Effect of Porosity on Thermal Stress Fracture

Sintered alumina with from 4 to 50% porosity was prepared by incorporating crushed naphthalene in a casting slip. Thermal stress resistance was determined from the radial temperature difference required to fracture hollow cylinders under steady‐state conditions. Thermal stress resistance decreases with increasing porosity; resistance to thermal stresses at 50% porosity is about one‐third of that estimated for zero porosity. Summary and Conclusions Samples with controlled porosity were prepared by incorporating naphthalene flakes in an alumina casting slip. Samples were prepared and fired under identical conditions so that the continuous solid phase was identical for all samples. Steady‐state thermal stress tests described lead to the following conclusions: (1) Resistance to thermal stress, R (maximum steady‐state Δ t for fracture), decreases with increasing porosity. The resistance for a sample with 50% porosity is about one‐third that for dense samples. (2) Resistance to thermal stress determined by the maximum heat flow permissible, R' , decreases more rapidly than R because increasing porosity lowers the thermal conductivity. This factor for a sample with 50% porosity is about one‐sixth the value for dense samples. This is the factor generally applicable to thermal shock conditions. (3) Resistance to thermal fracture with a constant rate of surface temperature rise is directly proportional to R , since thermal diffusivity is unaffected by porosity.