Sliding friction and wear of structural ceramics

A study has been made of the unlubricated sliding friction and wear behaviour of various like and unlike combinations of four materials -sintered-SiC, graphitized SiC, siliconized SiC and a Y₂O₃-stabilized ZrO₂ — in air at room temperature under dynamic conditions approximating the motion of a piston in the cylinder of an idling automotive engine. The steady state friction coefficient _f is largely independent of the initial surface finish, but is in all cases unacceptably high (0.25 _f 0.50) for engine applications. When the running-in process involves smoothing of the wear surfaces, the coefficient of friction decreases asymptotically towards its steady state value; and when running-in involves roughening, this coefficient usually increases asymptotically as the wear surfaces develop their steady state topographies. Friction couples containing siliconized SiC were the only exceptions to this pattern of behaviour. In every case the high steady state friction is accompanied by considerable wear. The results suggest that ceramic components will not be used unlubricated in reciprocating situations.