Friction and wear is a common phenomenon in nature. Friction is a phenomenon which is caused by the interaction of atoms or molecules between two mating surfaces due to the contact of micro areas. And wear refers to the phenomenon of the continuous loss of material on two mating surfaces due to relative motion. As long as there is a relative motion between the surface of the object will inevitably appear friction, friction is necessarily accompanied by wear, can produce wear working conditions including sliding, vibration, impact, abrasion, erosion, etc.. However, due to the complexity of wear causes and the uncertainty of wear types, it is necessary to analyze the working environment of parts when choosing wear-resistant coatings. Thermal spraying technology can increase the wear resistance of soft or worn matrix.
In general, thermal spray coated structures have higher wear resistance than casted or forged structures of similar materials. This is because in the process of thermal spraying particles undergo high-speed quenching, the formation of a special structure with certain pores, in the metal coating, deformation particles will also form a small amount of oxide. The micro-pore structure of the coating not only helps to maintain the lubrication film on the surface of the parts, but also can accommodate the debris caused by wear, which is beneficial to improve the wear resistance of the parts surface.
According to the wear process and failure mechanism of friction surface, wear can be divided into abrasive wear, adhesive wear, corrosion wear, fatigue wear, fretting wear, erosion wear and high temperature wear. High temperature wear is essentially the synthesis of adhesive wear and abrasive wear.
The requirements of wear-resistant coatings depend on the mechanical and chemical compatibility between the wear-resistant coatings and the base material, the direction and size of the applied load, and the performance of the coating itself. Depending on the application of the wear-resistant coating, the coating hardness, chemical stability, the yield strength of the coating, the ability to resist crack nucleation and growth and other factors all affect the wear-resistant performance of the coating.
The requirement of wear-resistant coating is to ensure that the coating and the substrate have enough bonding strength. Therefore, the selection and design of the substrate material and the coating material should be based on the premise of ensuring the firm bonding of the coating.
1) The matrix shall have no deformation. When the wear-resistant coating is used in high load condition, the matrix should have enough hardness and yield strength to support the coating without deformation.
2) Elastic modulus matching between the coating and the base material. In the case of elastic strain, if the elastic modulus of the coating and the matrix does not match, a precipitous stress will be generated at the interface between the coating and the matrix during loading. If the rigidity of the coating is greater than that of the substrate, the stress in the coating will increase. With the increase of load and the difference of elastic modulus between coating and matrix, the stress increases
3) Rigid matching between coating and base material. In order 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 the rigid coating is deposited on the less rigid matrix material. Due to the mismatching of rigidity, the tensile stress in the coating will increase, which will lead to the formation of cracks in the coating and spread to the matrix, thus causing the early failure of the coating.
4) The matching of thermal expansion coefficient. If the thermal expansion coefficient of the coating does not match that of the base material, stresses will occur due to volume changes. In general, the coating is very thin compared with the substrate, so the thermal expansion of the substrate is basically not 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 of thermal expansion between the coating and the matrix is basically concentrated in the coating. The greater the difference in thermal expansion coefficient, the greater the stress in the coating will be, the greater the tendency to crack or even spalling. This is one of the main reasons why many wear-resistant coatings fail prematurely before they are subjected to serious wear.
5) the affinity between the coating and the base material. The affinity between coating and matrix, namely chemical bonding ability, directly affects the bonding strength between coating and matrix. In general, when the solid solubility of one compound in another is low, the bonding strength between them is also weak. Only when there is a great chemical affinity between the coating and the matrix without the brittle interface phase, the bonding strength of the coating and the matrix is high and the wear-resisting coating can be fully played.
