Structure of Thermal Spray Coatings

       The structure of the coating is obviously different from that of the sprayed material. This is due to the difference in process methods. From the principle of thermal spray coating formation, it can be known that the coating structure is a layered structure formed by countless deformed and flat particles interlaced in a wave-like accumulation. This is because during the spraying process, the molten or semi-molten particles react with the spraying working gas and the surrounding atmosphere, so that surface oxides will appear after the spraying material is sprayed. At the same time, the superposition of the deformed flat pullers produces a bridging effect, which inevitably causes small pores in the coating. Therefore, the typical structure of the coating is a layered structure formed by the accumulation of deformed flat and fine coating materials, with some pores and oxides in the middle. The amount of pores and oxides depends on the spraying process method and spraying process parameters. Among the thermal spraying process methods, high-energy high-speed plasma spraying, low-pressure and controlled atmosphere plasma spraying, high-speed flame spraying, cold-air dynamic spraying, etc., can effectively control and eliminate the generation of oxide inclusions and pores. Improve coating structure and performance.



      The bonding of the coating includes the bonding between the coating and the surface of the substrate (that is, the so-called bonding strength of the coating) and the cohesion between the particles of the coating and the particles (that is, the bonding strength of the coating itself). Generally speaking, the bonding strength of the coating itself is higher than the bonding strength of the coating and the substrate, but it belongs to the physical and chemical bonding. This physical-chemical combination includes the following ways:



      (1) Mechanically combined sprayed particles impact the surface of the substrate to produce deformation, inlay, fill, and bite on the surface of the substrate: form a mechanical anchoring effect with the substrate. Most coating bonds exist in this manner.



      (2) Physical bonding The bonding of the particles to the surface of the substrate is composed of van der Waals forces or secondary valence bonds.



      (3) Metallurgical-chemical bonding Metallurgical-chemical bonding is much stronger than mechanical bonding and physical bonding. It consists of three parts: van der Waals force (on a clean substrate surface, the intermolecular attraction between the particles at the contact point between the coating particles and the substrate surface, the atomic distance reaches the atomic and molecular distance), chemical bond force (the bonding force of the chemical bonds formed when the atomic distance between the coating atoms and the substrate atoms or the coating particles reaches the value of the atomic lattice constant) and the microdiffusion force (elements in the coating particles and in the matrix material) The elements of mutual diffusion under certain conditions) is a type of combination in which diffusion and alloying occur on the surface of the coating material and the base material, including the formation of intermetallic compounds or solid solutions on the bonding surface.



      On the coating of the same specimen, the above three bonding methods may exist at the same time, but the mechanical bonding is the main one.