Control Principle of Thermal Spray Process

In general, an essential target in the research and development of material process lies in how to establish the controlling principle on a designated material process. In this article, the current situation in the controlling of an ordinary thermal spray process, which is a representative of a thick coating formation process using particle deposition, is reviewed. Precise observation results were introduced to the ordinary thermal spray process. In the flattening behavior of the sprayed particle onto the substrate surface, critical conditions were recognized in both the substrate temperature and the ambient pressure. A transition temperature, Tt, and a transition pressure, Pt, were defined and introduced, respectively, for these critical conditions. Three-dimensional transition curvature, by combining both the Tt and the Pt dependence, was proposed as a control principle of the thermal spray process. Furthermore, particle melting in the ordinary thermal spray process has been recognized as a negative process. To overcome this problem, a new direction in coating technology development by using non-fusion solid particles has been tried recently, which is typically called the cold spray or the aerosol deposition process. As these processes have common characteristic for all, namely, a thick coating formation by the deposition of several micron-sized particles, the present state of the art, academic/technological issues, and the prospects for the future of these coating formation processes is comprehensively summarized in this article.