APPLICATION OF THE ULTRA-THIN ELASTOHYDRODYNAMIC OIL FILM THICKNESS TECHNIQUE TO THE STUDY OF AUTOMOTIVE ENGINE OILS

Recent advances in the measurement of elastohydrodynamic (EHD) oil film thickness have enabled films of only a few molecules width to be measured with good accuracy. As a result, the range of temperature and viscosity that may be permitted in EHD experiments has been significantly extended and the practical value of the information obtained correspondingly increased. This paper explores the potential value of such techniques for investigating the viscometric behaviour of lubricants at high pressure and shear rate. It is shown that the film-forming behaviour of a viscometrically well-characterised Newtonian lubricant continues to obey classical EHD theory at film thicknesses as low as 10 nm. The level of agreement is close enough for pressure-viscosity characteristics to be deduced directly from film thickness information through the application of EHD theory. For polymer-containing, non-Newtonian lubricants, a high shear viscosity can be determined from film thickness data if pressure-viscosity characteristics are first established. A suitably structured sequence of experiments is described which yield estimates of both properties. To help interpret the viscometric information thus obtained, inlet zone viscosities are compared both with conventionally determined high shear viscosities and with the values predicted by a model of polymer solution behaviour.