Effect of grain size on R-curve behaviour of alumina ceramics

Alumina samples with average grain sizes ranging from 2.6 to 67.4 μm were prepared by sintering at 1500–1900°C for 2–20 h in high vacuum. Crystallographic and thermal expansion mismatch between adjacent grains during cooling involved residual stresses in these ceramics. The effect of these stresses on fracture behaviour of alumina ceramics was investigated by testing controlled crack growth during three point bending of single-edge-notched samples. After initiation, the crack grew slowly by repeated loading and unloading. The crack length c, was measured and registered in situ by means of a CCD camera coupled to an appropriate microscope, which was fitted to the test equipment by a system of elevator stages driven by stepping motors. The force P, necessary to produce an increasing crack length was computer controlled. The stress intensity factor K_I, was calculated from values of the crack length c, and force P. The data of K_I=f(c) obtained in the range of crack lengths studied were fitted by a linear function y=ax+b. As a result, the slope was used as a parameter describing R-curve behaviour of ceramics. The tests showed that R-curve behaviour of alumina ceramics strongly increases with the increase of Al₂O₃ grain size. This phenomenon was explained by analysis of microstructures and residual stresses found in ceramics by piezospectroscopic measurements. In several samples the crack growth tests were performed without unloading. The time dependent displacement d of the sample was measured and recorded together with values of force P. The stress intensity factor K_I, maximal stress intensity factor K_Imax, resistance to crack initiation K_Ii, and work-of-fracture γ_F, were inferred from measured data.