The problem of thermal stability of ceramic materials

Abstract

The thermal stability of ceramic materials is considered from the standpoint of fracture mechanics as a property determined by the capacity of the structure to resist the appearance and propagation of cracks on critical defects under the effect of thermal stresses. A method is suggested for comparative evaluation of the thermal stability of ceramics according to which specimens with a notch simulating a structural defect responsible for fracture are subjected to thermal shock. The absence of induced defects in the region adjoining the tip of the notch is a necessary condition for providing reproducible results. The resistance to thermal shock is determined from the relative decrease in the crack resistance after the thermal shock, the “insensitivity” of the structure to defects, and the degree of their accumulation in the region of the tip of the notch as a result of the thermal shock. The first and second criteria for evaluation of thermal stability involve the coefficient of relaxation of thermal stresses and the ultimate bending strength of the notched specimen. A ceramics of ZrO₂ partially stabilized by 3 and 12% Y₂O₃ and a cermet with a composition of ZrO₂-3 vol.% Y₂O₃-50 vol.% Cr are chosen for studying thermal stability by the method developed. Calculated results on the thermal stability of the cermet are compared with results obtained directly by thermocycling with variation of the metal content from 10 to 50%. The maximum mechanical properties are shown to correspond to a metal content of 40% due to formation of double-skeleton structure in the cermet. The method can be helpful for evaluating the initial stage of fracture caused by a thermal shock in structural ceramics.