Synthesis of Porous Silicon Nitride Ceramic with a Compositional Gradient

Abstract

This paper summarises results of a study on graded surface layer composition in porous silicon nitride ceramics with a nominal Yb-silicate, Yb2SiO5-additive content of 3.2 mol%, corresponding to 10wt%. A compositional gradient of the additive is formed within the sintered part, depending upon the sintering conditions. The thickness and the phase content of the compositional gradient is significantly affected by the nitrogen activity of the sintering atmosphere. When argon gas is used during heating up and replaced by nitrogen upon soaking, the surface of the sintered ceramic is free of Yb2SiO5, whereas in N2-atmosphere a silicate-rich coating is found as surface layer of the porous silicon nitride ceramic. In presence of carbon and nitrogen, the surface is covered by SiC. In particular the Yb-silicate enriched surface layer can be used as base for an Environmental Barrier Coating, EBC, to protect the interior of the ceramic from hot gas corrosion.