The influence of diamond grain sizes on the tribological performance and ultimate loading capacity of polycrystalline diamond self-mated friction pairs under water lubricated condition

To investigate the effect of diamond grain sizes on the tribological properties of polycrystalline diamond (PCD) self-mating pairs, five types of PCDs with average grain sizes of 10, 35, 60, 85, and 110 μm were developed. The surface microstructure of the PCDs was observed using scanning electron microscopy and laser confocal microscopy. The friction and wear behaviors, as well as the ultimate loading capacity of PCD self-mated ring-ring conformal friction pairs, were tested under water lubrication conditions. The worn surfaces were analyzed, and the wear rate was calculated. The results show that diamond grain size has a significant impact on the friction and wear performance of PCDs. Under a specific pressure of 5 MPa, the wear rate of PCD friction pairs decreases as the diamond grain size increases. For the stationary ring and the rotary ring with a diamond grain size of 110 μm, the wear rates are the lowest, at 0.045 μm/h and 0.075 μm/h, respectively. However, under a specific pressure of 10 MPa, the wear rate first decreases and then increases with increasing diamond grain size. The friction pairs with an average diamond grain size of 60 μm exhibit the most stable low friction coefficients and the lowest overall wear rate. The PCD friction pairs with average diamond grain sizes of 10, 35, and 60 μm mainly suffer from abrasive wear, while those with 85 and 110 μm grain sizes experience diamond grain fracture and detachment. There is a negative exponential relationship between the ultimate loading capacity and the sliding speed of the friction pairs. The PCD friction pair with an average diamond grain size of 60 μm has the highest ultimate loading capacity. These findings provide valuable insights for designing PCD self-mated sliding bearings that operate in water-lubricated environments.