A TRANSMISSION ELECTRON-MICROSCOPY STUDY OF WEAR OF MAGNESIA PARTIALLY-STABILIZED ZIRCONIA

The role which the stress assisted transformation of the tetragonal to monoclinic phase plays in the wear of zirconia ceramics remains unclear. This paper discusses the use of detailed transmission electron microscopy of the worn surface to assess the role of transformation in the sliding wear of magnesia partially stabilised zirconia against a steel counterface. Three mechanisms were identified for the wear of the ceramic. Firstly, transformation occurred cooperatively in bands along the [100] directions which led to extensive microcrack formation. Coalescence of the microcracks promoted preferential wear from the bands. Thus, transformation of the tetragonal to the monoclinic phase had led to an increased wear rate. Secondly, clear experimental evidence was found of a tribochemical wear mechanism of the zirconia. In such areas, the transformation of the tetragonal to monoclinic phase played a minor role and was only found to a depth of 200 nm. Finally, abrasive grooving was shown to cause extensive plastic deformation. Interestingly, no evidence of fracture or transformation was found at the abrasive grooves. Thus, this work has demonstrated that under these particular sliding conditions, transformation toughening can have a detrimental as well as a beneficial effect on the tribological properties of zirconia.