Dip Process Thermal-Barrier Coatings for Superalloys.

A new concept of growing a ceramic-based thermal barrier coating on gas turbine alloys was investigated. This process involves hot dipping alloy substrates in low-melting cerium-nickel or zirconium-nickel eutectics. Cerium oxide CeO2 or zirconium oxide ZrO2 were then grown by selective oxidation to form an external thermal barrier layer above an inner composite CeO2 or ZrO2substrate alloy layer. The microstructure and chemical composition of the thermal barriers were studied as a function of composition of melts and substrates, dipping temperature and oxidation conditions. This evaluation has led to a good understanding of the factors that control the formation of desirable coatings and to the specification of the conditions to produce them. An important advantage of ZrO2-baed coatings over CeO2-based coatings produced by this technique is that ZrO2 grows totally as an overlay layer with little or no influence of elements from the substrate. In contrast, CeO2-based coatings grow completely within the surface zone of the coated substrate, and thus their properties can be affected by substrate constituents. The results of this progam suggest that the dip process could be an important alternative approach to plasma spraying for producing thermal barrier coatings on superalloys.