Carbon dioxide (CO 2 ) 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 CO 2 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 CO 2 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 CO 2 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 CO 2 is a viable refrigerant from a tribology point of view.

In the magnetic storage industry, thin film carbon overcoats play a critical role in reducing magnetic and physical spacing between the recording slider and the rotating disk so that the information stored per unit area is maximized. Thin film carbon overcoats have been improved such that they exhibit higher hardness with lower thickness of few nanometers and still being able to perform reliably. In this paper, nanoindentation and nanoscratch techniques to measure nanomechanical properties, namely hardness, elastic modulus and shear strength of thin solid films were presented along with a recently developed high resolution force transducer. Nanowear behavior characterization techniques were also examined and were applied to commercially available magnetic disks to characterize their wear behavior. It was shown that the properties and wear behavior of sub-10-nm thick film carbon overcoats were reliably measured. These techniques could be applied to different thin solid films on substrates, and they are not restricted to magnetic storage systems.