Encapsulation of sacrificial silicon containing particles for SH oxide ceramics via a boehmite precursor route

Abstract

Easy crack propagation in oxide ceramic coatings limits their application in high temperature environment (e.g. such as engines and gas turbine components) [1]. In order to overcome this problem, incorporation of sacrificial particles into an oxide ceramic coating may be a viable option. Particles of silicon compounds, such as: Si3N4, SiC, MoSi2, TaSi2 and WSi2 are attractive due to their unique features of producing a reaction product (i.e. SiO2) which fills the crack and bonds well to the surrounding matrix and exhibit, a relatively low density and melting point above that of the matrix [2]. To achieve a better control of the self-healing process with silicon compound particles a gas tight encapsulation procedure of these particles is a crucial prerequisite in order to avoid premature oxidation in the absence of a local crack. One possible route is the encapsulation of these particles by a sol-gel method [3, 4]. The dispersed healing particles were coated by Al2O3 layer prepared from boehmite sols, using two different alumina precursors (salt and alkoxide precursors). The resulted embedded sols were annealed in reducing atmosphere at temperatures above 1200 °C in order to achieve a dense ?-Al2O3 microcapsule shell wall. The morphology, structure and composition of the encapsulation are determined with advanced analysis techniques. The thermal stability of the encapsulated healing particles is studied by high temperature exposure in air.