Investigation of environmentally acceptable lubricants with regard to their degradation-related change in tribological performance

Every year, significant amounts of non-biodegradable, toxic lubricants enter the environment. While legislative measures are being taken to reduce the environmental impact of this pollution, demand for environmentally acceptable lubricants (EAL) is increasing. These EALs differ in terms of their base oil and additives, thereby having a significant influence on the degradation behavior and degradation-related changes of tribological performance like oil film formation or boundary layer formation. However, the sensitivity of EALs to different degradation paths is not yet known. Therefore, the aim of this study is to determine the sensitivity of different lubricants to degradation based on their change in tribological performance. For this purpose, an ester- and polyglycol-based EAL as well as a mineral oil (polyalphaolefin) were degraded synthetically by oxidative and hydrolytic degradation. The degraded lubricants are visually, rheologically and chemically analyzed. In addition, the tribological performance in terms of oil film formation and tribological boundary layer formation was studied in a ball-on-disc tribometer. Afterwards the results are compared to their corresponding fresh lubricants. For the tested ester-based EAL no significant sensitivity to degradation could be shown, as no significant change in tribological performance occurred. Both polyalphaolefin and polyglycol proved to be sensitive to degradation leading to a significant reduction in the tribological boundary layer thickness. These results show, that mineral oil-based lubricants and EALs, as well as EALs themselves, differ in terms of their sensitivity to degradation. These effects must be considered in qualification process of lubricants.