Research of effect of abnormally growth grains on mechanical properties of aluminum oxide-based ceramics

Alumina ceramic specimens with a submicron equiaxed grain microstructure exhibit the highest mechanical performance. However, for certain ratios of fine and coarse grains, the presence of abnormally grown grains may be regarded not as a defect but as a strengthening phase, enabling improved properties, in particular high flexural strength. The aim of this work is to investigate the effect of the volume fraction of abnormally large grains on the strength and fracture toughness of alumina-based ceramics. Abnormal grain growth was achieved during pressureless sintering of submicron α-Al₂O₃ powder. It is shown that, under sintering at a constant heating rate up to a relative density of ~97%, the microstructure consists of equiaxed grains (size ~0.7 μm) and abnormally grown grains (5 vol. %, size >10 μm). By varying the sintering parameters, ceramics with different volume fractions of abnormally grown grains were produced. Increasing the fraction of abnormally grown grains from 5 to 15 vol. % increases the strength and fracture toughness by more than 1.5 times. The improved mechanical properties are attributed to deflection of the crack path from a straight trajectory as the crack bypasses abnormally large grains. The results obtained and the proposed approach may be used to optimise pressureless sintering routes for enhancing the mechanical properties of ceramics.

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