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

Alumina samples with average grain sizes ranging from 2.6 to 67.4 mum were prepared by sintering at 1500-1900 degreesC for 2-20 h in high vacuum. Crys tallographic and thermal expansion mismatch between adjacent grains during cooling involved residual stresses in these ceramics. The effect of these s tresses on fracture behaviour of alumina ceramics was investigated by testi ng controlled crack growth during three point bending of single-edge-notche d 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 ra nge of crack lengths studied were fitted by a linear function y = ax + b. A s 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 st rongly increases with the increase of Al2O3 grain size. This phenomenon was explained by analysis of microstructures and residual stresses found in ce ramics by piezospectroscopic measurements. In several samples the crack gro wth 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, r esistance to crack initiation K-Ii, and work-of-fracture gamma (F), were in ferred from measured data. (C) 2000 Elsevier Science Ltd. All rights reserv ed.