Bonding thermosprayed silicon to glass-ceramic

Many materials, among them silicon, fail to form adherent coatings when thermosprayed onto smooth oxide glasses and glass-ceramics. Mechanical bonding is usually possible if the substrate surface is abraded before spraying, but this process also weakens the crack-sensitive substrate. This paper gives the results of examining alternate surface preparations which create a roughened substrate without the usual weakening of the finished article. The methods rely on the ability of mechanically damaged glass articles to heal, i.e. to decrease the flaw population during crystallization. The surface roughness essential to the mechanical bond is produced when the article is in the green or glassy state, and in a subsequent processing step the glass is converted to glass-ceramic by a crystallization heat treatment. The simplest application is the fabrication of roughened durable ceramic substrates for spraying after the crystallization step. Profilometry and measurements of silicon metal coating adhesion to a lithium aluminosilicate glass-ceramic indicate that for hard glasses the crystallization step does not reduce the surface roughness. When high temperature materials are to be used as coatings, the spraying step can be performed while the substrate in the glassy state, and then the entire article is subjected to the crystallization heat treatment. For a silicon-coated glass-ceramic, this method produces significantly improved adhesion over simple mechanical bonding.