Effect of substrate surface conditions and impact velocity of sprayed particles on coatings produced by plasma spraying

In high temperature plasma spraying, surface conditions of the substrate and the impact velocity of sprayed particles are particularly important in the consideration of factors influencing the resultant coatings. In this study an induction plasma torch was used to spray aluminum, tungsten and nickel particles onto flat (111) single-crystal silicon substrates with different surface preparations. In addition to experiments with silicon substrates with clean surfaces and substrates with previously evaporated aluminum over oxide coatings, oxide films and water vapor, hydraulic oil and grease contaminant films were used to investigate the influence of these surface conditions on adhesion formation. The impact velocity of the sprayed particles was controlled by controlling the speed of a centrifugal disc on which the substrate was installed to traverse the plasma effluent. Scanning electron microscopy was used extensively to characterize features of the particle-substrate interaction and to compare the individual behaviors of the metal particles used. Microcavitations and dislocations resulting from the plastic deformation of the substrate were revealed by the successive use of a metal and a special dislocation etchant.