proposed processing method involves dipping of pre-sintered YTZP specimens in stable alumina suspensions and co-sintering of the dipped specimens. The influence of the processing parameters on the macro and microstructure of the materials has been established. Berkovich indentation has been performed to determine the Young's modulus of the substrates and coatings. The structural integrity of the coatings has been analysed using scratch tests. The Young's modulus. The optimised specimens present high resistance to scratch up to loads of 150N." ] "es" => array:2 [ "titulo" => "Resumen" "resumen" => "En este trabajo, se proponen los materiales YTZP recubiertos de alúmina como instrumento para combinar la fiabilidad mecánica de YTZP, con la rigidez y la dureza de la alúmina. Adicionalmente, con este diseño, el material cerámico se ve reforzado por las tensiones residuales de compresión que aparecen en el recubrimiento de alúmina durante el enfriamiento desde la temperatura de sinterización, debido a la diferencia en coeficientes de expansión térmica entre la alúmina y la YTZP.El método de procesamiento propuesto implica la inmersión de muestras de YTZP pre-sinterizadas, con un 42% de porosidad abierta, en suspensiones de alúmina estables, y la posterior co-sinterización de los materiales recubiertos.En este trabajo se ha establecido la influencia de los parámetros de procesamiento en la macro y microestructura de los materiales. Así mismo, se ha determinado el módulo de elasticidad de los sustratos y de los recubrimientos mediante ensayos de nanoindentación con puntas de tipo Berkovich. Finalmente, se ha analizado la integridad estructural de los recubrimientos mediante ensayos de rayado. Los recubrimientos optimizados presentan alta resistencia al rayado bajo cargas de hasta 150N." ] ] "multimedia" => array:9 [ 0 => array:7 [ "identificador" => "fig0005" "etiqueta" => "Fig. 1" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr1.jpeg" "Alto" => 1866 "Ancho" => 990 "Tamanyo" => 114106 ] ] "descripcion" => array:1 [ "en" => "Characteristic cross sections of sintered specimens fabricated from suspensions with 10vol% of Al2O3. SEM micrographs of polished sections." ] ] 1 => array:7 [ "identificador" => "fig0010" "etiqueta" => "Fig. 2" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr2.jpeg" "Alto" => 2197 "Ancho" => 3167 "Tamanyo" => 340387 ] ] "descripcion" => array:1 [ "en" => "Characteristic cross sections of the sintered specimens. Specimens fabricated from suspensions with different solids loadings of Al2O3. Two different batches are compared for each solids loading. Low magnification SEM micrographs of polished sections. (a–b) Solid loading 1vol% Al2O3. (c–d) Solid loading 3vol% Al2O3. (e–f). Solid loading 5vol% Al2O3." ] ] 2 => array:7 [ "identificador" => "fig0015" "etiqueta" => "Fig. 3" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr3.jpeg" "Alto" => 1037 "Ancho" => 1371 "Tamanyo" => 45094 ] ] "descripcion" => array:1 [ "en" => "Evolution of Al2O3 coating thickness with the solid loading for the 4 batches of samples." ] ] 3 => array:7 [ "identificador" => "fig0020" "etiqueta" => "Fig. 4" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr4.jpeg" "Alto" => 2010 "Ancho" => 1650 "Tamanyo" => 161077 ] ] "descripcion" => array:1 [ "en" => "Cross sections of sintered specimens showing the characteristic defects found. SEM micrographs of polished sections. (a) Protruding volume. Specimen fabricated using a solid loading 1vol% Al2O3. (b) Detached volumes. Specimen fabricated using a solid loading 5vol% Al2O3." ] ] 4 => array:7 [ "identificador" => "fig0025" "etiqueta" => "Fig. 5" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr5.jpeg" "Alto" => 1055 "Ancho" => 2840 "Tamanyo" => 89143 ] ] "descripcion" => array:1 [ "en" => "Properties determined by nanoindentation as a function of the solid loading, SL, of the alumina suspension used to shape the coatings. Data previously obtained for 10vol% solids are plotted for comparison. (a) Young's modulus of the coating, Ec, and the substrate, Es. (b) Hardness of the coating, Hc, and of the substrate, Hs." ] ] 5 => array:7 [ "identificador" => "fig0030" "etiqueta" => "Fig. 6" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr6.jpeg" "Alto" => 3352 "Ancho" => 1650 "Tamanyo" => 668588 ] ] "descripcion" => array:1 [ "en" => "Characteristic results of scratch tests for specimens shaped. Optical microscopy micrograph and COF plots. The central part of the residual groove presents bright contrast and chipping is revealed by the dark zones at the edges of the groove. (a) Solid loading 1vol% Al2O3. (b) Solid loading 3vol% Al2O3. (c) Solid loading 5vol% Al2O3." ] ] 6 => array:7 [ "identificador" => "fig0035" "etiqueta" => "Fig. 7" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr7.jpeg" "Alto" => 3200 "Ancho" => 2847 "Tamanyo" => 801669 ] ] "descripcion" => array:1 [ "en" => "Scanning electron microscopy micrographs of scratch tested specimens. (a–c) Secondary electrons. (d–f) Back scattered electrons: the white phase is the zirconia substrate. (a and d) Solid loading 1vol% Al2O3. (b and e) Solid loading 3vol% Al2O3. (c and f) Solid loading 5vol% Al2O3." ] ] 7 => array:7 [ "identificador" => "tbl0005" "etiqueta" => "Table 1" "tipo" => "MULTIMEDIATABLA" "mostrarFloat" => true "mostrarDisplay" => false "tabla" => array:1 [ "tablatextoimagen" => array:1 [ 0 => array:2 [ "tabla" => array:1 [ 0 => """ <table border="0" frame="\n \t\t\t\t\tvoid\n \t\t\t\t" class=""><thead title="thead"><tr title="table-row"><th class="td" title="table-head " align="left" valign="top" scope="col" style="border-bottom: 2px solid black">Al2O3 solid loading (vol%) \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="top" scope="col" style="border-bottom: 2px solid black">Frictional force (N) \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="top" scope="col" style="border-bottom: 2px solid black">Friction coefficient \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="top" scope="col" style="border-bottom: 2px solid black">Load for penetration depth oscillation (N) \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="top" scope="col" style="border-bottom: 2px solid black">Load for starting of chipping (N) \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="top" scope="col" style="border-bottom: 2px solid black">Maximum depth (μm) \t\t\t\t\t\t\n \t\t\t\t</th></tr></thead><tbody title="tbody"><tr title="table-row"><td class="td" title="table-entry " align="char" valign="top">1 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">≈25 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">≈0.15 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">15 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">≈110 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">35–45 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="top">3 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">≈30 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">≈0.20 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">30 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">≈60 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">50–64 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="top">5 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">≈30 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">≈0.20 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">30 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">≈95 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">50–55 \t\t\t\t\t\t\n \t\t\t\t</td></tr></tbody></table> """ ] "imagenFichero" => array:1 [ 0 => "xTab868858.png" ] ] ] ] "descripcion" => array:1 [ "en" => "Typical values of the quantitative parameters recorded during the scratch tests shown in <a class="elsevierStyleCrossRef" href="#fig0030">Fig. 6</a>." ] ] 8 => array:7 [ "identificador" => "tbl0010" "etiqueta" => "Table 2" "tipo" => "MULTIMEDIATABLA" "mostrarFloat" => true "mostrarDisplay" => false "tabla" => array:1 [ "tablatextoimagen" => array:1 [ 0 => array:2 [ "tabla" => array:1 [ 0 => """ <table border="0" frame="\n \t\t\t\t\tvoid\n \t\t\t\t" class=""><thead title="thead"><tr title="table-row"><th class="td" title="table-head " align="left" valign="top" scope="col">Al2O3 solid loading (vol%) \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="top" scope="col">E substrate (GPa) \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="top" scope="col">E coating (GPa) \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="top" scope="col">Coating thickness (μm) \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " colspan="2" align="center" valign="top" scope="col" style="border-bottom: 2px solid black">Calculated residual stress (MPa)</th><th class="td" title="table-head " align="center" valign="top" scope="col">Calculated porosity (%) \t\t\t\t\t\t\n \t\t\t\t</th></tr><tr title="table-row"><th class="td" title="table-head " align="" valign="top" scope="col" style="border-bottom: 2px solid black"> \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="" valign="top" scope="col" style="border-bottom: 2px solid black"> \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="" valign="top" scope="col" style="border-bottom: 2px solid black"> \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="" valign="top" scope="col" style="border-bottom: 2px solid black"> \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="top" scope="col" style="border-bottom: 2px solid black">Substrate \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="center" valign="top" scope="col" style="border-bottom: 2px solid black">Coating \t\t\t\t\t\t\n \t\t\t\t</th><th class="td" title="table-head " align="" valign="top" scope="col" style="border-bottom: 2px solid black"> \t\t\t\t\t\t\n \t\t\t\t</th></tr></thead><tbody title="tbody"><tr title="table-row"><td class="td" title="table-entry " align="char" valign="top">1 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">261±9 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">323±29 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">12.3±0.9 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">5 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">−1160 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">10 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="top">3 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="" valign="top"> \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">386±26 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">38±2 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">17 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">−1360 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">4 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="top">5 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="" valign="top"> \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">403±31 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">65±8 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">30 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">−1380 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">2.5 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="table-entry " align="char" valign="top">10 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="" valign="top"> \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">≈330 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">≈250 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">100 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">−1100 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="table-entry " align="char" valign="top">9 \t\t\t\t\t\t\n \t\t\t\t</td></tr></tbody></table> """ ] "imagenFichero" => array:1 [ 0 => "xTab868857.png" ] ] ] ] "descripcion" => array:1 [ "en" => "Average values of the parameters used to calculate residual stresses and porosity and calculated residual stress and porosity values." ] ] ] "bibliografia" => array:2 [ "titulo" => "References" "seccion" => array:1 [ 0 => array:2 [ "identificador" => "bibs0005" "bibliografiaReferencia" => array:39 [ 0 => array:3 [ "identificador" => "bib0160" "etiqueta" => "[1]" "referencia" => array:1 [ 0 => array:2 [ "contribucion" => array:1 [ 0 => 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"vista" => "all" ] ] ] "idiomaDefecto" => "en" "url" => "/03663175/0000005400000004/v1_201508140009/S0366317515000679/v1_201508140009/en/main.assets" "Apartado" => null "PDF" => "https://static.elsevier.es/multimedia/03663175/0000005400000004/v1_201508140009/S0366317515000679/v1_201508140009/en/main.pdf?idApp=UINPBA00004N&text.app=https://www.elsevier.es/" "EPUB" => "https://multimedia.elsevier.es/PublicationsMultimediaV1/item/epub/S0366317515000679?idApp=UINPBA00004N"]

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Processing, nanoindentation and scratch testing of alumina-coated YTZP

Procesamiento, nanoindentación y rayado de materiales de YTZP recubiertos con alúmina

Jorge Vallea,b, Marc Angladab, Begoña Ferraria, Carmen Baudína,

Corresponding author

cbaudin@icv.csic.es

Corresponding author.

a Instituto de Cerámica y Vidrio, CSIC, Madrid, Spain

b Department of Materials Science and Engineering/ETSEIB, Barcelona, Spain

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<a class="elsevierStyleCrossRef" href="#bib0165">&#91;2&#93;</a> which may limit their use in some structural applications&#46; Moreover&#44; they present limitations such as environmentally assisted slow crack growth and metastable transformation of the tetragonal phase to the monoclinic phase under wet environments from room temperature up to 400<span class="elsevierStyleHsp" style=""></span>&#176;C&#46; Transformation results in surface roughening and microcracking&#44; which leads to lower wear resistance <a class="elsevierStyleCrossRefs" href="#bib0165">&#91;2&#8211;6&#93;</a>&#46; On the other hand&#44; alumina offers a unique combination of high hardness &#40;<span class="elsevierStyleItalic">H</span><span class="elsevierStyleInf">V</span><span class="elsevierStyleHsp" style=""></span>&#8776;<span class="elsevierStyleHsp" style=""></span>18&#8211;20<span class="elsevierStyleHsp" style=""></span>GPa&#41; <a class="elsevierStyleCrossRefs" href="#bib0165">&#91;2&#44;7&#93;</a>&#44; corrosion resistance&#44; thermal stability and high wear resistance <a class="elsevierStyleCrossRef" href="#bib0195">&#91;8&#93;</a>&#46; However&#44; it has relatively low fracture toughness &#40;&#8776;2&#46;5&#8211;4<span class="elsevierStyleHsp" style=""></span>MPa<span class="elsevierStyleHsp" style=""></span>m<span class="elsevierStyleSup">1&#47;2</span>&#41; and low and variable strength <a class="elsevierStyleCrossRef" href="#bib0200">&#91;9&#93;</a>&#46;</p><p id="par0010" class="elsevierStylePara elsevierViewall">One approach to solve limitations of both alumina and YTZP ceramics is the development of alumina-zirconia composites to combine the advantageous characteristics of both phases&#46; In particular&#44; zirconia-toughened alumina &#40;ZTA&#41; with low zirconia content as second phase &#40;&#60;15<span class="elsevierStyleHsp" style=""></span>vol&#37;&#41; shows higher fracture toughness and hardness than alumina and YTZP&#44; respectively <a class="elsevierStyleCrossRefs" href="#bib0165">&#91;2&#44;10&#8211;14&#93;</a>&#46; However&#44; YTZP as second phase in ZTA composites is under tension <a class="elsevierStyleCrossRef" href="#bib0225">&#91;14&#93;</a>&#44; so it is still liable to ageing in contact with wet environments <a class="elsevierStyleCrossRefs" href="#bib0165">&#91;2&#44;10&#44;13&#93;</a>&#44; although significant resistance to ageing may be achieved <a class="elsevierStyleCrossRef" href="#bib0165">&#91;2&#93;</a>&#46; Another design strategy&#44; for improving the surface hardness of YTZP materials while avoiding the problem related to phase transformation&#44; is to produce laminated structures designed to induce compressive residual stresses at the surface by combining ceramics with different thermal expansion coefficients&#46; For instance&#44; alumina-YTZP symmetrical laminated structures in which surface alumina layers are subjected to compressive residual stresses show hardness values similar to those of single phase alumina combined with improved apparent surface fracture toughness and fracture strength <a class="elsevierStyleCrossRefs" href="#bib0230">&#91;15&#8211;17&#93;</a>&#46; These laminated structures have a great potential for applications involving wear processes <a class="elsevierStyleCrossRef" href="#bib0245">&#91;18&#93;</a>&#46;</p><p id="par0015" class="elsevierStylePara elsevierViewall">In a previous work <a class="elsevierStyleCrossRef" href="#bib0250">&#91;19&#93;</a>&#44; alumina-coated YTZP materials were proposed as means to improve the wear resistance of YTZP&#46; YTZP specimens coated by homogeneous alumina layers of about 250&#8211;200<span class="elsevierStyleHsp" style=""></span>&#956;m were obtained by dipping pre-sintered YTZP substrates with open porosity in alumina suspensions and subsequent sintering&#46; The wear resistance of the coated specimens was much higher than that of monolithic YTZP&#46; However&#44; the coatings presented some porosity resulting in Young&#39;s modulus &#40;Berkovich nanoindentation&#58; <span class="elsevierStyleItalic">E</span><span class="elsevierStyleHsp" style=""></span>&#8776;<span class="elsevierStyleHsp" style=""></span>330<span class="elsevierStyleHsp" style=""></span>GPa&#41; and hardness &#40;Vickers Hardness<span class="elsevierStyleHsp" style=""></span>&#8776;<span class="elsevierStyleHsp" style=""></span>16&#46;6<span class="elsevierStyleHsp" style=""></span>GPa&#59; Berkovich nanoindentation&#58; <span class="elsevierStyleItalic">H</span><span class="elsevierStyleHsp" style=""></span>&#8776;<span class="elsevierStyleHsp" style=""></span>22<span class="elsevierStyleHsp" style=""></span>GPa&#41; <a class="elsevierStyleCrossRef" href="#bib0255">&#91;20&#93;</a> lower than those of a reference dense alumina &#40;Berkovich nanoindentation&#58; <span class="elsevierStyleItalic">E</span><span class="elsevierStyleHsp" style=""></span>&#61;<span class="elsevierStyleHsp" style=""></span>422<span class="elsevierStyleHsp" style=""></span>&#177;<span class="elsevierStyleHsp" style=""></span>10<span class="elsevierStyleHsp" style=""></span>GPa&#44; <span class="elsevierStyleItalic">H</span><span class="elsevierStyleHsp" style=""></span>&#61;<span class="elsevierStyleHsp" style=""></span>26<span class="elsevierStyleHsp" style=""></span>&#177;<span class="elsevierStyleHsp" style=""></span>1<span class="elsevierStyleHsp" style=""></span>GPa&#59; Vickers Hardness<span class="elsevierStyleHsp" style=""></span>&#61;<span class="elsevierStyleHsp" style=""></span>19&#46;6<span class="elsevierStyleHsp" style=""></span>&#177;<span class="elsevierStyleHsp" style=""></span>0&#46;6<span class="elsevierStyleHsp" style=""></span>GPa&#41; <a class="elsevierStyleCrossRefs" href="#bib0260">&#91;21&#44;22&#93;</a>&#46; The fact that coating was done only on one face of relatively large thickness specimens was identified as responsible for such poor mechanical performance&#46;</p><p id="par0020" class="elsevierStylePara elsevierViewall">In this work&#44; the processing parameters have been changed focusing coatings with higher density&#46; In order to facilitate the compatibility of shrinkages during sintering and thermal expansion strains on cooling&#44; symmetric specimens have been prepared using thinner discs and suspensions with lower solid contents&#46; As terms of comparison&#44; specimens using the suspension with high solid loading previously used have been processed and characterised&#46;</p></span><span id="sec0010" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleSectionTitle" id="sect0030">Experimental</span><p id="par0025" class="elsevierStylePara elsevierViewall">High purity commercial zirconia powders &#40;TZ-3YS&#44; Tosoh&#44; Japan&#41; were isostatically pressed &#40;200<span class="elsevierStyleHsp" style=""></span>MPa&#41; in the shape of cylinders&#46; After a thermal treatment at 1150<span class="elsevierStyleHsp" style=""></span>&#176;C for 2<span class="elsevierStyleHsp" style=""></span>h in air&#44; the cylinders were cut into discs of approximately 10<span class="elsevierStyleHsp" style=""></span>mm in diameter and 4<span class="elsevierStyleHsp" style=""></span>mm in thickness and rectified for surface conditioning&#46; Open porosity of these pre-sintered zirconia discs was approximately 43&#37;&#46;</p><p id="par0030" class="elsevierStylePara elsevierViewall">Suspensions of alumina &#40;Condea&#44; HPA05&#44; USA&#41; were prepared in ethanol to solid loadings of 2&#44; 3&#44; 5 and 10<span class="elsevierStyleHsp" style=""></span>vol&#37; adding 0&#46;5<span class="elsevierStyleHsp" style=""></span>wt&#46; &#37;&#44; on a dry solids basis&#44; of PEI &#40;Polyethylenimine&#44; Mw 2000&#44; Aldrich&#41; as dispersant&#44; and sonicated for 2<span class="elsevierStyleHsp" style=""></span>min &#40;Ultrasonication Probe&#44; UP 400S&#44; Hielscher&#44; Germany&#41;&#46; The coatings were deposited on both sides of the YTZP discs&#44; using a lift to dip and withdraw the sample in the alumina suspension&#46; The system is provided with a clam which allows grabbing the disc letting free both sides of the disc and masking edges&#46; The YTZP substrates were dipped and maintained in the alumina suspensions for 20<span class="elsevierStyleHsp" style=""></span>s&#44; after which they were withdrawn at 7<span class="elsevierStyleHsp" style=""></span>mm&#47;s&#46;</p><p id="par0035" class="elsevierStylePara elsevierViewall">After drying in air at room temperature&#44; the coated compacts were sintered at 1280<span class="elsevierStyleHsp" style=""></span>&#176;C during 4<span class="elsevierStyleHsp" style=""></span>h and 1500<span class="elsevierStyleHsp" style=""></span>&#176;C for 2<span class="elsevierStyleHsp" style=""></span>h&#44; using 2<span class="elsevierStyleHsp" style=""></span>&#176;C<span class="elsevierStyleHsp" style=""></span>min<span class="elsevierStyleSup">&#8722;1</span> as heating and cooling rates&#46; Pre-sintering and sintering temperatures were selected from sintering studies to reach full density of reference single-phase monolithic YTZP and alumina compacts while avoiding undesirable grain growth <a class="elsevierStyleCrossRef" href="#bib0265">&#91;22&#93;</a>&#46; Final dimensions of the sintered discs were diameter <span class="elsevierStyleItalic">&#966;</span><span class="elsevierStyleHsp" style=""></span>&#8776;<span class="elsevierStyleHsp" style=""></span>9&#46;5<span class="elsevierStyleHsp" style=""></span>mm and thickness&#44; <span class="elsevierStyleItalic">t</span><span class="elsevierStyleHsp" style=""></span>&#8776;<span class="elsevierStyleHsp" style=""></span>3<span class="elsevierStyleHsp" style=""></span>mm&#46;</p><p id="par0040" class="elsevierStylePara elsevierViewall">The three series of specimens corresponding to the three different solids loading in the suspensions &#40;2&#44; 3 and 5<span class="elsevierStyleHsp" style=""></span>vol&#37;&#41; were fabricated in four different batches&#44; named S0&#44; S1&#44; S2 and S3&#44; to investigate reproducibility of the shaping process&#46;</p><p id="par0045" class="elsevierStylePara elsevierViewall">Diamond machined cross sections of the sintered coated specimens were characterised by scanning electron microscopy &#40;Tabletop Microscope TM-1000&#44; Hitachi&#44; Japan&#41; after polishing with diamond past of sizes down to 3<span class="elsevierStyleHsp" style=""></span>&#956;m&#46; The thickness of the coatings was measured directly from the micrographs&#46;</p><p id="par0050" class="elsevierStylePara elsevierViewall">Young Modulus and hardness determinations were carried out in the cross sections&#46; Berkovich nanoindentations of 500<span class="elsevierStyleHsp" style=""></span>nm depth were done &#40;MTS Nanoindenter XP&#44; USA&#41; and the load-penetration depth curves during the tests were recorded&#46; Data were taken in continuous stiffness measurement &#40;CSM&#41; mode&#44; in which a simultaneous oscillating force is applied which is several orders of magnitude lower than the nominal load&#44; allowing measuring the properties in the whole range of penetration&#46; Hardness values and Young&#39;s modulus given are those corresponding to maximum penetration depth and they were calculated using the method of Oliver and Pharr <a class="elsevierStyleCrossRef" href="#bib0270">&#91;23&#93;</a>&#46; For the zirconia substrates&#44; the results presented are the average of 9 values determined in an arrangement of 3x3 indentations separated 25<span class="elsevierStyleHsp" style=""></span>&#956;m&#46; For each alumina coating&#44; the values reported are the average and standard deviation of 6 randomly located indentations&#44; avoiding defects like pores and grain pullout induced by polishing&#46;</p><p id="par0055" class="elsevierStylePara elsevierViewall">Scratch tests were performed in a CSM Revetest scratch tester &#40;CSM Instruments SA&#44; Switzerland&#41;&#44; using a Rockwell indenter &#40;120&#176; diamond cone&#41;&#46; The length of each scratch was 5<span class="elsevierStyleHsp" style=""></span>mm&#44; and the load was increased linearly from 0 to 150<span class="elsevierStyleHsp" style=""></span>N&#46; The chipping load was taken from observations of the scratch track in the optical microscope &#40;Olympus LEXT&#44; Japan&#41;&#44; identifying the point where chipping started&#46; For each processing condition&#44; reported values are the average of three tests and errors are the standard deviations&#46;</p></span><span id="sec0015" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleSectionTitle" id="sect0035">Results</span><span id="sec0020" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleSectionTitle" id="sect0040">Microstructure and mechanical properties</span><p id="par0060" class="elsevierStylePara elsevierViewall"><a class="elsevierStyleCrossRef" href="#fig0005">Fig&#46; 1</a> shows characteristic cross sections of the specimens prepared using the highest solid loading &#40;10<span class="elsevierStyleHsp" style=""></span>vol&#37;&#41;&#46; In general&#44; the coatings presented high density and its thickness was around 250<span class="elsevierStyleHsp" style=""></span>&#956;m&#46; However&#44; delamination was observed at the coating-substrate interfaces&#46; Additionally&#44; transverse cracks that even entered the substrates and cracks parallel to the interfaces were found in the coatings &#40;<a class="elsevierStyleCrossRef" href="#fig0005">Fig&#46; 1</a>c&#41;&#46; These later two kinds of crack were very open and presented non flat faces&#46;</p><elsevierMultimedia ident="fig0005"></elsevierMultimedia><p id="par0065" class="elsevierStylePara elsevierViewall"><a class="elsevierStyleCrossRef" href="#fig0010">Fig&#46; 2</a> shows characteristic cross sections of the sintered specimens prepared using solid loadings of 1&#44; 3 and 5<span class="elsevierStyleHsp" style=""></span>vol&#37;&#44; and the measured thickness values are plotted in <a class="elsevierStyleCrossRef" href="#fig0015">Fig&#46; 3</a>&#46; Thickness of the coatings was coincident at both sides of the specimens and reproducibility was found for different batches&#46; Thickness increased smoothly with solids loading of the suspension up to 5<span class="elsevierStyleHsp" style=""></span>vol&#37;&#46; From 5 to 10<span class="elsevierStyleHsp" style=""></span>vol&#37; a sharp increase of thickness was found&#46; The associated increase of the dispersion of data reveals the decrease of the reliability of the shaping process&#46;</p><elsevierMultimedia ident="fig0010"></elsevierMultimedia><elsevierMultimedia ident="fig0015"></elsevierMultimedia><p id="par0070" class="elsevierStylePara elsevierViewall">In general&#44; defect free and dense coatings and good joining with the substrate were obtained &#40;<a class="elsevierStyleCrossRef" href="#fig0010">Fig&#46; 2</a>&#41;&#46; The most frequent defects were material volumes protruding even detached from the coatings as those shown in <a class="elsevierStyleCrossRef" href="#fig0020">Fig&#46; 4</a>&#46;</p><elsevierMultimedia ident="fig0020"></elsevierMultimedia><p id="par0075" class="elsevierStylePara elsevierViewall">Young&#39;s modulus and hardness of the substrates and the coatings as a function of the solid loading are plotted in <a class="elsevierStyleCrossRef" href="#fig0025">Fig&#46; 5</a>&#46; There are no significant differences between the values for substrates&#46; Results for the coatings present higher relative variability &#40;&#62;2&#215;&#41; than for the substrates&#46; Both properties are significantly lower for the coatings prepared with the suspension with the lowest and the largest solid loadings &#40;1 and 10<span class="elsevierStyleHsp" style=""></span>vol&#37;&#41; and similar for 3 and 5<span class="elsevierStyleHsp" style=""></span>vol&#37;&#46; The small increase from 3 to 5<span class="elsevierStyleHsp" style=""></span>vol&#37; stays inside the variability of the results&#46;</p><elsevierMultimedia ident="fig0025"></elsevierMultimedia></span><span id="sec0025" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleSectionTitle" id="sect0045">Scratch tests</span><p id="par0080" class="elsevierStylePara elsevierViewall">Characteristic results of scratch tests are shown in <a class="elsevierStyleCrossRef" href="#fig0030">Fig&#46; 6</a>&#46; Optical microscopy micrographs of the residual scratch tracks and the corresponding plots of the evolution of different quantitative parameters as a function of the increasing normal load are provided&#46; The localisation of the first chipping together with the corresponding value of the normal load is signalled in each micrograph&#46; Typical values of the plotted parameters for the different shaping conditions are summarised in <a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>&#46;</p><elsevierMultimedia ident="fig0030"></elsevierMultimedia><elsevierMultimedia ident="tbl0005"></elsevierMultimedia><p id="par0085" class="elsevierStylePara elsevierViewall">In all specimens&#44; scratch grooves could be optically observed from the beginning of the scratch test where the smaller loads were applied&#46; For 1<span class="elsevierStyleHsp" style=""></span>vol&#37; specimens the whole residual groove presented bright contrast and limited lateral cracking was revealed by the dark zones at the edges of the track from loads about 110<span class="elsevierStyleHsp" style=""></span>N &#40;<a class="elsevierStyleCrossRef" href="#fig0030">Fig&#46; 6</a>a&#41;&#46; Extensive lateral cracking was observed from loads about 60 and 95<span class="elsevierStyleHsp" style=""></span>N for 3 and 5<span class="elsevierStyleHsp" style=""></span>vol&#37; specimens&#44; respectively&#46; In these later cases&#44; the bright contrast eventually disappeared under the extensive lateral cracking &#40;<a class="elsevierStyleCrossRef" href="#fig0030">Fig&#46; 6</a>b and c&#41;&#46;</p><p id="par0090" class="elsevierStylePara elsevierViewall">The penetration depths increased along the scratch lengths&#46; For 3 and 5<span class="elsevierStyleHsp" style=""></span>vol&#37;&#44; the low-load portions of the penetration depth profiles were smooth&#44; with negligible fluctuations up to loads about 7<span class="elsevierStyleHsp" style=""></span>N where a sudden valley appeared in the profiles &#40;<a class="elsevierStyleCrossRef" href="#fig0030">Fig&#46; 6</a>b and c&#44; <a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>&#41;&#46; For these specimens&#44; numerous clearly defined valleys appeared for applied loads higher than 30<span class="elsevierStyleHsp" style=""></span>N&#46;</p><p id="par0095" class="elsevierStylePara elsevierViewall">For specimens shaped with 1<span class="elsevierStyleHsp" style=""></span>vol&#37; solid loading &#40;<a class="elsevierStyleCrossRef" href="#fig0030">Fig&#46; 6</a>a&#44; <a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>&#41;&#44; the penetration depth curves showed defined valleys practically from the beginning of loading and especially from 15<span class="elsevierStyleHsp" style=""></span>N&#46; It has to be remarked that the actual residual depths of the grooves are smaller than the measured maximum penetration depths due to elastic recovery of the material when unloading&#46;</p><p id="par0100" class="elsevierStylePara elsevierViewall">The frictional force is a tangential force induced by the applied normal load and the displacement of the contact between the conical tip and the sample surface when the tip is moving forward&#46; The coefficient of friction &#40;COF&#41; is the ratio of this force to the applied normal load&#46; Both parameters presented typical uncertainties in the low load region&#44; due to the instability of the contact when the conical pointer is beginning to plow into the specimen <a class="elsevierStyleCrossRefs" href="#bib0275">&#91;24&#44;25&#93;</a>&#46; After this initial transient interval&#44; they increased continuously with the applied force presenting some oscillations until a force of around 30<span class="elsevierStyleHsp" style=""></span>N was reached&#46; Then&#44; the frictional force increased continuously with the applied force and presented small singular peaks and valleys practically through the whole test&#46; Maximum values reached at the end of the test were similar for 3 and 5<span class="elsevierStyleHsp" style=""></span>vol&#37; and higher than those for 1<span class="elsevierStyleHsp" style=""></span>vol&#37;&#46; The COF behaved accordingly and the final COF values were also similar and the highest for 3 and 5<span class="elsevierStyleHsp" style=""></span>vol&#37;&#46;</p><p id="par0105" class="elsevierStylePara elsevierViewall"><a class="elsevierStyleCrossRef" href="#fig0035">Fig&#46; 7</a> shows characteristic scanning electron micrographs of scratch portions about 1&#46;2<span class="elsevierStyleHsp" style=""></span>mm from the end of the scratch track of tested specimens&#46; The characteristics of the damage are revealed by the images obtained in back scattered mode &#40;<a class="elsevierStyleCrossRef" href="#fig0035">Fig&#46; 7</a>d&#8211;f&#41;&#46; All the specimens presented extensive chipping at both sides of the scratch grooves at high loads&#46; For specimens shaped using the lowest solid contents &#40;1 and 3<span class="elsevierStyleHsp" style=""></span>vol&#37;&#41;&#44; the zirconia substrates are revealed while for the ones shaped using 5<span class="elsevierStyleHsp" style=""></span>vol&#37; the substrate is practically not detected meaning that chipping took place in the coating without hardly reaching the substrate&#46;</p><elsevierMultimedia ident="fig0035"></elsevierMultimedia></span></span><span id="sec0030" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleSectionTitle" id="sect0050">Discussion</span><span id="sec0035" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleSectionTitle" id="sect0055">Microstructure and mechanical properties</span><p id="par0110" class="elsevierStylePara elsevierViewall">Results described above show that it has been possible to process specimens with crack-free alumina coatings using suspensions with 1&#8211;5<span class="elsevierStyleHsp" style=""></span>vol&#37; solid loading &#40;<a class="elsevierStyleCrossRefs" href="#fig0005">Figs&#46; 1&#8211;4</a>&#41; whereas extensive cracking appeared for 10<span class="elsevierStyleHsp" style=""></span>vol&#37; suspensions &#40;<a class="elsevierStyleCrossRef" href="#fig0005">Fig&#46; 1</a>&#41;&#46; The obtained coatings are reproducible in thickness and density for concentrations within the range 1&#8211;5<span class="elsevierStyleHsp" style=""></span>vol&#37; of dispersed alumina in ethanol suspension&#46; Thickness increases from values below 80<span class="elsevierStyleHsp" style=""></span>&#956;m for 5<span class="elsevierStyleHsp" style=""></span>vol&#37; suspensions &#40;<a class="elsevierStyleCrossRefs" href="#fig0010">Figs&#46; 2 and 3</a>&#41; up to 250<span class="elsevierStyleHsp" style=""></span>&#956;m for a 10<span class="elsevierStyleHsp" style=""></span>vol&#37; suspension&#46; Coating growth has not a linear evolution with the concentration of the suspension&#46;</p><p id="par0115" class="elsevierStylePara elsevierViewall">As discussed in the introduction&#44; residual stresses would be expected to develop during cooling from the sintering temperature in the substrate as well as in the coating due to thermal expansion mismatch between YTZP and alumina&#46;</p><p id="par0120" class="elsevierStylePara elsevierViewall">Expected values for residual stresses can be obtained using the simple model of a symmetric laminate the thermal expansion coefficient of alumina&#44; <span class="elsevierStyleItalic">&#945;</span><span class="elsevierStyleInf">A</span>&#44; and YTZP&#44; <span class="elsevierStyleItalic">&#945;</span><span class="elsevierStyleInf">Z</span>&#44; &#40;average from 25 to 1000<span class="elsevierStyleHsp" style=""></span>&#176;C&#58; <span class="elsevierStyleItalic">&#945;</span><span class="elsevierStyleInf">A</span><span class="elsevierStyleHsp" style=""></span>&#8776;<span class="elsevierStyleHsp" style=""></span>8&#46;2<span class="elsevierStyleHsp" style=""></span>&#215;<span class="elsevierStyleHsp" style=""></span>10<span class="elsevierStyleSup">&#8722;6</span><span class="elsevierStyleHsp" style=""></span>K<span class="elsevierStyleSup">&#8722;1</span> and <span class="elsevierStyleItalic">&#945;</span><span class="elsevierStyleInf">Z</span><span class="elsevierStyleHsp" style=""></span>&#8776;<span class="elsevierStyleHsp" style=""></span>1 0&#46;6<span class="elsevierStyleHsp" style=""></span>&#215;<span class="elsevierStyleHsp" style=""></span>10<span class="elsevierStyleSup">&#8722;6</span><span class="elsevierStyleHsp" style=""></span>K<span class="elsevierStyleSup">&#8722;1</span>&#41; <a class="elsevierStyleCrossRefs" href="#bib0285">&#91;26&#8211;28&#93;</a> the experimental Young&#39;s modulus of the YTZP substrates and the coatings&#44; and the mean thicknesses for the substrate&#44; <span class="elsevierStyleItalic">t</span><span class="elsevierStyleInf">s</span><span class="elsevierStyleHsp" style=""></span>&#8776;<span class="elsevierStyleHsp" style=""></span>3<span class="elsevierStyleHsp" style=""></span>mm&#44; and the coatings &#40;<a class="elsevierStyleCrossRef" href="#tbl0010">Table 2</a>&#44; <a class="elsevierStyleCrossRef" href="#fig0025">Fig&#46; 5</a>&#41;&#46; 1200<span class="elsevierStyleHsp" style=""></span>&#176;C can be selected as limit temperature for accommodation of stresses because no stress relaxation by diffusional creep occurs in fine-grained alumina at temperatures lower than 1200<span class="elsevierStyleHsp" style=""></span>&#176;C <a class="elsevierStyleCrossRef" href="#bib0300">&#91;29&#93;</a>&#46;</p><elsevierMultimedia ident="tbl0010"></elsevierMultimedia><p id="par0125" class="elsevierStylePara elsevierViewall">High compressive stresses will be developed in the coatings while substrates will be subjected to very low tensile stresses &#40;<a class="elsevierStyleCrossRef" href="#tbl0010">Table 2</a>&#41;&#46; In principle&#44; compressive stresses would not lead to cracking of the coatings and the expected tensile stresses are not sufficient to originate cracking of the YTZP substrates&#46; The observed delamination in the specimens with the thickest coatings &#40;<a class="elsevierStyleCrossRef" href="#fig0005">Fig&#46; 1</a>&#41; can be attributed to the presence of high shear stresses at the interfaces&#46; The non flat faces of the additional transverse and parallel cracks with large openings reveal that they were probably formed during sintering by a combination of stresses due to thermal expansion and sintering rate mismatches&#46;</p><p id="par0130" class="elsevierStylePara elsevierViewall">Cracks were not observed in the specimens with thinnest coatings &#40;<a class="elsevierStyleCrossRef" href="#fig0010">Fig&#46; 2</a>&#41;&#46; Even though the necessary condition for delamination should be fulfilled due to the high shear stresses expected at the interfaces&#44; the critical relationship to produce cracking between thickness and residual stresses proposed by Evans <a class="elsevierStyleCrossRef" href="#bib0300">&#91;29&#93;</a> is not reached in the thinner coatings&#46;</p><p id="par0135" class="elsevierStylePara elsevierViewall">Young&#39;s modulus&#44; <span class="elsevierStyleItalic">E</span>&#44; and hardness&#44; <span class="elsevierStyleItalic">H</span>&#44; of the substrates were of the same level as those determined for dense YTZP materials by nanoindentation &#40;<span class="elsevierStyleItalic">E</span><span class="elsevierStyleHsp" style=""></span>&#61;<span class="elsevierStyleHsp" style=""></span>246<span class="elsevierStyleHsp" style=""></span>&#177;<span class="elsevierStyleHsp" style=""></span>12<span class="elsevierStyleHsp" style=""></span>GPa&#44; <span class="elsevierStyleItalic">H</span><span class="elsevierStyleHsp" style=""></span>&#61;<span class="elsevierStyleHsp" style=""></span>16&#46;9<span class="elsevierStyleHsp" style=""></span>&#177;<span class="elsevierStyleHsp" style=""></span>0&#46;9<span class="elsevierStyleHsp" style=""></span>GPa <a class="elsevierStyleCrossRef" href="#bib0275">&#91;24&#93;</a>&#46;</p><p id="par0140" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleItalic">E</span> and <span class="elsevierStyleItalic">H</span> values for the coating shaped from the lowest solid content suspension were the lowest and similar to those previously obtained for non-optimised coatings discussed in the introduction <a class="elsevierStyleCrossRef" href="#bib0255">&#91;20&#93;</a>&#46; Both properties were higher and similar for 3 and 5<span class="elsevierStyleHsp" style=""></span>vol&#37; These later values were slightly lower than those obtained using the same experimental procedure for the reference alumina with 99&#37; of theoretical density &#40;<span class="elsevierStyleItalic">E</span><span class="elsevierStyleHsp" style=""></span>&#8776;<span class="elsevierStyleHsp" style=""></span>422<span class="elsevierStyleHsp" style=""></span>GPa&#44; <span class="elsevierStyleItalic">H</span><span class="elsevierStyleHsp" style=""></span>&#8776;<span class="elsevierStyleHsp" style=""></span>26<span class="elsevierStyleHsp" style=""></span>GPa&#41; <a class="elsevierStyleCrossRef" href="#bib0260">&#91;21&#93;</a>&#46;</p><p id="par0145" class="elsevierStylePara elsevierViewall">The differences between the values of E and H of the coatings and those of the reference alumina should be related to the higher porosity of the former&#46; Even though it has been reported that tensile residual stresses can affect the measured <span class="elsevierStyleItalic">E</span> and <span class="elsevierStyleItalic">H</span><a class="elsevierStyleCrossRefs" href="#bib0305">&#91;30&#44;31&#93;</a>&#44; this effect should necessarily be very small for the coatings studied here due to the absence of pile up&#44; as observed in other ceramics <a class="elsevierStyleCrossRef" href="#bib0315">&#91;32&#93;</a>&#46; Porosity&#44; <span class="elsevierStyleItalic">P</span>&#44; in the coatings can be evaluated from the experimental Young&#39;s modulus values using the well known exponential relation between porosity and Young&#39;s modulus&#58;E&#61;E0exp&#40;&#8722;bP&#41;with <span class="elsevierStyleItalic">E</span><span class="elsevierStyleInf">0</span> the Young&#39;s of the fully dense material&#44; <span class="elsevierStyleItalic">b</span>&#44; a parameter which depends on the spatial configuration of the pores assembly and the material&#44; and <span class="elsevierStyleItalic">P</span>&#44; the volume fraction of porosity&#46;</p><p id="par0150" class="elsevierStylePara elsevierViewall">This relationship was originally obtained empirically and derived latter from MSA &#40;Minimum Solid Area&#41; models&#46; These MSA models were proved to be valid up to levels of porosity of more than 20&#37; in the case of spherical pores in cubic stacking <a class="elsevierStyleCrossRefs" href="#bib0320">&#91;33&#44;34&#93;</a>&#46;</p><p id="par0155" class="elsevierStylePara elsevierViewall">From the Young&#39;s modulus value obtained for the reference alumina using the same experimental method &#40;<a class="elsevierStyleCrossRef" href="#bib0260">&#91;21&#93;</a>&#44; 422<span class="elsevierStyleHsp" style=""></span>GPa&#41; and its porosity &#40;1&#37;&#41; and the value <span class="elsevierStyleItalic">b</span><span class="elsevierStyleHsp" style=""></span>&#61;<span class="elsevierStyleHsp" style=""></span>3 determined for alumina with spherical pores <a class="elsevierStyleCrossRefs" href="#bib0330">&#91;35&#44;36&#93;</a> it can be inferred the Young&#39;s modulus for the totally dense alumina&#44; <span class="elsevierStyleItalic">E</span><span class="elsevierStyleInf">0</span><span class="elsevierStyleHsp" style=""></span>&#8776;<span class="elsevierStyleHsp" style=""></span>435<span class="elsevierStyleHsp" style=""></span>GPa&#46; From the same relationship and the experimental <span class="elsevierStyleItalic">E</span> values&#44; the porosity of the coatings fabricated using the lowest solid content should be the highest and decrease for 3 and 5<span class="elsevierStyleHsp" style=""></span>vol&#37; &#40;<a class="elsevierStyleCrossRef" href="#tbl0010">Table 2</a>&#41;&#46;</p><p id="par0160" class="elsevierStylePara elsevierViewall">Summarising&#44; increasing solid content in the suspension used to shape the coatings leads to increasing coating density&#46; The sharpest increase occurs when the solid content increases from 1 to 3<span class="elsevierStyleHsp" style=""></span>vol&#37;&#44; further raise of the solid content level to 5<span class="elsevierStyleHsp" style=""></span>vol&#37; leads to slight density improvements&#46; Moreover&#44; a decrease of the density and cracking of the coatings occur when more concentrated suspensions &#40;10<span class="elsevierStyleHsp" style=""></span>vol&#37;&#41; are used&#46; Young&#39;s modulus and hardness increase with increasing density of the coatings&#46;</p></span><span id="sec0040" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleSectionTitle" id="sect0060">Scratch tests</span><p id="par0165" class="elsevierStylePara elsevierViewall">Induced scratch damage is a complicate phenomenon that includes a combination of processes&#44; elastic and plastic deformation and brittle fracture&#46; Grain dislodgement&#44; microcracks&#44; radial cracks and lateral cracks with associated chipping contribute to brittle fracture damage&#46; Due to the limited slip systems in ceramics&#44; the main deformation mechanism is brittle fracture&#44; leading to the formation of chips along the scratch grooves&#44; as it has been reported for bulk alumina <a class="elsevierStyleCrossRefs" href="#bib0275">&#91;24&#44;25&#44;37&#93;</a>&#46; Plastic deformation in alumina is limited to the initial stages of the process&#46; The behaviour of YTZP under scratch is different from that typical of ceramics as extensive plastic deformation and phase transformation might occur under localised loads <a class="elsevierStyleCrossRefs" href="#bib0345">&#91;38&#44;39&#93;</a>&#46;</p><p id="par0170" class="elsevierStylePara elsevierViewall">For the three alumina coatings studied in the present work&#44; the characteristic features of the scratch grooves and the dependence of the quantitative parameters with increasing loads are similar to those reported for different bulk alumina materials <a class="elsevierStyleCrossRefs" href="#bib0275">&#91;24&#44;25&#44;37&#93;</a>&#46; An increasing residual scratch groove width with load is observed in all tests&#46; The central part of the groove is plastically deformed and surrounded by brittle damage whose extension depends on the scratch load&#46; Brittle damage is often associated to lateral cracking and chipping&#46;</p><p id="par0175" class="elsevierStylePara elsevierViewall">Alumina deformation can be purely elastic-plastic at the initiation of the process&#44; which gives smooth penetration depth profiles for low depths&#46; The appearance of a sudden valley in the profiles indicates the presence of brittle fracture&#46; Further brittle damage is revealed by the irregular oscillation of the profile under continuously increasing load&#46; Fluctuations are also observed in the frictional force&#44; which increases with the applied load&#46; Oscillations in the penetration depth and force profiles reflect the intermittent nature of material removal during scratching&#46; Large fluctuations usually reflect the removal of large chips <a class="elsevierStyleCrossRef" href="#bib0340">&#91;37&#93;</a>&#46;</p><p id="par0180" class="elsevierStylePara elsevierViewall">For the lowest density coating &#40;1<span class="elsevierStyleHsp" style=""></span>vol&#37;&#44; <a class="elsevierStyleCrossRef" href="#fig0030">Fig&#46; 6</a>a&#44; <a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>&#41; brittle damage revealed by valleys in the penetration occurred from the beginning of the tests&#46; It was more significant from lower loads than for the denser coatings&#46; Maximum penetration depth was the lowest and the load for chipping was the highest&#46; The zirconia substrate could be observed in some zones &#40;<a class="elsevierStyleCrossRef" href="#fig0035">Fig&#46; 7</a>a and d&#41; indicating total detachment of the coating&#46;</p><p id="par0185" class="elsevierStylePara elsevierViewall">Brittle damage in the high density coatings &#40;3 and 5<span class="elsevierStyleHsp" style=""></span>vol&#37;&#44; <a class="elsevierStyleCrossRef" href="#fig0030">Fig&#46; 6</a>b and c&#44; <a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>&#41; started at higher loads and initially the severity of the damage&#44; measured by the penetration depth&#44; remained of the same order as for the low density coating&#46; However&#44; severe chipping occurred for higher loads&#44; especially for the 5<span class="elsevierStyleHsp" style=""></span>vol&#37;&#44; for which the zirconia substrate was clearly revealed &#40;<a class="elsevierStyleCrossRef" href="#fig0035">Fig&#46; 7</a>b and e&#41;&#46;</p><p id="par0190" class="elsevierStylePara elsevierViewall">The above discussed observations can be related to scratch damage processes described as follows&#46; In the low density coating&#44; sinking of the porous material under the localised stresses as well as fracture initiation from pores subjected to the shear stresses originated by the tangential loads occurred from the beginning&#46; Both processes can take place from relatively low loads and will originate material detachment from the coating&#46; As load increases&#44; the depth of the groove increases reaching values of the order of the coating thickness &#40;&#8776;12<span class="elsevierStyleHsp" style=""></span>&#956;m&#44; <a class="elsevierStyleCrossRef" href="#fig0025">Fig&#46; 5</a>&#41;&#46; At this point the YTZP substrate is in direct contact with the indenter and will experience plastic deformation which accommodation would require lateral cracking &#40;<a class="elsevierStyleCrossRefs" href="#fig0030">Figs&#46; 6 and 7</a>a and d&#41;&#46;</p><p id="par0195" class="elsevierStylePara elsevierViewall">The highest density coatings fabricated from suspensions with 3 and 5<span class="elsevierStyleHsp" style=""></span>vol&#37; solid loading &#40;porosities<span class="elsevierStyleHsp" style=""></span>&#61;<span class="elsevierStyleHsp" style=""></span>4 and 2&#46;5<span class="elsevierStyleHsp" style=""></span>vol&#37;&#44; <a class="elsevierStyleCrossRef" href="#tbl0010">Table 2</a>&#41; did not experience sinking under the applied loads and their resistance to tangential stresses was higher&#46; Consequently&#44; the specimens presented relatively small maximum penetration depths &#40;&#8776;50&#8211;64<span class="elsevierStyleHsp" style=""></span>&#956;m&#44; <a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>&#41; which for the densest material &#40;5<span class="elsevierStyleHsp" style=""></span>vol&#37;&#41; were even smaller than the coating widths &#40;&#8776;60&#8211;65<span class="elsevierStyleHsp" style=""></span>&#956;m&#41;&#46; Extensive chipping in these cases comes from deformation of the coatings and not of the substrates &#40;<a class="elsevierStyleCrossRefs" href="#fig0030">Figs&#46; 6b&#44; c and 7b&#44; f</a>&#41;&#46; For the coatings with the lowest porosity &#40;2&#46;5&#37;&#44; <a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>&#41;&#44; substrates remained coated after testing&#46;</p></span></span><span id="sec0045" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleSectionTitle" id="sect0065">Conclusions</span><p id="par0200" class="elsevierStylePara elsevierViewall">It is possible to obtain reliable alumina coatings on zirconia specimens by dipping porous zirconia specimens into stable alumina suspensions in such a way that a symmetric tri-layer structure is obtained&#46;</p><p id="par0205" class="elsevierStylePara elsevierViewall">Optimum processing can be manipulated in terms of the solid content of the alumina suspension which determines the density of the coating&#46;The optimised specimens present high resistance to scratch up to loads of 150<span class="elsevierStyleHsp" style=""></span>N&#46;</p></span></span>"
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              "titulo" => "Microstructure and mechanical properties"
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            2 => "Coatings"
            3 => "Nanoindentation"
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        "resumen" => "<span id="abst0005" class="elsevierStyleSection elsevierViewall"><p id="spar0005" class="elsevierStyleSimplePara elsevierViewall">In this work&#44; alumina-coated YTZP materials are proposed as means to combine the mechanical reliability of YTZP with the stiffness and hardness of alumina&#46; Additionally&#44; compressive stresses are developed in the alumina coating when cooling from the sintering temperature due to the thermal expansion mismatch between alumina and YTZP&#46;</p><p id="spar0010" class="elsevierStyleSimplePara elsevierViewall">The proposed processing method involves dipping of pre-sintered YTZP specimens in stable alumina suspensions and co-sintering of the dipped specimens&#46; The influence of the processing parameters on the macro and microstructure of the materials has been established&#46; Berkovich indentation has been performed to determine the Young&#39;s modulus of the substrates and coatings&#46; The structural integrity of the coatings has been analysed using scratch tests&#46; The Young&#39;s modulus&#46; The optimised specimens present high resistance to scratch up to loads of 150<span class="elsevierStyleHsp" style=""></span>N&#46;</p></span>"
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        "resumen" => "<span id="abst0010" class="elsevierStyleSection elsevierViewall"><p id="spar0015" class="elsevierStyleSimplePara elsevierViewall">En este trabajo&#44; se proponen los materiales YTZP recubiertos de al&#250;mina como instrumento para combinar la fiabilidad mec&#225;nica de YTZP&#44; con la rigidez y la dureza de la al&#250;mina&#46; Adicionalmente&#44; con este dise&#241;o&#44; el material cer&#225;mico se ve reforzado por las tensiones residuales de compresi&#243;n que aparecen en el recubrimiento de al&#250;mina durante el enfriamiento desde la temperatura de sinterizaci&#243;n&#44; debido a la diferencia en coeficientes de expansi&#243;n t&#233;rmica entre la al&#250;mina y la YTZP&#46;</p><p id="spar0020" class="elsevierStyleSimplePara elsevierViewall">El m&#233;todo de procesamiento propuesto implica la inmersi&#243;n de muestras de YTZP pre-sinterizadas&#44; con un 42&#37; de porosidad abierta&#44; en suspensiones de al&#250;mina estables&#44; y la posterior co-sinterizaci&#243;n de los materiales recubiertos&#46;</p><p id="spar0025" class="elsevierStyleSimplePara elsevierViewall">En este trabajo se ha establecido la influencia de los par&#225;metros de procesamiento en la macro y microestructura de los materiales&#46; As&#237; mismo&#44; se ha determinado el m&#243;dulo de elasticidad de los sustratos y de los recubrimientos mediante ensayos de nanoindentaci&#243;n con puntas de tipo Berkovich&#46; Finalmente&#44; se ha analizado la integridad estructural de los recubrimientos mediante ensayos de rayado&#46; Los recubrimientos optimizados presentan alta resistencia al rayado bajo cargas de hasta 150<span class="elsevierStyleHsp" style=""></span>N&#46;</p></span>"
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          "en" => "<p id="spar0030" class="elsevierStyleSimplePara elsevierViewall">Characteristic cross sections of sintered specimens fabricated from suspensions with 10<span class="elsevierStyleHsp" style=""></span>vol&#37; of Al<span class="elsevierStyleInf">2</span>O<span class="elsevierStyleInf">3</span>&#46; SEM micrographs of polished sections&#46;</p>"
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          "en" => "<p id="spar0035" class="elsevierStyleSimplePara elsevierViewall">Characteristic cross sections of the sintered specimens&#46; Specimens fabricated from suspensions with different solids loadings of Al<span class="elsevierStyleInf">2</span>O<span class="elsevierStyleInf">3</span>&#46; Two different batches are compared for each solids loading&#46; Low magnification SEM micrographs of polished sections&#46; &#40;a&#8211;b&#41; Solid loading 1<span class="elsevierStyleHsp" style=""></span>vol&#37; Al<span class="elsevierStyleInf">2</span>O<span class="elsevierStyleInf">3</span>&#46; &#40;c&#8211;d&#41; Solid loading 3<span class="elsevierStyleHsp" style=""></span>vol&#37; Al<span class="elsevierStyleInf">2</span>O<span class="elsevierStyleInf">3</span>&#46; &#40;e&#8211;f&#41;&#46; Solid loading 5<span class="elsevierStyleHsp" style=""></span>vol&#37; Al<span class="elsevierStyleInf">2</span>O<span class="elsevierStyleInf">3</span>&#46;</p>"
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          "en" => "<p id="spar0050" class="elsevierStyleSimplePara elsevierViewall">Properties determined by nanoindentation as a function of the solid loading&#44; SL&#44; of the alumina suspension used to shape the coatings&#46; Data previously obtained for 10<span class="elsevierStyleHsp" style=""></span>vol&#37; solids are plotted for comparison&#46; &#40;a&#41; Young&#39;s modulus of the coating&#44; Ec&#44; and the substrate&#44; Es&#46; &#40;b&#41; Hardness of the coating&#44; Hc&#44; and of the substrate&#44; Hs&#46;</p>"
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          "en" => "<p id="spar0055" class="elsevierStyleSimplePara elsevierViewall">Characteristic results of scratch tests for specimens shaped&#46; Optical microscopy micrograph and COF plots&#46; The central part of the residual groove presents bright contrast and chipping is revealed by the dark zones at the edges of the groove&#46; &#40;a&#41; Solid loading 1<span class="elsevierStyleHsp" style=""></span>vol&#37; Al<span class="elsevierStyleInf">2</span>O<span class="elsevierStyleInf">3</span>&#46; &#40;b&#41; Solid loading 3<span class="elsevierStyleHsp" style=""></span>vol&#37; Al<span class="elsevierStyleInf">2</span>O<span class="elsevierStyleInf">3</span>&#46; &#40;c&#41; Solid loading 5<span class="elsevierStyleHsp" style=""></span>vol&#37; Al<span class="elsevierStyleInf">2</span>O<span class="elsevierStyleInf">3</span>&#46;</p>"
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          "en" => "<p id="spar0060" class="elsevierStyleSimplePara elsevierViewall">Scanning electron microscopy micrographs of scratch tested specimens&#46; &#40;a&#8211;c&#41; Secondary electrons&#46; &#40;d&#8211;f&#41; Back scattered electrons&#58; the white phase is the zirconia substrate&#46; &#40;a and d&#41; Solid loading 1<span class="elsevierStyleHsp" style=""></span>vol&#37; Al<span class="elsevierStyleInf">2</span>O<span class="elsevierStyleInf">3</span>&#46; &#40;b and e&#41; Solid loading 3<span class="elsevierStyleHsp" style=""></span>vol&#37; Al<span class="elsevierStyleInf">2</span>O<span class="elsevierStyleInf">3</span>&#46; &#40;c and f&#41; Solid loading 5<span class="elsevierStyleHsp" style=""></span>vol&#37; Al<span class="elsevierStyleInf">2</span>O<span class="elsevierStyleInf">3</span>&#46;</p>"
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                  <table border="0" frame="\n
                  \t\t\t\t\tvoid\n
                  \t\t\t\t" class=""><thead title="thead"><tr title="table-row"><th class="td" title="table-head  " align="left" valign="top" scope="col" style="border-bottom: 2px solid black">Al<span class="elsevierStyleInf">2</span>O<span class="elsevierStyleInf">3</span> solid loading &#40;vol&#37;&#41;&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</th><th class="td" title="table-head  " align="center" valign="top" scope="col" style="border-bottom: 2px solid black">Frictional force &#40;<span class="elsevierStyleItalic">N</span>&#41;&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</th><th class="td" title="table-head  " align="center" valign="top" scope="col" style="border-bottom: 2px solid black">Friction coefficient&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</th><th class="td" title="table-head  " align="center" valign="top" scope="col" style="border-bottom: 2px solid black">Load for penetration depth oscillation &#40;<span class="elsevierStyleItalic">N</span>&#41;&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</th><th class="td" title="table-head  " align="center" valign="top" scope="col" style="border-bottom: 2px solid black">Load for starting of chipping &#40;<span class="elsevierStyleItalic">N</span>&#41;&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</th><th class="td" title="table-head  " align="center" valign="top" scope="col" style="border-bottom: 2px solid black">Maximum depth &#40;&#956;m&#41;&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</th></tr></thead><tbody title="tbody"><tr title="table-row"><td class="td" title="table-entry  " align="char" valign="top">1&nbsp;\t\t\t\t\t\t\n
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Article information

ISSN: 03663175
Original language: English

DOI:10.1016/j.bsecv.2015.07.002

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Year/Month	Html	Pdf	Total

2024 October	10	4	14
2024 September	27	2	29
2024 August	22	3	25
2024 July	18	2	20
2024 June	16	2	18
2024 May	20	1	21
2024 April	18	10	28
2024 March	25	9	34
2024 February	11	1	12
2024 January	35	7	42
2023 December	36	9	45
2023 November	43	10	53
2023 October	61	11	72
2023 September	23	5	28
2023 August	25	12	37
2023 July	20	10	30
2023 June	23	12	35
2023 May	43	7	50
2023 April	53	3	56
2023 March	52	1	53
2023 February	52	6	58
2023 January	50	4	54
2022 December	48	9	57
2022 November	51	22	73
2022 October	49	8	57
2022 September	41	7	48
2022 August	33	11	44
2022 July	33	10	43
2022 June	32	11	43
2022 May	32	6	38
2022 April	29	9	38
2022 March	42	22	64
2022 February	49	13	62
2022 January	66	7	73
2021 December	42	19	61
2021 November	27	8	35
2021 October	39	15	54
2021 September	34	17	51
2021 August	43	7	50
2021 July	36	15	51
2021 June	36	12	48
2021 May	48	7	55
2021 April	117	32	149
2021 March	44	14	58
2021 February	48	10	58
2021 January	54	21	75
2020 December	51	18	69
2020 November	37	18	55
2020 October	29	12	41
2020 September	19	18	37
2020 August	35	4	39
2020 July	19	13	32
2020 June	26	10	36
2020 May	22	4	26
2020 April	20	11	31
2020 March	22	4	26
2020 February	24	5	29
2020 January	24	7	31
2019 December	26	7	33
2019 November	26	8	34
2019 October	30	7	37
2019 September	21	12	33
2019 August	19	3	22
2019 July	22	9	31
2019 June	61	28	89
2019 May	121	26	147
2019 April	63	28	91
2019 March	13	13	26
2019 February	24	9	33
2019 January	13	9	22
2018 December	22	8	30
2018 November	49	21	70
2018 October	36	17	53
2018 September	17	22	39
2018 August	13	1	14
2018 July	7	6	13
2018 June	19	6	25
2018 May	20	2	22
2018 April	9	0	9
2018 March	8	0	8
2018 February	18	11	29
2018 January	20	2	22
2017 December	13	0	13
2017 November	20	0	20
2017 October	19	6	25
2017 September	18	0	18
2017 August	9	5	14
2017 July	13	6	19
2017 June	26	6	32
2017 May	23	1	24
2017 April	18	0	18
2017 March	19	25	44
2017 February	20	12	32
2017 January	16	3	19
2016 December	50	8	58
2016 November	61	10	71
2016 October	59	7	66
2016 September	66	8	74
2016 August	62	5	67
2016 July	54	2	56
2016 June	52	14	66
2016 May	36	23	59
2016 April	44	17	61
2016 March	64	25	89
2016 February	50	25	75
2016 January	78	21	99
2015 December	45	23	68
2015 November	59	27	86
2015 October	84	40	124
2015 September	77	26	103
2015 August	18	12	30

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