Thermal-stress resistance and fracture toughness of two tool ceramics

The thermal-stress resistance and fracture strength behaviour of two oxide ceramics (a hot-pressed pure Al₂O₃ and a composite ceramic NTK-HC2) subjected to severe thermal shocks have been investigated. The damage resistance parameter (K_IC/σ_f)² for both ceramics is also determined for a wide range of temperatures (25° to 800° C) and cross-head rates (10^-2 cm min^-1 ∼ 2.5 m sec^-1). Fracture strength behaviour of these two oxide ceramics is shown to follow Hasselman's model where the instantaneous strength loss at the critical quenching temperature may be calculated using appropriate (K_IC/σ_f)² values to give good agreement with experimental results. Repeated shocks show some further degradation in the retained strength for both ceramics so that these materials are susceptible to thermal fatigue. It is found that both materials possess similar resistance to crack initiation (i.e. similar ΔT_c and retained strength after shocking through ΔT_c) but the pure oxide ceramic has higher resistance to crack propagation.