In addition to the six types of coating characteristic inspection such as: appearance inspection, thickness inspection, corrosion resistance inspection, porosity inspection, bonding strength inspection and hardness inspection, other important inspections include internal stress (residual stress) inspection, wear resistance inspection, Heat resistance and insulation test, etc.
coating internal stress
For thermal spray coatings, the internal stress of the coating is represented as residual stress, which is essentially thermal stress. It is due to uneven heating, uneven temperature distribution, thermal expansion and contraction in the coating and between the coating and the substrate, resulting in temperature strain, thereby forming temperature stress.
During the spraying process, the high-temperature coating material particles move at high speed, shoot to the surface of the substrate, and gradually deposit on it to form a coating. After the high-speed and high-temperature material particles reach the surface of the substrate, they are suddenly cooled, shrink sharply, and deform, and the deformation is uneven, which will generate stress in the coating and between the coating and the substrate. The magnitude and direction of the stress depend on the spraying temperature (relevant instrument: temperature measuring paper), the substrate preheating temperature, the compactness of the coating and the thermodynamic properties of the material, etc. The residual stress of the coating causes the coating to deform, wrinkle, crack and even peel. For thin sheet metal coatings, buckling deformation also occurs.
For electroplating, when the electroplating layer is formed, due to the defects in its metal lattice, internal stress is formed.
It can be seen from the above that due to the different implementation methods of the coating, the mechanism of the stress is also different.
