D. Wei et al /Ceramics Intemational 32(2006)549-554 55 Fine flake 100μr 20 Fig. 7. SEM micrographs of the worn surfaces of monolithic Si3 N4 and 10 vol. ch-BNSi3 N4 ceramic composite:(a),(c) SNO and(b),(d)SN10 3.3. Effect of h-BN on the tribological properties However, the characteristic of spalling morphology on worn surface changes from fine flakes to large flakes by 3.3.. Friction coefficient incorporating 10 vol %h-BN into the Si3N4 ceramic matrix Fig. 6 shows the relation of the friction coefficient of h- as shown in Fig. 7a and b. In addition, no coarse cracks BN/Si3 N4 ceramic composites versus h-BN content. In all observed on the worn surface of SNo but on that of SN10 as cases the friction coefficient rose quite quickly at the start of shown in Fig. 7c and d the test before reducing to a level which remained relatively le formation of coarse cracks on the worn surface of sn constant for the remainder of the test. It can be seen that the can be explained by the surface pores and weak interface friction coefficient of h-BN/Si3 N, ceramic composites has a between h-BN and B-Si3N4 grains. Fig. 8 shows the SEM tendency to decrease with increasing h-BN content. micrograp ohs of the polished surfaces of SNO and SN10. No h-bn has a lower friction coefficient and lubrication evident pores are observed on the polished surface of SNO as action,thereby the friction coefficient of h-BN/Si3N4 shown in Fig. &a. However, as can be seen from Fig 8b, pores ceramic composites decreases with increasing h-bn are produced by incorporating 10 vol %h-Bn into Si3 n4 content. In addition, the reductions in diameter and aspe ceramic matrix, which is caused by the reduction in its sinter ratio of elongated B-Si3 N4 grains also result in the decrease ability. The pores of SN10 lead to the stress concentration, of friction coefficient. Investigations of ZrO2, Al2O3 and thereby microcracks are easily produced from thesites of pores ZrO2-Al2O3 ceramics system have shown that the frictional existed by applying stress. In addition, the weak interface also ceramic is more smooth and the friction coefficient is lower than that of coarse-grains ceramic [23-25]. Therefore, the reduced B-Si3N4 grain size is also the factor to decrease the friction coefficient of h-BN/ Si3N4 ceramic composites(see Fig. 6) 3 3.2. Wear behaviour The wear behaviour of sno and nio were investigated preliminarily. Fig. 7 shows the SEM micrographs of the worn surfaces of SNO and SN10 A surface spalling mode of failure is observed on the worn surfaces of sno and sn10 Fig 8. SEM micrographs of the polished surfaces of (a) SNO and(b)SN103.3. Effect of h-BN on the tribological properties 3.3.1. Friction coefficient Fig. 6 shows the relation of the friction coefficient of h￾BN/Si3N4 ceramic composites versus h-BN content. In all cases the friction coefficient rose quite quickly at the start of the test before reducing to a level which remained relatively constant for the remainder of the test. It can be seen that the friction coefficient of h-BN/Si3N4 ceramic composites has a tendency to decrease with increasing h-BN content. h-BN has a lower friction coefficient and lubrication action, thereby the friction coefficient of h-BN/Si3N4 ceramic composites decreases with increasing h-BN content. In addition, the reductions in diameter and aspect ratio of elongated b-Si3N4 grains also result in the decrease of friction coefficient. Investigations of ZrO2, Al2O3 and ZrO2–Al2O3 ceramics system have shown that the frictional surface of fine-grains ceramic is more smooth and the friction coefficient is lower than that of coarse-grains ceramic [23–25]. Therefore, the reduced b-Si3N4 grain size is also the factor to decrease the friction coefficient of h-BN/ Si3N4 ceramic composites (see Fig. 6). 3.3.2. Wear behaviour The wear behaviour of SN0 and SN10 were investigated preliminarily. Fig. 7 shows the SEM micrographs of the worn surfaces of SN0 and SN10. A surface spalling mode of failure is observed on the worn surfaces of SN0 and SN10. However, the characteristic of spalling morphology on worn surface changes from fine flakes to large flakes by incorporating 10 vol.%h-BN into the Si3N4 ceramic matrix as shown in Fig. 7a and b. In addition, no coarse cracks are observed on the worn surface of SN0 but on that of SN10 as shown in Fig. 7c and d. Theformation ofcoarsecracks ontheworn surfaceofSN10 can be explained by the surface pores and weak interface between h-BN and b-Si3N4 grains. Fig. 8 shows the SEM micrographs of the polished surfaces of SN0 and SN10. No evident pores are observed on the polished surface of SN0 as shown in Fig. 8a. However, as can be seen from Fig. 8b, pores are produced by incorporating 10 vol.%h-BN into Si3N4 ceramic matrix, which is caused by the reduction in its sinter ability. The pores of SN10 lead to the stress concentration, therebymicrocracksareeasilyproducedfromthesitesofpores existed by applying stress. In addition, the weak interface also D. Wei et al. / Ceramics International 32 (2006) 549–554 553 Fig. 7. SEM micrographs of the worn surfaces of monolithic Si3N4 and 10 vol.%h-BN/Si3N4 ceramic composite: (a), (c) SN0 and (b), (d) SN10. Fig. 8. SEM micrographs of the polished surfaces of (a) SN0 and (b) SN10