Tribological behaviour of plasma-sprayed zirconia coatings

Various layers of zirconia, coated on cast iron plates and steel discs by plasma spraying, were studied to gain a better understanding of their tribological behaviour. Coatings selected for this investigation were zirconia-5% CaO, zirconia-8% yttria, zirconia-20% yttria and alumina-zirconia coatings. Wear tests were conducted under conditions of dry contact with reciprocating motion at a temperature of 200°C. Measurements of friction and wear rate were made in association with sliding distance for various friction pairs: self-mated contact between zirconia-sprayed specimens, sprayed plate against chromium-plated disc, and uncoated cast iron plate against chromium-plated disc which represents the contact between cylinder liner and piston ring in diesel engines. To identify the wear mechanism, an X-ray diffractometer, scanning electron microscope, transmission electron microscope, surface roughness tester and micro-hardness tester were used. 92%-8% zirconia-yttria, 80%-20% zirconia-yttria and alumina-zirconia composite coatings exhibited excellent wear resistance. Wear debris were considerably involved in the wear process in the steady state. It is shown that the plastic deformation and forming of a smooth film by material transfer on the sliding surface strongly influenced the wear rate and friction coefficient. Phase transformation in the contact region was discussed. Phase transformation from the tetragonal to monoclinic phase was not noticeable but the transformation of the tetragonal phase to the t′ tetragonal phase was observed to influence the tribological behaviour of the coatings.