Surface and sub-micron sub-surface evolution of Al390-T6 undergoing tribological testing under submerged lubrication conditions in the presence of CO2 refrigerant

Carbon dioxide (CO2) with its environmental benefits is considered a good replacement for commonly used synthetic refrigerants. In this study, the surface and sub-surface changes in simulated CO2 environment during the initial or transient stages of a sliding contacting interface were investigated. Pin-on-disk configurations involving Al390-T6 disks in contact with 52100 steel pins were used in controlled tribological experiments using a High Pressure Tribometer. In order to evaluate the effectiveness of CO2 refrigerant, comparative tribological experiments involving a conventional refrigerant and different commonly used lubricants were initially performed in a step-increasing load manner under submerged lubricated conditions. Subsequent detailed experiments for investigating the surface and sub-surface changes were performed in the presence of CO2 refrigerant and the best performing lubricant, polyalkyline glycol. Burnishing was observed on the surfaces during the transient (evolutionary) stage, which indicated asperity contacts due to the breaking of the elasto-hydrodynamic lubrication film. In order to quantify the surface and sub-micron sub-surface changes that occurred during this transient stage of tribological operation, several analytical tasks were performed, which involved the measurements of nanomechanical properties, chemical compositions of the topmost 200 nm surface layer, and surface roughness. Such studies of detailed evolutionary changes that occurred during the transient stage of a tribopair shed light on the complex interactions between surface and sub-surface changes that determine whether successful tribological conditions will eventually be achieved. Based on the analyses presented in this work, it is concluded that CO2 is a viable refrigerant from a tribology point of view.