Sol infiltration and heat treatment of alumina–chromia plasma-sprayed coatings

Alumina–chromia coatings deposited on alumina substrates by plasma spraying were subjected to various combinations of thermal and chemical treatments in an attempt to improve their high-temperature stability. The chemical treatment consisted of infiltrating the coatings with a SiO2-containing sol. The thermal treatments involved subjecting the coatings to thermal cycles in a furnace prior to and following the infiltration. Observation of the microstructure of these samples revealed that the introduction of silica, which reacted with alumina to produce mullite during heat treatment, reduced cracking within the coating and between the coating and substrate.