Ionic Liquid Additives in Water-Based Lubricants for Bearing Steel – Effect of Electrical Conductivity and pH on Surface Chemistry, Friction and Wear

Water-based lubricants have the potential to become the largest environmentally friendly lubricants in applications such as electric vehicles and the newly emerging green technologies of the future due to their inherent low viscosity and cooling properties. In order to be environmentally acceptable (EAL), both base lubricants and additives should comply with biodegradability, non-toxicity, and non-bioaccumulation requirements. Additives for water-based lubricants should ideally be polar and soluble in water and, at the same time, should not increase the electrical conductivity to critical levels for corrosion. However, most additives used in synthetic or mineral oils are non-polar. Ionic liquids have recently gained attention as lubricant additives due to their high polarity, making them highly surface-active (i.e. high tendency to adsorb on metal surfaces). However, they are seen as highly corrosive for many metal alloys. In this work, a water-glycol lubricant containing two different ionic liquids has been investigated as a potential green lubricant for a bearing steel AISI 52100 with accurate control on electrical conductivity and pH. The selected ionic liquids were tributylmethylphosphonium dimethylphosphate (PP) and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate (BMP). The tribological behaviour of the ionic liquids was compared with a well-known organic friction modifier, dodecanoic acid (C12). The ionic liquids showed lower friction and wear rate than the water-based lubricant alone. However, they showed higher friction than the lubricant formulated with C12, in which PP gave lower friction than BMP due to low pH. A detailed subsurface analysis of the wear track using scanning-transmission electron microscopy (STEM) showed that a thick oxide tribofilm was built on the wear track for both lubricants formulated with ionic liquids due to high electrical conductivity. This tribofilm gave beneficial effect on wear. Although PP and BMP gave thicker tribofilms than C12, it was not durable, resulting in cracking and detachment.

Review of automotive brake lining materials and their tribological properties

Brake lining material properties are considered an essential aspect of the safe operation of vehicles. The presented paper aims to present a comprehensive review of brake lining materials and their tribological properties including the proposed materials, their advantages and disadvantages, analysis of results, and test rig experiments. The first section of the paper includes brief information on brake lining materials, friction modifier additives, and recent developments in friction additives. The second section discusses the tribological performance of brake lining materials. Furthermore, the key mechanisms of the friction layer produced on the worn brake lining surface have been explained microscopically (HRTEM). Part of this review is devoted to demonstrating current research gaps and challenges related to brake performance in automotive for further research. In brief, this review study is highly significant, as it provides more detailed information regarding the performance of brake lining materials.

Research and development of commercially viable lubricants to intensify working life of marine diesel engines and cylinder piston group in internal combustion engines

The paper presents the results of studying the influence of variable characteristics of lubricating oils for marine diesel engines (concentration of layered friction modifier in lubricating oil, viscosity of lubricating oil, contact pressure in the friction zone) on the tribological parameters of parts of the cylinder-piston group of marine diesel engines. There are considered the aspects of increasing the reliability and wear resistance of the cylinder-piston group of marine diesel engines when a layered friction modifier is added to the base lubricating oil in a concentration of 1.5 vol.%. There have been carried out the comparative tribological studies of M-16G2CS lubricating oil including an additive based on molybdenum diselenide. Dependences of the wear of parts of the cylinder-piston group of a marine engine on different parameters of the studied lubricants are shown. The wear rate of experimental samples in conditions of variable characteristics of lubricants has been studied. According to the tribological studies of lubricants and structural materials, the tribological rating of lubricating compositions containing M16G2CS marine engine oil as a base and a layered friction modifier - molybdenum diselenide as a tribologically active additive was built. The test tool for the antiwear ability of lubricants is a friction machine of an original design with abraded samples according to the “ball-cylinder” contact scheme. Mechanical mixing of the lubricating medium of “oil + additive” type on the RPU-0.8-55A rotary-pulsating unit was one of the variable parameters in the tests. The tribological efficiency of the studied antiwear additive varies depending on the type of mixing of the additive solution before adding to the base lubricating oil and makes 13-54% (the difference in the diameter of the wear spot of the sample) for mechanical mixing, and for rotary-pulsation mixing - 45-56%.

The Effect of Additive Chemical Structure on the Tribofilms Derived from Varying Molybdenum-Sulfur Chemistries

Molybdenum disulfide (MoS2) is an effective friction modifier that can be formed on surfaces from oil-soluble lubricant additives. Different additive chemistries can be used to form MoS2 on a surface. The tribofilms formed from three different molybdenum additives (MoDTC Dimer, MoDTC Trimer, and molybdate ester) were studied in additive monoblends and fully formulated systems. The resulting tribofilms were then characterized by Raman spectroscopic spatial mapping, XPS, and FIB-TEM. The distribution of MoS2 on the surface was much more sparse for the molybdate ester than the other additives. No crystalline molybdenum oxides were observed by Raman spectroscopy, but their presence was inferred from XPS analysis. XPS analysis showed very similar distributions of Mo oxidation states from each additive, such that the chemical nature of the films formed from all of the additives is likely similar. Each of the additive tribofilms was observed to have MoS3 vibrations in Raman and persulfide XPS peaks associated with amorphous MoS3, as such this species is presented as a common frictional decomposition product for all the additives. The MoDTC trimer is more able to produce this amorphous species on the contacting surfaces due to its structural similarities to the co-ordination polymer MoS3. Graphical Abstract

Towards a Polymer-Brush-Based Friction Modifier for Oil

To meet the need for oil-compatible friction modifier additives that can significantly reduce energy consumption in the boundary-lubrication regime, a macromolecular design approach has been taken. The aim was to produce a lubricious polymer film on the sliding surfaces. A series of readily functionalizable block copolymers carrying an oleophilic poly(dodecyl methacrylate) block and a functionalizable poly(pentafluorophenyl methacrylate) block of various lengths was synthesized by means of reversible addition-fragmentation chain-transfer (RAFT) polymerization. The poly(pentafluorophenyl methacrylate) block was used to attach surface-active nitrocatechol anchoring groups to the polymer. The friction-reduction properties of these polymers were assessed with 0.5 wt% solutions in hexadecane by means of rolling-sliding macroscopic tribological tests. Block copolymers with roughly equal block lengths and moderate molecular weights were significantly more effective at friction reduction than all other architectures investigated. They also displayed lower friction coefficients than glycerol monooleate—a commercially used additive. The film-formation ability of these polymers was examined using a quartz-crystal microbalance with dissipation (QCM-D), by monitoring their adsorption onto an iron oxide-coated QCM crystal. The polymer with highest lubrication efficiency formed a thin film of ~ 17 nm thickness on the crystal, indicating the formation of a polymer brush. Interferometric rolling-sliding experiments with the same polymer showed a separating film thickness of ~ 20 nm, which is consistent with the QCM-D value, bearing in mind the compression of the adsorbed layers on the two sliding surfaces during tribological testing. Graphical Abstract

Effects of annealing treatment on tribological behavior of tungsten-doped diamond-like carbon film under lubrication (Part 2): Tribological behavior under MoDTC lubrication

Molybdenum dialkyldithiocarbamate (MoDTC) is widely used as a friction modifier in engine lubricating oil. Under MoDTC lubrication, the friction and wear behaviors of tungsten-doped diamond-like carbon (W-DLC) films annealed at 100–400 °C were discussed and evaluated using scanning electron microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy. Under (polymerized alpha olefin) PAO + MoDTC lubrication, the coefficient of friction of all samples decreased, but the wear rates of the W-DLC films annealed at 300 °C increased significantly. By interacting with zinc dialkyldithiophosphate (ZDDP), the wear rates of W-DLC films annealed at different temperatures declined significantly owing to the formation of dense phosphate tribofilms on the worn surfaces.

Gas Phase Lubrication Study with an Organic Friction Modifier

Friction modifier additive technologies play a crucial role in controlling friction and wear of lubricated tribological systems. Novel additives are usually evaluated using formulations of varying concentrations. It can be very difficult to understand the underlying mechanisms in those laboratory tests because of the interaction of base oil with the additives. It thus can be insightful to perform model experiments in a controllable atmosphere. This can be achieved for instance by integrating a tribometer into a vacuum system comprising in-situ surface analytical methods.In this work, a nitrogen containing organic friction modifier is adsorbed from the gas phase onto a Fe2O3 surface. Different coating thicknesses are prepared by varying the duration of the vapor deposition, so that the influence of the coating thickness on the friction behavior can be investigated. The chemical composition of the coated surfaces is also analyzed by coupling to an XPS photoelectron spectrometer.Contrary to the assumption that layers are formed, this friction modifier accumulates in droplets on the Fe2O3 surface. The number of droplets as well as the radii of droplets increase with evaporation time. The chemical composition of the additive does not change as a result of the gas phase deposition. In the friction tests, the smallest friction values are found for a very low coverage of droplets. For larger droplets, friction increases due to a capillary neck of additive that forms between the sliding surfaces and is dragged along during the friction test.Using gas phase adsorption of a nitrogen containing organic friction modifier it was possible to establish a correlation between droplet morphology and the friction behavior.

Effects of solid friction modifier on friction and rolling contact fatigue damage of wheel-rail surfaces

In railway network, friction is an important factor to consider in terms of the service behaviors of wheel-rail system. The objective of this study was to investigate the effect of a solid friction modifier (FM) in a railway environment. This was achieved by studying the friction, wear, and rolling contact fatigue (RCF) damage on the wheel-rail materials at different slip ratios. The results showed that when a solid FM was applied, the friction coefficient decreased. After the solid FM was separated from the wheel-rail interface, the friction coefficient gradually increased to its original level. With the application of the solid FM, the wear rates of the wheel-rail decreased. In addition, the thickness and hardness of the plastic deformation layers of the wheel-rail materials were reduced. The worn surfaces of the wheel-rail were dominated by pits and RCF cracks. Without the FM, RCF cracks ranged from 84 to 120 µm, and subsurface cracks were generated. However, with the FM, RCF cracks ranged from 17 to 97 µm and no subsurface cracks were generated. These findings indicate possible methods of improving the performance of railway rolling stock by managing friction, and reducing wear and permanent RCF damage affecting both the wheels and rails.