Erosion and corrosion of PSZ-zirconia and the t-m phase transformation

Partially stabilised zirconia (PSZ) has received special attention due to its high strength, high corrosion and erosion resistance, and high resistance to fracture. The phase transformations of ZrO2 in its application process have long been known to be important. Various physical, hydrothermal, and chemical effects can cause the phase transformations. Although many advances have been made in recent years in the understanding of the effects, foundations and mechanisms of the ZrO2 phase transformation, however, no study has yet reported on the phase transformation from t-ZrO2 to m-ZrO2 in corrosion solution at room temperature. In the present study, both the corrosion resistance of ZrO2 and the ZrO2 phase transformation in a corrosive solution (1.5% HF + 5% HCl) used in the oil industry to clean downhole pipes were investigated. The erosion behaviours of PSZ-ZrO2 before and after corrosion have been studied using a SiC/water slurry jet impingement test rig with the aim of providing some information on the material's response to erosion attack. SEM and X-ray were used to characterise the microstructure of the corroded layer, and phase change after corrosion. After corrosion, the peaks of monoclinic structures increase dramatically; ZrCl4 or ZrF4 were not detected. By increasing the length of corrosion time, the monoclinic structure is markedly increased. Thus, the corroded layer is the main source contributing to the increase in the monoclinic phase. The possible reaction steps in HF + HCl solution are discussed. The mechanism of t-m phase transformation is discussed in this work. From the thermodynamic viewpoint, a phase structure would tend to transform from the metastable phase to the more stable under the attack of a corrodent, or under impact stress or tensile stress. However, the difference between the two cases is that during the transformation induced by impacting stress, part of the t-ZrO2 transforms into m-ZrO2 and microcracks develop, which consume crack propagation energy and release stress concentration, while t-m transformation due to the corrosion process forms a porous m-zirconia layer. Microstructural evaluation as well as chemical analysis shows that the surface structure of PSZ-ZrO2 changes and becomes more porous during corrosion process, not only due to the t-m phase transformation, in which cracks and voids are anticipated owing to 3%-5% volume change; but also due to the effect of higher corrosion rates of additives in PSZ-ZrO2 as well. The erosion behaviours of PSZ-ZrO2 before and after corrosion, using a SiC/water slurry jet, are also compared with other ceramic materials. (C) 1999 Elsevier Science S.A. All rights reserved.