Silicon carbide particle embedded magnesium oxychloride cement composite bricks for polishing of porcelain stoneware tiles

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1 Silicon carbide particle embedded magnesium oxychloride cement composite bricks for polishing of porcelain stoneware tiles A. Ozturk* and M. Timucin Silicon carbide (SiC) particle embedded magnesium oxychloride cement (MOC) composite polishing bricks developed for polishing of porcelain stoneware tiles were produced by incorporating 600 and 1200 grit SiC particles to the MOC paste in the amounts of 20, 25 and 30 wt-%. Density, compressive strength, abrasion resistance and polishing properties of the bricks were determined with respect to the amount and particle size of the SiC powder. SiC particle embedment enhanced density, compressive strength and abrasion resistance of the neat MOC paste. Polishing was accomplished both in laboratory scale and in a typical online industrial scale. The polishing performance of the bricks was evaluated in terms of mean surface roughness and optical gloss of ceramic tiles, and abrasive brick consumption occurred during polishing. Scanning electron microscopy examinations revealed evidences of the reasons that 25 wt-%sic particle embedded MOC bricks have the best qualifications in terms of abrasion resistance and polishing performance. Keywords: Polishing, Cement, Porcelain, Wear parts, Tile Introduction Surface grinding and polishing are critical and very expensive steps in the production of unglazed ceramic tiles. 1 3 They are performed to improve the aesthetic and performance of the tile. 4 A very smooth glossy surface finish could be achieved with proper polishing treatment. 5 Surface roughness and glossiness are the most important quality criteria in high specification architectural applications. Highly polished, unglazed porcelain ceramic tiles are being increasingly used in such applications as they show excellent performance, including good mechanical strength and abrasion, chemical, stain and frost resistance, as well as aesthetic advantages over glazed ceramic tiles. 6 9 However, several difficulties arise during the polishing process. Irreversible surface damage, mainly due to the opening of the closed pores occurring into the tile body, is an unavoidable drawback of surface polishing. 1,4,9,10 If process parameters are not applied properly, then the surface quality of the ceramic tile deteriorates due to scratches, grooves, cuts and subsurface cracking, and the polishing cost increases due to the unnecessarily high wear of the grinding/polishing tools, high energy consumption and excessive numbers of rejected products. 1,2,6 The success of grinding/polishing depends not only on the surface quality of the tile before the polishing process but also on the characteristics of the tile as well as the performance of the polishing tool. 3 Department of Metallurgical and Materials Engineering, Middle East Technical University, Ankara 06531, Turkey *Corresponding author, abdullah@metu.edu.tr The main abrasive used in the grinding step of porcelain tiles is silicon carbide (SiC) particle embedded in an organic or inorganic matrix of variable composition. 11 Commonly, magnesium oxychloride cement (MOC) is used as matrix material 6,12 due to its high early strength, good abrasion resistance and good bonding ability to aggregates Even though SiC particle embedded MOC composite bricks are the main abrasives used in the polishing of porcelain stoneware tiles, there is only little information on the polishing performance of these bricks with respect to their chemical compositions. 12 An understanding of the polishing performance of these bricks is important also for correlating the abrasion and polishing behaviour with the abrasive particle size used in the brick. Hence, this investigation has both scientific and practical significance. The purpose of this study was to determine the effects of the amount and particle size of SiC powder on the abrasion and polishing performance of SiC particle embedded MOC composite bricks used for polishing of porcelain stoneware tiles. The polishing performance of the bricks was evaluated in terms of surface roughness and gloss of the polished ceramic tiles together with weight loss of the polishing brick occurring during polishing. The microstructures of the polished ceramic tiles were investigated by scanning electron microscopy (SEM) to elucidate the observed behaviour. Experimental Materials The previous study conducted by the authors revealed that the MOC brick composed of MgO/MgCl 2 /H 2 O ß 2011 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 1 June 2011; accepted 19 July Advances in Applied Ceramics 2011 VOL 110 NO 7 DOI / Y

2 molar ratio of 7 : 1 : 11 has the best qualifications in terms of polishing performance. 16 Therefore, a neat MOC paste having MgO/MgCl 2 /H 2 O molar ratio of 7 : 1 : 11 was selected as the binding medium for the SiC particles. The neat MOC paste was prepared by mixing proper amounts of caustic calcined MgO powder (KUMAS), MgCl 2.6H 2 O salt (Nedmag Co.) and tap water in accordance with the production procedure of the MOC pastes given by Ozer et al. 16 In addition, the neat MOC paste, six different SiC particle embedded MOC composite bricks with regard to the amount and particle size of the SiC powder were prepared. The SiC particle embedded MOC composite bricks were formed by incorporating 600 and 1200 grit SiC particles to the MOC paste in the amounts of 20, 25 and 30 wt-%. The formation procedure of the composite test specimens was similar to the procedure of the neat MOC bricks given by Ozer et al. 16 The grit number of the SiC powders and the SiC content in weight percentage of the bricks prepared are shown in Table 1. Hereafter, the bricks will be referred to as the codes given in Table 1. Two different particle sizes (600 and 1200 grit) of SiC particles supplied by Eczacıbaşı Esan AŞ, Turkey, were employed for the preparation of composite abrasive bricks. The grit number describes the abrasive particle size using the standard Federation of European Producers of Abrasive designation. The average particle size of the 600 and 1200 grit SiC particles corresponds to 9?3 and 3 mm respectively. The chemical compositions in weight percentage and the properties of the SiC particles used in this study are given in Table 2. The nominal dimensions of the cubic shape specimens used for the abrasion tests were cm. The specimens for polishing tests were in cylindrical shape and had height and cross-sectional area of 4 cm and 8cm 2 respectively. In addition to the laboratory scale polishing specimens described above, industrial size polishing specimens (blocks) made from 25 wt-% of 600 and 1200 grit SiC particle embedded MOC composites were prepared to evaluate the polishing performance of brick samples in a typical industrial polishing process of porcelain stoneware tiles. The industrial size abrasive blocks were cast in approximately rectangular shape. The blocks were in the same dimensions with commercially available 600, 1200 and 1600 grit abrasive blocks supplied Table 1 The SiC content and the codes of the bricks SiC grit number SiC content of the brick (wt %) Brick code C C C F F F from Luna Abrasivi Srl, Italy. The commercial abrasives are identified only for comparison purpose. They do not imply endorsement by the authors or institutions supporting this work. The composite bricks and blocks were kept 28 days in plastic moulds, in accordance with the TS 1769 standard. The relative humidity of the conditioning room was,50%, and the temperature was in the range of 20 25uC. After 28 days of conditioning period, the bricks and blocks were ready for the tests. Testing methods and equipment The testing procedure for the abrasion resistance and polishing performance of the SiC particle embedded MOC composite bricks and the equipment used in this study were the same as that given by Ozer et al. 16 Compression tests were performed using a compression test machine (Baz Makine AŞ) in accordance with the TS EN196-1 standard. The tests were conducted at a loading speed of 240 kg s 21. Six specimens in cube shape (76767 cm) were tested for all the bricks. The specimens were loaded from the top until fracture. The fracture load of the specimens was recorded. The compressive strength of the bricks was calculated using the following equation s~ F (1) A where s represents compressive stress, F is the maximum force applied and A is the cross-sectional area of the sample. The average value of the six determinations was taken as the compressive strength of the brick. The abrasion test (TS 669 standard) was performed on cube shape test specimens using an abrasion test machine (Atom Teknik AŞ). Three cube shaped specimens of the brick samples were involved in the abrasion tests. The initial weight and height of the specimens were recorded before the test. The weight of specimen was measured accurately ( 0?001 g) using an analytical balance (Mettler Toledo). The height of the specimen was measured ( 0?1 mm) using a micrometer. The test procedure was based on abrasion of the bottom surface of the test specimen on a rotating disc by means of 20 g of alumina powder laid on the disc. The abrasion occurred due to the frictional motion of the alumina powder between the disc and the specimen in a circular direction. The final weight and height of the specimens were recorded. The difference in weight and height of the specimen before and after abrasion test was taken to determine the abrasion resistance. Polishing tests were performed on coarse ground (46 grit calibrated) porcelain stoneware tiles supplied by Kaleseramik AŞ, Turkey. The nominal dimensions of the tiles in length6width6depth were cm respectively. The polishing processes began with the application of 600 grit SiC particle embedded MOC composite bricks (coarse bricks). The bricks comprise 20, 25 and Table 2 Chemical composition and properties of the SiC particles SiC grit size Chemical composition (wt%) SiC Fe 2 O 3 C (active) Mineral Type Surface area (m 2 /g) Mohs Hardness ,1. 3,0. 3 a-sic ,1. 4,0. 3 a-sic Advances in Applied Ceramics 2011 VOL 110 NO 7 401

3 1 Variation in density of SiC particle embedded MOC bricks with SiC powder addition 2 Variation in compressive strength of SiC particle embedded MOC bricks with SiC powder addition 30 wt-%sic powder and are coded as 20-C, 25-C and 30- C respectively. The average surface roughness R a of the working surface of all tiles was measured before and after application of each brick. When the tests with coarse bricks were complete, the polishing procedure continued with the application of 1200 grit SiC particle embedded MOC composite bricks (fine bricks) on the same tile. These bricks comprise 20, 25 and 30 wt-%sic powder and are coded as 20-F, 25-F and 30-F respectively. Before the application of fine bricks, the average surface roughness of the tiles was brought to almost the same roughness value (0?56 0?03 mm) to get a comparable data. In order to achieve an equivalent degree of roughness on the tiles before the application of fine bricks, the tiles polished by bricks 20-C and 30-C were additionally polished by brick 25-C for 5 min. After that, the polishing tests were performed using fine bricks. After completion of the polishing test, a circular polished path of,5 cm wide and 15 cm outer diameter was obtained on the polished tile. The cylindrical polishing test specimens were weighed accurately before and after each individual polishing test. The difference between the initial and final weights was taken as the weight loss of the brick. The arithmetic average of weight loss that occurred in nine bricks was evaluated as the brick consumption that occurred during the polishing test. The surface properties, namely surface roughness R a and gloss G, of the tiles were measured using a stylus profilometer (Surtronic 3z; Taylor Hobson) and an optical glossmeter (Novogloss, 60u measuring angle) before and after each individual polishing test. A TalyProfile Lite version: 3?1?4 (Taylor Hobson) software program was employed to trace the surface profile of the tiles. Seven measurements were performed at evenly spaced positions around the polishing path. The arithmetic average of seven values was taken to establish the average roughness and gloss. In addition to laboratory scale polishing described above, industrial scale composite blocks containing 25 wt-% 600 or 1200 grit SiC particles were prepared and tested online, similar to a typical industrial polishing process of porcelain stoneware tiles in Kaleseramik AŞ, Turkey. The blocks were weighed accurately before and after the polishing test. The difference between the initial and final weights was taken as the weight loss of the block. The arithmetic average of weight loss that occurred in six blocks was evaluated as the abrasive consumption that occurred during online polishing. The surface properties the tiles were measured before and after online industrial polishing tests. Seven measurements were performed at evenly spaced positions on the polished tile. The arithmetic average of seven values was taken to establish the average roughness and gloss. Variability in the results was,10% of the measured values for surface roughness and gloss. The microstructural features of the SiC particle embedded MOC composite bricks and of the working surface of polished ceramic tiles were examined using an SEM (JEOL 6400) equipped with an energy dispersive spectroscope (EDS). Before SEM analysis, the bricks and tiles were washed, dried and sputtered by gold. Scanning electron microscopy examination of the brick samples was performed on the gold coated fracture surfaces. Elemental analysis of the samples was performed using EDS. Results and discussion Density The density of the neat MOC paste is 1?99 g cm The density of various amounts of SiC particle embedded bricks was greater than that of the neat MOC paste. The variations in density of the 600 and 1200 grit SiC particle embedded MOC composite bricks with SiC addition are shown in Fig. 1. The error bars in the figure represent the standard deviations from the average values. The density of the neat MOC paste increased from 1?99 to 2?04 g cm 23 when 20 wt-% of the 600 grit SiC powder is embedded. The densities of the 25 and 30 wt-% 600 grit SiC particle embedded polishing bricks were 2?05 and 2?06 g cm 23 respectively. The density of the 20, 25 and 30 wt-% 1200 grit SiC powder embedded polishing bricks were 2?04, 2?05 and 2?05 g cm 23 respectively. The increase in density with SiC particle addition is attributed to the increase in the mass of the brick without changing the volume much. Density measurements also revealed that when the SiC particle addition is increased beyond 25 wt-%, a further increase in density was not attained in the fine bricks. The change in density of the fine bricks containing 25 and 30 wt-%sic particle was within the error limits. This phenomenon is probably 402 Advances in Applied Ceramics 2011 VOL 110 NO 7

4 3 Variation in weight and height losses that occurred during abrasion test of 600 grit SiC particle embedded MOC composite bricks with SiC powder addition related to the viscosity and porosity of the MOC paste. Fine powder addition to cement pastes increases the viscosity. 17 High viscosity causes air bubbles to be entrapped in the paste but not reach the free surface. Consequently, the addition of SiC particles beyond 25 wt-% increases the viscosity, which in turn hinders the increase in density. A similar phenomenon of viscosity is observed for a cementitious material by Zhang and Han, 18 who stated that ultrafine admixtures, such as silica fume and power plant fly ash, are effective on the rheological properties of cement paste. A low amount of ultrafine powder addition to cement paste was found to improve the rheological properties of cement paste in terms of viscosity and yield strength. Compressive strength The variation in compressive strength of the 600 and 1200 grit SiC particle embedded MOC composite bricks with SiC powder addition is shown in Fig. 2. The compressive strength of the neat MOC paste (74?2 MPa) is also shown in the figure for comparison purpose. Experimental results reveal that the compressive strength of the neat MOC paste increases when 600 and 1200 grit SiC powders are embedded. The compressive strength of the neat MOC paste increased to 92?4 MPa when 20 wt-% of the 600 grit SiC powder is embedded. The compressive strengths of the 25 and 30 wt-% 600 grit SiC particle embedded MOC bricks were 98?5 and 97?2 MPa respectively. The compressive strength of the 1200 grit SiC particle embedded MOC bricks exhibited a similar tendency with the 600 grit SiC particle embedded MOC bricks. The compressive strengths of the 20, 25 and 30 wt-% 1200 grit SiC particle embedded MOC bricks were 89?4, 95?6 and 94?0 MPa respectively. The compressive strength of the bricks exhibited retardation over 25 wt-%sic addition. Specifically, when the SiC particle content is increased beyond 25 wt-%, an increase in compressive strength was not acquired. The increase in compressive strength due to SiC powder embedment is attributed to the occurrence of energy absorbing mechanisms such as crack branching and crack deflection in the bricks during fracture. Aggregate additions to cement pastes were studied by Bouzoubaa et al., 19,20 who recognised that additions of coarse and fine aggregates enhance the compressive 4 Variation in weight and height losses that occurred during abrasion tests of 1200 grit SiC particle embedded MOC composite bricks with SiC powder addition strength of cements. The effect of fine SiC powder addition on the compressive strength of SiC particle embedded MOC bricks is in good agreement also with the results reported by Sant et al. 17 and Pandey et al. 21 For a given amount of SiC addition, the 600 grit SiC particle embedded MOC composite bricks (coarse bricks) offered better compressive strength than the 1200 grit SiC particle embedded MOC composite bricks (fine bricks). The difference in the increase in compressive strength between coarse and fine bricks is tied to the differences in particle size and surface area of the SiC powders. The particle size of the 600 grit SiC powder is twice that of the 1200 grit SiC powder, and the specific surface area of the 600 grit SiC powder is four times larger than that of the 1200 grit SiC powder, as shown in Table 2. Finer particles of SiC powder trap more capillary water and yield microporosities in the cement structure. Coarser particles of the SiC powder, on the other hand, contribute soundness of the three-dimensional MOC network by filling voids and increase the compressive strength. 22 Abrasion resistance The abrasion resistance of the neat MOC paste and SiC particle embedded MOC composite bricks was measured according to dry abrasion test. Results revealed that the abrasion resistance of SiC particle embedded MOC bricks was better than that of the neat MOC paste. The changes in weight and height loss of the 600 and 1200 grit SiC particle embedded MOC composite bricks with SiC powder addition are shown in Figs. 3 and 4 respectively. The weight and height losses due to the abrasion of neat MOC bricks were 31?6 g and 3?1 mm respectively. 16 The abrasion resistance in terms Table 3 Surface properties and consumption of the bricks Brick Surface roughness (mm) Surface gloss 20- C C C F F F Brick consumption (g) Advances in Applied Ceramics 2011 VOL 110 NO 7 403

5 5 Representative surface roughness profiles of porcelain stoneware tiles after being polished by a 25-C (mean roughness is 0?56 mm) and b 25-F (mean roughness is 0?27 mm) of the weight and height losses of the neat MOC paste increases as the SiC powder addition is increased up to 25 wt-%. In addition,.25 wt-% of SiC powder additions deteriorates the abrasion resistance of the MOC paste. The decrease in the abrasion resistance with increasing amount of SiC powder is probably due to the decrease in the amount of binder phase, which results in weakening of the structure. Experimental findings of abrasion tests are in good agreement with the results presented by Bouzoubaa et al. 19 The weight loss of neat MOC paste decreased from 31?6 to16?8 g when 20 wt-% of 600 grit SiC powder was admixed. The weight losses of the 25 and 30 wt-% of 600 grit SiC particle embedded MOC composite bricks were 12?1 and 13?4 g respectively. The height losses in the coarse bricks were 1?6, 1?1 and 1?3 mm for the 20, 25 and 30 wt-%sic powder additions respectively. The fine bricks displayed a similar abrasion behaviour with the coarse bricks. The weight losses of the 20, 25 and 30 wt-% 1200 grit SiC particle embedded MOC composite bricks were 20?7, 18?5 and 19?3 g respectively. The height losses in the fine bricks were 2?8, 1?8 and 1?9 mm for the 20, 25 and 30 wt-%sic powder additions respectively. Table 4 Results of online industrial polishing of porcelain stoneware tiles Block code Average surface roughness (mm) Average surface gloss grit* F grit* C grit* * Commercially available. Block consumption (g) Polishing The values for the average surface roughness and the average surface gloss of the porcelain stoneware tiles after being polished by different abrasive bricks are presented in Table 3. The variability in the results associated with the tile and abrasive bricks, measurement procedure and methods was,10% of the measured value for both roughness and gloss. The values next to the data points in the table are the standard deviations from the averages. The values for the average surface roughness and average surface gloss of the 46 grit calibrated porcelain stoneware tiles before polishing were 2?41 mm and 2 respectively. Polishing tests revealed that the average surface roughness decreased but the average surface gloss increased sequentially when the grit number of the SiC particle is increased. Figure 5 illustrates representative surface profiles of the tiles upon completion of 600 and 1200 grit polishing tests. The 600 grit SiC particle embedded MOC composite bricks were more effective on surface smoothening than the gloss. However, the 1200 grit SiC particle embedded MOC composite bricks had a considerable effect on the gloss yet the corresponding change in roughness was minor. No significant change in the surface roughness and gloss of the porcelain stoneware tiles was observed within the bricks composed of the same grit SiC particles. The values for different amounts but the same grit SiC particle embedded MOC composite bricks were more or less the same. The results imply that, within the compositions studied, the size of the SiC particles in MOC composite bricks is more noteworthy than their amount in improving the surface properties of the porcelain stoneware tiles. The values for the consumption ensued in the brick after polishing the tile are presented also in Table 3. Variability in the results associated with the measurement procedure was,5% of the measured value. When a comparison is made between the values of the abrasive consumption that occurred in the coarse and fine bricks containing the same amount of SiC powder, it is seen 404 Advances in Applied Ceramics 2011 VOL 110 NO 7

6 6 Representative surface roughness profiles of porcelain stoneware tiles after being online polished by blocks of a 25-C (mean roughness is 0?421 mm) and b 25-F (mean roughness is 0?260 mm) that the erosion that occurred in the coarse bricks was nearly three times greater than that of the fine bricks. It was realised earlier that the abrasion resistance of coarse powder embedded bricks was better than the fine powder embedded bricks. The difference in erosion between different size SiC particle embedded MOC composite bricks is not due to the abrasion resistance of the bricks but due to the initial surface roughness of the porcelain tile. 2 The average surface roughness of the tile before and after the application of coarse bricks was 2?25 and 0?56 mm respectively. It became 0?27 mm after the application of fine bricks. It is obvious that most of the roughness reduction (smoothening) of the porcelain tile surface is achieved in 600 grade polishing process. As a result, material removal and erosion in coarse bricks were more than fine bricks, although coarse bricks had greater abrasion resistance, as stated earlier. Polishing tests also revealed that the coarse or fine bricks containing 25 wt-%sic particles exhibited the best polishing performance in terms of roughness reduction, glossiness and abrasive consumption. There are no data reported in the open literature on the polishing performance of SiC particle reinforced abrasive bricks with respect to abrasive composition in order to support and/ or compare this judgment. Brick 30-F exhibited slightly more consumption than brick 25-F, although both bricks resulted in more or less the same smoothness on the tile. The reason of high abrasive consumption in brick 30-F as compared to brick 25-F is attributed to the slightly higher density of brick 30-F. Therefore, more material removal occurs in brick 30-F, although almost the same volume wears out in both bricks. The results of 10 min online industrial polishing of the porcelain stoneware tiles with neat MOC blocks and with the SiC particle embedded MOC composite blocks as well as with the commercially available abrasive blocks supplied from Luna Abrasivi Srl, Vezzano Ligure, Italy, are given in Table 4. It is clear that the roughness of the tile surface polished by 600 and 1200 grit SiC particle embedded MOC composite blocks produced in this investigation was inferior, but the gloss was superior to the tiles polished by the commercially available abrasive 7 Image (SEM) of composite brick containing 25 wt-% of 600 grit SiC particles 8 Image (SEM) of composite brick containing 25 wt-% of 1200 grit SiC particles Advances in Applied Ceramics 2011 VOL 110 NO 7 405

7 9 Image (SEM) showing distribution of porosities in microstructure of brick 25-F blocks. However, abrasive consumptions that occurred during online polishing of the tiles in the 600 and 1200 grit SiC particle embedded MOC composite blocks were more than the commercial blocks having the same grit number. The differences in the surface properties of the tiles and abrasive consumption in the commercial and experimental bricks are attributed to the dissimilarities in the properties of the SiC particles used in both bricks. The surface profiles of the tiles after online polishing by blocks 25-C and 25-F are shown in Fig. 6. The average surface roughnesses of the tiles online polished by blocks 25-C and 25-F were 0?421 and 0?26 mm respectively. The appearance of surface quality, in terms of roughness and gloss, of the tiles polished using bricks 25-C and 25-F in laboratory scale polishing set-up and of the tiles polished using blocks 25-C and 25-F in industrial online polishing was comparable, too. The results reveal that the laboratory scale polishing set-up constructed simulates intimately the important features of the industrial polishing conditions. Microstructure of SiC particle embedded MOC composite bricks The general features of the microstructure of the neat MOC brick were presented by Ozer et al. 16 A representative microstructure of the 25 wt-% of 600 and 1200 grit SiC particle embedded MOC composite bricks is shown in Figs. 7 and 8 respectively. The microstructure of the composite bricks containing 20 and 30 wt-% of 600 and 1200 grit SiC particles was similar in appearance to those shown in Figs. 7 and 8. The SEM investigations revealed that the composite bricks have a porous microstructure. Closed porosities were distributed randomly in the structure and were as big as 100 mm in size, as seen in Fig. 9. The SiC particles were surrounded by the cement matrix. It is seen that there is a good distribution of SiC particles and no aggregation or settlement in the material. The bonding between SiC particles and cement matrix is poor, and this is indicated by the smooth appearance of the SiC particle surfaces shown to protrude above the fracture plane of the matrix. No matrix could be seen attached to the surface of the pulled out SiC particles. A large number of SiC particles beneath the fracture surface of the matrix separated from the cement matrix. The rough fracture surface is due to the crack propagation in material below and above the main crack plane. Ridges are formed by the merging of two crack planes at slightly different levels. This occurs when the crack front passes around a particle, splitting the crack, which then travels on two different planes. The deflected crack merges again after passing the particle forming a ridge. A rough structure with prominences and depressions caused by variations in the crack propagation above and below the main crack is apparent in the microstructure. Consistent with the results given by Deng and Zhang, 23 the morphology of crystalline phase development in the MOC phase was generally in needle shape. The needle shaped crystals were observed mostly in the microporosities. The SEM examination also reveals that the porosity of the composite bricks decreases as the SiC particle is increased. Surface microstructure of polished ceramic tiles The surface of the ceramic tile polished by the SiC particle embedded MOC composite bricks was examined using SEM to understand and assess the polishing performance of abrasive bricks. The images in Fig. 10 show the ceramic tile surface after being polished by the 600 grit SiC particle embedded MOC composite bricks containing various amounts of SiC. Microstructural analysis of the polished surfaces revealed that the tiles have a homogeneous microstructure exhibiting open and closed porosities, scratches and polishing traces. Pores with spherical morphology with a mean value of 10 mm distributed uniformly. The formation of a liquid phase fills the voids during sintering; thus, a very homogeneous microstructure is obtained. 24,25 The images in Fig. 11 show the ceramic tile surface after being polished by the 1200 grit SiC particle embedded MOC composite bricks containing various amounts of SiC. The microporosities and polishing traces are visible in the images. When a comparison is made between the images shown in Figs. 10 and 11, it is 10 Images (SEM) of ceramic tile surface after being polished by coarse bricks of a 20-C, b 25-C and c 30-C 406 Advances in Applied Ceramics 2011 VOL 110 NO 7

8 11 Images (SEM) of ceramic tile surface after being polished by fine bricks of a 20-F, b 25-F and c 30-F obvious that the tiles polished using coarse bricks possess more and better noticeable traces than the ones polished using fine bricks. Consistent with the results obtained from polishing tests, SEM examinations of the polished ceramic tiles revealed that the SiC particle embedded MOC composite bricks containing 25 wt-%sic particle exhibited the best qualifications in terms of polishing performance. Conclusion The particle size and the amount of SiC powder have a profound effect on density, compressive strength and abrasion resistance of SiC particle embedded MOC composite bricks. The addition of 600 and 1200 grit SiC particles up to a limiting value of 25 wt-% enhances the compressive strength and abrasion resistance, but further addition has the opposite effect. For a given amount of addition, coarse SiC powders are more effective than fine SiC powders in the abrasion of porcelain stoneware tiles. The SEM examinations of the polished surface of porcelain stoneware tiles revealed that the 25 wt-%sic particle embedded MOC composite bricks have the best qualifications in terms of polishing performance. Acknowledgements The present work is the outcome of the SANTEZ project conducted together with Kaleseramik AŞ and supported by the Scientific and Research Council of Turkey (TUBITAK; project no. 105M140). The authors would like to acknowledge TUBITAK and Kaleseramik AŞ for their financial and technical support respectively. A special thank goes to Mr M. Said Ozer for his conscientious and faithful assistance. References 1. L. Esposito, A. Tucci and D. Naldi: The reliability of polished porcelain stoneware tiles, J. Eur. Ceram. Soc., 2005, 25, I. M. Hutchings, K. Adachi, Y. Xu, E. Sanchez, M. J. Ibanez and M. F. 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