Improved tribological properties of Si₃N₄/GCr15 sliding pairs with few layer graphene as oil additives

The tribological behaviours of two types of tribo-pairs (Si₃N₄/GCr15, GCr15/GCr15) lubricated with 4010 aviation lubricants (4010AL) containing different concentrations (0–0.3 wt%) of few layer graphene (FG) were investigated using a four-ball testing machine. Atomic force microscopy (AFM) and Raman spectroscopy were employed to characterise the structure and morphology of FG. Laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy were employed to investigate the worn surface. The test results demonstrated that the frictional properties of the Si₃N₄/GCr15 pairs were significantly improved after adding FG into the base lubricating oil. With the lubricating oil containing 0.075 wt% FG, the friction coefficient and wear scar diameter were reduced by 27% and 43%, respectively. On rubbing surfaces, disordered graphene was obtained as examined by EDS and Raman measurements. The lubrication mechanism was investigated to explain the enhancement in the friction-reduction and anti-wear properties, which can be attributed to shearing, tearing, and filling of FG as well as its adhesion to rubbing surfaces, and the mechanical contact between frictional interfaces was avoided.