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D.-S. Lim et al./Wea225-2291999868-873 873 scratch the whisker as well as the alumina matrix by three smaller than that parallel to the direction in spite of the body wear. No visible inter-and intra-granular cracks were fact that the friction coefficients showed the opposite present. High compressive residual stress prevented the whiskers from being pulled out during the tests. Even though it should depend on the crystallographic orienta 4. Conclusions ion, elastic modulus of Sic was comparable to that of alumina. So, the grain boundary was thought to experience The monolithic alumina showed smallest wear because small deformation resulting from the difference in elastic of the smallest matrix grain size. Depending on sliding deformations of the matrix and the whisker under the orientation, friction and wear of the samples varied as contact stress. When whiskers were lying normal to the follows The friction coefficient was higher on the surface sliding direction in sample T10, they eliminate fine as- where more whiskers exposed their basal plane. Sliding perites on the contact surface of the ball. The longer the normal to the whisker length direction caused less wear projection length of the whiskers normal to the sliding than that parallel to it. As the whisker content increased direction, the larger the area covered with the debris layer from 10 vol. to 20 vol% both friction coefficient and as shown in Fig. 6(b). So, the contact surface became wear decreased due to the smaller matrix n size ther and the friction coefficient lower as shown in Even though whiskers were randomly oriented in the contact surface of sample w10 and were aligned Acknowledgements parallel to the sliding direction, the friction coefficients were quite similar to each other. It implied that the fine The authors(Korea University) would like to acknowl- lying normal to the sliding direction to be removed, and Korea c.hancial support for this study provided by the asperites on the ball took many passes over the whisker the friction stayed at a high value. When the whisker through the Ceramic Processing Research Center(CPRc) content increased from 10 vol% to 20 vol % the friction at Han Yang University coefficients decreased Wear also depended on sliding direction with respect to orientation of the whiskers as shown in Fig 3. Hardness of references single crystal Sic depends on the crystallographic orienta tion and plane. The basal plane exhibited the highest hardness and higher hardness was reported in the direction Ceran.Soc.74(1991)255-2 normal to the c-axis than parallel to it on prismatic plane 2] K.T. Farber, A.G. Evans, Crack deflection processes: 1. Theory, [12]. Based on the above information, sliding in the direc- cta Metal.31(1983)565-576 tion normal to the lamination direction should have the F. Shuster. CW whiskers of highest hardness because it involved more with alumina-SiC whisker cutting tools, Ceram. Eng. Sci. Proc. 9 contacts with basal plane of the SiC. So, the smallest wear [4] C.S. Yust, Tribological behavior of whisker-reinforced ceramic com in the direction normal to lamination direction was due to posite materials, in: S. Jahanmir(Ed. ) Friction and Wear of Ceram- highest hardness of the sic whiskers that slowed down the of shown in Fig. 6, the whiskers stood [5]C. DellaCorte, Tribological characteristics of silicon carbide p higher than the surface. The harder the whiskers were whisker-reinforced alumina at elevated temperatures, Friction and ar of Ceramics, Marcel-Dekker, New York, 1994, pp. 225-59 the longer they last and the slower the wear progressed [6] D.S. Park, C.-W. Kim, C. Park, Self-reinforced silicon nitride But the long lasting sharp whiskers increased the friction ning unidirectionally oriented silicon nitride whisker coefficients due to rougher contact surface. Sliding in tape seeds, to be published in Ceram. Eng. Sci. Proc casting direction involved contacts with whiskers length [7] D-S. Park, B.-D. Han, D-S. Lim, L-w. Yeo, A study on wear and erosion of sialon-Si3N4 whisker ceramic composites, Wear 203-204 direction where the hardness was lowest. The exposed (1997)284-290 whiskers were worn out faster than those of other orienta [8]M. Moser, Microstructure of Ceramics-Structure and Properties of tions, and the biggest wear was measured. The wear scar nding Tools, Akademiai Kiado, Budapest, 1980, p. 136 diameter of the ball was the largest and smallest in the [9 M. Yasuoka, K. Hirao, S. Kanzaki, Effects of TIN direction parallel to the whisker in the article size on mechanical pre direction on the tape surface, respectively. Sliding in the s,J. Ceran.Soc.Jpn.100(1992)617-620 [10l S.J. Cho, C.-D. Um, S.-S. Kim, Wear and wear transition in silicon direction normal to the lamination direction resulted in the carbide ceramics during sliding, J. Am. Ceram. Soc. 79(1996) smallest wear on the composite sample, but it caused more 1247-1251 vear of the ball than sliding normal to the whisker length [11] S.J. Cho, B. Hockey, B.R. Lawn, S.J. Bennison, Grain-size and direction. As the whisker content increased from 10 vol% R-curve effects in the abrasive wear of alumina, J. Am. Ceram Soc. 72(1989)1249-1252 to 20 vol % wear of the sample and the ball decreased. [12] T K. Miyoshi, D.H. Buckely, Ceramic wear in indentation and However. wear normal to whisker length direction was still sliding contact, ASLE Trans. 28(1985)296-302D.-S. Lim et al.rWear 225–229 1999 868–873 ( ) 873 scratch the whisker as well as the alumina matrix by three body wear. No visible inter- and intra-granular cracks were present. High compressive residual stress prevented the whiskers from being pulled out during the tests. Even though it should depend on the crystallographic orientation, elastic modulus of SiC was comparable to that of alumina. So, the grain boundary was thought to experience small deformation resulting from the difference in elastic deformations of the matrix and the whisker under the contact stress. When whiskers were lying normal to the sliding direction in sample T10, they eliminate fine asperites on the contact surface of the ball. The longer the projection length of the whiskers normal to the sliding direction, the larger the area covered with the debris layer as shown in Fig. 6 b . So, the contact surface became Ž . smoother and the friction coefficient lower as shown in Fig. 1. Even though whiskers were randomly oriented in the contact surface of sample W10 and were aligned parallel to the sliding direction, the friction coefficients were quite similar to each other. It implied that the fine asperites on the ball took many passes over the whisker lying normal to the sliding direction to be removed, and the friction stayed at a high value. When the whisker content increased from 10 vol.% to 20 vol.%, the friction coefficients decreased. Wear also depended on sliding direction with respect to orientation of the whiskers as shown in Fig. 3. Hardness of single crystal SiC depends on the crystallographic orientation and plane. The basal plane exhibited the highest hardness and higher hardness was reported in the direction normal to the c-axis than parallel to it on prismatic plane w x 12 . Based on the above information, sliding in the direction normal to the lamination direction should have the whiskers of highest hardness because it involved more contacts with basal plane of the SiC. So, the smallest wear in the direction normal to lamination direction was due to highest hardness of the SiC whiskers that slowed down the progress of wear. As shown in Fig. 6, the whiskers stood up higher than the surface. The harder the whiskers were, the longer they last and the slower the wear progressed. But the long lasting sharp whiskers increased the friction coefficients due to rougher contact surface. Sliding in tape casting direction involved contacts with whiskers length direction where the hardness was lowest. The exposed whiskers were worn out faster than those of other orientations, and the biggest wear was measured. The wear scar diameter of the ball was the largest and smallest in the direction parallel to and normal to the whisker in the direction on the tape surface, respectively. Sliding in the direction normal to the lamination direction resulted in the smallest wear on the composite sample, but it caused more wear of the ball than sliding normal to the whisker length direction. As the whisker content increased from 10 vol.% to 20 vol.%, wear of the sample and the ball decreased. However, wear normal to whisker length direction was still smaller than that parallel to the direction in spite of the fact that the friction coefficients showed the opposite. 4. Conclusions The monolithic alumina showed smallest wear because of the smallest matrix grain size. Depending on sliding orientation, friction and wear of the samples varied as follows. The friction coefficient was higher on the surface where more whiskers exposed their basal plane. Sliding normal to the whisker length direction caused less wear than that parallel to it. As the whisker content increased from 10 vol.% to 20 vol.%, both friction coefficient and wear decreased due to the smaller matrix grain size. Acknowledgements The authors Korea University would like to acknowl- Ž . edge the financial support for this study provided by the Korea Science and Engineering Foundation KOSEF Ž . through the Ceramic Processing Research Center CPRC Ž . at Han Yang University. References w x 1 P.F. Becher, Microstructural design of toughened ceramics, J. Am. Ceram. Soc. 74 1991 255–269. Ž . w x 2 K.T. Farber, A.G. Evans, Crack deflection processes: I. Theory, Acta Metal. 31 1983 565–576. Ž . w x 3 E.R. Billman, P.K. Mehrotra, A.F. Shuster, C.W. Beegly, Machining with alumina–SiC whisker cutting tools, Ceram. Eng. Sci. Proc. 9 Ž . 1988 543–552. w x 4 C.S. Yust, Tribological behavior of whisker-reinforced ceramic composite materials, in: S. Jahanmir Ed. , Friction and Wear of Ceram- Ž . ics, Marcel-Dekker, New York, 1994, pp. 199–223. w x 5 C. DellaCorte, Tribological characteristics of silicon carbide whisker-reinforced alumina at elevated temperatures, Friction and Wear of Ceramics, Marcel-Dekker, New York, 1994, pp. 225–59. w x 6 D.-S. Park, C.-W. Kim, C. Park, Self-reinforced silicon nitride composite containing unidirectionally oriented silicon nitride whisker seeds, to be published in Ceram. Eng. Sci. Proc. w x 7 D.-S. Park, B.-D. Han, D.-S. Lim, I.-W. Yeo, A study on wear and erosion of sialon–Si N whisker ceramic composites, Wear 203–204 3 4 Ž . 1997 284–290. w x 8 M. Moser, Microstructure of Ceramics—Structure and Properties of Grinding Tools, Akademiai Kiado, Budapest, 1980, p. 136. w x 9 T. Nagaoka, M. Yasuoka, K. Hirao, S. Kanzaki, Effects of TiN particle size on mechanical properties of Si N rTiN particulate 3 4 composites, J. Ceram. Soc. Jpn. 100 1992 617–620. Ž . w x 10 S.-J. Cho, C.-D. Um, S.-S. Kim, Wear and wear transition in silicon carbide ceramics during sliding, J. Am. Ceram. Soc. 79 1996 Ž . 1247–1251. w x 11 S.-J. Cho, B. Hockey, B.R. Lawn, S.J. Bennison, Grain-size and R-curve effects in the abrasive wear of alumina, J. Am. Ceram. Soc. 72 1989 1249–1252. Ž . w x 12 T.K. Miyoshi, D.H. Buckely, Ceramic wear in indentation and sliding contact, ASLE Trans. 28 1985 296–302. Ž