Knowledge

Optical interferometry has proved to be a valuable experimental tool in the study of elastohydrodynamic lubrication (EHD). It is a technique that gives detailed information on the lubricant film distribution within the contact; however, the sensitivity is limited and it is only recently, with the development of the spacer layer optical technique, that the study of the thin film lubrication regime has been possible. The limitation of the spacer layer technique is that generally only one measurement is taken from the center of the contact. The next logical step in the development of this technique is, therefore, a system that combines the mapping capabilities of the original optical method with the thin film capabilities of the spacer layer approach.

This paper describes the development of a contact mapping technique that uses the spacer layer approach to visualize, and measure, thin lubricant films in concentrated contacts. The development of the technique is described and its application to both static and moving contacts reported. Thin EHD films (down to 10 nm) have been measured and mapped.

Keywords: Elastohydrodynamic Lubrication, Optical Interferometry, Surface Films/Coatings

DOI Link: https://doi.org/10.1080/10402009608983612

The elastohydrodynamic traction coefficient (tc) properties of nine seed oils of varying chemical structures, polyalphaolefin oil (PAO) and hexadecane, were investigated using a ball-on-disk traction apparatus. The seed oils were castor oil, a triglyceride with hydroxyl functional group; jojoba, a monoglyceride; and seven triglyceride seed oils with varying fatty acid compositions. Two types of experiments were conducted at constant temperature (40 or 100°C) and constant load (10, 20, 30, or 40 N): tc as a function of slide-to-roll ratio (srr) at 1 m/s entrainment speed (u); and tc as a function of u at 50% srr. In both types of experiments, tc increased with decreasing temperature, increasing load, and increasing srr. All u vs. tc experiments gave the familiar Stribeck-type profiles. A maximum in tc values was observed in some srr vs. tc experiments. Regression analysis showed excellent agreement between limiting tc (tc at 1 m/s u and 50% srr) values from these two types of experiments. Hexadecane and PAO displayed higher tc values than the seed oils, even though their viscosities were up to 80 and 7 times lower, respectively, than that of seed oils. This observation cannot be rationalized using molecular structure arguments. The results were attributed to differences in polarity between the two groups of oils. Unlike PAO and hexadecane, seed oils are polar, adsorb on friction surfaces, and lower boundary friction, which contributes to the lowering of tc in the EHD regime.

Keywords: Castor Oil, Jojoba, Hexadecane, Polyalphaolefins, Seed Oils, Slide-to-Roll Ratio, Limiting Traction Coefficient, Elastohydrodynamic Traction Coefficient
DOI Link: https://doi.org/10.1080/10402001003599769

Non-additized batches of lubricating greases were tribologically characterized through traction coefficient and Stribeck curves at different operating conditions on a ball-on-disc test rig, ensuring that the contact was fully flooded. The tests were performed at constant load, but different operating temperatures while varying the slide-to roll ratio (SRR) or the rolling entrainment speed.

The results were correlated to the greases׳ formulation in terms of base oil viscosity, thickener and/or elastomer content. A relationship between the coefficient of friction (COF) of greases formulated with different thickener content was found and the thickener influence on the COF was addressed.

Keywords: Polymer greases, Coefficient of friction, Stribeck curves

DOI Link: https://doi.org/10.1016/j.triboint.2015.09.027

The viscosity of liquids governs crucial physical and engineering phenomena, ranging from diffusion and transport processes of nutrients and chemicals, to the generation of friction and the physics of damping. Engineering fluids frequently experience local conditions that change their bulk rheological properties. While viscosity data can easily be acquired using conventional rheometers, the results are not always applicable to fluids under engineering conditions. This is particularly the case for fluids being sheared at high pressure under severe confinement, which experience very high shear stresses and often show extensive shear thinning. There is a lack of suitable methods for measuring fluid viscosity under such conditions. This work describes a novel in situ viscosity measurement technique to fill this gap. It involves the quantification of the fluorescence lifetime of a fluorescent dye that is sensitive to viscosity. The capability of the developed technique is verified by taking measurements in submicron thick films of two model fluids confined in a ball on flat contact. Viscosity measurements were successfully performed at pressures up to 1.2 GPa and shear rates up to 10⁵ s⁻¹. Spatial heterogeneity in viscosity caused by variations in pressure within the thin fluid film could be observed using the technique. It was also possible to detect differences in the rheological responses of a Newtonian and a non-Newtonian fluid. These first in situ high pressure, high shear viscosity measurements demonstrate the versatility of the proposed technique in providing information on the viscosity in conditions where contemporary techniques are insufficient. More importantly it highlights the complexity of the rheology of engineering fluids and provides a means of verifying existing theories by performing in situ measurements. Information on local viscosity is crucial for understanding the physics of confined fluids and to facilitate improvements in engineering technology.

Keywords: High pressure, EHD, Shear stress
DOI Link: https://doi.org/10.1039/C5RA19245E

The film thickness and the traction curves of four fully formulated wind turbine gear oils were measured on a ball-on-disc device. All oils have the same viscosity grade (ISO VG 320) and different formulations: ester, mineral, PAO and mineral+PAMA.

Film thickness and traction coefficient results will be presented. The film thickness measurements were compared with predictions using film thickness equations from the literature.

Keywords: Wind turbine gear oils, Film thickness, Coefficient of friction, Stribeck curve
DOI Link: https://doi.org/10.1016/j.triboint.2015.01.014

In this work, the ability of ashless anti-wear additives to form protective tribofilms on diamond-like carbon (DLC) and steel surfaces was investigated and compared to the reactions of ZnDTP. Reciprocating sliding tests were performed under mild tribological conditions for steel/steel and DLC/DLC contacts to avoid wearing through the DLC coating. A comparison of the friction behavior of ZnDTP with two ashless additives, a butylated triphenyl phosphorothionate (b-TPPT) and an amine phosphate (AP), indicated that the latter additive behaved in a different manner to the first two. b-TPPT shows the lowest friction coefficient for DLC/DLC contacts whereas the AP give the lowest friction coefficient for steel sliding against steel. AFM and ESEM were performed to analyze the surfaces and showed the build up of tribofilms from ZnDTP and b-TPPT onto both the steel and DLC surfaces. No evidence of such a film formation on DLC was observed when the AP were used.

Keywords: DLC, ZnDTP, Ashless additives, Amine phosphate, Phosphorothionate, Reciprocating sliding

DOI Link: https://doi.org/10.1016/j.triboint.2008.03.004

Increasing use of DLC coatings in oil-lubricated, mechanical systems has led to a great deal of interest in optimising the interactions between lubricant additives and coatings. We have looked at the formation of a tribofilm from ZnDTP on diamond-like carbon (DLC) when sliding in oil against DLC on a polished steel substrate under very low wear conditions. It was found that the tribofilm formed in the absence of iron adhered very weakly to the surface and could be removed by washing in an ultrasonic bath. The wear was too low to be detected with laser profilometry. AFM imaging of the surfaces of steel and DLC after sliding of steel/steel and DLC/DLC showed that the tribofilm formed on DLC had a very different morphology to that formed on steel. EDX was used to demonstrate that the tribofilm contained zinc, sulphur and phosphorus. ToF-SIMS measurements showed that iron was neither present at the DLC surface nor in the tribofilm.

Keywords: Diamond-like carbon, ZnDTP, Friction, Tribofilm, Tribochemistry

DOI Link: https://doi.org/10.1016/j.wear.2007.03.012

Methyl and ethyl monoalkyl esters of various vegetable oils were produced for determining the effects of type of alcohol and fatty acid profile of the vegetable oil on the lubricity of the ester. Four methyl esters and six ethyl esters were analyzed for wear properties using the American Society for Testing and Materials method D 6079, Evaluating Lubricity of Diesel Fuels by the High-Frequency Reciprocating Rig. Ethyl esters showed noticeable improvement compared to methyl esters in the wear properties of each ester tested. No correlation was found between lubricity improvement and fatty acid profile of the ester, except that esters of castor oil had improved lubricity over other oils with similar carbon chain-length (C₁₈) fatty acids.

Keywords: Biodiesel fuel, engine wear, fatty acid, lubricity, monoalkyl esters, vegetable oil
DOI Link: https://doi.org/10.1007/s11746-001-0307-y

This paper studies the use of 3 ionic liquids ([(NEMM)MOE][FAP], [BMP][FAP] and [BMP][NTf₂]) as neat lubricant within steel–steel contact conditions. Tribological tests (at 40 and 100 °C) were conducted in a HFRR tribometer and hence a complementary study was developed using a MTM tribometer. The wear surface on the discs was measured after the HFRR tests by confocal microscopy and also analyzed by SEM and XPS. The [BMP][NTf₂] showed the lowest friction coefficient in the MTM and HFRR tests at 40 °C but at 100 °C its tribological behavior worsened due to its lowest viscosity. Similar results were found for wear behavior. Both antifriction and antiwear results were related to the tribofilms formation from the ECR and XPS measurements.

Keywords: Ionic liquids, Lubrication, Wear, Friction coefficient
DOI Link: https://doi.org/10.1016/j.triboint.2013.12.007