Factors Influencing The Thermal Shock Behavior of Ceramics

Abstract

The application of ceramics often requires resistance to tensile stresses developed by steep thermal gradients. Among the most popular test techniques to evaluate thermal stress/shock resistance is the down-quench test where the strength retained is determined after the sample is quenched from an elevated temperature into a fixed temperature bath. When the temperature difference is sufficient to cause a loss in strength, this temperature difference represents the critical temperature drop, ΔT_c The thermal shock resistance becomes greater as ΔT_c increases. However, heat conduction within the sample during quenching results in temperature (and surface tensile stresses) gradients that are less in thin samples as compared to thick samples. In fact, the ΔT_c values for a given material will decrease and then reach or approach a constant value with increase in sample thickness.

Only by addressing these issues can the influence of material properties on the thermal resistance of ceramics be considered. Increases in the fracture toughness and thermal conductivity yield significant increases in ΔT_c values as shown in SiC whisker-reinforced alumina versus unreinforced alumina. Changes in the thermal shock damage from large cracks in alumina to a high density of microcracks in zirconia toughened alumina composites are indicated as one source of their improved thermal shock resistance.