Load Capacity and Durability of H-DLC Coated Hydrodynamic Thrust Bearings

Abstract

Hydrogenated diamondlike carbon (H-DLC) coatings provide excellent wear resistance and low friction for bearing applications. However, the use of such coatings with aqueous lubricants could pose some difficulties due to the hydrophobic nature of the surface. A thrust bearing tribometer was used to compare performance of hydrophilic and hydrophobic surfaces in hydrodynamic lubrication with a mixture of water and glycerol as the lubricant. Hydrophobic surfaces on both runner and bearing were achieved with the deposition of H-DLC films on titanium alloy surfaces. Hydrophilic surfaces were created through modification of H-DLC surface with covalently bonded heparin. Several possible combinations of hydrophobic and hydrophilic surface conditions were used on the bearing and runner surfaces to provide full-wetting, partial-wetting, and half-wetting conditions. The experimental results confirmed that load support is still possible, when the bearing is half-wetted or partially wetted. However, the full-wetted bearing combination (i.e., Reynolds no-slip boundary condition) provided the highest load support. Introduction of slip at the surface resulted in a lower measured torque. Heparin treatment resulted in a lower than expected static friction and friction in full lubrication regime. The durability of coated surfaces was evaluated in a series of start–stop tests and in impact tests. The results confirmed that the coatings are stable and survive the test regiment that exceeded 50 test cycles; whereas the uncoated titanium alloy bearing surfaces were damaged after ten test cycles.

The Effect of DLC Coating Thickness on Elstohydrodynamic Friction

“…Such measurements have included contact angle, surface energy, and spreading parameter. However, results in the literature have been nonconclusive in the sense that some authors have correlated the friction reduction to poor wetting (large contact angle) [13,23,25,29,41,42,49] and sometimes with surface related properties such as hydrophobicity and low surface energy [14,27]. Recently, Kalin and Polajnar presented a series of studies [26,27] where they conclude that there is poor correlation between contact angle and friction reduction with surface coatings in full film EHL.…”

“…The phenomena of reducing friction with DLC coatings in full film conditions where there are no asperity contacts between the mating surfaces have by several authors been explained as solid-liquid slip [14,25,29]. The principle of the theory being that the often assumed no-slip boundary condition between the liquid and the solid is violated.…”

“…Several authors have experimentally observed a reduction in friction with DLC-coated surfaces in full film EHL [6,7,14,27,48]. The friction reduction has been explained by several authors as an effect of boundary slip, or solid-liquid interface slip [14,27,29] a phenomena thoroughly discussed in literature [12,25,37,38,41,42,46,47,49], where some of the work is based on atomically smooth surfaces. However, it is still not entirely clear how the mechanism of slip works, and several hypothesis can be found in the literature.…”

“…However, it is still not entirely clear how the mechanism of slip works, and several hypothesis can be found in the literature. In many cases, poor wetting or high contact angle is proposed as the main feature to promote slip [13,23,25,29,41,42,49] and in other cases low surface energy [14,27]. However, using only surface energy as a means to determine the potential of solid-liquid slip may not be suitable since surface energy is a material property and tells nothing about the interaction of a specific material with a specific fluid.…”

Thermal effects influencing stability and performance of coatings in automotive applications

“…As a possible explanation, a boundary slip based on partially wetted surfaces is discussed [14,[16][17][18][19][20][21][22][23][24][25][26]. In the here presented investigations, a full lubrication can be assumed as given.…”