Knowledge

This study aims to introduce the effects of applied voltages on steel components, with a particular focus on friction, tribofilm formation and wear and aid in the creation of lubricant test method guidelines for electrified systems.

A very detailed paper on a hugely important subject, tribofilm formation and activation. The term “mechanochemsity” ( the concept of using mechanical force to initiate chemical reactions ) aptly describes this research area, which intriguingly concludes that the shear stress experienced by lubricants primarily controls the rate of additive film formation. The paper presents clear MTM mapper experimental data which is then used to discuss, in depth why and when lubricant additive films form.
Open Access Link:
https://link.springer.com/article/10.1007/s11249-020-1281-5

Cutting edge MTM nanoparticle research carried out at the highly respected Carpick laboratory. The paper showed how relatively large nanoparticle of Zirconium dioxide (ZrO2), combined with organic capping ligands can form robust tribofilms under a wide range of contact conditions. Impressively significant   wear reduction was also reported when the nanoparticles were blended in commercial formulations.

Open Access Link:
https://link.springer.com/article/10.1007%2Fs11249-020-01346-1

There is great interest in using hydrogen as a gaseous fuel in combustion engines to eliminate CO₂ emissions.  Unfortunately, hydrogen gas is a poor lubricant for most engineering metals and an effective lubrication solution for pumping and injecting hydrogen is required. This study explores the possibility of additivating hydrogen with a low concentration of a lubricious gas to reduce friction and wear. We find that unsaturated hydrocarbon gas additives form protective carbon-based tribofilms, while gaseous ammonia and amine additives form nitrogen-based films on steel surfaces during rubbing in additivated hydrogen. Gaseous amines are particularly effective in reducing friction and wear, even at concentrations as low as 100 ppm mole/mole. This demonstrates that the addition of a small concentration of lubricious gas is a feasible way to improve the lubricity of gaseous hydrogen.

Keywords: Gas-phase Lubrication, Hydrogen, Tribofilm formation, Gaseous Additives

Many previous researchers have reported the formation of carbonaceous tribofilms from organic lubricants on rubbing metallic surfaces. This paper shows that a very important factor in the formation of such tribofilms is the presence or absence of molecular oxygen. When steel surfaces are rubbed in saturated hydrocarbon lubricants in the absence of oxygen, for example in nitrogen or hydrogen gas, carbonaceous films form very readily, resulting in low friction and wear. However, when a significant amount of oxygen is present, as is the case in air, carbonaceous tribofilms are not generally formed, so friction and wear are very high, with values comparable to those seen when no lubricant is present. In situ Raman analysis combined with gas-switching experiments show that the carbonaceous films formed during rubbing when no oxygen is present are rapidly removed during rubbing in air, while tests in which lubricant is removed during a test in N₂ indicate that the films are quite weak. This suggests that these carbonaceous films are being continually removed and replenished during rubbing in oxygen-free conditions. It is proposed that these carbonaceous films are formed from hydrocarbyl free radicals that are generated mechanochemically from hydrocarbon molecules during rubbing. In the absence of oxygen, these free radicals then react together to form a carbonaceous film. However, when oxygen is present, the hydrocarbyl free radicals react extremely rapidly with oxygen molecules to produce hydroperoxyl free radicals and so are no longer available to generate a carbonaceous tribofilm.

Keywords: Carbon film, Free radical, Propane, Hexacane, Isooctane, Oxygen, Tribooxidation, Lubricant Inerting

Hydrogen applications face sealing challenges. Often, these seals are made of PTFE and work under fretting conditions. Experiments in this study were conducted with PTFE against 52100 steel under varying humidity. Results show significant wear of PTFE in hydrogen, regardless of humidity, with PTFE transferring onto steel surfaces. FTIR and XPS analyses reveal that hydrogen's reducing effect suppresses surface metal oxide formation, inhibiting tribochemical reactions. However, when hydrogen is diluted with nitrogen, PTFE's tribological performance improves, though tribochemistry is not promoted. Only with oxygen and water vapour do tribochemical reactions occur in H₂, where metal fluoride is first formed, followed by carboxylate chelates. These findings highlight the critical role of hydrogen's reducing nature, offering insights for developing advanced sealing materials.

Keywords: PTFE, Sealing Materials, Hydrogen, Tribochemical Reaction, Fretting

Synthetic paraffinic kerosene (SPK) is an ultra-clean fuel with low aromatic content and negligible quantities of sulfur compounds. Although, SPK has a good potential to replace the conventional fuel Jet A-1, it also has some deficiencies. One of them is the low lubricity compared to its conventional counterpart Jet A-1. To improve the lubricity of SPK, three selected additives have been mixed with SPK at different concentrations. The lubricity of the samples was determined experimentally and the samples that meet the industry specifications have been studied further. The effect of the additives on the physicochemical properties, such as, density, flash point, freezing point, viscosity, and heat content, were investigated. Linoleic acid was found to be an excellent lubricity improver even at a very low concentration and its negative impact on the other physicochemical properties was found to be insignificant. Ethyl oleate also demonstrated significant improvement in lubricity at low concentrations but had a negative impact on the fuel’s freezing point at high concentrations. Quinoline, at high concentrations, elevated the blend’s freezing point above the acceptable limits. In parallel to the experimental campaign, a pre-existing mathematical modelling tool was utilized to predict the properties of interest. The lubricity model was successfully introduced into the mathematical model in order to improve the capabilities of the model. Linoleic acid sample showed the best improvement in lubricity of SPK with wear scar diameter of 417 μm; well below the ASTM D7566 maximum limit of 850 μm. The dual nature of this study facilitated the optimization of the physicochemical properties of the fuel samples.

The article studies the friction coefficient in elastohydrodynamic lubrication (EHL) by means of analytically obtained equations for different contact geometries. The introduction of some simplifications allows for the simultaneous consideration of piezoviscous, pseudoplastic and thermal phenomena, resulting in complete and realistic models, which provide results in a quick and easy manner. The predictive potential of this analytical approach is analyzed by comparing the estimates of friction with full-EHL simulations and experimental data under different operating conditions. The results obtained allow us to discuss the influence of some assumptions taken into account and the scope of applicability of the models, in order to determine their usefulness and limitations.

Keywords: elastohydrodynamic lubrication; friction; analytical; pressure-viscosity coefficient

Scuffing is becoming a quite common failure mode in gears and bearings. It has been shown that AW/EP additives are effective in preventing scuffing, but only if they are able to form a thick tribofilm before encountering severe scuffing-type conditions. This study has employed a contra-rotating, step-sliding speed scuffing test to explore the impact of PMAs on the ability of ZDDP and a commercial SP additive-containing package to prevent scuffing when subjected to immediately severe conditions. It is found that some PMAs can greatly enhance the anti-scuffing performance of these AW/EP additives. They do this by forming thick, adsorbed boundary films that can withstand high speed sliding conditions and protect the rubbing surfaces long enough for tribofilms to form.

Keywords: Scuffing, Tribofilm, PMA, ZDDP