1. Formation of thermal spray coating
During thermal spraying, the particles of the coating material are heated to a molten state or a highly plastic state by a heat source. Under the thrust of the external gas or the flame itself, they are atomized and sprayed to the surface of the substrate at a high speed, and the particles of the coating material collide violently with the substrate. The deformation and flattening are deposited on the surface of the substrate, and at the same time, the particles are rapidly cooled and solidified, and the particles are deposited in layers to form a coating.
2 Structural characteristics of thermal spray coating
The formation process of thermal spray coating determines the structural characteristics of the coating. The spray coating is a layered structure in which countless deformed particles are interlaced and stacked together in waves. There are inevitably some pores and pores between the particles in the coating. voids, accompanied by oxide inclusions.
3. The bonding mechanism of thermal spray coating
The bonding of the coating includes the bonding of the coating to the substrate and the bonding within the coating. The bonding force between the coating and the surface of the substrate is called the bonding force, and the bonding force inside the coating is called the cohesive force. The binding mechanism between the particles and the substrate in the coating and the binding mechanism between the particles are still inconclusive, and it is generally believed that there are the following ways.
(1) Mechanical combination
The particles that collide into a flat shape and are undulating with the surface of the substrate and the uneven surface are fitted with each other, and are combined by the mechanical interlocking of the particles (anchor effect). Generally speaking, the combination of the coating and the substrate is mechanically combined. main.
(2) Metallurgy-chemical combination
This is a type of bonding when metallurgical reactions such as diffusion and alloying occur between the coating and the substrate surface. When remelting or spray welding is carried out after spraying, the bonding between the spray welding layer and the substrate is mainly metallurgical bonding.
(3) Physical combination
The bond between the particle and the surface of the matrix formed by van der Waals forces or subvalent bonds.
4. Residual stress of coating
When the molten particles collide with the surface of the substrate, they are chilled and solidified while being deformed, resulting in shrinkage stress. The outer layer of the coating is under tensile stress, and the substrate sometimes includes the inner layer of the coating, which produces compressive stress. This residual stress in the coating is caused by thermal spray conditions and differences in the physical properties of the spray material and the base material. It affects the quality of the coating, limits the thickness of the coating, and measures should be taken in the process to eliminate and reduce the residual stress of the coating.