Effect of surface degradation on wear behaviour of 3Y-TZP ceramics

Abstract

3Y-TZP ceramics used in biomedical applications, such as total hip replacement and dental restorations, are of large technological interest. The tendency of these ceramics to transform from tetragonal to monoclinic phase in the surface at temperatures close to room temperature is of huge concern, and is referred as hydrothermal degradation or low temperature degradation in aqueous environments. Due to this degradation many in vivo failure of femoral heads occurred in hip prosthesis in recent years. The goal of this work is to study the effect of hydrothermal temperature degradation on Zirconia/Zirconia wear behaviour under dry sliding conditions, analyse the wear rates and also to detect the surface degradation by means of variation in friction and wear behaviour. Zirconia ceramics doped with 3 molar % of yittria are used in this study. Samples are made using 3Y-TZP powder, cold pressed and sintered at 1450oC. Sintered samples are subjected to low temperature degradation for 10 & 60 hours in autoclave at temperature 131oC and pressure 2 bars. As a first approach to investigate the wear behaviour of the sintered, 10 and 60 hour degraded samples, single pass scratch experiments were performed using an automatic scratch tester. Blunt and sharp indenters were used for increasing loads and different constant loads respectively. Ball on disc tribology tests were performed using Zirconia/Zirconia couple. All the tribology tests are done in an automated tribometer by varying sliding distance and keeping the sliding velocity and load constant. The wear micromechanisms are observed using scanning electron microscope and profiles are obtained using optical interferometer. A single pass scratch test can detect the surface degradation with a clear variation in acoustic emission under sharp indenter tests, but coefficient of friction does not vary. In case of blunt indenter test coefficient of friction for 60hour sample exhibits a rise at load around 75N, which is not seen in sintered sample. The wear micromecahnisms observed are plastic ploughing in sintered sample and chipping associated with severe grain pullout in 60 hours degraded sample as is the same case in 10 hour degraded sample with mild grain pullout. Under tribological tests the wear micromecanisms are abrasion and plastic deformation in sintered sample and grain pullout with intergrannular fracture in 60 hour degraded sample. The wear rates under both experiments show a clear variation in all samples. By showing variation in coefficient of friction, acoustic emission, wear micromechanisms and wear rates a simple method has been established to detect the surface degradation.