2.2 Transformation of tissue structure after heat treatment After conventional heat treatment, the morphology of the coating did not change significantly. Even after high-temperature heat treatment, an oxide film with a thickness of about 10 μm was formed on the surface of the coating. XRD analysis showed that the phase structure of the coating after heat treatment changed radically. The coating had changed from the amorphous phase to the crystalline phase, mainly crystalline Ni, and a small amount of second phase Ni3P. Very fine Ni3P is dispersed on the Ni base. After analyzing the interplanar spacing, the measured interplanar spacing is larger than the standard, indicating that the second phase Ni3P precipitates in a coherent relationship with the parent phase Ni, causing lattice distortion. The generation of coherent distortion hinders the movement of dislocations, thereby strengthening the coating after heat treatment. From the original amorphous structure to the crystalline composite phase (Ni+Ni3P), the hardness is increased to HV920-1050, the corresponding wear resistance is also greatly improved, the stable wear phase is prolonged, and the wear amount is reduced. 2.3 Change of stress state after heat treatment The change in internal stress of the coating is closely related to the transformation of its structure. At lower temperature heat treatment, such as 200 ° C × 1 h, although the XRD diffraction peak of the coating is not significantly crystallized, the gas absorbed inside the coating overflows, and the coating structure may have a relaxation phenomenon, and the tensile stress in the coating is to some extent. It was lowered. When the temperature rises, the plating begins to crystallize, and the crystallization reaction is completed to about 400 ° C, which causes the defects of the plating layer to increase, and the precipitation of the Ni 3 P phase causes the volume of the coating to shrink and the tensile stress increases. When the temperature rises to about 600 ° C, the crystal grains grow and the tensile stress of the plating layer increases. Due to the difficulty in characterizing the interfacial adhesion and stress in the coating, most of the research on internal stress and bonding strength remains in qualitative and semi-quantitative research. There is little literature in this area. 2.4 Effect of heat treatment on the bonding strength of the coating The change in bond strength between the coating and the substrate is mainly related to the diffusion of elements at the interface between the two. The literature studies the diffusion of steel-based Ni-P coating and steel substrate. The electroless plating and the substrate are mainly physically combined in the plated state. After subsequent heat treatment, the diffusion of the coating and the substrate is intensified, and a strong chemical bond is formed between the coating and the substrate. Thereby greatly improving the bonding force between the steel base and the electroless plating. The literature also pointed out that high temperature and long-term heat treatment, a thick and continuous strong Ni-Fe diffusion layer is formed between the coating and the steel substrate, which strengthens the bonding between the coating and the substrate. In addition, the bonding force between the plating layer and the substrate is also related to the lattice type of the two. If the lattice type of the two is the same, the lattice constant is close, and the two phases are easy to form a coherent relationship at the interface, and the combination of the two is firm, otherwise it is poor. In addition, the bonding strength of the nickel-phosphorus alloy coating to the substrate is also affected by the plasticity of the coating itself. The nickel-phosphorus alloy coating in the plated state is very brittle, and cracks, peels and peels off as the matrix deforms. After heat treatment, especially heat treatment higher than the crystallization temperature, the plasticity of the nickel-phosphorus alloy can be significantly improved, thereby improving the bonding force between the plating layer and the substrate. Previous page next page High Power Copper Wire Roller Shutter Opener Roller Door Controller,Roller Garage Door Opener,Roller Door Automatic Opener,High Power Copper Wire Roller Shutter Opener GUANGDONG BEIDI SMART SCIENCE & TECHNOLOGY CO LTD , https://www.beidimotor.com
If the heat treatment temperature is continuously increased, the amount of Ni3P phase will gradually increase, the phosphorus content in nickel will further decrease, the coherent distortion will be more serious, and the hardness will continue to rise. The heat treatment at 400 °C × 1 h can reach a hardness of 1040 HV. After that, the heat treatment temperature is increased, the coherent relationship is broken, the elastic stress field is relaxed, the Ni3P phase is coarsened, the hardness is gradually decreased, and the crystal grains are also coarsened.