Polysilazane derived ceramic oxidation barrier coatings on MoSiB alloys

Reference	Presenter	Authors (Institution)	Abstract
18-021	Iryna Smokovych	Smokovych, I.(Otto-von-Guericke University Magdeburg); Scheffler, M.(Otto-von-Guericke University Magdeburg);	Due to a combination of sufficient strength and fracture toughness at low temperature and an excellent high temperature strength and creep resistance, alloys of the Mo-Si-B system are the most promising class of materials for increasing efficiency in gas fired combustion turbines. They show very high creep resistance up to 1300 °C and high static and cyclic strength of up to about 1100 °C. However, these alloys tend to oxidize between 700 °C and 800 °C because of MoO3 molybdenum oxide formation, which quickly evaporates between 400 °C and 800 °C; this leaves behind open porosity and easy oxygen penetration into the alloy’s matrix. Glassy SiO2-B2O3 passive oxide layers on Mo-Si-B alloy itself occur between 650 °C and 750 °C, when the evaporation of MoO3 has already started, and the viscosity of the glass phase is too high to file all gaps after MoO3 volatilization. That is way the generation of the additional protective layer from the outside on the base of SiO2 or SiON compounds, which have a high thermal stability, could be a profitable alternative for improving the oxidation resistance of Mo-Si-B alloys at high temperatures. Polysilazanes precursors were applied and thermally converted as coatings on plain Mo (99.95 %) substrates. First investigations with a polysilazane (PHPS NN 120-20) as a preceramic polymer and Si and B powders as active fillers lead to an intact, crack-free coating after cross-linking at 110 °C and pyrolysis in nitrogen at 1000 °C; a dip-coating process was applied. Amorphous SiOxNy phase and free Si were detected by Raman microscopy. The formation of elemental Si during thermal treatment facilitates the formation of additional SiO2 following a parabolic oxidation kinetic. This layer might act as an additional passivation layer on Mo providing its alloys a thermal stability in air at 1100 °C up to 100 hours.
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