D.-S. Lim et al./Wear251(2001)l452-1458 tribochemical layer and the plastically deformed layer in the micrographs of the damaged surface. However, evidence of 8 roll formation confirms formation of films(Fig. 9). Similar 6 Si roll formations have been found on silicon nitride. alumina these soft film and rolls has been found to lower friction and wear of sliding ceramics. EDS analysis of unworn and worn surfaces also confirm that silicon rich films formed on the worn surface. As shown in Fig. I l, more rolls ar 86420 found on the worn surface at 673 K. At 673 K, the number of rolls seems to increase with the whisker contents( Fig 8 Higher temperatures facilitate faster chemical reaction and also decreases the viscosity of the glassy surface film. If 0.00.51.01.52.02,53.0354,0455.0 the reaction layer is liquid at the sliding temperatures, roll formation would be difficult but the friction might be kept lower. Therefore. the lowest friction coefficient and the least Fig. 11. EDS analysis of(a) unwom and(b) worn area. A smooth spot roll formation at 873 K for sample T20 might be due to the of the T10 sample tested at 873K was selected for worn area analysis. lowering viscosity of the reaction layer. Yust and Allard showed that even at a moderate load and sliding speed, the above 800C due to the plastic deformation accompanied by frictional temperature for SiC whisker reinforced alumina recrystallization [8]. Tested temperature is so much lower composite could rise above the test temperature by about than brittle to plastic transition temperature that the dom- 400-500oC, based on their calculation [ 14]. This temper nant mechanism might be brittle fracture and both of the ature can greatly influence tribological performance by the modification of the reaction film ture. In case of the whisker reinforced The whiskers of samples T10 and T20 were aligned with composites, the friction coefficients were decreased and the tape casting direction. Sliding on sample T10 and was the wear rates were slightly increased. This results suggest carried out in three directions with respect to whisker ori that SiC whiskers somehow lower the friction coefficient entation, but the whiskers were well aligned Samples T10 at higher temperature Miyoshi and Buckley reported that and T20 showed different friction coefficients depending the friction coefficient of the silicon carbide decreased with on the orientation and surface of the test. For sample TI an increase in temperatures from 400 to 600c due to the at 403 K, the lowest friction coefficient was obtained in the gradual removal of the contaminants of carbon and oxygen direction normal with the tape casting direction on the tape from the surface [9]. Above 800 C, the friction coefficient surface, and highest in the direction normal with the lami- decreased rapidly with an increase of temperature due nation direction, as shown in Fig. 5. Slightly higher friction graphitization of the silicon surface. Removal of contar oefficients in the direction normal with the tape casting inants and the graphitization might contribute to lower direction were noticed for sample T20 at 403 K. However, friction coefficient of Sic reinforced alumina composites. the friction coefficients depending on different sliding di The analysis of the worn surface indicated that a tribo- rections are maintained to be approximately 0.35 and 0.27 chemical layer can be formed, as evidenced by observed at 873 K for T10 and T20, respectively. This result indicates roll debris. Roll debris has been observed for silicon-based that anisotropic effect on the friction diminishes at 873K ceramics and Sic whisker reinforced alumina composites The frictional surface must be covered in a similar manner [5, 10]. The formation of the tribochemical layer can be pro- in spite of the different whisker orientation. However, the moted from the oxidation of whiskers or reaction products effect of sliding direction on wear rate at 873 and 403K between the whisker and the matrix as suggested by Della- show similar trends. Cyfika and Hombogen also reported Corte [5]. The formation of a tribochemical layer will lower that wear resistance showed a much more pronounced the friction and wear rate as whisker contents increased, as anisotropy than friction [15] shown in Figs. 3 and 4. The monolithic alumina shows the least amount of wear at 403K. as shown in Fig 3. This low temperature result was explained by the dominant effect of 5. Conclusions the matrix grain size in a previous publication [4]. Above 673K, the formation of a tribochemical layer plays more The dominant wear and friction mechanism change with mportant role in the friction and wear of the SiC whisker increase in test temperature. The matrix grain size is the reinforced alumina composite. Evidence for this formation dominant mechanism at a relatively low temperature but the of a tribochemical layer or roll formation, is easily seen reaction layer due to the whiskers can be considered to be the on the worn surfaces of Sic whisker reinforced alumina dominant mechanism for the high temperature sliding wear composites(Figs. 7 and 8). It is difficult to distinguish the of the Sic whisker reinforced alumina composite. AboveD.-S. Lim et al. / Wear 251 (2001) 1452–1458 1457 Fig. 11. EDS analysis of (a) unworn and (b) worn area. A smooth spot of the T10 sample tested at 873 K was selected for worn area analysis. above 800◦C due to the plastic deformation accompanied by recrystallization [8]. Tested temperature is so much lower than brittle to plastic transition temperature that the dom￾inant mechanism might be brittle fracture and both of the friction coefficients and the wear rates were increased with increasing temperature. In case of the whisker reinforced composites, the friction coefficients were decreased and the wear rates were slightly increased. This results suggest that SiC whiskers somehow lower the friction coefficient at higher temperature. Miyoshi and Buckley reported that the friction coefficient of the silicon carbide decreased with an increase in temperatures from 400 to 600◦C due to the gradual removal of the contaminants of carbon and oxygen from the surface [9]. Above 800◦C, the friction coefficient decreased rapidly with an increase of temperature due to graphitization of the silicon surface. Removal of contam￾inants and the graphitization might contribute to lower friction coefficient of SiC reinforced alumina composites. The analysis of the worn surface indicated that a tribo￾chemical layer can be formed, as evidenced by observed roll debris. Roll debris has been observed for silicon-based ceramics and SiC whisker reinforced alumina composites [5,10]. The formation of the tribochemical layer can be pro￾moted from the oxidation of whiskers or reaction products between the whisker and the matrix as suggested by Della￾Corte [5]. The formation of a tribochemical layer will lower the friction and wear rate as whisker contents increased, as shown in Figs. 3 and 4. The monolithic alumina shows the least amount of wear at 403 K, as shown in Fig. 3. This low temperature result was explained by the dominant effect of the matrix grain size in a previous publication [4]. Above 673 K, the formation of a tribochemical layer plays more important role in the friction and wear of the SiC whisker reinforced alumina composite. Evidence for this formation of a tribochemical layer or roll formation, is easily seen on the worn surfaces of SiC whisker reinforced alumina composites (Figs. 7 and 8). It is difficult to distinguish the tribochemical layer and the plastically deformed layer in the micrographs of the damaged surface. However, evidence of roll formation confirms formation of films (Fig. 9). Similar roll formations have been found on silicon nitride, alumina and silicon carbide composites [11–14]. The formation of these soft film and rolls has been found to lower friction and wear of sliding ceramics. EDS analysis of unworn and worn surfaces also confirm that silicon rich films formed on the worn surface. As shown in Fig. 11, more rolls are found on the worn surface at 673 K. At 673 K, the number of rolls seems to increase with the whisker contents (Fig. 8). Higher temperatures facilitate faster chemical reaction and also decreases the viscosity of the glassy surface film. If the reaction layer is liquid at the sliding temperatures, roll formation would be difficult but the friction might be kept lower. Therefore, the lowest friction coefficient and the least roll formation at 873 K for sample T20 might be due to the lowering viscosity of the reaction layer. Yust and Allard showed that even at a moderate load and sliding speed, the frictional temperature for SiC whisker reinforced alumina composite could rise above the test temperature by about 400–500◦C, based on their calculation [14]. This temper￾ature can greatly influence tribological performance by the modification of the reaction film. The whiskers of samples T10 and T20 were aligned with the tape casting direction. Sliding on sample T10 and was carried out in three directions with respect to whisker ori￾entation, but the whiskers were well aligned. Samples T10 and T20 showed different friction coefficients depending on the orientation and surface of the test. For sample T10 at 403 K, the lowest friction coefficient was obtained in the direction normal with the tape casting direction on the tape surface, and highest in the direction normal with the lami￾nation direction, as shown in Fig. 5. Slightly higher friction coefficients in the direction normal with the tape casting direction were noticed for sample T20 at 403 K. However, the friction coefficients depending on different sliding di￾rections are maintained to be approximately 0.35 and 0.27 at 873 K for T10 and T20, respectively. This result indicates that anisotropic effect on the friction diminishes at 873 K. The frictional surface must be covered in a similar manner, in spite of the different whisker orientation. However, the effect of sliding direction on wear rate at 873 and 403 K show similar trends. Cyffka and Hornbogen also reported that wear resistance showed a much more pronounced anisotropy than friction [15]. 5. Conclusions The dominant wear and friction mechanism change with increase in test temperature. The matrix grain size is the dominant mechanism at a relatively low temperature but the reaction layer due to the whiskers can be considered to be the dominant mechanism for the high temperature sliding wear of the SiC whisker reinforced alumina composite. Above