Method of forming a protective coating on a surface of a ceramic substrate

A method for forming a protective coating on a surface of a ceramic substrate, said method comprising the steps of:

• providing a powder comprising a rare-earth oxide, alumina, and silica and having a melting point;
• etching the surface of the ceramic substrate,
• applying the powder on the etched surface of the ceramic substrate in an amount of from about 0.001 to about 0.1 g/cm2 to reduce capture of bubbles from off-gassing of the ceramic substrate;
• heating the powder on the surface of the ceramic substrate for a time of from about 5 to about 60 minutes to a temperature at or above the melting point such that the powder melts and forms a molten coating on the surface of the ceramic substrate that has a minimized number of bubbles from the off-gassing of the ceramic substrate;
• and
• cooling the molten coating to ambient temperature to form the protective coating disposed on and in direct contact with the surface of the ceramic substrate,
• wherein the protective coating has a thickness of less than about 1 mil, and
• wherein there is no layer disposed between the protective coating and the surface of the ceramic substrate.