Sample preparation factors
Surface treatment of substrate:
Cleanliness level: If there are impurities such as oil stains, dust, rust on the surface of the substrate, it will seriously affect the contact and adhesion between the coating and the substrate. During testing, it is easy to cause the coating to peel off prematurely, resulting in a lower measured bonding strength value. For example, if the surface of a metal substrate is not completely cleaned of oil stains, even if the adhesion of the coating itself is still acceptable after coating, the oil stains may become a weak link during tensile testing and other inspections, leading to the separation of the coating from the substrate.
Roughness: The roughness of the substrate surface has a significant impact on the detection of bonding strength. Appropriate roughness can increase the contact area between the coating and the substrate, form a mechanical interlocking effect, and help improve the bonding strength. However, if the roughness is excessively uneven, it may lead to uneven thickness of the coating during coating, which in turn affects the accuracy of the detection results; However, a too small roughness may result in insufficient adhesion between the coating and the substrate, leading to a detected bonding strength lower than the actual level.
Surface activation degree: For some substrates that have undergone chemical activation treatment, poor activation effect can affect the chemical reaction between the coating and the substrate, thereby reducing the bonding strength. For example, when treating an aluminum substrate with alkaline solution, if an ideal active alumina hydrate layer is not formed, the adhesion between the coating and the substrate will be poor in subsequent testing.
Coating preparation process parameters:
Coating method: different coating methods (such as thermal spraying, sol gel method, chemical vapor deposition, etc.) will cause differences in the structure and density of the coating, which will affect the bond strength test results. For example, during thermal spraying, if the spraying parameters (such as spraying distance, angle, speed, etc.) are not appropriate, it can cause uneven particle accumulation and high porosity in the coating, resulting in a weak bond between the coating and the substrate. This can easily lead to peeling during the testing process, resulting in a lower measured bond strength.
Coating thickness: The coating thickness is directly related to the detection of bonding strength. Excessive thickness of the coating may result in significant internal stress, which can easily cause cracking, peeling, and other issues during testing, affecting the accuracy of the results; However, overly thin coatings may not effectively cover the substrate or be unable to withstand the load during the testing process, which can also result in the measured bonding strength not matching the actual situation.
Drying and sintering conditions (for some coatings): For coatings prepared by the sol-gel method, if the heating rate, holding time, drying environment and other conditions in the drying and sintering process are unreasonable, the coating will have cracks, holes and other defects, reducing the bonding strength between the coating and the substrate. These defects are easy to become the starting point of coating peeling during testing.
Testing equipment and environmental factors
Accuracy and calibration of testing equipment:
Mechanical testing equipment (such as tensile testing machines, shear testing machines, etc.): Insufficient accuracy of the equipment, such as inaccurate force sensors, can lead to errors in the measured load values, thereby affecting the calculation results of the bonding strength. If the equipment is not calibrated regularly, this error may accumulate continuously, making the detection data unreliable. For example, in tensile testing, when the error of the force sensor is large, the maximum tensile force value recorded when the coating peels off from the substrate is inaccurate, and thus the correct bonding strength value cannot be obtained.
Non destructive testing equipment (such as ultrasonic testing equipment, infrared thermal imaging equipment, etc.): Its sensitivity, resolution, and other performance indicators will affect the detection effect of the interface between the coating and the substrate. If the sensitivity of the device is not sufficient, it may not be able to accurately capture small defects or signal changes at the binding interface, and can only provide vague qualitative judgments, which is not conducive to accurately evaluating the binding strength.
Environmental temperature and humidity:
Temperature: Environmental temperature has an impact on the material properties of coatings and substrates, especially for some temperature sensitive coatings (such as certain polymer coatings), which may become softer and less viscous at high temperatures, resulting in lower values in bond strength testing; The low temperature environment may make the coating brittle, making it more prone to cracking during the testing process, which affects the accuracy of the results.
Humidity: High humidity environments can easily cause coatings to absorb water, changing their physical and chemical properties. For some substrates that are prone to moisture (such as metal substrates that are prone to rusting), it can also affect the bonding between the coating and the substrate, resulting in a discrepancy between the detected bonding strength and the actual situation.
Detecting operational factors
Loading rate (for mechanical testing methods):
In mechanical tests such as tensile and shear tests, a loading rate that is too fast can prevent the coating from uniformly transmitting stress, which can easily cause the coating to suddenly fracture or peel off from the substrate, and the measured bonding strength values may be lower; However, if the loading rate is too slow, it may introduce some other factors (such as long-term stress causing creep of the coating) that affect the detection results. Therefore, it is necessary to control the loading rate reasonably according to the corresponding standards and test requirements to obtain accurate bonding strength data.
Detection location and quantity:
Improper selection of detection location can lead to result deviation. For example, detecting areas with local defects on the coating surface will inevitably result in lower bonding strength values, which cannot represent the overall bonding between the coating and the substrate. At the same time, the insufficient number of tests also makes it difficult to accurately reflect the overall bonding level between the coating and the substrate. Generally, it is necessary to conduct multiple tests in different parts according to statistical principles, take the average value and other statistical data to more scientifically evaluate the bonding strength.

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