Abstract
The development of thermal barrier coatings for surface protection of components exposed to high temperature engine service has been a critical and significant requirement in the advancement of aerospace and land based turbine technology. The presence of a protective ceramic barrier between the severe temperatures of the engine exhaust gases and the super-alloy turbine blade material has resulted in an increase in engine operating temperatures and a subsequent increase in fuel efficiency. The high velocity oxygen fuel (HVOF) process has been used for over 20 years to deposit hard wear and erosion resistant coatings with dense microstructure for surface protection of components. Due to the fuel gas combustion limitations of HVOF the process has failed to deposit higher melting point ceramic coatings for thermal barrier protection. This work covers the use of acetylene, a high flame temperature hydrocarbon, to generate sufficient thermal energy to soften yttria stabilised zirconia powder and produce dense HVOF coatings. Improvements in equipment design for an acetylene supply system to generate the pressure and flow necessary for HVOF were investigated and implemented. Optimisation of the HVOF process resulted in an acceptable deposition rate. Three yttria stabilised zirconia powders with different morphologies were successfully deposited and the effect on coating characteristics examined. These HVOF coatings were found to be both hard and dense with improved high temperature erosion resistance and surface roughness, comparing favourably with the current state of the art coatings produced by plasma spray and EB-PVD techniques. The success of these coatings has resulted in HVOF yttria stabilised zirconia coatings being introduced on the fins and fences of the high pressure turbine blades in the Rolls Royce Trent 880 series engine.