Comprehensive study on the effect of HVOF processing parameters and particle size on high-temperature properties of NiCoCrAlYTa coatings

This paper investigates the microstructural and high-temperature properties of NiCoCrAlYTa coating deposited by High-Velocity Oxy-Fuel spraying (HVOF) by focusing on the optimization of process parameters and particle size effect. Since the NiCoCrAlYTa powder has a high amount of fine particles (5–38 μm), spraying of these fine particles due to the high surface-to-volume ratio is associated with an increase in particle surface energy, which can lead to in-flight oxidation of particles. To evaluate the oxidation behavior of these particles, the oxygen/fuel ratio, spray distance, powder size, and spraying under shrouded condition was considered as influencing factors. The formation of unmelted particles and in-flight oxidation can strongly be influenced by the oxygen and fuel flows, which determine the velocity of sprayed particles, and the temperature of the jet, respectively. Among the applied coatings, at a constant spraying distance of 300 mm, the sample with an oxygen flow rate of 900 SLPM and a fuel flow rate of 400 ml/min obtained a lower oxide content equal to 11.7 %. Different spraying distances can also play a significant role in the final microstructure evolution of the coating. Under constant flow rates of oxygen and fuel, the spraying distance was changed from 300 mm to 250 mm and 350 mm, which resulted in oxide content change from 11.7 % to 14.3 % and 7.8 %, respectively. Under the same spraying condition, increasing the sprayed particle sizes from 5–38 μm to 25–38 μm, the oxide content decreased from 7.8 % to 1.8 %. It is because smaller particles can expose to higher temperatures and cause them to heat up faster, resulting in more intense in-flight oxidation of the particles. The present study aims to provide a comprehensive understanding of the influence of spraying conditions and particle size distribution on the performance and durability of the NiCoCrAlYTa coating by evaluating the isothermal oxidation, thermal shock behavior, and high-temperature erosion.