Knowledge

In the bearings’ segment of machine tools, there is a strong demand for high-performance steel solutions. The bearings may operate under severe conditions of contact pressures of up to 3 GPa; rotating at speed factors in excess of 3 million ndm. Such conditions pose a high risk of bearing seizure failure. Improving lubrication conditions is complex as the bearing operating temperature must be well controlled. Adhesive wear was found to occur in hybrid steel-ceramic contacts. Another relevant failure mode is micropitting. It is demonstrated that macroscopic hardness is insufficient to predict the resistance of steel microstructures to surface-initiated fatigue. In this regard, strain-hardening and the breadth of the range of hardness values of microstructure phases play an important role.

Nowadays, grease lubrication is frequently used in rolling element applications such as bearings or constant velocity joints. The advantage of grease is to supply lubrication to the application without leaking thanks to its consistency. Nevertheless, starvation can occur leading to damage such as scuffing.
In the present study, starvation is analyzed using the Starvation Degree parameter through tests with different operating conditions and different types of greases.

There is a high demand for environmentally friendly lubricants in order to support a transition to sustainable transport and manufacturing since conventional mineral oils derived from fossil sources are inherently harmful for the environment. Glycerol aqueous solutions have the potential to be used as environmentally friendly base fluids, due to their high solubility in water, and non-toxicity. In this investigation a micropitting test rig (MPR), was used to study the friction, wear and micropitting behaviour of Glycerol-based lubricants in a rolling/sliding contact. Micropitting and wear profiles were analysed through optical profilometry, and the morphology and evolution of micropits were studied trough scanning electron microscopy (SEM). The results showed that the steel-steel contact lubricated with a Glycerol-water-glycol lubricant reduced mild-wear, promoting micro-pitting as a main failure mode at low sliding levels compared to a commercial fully formulated gear oil. It was also shown that friction was significantly lower for the Glycerol-water and Glycerol-water-glycol lubricants which is mainly attributed to an effect of a low pressure-viscosity coefficient.

In this work the active control of friction was explored using various methodologies employing quartz crystal microbalance (QCM) as the primary technique. QCM was used to examine the friction of both dry and lubricated contacts in boundary and hydrodynamic lubrication regimes. The hydrodynamic lubrication regime was explored applying fields perpendicular to a QCM immersed in nanoparticle and molecular suspensions. These studies established that QCM is capable of monitoring how friction can be tuned at a solid-liquid interface in a lubricant with charged constituents in suspension. In particular, an electric field was perpendicularly applied across two Pt QCMs submerged in nanoparticle (NP) suspensions, where the NPs studied were 30 nm diameter TiO2 , 5 nm diameter TiO2 , and 30 nm diameterAl2O3 . It was found that the fields observably pushed the NPs towards or away from the QCM surface and had a measurable effect on the friction of the system. Additionally, the electrochemical properties of these systems were examined using two- and three-probe cyclic voltammetry. It was observed that the 30 nm TiO2 suspension showed electrochemical properties consistent with reversible electrophoretic deposition.

This work critically scrutinizes and compares the tribological performance of randomly distributed surface pores in sintered materials and precisely tailored laser textures produced by different laser surface texturing techniques. The pore distributions and dimensions were modified by changing the sintering parameters, while the topological features of the laser textures were varied by changing the laser sources and structuring parameters. Ball-on-disc tribological experiments were carried out under lubricated combined sliding-rolling conditions. Film thickness was measured in-situ through a specific interferometry technique developed for the study of rough surfaces. Furthermore, a machine learning approach based on the radial basis function method was proposed to predict the frictional behavior of contact interfaces with surface irregularities. The main results show that both sintered and laser textured materials can reduce friction compared to the untextured material under certain operating conditions. Moreover, the machine learning model was shown to predict results with satisfactory accuracy. It was also found that the performance of sintered materials could lead to similar improvements as achieved by textured surfaces, even if surface pores are randomly distributed and not precisely controlled.

The tribological properties of ionic liquids (ILs) have been widely investigated, and their excellent performance has been acknowledged by many researchers. However, their lubricity is closely linked to operating conditions. For wider applications, the complexity of their behavior must be understood. In this study, we compared the tribological properties of ILs at temperatures of 30 °C and 80 °C. Two ammonium ILs with different cation side-chain lengths, methyl-trioctylammonium [N1888] and tributylmethylammonium [N4441] and the same anion bis(trifluoromethylsulfonyl)imide [NTf2] were selected. Their tribological properties were investigated in the lubrication of three different friction pairs: bearing steel–bearing steel E 52100, bearing steel–stainless steel 304, and bearing steel–aluminum alloy 6082 T6. Tribotests were performed using a ball-on-plate reciprocating tribometer for 30 min at 4 N, 15 Hz, and a 1 mm stroke length. The worn surfaces were analyzed using optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results indicated diminished lubricity at higher test temperatures. The performance difference between the investigated temperatures depended on the friction pair, being almost negligible in bearing steel–bearing steel friction pairs and significantly different for bearing steel–aluminum friction pairs. The wear reduction ability was more sensitive to the test temperature than friction did. The investigated ILs were superior to the 5W–40 motor oil at both temperatures.

Zinc dialkyldithiophosphate (ZDDP) is added to engine lubricants to reduce wear and ensure reliable operation. ZDDP reacts under rubbing conditions to form protective zinc/iron phosphate tribofilms on steel surfaces. Recently, it has been demonstrated that this process can be promoted by applied stresses in lubricated contacts, as well as temperature, and is thus mechanochemical in origin. In this study, a tribology test rig, capable of applying very high loads, has been developed to generate ZDDP tribofilms under full-film elastohydrodynamic lubrication (EHL) conditions in steel/steel ball-on-disk contacts. This provides a well-defined temperature and stress environment with negligible direct asperity contact in which to study mechanochemical processes. ZDDPs with branched primary and secondary alkyl substituents have been studied in three base oils, two with high EHL friction and one with low EHL friction. In the high EHL friction base oils, the tribofilm growth rate increases exponentially with shear stress and temperature for both ZDDPs, as predicted by a stress-augmented thermal activation model. Conversely, under otherwise identical conditions, negligible ZDDP tribofilm formation takes place in the low EHL friction base oil. This confirms that the ZDDP reaction is driven by macroscopic shear stress rather than hydrostatic pressure. The secondary ZDDP forms tribofilms considerably faster than the primary ZDDP under equivalent conditions, suggesting that the initial decomposition reaction is the rate-determining step for tribofilm formation. The rate of tribofilm growth is independent of ZDDP concentration over the range studied, indicating that this process follows zero-order kinetics. Under full-film EHL conditions, ZDDP tribofilm formation is promoted by macroscopic shear stress applied through the base oil molecules, which induces asymmetric stress on adsorbed ZDDP molecules to promote their decomposition and initiate rapid phosphate polymerization.

This work investigates the effects of kinematic conditions, configuration, viscoelasticity, and lubricant viscosity on friction in lubricated compliant contacts. Experimental data were also used to develop a numerical simulation capable of predicting fluid friction in compliant contacts. Mini Traction Machine (MTM) in the ball-on-disc configuration was used to successfully gain insight into the behaviour of compliant contacts, allowing the investigation of the mentioned effects. The findings have confirmed that viscoelastic effects are present in all configurations, being soft-on-hard (S/H), hard-on-soft (H/S) and soft-on-soft (S/S), where they seem to be more profound in the configurations using compliant discs. The experimental data also suggest that the slide-to-roll ratio affects rolling friction in all configurations which is contrary to current literature.

There is a push toward using biobased lubricants due to their lower ecological impact. Unfortunately, despite some favorable properties, lipids have drawbacks, most notably oxidative stability and cold-flow properties, that hamper their use as high-quality lubricants. To overcome the drawbacks, researchers seek appropriate chemical modifications. In this chapter, we discuss hydrophosphonylation of lipids for obtaining better biobased lubricant – reaction chemistry, analysis, and tribological properties of the product. The synthesized lipid phosphonates, especially from di-n-butyl phosphite, have better oxidative stability, good cold flow properties, low compressibility (high bulk modulus), and low traction. While the viscosity index is worsened (lowered), it is still acceptable. The phosphonates also showed good wear reduction in a four-ball tester, when used as additive. These properties indicate that phosphonates from lipids have potential as biobased lubricants, and especially as hydraulic fluids.