**2. Thermal spray processes and coatings´ microstructure**

All together thermal spray processes make use of heat and kinetic energy to warm-up and propel feedstock material to build up a coating on the substrate. Often the goal is to melt the feedstock thoroughly due to reach a dense microstructure, but in some cases the feedstock impinges in solid state and is deformed by the kinetic energy as the particles reach supersonic velocity before impact. Dependent on the source of energy distinctly different process characteristics and therefore visibly diverse microstructure and properties of the coatings can be obtained.

In the norm DIN EN 657 "Thermal Spraying" the different processes are distinguished by the means of the energy source. The processes being widely in operation are based on the energy sources flames and electric or gas discharges. Although the lasers assisted spraying techniques are coming more and more into operation, they cover only a small segment compared to the conventional techniques. By means of molten bath and "cold" or, in other terms, kinetic spraying only metallic feedstock can be used. Therefore both processes are not covered in this chapter. In fact the focus of this chapter lies on workings done in the fields of thermal spraying by means of Atmospheric plasma spraying (APS) as well as High velocity oxyfuel spraying (HVOF, see markings in **Figure 1** on the following page).
