Choosing a suitable coating bonding strength testing method requires comprehensive consideration of multiple factors. The following are some key considerations and corresponding selection suggestions:
Coating's own characteristics
Coating thickness:
Thicker coatings (millimeter level and above): For some thick ceramic coatings prepared by thermal spraying, direct mechanical testing methods such as tensile testing and shear testing can be prioritized. These methods can withstand large loads and are suitable for quantitative measurement of the bonding strength between thick coatings and substrates. They can intuitively reflect the peeling of coatings from substrates under large external forces.
Thinner coatings (at the micrometer or even nanometer level): such as nanofilm coatings, organic thin film coatings on electronic device surfaces, etc., indentation testing methods, nanoindentation testing methods, etc. are more suitable. They can precisely control the loading force at a small scale, detect thin coatings, avoid coating damage caused by conventional high load tests, and accurately analyze the bonding performance of thin coatings under small forces.
Coating hardness:
Hard coatings, such as ceramic coatings, hard metal coatings, etc., are commonly used in scratch testing and indentation testing. When hard coatings are subjected to scratches or indentations, the peeling, cracking, and other phenomena around them can clearly reflect the bonding with the substrate. Moreover, these two methods are relatively easy to operate and effective in capturing the mechanical response characteristics of hard coatings.
Soft coating: For some soft polymer coatings, special attention should be paid to controlling the loading rate and other parameters during tensile or shear testing to avoid affecting the accuracy of the test results due to excessive deformation of the coating. Non destructive testing methods such as infrared thermography can also be considered to indirectly evaluate the bonding strength by observing the bonding performance between the coating and the substrate during the heat transfer process.
Characteristics of matrix material
Base material:
Metal substrate: If the substrate is metal, such as steel, aluminum alloy, etc., various testing methods are applicable, such as tensile testing, shear testing, eddy current testing (for conductive metal substrates), etc. Because the mechanical properties of metal substrates are relatively stable, they can be well combined with various mechanical testing methods. At the same time, eddy current testing can effectively detect the bonding of coatings by utilizing the conductive properties of metals.
Ceramic substrate: For ceramic substrates, due to their high brittleness, it is important to avoid using high load mechanical tests that may cause substrate fracture when selecting testing methods. Non destructive testing methods such as ultrasonic testing, radiographic testing, etc. can be considered to evaluate the bonding strength by detecting the internal bonding interface and reduce the risk of damage to the substrate and coating.
Composite material matrix: The structure of composite material matrix is relatively complex, and the material properties of different phases vary greatly. In this case, it is advisable to use detection methods with strong resolution of the internal structure of the material, such as laser-induced breakdown spectroscopy (which can analyze the distribution of interface elements), X-ray detection (which can observe the internal structure), etc., in order to accurately determine the bonding state between the coating and various parts of the matrix.
Matrix shape and size:
For substrates with regular shapes and large sizes, such as flat metal plates, large ceramic components, etc., a wide range of testing methods can be selected, including tensile testing, bending testing, infrared thermal imaging testing, etc., which can be well implemented, making it easy to operate and fix the sample for testing.
For substrates with complex shapes and small sizes, such as mechanical components and microelectronic devices with complex surfaces, non-destructive testing methods are more suitable, such as ultrasonic testing and laser speckle detection. These methods are not limited by complex shapes and small sizes and can be used for testing without damaging the substrate and coating. Although some can only be qualitatively evaluated, they can also meet the testing needs of such special substrates.
Coating usage conditions and testing purposes
Operating conditions:
Coatings used in high-temperature environments, such as coatings on hot end components of aircraft engines, are more suitable for thermal shock testing combined with other testing methods (such as tensile testing). The thermal shock test can simulate the actual working conditions of high temperature alternation, investigate the bonding between the coating and the substrate under thermal stress, and further quantify the bonding strength through conventional mechanical testing, comprehensively evaluating the bonding performance of the coating under high temperature working conditions.
Coatings that mainly withstand shear forces, such as coatings on the surface of mechanical transmission components, use shear testing as a key detection method to accurately reflect the bonding strength of the coating under shear forces in actual work, ensuring that the coating will not peel off when the component is subjected to corresponding external forces.
Coatings that have been exposed to corrosive environments for a long time: In addition to paying attention to the corrosion resistance of the coating itself, the bonding strength between the coating and the substrate is also important. Non destructive testing methods such as ultrasonic testing and infrared thermography can be used to regularly detect poor bonding caused by corrosion and other factors in a timely manner, ensuring that the coating continues to play a protective role.
Purpose of testing:
Qualitative evaluation of coating adhesion: If you only want to roughly understand whether there are bonding defects between the coating and the substrate, such as delamination, detachment, etc., non-destructive testing methods such as infrared thermography detection and laser speckle detection can meet the needs. They can quickly provide a qualitative judgment of the coating adhesion status, which is convenient for preliminary screening.
Quantitative determination of bonding strength values: When precise bonding strength data is required for quality control, design verification, and other purposes, direct mechanical testing methods such as tensile testing and shear testing should be selected to obtain specific bonding strength values through accurate load measurement and corresponding calculations.

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