| Nanoindenter-based scratch tests are simple, rapid, and reliable method to evaluate the tribological properties of materials at the microscale. It not only works well on dry polymer surfaces, but also can evaluate thin fluid film lubrication on polymer surfaces. By using different experimental tips (different shape and radius) on the same polymer surfaces, different coefficient of friction results are obtained. Because of the different molecular structures, different polymer surfaces show different tribological properties. Furthermore, the relationship between the adsorption of lubricant and the lubricity of lubricant has been studied. Lubrication is an extremely complex phenomenon and it can be influenced dramatically by the lubricant, including the lubricants' molecular weight, molecular structure (both its chemical structure and its geometry (et al., di-block versus tri-block) and chain length, the solvent, the substrates, including the substrates' roughness and chemistry, the number of lubrication layers, the sliding velocity, and applied load. In addition, the lubrication properties of synovial fluid and its components (sodium hyaluronate (HA), proteins, and phospholipids) have also been studied. The concentration and molecular weight of sodium hyaluronate both have great effect on its lubricity. The concentrations of gamma-globulins and albumin also have effect on their lubricating properties. With just one type of protein or with excessive proteins, the solutions looses lubricity. Addition of HA can improve the lubricating properties of some protein solutions. DPPC (dipalmitoyl phosphatidylcholine) also shows some lubricity for polyethylene. However, without HA, the concentration of DPPC has no effect on its lubricity at the microscale. Addition of HA can improve the lubricity of DPPC solution when the concentration of DPPC is high. Depending on the concentration, every component has some lubricity, but working synergistically at the concentrations found in typical, healthy synovial fluid can greatly improve their lubricity. Indeed, the Synovial fluid model, which has gamma-globulins at 7 mg/mL, albumin at 11 mg/mL, HA (MW: 2.0 MDa) at 3 mg/mL, showed the best lubricity. |
