The Effect of Lubricant’s Viscosity on Reducing the Frictional-Induced Fluctuation on the Onset of Friction

The initial friction stage between the contacting materials would generate a maximum shear force and an unstable fluctuating time, which had a negative effect on the entire frictional system, especially at low temperature conditions. In order to decrease the occurring shear force and fluctuating time on the onset of friction, two different lubricating oils were applied in this study to investigate the influence of lubricant’s viscosity on these friction behaviors. The frictional experiments were conducted between the steel ball and the 40CrMnMo, and special attention was paid to the relationship between maximum friction force, fluctuating time, frictional vibration and the initial lubricant temperature. The results showed that the friction force first increased to the maximum value and then experienced an oscillation damping period (fluctuating time) before it reached a stable state. And this fluctuating behavior caused corresponding vibrations on the initial contacting. However, compared to the high viscosity lubricating oil (HO), the low viscosity lubricating oil (LO) contributed to more than 50% reductions on max friction force, fluctuating time and vibration at the cold start (0 °C). Moreover, the weakened initial frictional fluctuation was conducive to generating a low and stable friction coefficient (COF) and wear loss of the long-term test. The discrepancy on lubricating performance was that the low viscosity provided high fluidity, which allowed rapid distribution of the lubricant between the contacting surfaces and formed an intact lubricating film. Similarly, the high temperature decreased the viscosity of HO and thus led to satisfactory friction reductions. The knowledge gained herein provides a supporting theory on the design and preparation of a lubricating oil with high performance.