Evaluation of the Rolling Contact Fatigue Performance of Tribological Surface Coatings

Dr. J. Eichler
Dr. A. Leyland
Prof. A. Matthews
Dr. G. Doll, Timken Research (USA)

Rolling element bearings are one of the most commonly-encountered machine components. Modernsteel processing methods have all but eliminated material defects and inclusions from high quality bearing steels – and this has shifted the rolling contact fatigue wear mode to surface initiated failure. In this wear regime, dents caused by debris or other damage mechanisms can initiate microcracks which then propagate to damage the bearing surfaces. A modern bearing under light, constant load - and running in the full film thickness lubrication regime - would be expected to last almost indefinitely; however, most bearings operate under more extreme conditions. The drive to increase power density means that components are running faster, at increased loads and at higher temperatures. One way to extend the life of bearings running under severe conditions (such as those of starved lubrication) is to use a tribological coating.

 

This project examined the effect on temperature and torque of combinations of vibratory superfinishing and low-friction coatings applied to the raceways of a heavy duty thrust ball bearing under oil starvation conditions. Normally, rolling elements benefit from a film of lubricant which prevents adhesion and wear between contacting faces under high contact stresses. With little or no lubricant film, an unmodified bearing will quickly fail. A practical example of where such problems can occur is that of wind turbines, where increasingly high torque output loads delivered at low angular velocities present challenges in maintaining adequate lubricant supply.

 

A major aspect of this project was the design and fabrication of a bespoke, computer-controlled bearing test facility (shown diagrammatically in figure 2). Instrumented with sensors for measuring bearing torque, temperature, speed and vibration, this equipment is capable of accurately detecting the onset of bearing failure.

 

Using this bearing test facility, the rolling contact fatigue performance of Cr2N and WC/a-C:H coatings deposited by PVD and hybrid PVD-CVD processes, respectively, were tested. In addition, the effect of vibratory superfinishing on rolling behaviour, both in isolation and as a surface pre-treatment prior to PVD coating, was examined.

 

In this investigation, the WC/a-C:H coating offered the best performance under lubrication starvation conditions. This coating is likely to function as a barrier to adhesive interactions between the raceway and rolling element, thereby increasing bearing life. Evidence of carbonaceous material on the rolling elements was found. This transferred material is likely to act as a solid lubricant, delaying bearing failure under boundary lubricated conditions. The performance of the superfinished specimens was unexpectedly poor. Superfinishing alone could not provide appreciable improvements in performance and also reduced the effectiveness of WC/a- C:H coatings when used as a surface pre-treatment. The superfinished substrate topography is unlikely to have causedpoor coating adhesion. However, the modified raceway surface will result in a larger contact area which could promote spinning of the rolling elements, increasing the frictional torque and raising shear stresses in the coating. Experimentation with the ceramic media size, type and process duration is likely to yield better superfinishing results and will be the subject of future investigations.