In addition to the above six inspections of coating properties: appearance inspection, thickness inspection (applicable instrument: coating Thickness Gauge), corrosion resistance inspection, porosity inspection, bonding force inspection and hardness inspection, other important inspections include internal stress ( Residual stress) inspection, wear resistance inspection, heat resistance and heat insulation inspection, etc.
1. 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, 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.
2. Inspection of internal stress
1) Bending rate test method This method is suitable for testing the residual stress in thermal sprayed coatings. 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.
2) Rectangular sample is sprayed on the rectangular test plate, the remaining material is sprayed, bending deformation occurs, the radius of curvature of the two materials is measured respectively, and the arithmetic average value is taken, then the coating is separated, the internal stress is released, the residual strain is measured, and finally the residual strain is calculated. stress. As shown in Figure 11-71.
When doing the bending test of the sprayed coating of the sample, it is assumed that the bending meets the pure bending assumption in material mechanics, that is, the flat section assumption. The same section remains unbent before and after deformation, 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.
The coating should be
3) Spiral contraction instrument method The spiral contraction instrument method uses the change of the radius of curvature of the spiral metal sheet sample during electroplating for testing. A stainless steel spiral belt of a certain specification is weighed after the surface is cleaned, dried, and the inner wall is insulated. One end of the spiral belt is fixed on the instrument, and the other end is a free end. Install the spiral belt together with the instrument on the top of the electrolytic tank, so that the spiral belt is immersed in the plating solution for electroplating. After the coating is deposited on the outer wall of the spiral belt, due to the internal stress of the coating, the radius of curvature changes, driving the spiral piece to another The gear connected at one end rotates, and the displacement is amplified, and the corresponding value is read from the pointer position of the instrument dial. Weigh the weight of the spiral test piece before and after electroplating, calculate the average thickness of the electroplating layer, and then get the internal stress of the coating at a certain thickness.
4) Resistance strain method The resistance strain method is to measure the internal stress of the test piece by using the resistance value change caused by the stretching and contraction of the resistance wire. On the surface to be tested of a carbon steel test piece with a size of 100×20×2mm (required that its roughness R a ≤0.4), use No. 88 adhesive to paste the resistance strain gauge, weld a certain length of wire, and then use Coat the back of the sample and the strain gauge with the corresponding insulating paint, and the joints are strictly insulated (up to 50MΩ insulation resistance). After the double bridge balance is carried out by the resistance strain gauge, it is placed in the electrolytic cell for electroplating according to the specified current density and time. When the metal coating is deposited on one side of the sample, due to the internal stress of the coating, the strain gauge will shrink accordingly, and the resistance value will change. After plating, take out the sample, clean and dry it, and then measure the indicated value of the strain on the strain gauge. Calculate the internal stress of the coating according to the following formula:
During the test, the following items should be paid attention to:
(1) Adhesives and insulating coatings should be selected according to the 01 value and temperature of the plating solution used;
(2) The strain gauge should be tightly attached, and there should be no air bubbles between it and the test piece;
(3) The sample should be fully cleaned before electroplating, but the strain gauge and insulation layer should not be damaged, deformed, broken, and short circuit and open circuit should be prevented;
(4) In order to prevent the strain value from changing at any time, the measurement time interval after plating should be consistent each time.
