The following are some common non-destructive testing methods that can be used to evaluate the bonding strength of coatings, in addition to indentation testing:
Ultrasonic testing method
Principle: When ultrasonic waves propagate at the interface between the coating and the substrate, reflection, refraction, scattering and other phenomena will occur due to the differences in acoustic properties of the interface. When the coating is well bonded to the substrate, ultrasonic propagation is relatively smooth and signal changes are small; If the combination is poor, such as the presence of gaps, delamination, etc., ultrasound will show obvious abnormal signals such as reflection and scattering enhancement at the interface. By using professional ultrasonic testing equipment to receive and analyze these signals, it is possible to qualitatively evaluate the bonding status between the coating and the substrate, and to some extent infer the bonding strength.
Advantages: It belongs to non-contact and non-destructive testing, which will not cause damage to the coating and substrate being tested. It can be used to detect coating components already installed on equipment, and the testing speed is relatively fast, suitable for batch testing. It can quickly screen out coatings that may have bonding problems.
Limitations: It is difficult to achieve accurate numerical determination of binding strength, often only providing a rough judgment of the binding situation, and further quantitative analysis is needed in conjunction with other detection methods; Moreover, for components with complex shapes and uneven surfaces, the accurate analysis of detection signals will be affected to some extent.
Eddy current testing method
Principle: When an alternating magnetic field is applied to the coating on a conductive substrate (when the substrate under the coating is a conductive material), eddy currents are induced in the substrate, and the bonding state between the coating and the substrate affects the distribution and magnitude of the eddy currents. By detecting the induced magnetic field changes generated by eddy currents through the detection of coils, the presence of bonding defects such as delamination and porosity between the coating and the substrate can be analyzed to indirectly evaluate the bonding strength.
Advantages: Fast detection speed, easy operation, also belongs to non-destructive testing, can quickly scan and detect large-area coated components, especially suitable for the detection of surface coatings on metal substrates, and has good detection effect on components with regular shapes.
Limitations: It is only applicable to the detection of coatings on conductive substrates and cannot be applied to non-conductive substrates such as ceramic substrates; Moreover, it is sensitive to factors such as coating thickness and substrate material, and requires accurate calibration of detection parameters to ensure the accuracy of detection results.
Radiographic testing method
Principle: By using X-rays, gamma rays, and other penetrating materials to penetrate coatings and substrates, due to the varying degrees of absorption and attenuation of radiation by different substances (coatings, substrates, and possible bonding defects), by receiving the passing radiation on the other side and using imaging equipment to convert the intensity distribution of the radiation into a visual image, it is possible to observe whether there are defects such as pores and delamination at the interface between the coating and substrate, and infer the bonding strength.
Advantages: It can visually present the internal structure of the coating and substrate, and the detection results are relatively intuitive and vivid. It also has a certain ability to discover small defects hidden inside, and is suitable for the detection of various material substrates and coatings.
Limitations: Radiation has radiation properties and requires strict protective measures and operating standards, resulting in relatively high testing costs; And for some very thin coatings or small bonding defects, the detection resolution may be limited and sometimes difficult to accurately detect.
Infrared thermography detection method
Principle: Heating the surface of the coating. When the coating is well bonded to the substrate, the heat transfer is relatively uniform and the surface temperature distribution is more regular; If there is poor bonding, such as gaps, delamination, etc., the transfer of heat in these areas will be hindered, leading to abnormal changes in surface temperature. By using an infrared thermal imaging device to capture the temperature distribution on the surface of the coating, and analyzing the temperature difference image, the bonding state between the coating and the substrate can be determined, thereby qualitatively evaluating the bonding strength.
Advantages: The detection operation is relatively simple, able to quickly scan large areas of coating surfaces, intuitively see temperature abnormal areas, and locate possible bonding problems. The equipment is portable and easy to detect on site.
Advantages: This method can only qualitatively determine the bonding between the coating and the substrate, and it is difficult to accurately provide specific numerical values for the bonding strength; Moreover, it is easily affected by external environmental factors such as temperature and thermal interference, and the accuracy of the detection results requires reasonable control of the detection environment to ensure.
Laser speckle detection method
Principle: By irradiating the surface of the coating with a laser, different speckle patterns will be generated in the reflected light due to the different bonding states between the coating and the substrate. By recording and analyzing the changes in these speckle patterns, it is possible to understand whether there is relative displacement, delamination, or other conditions between the coating and the substrate, in order to evaluate the bonding strength between the two.
Advantages: It has the advantages of full field measurement, non-contact, non-destructive testing, etc. It can monitor the dynamic changes of the bonding between the coating and the substrate in real time, which is very helpful for some coating components that need to investigate the changes in their bonding state during use.
Limitations: High stability is required for the detection environment, as external factors such as vibration and lighting can easily interfere with the accurate analysis of speckle patterns. Moreover, the interpretation and quantitative analysis of detection results are relatively complex and require professional technical and software support.

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