For the thermal spraying Xu layer, the internal stress of the coating is shown as residual stress, and its essence is thermal stress. It is due to uneven heating, uneven temperature distribution, thermal , resulting in temperature strain, thereby forming temperature stress.
During the spraying process, the particles of the high-humidity coating material move at a high speed. They shoot towards 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, the substrate preheating temperature, the compactness of the coating and the thermodynamic properties of the material. The residual stress of the coating causes the coating to deform, wrinkle, crack, or even peel. For thin metal coatings, deformation will also occur.
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 stress mechanism is also different.
Internal Stress Test
1) Exaggerated curvature test method This method is suitable for testing the residual stress in the thermal spray coating. Due to the different shapes of the samples, they can be divided into rectangular samples and circular samples, so the specific methods are also different.
(1) The rectangular sample is sprayed on the rectangular test plate, and the bending deformation occurs. The radii of curvature of the two materials are measured respectively, and the arithmetic average value internal stress is released, the residual strain is measured, and finally the residual strain is calculated. stress. As shown in Figure 17-40 , when doing the bending test of the sprayed coating of the sample, it is assumed that the bending meets the pure exaggeration assumption in material mechanics, that is, the plane section is the most determined. The same section remains unbent before and after , only rotates and does not displace. It can also be interpreted as maintaining a layered state within the matrix, within the coating and between them, without misalignment between layers, and without detachment of the upper and lower layers.
