DIRECT OBSERVATION OF BOUNDARY-LAYERS

By using a combination of spacer layer optical interferometry and spec tral analysis, it has recently become possible to measure lubricant fi lms down to less than 2 nm thickness in high-pressure, rolling ball on flat contacts. This enables the presence and some of the properties o f boundary lubricating films to be observed directly for the first tim e in realistic contact conditions. This paper describes some recent wo rk using this approach. It is shown that highly refined, nonpolar base fluids exhibit classical hydrodynamic behavior down to less than 2 nm film thickness, whereas fluids such as esters give enhanced film thic kness in this very thin film region. Work on polymer solutions and bas e fluid mixtures suggests that the main way that such fluids form boun dary layers is by preferential adsorption on solid surfaces of polar m oieties from solution. This has the effect of changing the composition and thus effective viscosity of lubricant layers close to solid surfa ces, thereby leading to a hydrodynamic response different from that pr edicted from the bulk rheological properties of the blend.