Development and characterization of an environmental barrier coating system by PVD for Mo-based alloys

Abstract

Mo-Si-based alloys are being considered as high-temperature structural materials to replace Ni-based superalloys in the hot section of gas turbines due to their high melting point of over 2000 °C and superior mechanical properties. As a result, significantly higher operating temperatures could be achieved in comparison to state-of-the-art Ni-based superalloys. However, their oxidation behaviour is inadequate, as MoO3 evaporates at temperatures below 1000 °C and rapid oxide growth occurs at temperatures above 1000 °C. By applying protective coatings against oxidation and water vapor corrosion, so-called environmental barrier coatings (EBC), a sufficient resistance of the alloys against environmental threats over a wide temperature range can be ensured. In this work, a multisource sputter coater was used to deposit a four-layer PVD EBC system on Mo-Si based alloys. Oxidation protection was provided by a dual layer coating system containing a graded Mo-Si interlayer to compensate for differences in the coefficient of thermal expansion (CTE), and a pure Si top layer to serve as a reservoir for the desired protective thermally grown oxide layer of SiO2. Ytterbium monosilicate (YbMS) is well known for excellent water vapor resistance and provides a CTE close to that of Mo-Si-based alloys. In order to restore the thermodynamic equilibrium between the oxidation protective Si layer and the YbMS, another intermediate layer of ytterbium disilicate (YbDS) was deposited. The EBC system on different Mo-Si-based alloys was tested at 800 °C and 1200 °C in laboratory air where it successfully improved the oxidation resistance. The emphasis was on the chemical reactions and diffusion processes at the interfaces of the coating system, which were analysed by scanning and transmission electron microscopy as well as electron dispersive spectroscopy, respectively. Due to the specific design of the multi-layer coating, all interfaces were stable, but some interdiffusion occurred mainly between the protective dual layer coating system and the alloys by forming an interdiffusion zone after prolonged oxidation. In addition, phase formation was evaluated using high temperature X-ray diffraction techniques.