Microstructure dependent hardness and fracture behavior in liquid-phase-sintered Al2O3

The liquid-phase-sintered Al2O3 (LPS) derived from commercial powders of different particle size, e.g. coarse (70–100 μm), medium (3.6–7.0 μm) and reactive (<2.0 μm) showed a substantial differences in Vickers indentation fracture behavior depending upon their grain size distribution and thermal expansion coefficient mismatch between the matrix grain and the intergranular phases. A high true hardness and a low indentation crack length was observed in the case of the LPS with reactive powder which was attributed due to enhanced dissolution of Al2O3 into the glassy phase. A high flexural strength was achieved with the LPS of medium powder. A high Kic-short always resulted either due to (i) the MgO/(CaO+BaO+KNaO) ratio of nearly 1 in the chemical composition of LPS, or (ii) higher modulus of elasticity to hardness ratio, or (iii) reinforcement of coarse grains (>12 μm) in the fine-grained (∼2 μm) microstructure. The crack path was predominantly intergranular at lower MgO/(CaO+BaO+KNaO) ratio (<1.0) for the indentation load in between 9.81 and 49.03 N, whereas it was transgranular at a higher ratio (∼1.6). A low Kic-short was observed due to precipitation of anorthite phase in the LPS with a high MgO/(CaO+BaO+KNaO) ratio. Finally the sintered density of 91–94 wt% LPS materials comprising of all powders produced a linear relationship with both the hardness and the modulus of elasticity.