Dependence of Adhesion Strength of Plasma Spray on Coating Surface Properties

Abstract

It is well known that thermal plasma spray coatings have diversified utilizations, but not vividly in use, mainly because of high cost of spray grade powder, for development of thick coating > 50 µm on metal substrates. Present investigation aims at depositing plasma spray coating on metals using low-cost/waste materials. A mixture of fly-ash + 20 weight percent of Quartz + 20 weight percent of illmenite is used for coating on copper and mild steel substrates. The powder mixture is plasma sprayed by different operating power levels (11 to 21 kW) of the plasma torch. The coatings are characterized for their adhesion strength, porosity and deposition efficiency. It is observed that with increasing operating power level, the adhesion strength increases up to a certain power level, and then it remains nearly steady with further increase in torch input power. A maximum of 6.56 MPa adhesion strength is obtained on the coating developed on mild steel. It is observed that coating adhesion is more in case of mild steel than that of copper substrate. The amount of porosity in the coating decreases with increase in operating power level. Coating deposition efficiency also increases with increase in operating power of the plasma torch, up to a certain power level, and then becomes stagnant with further increase in input power. The substrate morphology of the coating observed under scanning electron microscope indicates the difference in particle size and inter-particle adhesion, which collaborate to the mechanical properties, i.e., adhesion strength and level of porosity.