Study of the residual stress field around Vickers indentations in a 3Y-TZP

Indentation cracks are often used as initial flaws in ceramics for different mechanical tests because of their unique advantages. The residual stresses around the indent due to the elastic/plastic contact must generally be relaxed, which can be conducted by heat treatment. The stress field around Vickers indentation and conditions of annealing have been analysed on a Y-TZP material. Different heat treatments from room temperature up to 1200 degrees C have been conducted. The residual stress field has been characterized by different methods. First, stable crack propagation in the residual stress field has been conducted by applying a bending stress to the indented specimens. The apparent toughness has also been measured by conducting fast fracture on indented specimens. Finally the residual stress present around the indentation has been measured by making micro-indentations and by measuring the crack lengths. The results show that at intermediate temperatures up to 600 degrees C an apparent stress relaxation occurs due to the tetragonal to monoclinic phase transformation which induced a superimposed compressive stress. The higher temperature of 1200 degrees C effectively leads to a real stress relaxation without healing the crack.