The application of manganese phosphate conversion coating process in improving the surface strength of transmission parts first appeared in the improvement of the fatigue life of bearing rollers. The manganese phosphate conversion coating obtained after phosphating treatment on the surface of automobile gears can effectively reduce the friction coefficient of the friction pair surface and has good anti bite or scratch performance.
In Japan, manganese phosphate conversion coating technology was applied to automotive automatic transmission gear for the first time in the research and practice of high-strength automobile. Shi wankai and ZHY gear of Chongqing University prepared ultra-fine manganese phosphate conversion coating on the surface of 20Cr gear. The relationship between phosphating grain size and oil storage characteristics of pores on the surface of the coating was studied. The results show that the ultra-fine manganese phosphate conversion coating on the steel surface has obvious antifriction and wear resistance under the condition of surface oil immersion lubrication. Wang et al. Studied the influence of post-treatment and lubrication of phosphating film on its tribological characteristics.
The manufacturing process of manganese phosphate conversion coating mainly includes pretreatment of automobile gear surface with degreasing agent in degreasing tank at the temperature of 70 ~ 95 ℃, water cleaning, and then surface adjustment at the treatment temperature of 40 ~ 80 ℃. The temperature of phosphating treatment is 80 ~ 100 ℃, the acid ratio is controlled at 5.6 ~ 6.2, and the treatment time is 10 ~ 15 min.
The process flow is shown in Figure 1. The coating is formed through the following chemical reactions:
After manganese phosphate conversion coating treatment, a soft layer of several microns is generated on the surface of automobile gears, which fills most of the concave convex cutting ripples on the surface of automobile gears, reduces the local maximum meshing contact stress and metal surface friction coefficient, and improves the oil film condition and lubrication condition of automobile gears during meshing. By controlling the process parameters of manganese phosphate conversion coating to affect the density and grain size of the coating, the ultra-fine manganese phosphate conversion coating can be obtained, and the tooth surface will produce 3 ~ 5 after treatment μ M, the density of the resulting coating is about 2.2 g / m2. After treatment, the surface morphology is observed by SEM, as shown in Figure 2. The selection of ordinary manganese phosphate coating treatment and ultra-fine manganese phosphate coating treatment needs to be determined in combination with automobile gear processing technology and actual working conditions.
Figure 3 shows the formation of oil film by cylindrical roller. The formation of oil film of cylindrical roller is observed by separation voltage resistance method. 0 V is full contact and 0.1 V is complete separation. The separation voltage of cylindrical roller after manganese phosphate coating treatment begins to rise after 30 min, and its oil film formation ability is significantly better than that of untreated cylindrical roller.
The tribological properties of the coating under certain working conditions were evaluated by friction and wear test. As shown in Figure 4, according to the working load and lubrication conditions of automobile gears, the friction and wear test of manganese phosphate coated specimens is carried out. The ultra-fine manganese phosphate conversion coating has good continuous friction reduction and wear resistance characteristics under low load and high load conditions.
Figure 5 shows the surface morphology pitting corrosion of two groups of at gears under the same test conditions according to the durability test requirements of the automatic transmission provided by Japan jatco automatic transmission company to Volkswagen. The two groups of automobile gear pairs have undergone the same processing technology. Group B automobile gear pairs have been treated with manganese phosphate conversion coating, while group a automobile gear pairs have not been treated.
As shown in Figure 6, the experiment is carried out under the condition that the maximum pressure Pmax at the contact point of the tooth surface is 2000 MPa and the maximum sliding speed of the tooth root is 7.8 M / s. The horizontal axis represents the number of meshing cycles and the vertical axis shows the pitting area rate under each cycle. Group a automobile gear in 3 × Pitting corrosion damage occurred under 106rev cycle, while group B automobile gear was at 1.1 × The Rev cycle has not yet occurred after pitting. The test shows that the manganese phosphate conversion coating increases the pitting fatigue life of transmission gears by 3 to 4 times.
