Influence of Y_2O_3 Addition on the Transformation Behavior of ZrO_2 in Al_2O_3-ZrO_2 Composites

The effect of Y2O3 content on the stress-induced transformation of ZrO2 in sintered Al2O3-ZrO2 (Y2O2: 0-3mol%) composites was studied by using laser Raman spectroscopy and X-ray diffraction method, and their mechanical properties, such as Young's modulus, bending strength, Vickers hardness and fracture toughness, were measured. The tetragonal phase of ZrO2 was 54mol% in the Y2O3-free composite, and increased with increasing Y2O3 content. From SEM observation, the particle size of ZrO2 in the composites was from 0.9 to 1.2μm. The Raman measurement of ZrO2 phases on the fracture and tensile surfaces after bending test indicated that the stress-induced transformation proceeded more extensively in the composites with low Y2O3 contents and near the fracture surfaces. Consequently, fracture toughness increased with decreasing Y2O3 content, and this tendency was related to the stress-induced transformation and the presence of microcracking. On the other hand, bending strength, Vickers hardness and Young's modulus decreased with decreasing Y2O3 content due to microcracking in Al2O3-ZrO2 composites. The results obtained lead to the conclusion that the stress-induced transformation behavior is explained by the constraint of ZrO2 particles in the matrix of Al2O3, and is influenced by the stabilization of tetragonal ZrO2 by Y2O3.