Knowledge

Robust, chromium, semi-reflective coatings have been applied to transparent polymethylmethacrylate and polyurethane discs and this has enabled conventional, normal incidence optical interferometry to be used to measure lubricant film thickness in soft EHL conditions for the first time. High quality interferograms comparable to those obtained from coated glass discs are obtained. Measured film thickness has been compared with existing soft EHL film thickness equations obtained using computer modelling and revised central and minimum film thickness equations have been proposed. These film thickness measurements and measurement technique have applicability to our understanding of the performance and design of lubricated gears and bearings manufactured from polymeric materials.

The film thickness in elastohydrodynamic (EHD) conditions for soybean oil (SBO), oleic estolide ester (EST) and their binary blends with polyalphaolefins (PAO2 or PAO40) were studied at 30 and 100 °C. Changes with time, for up to 200 min, were monitored. SBO and its blends with the lower viscosity PAO2 showed initially good agreement with the Hamrock–Dowson (H–D) equation down to 1–3 nm film thickness. 60 min or more after the start of the measurements, boundary layers with thickness up to 4.7 nm were observed. The blend of SBO with the more viscous PAO40 showed initially a good agreement with H–D at 100 °C. Negative deviations in film thickness were observed 15 min after the start of the measurements. At extended periods of time, up to 200 min, they were less pronounced but still detectable. EST–PAO2 blend showed initially formation of boundary layers with thickness around 2 nm. The boundary layer at 30 °C did not change for 200 min, while at 100 °C showed a decrease in thickness and/or viscosity with time. The EST and the EST–PAO40 blends showed good agreement with the H–D equation and did not display a boundary or fractionation layer within 200 min.

Boundary film formation of ionic liquid (IL) 1-hexyl-3-methylimidazolium tetrafluoroborate, [HMIM][BF₄], as an additive of hydrocracked mineral oil is evaluated for a steel–steel contact. Accelerated wear testing was carried out using a high frequency reciprocating rig (HFRR) under these test conditions: maximum contact pressure of 1.04 GPa, two different temperatures (40 and 100 °C) and three different times (300, 1800 and 3600 s). Wear volumes were measured using a non-contact 3D profilometer while worn surfaces were characterized using XPS. Furthermore, electrical contact resistance (ECR) was used as qualitative indicator of the formation of electrically insulating films in the sliding contact.

Experiments show that the rate of boundary film formation of base oil-ionic liquid blend is faster than neat base oil. Moreover, ECR was in good agreement with film formation and friction behaviour. Ionic liquid as additive not only decreases the time of running-in but also the time of wear-in. Results of neat base oil show that wear-in was not reached during any duration of tests. The improved friction and wear results for the blend are closely related to the boundary film formation on the worn surfaces due to the reactivity of the anion with the steel surfaces.

This work shows the influence of solid–liquid interactions between engineering surfaces (steel and several types of DLC coatings) and lubricating oil (polyalphaolefin, PAO) on the coefficient of friction in the elastohydrodynamic lubrication (EHL) regime. Specifically, it confirms that the spreading parameter, rather than the contact angle, is the relevant parameter to evaluate the wetting behaviour of these surfaces with oils. Both the spreading parameter and the surface energy correlate very well with the friction in the EHL regime and can predict its behaviour. In particular, the polar component of the surface energy was found to correlate almost perfectly with the friction behaviour (a Pearson’s linear correlation coefficient of 0.999). By tailoring the wetting and surface energy—achieved by varying the DLC/DLC contacts with different types of DLC coatings—the coefficient of friction in the EHL regime was reduced by more than 30 % compared to steel/steel contacts. Poor wetting of the DLC coatings with a low surface energy is reflected in low values of the spreading parameter, which indicates easier slip of the lubricant over the solid surface due to shear action, and this leads to a lower viscous friction. A “Slip-inducing interaction model based on surface forces” is presented to explain why oil slip is promoted, particularly at surfaces with a low polar surface energy. The model suggests that a small number of permanent polar interactions, i.e. a larger proportion of intermittent dispersive interactions, results in less adhesive interactions between the predominantly non-polar liquid (oil) and the low polar surface (DLC), which enables easier slip at the solid–liquid interface.

Emulsions, consisting of a small volume of oil dispersed in water in the form of small particles, are popular lubricants for metal rolling and some machine design applications. A number of mechanisms have been suggested for the lubricating behavior of emulsions, among which plate-out, starvation, and dynamic concentration are of particular interest here. At low speeds, the emulsion provides essentially the same lubricating ability as neat oil for a point contact, consistent with plate-out. At some critical speed, the emulsion behavior departs from the neat oil, associated with starvation of the inlet zone. At a second critical speed, dynamic concentration becomes the important mechanism. This article measures the film thickness and traction coefficients of oil-in-water emulsions in the different regimes of behavior and compares the results to existing theoretical understanding. The effect of droplet size is isolated as a causative element in fluid film formation.

This paper describes a study of point contact elastohydrodynamic (EHD) lubrication behavior at high speeds (up to 20 m s⁻¹). Central film thicknesses were measured by optical interferometry device. The influence of slide-roll ratio and operating temperature on the central film thickness was determined. The influence of thermal effects on the reduction of film thickness was discussed via the analysis of numerical simulation method considering thermal effects. Subsequently, the experimental data was used to amend a set of unified parameters for the thermal corrections for different types of oil at high speeds.

The essential functions of an automotive gear lubricant viscosity modifier (VM) are to maintain fluid film protection of gears and bearings as the lubricant warms to operating temperature, to improve cold temperature flow for efficient lubrication in winter and to minimise viscosity loss in a high shear, high load environment. Although a number of different VM technologies can be considered appropriately resistant to permanent shear for automotive gear oils use, their effect on fluid efficiency can vary widely. This paper outlines the study of a series of different VM technologies assessing relationship of operating temperature, operating viscosity and axle efficiency under different load and speed regimes. The fluids presented were formulated to equal kinematic viscosity at 100 °C but vary widely in viscosity index (VI), elastohydrodynamic (EHD) traction and EHD film thickness. The differences observed during efficiency testing were qualitatively related to the rheological properties of the VM technology present and further related to the operating temperature and operating viscosity.

Fire resistant hydraulic fluids tend to show significantly poorer tribological performance in hydraulic systems than conventional mineral oil-based fluids. There have recently been performance problems associated with increases of operating temperatures of mining hydraulics. This paper describes measurements of the elastohydrodynamic and boundary film-forming properties of a range of different hydraulic fluid types at temperatures up to 80°C. These are compared with friction and wear results obtained using the same fluids.

We present a detailed experimental investigation on viscoelastic rolling contacts. The tests focus on contact area, penetration and viscoelastic dissipation measurements between a nitrile rubber ball rolling on a glass disc. Each of the measured parameters is shown to be dependent on the rolling speed and normal load and has, therefore, been used to assess the main differences between viscoelastic and linear elastic rolling contacts. Experimental outcomes are compared with numerical predictions of the theory proposed by Carbone and Putignano (J Mech Phys Solid, 2013). A good agreement is found between experiments and theoretical predictions, thus demonstrating the validity of the numerical approach. This has important implications for modelling the behaviour of real viscoelastic materials, whose response is characterised by a wide distribution of relaxation times. The presented methodologies and results can be applied directly or are of relevance to a number of engineering components, such as tires and seals.

Keywords: Viscoelastic solids, Rolling friction, Contact mechanic

DOI Link: 10.1007/s11249-013-0151-9