Co-spraying of alumina–titania: correlation of coating composition and properties with particle behaviour in the plasma jet

Plasma-sprayed coatings of alumina–titania (60:40 by weight) have been prepared under different operating parameters. The coatings have been characterised with respect to chemical composition, phase composition, wear resistance and microhardness. It was observed that the chemical composition of coatings prepared at low input power is significantly different from that of the feedstock powder. The coating composition progressively approaches that of the feedstock powder at higher power input. Numerical process simulation has been carried out using a one-dimensional model to illustrate the importance of plasma–particle interaction during co-spraying of alumina–titania powder blend. Experimental results compare very well with the results predicted by the model. At lower power levels, titania particles are melted, whereas alumina particles remain unmelted. This is expected to give a coating composition quite different from that of the feedstock powder. With an increase in the plasma power, alumina particles are also heated to their melting point, and by choosing the proper combination of power and particle size, the spray process can be optimised to obtain a targeted coating composition.