Effects of Interfacial Velocity or Temperature Slips on Finite Line Contact Thermal EHL in Simple Sliding Motion

With the development of surface engineering technology, plenty of hydrophobic and oleophobic surfaces are designed so that the occurrence of interfacial slips has become quite common. For an interface in elastohydrodynamic lubrication (EHL) contact with obvious thermal rise, temperature slip should take place accompanying with the occurrence of velocity slip. Thus, for a thermal EHL contact, friction coefficient is possible to drop below 0.01 and reach a superlubricity level.

In this study, based on a finite line contact simple sliding thermal EHL problem, numerical analyses were completed to reveal the changes in the lubrication characteristics when velocity or temperature slip occurred on the moving and stationary surfaces, respectively, together with the influence of velocity change on lubrication state in the contact zone. The work was also aimed to explore the reduction of the friction coefficient as well as the possible occurrence of superlubricity.

The geometrical modeling of the finite line contact was based on the authors' previous design of the roller ends to obtain a beneficial lubrication state at the end of the contact zone that was least susceptible to severe stress concentration phenomena. The modified generalized Reynolds equation taking into account the interfacial velocity slip, the Ree-Eyring fluid flow model and respectively 4types of interfacial slip models were used to establish the mathematical models. The Reynolds equation was solved by the multigrid method, the elastic deformation by the multigrid integration method, and the temperature field by the column-column scanning technique.

Velocity slip at the moving surface significantly reduced the lubricant flow rate in the contact zone, while velocity slip at the stationary surface acted the opposite way. All 4 slip models reduced the oil film thickness while the effect of the velocity slip on the moving surface was the most obvious. Velocity and temperature slips on the moving surface and velocity slip on the stationary surface all reduced the maximum thermal rise in the contact area, but temperature slip on the stationary surfaces significantly increased the maximum thermal rise. Velocity slip on both the moving and the stationary surfaces could reduce the friction coefficient to below 0.01 thus a superlubricity level was reached under low-medium speed conditions, or if the velocity slip length was long enough. Moreover, temperature slip on the stationary surface could smoothly reduce the friction coefficient to reach a superlubricity level.

Velocity slip on the moving surface significantly reduced the film thickness in the contact zone, and temperature slip on the moving surface led to an increase in the friction coefficient in the contact zone, so it was suggested to avoid interfacial slip on moving surface. Velocity slip on the stationary surface played a major role under low speed condition while temperature slip on the stationary surface was dominating under high speed condition, thus the friction coefficient could be reduced with the aid of interfacial slip on the stationary surface.