Study of transformation induced intergranular microcracking in tetragonal zirconia polycrystals with the phase field method

Abstract

The superior fracture toughness of yttria stabilized tetragonal zirconia polycrystals (TZPs) originates from the stress-induced tetragonal to monoclinic (t-m) phase transformation in the tensile stress field around a propagating crack front. However, spontaneous phase transformation can take place when TZPs are exposed to humid environments, which is accompanied by intergranular microcracking and results in the loss of mechanical properties. This phenomenon, usually called low temperature degradation (LTD) or aging, is considered as a main drawback of TZPs for their clinical applications such as femoral heads and dental implants. In order to explore the mechanisms behind transformation induced intergranular microcracking, a coupled phase field model is developed in this paper, which can simulate the t-m phase transformation and intergranular microcracking simultaneously. The phase field model is validated by studying the microcrack nucleation from a single twin variant with a pure shear eigenstrain. The length of the nucleated microcrack obtained with the phase field modelling matches well with the theoretical prediction. The influence of the dilatational eigenstrain on microcrack nucleation and microcrack nucleation from a monoclinic twin are fully discussed. After that, phase field simulations are conducted to study t-m transformation induced intergranular microcracking in polycrystalline tetragonal zirconia ceramics. It is found that the width of the martensitic variants and their incidence angles to the grain boundary have significant influences on microcrack nucleation. The microstructure of the martensitic twins and the distribution of the intergranular microcracks are fully discussed. The results are helpful to understand the degradation mechanisms of TZPs after the hydrothermal aging.