Cracking Resistance of Alumina-Based Polyphase Ceramics

Abstract

The cracking resistance of four alumina-based products is investigated. They are the following: 1 sintered product Al₂O₃-Cr₂O₃, 1 sintered product Al₂O₃-mullite, 1 sintered product Al₂O₃-SiC, 1 molten-cast product Al₂O₃-ZrO₂-SiO₂. The cracking tests were conducted under 4-point bending on SENB* specimens of 230 × 50 × 12,5 mm³ with a notch depth of 13 mm. The appearance of the first cracks and the evolution of cracking during the test were followed by acoustic emission. The graph of the functions K_I = f(a) and V = f(K_I) of each product was plotted from tests at imposed cracking and delayed cracking rates (under relaxation and creep). The critical stress intensity factor for initiation K_Ii varies from 0,7 to 2,7 MPa√m and that corresponding to the crack propagation plateau K_Ip from 3,2 to 6,5 MPa√m. The sintered products Al₂O₃-SiC and Al₂O₃-mullite exhibit the most favourable K_Ip/K_Ii ratio as well as the best delayed cracking range, the cracking rate being independent of KI_I. Acoustic emission measurements show a very good correlation between K_I and the logarithm of the acoustic emission rate. The acoustic emission rate moreover makes it possible to determine the number of microfractures occurring per unit of crack length.

SENB: single edge notched beam