The following are some factors that can affect the bonding strength test results of high-temperature resistant coatings:
1、 Factors related to sample preparation
Characteristics of base material:
Material uniformity: If the substrate material itself has defects such as uneven composition, internal pores or inclusions, it can cause uneven local stress when the coating is bonded to the substrate. During the testing process, it is easy for the coating to peel off prematurely, resulting in low and unstable bonding strength data, which affects the accuracy of the results. For example, if impurities are mixed into the metal substrate during the casting process, the degree of adhesion between it and the coating will vary in different parts.
Surface roughness: The surface roughness of the substrate has a significant impact on the results of the bonding strength test. Overly rough surfaces may result in uneven coating thickness, with stress concentrated on thinner or protruding parts of the coating during testing, which can easily cause premature coating detachment and lower bond strength test values; However, a surface that is too smooth is not conducive to coating adhesion and can also affect bonding. For example, before the preparation of thermal spray coating, if the roughness of the substrate sandblasting treatment does not meet the process requirements, whether it is too coarse or too fine, it will interfere with the subsequent bonding strength test results.
Surface cleanliness: If there are residual impurities such as oil stains, oxide scales, dust, etc. on the surface of the substrate, it will hinder the formation of good chemical bonding or mechanical interlocking between the coating and the substrate, making the coating bonding weak. During testing, the coating is prone to detach from the parts with impurities, reducing the bonding strength test value. If the substrate has not been thoroughly cleaned or rust removed before coating, this situation may occur.
Coating preparation process:
Uneven thickness: During the preparation process, if the thickness of the coating is not well controlled, there may be local excessive or insufficient thickness. During testing, the thick coating may easily peel off due to high internal stress, while the thin coating may have weak bonding due to ineffective coverage of the substrate, which in turn affects the accurate measurement of bonding strength. For example, when using the brushing process to prepare coatings, if the brushing technique is not uniform, it is easy to cause inconsistent coating thickness.
Density difference: The density of the coating varies, resulting in different internal porosity and particle bonding, which can affect the bonding with the substrate and the ability to withstand stress during testing. Coatings with high porosity have a loose internal structure and are more prone to cracking and expansion when subjected to tensile, shear, and other stresses, resulting in coating detachment and lower measured bonding strength. For example, in plasma spraying technology, if the spraying parameters are not reasonable, the density of the prepared coating may be poor.
2、 Testing equipment and environmental factors
Equipment accuracy and calibration:
Force measurement error: If there is a significant error in the force sensor of the tensile testing machine, shear testing instrument and other equipment used for testing, it will result in inaccurate recorded applied force data, leading to deviation in the calculated bond strength results. For example, if the equipment is not regularly calibrated and the force value displayed is higher or lower than the actual value, it will affect the correct judgment of the coating bonding strength.
Displacement measurement accuracy: In some testing methods that require displacement measurement (such as scratch test, indentation method, etc.), the accuracy of displacement measurement is insufficient, which can affect the judgment of key parameters such as coating failure process and critical load, and thus affect the evaluation of bonding strength. For example, displacement sensors have low resolution and cannot accurately capture the displacement changes at the moment of coating peeling, which may result in inaccurate test results.
Changes in environmental conditions:
Temperature effect: When the ambient temperature fluctuates greatly, on the one hand, it will change the physical properties of the sample, such as the brittleness of the coating, the thermal expansion coefficient of the substrate, etc., causing the coating to perform differently from expected during testing; On the other hand, it may also affect the operational stability of the testing equipment, for example, the performance of some electronic components of the equipment may deteriorate in high temperature environments, affecting measurement accuracy and ultimately leading to errors in the results of the combined strength test.
Humidity influence: The level of humidity can affect the surface condition of the sample. A high humidity environment may cause the sample surface to adsorb moisture, which in turn affects the bonding interface between the coating and the substrate. During testing, coating detachment is more likely to occur, resulting in lower bonding strength test results, especially for some humidity sensitive coating materials.
3、 Test operational factors
Sample installation situation:
Uneven force distribution: During tensile and shear tests, if the specimen is not installed correctly on the equipment, resulting in uneven force distribution, such as eccentricity during the installation of the tensile specimen, it will cause the coating to bear excessive stress locally and peel off prematurely. The measured bonding strength value will be lower than the actual value, which cannot truly reflect the bonding between the coating and the substrate.
Loose fixation: If the sample is not firmly fixed, it may become loose or displaced during the testing process, which can also interfere with the test results and make the data unreliable. For example, in scratch testing, if the sample is not fixed properly and shakes during the scratching process, it can affect the stability of the scratch and the observation of coating damage.
Test parameter settings:
Unreasonable loading rate: Improper setting of loading rate in tensile testing, scratch testing, and other tests can affect the failure mode of the coating and corresponding critical load parameters. If the loading rate is too fast, the coating may undergo brittle fracture before sufficient deformation, resulting in low measured bonding strength; The slow loading rate may introduce additional influencing factors, such as long-term effects of environmental factors on the sample, which can lead to inaccurate results.
Other parameter deviations: Inaccurate parameter settings such as temperature range, heating and cooling rate, and cycle times in thermal shock testing cannot truly simulate the high temperature environment and thermal stress conditions faced by the coating. The observed phenomena such as coating peeling and cracking cannot accurately reflect its bonding strength level, thereby affecting the validity of the test results.
4、 Data processing and analysis factors
Insufficient number of repeated tests: If only a small number of tests are conducted, the results of a single test are easily affected by accidental factors, such as small defects in the sample itself, temporary errors in testing operations, etc., which cannot effectively reflect the true situation of coating bonding strength, resulting in a lack of representativeness and accuracy in the final results.
Improper handling of abnormal data: In the data obtained from multiple tests, if abnormal data caused by sample preparation defects, testing operation errors, etc. are not correctly identified and removed, but are included in statistical analysis, it will cause deviations in the calculation results of average bonding strength and affect the accurate evaluation of coating bonding strength.

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