| Silicon-oxygen or 'siloxane' based polymers are known for low viscosity-temperature dependence, low glass transition temperature and high oxidative stability due in part to strong and flexible molecular bonds. The unique properties of siloxanes have led to evaluation of their performance as lubricants. Through collaboration with Prof. Marks' Chemistry group on synthesis and Prof. Chung's Materials Science and Engineering group on surface science, we designed, synthesized and tested a number of polysiloxanes and compared their performances to several commercially available polysiloxanes, polyalphaolefin, and polyether lubricants.;Molecular masses and chemical structures were determined by gel permeation chromatography and nuclear magnetic resonance, respectively. Density and viscosity were measured over a temperature range of 298 to 398K. Elastohydrodynamic film thickness, friction, and wear measurements were made at loads and speeds that represent the boundary, mixed and full film lubrication regimes. The investigations reveal that lubricant film formation and friction performance vary significantly with different length, branch content, and atomic constituents.;The experimental results were used in conjunction with theories of rheology and lubrication to develop a molecular-rheological modeling system that uses polysiloxane alkyl branch length L, pendant type J, percent of branch functional monomers Q, and degree of polymerization DP to predict viscosity, pressure-viscosity index, shear modulus, and limiting shear stress over a range of temperatures and pressures. The rheological properties are used to project film formation, boundary friction, and film friction over a range of slide-to-roll ratios and entrainment speeds. A mathematical bridge is thereby built to relate molecular structure to tribological performance, considering non-Newtonian characteristics.;An optimization algorithm has been developed to predict desired lubricant structures with a set of rheological and tribological characteristics. The boundary friction due to asperity interaction and film friction due to viscous dissipation were used as constraints to develop both Newtonian traction and non-Newtonian energy-efficient lubricants fluids with improved wear protection. The insights into the structural-functional characteristics of siloxanes facilitate the design of advanced lubricants to maximize film formation yet vary hydrodynamic friction for a range of engineering applications. |
