Fracture toughness and thermal shock of tool and turbine ceramics

The behaviour of some commercial tool carbides and turbine ceramics has been investigated in regard to resistance to crack initiation, crack propagation and retained strength after thermal shock. New data are provided, particularly measurements of the fracture toughness of these materials at actual operating temperatures (up to 1200 ~ C). Many of the materials did not follow the generally accepted Hasselman theory for thermal shock in ceramics, and instead of showing a discontinuity in retained strength at some critical quenching temperature difference, their residual strengths fell gradually at temperatures lower than their supposed critical quenching temperature. This behaviour is explicable when high temperature toughnesses, strengths and moduli are used in the damage resistance parameter (ER/o~). It seems that materials not following the Hasselman model suffer cumulative damage with increasing number of shocks. Sub-critical crack growth occurs even if (Kic/af)2 values are constant, and such damage, which reduces the room temperature retained strength, is enhanced by (Kin/of)2 decreasing at temperatures below ATc. In contrast, materials obeying Hasselman's model appear to have a constant (Kic/af) 2 below ATc and for some temperature range above. Only then are "one-shock" characterizations of materials possible, otherwise, the retained strength depends upon the number of prior shocks. Experiments are also reported which describe the effects of rate of testing on the unshocked and shocked mechanical properties of ceramics. Oxidation is shown to influence the results in a manner not obvious from single shock tests.