How do coating cracks occur?

       Although the time from the molten or semi-molten state to the complete solidification of the sprayed particles is very short, during the instantaneous cooling process, the temperature of the edge of the particles is inconsistent with the temperature of the core, and the flat flake particles shrink and curl. Crack-like pores are generated in the coating, and the subsequent spray particles can only cover these pores but not completely fill them. The shrinkage and curling of thin and flat flake particles are often constrained by the surrounding flake particles during the formation of the spray coating, and the thermal stress in the single flake particle cannot be completely released. The accumulation of residual stress in many particles causes the coating to produce Large residual stress, thus greatly reducing the strength of the coating itself and the bonding strength of the coating and the substrate.



    Whether the microcracks in the coating are extended or not depends on the state and size of the residual stress in the coating. When the internal stress is in the state of tensile stress, as the thickness of the coating increases, the residual tensile stress will increase and the microcracks will expand and deepen, eventually leading to the cracking or peeling of the coating. The internal stress of most thermal spray coatings is in the state of tensile stress, which limits the thickness of the coating. However, some spraying materials adopt high-speed flame spraying process, and the internal stress of the prepared coating is compressive stress, and the thickness of the coating can reach more than ten millimeters. In addition, when the coating is affected by external force and external environment, it will also induce the expansion of micro-cracks, such as machining, rapid cooling and rapid heating temperature changes during use.