US Patent 7306860 Protective coating for oxide ceramic based composites

A layered structure includes a substrate comprising a layer of an oxide/oxide ceramic based composite material, a first oxide layer disposed directly on the substrate and formed from a material that has no greater than about 10% porosity and is substantially impermeable by water vapor, and a second oxide layer disposed directly on the first oxide layer and having a greater porosity than the first oxide layer. Either or both the first and second oxide layers of the coating system may be deposited using a plasma spraying process, a slurry deposition process which is followed by a sintering step, or an EB-PVD process.