Study of phase changes in plasma sprayed deposits

Abstract

The formation of a plasma-sprayed coating that exhibits predictable properties requires the control of many process variables. The phase changes that take place during plasma spraying are significant material variables that should be controlled. Several different materials were deposited in air with a water-stabilized plasma torch (model PAL 160). Usually, air was used as a carrier gas for the powder; however, argon was also used for some coatings. The injected powders (NiAl, Ni, ZrSiO4-based, Al2O3-based, etc.) as well as the coatings were studied for, among other properties, their structure, particle size, microhardness, and chemical and phase composition. Phase changes induced by the different cooling rates of molten particles after their impact on a substrate are illustrated for ZrSiO4. It has also been found that the oxidizing power of the water-stabilized torch is less than previously believed. For example, coatings produced with nickel powder injected with argon as the carrier gas exhibited almost no oxides. Significant element redistribution during plasma spraying was demonstrated with a two-phase NiAl feedstock powder. The coating exhibited almost all the phases that are present in the binary NiAl alloy as well as envelopes of oxides and traces of amorphous phase.