Knowledge

The lubricant solution used in tribological testing of total knee replacements must have a 20 g/L (ISO 14243-3:2014) overall value of protein concentration; bovine serums (BSs) are currently used as base fluids to prepare such solution. BSs normally have a higher concentration of bovine serum albumin (BSA) and minor concentration of globulins (α, β and γ). It has been observed that these proteins can be degraded and adsorbed onto the surface of the materials during tribological tests. Therefore, this research is aimed at analyzing the proteins of denatured lubricant BSA-based solutions having a protein concentration of 20 g/L, after 6 h of tribological testing under different conditions. The testing tribological parameters considered were the applied load (L), sliding-to-rolling ratio (SRR), and entrainment speed (V_m). Four combination of parameters were set: (a) 8N-18-80 mm/s, (b) 2N-18-80 mm/s, (c) 8N-0.2-20 mm/s, and (d) 2N-0.2-20 mm/s. All tests carried out at 37 °C. Samples of the solutions were analyzed before and after testing by UV-Vis spectroscopy (range 200-300 nm), Bradford and Ellman methods. The UV-Vis spectroscopy analyses allowed to determine possible change suffered of the aromatic residues (residues exposed or covered cause an increase or decrease of absorbance). Bradford analysis of solutions indicated similar content of proteins. Therefore, basic amino acid residues in proteins interact and reorganize. Moreover, disulfide bonds involved in tertiary structure of protein remains stable. Finally, COF (Coefficient friction) is dependent on both, protein state as well as the tribological parameters.

A research effort was conducted to investigate wear mechanisms of a sintered tribofilm formed by a zirconia nanoparticle antiwear additive. Spherical five nanometer diameter zirconium oxide (ZrO₂) nanoparticles were dispersed in polyalphaolefin (PAO) synthetic base oil and used to generate approximately 120 nm thick tribofilms on AISI 52100 steel counterfaces in a ball-on-disk tribometer under boundary lubrication conditions. The sintered tribofilms were subsequently worn using the same conditions without the nanoparticle additive. Semi-in situ measurements of tribofilm thickness were conducted with an optical interference-measuring device to observe tribofilm growth and subsequent tribofilm wear. A rapid initial wear process was observed, however, a thinner tribofilm provided enduring protection of the substrate steel for the 120 min test duration. Without nanoparticles to replenish the tribofilm, the wear was deconvoluted from growth. Comparatively, lack of a tribofilm would result in scuffing within 10 min when testing the unadditized PAO base oil. Scanning electron microscopy and profilometry suggest the tribofilms were smoothened by the wear process. Evidence of crack formation between pores of the tribofilm illustrate a wear mechanism for the tribofilms. The cracks were concentrated where the contact pressure was the largest. Mild abrasive wear appeared as minor furrows in the sliding direction throughout the full width of the tribofilm.

To reduce friction, especially under high-temperature conditions, an oil-soluble polymeric friction modifier (polymeric FM) with a methacrylate backbone and hydroxyl groups has been developed. It was designed to have high adsorption performance and to increase in size when dissolved in base oil as the temperature is increased. To investigate the temperature dependence of the structural and tribological characteristics of the adsorbed layer formed on a metal surface by the polymeric FM, neutron reflectometry measurements and nano-to-macro tribological tests were conducted. The measurements revealed that the polymeric FM adsorbed efficiently on a Cu surface and formed a 6.0-nm-thick adsorbed layer at 23 °C. This adsorbed layer was highly swollen, and its thickness became about three times larger at 100 °C. Nanoscale friction tests using an atomic force microscope showed that the swollen-state adsorbed polymeric FM layer exhibited low-friction and surface-protection performance at 100 °C. Macroscale friction tests revealed the tribological behaviour of an adsorbed polymeric FM layer in elastohydrodynamic lubrication and mixed lubrication regimes. At higher temperatures, the increase in shear resistance due to the effect of thin-film lubrication was suppressed by the weaker segment–segment interaction, causing the boundary contact of the adsorbed polymeric FM layer to have low frictional properties on both ball and disc surfaces. The low-friction mechanism of the adsorbed polymeric FM layer at higher temperatures was justified by associating the temperature with the layer thickness.

This paper reviews the structural and group compositions of three winter base diesel fuels and their influence on the low-temperature and lubricating properties of the fuels. It is shown that a high content of saturated hydrocarbons, primarily medium-molecular-mass n-alkanes, and arenes with a higher proportion of substitution worsens the low-temperature properties. A decrease in the proportion of medium-molecular-mass alkanes and even a slight increase in the content of bi- and polycyclic aromatic hydrocarbons degrades the lubricating properties of the fuel. The influence of the component composition of the diesel fuels on the effectiveness of anti-wear and depressor-dispersing additives was noted. A study of the compatibility of additives of different functionalities revealed that an anti-wear additive based on tallow-oil fatty acids did not affect the activity of a depressant-dispersing additive while the combined use of these additives slightly worsened the lubricating properties but did not shift this indicator beyond the established standards.

Two different lubricants containing zinc dialkyldithiophosphate (ZDDP) additive were tested in a rolling–sliding contact test rig (micropitting rig) at different relative humidities. The effect of relative humidity on the bulk properties (e.g., viscosity, water concentration, water saturation level) of the lubricants and their tribological performance (e.g., friction, wear, micropitting level) as well as the related tribochemistry was extensively explored. Relative humidity had a limited effect on the viscosity of the tested lubricants. However, the friction and micropitting level decreased, while the wear increased at higher relative humidity. This increased wear was attributed to a thinner tribofilm and shorter chain length of the polyphosphates derived from the ZDDP additive. Hydrolysis of the ZDDP additive occurred, and the polar water molecules limited the access of the ZDDP additive to the substrate. The different polarities of the two base oils (Ester, polyalphaolefin) also led to different tribological and tribochemical performance.

A phosphonium-phosphinate ionic liquid (IL) was studied as a lubricant additive for rolling-sliding contacts. The bench-scale test was designed to simulate automotive rear axle operation during cold start, highway towing, and overload conditions. Adding such an IL (2%) into a base oil significantly reduced wear loss and rolling contact fatigue, e.g., microcracking and micropitting, but made the vibrational noise notably higher under a low (−1.5%) sliding roll ratio (SRR). Worn surface characterization revealed an interesting texture pattern with alternating smoother plateaus and rougher valleys, which is believed to cause the high vibration. No increased vibration was observed at a high (−30%) SRR, possibly because the more aggressive sliding abrasion prevented such a surface texture from forming.

Tribological performance of an organic friction modifier (glycerol monooleate – GMO), an anti-wear additive (zinc dialkyldithiophosphate – ZDDP) and its combination was studied using a ball-on-disc tribometer equipped with optical interferometry at 90 °C and 140 °C. Higher temperatures were needed for the formation of additive layers and chemically reactive anti-wear protective layers. Comparison of Stribeck curves obtained after different rubbing durations showed that GMO reduced friction at low speed sliding-rolling contact. Despite notable decrease in the base oil viscosity with increase in temperature — in turn, increasing the severity of asperity contact — GMO exhibited enhanced frictional performance. The presence of ZDDP alone in the formulation led to significant increase in friction at both low and high temperatures, with the thickness of tribofilm unaffected after prolonged rubbing. The addition of friction modifier to ZDDP-based formulation reduced the friction in the boundary lubrication regime where longer rubbing aided in effective friction reduction at higher temperature. For the binary additive system, the friction coefficient was found to lie between the values corresponding to the single additives. The observed tribological response is attributed to preferential adsorption of the friction modifier over ZDDP and/or ZDDP decomposition products. This notion was corroborated by the results obtained from static and dynamic time-of-flight secondary ion mass spectrometry (ToF-SIMS) and energy-dispersive X-ray spectroscopy (EDX) analysis of the disc surface post tribological measurement. The concentration gradient of different chemical species detected in the tribofilm correlated with the frictional performance of the additives. Analysis of surface roughness and wear scar width showed an improvement in wear performance at higher temperature suggesting the friction modifier and anti-wear additive adsorbed on the surface providing a mechanical barrier. A synergistic effect on the wear performance was observed for GMO + ZDDP-based formulations, which resulted in a smaller wear scar compared to the individual additives.

The performance of lubricant additives, such as organic friction modifiers (OFMs), depends critically on their ability to adsorb onto the surfaces of moving components and form protective self-assembled layers (SAMs). Therefore, understanding the relationship between the concentration of the additive in the base oil and the resulting surface coverage is extremely important for lubricant formulations, as well as many other surfactant applications. Here, we use molecular dynamics (MD) simulations to study the adsorption isotherms of three different OFMs, stearic acid (SA), glycerol monoostearate (GMS), and glycerol monooleate (GMO), onto a hematite surface from hydrocarbon solvents, n-hexadecane and poly-α-olefin (PAO). First, we calculate the potential of mean force (PMF) of the adsorption process using MD simulations with the adaptive biasing force (ABF) algorithm. Our MD simulations show that SA has the weakest adsorption energy on hematite, followed by GMS, and finally GMO, due to the increasing number of functional groups available to bind to the surface. We also estimate the area occupied by each OFM molecule on the surface in the high-coverage limit using MD simulations of the annealing of OFM films with different initial surface coverages. We obtain a similar hard-disk area for GMS and GMO, but a lower value for SA, which is due to its smaller headgroup size. Based on the adsorption energy and surface area, we determine the corresponding adsorption isotherms using the molecular thermodynamic theory (MTT), which agree well with one available experimental data-set for SA. Two other experimental data-sets for SA require lateral interactions between surfactant molecules to be accounted for. SA forms monolayers with lower surface coverage than GMO and GMS at low concentrations (due to a smaller adsorption energy), but also has the highest plateau coverage (due to a smaller hard-disk area). We validate the adsorption energies from the MD simulations using high frequency reciprocating rig (HFRR) friction experiments with different concentrations of the OFMs in PAO. We use the Jahanmir and Beltzer model to estimate the surface coverage at each concentration and the adsorption energy of each OFM from the HFRR friction data. For OFMs with saturated tailgroups (SA and GMS), we obtain good agreement between the predictions made by the simulations and the experiments. The MD simulation and experimental results deviate for OFMs containing Z-unsaturated tailgroups (GMO), with the former suggesting stronger adsorption for GMO than GMS, while the latter predicts the opposite trend. We suggest that this is can be attributed to the higher steric barrier of adsorption of the OFMs with kinked Z-unsaturated tailgroup through a partially formed monolayer, an aspect which was not captured in the current simulations. This study demonstrates that MD simulations with the ABF algorithm, alongside MTT, are an accurate and efficient tool to predict adsorption isotherms at solid-liquid interfaces.

This study explored the feasibility of using a phosphonium phosphate ionic liquid as a candidate anti-wear and anti-pitting additive for rear axle lubricant. This particular IL was first added to a VHVI8 base oil at 2–3% concentration and demonstrated effective surface protection for wear and micro-cracking under rolling-sliding contacts. The promising results directed to a step further to produce a series of IL-containing low-viscosity (about a half of SAE 75W-90) fully formulated gear oils. Selected IL-containing experimental oils showed superior mitigation of rolling contact fatigue to a commercial SAE 75W-90 gear oil in bench-scale rolling-sliding tests. Full-scale hub dynamometer tests were then conducted and demonstrated more than 3% power output and torque generation for an IL-containing low-viscosity gear oil benchmarked against commercial baselines.