Structure and High-Temperature Creep of Oxide Ceramics. Properties of Diffusion Path and Their Influence on Creep

Abstract

Lifetime of ceramic workpieces at elevated temperatures is limited by their creep resistance and damage tolerance. Ceramic materials are usually composed of one or several crystalline phases, glass phases, and pores and have a complex multi-level hierarchical structure. Creep of ceramics occurs primarily due to a deformation of a substance in the intercrystal line area. At high temperatures, in a stable creep regime distant from the point at which a fracture starts, the mechanism of this deformation is the bulk diffusion (NabarroHerring-Lifshitz’s bulk diffusion). For most of oxide ceramics, the principle type of diffusion is the vacancy diffusion. Quantitative parameters of the process (activation energy and creep flow rate) are defined by a movement of vacancies from their sources to sinks (diffusion path). Thus, properties of the diffusion path control the creep. The diffusion path could go through the bulk of a crystal or through a substance in the intercrystal area of the ceramic material. A type of the diffusion path chosen by the system depends primarily on amount, composition, and plastic properties of the intercrystal line substance. From viewpoint of this generalised approach, various cases of creep behaviour are discussed and parameters affecting creep properties of ceramics, such as crystal size, content of impurities or artificial additives, porosity, concentration of point defects, etc., are considered on the base of selected experimental results for ceramics based on aluminum, magnesium, and cerium oxides, mullite as well as industrial refractors.