Coating materials research to extend engines service life
Nickel-based superalloys have contributed significantly to the ability of modern gas turbine engines to operate at high fractions of their melting temperatures and to sustain large mechanical loads. The addition of thermal barrier coatings (TBCs) has provided an additional increase in the capability of superalloys to sustain repeated and prolonged exposures to high temperature corrosive operating environments. TBCs comprise advanced ceramics such as yttria-stabilised zirconia, which exhibit very low thermal conductivity, and a bond coat providing protection from oxidation and corrosion to the underlying metal substrate. Research work within the HIPERCOAT project aimed to address potential improvements in the thermal capability of ruthenium-containing bond coat layers by defining an appropriate processing path for their fabrication. Microstructural evolution at various stages of the coating process of ruthenium and platinum-modified bond coat layers were studied in detail. A science base was built to guide the evolution of bond coat materials and assess their potential. Through vapour-phase inter-diffusion of aluminium and nickel from the substrate, alloy ruthenium and aluminium-rich layers were produced by scientists at the University of Michigan. A series of diffusion couple experiments showed the layers' arrangement to be process-dependent with low concentration ruthenium-containing bond coatings producing an exterior NiAl layer with an interior RuAl layer. Further information regarding the phase stability and diffusional characteristics of the ruthenium additions to nickel-based single crustal superalloys is needed for tailoring the properties of continually advancing thermal barrier coatings. Comprehensive modelling of stresses evolution and other deformation mechanisms in these multilayer structures during thermal cycling will be an integral element for predicting the life of a coating system.