Dip process thermal barrier coatings for gas turbines

A new concept to apply zirconia-based thermal barrier coatings on cobalt base alloys has been developed. Contrary to plasma spraying or electron beam vaporization, the new process produces a dense and highly adherent zirconia coating that resists thermal cycling and penetration by corrosive molten salts. The new method is based on thermally growing a ZrO₂-based layer from a Zr-rich alloy, predeposited on a Y-rich substrate by hot dipping. The coating consists of an outer ZrO₂/Y₂O₃ layer and an inner oxide-metal composite layer next to the substrate surface. The outer oxide layer acts as a thermal barrier, while the inner layer acts as a graded seal that improves the adhesion of the coating to the substrate. Thermal cycling experiments showed that the coating has a good resistance to spallation between room temperature and 1100 C.