Effect of spray parameters on residual stress build-up of HVOF sprayed aluminium/tool-steel functionally graded coatings

The High Velocity Oxy–Fuel (HVOF) process is one of the most versatile thermal spray technologies and has found use in many industries due to its flexibility and cost effectiveness. It is normally used to protect components against wear, heat and/or corrosion and has the potential to produce functionally graded coatings. Gradual changes of microstructure in functionally graded coatings reduce possible residual stress build-up by gradually attaining property changes between the substrate and the coating. Effectively this is done by depositing layers of coatings of varying composition. The present study investigates the effect of spray parameters on residual stress build-up in aluminium/tool-steel functionally graded coatings (FGC). A simple 33 factorial design of experiments was employed to establish the effects of spray parameters on residual stress. Residual stress was measured using Clyne's analytical method. Parameters such as flow rate ratio of the oxygen to propylene, flow rate of the compressed air and spray distance were varied during coating deposition. Apart from the thickness of the coated sample, the spray distance showed greater effect on residual stress build-up in the graded coatings compared to flow rate ratio of the oxygen to propylene, and flow rate of the compressed air. Finally a set of values of spray parameters giving the best compromise between low residual stress and high deposited coating thickness were identified. This paper concluded that the optimised set of parameters for FGC resembles the parameters recommended for the deposition of the powder with the lowest melting and boiling point; in this case aluminium.