“The Behaviour of Lubricant Blends in Elastohydrodynamic Contacts”
Blends of lubricant base stocks are commonly employed to produce lubricants with optimised performance. However, the influence of blending on lubricant behaviour within high pressure, mechanical contacts, particularly the film forming capacity and friction, have largely gone unstudied. This thesis examines both of these aspects for a range of base fluids and their blends.
Film thickness generation and fluid friction are reviewed in the context of elastohydrodynamic (EHD) lubrication. Various models addressing the origin and mechanism of liquid viscosity are reviewed. The derivation of viscosity as a thermodynamic property is also reviewed and its relevance to the current study is discussed. Consideration is also given to two commonly accepted yet contrasting models of EHD traction, resulting in one being adopted for this research.
The relationship of molecular structure to film generation is examined experimentally by studying a number of single component fluids with widely varying chemical structures. It is shown that pressure-viscosity coefficients derived from film thickness are strongly related to the respective chemical “family” of the lubricant.
EHD friction (traction) measurements are also made and analysed in order to establish a reliable method for comparing the influence of fluid composition on traction. The method, based on a well regarded fluid model, allows accurate description of full fluid traction by means of the Eyring stress and pressure-viscosity coefficient in the central EHD contact.
Binary blends of well-defined base fluids are studied. It is found that the pressure- viscosity coefficient, as derived by the aforementioned methods, varies nonlinearly with the composition and tends toward the lower value of the individual components, at times attaining values lower than either of the individual components.