Effect of water vapour content on oxidation of Ni-based superalloys.

Kurzfassung

One of the major challenges in use of superalloys and/or protective coatings for modern aerospace gas turbine engine applications is understanding the implications of service life conditions on high temperature oxidation of turbine components, e.g.: high operation temperatures and water vapour. The current study is an effort to understand the fundamentals of isothermal oxidation mechanism of Ni-based superalloys which would be implemented in turbine section of aero-engine under the influence of water vapour content at high temperatures. Ni-based superalloys namely Ni-8Cr-5.5Al and Ni-12Cr-5.9Al are oxidised at 900 °C and 1000-1050 °C for 100 hours under three distinct water vapour contents corresponding to the gas composition of present and futuristic engines namely, conventional engine (0-5 wt.% water vapour), H2 burn engine (10-15 wt.% water vapour) and WET engine (>25 wt.% water vapour). In addition well-known high temperature oxidation protective bond coat material was exposed to water vapour at high temperature to elucidate the change in oxidation mechanisms. Oxidation kinetic study was performed up to 100 hours on NiCoCrAlY bond coated IN718 under >25 wt.% water vapour at 1000-1050 °C and 1100 °C. In addition, a NiCoCrAlY sample was exposed to 0-5 wt.% water vapour at 1000-1050 °C for 100 hours for comparison with the other two conditions.The important observations under the presence of water vapour were: A noticeable change in mass gain and oxide scale formation w.r.t water vapour content, formation of thicker NiO scales (i.e., breakawayoxidation) for alloy Ni-8Cr-5.5Al at both test temperatures with an exception of external HfO2 formation under 10-15 wt.% water vapour at 1000-1050 °C, Ni,(Al,Cr)O spinel phase formation is observed in alloy Ni-12Cr-5.9Al at both test temperatures after 100 hours with decreased continuity of scale formation on increasing water vapour content. In both the alloys, no continuous external protective Al2O3 scale formation was observed on the surface rather continuous/ discontinuous internal Al2O3 scale was formed. NiCoCrAlY bond coat exhibited parabolic kinetics both at 1100 °C and 1000-1050 °C under presence of >25 wt.% water vapour. β-phase depletion depth also exhibited parabolic kinetics both at 1100 °C and 1000-1050 °C under presence of >25 wt.% water vapour. At 1100 °C, external yttrium enriched oxide formation on top of Al2O3 was observed. Another peculiar observation for oxidation of 2 NiCoCrAlY bond coats is, the scale thickness is observed to increase with respect to temperature as well as water vapour content both after 100 hours. No dramatic changes were noticed in the protective nature of the coating.