The sliding wear of plasma sprayed alumina

The wear behaviour of vacuum plasma sprayed (VPS) and air plasma sprayed (APS) alumina coatings was investigated against a number of sliding counterfaces and compared with that of bulk material. Both reciprocating wear tests using diamond cones and sapphire spheres and sphere-on-disc tests using sapphire and steel spheres against plasma sprayed flats were performed at a range of loads to investigate wear rates and determine the operating wear mechanisms and how these are changed as a function of variations in the deposition process. As expected, the VPS material shows much better wear performance compared with the APS coatings but both are poor when compared with the bulk material. Examination of the wear scars shows that though the failure is predominantly brittle, with fracture both within the splats and at the intersplat boundaries, there is some evidence for plastic deformation in all the coatings investigated. With the steel counterface considerable transfer of steel to the coating occurs during wear which has an effect both on the measured friction and on the operating wear mechanisms. These wear results are correlated with the microstructure and properties of the alumina prior to testing.