Development of CoNiCrAlY oxidation resistant hard coatings using high velocity oxy fuel and cold spray techniques

•Coating of superalloy CoNiCrAl powder on turbine blade for oxidation/corrosion resistance at higher temperature is studied.•Thermal spray techniques CCGD and HVOF process are used to coat superalloy powder.•Bond coats deposited by CGDS technique were harder and denser.•The hardness of the CGDS sprayed with helium is high.•The oxide scale for the CGDS coatings composed of alumina only.

High velocity oxy fuel (HVOF) and cold spray techniques have been used to form oxidation resistant hard coatings for bond coat application for gas turbine blades. Though both techniques accelerate powder particles with high kinetic energy, the coatings formed differ considerably in their microstructures. In the former, high pressure is created by burning gases such as acetylene, propane or kerosene at high pressure and generating high temperature (3500 to 4500 °C) in the gun. This gives high acceleration to powder particles which melt and deposit on substrate layer by layer with splat cool mechanism. On the other hand, large kinetic energy (KE) is generated in cold spray by passing carrier gases such as He or N2 through a converging–diverging nozzle, with a lower gun temperature of around 500 °C. Here the particles are not liquid droplets because of lower temperature and the deposition mechanism is not a splat cooling, but a high impact of solid particles, which results in plastic deformation, making a very adherent coating.In this work, CoNiCrAlY powder was used. Presence of Al creates suitable deformation properties on impact. The powder was deposited on 316L SS substrate using HVOF and also by cold gas dynamic spray (CGDS) deposition process. In the latter, there are two processes, which give two different kinetic energies, depending upon the gas used, nitrogen or helium. The latter gives much higher KE. The deposited coatings were characterized and subjected to wear and oxidation studies. One of the interesting results was that the oxidation resistance of CGDS coating was much superior to that HVOF coated stainless steel with low wear rate and better friction behavior.