Variation of Microstructure upon Residual Stress, as Obrained by the Vickers Indentation Method in Al2O3 Ceramics with Differing Aspect Ratios
Tarou TOKUDA, Mitsuo KIDO, Gonojo KATAYAMA, Takahiro YUKI and Kouji MUKAI
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 (KC) 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 ƒÓ' was defined as the ratio of the value determined by the ID method and the way that tensile stress ƒÐT or compressive stressƒÐC varied with the aspect ratio. This alteration appears commonly in the case of tensile stress. It was thought that the origin of changing ƒÓ' (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. Key Words:Indentation, Fracture toughness, Residual stress, Microstructure, Ceramics