Fundamentals of Thermal Spray

       ⒈The basic process of thermal spraying



①The spray material is heated to a molten or semi-melted state



②The droplets of the spray material are atomized



③Atomized or softened fine particle jet flying



④ Tiny particles hit the surface of the substrate and form a coating



⒉ Coating structure



Thermal spray coating is to heat a certain material to a molten or thermoplastic state by a combustion flame or a plasma heat source to form a cluster of high-speed molten particle flow (droplet flow), and the droplets sequentially hit the substrate or the surface of the formed coating. It is formed by the lateral flow of particles, flattening, rapid solidification and cooling, and continuous accumulation.



Since the thermal spray coating is a typical layered structure, the properties of the coating are directional. There is a significant difference in vertical and parallel coatings for upward performance. Appropriate treatment and remelting of the coating can both transform the layered structure into a homogeneous one and eliminate oxide inclusions and pores in the layered structure.



⒊ Coating combination



The bonding of the coating includes the bonding of the coating to the surface of the substrate and the cohesive bonding of the coating. The binding strength of the former is called binding force. The bond strength of the latter is called cohesion.



There are three general combinations of coatings:



①Mechanical bonding When the molten particles hit the surface of the substrate and rapidly cool and solidify, they will bite the uneven substrate due to shrinkage, forming a mechanical bond.



②Physical bonding The van der Waals force between the molecules (atoms) is the bonding method for the spray coating to adhere to the surface of the substrate.



③ Metallurgical bonding When the molten fine particles hit the surface of the substrate at a high speed, the interface between the coating and the substrate appears diffusion and alloying.



⒋ Residual stress of coating



     In general, thermal spray coatings have obvious residual stress. When the molten particles collide with the surface of the matrix at a high speed and rapidly cool and solidify while producing deformation, tensile stress will be generated inside the particles and compressive stress will be generated on the surface of the matrix.