Abstract
The development of a high speed pin-on-disc wear testing rig has been undertaken and it has enabled the wear behaviour of zirconia sliding against silicon carbide to be examined. Sliding wear experiments were conducted for zirconia pins and silicon carbide discs under both water-lubricated and unlubricated conditions. Under water-lubricated conditions, tests at a relative sliding speed of 6 m s-1 showed that wear was geometry dependent and the exact alignment of the pin was crucial. At this speed it was possible, with pins polished in-situ on the rig, to achieve hydrodynamic lubrication (HDL) which resulted in low wear. Increasing the nominal contact pressure to 70 MPa resulted in the breakdown of the lubrication and led to high wear coefficients (e.g. 1.1 x 10-6 mm3 N-1 m-1). Under unlubricated conditions, the zirconia pin wore faster than with water lubrication present, the wear rate increasing with speed and nominal contact pressure in the range 1-6 m s-1 and 2-14 MPa. Wear coefficients ranged from 1.4 x 10-6 to 5.1 x 10 -5 mm3 N-1 m-1. The wear tests were followed by examination of the worn surfaces, using a variety of techniques including reflected light and scanning electron microscopy, in order to elucidate likely wear mechanisms. These techniques revealed that there was some degree of commonality between water-lubricated and unlubricated tests, suggesting a universal mechanism which operates over a broad spectrum of testing conditions. The electron microscopy study, combined with observations in the literature, led to the development of a physical model for the wear mechanism, including surface modification and material removal. During the initial stages of wear, and under mild testing conditions, grooves were formed on the surface by plastic deformation due to counterface asperities and trapped debris. The surface was further smeared and deformed as the test proceeded. A deformed surface layer built up which caused intergranular cracks parallel to the surface, at a depth of approximately 3 m. Wear occurred when these cracks linked up with the surface, a process which may be helped by the formation of a network of cracks parallel and normal to the sliding direction. The mechanism of formation of these crack networks remains controversial but is thought to involve thermal shock. Once material is removed from the surface, it is either thrown out of the contact or trapped in it causing further damage.