A new concept for thermal protection of all-mullite composites in combustion chambers

A new thermal protection concept for all-mullite composite shingles based on a thermally-sprayed mullite layer is described. Because of the insufficient thermal long-term stability of the Nextel™ 720 fibers in the 1273 plus regime, Nextel™ 720 fiber-based ceramic composites are protected by a flame-sprayed mullite coating in order to prevent the composite from thermal degradation in service. The protection layer is deposited on the front side of the ceramic shingle facing the hot gas stream. Front and back sides of the shingles are cooled through film and convection cooling, respectively. Reducing both the composite material and the protection layer to a single phase (mullite) system is a simple, but highly efficient approach to keep thermal and elastic misfit strains at the interface at reasonably low levels. Due to the porous grain texture intrinsic to thermally-sprayed materials, thermal conductivity of the protection layer is low, yielding a considerable thermal insulation effect depending on the layer thickness and the particular heat flow scenario of the combustion chamber. The microstructure/property relationship of the thermal protection layer and its interaction with the underlying composite are discussed focussing on the constraints of real combustion chamber operation conditions.