Patent ID: 7687105

Claim:
A process of forming a protective coating on a silicon-containing surface, the protective coating consisting essentially of barium oxide, strontia, alumina, and silica, and incidental impurities so as to have a barium-strontium aluminosilicate composition, the protective coating having at least an outer surface region that consists essentially of one or more stoichiometric crystalline phases of barium-strontium aluminosilicate of which at least one is a celsian crystalline phase, at least the outer surface region being substantially free of a nonstoichiometric and silica-lean second crystalline phase of barium-strontium aluminosilicate, the process comprising: depositing the protective coating by spraying a powder that contains non-stoichiometric amounts of barium oxide, strontia, alumina, and silica, wherein silica constitutes more than 50 molar percent of the powder and/or strontia constitutes less than 6.25 molar percent of the powder, and wherein silica volatilizes during the spraying of the powder so that at least the outer surface region of the protective coating consists essentially of BaO, SrO, Al2O3 and SiO2 in approximately stoichiometric amounts for barium-strontium aluminosilicate; and then heat treating the protective coating so that at least the outer surface region of the protective coating consists essentially of the one or more stoichiometric crystalline phases of barium-strontium aluminosilicate and is substantially free of the nonstoichiometric and silica-lean second crystalline phase.