Influences of Metal Coating on Graphite Surface on Braking Tribological Behavior of Copper-Based Powder Metallurgical Friction Material

Using spark plasma sintering method, two kinds of copper-based powder metallurgical friction materials were prepared with Cu-coated graphite or uncoated graphite as lubricant. After testing and comparing the microstructure, mechanical property and physical property of the two materials, the friction and wear properties under different conditions were tested on the MM1000-Ⅱ inertial braking test bench, and microscopic analyses on the features of friction surfaces and their 3D morphology, major elements distribution of friction surface and subsurface, wear debris and surface phases of the two materials were carried out. The influences of metal coating on graphite surface on braking tribological behavior of copper-based powder metallurgical friction material were studied, and the reason for the differences in braking tribological behavior between the two materials was explained based on the thermodynamics theory. The results showed that the Cu-coating treatment of graphite can strengthen the interface bond between graphite and copper-matrix and significantly improve the hardness, density and thermal conductivity of material. With the increase of braking speed, the average friction coefficient and wear rate of the two materials decreased. Under the same braking conditions, using Cu-coated graphite as the lubricant can reduce the friction coefficient and wear of material, and the geometric quality of friction surface of material was also better. At the low braking speed, for the material containing Cu-coated graphite, only mild abrasive wear and adhesive wear occurred on the surface. However, for the material containing uncoated graphite at the low speed, except abrasive wear and adhesive wear, severe delamination also occurred on the surface. For both of the two materials, increasing the braking speed can promote the formation of friction films on their surfaces, which can reduce the materials wear. However, the friction films formed on surfaces showed significant difference in their formation mechanism from Cu-coated graphite to uncoated graphite. For Cu-coated graphite as lubricant, the friction film on the material surface was mainly composed of oxide film. For uncoated graphite as lubricant, due to the presence of more graphite on the surface of the material, which had a strong inhibition effect on the oxidation reaction, graphite-rich film was formed on the surface, and the protecting effect of graphite film on the material surface was not as good as that of oxide film.