Influence of steel composition on the formation and effectiveness of lubricant boundary layers in tribological contacts

Abstract

Boundary lubricating films provide surface protection in rolling-sliding contacts under conditions where adequate fluid films cannot be formed. These boundary films are formed through the interaction of lubricant additives and rubbing surfaces Their effectiveness is crucial for reliable operation of most mechanical systems, such as engines, bearings and gears. With the current trend of reducing lubricating fluid film thickness in modern machines, due to decreases in oil viscosity and increases in power density in the search for improved efficiency, effective operation of such boundary films is further gaining in significance. Therefore, it is not surprising that a substantial body of research exists on additive formulations and formation of boundary lubricant films. However, it is notable that previous studies were almost exclusively conducted using AISI 52100 bearing steel as the rubbing material; this is most likely because boundary lubrication studies are most often conducted using ball-on-disc tribometers and commonly available steel ball specimens are almost invariably those used in ball bearings and hence made of bearing steel. The obvious problem with this approach is that lubricants formulated based on the results of such tests are subsequently used in systems that are primarily made of other steels, for example case carburised gear steels in a gearbox, and the assumption is made that boundary lubrication will be just as effective as was found in tests with 52100 bearing steel. Present work attempts to address this issue by providing new understanding on the influence of steel composition on the formation and effectiveness of lubricating boundary films. 52100 bearing steel, 16MnCr5 case carburised gear steel, M2 tool steel and 440C stainless steel specimens are tested in rolling-sliding ball-on-disc contacts lubricated with custom-made oils with systematically varied additive package. The use of custom-made oils enables a valid comparison to be made and concrete conclusions to be drawn. In parallel, surfaces that have been ion-implanted with a single metallic element and surface coatings made of similar elements were also tested to gain clearer understanding of metal – lubricant interactions. A selection of additive types and chemistries were tested, with the primary focus on ZDDP and ashless anti-wear additives.

It is found that under mixed and boundary lubrication conditions, formation of ZDDP anti-wear films is not influenced by steel composition, specimen surface roughness or contact pressure magnitude. Similar film thickness, final surface roughness and friction was observed with all materials tested, with little or no wear observed. On the other hand, the results show that the thickness and effectiveness of ashless anti-wear films is influenced by steel composition, specimen roughness and contact pressure level: These additives build films of varying film thickness depending on the type of steel present. The exact influences depend on the type of the ashless additive (ester based or acid based etc.) but for example, M2 steel promoted ashless film formation and 440C retarded ashless film formation. This behavior is attributed to the presence of different alloying elements in different steels.

The XPS (X-Ray Photoelectron Spectroscopy) analysis of tested surfaces reveals that high amounts of metal oxides, such as chromium oxide, are detected on the surface when only a thin tribo-film was formed, whereas a high amount of phosphates is present on the surface when thick tribo-films are formed. Significantly, phosphates of different alloying elements were observed in tribo-films formed with ashless additives depending on steel composition.

These observations led to further studies using coated and ion-implanted 52100 steel surfaces where the coating or ion-implantation was done with a single metal each time, chosen based on the learnings obtained in the earlier tests with the four steels in relation to the major alloying elements present in them. Ni, Cr, W, Mo, V and Mn were of particular interest. The results with coatings and ion-implanted specimens are consistent, although the coating tests cannot be run for prolonged periods due to relatively early coating failures so the major observations presented here are based on the test with ion-implanted specimens. Both methods show that ashless anti-wear films form at a significantly faster rate in the presence of Nickel (Ni) where thick protective films are formed after only a few minutes of rubbing. The formation rate of these films is even faster than that of ZDDP films. Long term tests with ion-implanted specimens, that therefore run for a higher sliding distance, show that, although the formation rate is lower, the presence of Mo and W ions also eventually leads to thick films with ashless additives, albeit with somewhat higher tribo-film roughness and friction. On the other hand, presence of Cr ions in the surface acts to significantly retard the formation of ashless based anti-wear films.

These results suggest that ashless additive type needs to be carefully chosen to match the type of steel being lubricated to optimise the effectiveness of boundary lubrication. In addition, suitable application of ion implantation seems to offer a promising way of optimising the effectiveness of boundary lubrication with ashless based anti-wear additive packages which are becoming widely used in modern oils but can, with selected contact conditions and materials, be less effective than standard ZDDP based additives in terms of wear protection. The potential for the application of ion-implanted surfaces in real components is further enhanced by the fact that the ion–implantation process itself does not modify the original hardness and roughness of the surfaces so that the risk of any damage due to plastic deformation and fatigue is not increased.