The structure of a coating is ultimately determined by the mechanism of its formation. For metallic powders three major processes contribute to the formation of structure:-heating and melting (partial or complete), spherization; -moving to the surface of the specimen due to interaction with the oxygen of the air, oxide film formation; -dynamic contact with the surface of the base metal or an adjacent layer, deformation and spreading over the surface.The latter process leads to formation of three new types of boundaries in addition to inter-granular and inter-phase boundaries common to compact materials. In the "base metal-coating" composition the boundaries between the deformed particles, between the layers, and between the base metal and the coating are also formed.If the formation of the coating is accompanied by elevation of the base metal temperature (steel) up to 600--700 DC, a film ofwustite (FeO) or a combination of iron oxides (FeO, Fe203, Fe304) covers the surface of the metal. For steels of complex composition the films of the oxides of the alloying elements can also be formed. A film of oxide at the "base metal-coating" interface significantly reduces the strength of the coating. However, if the oxide film is broken upon the collision of the particle with the base metal, strong inter-atomic bonds will form at the point of contact. Oxide films decorating the shapes of particles are also quite common between the particles inside the layers and at the inter-layer boundaries.Powder particles often have nearly equilibrium, spherical shape. During the deposition of the coating the shape of microparticles changes due to imposed plastic deformation. The particles get stretched, become elongated and partially bent. From the point of view of spatial symmetry the system of surfaces becomes planeoriented. In practice this is seen as the presence of equal-axes grains at the longitudinal section, and pronounced spatial orientation (oblongness) of particles at two mutually perpendicular transverse sections. Undeformed particles of regular shape can also be found occasionally.L. I. Tushinsky et al., Coated Metal
