In the present study, four raw ceramic glazes with the addition of 5, 9, 13, and 17wt% respectively of ZrO2 at the expense of SiO2 (Table 1) were prepared by ceramic traditional route. The starting materials were feldspar (KAlSi3O8), quartz (SiO2), kaolin (Al2Si2O5(OH)4), wollastonite (CaSiO3), zinc oxide (ZnO), colemanite (2CaO–3B2O3–5H2O) (industrial grade-Clayman Supplies-UK), and ZrO2 (Sigma Aldrich). The detailed synthesis procedure has been described in our previous works [12,13]. In brief, the raw materials were wet milled (balls: material: water ratio of 1.5:1:0.6) up to 0.1% residue on a sieve of 63μm. The resulted slurry was applied on the unfired ceramic substrate followed by thermal treatment with a ramp rate of 10°C/min up to 1250°C for 1h plateau in an electrical furnace. The obtained samples were labeled as following: G5Z, G9Z, G13Z, and G17Z. The theoretical oxide composition of ceramic glazes expressed in weight % is presented in Table 1.

XRD patterns were collected using a Rigaku UltimaIV diffractometer (Cu Kα radiation, λ=1.5406Å) operating at 40kV and 30mA, in the range of 10–80° with a step size of 0.02 and a speed of 2°/min. Qualitative and quantitative phase analyses were performed using Rigaku's PDXL software, connected to ICDD PDF-2 database. The XRD patterns of the obtained ceramic glazes were refined by the whole Pattern Powder Fitting (WPPF) method using the split-pseudo Voigt function and the B-spline background model. The following parameters confirm the fitting quality of the experimental data: the goodness of fit S that should be closed to 1 for a good fit and, Rwp (weighted differences between measured and calculated values) that should close to or less than 10%.

The microstructure of the samples was analyzed using a Zeiss SUPRA 55VP Scanning Electron Microscope equipped with energy dispersive X-ray analysis (EDX) capability.

The 29Si solid-state MAS NMR spectra were recorded using a Varian Infinity Plus spectrometer with a 7.05T magnet at a Larmor frequency of 59.6MHz. The powder samples were packed into 7mm zirconia rotors and spun at 4kHz. All collected spectra were referenced to tetramethylsilane at 0ppm. Simulations using four and five Gaussian peaks were carried out using the Dmfit software [14].

27Al solid state MAS NMR spectra were acquired at MAS frequency of 18kHz on a Bruker Advance II+spectrometer equipped with a magnet of 14.1 T (Larmor frequency of 156.37MHz). The powder samples were packed into 3.2mm zirconia and Y3Al5O12 was used as a reference with the AlO6 resonance set to 0.7ppm.

Micro-Raman data were measured by means of a LabRam HR800 spectrometer from Jobin–Yvon–Horiba, equipped with a 532nm laser line and a grating of 1800 lines. The backscattered light was collected through the 50x LWD/0.55 objective (∼1.18μm spot size) of an Olympus microscope. To improve sensitivity in detecting surface structure of the glazes under investigation, UV-Raman spectra were collected by a 325nm laser line, through a 40×/0.47 NUV objective, from a sample spot size less than 1μm. Grating of 2400 lines was used in the latter configuration. Edge filter prevent us recording UV-Raman spectra at low wavenumbers.

Infrared transmission spectra were recorded using Nicolet 6700 spectrophotometer in the 400–1400cm−1 range. The same weight of the ceramic glaze powder was dispersed in KBr pellet.

The chemical resistance of the prepared ceramic glazes was evaluated based on DIN 12116 for acid resistance and ISO 695 for alkali resistance respectively. In brief for acid resistance evaluation, the glaze surface to be tested is boiled for 6h in 20% hydrochloric acid [c (HCI)=6mol/l], and the weight loss is determined in mg/100cm2. In the case of alkali resistance assessment (ISO 695), glaze surface is subjected to a 3h treatment in a boiling aqueous solution consisting of equal volumes of sodium hydroxide, c (NaOH)=1mol/l and sodium carbonate, c(Na2CO3)=0.5mol/l.

X-ray analysis of the prepared ceramic glazes was carried out to assess the crystallization of zircon. Fig. 1 shows the XRD patterns corresponding to ceramic glazes after thermal treatment at 1250°C. In all investigated glazes, the formation of zircon-ZrSiO4 (JCPDS card no. 00-006-0266) as a major phase occurs. In addition, anorthite-CaAl2Si2O8 (JCPDS card no.00-041-1486) and quartz-SiO2 (JCPDS card no. 01-0850794) were also identified beside the amorphous halo between 2theta 20–35°. As can be observed from Table 2 the amount of zircon continuously increases, from 35wt% (G3Z) to 86wt% (G17Z) with the increase of ZrO2 concentration. A similar trend was also observed for the glaze degree of crystallinity. A crystallinity of 87% was determined for the glaze with the highest ZrO2 content (17wt%).

Fig. 1.

XRD patterns of the studied ceramic glazes.

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Raman spectroscopy has enabled structural investigation of both crystalline and vitreous phases present in silicate glass-ceramics.

Spectral features of zircon, ZrSiO4, at 1009cm−1 (ν3 modes of SiO4 with B1g symmetry), 975cm−1 (ν1 modes with A1g symmetry), 440cm−1 (ν2 symmetric bending in isolated SiO4, Q°, with A1g symmetry) and 269cm−1 (B2g, ν2 bending) and less intense external modes (356, 224 and 202cm−1) [17,18] prevail in the Raman spectrum of G17Z in Fig. 4. More intense zircon spectral features are present in the UV-Raman spectra of the G (5/9/13/17) Z glazes either due to higher zircon content at the surface and/or resonant Raman effect (see inset of the Fig. 4). Lowering of the zircon symmetry in the G13Z spectrum caused shifting of the ν1 and ν3 modes toward lower wavenumbers (inset of the Fig. 4). Moreover, G17Z glaze contains both zircon and lowered symmetry zircon (the 1009cm−1 band and its shoulder at 1003cm−1). Lack of the doublet at ∼179/191cm−1 invalidates formation of monoclinic zirconia [19] in case of G17Z. Much wider and less intense peaks at 142, 266 and 478cm−1 (shoulder of the 467cm−1 peak) might originate from amorphous zirconia [19]. Presence of α-quartz in the G17Z might be signaled by the peak at 467cm−1[20].

Fig. 4.

Raman spectrum of G17Z ceramic glaze (* stands for zircon vibration modes). Inset represents UV-Raman spectra of the G(5/9/13/17)Z ceramic glazes over 850–1140cm−1 range.

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Despite low content of the B2O3 in the G (5/9/13/17) Z glazes, tinny bands at 1321 and 1466cm−1 might belong to BØ2O− linked to AlØ4− in aluminum metaborates (Ø is bridging oxygen) and BO stretching mode in (BO3)− metaborate rings and/or chains, respectively [21,23]. Wide peaks at 822, 871, 1059, 1089 and 1185cm−1 are attributable to symmetric SiO stretching of the Q0, Q3 and Q4 units (0, 3 and 4 stand for number of bridging oxygen per SiO4 tetrahedra) in silicate glasses [22–24]. The other two regions of the Raman spectra of the silicate glasses, e.g. <400cm−1 due to lattice modes and network modifying cations as well as 400–800cm−1 range originating from SiOSi bending modes among SiO4 tetrahedra [25], are rather overwhelmed by the strong zircon modes (Fig. 5).

Fig. 5.

FTIR spectra of the studied ceramic glazes.

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FTIR spectra displayed in Fig. 5 reveal the presence of three vibration bands corresponding to SiOSi bonds: the broad one at 1050cm−1(asymmetric stretching vibration), at 788cm−1 (symmetric stretching vibration) and 460cm−1 (bending vibrations) respectively [26]. The displacement of the first two bands regarding to those of silica (1080 and 800cm−1) is due to the presence of network modifiers (K2O, Na2O, CaO, ZnO) in ceramic glazes formulation. Along with the vibration bands related to the silica network, a vibration band at 606cm−1 attributed to SiOZr bond of ZrSiO4 was also noticed [27]. An increase of the band intensity located at 606cm−1 with the increasing amount of formed zircon was observed.

SEM investigations of the G5Z and G17Z ceramic glazes are shown in Figs. 6 and 7. The microstructure of the glaze surface is quite heterogeneous consisting of crystalline phases and residual glassy matrix which appears in dark contrast. The EDX analysis of the glassy matrix (Fig. 7b) exhibited the presence of silicon, aluminum and oxygen as predominant constituents. The white crystals are assigned to ZrSiO4 as a result of zirconia reaction with silica while the darker crystals (indicate by the circle in Fig. 7a) seem to correspond to quartz as was revealed by EDX element map (Fig. 7b) and in good agreement with XRD results. Si accumulation constitutes a part of the ceramic body as well as the glaze. The white crystals are all enriched in Zr indicating the formation of zircon. Moreover, the EDX spectra of the surface of ceramic glazes (Figs. 6c and 7c) confirm also zircon crystallization.

Fig. 6.

SEM image of the glaze ceramic/substrate interface for G5Z sample (a) EDX mapping for Zr, Si, Al, and O (b) and EDX analysis of the surface of ceramic glaze (c).

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Fig. 7.

SEM images of the glaze–ceramic substrate interface for G17Z sample (a) glaze surface (b) and the EDX analyses of the white crystals (c) and, glassy matrix (d).

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The chemical durability of the prepared ceramic glazes in acidic and alkaline solutions was determined according to DIN 12116 and ISO 695 respectively. The obtained results are presented in Table 5. As can be observed, a very good chemical resistance against the hydrochloric acid solution was obtained for G13Z and G17Z ceramic glazes with the highest zircon content and degree of crystallinity. In contrast, the G5Z and G9Z ceramic glazes with lower content of ZrSiO4 received chemical acid resistance classification (class 2). Standard DIN 12116 divides glass types into four classes. If half the weight loss of the surface removal is less than 0.7mg/100cm2 glass type is classified as class 1 highly acid resistance, above 0.7 up to 1.5mg/100cm2 class 2 acid resistance, above 1.5 up to 15mg/100cm2 slight acid attack and above 15mg/100cm2 high acid attack [27]. Further, all studied ceramic glazes received class 1 chemical alkali resistance (solution, consisting of equal volumes of sodium hydroxide and sodium carbonate). According to DIN ISO 695 if the weight loss of the glass after 3h of boiling is less than 75mg/100cm2, the glass is alkali resistance class 1. Between 75 and 175mg/cm2 slight alkali attack, above 175mg/cm2 high alkali attack [28].

An improvement of the chemical resistance to acid attack (from class 3 to class 2) was achieved in the present study compared to our previous work when ZrSiO4 was added as raw materials in glaze formulation [29]. It is known that the chemical resistance of ceramic glaze is related to its chemical composition and structure being a combination of the resistance of the glassy matrix, crystalline phases as well as the interfacial layer between them [30].

Depending on the properties as described above and exposure conditions (the type of solution, temperature, and time of exposure), the glaze surface can undergo ion exchange, dissolution or absorption reactions. Usually, ceramic glazes are resistant to acidic attack (excepting hydrofluoric acid) and alkalis attack. Alkali cations from the ceramic glaze network associated with less extent polymerized SiO4 tetrahedral network are released to the acidic solutions by ion exchange with H+ or H3O+[29]. This reaction leads to the formation of a silica gel layer on the surface that acts as a barrier for further reactions. Furthermore, the alkali ions compensate for the negative charge such as AlO4− and in this role are strongly bounded and cannot be easily detached during the corrosion process. The rate of the process is controlled by the diffusion rate of the ions, and decreases with time.

When the glaze surface is exposed to alkaline attack the dissolution of the glass network takes place by rupture of the SiO bonds at a constant rate [31].

The substitution of SiO2 by ZrO2 improves the chemical resistance of the glaze and makes it less sensitive to changes in pH solution. Zirconia can take part in glaze structure as a network former or network modifier leading to an increase of bridging oxygen atoms per silicon and as a result ceramic glazes in the presence of Zr4+ will have a more packed structure and higher resistance [32].