Thermodynamic conditions for cluster formation in supersaturated boundary layer during plasma spray-physical vapor deposition

Plasma spray-physical vapor deposition (PS-PVD) is used to produce columnar microstructure coatings for vapor materials under particular operating parameters. However, due to the particular conditions of PS-PVD, the state of vapor material transported in plasma jet is difficult to predict by experimental method. Moreover, the properties of plasma jet changes sharply when the hot plasma jet approaches a relatively low temperature substrate. It is more difficult to examine the behavior variation of vapor material by experiment. In this work, the thermodynamic condition for state variation of the vapor material along with methods to change this condition were studied. The distribution of plasma temperature near the substrate surface was obtained through simulation and the vapor pressure of vapor materials was calculated by the obtained temperature distribution. The supersaturated boundary layer was obtained from the distribution of the vapor pressure and partial pressure of the vapor atoms. The environment of this supersaturated boundary layer agreed with the thermodynamic conditions of the state variation of vapor materials. Further, this layer could be varied by changing the spraying distance and powder feed rate, thus allowing further control of the state variation of vapor materials. For PS-PVD process, especially for the deposition of columnar coating, it is very important to understand the properties of boundary layer and condensation conditions of vapor materials; this study provides the basis for future research on the transport of materials in the boundary layer.