The thickness of high-temperature resistant coatings has a complex impact on their bonding strength with the substrate, as follows:
The impact of thin coating
Difficulty in forming effective coverage: When the coating thickness is too thin, it may not be able to completely and uniformly cover the surface of the substrate, which can easily lead to local areas not being protected by the coating, resulting in the possibility of direct contact between the substrate and the external environment. For example, when coating high-temperature resistant coatings on the surface of some complex shaped metal components, if the coating is too thin, it may not fully cover the edges, grooves, and other parts of the component, resulting in a lack of integrity between the coating and the substrate, and the overall bonding strength is difficult to guarantee.
Unable to fully utilize the bonding effect: A coating that is too thin often means that it is difficult for it to form a sufficiently stable structure to interact well with the substrate, and functions such as chemical bonding and mechanical interlocking may not be fully utilized due to the coating being too thin. For example, in some cases where the bonding strength is enhanced by chemical reactions between the active ingredients in the coating and the substrate, a thin coating contains limited active ingredients and forms fewer chemical bonds with the substrate, resulting in lower bonding strength.
The impact of excessive coating thickness
Internal stress accumulation: As the coating thickness increases, larger internal stresses will gradually accumulate inside the coating. This is because during the curing, thermal expansion, or cooling shrinkage process of the coating, stress is generated between the layers and between the coating and the substrate due to the coordination of deformation. Thick coatings cannot effectively release these internal stresses. When the internal stress exceeds the bonding force between the coating and the substrate, it is easy to cause the coating to peel off or crack from the substrate, seriously damaging the bonding strength. For example, when some ceramic high-temperature resistant coatings are coated too thick, the internal stress generated by thermal expansion cannot be released during subsequent high-temperature use, often resulting in coating cracking, warping and other phenomena, which greatly reduce the bonding strength.
Easy to spread defects: Once small defects (such as pores, microcracks, etc.) appear locally in thicker coatings, these defects are more likely to expand and spread during use because there is more "space" inside the coating for their development. Moreover, the interaction between the layers of thick coating is relatively complex, and a problem in one area may affect the bonding between the surrounding area and the substrate, resulting in a decrease in the bonding strength between the entire coating and the substrate. For example, in high-temperature environments, microcracks in thick coatings may gradually extend under the action of thermal stress, ultimately causing the coating to detach from the substrate over a large area.
The importance of appropriate thickness
Ensure bonding stability: Choosing the appropriate coating thickness can meet performance requirements such as high temperature resistance while forming a relatively stable bonding state between the coating and the substrate. At this thickness, the coating can fully cover the substrate and exert its various bonding effects with the substrate (such as mechanical interlocking, chemical bonding, etc.), without affecting the bonding strength due to excessive internal stress. For example, for a high-temperature resistant silicone coating applied to metal components in a high-temperature furnace, it has been experimentally determined that when the thickness is between 100-200 microns, it can effectively resist high-temperature corrosion and maintain good adhesion with the substrate, ensuring long-term stable operation of the component.
Optimizing comprehensive performance: Appropriate coating thickness helps optimize the comprehensive performance of coatings, including bonding strength, high temperature resistance, corrosion resistance, etc. It can enable the coating to better cooperate with the substrate under different working conditions, avoid various failure situations caused by thickness issues, and extend the service life of the overall structure composed of the coating and substrate.
In short, the thickness of high-temperature resistant coatings needs to be determined comprehensively based on specific coating types, substrate materials, and usage conditions to ensure good bonding strength and excellent performance between the coating and substrate.

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High termperature curing polysilazane, pls check IOTA 9108, IOTA 9118.