The bonding form of thermal spray coating and substrate - metallurgical bonding

       Metallurgical bonding

 

    The metallurgical bond can be that the coating material and the substrate form a common grain at the interface (co-crystal), or only the grains are in contact and have grain boundaries at the interface (no common grain is formed), or they can react with each other to form intermetallic compounds. The case where common crystal grains are formed can be called "intracrystalline bonding", and the case where common crystal grains are not formed but only contact each other is called "intergranular bonding".



    Generally speaking, the bonding of the coating material and the substrate in the thermal spraying process rarely sees the effect of intragranular bonding. That is to say, the crystallization process of the coating material on the surface of the substrate is not mainly the epitaxy of the substrate lattice. The reason can be analyzed as follows. The first is the difference in materials. When spraying aluminum, ceramic materials or Ni-based, Co-based alloys on the steel substrate, the lattice types and lattice constants of the substrate and the coating are extremely mismatched, which makes it difficult to epitaxially crystallize on the substrate. But the most important factor is that the temperature of the substrate is too low. In the normal thermal spraying process, the temperature of the substrate is not higher than 200-30090. This reduces thermal deformation of the substrate and ensures that the chemical composition of the deposited layer does not deviate significantly during the spraying process. At such a low substrate temperature, the energy (including thermal and kinetic energy) possessed by the molten metal is not sufficient to overcome the potential barriers between atoms to the extent that intragranular bonds are formed. However, due to the exothermic effect of some coating materials themselves, local melting may also occur at some microscopic points, resulting in the formation of intragranular bonding at the points.



    Intergranular bonding is the most important bonding form between the thermal spray coating and the substrate. The high-speed moving molten droplets hit the surface of the substrate and quickly spread out into flakes. The cold bulk surface not only provides a great degree of subcooling for the crystallization of the droplets, but also provides a ready-made solid surface. The crystallization process of shape and growth rapidly takes place on the surface of the substrate. There is an obvious grain limit between the coating and the substrate, which cannot be epitaxy to each other. Due to the nature of this bonding, the bonding strength may be too high, and the intergranular bonding is the most important junction method between the thermal spray coating and the substrate, so the bonding strength between the thermal spray coating and the substrate is relatively low, and the value is only within the crystal. combined tenths.



    Another way of metallurgical bonding is that the coating reacts with the substrate to form intermetallic compounds. A prerequisite for this reaction is a diffusion process (the sintering process discussed later provides the conditions for the diffusion process). It is generally believed that the compounds formed by the diffusion between the substrate and the coating are beneficial to improve the bond strength. However, in the thermal spraying process, the time from the droplet contacting the surface of the substrate to completing the crystallization process is very short, generally not more than 0.1 s. Therefore, most researchers believe that there is no interdiffusion of substrate and coating and formation of compounds in this type of process. But some researchers believe that in such a short period of time, the process of diffusion and formation of compounds can still happen conditionally. For example, nickel-coated aluminum powder, a commonly used base material for thermal spraying, undergoes a violent exothermic reaction at 660-680 °C. Even after the droplet reaches the surface of the substrate, the exothermic reaction continues for several microseconds, and there is sufficient heat of heat to facilitate the reaction with the substrate, possibly forming some form of metallurgical bond to improve bond strength. Although other researchers believe that the increase in bond strength caused by the use of exothermic powders is not necessarily due to the generation of diffusible compounds, this improvement is an indisputable fact.