The following are some common methods for evaluating the bonding strength of high-temperature resistant coatings:
1、 Tensile testing method
Principle: Apply a tensile force perpendicular to the surface of the coated sample until the coating separates from the substrate, record the corresponding tensile force value, and then calculate the bonding strength based on the size and other parameters of the sample. This method intuitively reflects the adhesion ability between the coating and the substrate.
Operation example: Usually, the coating sample is prepared into a specific shape, such as cylindrical or rectangular, with one end fixed and the other end connected to the fixture of the tensile testing machine. Then, tension is applied at a certain loading rate, gradually increasing the tension until the coating peels off. The bonding strength is calculated based on the maximum tension value recorded by the testing machine and the cross-sectional area of the sample. For example, when testing the bonding strength of ceramic coatings on metal substrates, after preparing the sample according to standard specifications, load it at an appropriate speed on a tensile testing machine. If the maximum tensile force is F N and the effective cross-sectional area of the sample is S square meters, the bonding strength can be roughly calculated according to the formula "bonding strength=F/S" (unit: Pascal).
2、 Shear test method
Principle: Apply a shear force parallel to the interface between the coating and the substrate, causing the coating to undergo shear failure along the interface, in order to measure the bonding strength between the two. In practical applications, many components are subjected to shear stress, so this method is meaningful for evaluating the bonding performance of coatings under actual working conditions.
Operation example: A specialized shear specimen can be prepared, such as using a lap joint form of specimen. The coated part is overlapped with another base material, and then a shear force is applied to the overlapped part through a specialized shear testing device. When the coating is sheared down from the base, the maximum shear force at this time is recorded, and the shear bond strength is calculated based on the relevant dimensional parameters of the specimen (such as overlap area, etc.). This method is commonly used to evaluate the bonding performance of high-temperature resistant coatings on thin plate structures in the aerospace industry, to see if the coating is firm when subjected to shear forces during flight.
3、 Scratch test method
Principle: Use a high hardness indenter to scratch the surface of the coating while gradually increasing the load. Observe the cracking and peeling of the coating during the scratching process. By analyzing the damage mode of the coating at the scratch and the corresponding critical load parameters, indirectly determine the bonding strength between the coating and the substrate. Generally speaking, the higher the critical load, the better the bonding strength of the coating.
Operation example: Place the coated sample on the workbench of the scratch tester, adjust the indenter (such as a diamond indenter), set the parameters such as the length of the scratch and the loading rate, and then start the tester. The indenter will scratch the surface of the coating and continuously increase the pressure. The tester observes when the coating begins to peel off, crack propagation, and other phenomena, and records the corresponding critical load value. Different coating materials and substrate combinations have different critical load performances, which can be used to compare and evaluate the bonding strength. For example, when testing the high-temperature resistant coating on the surface of engine blades, scratch testing can quickly understand the approximate degree of coating adhesion, providing a basis for subsequent improvement.
4、 Thermal shock test combined with observation method
Principle: The coated sample is subjected to repeated rapid heating and cooling processes, also known as thermal shock, to simulate the temperature fluctuations in actual high-temperature environments. Due to differences in thermal expansion coefficients between the coating and substrate, if the bonding strength is insufficient during the thermal shock process, the coating is prone to peeling, cracking, and other phenomena. The bonding strength is evaluated by observing the integrity of the coating after thermal shock.
Operation example: Place the sample in a high-temperature furnace and heat it to the set high temperature value. After a certain period of time, quickly remove it and cool it at room temperature or low temperature environment. Repeat this process multiple times (such as 10 cycles, 20 cycles, etc.), and then use equipment such as optical microscope and electron microscope to observe whether there are cracks, peeling and other damages on the coating surface and interface. The more severe the damage, the lower the bonding strength. For example, for some high-temperature resistant coatings in high-temperature furnace linings, after multiple thermal shock cycles, if the coating still maintains good integrity, it can be basically judged that its bonding strength can meet the requirements for use in this high-temperature environment.
5、 Ultrasonic testing method
Principle: Use the reflection, refraction, scattering and other characteristic changes of ultrasound at the interface between the coating and the substrate to detect the bonding of the interface. When the coating is well bonded to the substrate, ultrasonic propagation is relatively stable; If the bonding is poor, abnormal acoustic signals will appear at the interface, and the bonding strength can be evaluated by analyzing these signals.
Operation example: Place the ultrasonic probe at a suitable position on the surface of the coating, emit ultrasonic waves into the interior of the coating, receive and analyze the reflected ultrasonic signals. Professional personnel can determine whether there are defects such as debonding and porosity between the coating and the substrate that affect the bonding strength based on the amplitude, phase, frequency and other characteristic changes of the signals. However, this method requires professional ultrasonic testing equipment and operators with certain analytical and interpretive abilities. It is commonly used for non-destructive testing of the bonding strength of high-temperature resistant coatings on large and complex structural components, such as the testing of high-temperature resistant coatings inside large chemical reaction vessels.
6、 Pressing method
Principle: Use a specific indenter to press the coating surface, and infer the bonding strength between the coating and the substrate by analyzing the deformation, peeling, and corresponding load displacement curves of the coating during the pressing process. Under different bonding strengths, the performance of coatings during compression varies. For example, coatings with higher bonding strengths are less likely to peel off.
Operation example: Using hardness testing equipment (such as modified devices such as Rockwell hardness tester, Vickers hardness tester, etc.), select a suitable indenter, slowly press it into the coating surface, record the load changes and displacement of the indenter during the pressing process, draw a load displacement curve, observe whether there is local peeling or peripheral warping of the coating during pressing, and comprehensively evaluate the bonding strength based on the characteristics and phenomena of the curve. For example, in the testing of high-temperature resistant coatings on some precision mechanical components, the pressing method can make preliminary qualitative judgments on the bonding strength of the coating without damaging the entire component.

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