INTRODUCTION: The accurate prediction of film thickness and friction, for a wide range of operating conditions and for different types of lubricants, is extremely important for controlling separation of the rubbing surfaces because it leads to an increase of the energy efficiency and longer life of mechanical components. During the last several decades, the progressive trend to operate machine elements in higher loads, temperatures and harsh conditions, in general, can be observed1,2. These conditions cause film thickness becomes significantly thinner. Because of that numerous current machine elements operate very often near the border of transition between full film lubrication, where rubbing surfaces are completely separated by a thick film, and mixed lubrication where the contact between opposite surfaces may occur. In this case, the proper understanding of film formation and friction origin is the key factor for longer life of machine elements operating under these conditions. In case of mixed lubrication, it is very complicated to estimate film thickness behavior, because of the occurrence of film lubrication and surface contact at the same time. The friction has not been widely studied in combination with film thickness. Thus, there is a lack of knowledge about friction on a border of mixed lubrication regime of non-conformal contacts.
The purpose of this study is to experimentally reveal the connection between the film thickness and the friction for a uniform structure of surface texture on the transition from EHL to boundary lubrication in non-conformal contacts. The current study tries to clarify processes which lead to the friction increase during this transition using base oils and special lubricants as reference.
METHODS: In-situ measurements were conducted by using a ball-on-disc rig. In this rig, a circular EHL contact is formed by loading a flat transparent disc made from optic glass against the ball with a uniform surface structure made from bearing steel (100Cr6). This apparatus uses optical interferometry technique for film thickness evaluation. The friction force and film thickness were measured simultaneously by using the build in torque sensor and thin film colorimetric interferometry3.During the experiments, the load and temperature were maintained at 44.5 N which corresponds to
0.6 GPa Hertzian pressure. Experiments were conducted for a wide variety of lubricants e.g. mineral SR 90, synthetic PAO 6 and special lubricants n-Hexadecane and OMCTS. Balls with different uniform structure were also created for those experiments. Therefore, the influence of surface roughness was also studied.
RESULTS: Experiments were focused on friction, the average and minimum film thickness in the contact area. In-situ measurements were conducted for a wide variety of created balls and lubricants. Film thickness development was studied in detail with friction information. Experimental results for higher speeds have shown average film thickness correspond with prediction, based on Hamrock and Dawson’s formula for circular EHL contact, quite well. However, after the certain point, the deviation from theory was detected. Results have shown there are several explanations of this deviation e.g. due to direct contact between opposite surfaces which can be clearly seen in Figure 1, or due to an ordering of surface molecules in case of OMCTS lubricant. The same effect was observed in previous study4.
Friction measurements have shown a significant difference between the first contact and increase in friction. The increase in friction occurs before the first contact between opposite surfaces is