Analysis of Influencing Factors in Testing and Evaluation of Thermal Spray Coating Quality

       The detection and evaluation of thermal spray coating quality is the basic basis for coating quality assurance and functional use, and is the evidence for technical arbitration. However, the thermal spray coating is a star-layered accumulation of molten particles, which is mainly mechanically combined with the substrate. The detection and evaluation of properties brings far more difficulties than the detection and evaluation of overall material properties.



This is mainly manifested in the following aspects:



1) The coating properties are strongly influenced by the spraying method and spraying process parameters. For the same spraying material, different spraying methods are used, and the coating properties such as bonding strength, hardness and porosity are obviously different. Even with the same spraying material and the same spraying method, due to the different spraying process parameters such as spraying power, spraying distance and spraying angle, the coating performance will be significantly different.



2) The coating properties are also affected by the test conditions. Using the same spraying material, the same spraying method, and the same spraying process parameters, but due to different test conditions, the detected coating properties may be different. For example, in the determination of the tensile strength of the coating, the bonding strength of the thin coating is higher than that of the thick coating; using different adhesives, the measured tensile bonding strength will also have a difference, spray the test sample It is easier to overheat than the sprayed workpiece, and the measured coating performance is lower than the actual coating performance.



3) Coating properties are related to sample preparation. The stress state distribution of the coating of the cylindrical specimen and the flat specimen is different, the former is compressive stress, and the latter is tensile stress at the edge. When the tensile strength test is performed, the flatness of the coating surface and the roughness of the surface of the counterpart will affect the test results. When testing the microstructure of the coating, whether the sample is prepared correctly or not has a great influence on the testing result. Due to the low bonding force between the hard phase in the coating structure and the substrate, improper sampling and polishing may cause the hard phase to fall off, which may be misjudged as coating pores.



4) The difference between the coating performance measured in the laboratory and the actual performance of the workpiece spray coating. Usually, the performance data of the coating measured using the sprayed specimen is higher than the actual performance of the sprayed coating on the workpiece.



5) Dispersion and statistics in spray coating performance testing. The structure of spray coating is basically composed of matrix phase, hard phase and pores. The coating is a polymer with a heterogeneous structure. In addition, the disk-like laminated structure formed by the deformation of the molten particles after hitting the matrix leads to the mechanical properties of the coating. Orientation or anisotropy, so properties measured at different parts of the coating may be inconsistent and reproducible.



Due to the above reasons, it is very difficult to evaluate the performance of thermal spray coatings. At the same time, the use conditions and working conditions of the coating are very different, and completely different requirements for the performance of the coating are put forward. For example, corrosion-resistant coatings require low porosity or no porosity, wear-resistant coatings with lubricating conditions require coatings to have a certain porosity to achieve the purpose of storing oil and lubrication, thermal barrier coatings require corresponding porosity to increase heat insulation Effect. Therefore, apart from the formulation of national standards for the common technology of coating bonding strength, there is no unified method and national standard for coating performance evaluation. Only the corresponding testing methods are determined according to the application requirements of the coating and the selected spraying process method. and corresponding technical indicators.