Enhanced tribological behaviors of sintered polycrystalline diamond by annealing treatment under humid condition

The tribological behaviors of the 700 degrees C annealed sintered polycrystalline diamond (PCD) at various relative humidity (RH) levels were systematically investigated. The comparison of tribological behaviors between the 700 degrees C annealed PCD and the pristine PCD was made to further understand the tribological mechanisms. The results reveal that the friction inducing carbonaceous transfer film and oxidation and hydrolysis induced tribochemistry reaction dominant the tribological behaviors of the annealed PCD at various RH levels. The low coefficient of friction (COF) obtained in dry environments is attributed to carbonaceous transfer film on the worn Si3N4 surface, which was formed by the layers shearing action of massive tiny diamond grains exfoliated from the annealed PCD surface. The graphitization, oxidation and stress relaxation of the PCD induced by the 700 degrees C annealing treatment make the tiny diamond grains more easily to exfoliate and be grinded on the Si3N4 interface. It facilitates the formation of friction reducing carbonaceous transfer film, and finally results in the 30% lower COFs than those of pristine PCD at low RH levels (5%-50% RH). Meanwhile, an enhanced wear resistance of PCD can be achieved after 700 degrees C annealing treatment. The tribochemistry reaction induced by the oxidation and hydrolysis of Si3N4 governs the tribological behaviors of the annealed PCD at high RH levels (60%-99.9% RH). It reveals higher COFs accompanied with serious wear of Si3N4 ball and nearly no wear loss of annealed PCD. The produced SiO2 and silicic acid embeds into massive spalling pits on the annealed PCD surface, resulting in slighter wear of the PCD and Si3N4 than that of the pristine PCD/Si3N4. These results propose that the tribological behaviors of PCD under humid environment can be significantly improved by the 700 degrees C annealing treatment.