Wear-resistant coating bond strength requirements

       The requirements for wear-resistant coatings depend on the mechanical and chemical matching of the wear-resistant coating and the base material, the direction and magnitude of the applied load, and the properties of the coating itself. Depending on the application of the wear-resistant coating, factors such as coating hardness, chemical stability, coating yield strength, and ability to resist crack nucleation and growth all affect the wear resistance of the coating.



The primary requirement for the wear-resistant coating is to ensure that the coating has sufficient bonding strength with the substrate. For this reason, the selection and design of the substrate material and the coating material should be based on ensuring that the coating is firmly bonded.



1) The substrate should be free of deformation. When the wear-resistant coating is used in high load conditions, the substrate should have sufficient hardness and yield strength to support the coating without deformation.



2) The elastic modulus matching between the coating and the base material. In the case of elastic strain, if the elastic moduli of the coating and the substrate do not match, a steep stress will be generated at the interface between the coating and the substrate under load. If the coating is stiffer than the substrate, the stress in the coating will increase. The stress increases as the load and the difference between the elastic moduli of the coating and the substrate increase.



3) The rigidity of the coating and the base material is matched. To make the hard wear-resistant coating have a long service life, the rigidity of the coating and the base material should be reasonably matched. If a highly rigid coating is deposited on a less rigid substrate. Due to the stiffness mismatch, the tensile stress in the coating increases, causing cracks to form in the coating and spread to the substrate, causing early failure of the coating.



4) The matching of thermal expansion coefficient. If the thermal expansion coefficients of the coating and the base material do not match, stress will arise due to the volume change. Usually, the coating is very thin compared to the substrate, so the thermal expansion of the substrate is not substantially affected by the thermal expansion of the coating, while the thermal expansion of the coating is strongly affected by the thermal expansion of the substrate. The thermal stress caused by the mismatch between the thermal expansion of the coating and the substrate is basically concentrated in the coating. The greater the difference in thermal expansion coefficients, the greater the stress in the coating and the greater the tendency to crack or even spall. This is one of the main reasons why many wear-resistant coatings experience premature spalling failure before they have suffered severe wear.



When the thermal expansion coefficient of the substrate is greater than the thermal expansion coefficient of the coating, the stress generated when the temperature is raised is tensile stress, on the contrary, if the thermal expansion coefficient of the coating is greater than that of the substrate, it is compressive stress.



The thermal expansion coefficients of most carbides, nitrides and cermet coatings are smaller than those of steel, only the thermal expansion coefficients of TiN, NbN and Cr3C2 are relatively close to those of high-speed steel, but the difference is still not small.



5) The affinity between the coating and the substrate material. The affinity between the coating and the substrate, that is, the chemical binding ability, directly affects the bonding strength between the coating and the substrate. Generally, when the solid solubility of one compound in another compound is low, the bond strength between them is also weak. Only when there is the greatest chemical affinity between the coating and the substrate without brittle interface phase, the bonding strength between the coating and the substrate is the greatest, and the wear-resistant coating can fully play its role.