Densification of powder rings constrained by dense cylindrical cores

Abstract

Alumina rings were slip cast around dense alumina discs, dried and then heated with controlled heating rates to 1550°C. Most specimens developed hair-line, radial cracks that would span the ring due to either drying (differential shrinkage < 1%) or heating to 800°C (densification shrinkage = 0.3%). This catastrophic cracking phenomena, which appeared to initiate from agglomerates, occurred when the ring was weakest, i.e. when its critical stress intensity factor was small. Constrained ring specimens that did not develop hair-line cracks (about 20%) could be heated without macrocracking at any heating rate to achieve the same relative density (> 0.97) at 1550°C as unconstrained rings subjected to an identical heating schedule. Parallel crack-like voids, normal to the tangential tensile stresses developed by constrained densification, and apparently associated with agglomerates, were observed in the constrained rings. Their opening displacement was proportional to the densification strain. Identical, 25 μm, spherical voids introduced into some cast specimens deformed to oblate spheroids consistent with the stress distribution associated with constrained densification. Crack-like voids were also associated with the deformed voids. Radial notches were required to induce macrocracking during constrained densification. Propagation of radial macrocracks initiated at a temperature (1300°C) near the maximum shrinkage strain rate exhibited by the powder compact. They extended with increasing temperature (or density) and their opening displacement was proportional to densification strain. Macrocracking phenomena were independent of heating rate. These results are discussed with relation to damage that might be produced in powder composites by constrained densification and the effect of this damage in reducing densification rates and resulting in end-point densities less than theoretical.