Phase changes during plasma spray deposition

Plasma sprayed coatings are now widely used in various fields of application and therefore a considerable effort is aimed toward the description, understanding, and control of particular processes and mechanisms governing the formation and properties of coating. Generally, there are several tens of process variables determining the output, but very often the struggle for a required property is solved only through a chemical modification of the injected powder. Concentrations of the accompanying stabilizing elements in a certain powder are optimized but frequently this is only from the equilibrium data while the plasma spraying is definitely a nonequilibrium process. Experimental evidence shows a great variety of phase changes taking place during the plasma duration. Sometimes they can be beneficial as in the case of the formation of amorphous phases for certain purposes. At other times they can be very detrimental as in the case of alpha/gamma Al2O3 formation. Hence it is very important to study all the possible mechanisms of phase changes during the plasma deposition (what can cause them, and at what stage of the process they can occur). A contribution in this direction is presented describing results obtained from detailed studies of several metallic as well as ceramic materials.