Friction and normal forces of model friction modifier additives in simulations of boundary lubrication

Effect of Loading on the Adhesion and Frictional Characteristics of Top Layer Articular Cartilage Nanoscale Contact: A Molecular Dynamics Study

Articular cartilage is a water-lubricated naturally occurring biological interface imparting unique mechanical and ultralow frictional properties in bone joints. Although the material of cartilage, synovial fluid composition, and their lubricating modes and properties have been extensively investigated at various scales experimentally, there is still a lack of understanding of load bearing, adhesion, and friction mechanisms of the cartilage-cartilage interface from an atomistic perspective under heavy loads. In this study, the effect of loading on adhesion and frictional behavior in articular cartilage is investigated with a proposed atomistic model for top layer cartilage-cartilage contact in unhydrated conditions using molecular dynamics (MD) simulations. Pull-off tests reveal that cohesive interactions occur at the interface due to formation of heavily interpenetrated atomistic sites leading to stretching and localized pulling of fragments during sliding. Sliding tests show that friction is load- and direction-dependent with the coefficient of friction (COF) obtained in the range of 0.20-0.75 at the interface for sliding in parallel and perpendicular directions to the collagen axis. These values are in good agreement with earlier nanoscale experimental results reported for the top layer cartilage-cartilage interface. The COF reduces with an increase in load and tends to be higher for the parallel sliding case than for the perpendicular case owing to the presence of the constant number of H-bonds. Overall, this work contributes toward understanding sliding in unhydrated biointerfaces, which is the precursor of wear, and provides insights into implant research.

Tribological Properties of Additive Manufactured Materials for Energy Applications: A Review

Recently, additive manufacturing (AM) has gained much traction due to its processing advantages over traditional manufacturing methods. However, there are limited studies which focus on process optimization for surface quality of AM materials, which can dictate mechanical, thermal, and tribological performance. For example, in heat-transfer applications, increased surface quality is advantageous for reducing wear rates of vibrating tubes as well as increasing the heat-transfer rates of contacting systems. Although many post-processing and in situ manufacturing techniques are used in conjunction with AM techniques to improve surface quality, these processes are costly and time-consuming compared to optimized processing techniques. With improved as-built surface quality, particles tend to be better fused, which allows for greater wear resistance from contacting tube surfaces. Additionally, improved surface quality can reduce the entropy and exergy generated from flowing fluids, in turn increasing the thermodynamic efficiency of heat-transferring devices. This review aims to summarize the process-optimizing methods used in AM for metal-based heat exchangers and the importance of as-built surface quality to its performance and long-term energy conservation. The future directions and current challenges of this field will also be covered, with suggestions on how research in this topic can be improved.