Variation of Microstructure Upon Residual Stress, as Obtained by the Vickers Indentation

Method in Al₂O₃ Ceramics with Differing Aspect Ratios

Abstract

Recently, an indentation (ID) method using a Vickers hardness tester has been specified as a standard by the Society of Materials Science, Japan (JSMS-SD-4-01) as the method for practical use. This method determined a base value (K_C) while measuring residual stress by the ID method. Porosity and other factors have been reported to cause problems when using the ID method on Al₂O₃, Si₃N₄ and ZrO₂ as structural ceramics, a fact which motivated the establishment of this new method. The value measured by this method might change not only in response to microstructural factors such as grain size and aspect ratio, but also with different types of materials. However, this problem has received little attention to date.
In this study, a prescribed bending moment was loaded on Al₂O₃ ceramics that differed in microstructure (grain size and aspect ratio). The bending stress value was considered to establish the level of residual stress, and was compared to the value obtained by the ID method. The variation of that relationship was examined as a function of the aspect ratio. The stress ratio φ' was defined as the ratio of the value determined by the ID method and the way that tensile stress σ_T or compressive stress σ_C varied with the aspect ratio. This alteration appears commonly in the case of tensile stress. It was thought that the origin of changing φ' (stress ratio) values was interaction across crack faces by grain bridging during cracking, which varied with the aspect ratio, making the crack propagation resistance value different in various materials.