Plasma sprayed mullite coatings on silicon-base ceramics

A method of applying a chemical and thermal barrier coating to a surface of a silicon-base ceramic substrate comprising the steps of

• etching said surface of the substrate in molten sodium carbonate,
• burning out residual carbon from said etched surface,
• dissolving silica on said etched surface in 10% HF,
• placing the substrate in a furnace,
• raising a temperature inside said furnace to uniformly heat said substrate prior to applying said coating,
• monitoring the temperature inside said furnace to determine when said substrate is heated to a first temperature greater than about 1000.degree. C.,
• plasma spraying mullite powder having a crystallization temperature of about 1000.degree. C. onto said etched surface of said substrate while maintaining said substrate at said first temperature whereby said mullite crystallizes as it solidifies at a rate that facilitates the formation of a crystalline mullite coating that is resistant to cracking and delamination, and,
• cooling the coated substrate to a second temperature that is ambient.