Processing of stepped-density alumina via gelcasting and reaction bonding techniques

Abstract

Ceramics that combine property gradients or steps in monolithic bodies are increasingly being sought as alternatives to ceramic composites made of multiple dissimilar materials. Furthermore, more sophisticated applications specify single components to serve many functions that often require disparate properties. For example, oxygen impermeability and high compliance may need to be coupled with low thermal conductivity in thermal barrier coatings or a thin, oxygen permeable membrane may need to be coupled to a rigid porous support for electrochemical gas extraction. Density graded materials are also of interest for structural applications, as one may couple a thin, dense wear resistant layer with a lightweight support structure or a porous, flaw tolerant surface with a dense, stiff support structure. However, graded density ceramics are difficult to process due to the need to accommodate differential volume changes upon firing. For example, a green body containing a particle size gradient would result in graded density due to higher densification of the small particle portion upon firing. However, due to the non-uniform volume changes throughout the specimen, sintering stresses occur which cause macroscopic cracking. This paper presents a novel method to produce stepped-density alumina bodies which maintain mechanical integrity by eliminating non-uniform volume changes. The processing instead relies on a reaction bonding technique.

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