GLASS-CERAMIC FOR CERAMIC TILE APPLICATIONS

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1 SINTERING, CRYSTALLIZATION AND PROPERTIES OF A Li 2 O-ZrO 2 -SiO 2 GLASS-CERAMIC FOR CERAMIC TILE APPLICATIONS A. P. Novaes de Oliveira, C. Leonelli and T. Manfredini Department of Chemistry, Faculty of Engineering, University of Modena, MODENA-Italy ABSTRACT In this work, sintering and crystallization behavior as well as properties of glass-ceramics belonging to the Li 2 O~ZrO 2 -SiO 2 system were investigated and compared with ceramic materials from the same application field produced by the same technology. By means of thermal shrinkage measurements, sintering was found to start at about 650 C and completed is a very short temperature interval ( T=100 C) in less than 30 min. Crystallization took place just after completion of sintering and was almost complete at 900 C. Secondary porosity prevailed over the primary porosity during crystallization stage, so that the relative density of samples was found to be 95 % at 800 C and 91 % at 900 C. The glass power compacts first crystallized into ZrSiO 4 and then into Li 2 Si 2 O 3 and ZrSiO 4 after crystallization process was essentially complete, so that, a crystallinity degree between 38 and 56 % in the C temperature range was obtained. Consequently, thermal expansion coefficients revealed decreasing values from 10.0 to 8.80 x 10 C -1. Chemical durability showed a reverse behavior, i.e., it was increased from 0.01 to 0.00 wt %, and from 0.l to 0.00 % wt in acid and base solutions respectively. For a given sintering temperature in the C temperature range, Vickers hardness showed values between 5.0 ± 0.1 and 6.1 ± 0.4 GPa, while Young s modulus (E), bending strength (σ) and abrasion resistance showed values between 92 ± 1.4 and 107 ±1.1 GPa, 128 ± 32 and 163 ± 8 MPa and 34 and 37 mm 3 respectively. 1. INTRODUCTION Unglazed ceramic tiles (like porcelainized stoneware) presenting moderate high sinterability, are very important low-porosity products used for interior and for outside P.GI - 193

2 applications as well as frost-resistant materials 1. They are being used in larger quantities and in ever wider. 3, 2, 1 fields of application and on the last decade they showed a relevant increasing of production and sale On the other hand, the research of new materials and/or new processing techniques have contributed on the development of alternative unglazed ceramic tiles, based on glass-ceramics 4. These materials show very good sinterability characteristics and improved mechanical (bending strength, abrasion resistance) and chemical properties. In fact, their use is very extensive in east-european, American and Asian (Japan) countries in constructions for covering large surfaces as floor and wall tiles. By careful choice of the glass composition and glass particle size, the manufacture of these glass-ceramics tiles may be realized economically using 1 ordinary equipment of a traditional ceramic plant. Furthermore, glasses which are usually capable of surface crystallization at relatively low viscosities may be successfully sintered before or. 5 simultaneously with crystallization in one firing cycle Thus, in this work a selected glass composition belonging to the Li 2 O-ZrO 2 -SiO 2 syste in agreement with our previous works 7, 6 was prepared. The glass system has been choised because in a given composition interval, zircon (ZrSiO 4 ), which is a crystalline phase with many unique properties (low thermal expansion, high refractive index, high hardness and abrasion resistance, very high chemical resistance, etc...) is one of the main products after the crystallization process. Sintering and crystallization behavior were studied by means of thermal shrinkage measurements, differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). At last, physical mechanical and chemical properties of sintered samples were also evaluated and the results compared with those of ceramic materials from the same application field and/or produced by the same technology. The aim of this study in particular is: Tu obtain a glass-ceramic by sintering of a glass powder, applying a single fast-firing thermal cycle (<50 min) at low temperatures (<950 C) with optimized properties of performance for ceramic tiles applications. 2. EXPERIMENTAL PROCEDURE 2.1. SAMPLE PREPARATION A batch to produce a glass with a composition of 9.55 wt % Li 2 O,90.45 wt % ZrO 2 +SiO 2, was prepared from well-mixed powders containing appropriate amounts of Li 2 CO 3, ZrSiO 4 and SiO 2 as raw materials. Subsequently, the batch has been placed in a zircon crucible and melted at 1500 C for 7 h in a gas furnace. One part of the melt was cast into water to provide a frit for milling and the other part was quenched onto a cold steel plate to provide a solid glass for crystallization studies. The glass frit was dried and dry-crushed in an alumina 523. Informazione, 28, 1993, G. Olivieri, Un decennio di Sviluppo del Gres Porcellanato. Ceramica Informaciòn, 153, 1990, D.Gardini, Gres fino Porcelanico. Ceramica. 2 International, T. Manfredini and G. C. Pellacani, Tile Whitewares. In: Engineered Materials Handbook, vol.4,ceramics and Glasses,ASM , J. M. Rincòn and M. Romero, Materiales de Construcciòn. Vol. 46, N , abril/junio-julio/septiembre. 4 (1985) , 20, Sci. E.M. Rabinovich, Rewiew-Preparation of glass by Sintering. J. Mater Soc., 79 A.P. Novas de Oliveira et. al., Physical Properties of Quenched Glasses in the Li2O-ZrO2-SiO2 System. J. Am. Ceram (1996). Glass. A.P. Novaes de Oliveira et. al., Effect of the Addition of ZrSiO4 on the Crystallization of the 30Li20/70SiO2 Powered. 7 Thermochimica Acta 286 (1996) P.GI - 194

3 ball-mill for 2 h, then sieved to yield a powder of particle size <100 µm. The crushed glass powder was then milled for 48 h in an aluminous porcelain mill containing alumina grinding media and water. The resulting slurry was dried to a humidity content of about 5.5 % and then granulated in a plastic cylinder for 10 min. Granule vvith sizes lower then 2 mm were obtained and the average particle size was found to be 10 µm by using a laser scattering particle sizer analyzer (Model Analysette 22, Fritsch). The granulated glass was then uniaxially pressed by means of an automatic hydraulic press at 40 MPa (green density about 1.45 g cm -3 ) in steel molds so that compacted samples with nominal dimensions of 100 mm x 50 mm x 10 mm, in the shape of discs with a diameter of 13 mm and a thickness of 5 mm respectively, were used. Before measuring, the samples were dried at 120 C for 2 h and cooled and stored in a dryer, so that the final humidity was constant at about 0.5 %, 2.2. CHARACTERIZATION SINTERING The compacted samples were isothermally sintered in a electric laboratory furnace (+5 C) at a heating rate of 30 C min -1 in air for appropriate time intervals (10-60 min) at T = 800, 850, 900 and 950 C. After sintering, samples were air-quenched to room temperature. No cracks were visually observed. Thermal shrinkage was measured by means of a hot stage microscope - HSM (trade name MISURA) at 10 C min -1 in air. In this case, the apparatus consisted of a horizontal tube furnace with specimen carriage, a light source to illuminate the field of view fully and uniformly, an electronic video camera and a special optical lens designed to obtain maximum resolution (10 µm). Drop profile (dimensional changes) and images can be printed out when the HSM analyses have been completed by a special computer program. The temperature is read a 2 C resolution and measured by means of a Pt/Pt-Rh thermo-element placed directly under the sample and digitalized. Densities for the green and sintered samples were measured by the Archimede s principle with mercury immersion at 25 C. Theoretical density was also measured by using a pycnometer. Sintered samples were cut and their cross sections were mounted in epoxy resin, then the surfaces were ground smooth, polished with l µ m alumina paste and etched (2 % HF, 25 s). Subsequently, all the samples were coated with a thin Pd-Au film for SEM observations (Model Philips XL-40) CRYSTALLIZATION The crystallization temperature of the glass powder was measured by differential thermal analysis (DTA) in air at a heating rate of 10 C min -1 using about 30 mg of powdered samples. A DSC model 404 high-temperature Netzsch thermoanalyser was used with an empty Pt crucible as reference material. To investigate the amorphous nature of the as-quenched glass and the crystalline phases of the sintered samples, powdered samples were analyzed with a Philips PW 3710 computer -assisted X-ray (Cu Kα) powder diffractometer (XRD). To demonstrate the mechanism of crystallization six disk samples each having a thickness of 2 mm and a diameter of 10 mm were heat-treated at 850 C for 30, 45, 60, 90 and 120 min respectively. Subsequently, the disks were cut and the thickness of the crystallized layers were measured by SEM observations. The crystallinity of the sintered and crystallized samples were also determined by applying the Chung's method 8 using α-al 2 O 3 as reference material. 8. F. H. Chung, Quantitative Interpretation of X-ray Diffraction Patterns of mixtures. 1. Matrix-Flushing Method for Quantitative Multicomponent Analysis. J. Appl. Cryst. (1974), 7, 519. P. GI - 195

4 MEASUREMENTS Water absorption, deep abrasion and bending strength were determined according to EN 99, EN 102 and EN 100 standard test methods respectively. Young's modulus (E) of sintered samples was determined as before mentioned from the average value of 10 specimens which was collected by the acoustic-resonance method (Grindo Sonic MK5 Instrument). Vickers microhardness, HV, measurements were performed applying a load of 200 g during 15 s by a Matsuzawa DMH2 hardness tester. In this case, samples were prepared according to the procedures used in the sample preparation for SEM observations. Each hardness value is the average of 10 indentations. Chemical resistance was evaluated according to JIS procedures which consists in the weight loss of the test piece of 15 x 15 x 10 mm after 650 h immersed in a 1 % H 2 SO 4 and a 1 % NaOH solutions respectively at 25 C. At last, linear thermal expansion of the sintered samples were measured on bars with dimensions of 45 x 5 x 5 mm, using a Model 402 Netzsch automated recording dilatometer at 10 C min RESULTS AND DISCUSSION 3.1. SINTERING BEHAVIOR The thermal linear shrinkage curve obtained by hot stage microscope measurements is shown in figure 1. Fig. 1. Thermal linear shrinkage of the glass powder studied, compacted at 40 MPa (heating rate 10 C min -1 ). From the figure it can be seen that sintering starts at about 650 C and is completed in a very short temperature interval ( T 100 C). Crystallization starts very soon after the completion of sintering; such a slight increase in volume is observed up to about 950 C when a new and strong increase in volume caused by the growth and/or melting of the crystalline phases takes place. In fact, as shows figure 2 which refers to the DTA curve of the glass powder, the same effects as mentioned above can be observed: an endothermal inflection at about 600 C related to the glass transition, an exothermal peak having maximum at about 860 C which is correlated with the crystallization processes and an endothermal effect having maximum at about 990 C relaled to the melting of crystalline P. GI 196

5 phases. The effect of sintering time and temperature on the densification is shown in figure 3. The green density is about 1.45 g cm -3 (the lower end of the ordinate axis) and examination of the fig. 3 shows that: the sintered density increases as the temperature decreases from 950 to 800 C and rapidly becomes almost constant (except at 950 C, where a decrease is observed) as the heating time increases. This indicates that most of the densification takes place in a very narrow range of heating time ( 10 min) in close agreement with the thermal linear shrinkage measurements and also with the relative density curve (figure 4) where it reaches a maximum of 95 % at 800 C and then decreases as the temperature increases. The decrease of the sintered density as the temperature increases is related to the crystallization processes, which will be indicated and discussed later. Fig. 2. DTA curve of the glass powder studied (heating rate 10 C min -1 ) Fig. 3. Density of sintered glass powder compacts υs. time for sintering temperatures of 800 C, 850 C, 900 C and 950 C. P. GI 197

6 Fig. 4. Relative density of glass powder compacts υs. temperature for sintering time of 10 min CRYSTALLIZAT1ON BEHAVIOR From fig. 5 which is related to the XRD it can be seen that the reflections associated with the sample sintered at 800 C for 10 min were assigned to the crystalline phase of zircon (ZrSiO 4 ), file JCPDS n Fig. 5. Powder XRD patterns of samples : (a) as-quenched glass; (b) sintered at 800 C for 10 min, (c) sintered at 850 C for 10 min and (d) sintered at 900 'C for 10 min. LS Li 2 Si 2 O 5 ; Z ZrSiO 4. On the other hand, in sample sintered at 850 C for 10 min, the reflections related to the crystalline phase of zircon increased in intensity and the crystalline phase of lithium disilicate (Li 2 Si2O5) took place, files JCPDS n , However, the reflections associated with the X-ray pattern of the sintered sample at 900 C for 10 min also increased in intensity, indicating that the crystallization process was almost completed, according to the DTA curve. Figure 6 reports the degree of crystallinity as a function of the temperature, and shows as total crystallinity has a maximum at about 900 C being almost constant for P. GI -198

7 temperatures higher than 900 C and lower than 950 C. In this temperature interval Li 2 Si 2 O 5 and ZrSiO 4 constitute about 25 wt % and 31 wt % respectively so that a total crystallinity of 56 wt % was obtained. However, for temperatures between 950 C and 980 C an increasing on total crystallinity is observed. This increasing is related to crystal growth. The XRD pattern shown in fig. 5 (a) regarding the as-quenched glass demonstrates its amorphous nature, since it is characteristic of the glassy state. Fig. 6. Crystallinity versus temperature: - Li 2 Si 2 O 5, - ZrSiO 4, - Total. Considering that the crystallization is completed at 900 C, as shown above, and to verify the time effect on the crystallization and also the mechanism of crystallization, glass bulk samples were subjected to heat-treatments. In this case, the glass samples were heated up to 850 C at a heating rate of 30 C min -1 for 30, 45, 60, 90 and 120 min respectively. Fig. 7. SEM photograph (300 x) of cross section of a bulk sample heat-treated at 850 C for 45 min shawing the crystalline layer. Figure 7 shows a SEM photograph of the cross section of a glass sample heat-treated at 850 C for 45 min, in which an uniform surface-crystallized layer on the glass core, is evident. P. GI - 199

8 Figure 8 shows the effect of the soaking time at the heating temperature of 850 C on the thickness of the crystallized layer. The thickness of the crystallized layer increases almost linearly and very fast from surface nuclei at last up to 120 min with a growth rate of 5.50 µm min -1. These results as were indicated, confirm a dominant surface mechanism of crystallization and appears that crystallization is almost complete after 30 min. Fig. 8. Effect of soaking time at 850 C on the thickness of the crystallized layer SINTERING AND CRYSTALLIZATION RELATIONSHIP SEM observations on etched surfaces of partially and fully sintered powder compacts provide a good insight into the process occurring during densification and crystallization. Consequently, figures 9 (a), (b) and (c) show the microstructures of samples heated for 10 min at 800, 850 and 950 C respectively. Fig. 9 (a). SEM photograph (3500 x) of a sample sintered at 800 C for 10 min. P. GI - 200

9 Figure 9 (a) shows that almost all of the fine scale porosity is eliminated in 10 min at 800 C so that the pore size and pore fraction represent the degree of densification. In fact, according to figures 3 and 4 densification at 800 C is already completed and it is almost the maximum, However, there is a clear evidence of crystallization which appears to have nucleated at the boundaries of the original glass powder particles, in agreement with the XRD of the figure 5 (b). Fig. 9 (b). SLM photograph (3500 x) of a sample sintered at 850 C for 10 min. Fig. 9 (c). SEM photograph (3500 x) of a sample sintered at 950 C for 10 min. P.GI - 201

10 As evidenciated in figure 9 (b) showing the sample sintered for 10 min at 850 C, although the crystalline phases, mainly zircon and lithium disilicate have been formed, the crystallization process is not finished, since some regions containing original glass powder particles (glassy phase) are visible, confirming the XRD pattern of the figure 5 (e) and the crystalline data of figure 6. In the samples sintered for 10 min at 900 and 950 C, here represented by figure 9 (e), no isolated regions containing glassy phase are observed, which indicates that the crystallization process is almost finished in agreement with the XRD pattern and crystallinity data of figures 5 and 6. On the other hand, by comparing figures 9 (a) with 9 (b) and (c) it can be seen that secondary porosity appears and is higher when the crystallization is completed. In this case, the number and size of the secondary pore are affected by the amount of crystals and their size, since for samples sintered at 900 C and specially at 950 C for 10 min (figure 9 (c)) there is a clear evidence of crystal growth and the porosity representing the densification increases over the time and temperature as shown in figures 3 and 4 respectively. These evidences well explain the decrease of the sintered density with the increase of temperature and time. 3.4 PROPERTIES Taking in account the sintering and crystallization behavior of the glass considered in this study, samples were prepared and sintered at 850", 900 and 950 C for 10 min respectively by applying an one-stage cycle. The sintered samples were then subjected to several measurements so that typical properties characteristic of traditional ceramic tiles were determined as shows table 1. Properties/feature Water absorption (100%) Total porosity (%) Density (g/cm 3 ) Crystallinity (wt %) Hardness (GPa) σ c (MPa) E (GPa) Deep abrasion (mm 3 ) Resistance to acid Resistance to alkalies CTE (x 10-6 C -1 ) Glass-ceramics of this study * Reference Materials Sintering Temperature/Time 850 C/ C/ C/10 M G NP GP >1 - <1 < < < ± ± ± ± ± ± ± ± 1 92 ± : 1 % H 2 SO C, 650 h, wt % loss for a sample size of 10x10x25 mm : 1 % NaOH, 25 C, 650 h, wt % loss for a sample size of 10x10x25 mm : C M : Marble; G : Granite; NP : Neopariés, GP: Porcelainized stoneware * Table 1. Properties of the studied glass-ceramics sintered at 850 C, 900 C and 950 C for 10 min compared with reference materials. From the table it can be seen that the results close agree with the sintering and crystallization behavior observed, i.e., the sintering temperature increasing ( C) J. M. RINCON AND M. ROMERO, Materials de Construcciòn. Vol. 46, N , abril/junio-julio/septiembre. 4 M. DONDI, B. FABBRI, T. MANFREDINI, G. C. PELLACANI, Microstructure and mechanical properties of porcelainized stoneware tiles. Fourth. 9 Euro Ceramics Vol. 11 pp , 1995 (1986) , pp. Z. SRNAD, in Glass-Ceramic Materials- Glass Science and Technology 8, Elsevier, New York, 10 P. GI - 202

11 produces a higher crystallinity and consequently according to the previous results a decreasing on densification which can be appreciate from results of total porosity and density. This behavior directly reflects on mechanical properties, especially on bending strength (σ c ) and Young's modulus (E), In this case, porosity and crystallinity play fundamental roles. The increasing of crystallinity give rise to higher mechanical properties values with respect to σ c E, deep abrasion and hardness. On the other hand, even considering that crystallinity is about constant for sintering temperatures between 900 and 950 C, the porosity was increased and this causes a decreases of mechanical properties. The chemical stability of glass-ceramics is also favorable. Only a small amount of mass ( wt %) was lost after treating a glass-ceramic which 1 % H 2 SO 4 and 1 % NaOH for 650 h for samples sintered at 850 C for 10 min, while for samples sintered at 900 and 950 C for 10 min respectively no lost were verified. The coefficients of thermal expansion (CTE) of the final glassceramics were 10.0, 9.9 and 8.8 x 10 C -1 according to the sintering temperatures (850, 900 and 950 C) respectively. The relatively high CTE specially at low sintering temperatures can be explained since lithium disilicate which is present in about the same proportions of zircon has a high CTE (about 11 x 10-6 C -1 ) against 4 x 10-6 C -1 of zircon. As sintered glass-ceramic materials can be considered composite materials with a microstructure constituted of fine crystals uniformly distributed and arbitrarily oriented throughout the residual phase containing pore, their properties are independent of the direction in which they are measured. In consequence of these typical features which are favorably reflected in a number of properties, glass-ceramic materials are differentiated from traditional ceramics as shown in table 1. In fact, the glass-ceramic studied can be distinguished from the traditional ceramics and natural materials by its properties and by its favorable processing properties, since it can be pressed into different shapes, even in a partially crystalline state. Finally, by using mechanical and density data of the studied glass-ceramic sintered at 900 C for 10 min and comparing with those of traditional ceramics and natural materials listed in table 1, it seens that the studied glass-ceramic is about 68 %, 69 %, 41 % and 45 % lighter in weight than marble, granite, porcelainized stoneware and Neoparies respectively which make this material a strong candidate as a ceramic file and also in a number of other applications with high performance. 4. CONCLUSIONS A lithium disilicate-zircon-based glass-ceramic has been produced by the sintering and crystallization of a suitable glass powder in the Li 2 O-ZrO : -SiO : System having composition 9.55 wt % Li 2 O, wt % ZrO 2 + SiO 2. Sintering was found to start at about 650 C and completed in a very short temperature interval ( T 100 C) and a heating time lower than 30 min. Surface crystallization took place just after sintering and was almost complete at about 900 C for a soaking time of about 10 min so that densification and crystallinity were 91 % and 56 % respectively. On heating, the glass powder compacts first crystallized into zircon (ZrSiO 4 ) and then into lithium disilicate (Li 2 Si 2 O 5 ) and zircon (ZrSiO 4 ) after the crystallization process was essentially complete, The microstructure consists of fine crystals uniformly distributed and arbitrarily oriented throughout the residual glassy phase as well as secondary porosity over the primary porosity. The measurements results related to properties and features demonstrated that the P. GI - 203

12 studied glass-ceramic is a strong candidate as a ceramic tile and also in a number of other applications with characteristics of high performance being 41 to 69 % lighter than traditional and natural materials. ACKNOWLEDGEMENTS Support for A. P, Novaes de Oliveira was provided by the "Fundação Coordenação de Aperfeiçoamento de Pessoal de Nìvel Superior (CAPES)" and by the Federal University of Santa Catarina, both Brazilian institutions. Additional support for the authors was provided by MURST, P. GI - 204