Regions of Local Densification and Their Role in Ceramics. Analysis of Fracture Surfaces as a Simple Tool to Study the Local Densification Regions in Oxide Ceramics

Abstract

Regions of local densification and their evolution upon sintering are one of the key issues in understanding the structure and properties of ceramics. As a probing tool to study the regions of local densification, a simple method based on analysis of fracture surfaces for ceramic green compacts is suggested. When samples used in the experiments are not very strong and their fracture starts at relatively low mechanical loads, cracks propagate through boundaries of the local densification regions. It allowed us to visualise the local densification regions and to estimate their size. The method was applied to a series of model samples obtained by a pressing of mixtures of coloured granules and fine powder of alumina as well as to a few real workpieces made from alumina ceramics. Upon thermal treatment of samples, with an increase in the temperature the fracture surfaces become, first, smoother and then rougher again. Such a behaviour can be explained by a relaxation of local stresses and a disappearance of legacy local densification regions due to diffusion processes and appearance of new ones due to a start of non-uniform local shrinkages. From viewpoint of nonequilibrium processes responsible for a formation of the local densification regions, the area of the smoothest fracture surface corresponds to a bifurcation point.