It is popular practice to aluminise and pre-coat the substrate material (generally a nickel or cobalt superalloy) with an MCrAlY bond-coat. The bond-coat is necessary to accommodate residual stresses that might otherwise develop in the coating system, caused by the metallic substrate and the ceramic TBC having different coefficients of thermal expansion.
TBCs are characterised by their very low thermal conductivity, the coating bearing a large temperature gradient when exposed to heat flow. The most commonly applied TBC material is yttria stabilized zirconia (YSZ) which shows resistance to thermal shock and thermal fatigue up to 1150°C. YSZ is generally deposited by plasma spraying and electron beam physical vapour deposition (EBPVD) processes. It can also be deposited by HVOF spraying for applications such as blade tip wear prevention, where the wear resistant properties of this material can also be used.
Thermal barrier coatings (TBCs) perform the important function of insulating components, like gas turbine and aeroengine parts, operating at elevated temperature. Typical examples are turbine blades, combustor cans, ducting and nozzle guide vanes. TBCs have made possible the increase in operating temperature of gas turbines.
Gas turbine transition duct