Effects of a second phase on the tribological properties of Al₂O₃ and ZrO₂ ceramics

The tribological properties of four different materials are investigated, tetragonal zirconia (Y-ZTP), Al₂O₃ dispersed in Y-TZP (ADZ), ZrO2 dispersed in Al₂O₃ (ZTA) and Al₂O₃ (with 300 ppm MgO). These materials are used as a cylinder sliding against a plate of Y-TZP (TZ-3Y)). Compared to Y-TZP, the wear resistance of ADZ composites is increased by a factor of 4-10. At a contact pressure of 230 MPa, a wear transition for Y-TZP is observed from plastic deformation to microchipping and microfracture due to the high interfacial temperature (450°C-550°C) generated by frictional heating. Because of the higher elastic modulus, hardness and fracture toughness at high temperature, ADZ composites show better wear resistance and a higher transition contact pressure (over 400 MPa) under the present conditions. For Al₂O₃, the transition from mild to severe wear occurs when the contact pressure is changed from 250 to 400 MPa. For ZTA ceramics, the wear behaviour does not change because of the presence of a compressive layer due to the zirconia phase transformation during sliding.In water the wear resistance for ADZ and ZY5 is almost two orders of magnitude higher than the results under dry conditions. Reduction of the interfacial temperature by using water and the formation of a hydroxide layer at the contact surface by the tribochemical reaction of water with the ceramic, as observed by XPS, gives a positive effect on wear resistance.