Synthesis, protection performance, and failure analysis of α-Al2O3/Mo(Si,Al)2 coatings

Abstract

To prepare the advanced cooling structure of hollow turbine blades, molybdenum metal core (MMC) has been proposed as a substitute for ceramic core substrate, but the protective coating needs to be redesigned to resist ablation during the shell burning process and erosion during the casting process. To this end, in this study, an α-Al2O3/Mo(Si,Al)2 barrier coating is fabricated on a molybdenum surface utilizing a two-step pack cementation process, which includes first siliconizing and then aluminizing, combined with pre-oxidation treatments. The pre-oxidation behavior of the new Mo(Si,Al)2 coating under isothermal oxidation (at 1300 °C) and step oxidation and its protection performance during the superalloy casting process are comprehensively examined. The results indicate that the as-deposited coating consists of an outer Mo(Si,Al)2 layer and an inner Al8Mo3 layer. After pre-oxidation, an admixture (SiO2▪Al2O3) monolayer is first formed on the surface of MMC. Then, internal Al2O3 emerges in the lower part of (SiO2▪Al2O3) mixture. Finally, the SiO2 → Al2O3 displacement reaction encapsulates MMC with an exclusive α-Al2O3 layer. Besides, the step oxidation process can obviously enhance the formation of external α-Al2O3 compared to the isothermal oxidation process. Unfortunately, after pre-oxidation treatment, a bulge arises at the interface of Mo(Si,Al)2 and Al8Mo3. The buckled coating induces partially destruction of the MMC coating by the alloy melt, whereas the intact coating can protect MMC during the casting process. Furthermore, the interfacial gap of MMC varies with the addition of alumina.