Friction Reduction Capability Research Articles

Influence of Few-Layer Graphene on Frictional Properties of Lithium Compound Grease

The frictional properties of lithium compound grease (LCG) with different percentage compositions of few-layer graphene (FLG) were investigated, and the mechanisms of temperature and loading effects on LCG containing FLG are also considered. The concluding effect shows that 1 wt% FLG is more appropriate for friction and wear modifiers for lithium compound grease at elevated temperatures and less suitable at ordinary temperatures. Thickener chemisorption film, FLG layering film, and tribo-reaction film consisting of FeO(OH), Fe2O3, Fe3O4, Li2O, and other oxides assist in the establishment of a lubricating boundary film on the friction interfaces lubricated with LCG containing FLG. The poor fluidity of lithium compound grease at low temperatures leads to poor dispersion of FLG, decreasing friction reduction capability. Under elevated temperature and low load condition, adding 1wt% FLG to LCG can only improve its wear-resistant property, the abrasion volume of steel plate reduced by 24.49%. Under elevated temperature and high load condition, adding 1wt% FLG to LCG can only enhance its anti-friction characteristics.. Conversely, FLG is unsuitable as an anti-friction and wear-resistant additive for LCG at low-temperature conditions.

Tribological Investigation of Textured Surfaces in Starved Lubrication Conditions

The present work investigates the friction reduction capability of two types of micro-textures (grooves and dimples) created on steel surfaces using a vertical milling machine. The wear studies were conducted using a pin-on-disc tribometer, with the results indicating a better friction reduction capacity in the case of the dimple texture as compared to the grooved texture. The microscopic images of the pin surface revealed deep furrows and significant damage on the pin surfaces of the groove-textured disc. An optimization of the textured surfaces was performed using an artificial neural network (ANN) model, predicting the influence of the surface texture as a function of the load, depth of cut and distance between the micro-textures.

Enhanced tribological properties of wind turbine engine oil formulated with flower-shaped MoS2 nano-additives

Wind energy has become one of the most viable green and sustainable energy sources. A major concern of this sector is the damage of mechanical components and energy losses caused by repeated friction and wear, which lead to reduced efficiency and increased maintenance costs. The development of new lubricant additives with high anti-wear properties and friction reduction capabilities is of paramount importance to ensure long service life and high efficiency of mechanical devices. Herein, flower-shaped MoS2 nano-additives of about 150 nm were prepared under mild conditions through a hydrothermal process. Octyl- and octadecyltrichlorosilane were used as surface hydrophobization agents to enhance the colloidal stability of MoS2 nano-additives formulations in wind turbine engine oil. Increasing the length of the alkyl chain grafted onto the particles improved the colloidal stability up to at least one year and, more importantly, resulted in the lowest friction and best wear reduction capabilities, as shown by the pin-on-disk tribometer and laboratory wear tests.

Investigating the friction reduction capability of dimpled surface using CNSL as lubricant

This work investigates the friction reduction capability of dimple – textured steel surface created using a vertical milling machine. A suitable geometry for creation of dimples was chosen and cashew nut shell liquid (CNSL) was used as lubricant. A L9 orthogonal array was created using Taguchi design of experiment where load, speed of disc and duration of the test were chosen as the parameters. A pin – on – disc tribometer is used to find the coefficient of friction (COF) between an EN 24 steel pin and the dimple – textured steel disc made of low carbon steel. ASTM G99 standard was used for the test and the lubricant was supplied intermittently. Optimization was carried out using Taguchi method, where the optimized condition of the input parameters influenced a substantial change of 32.38% in COF as compared to the non – textured disc surface. The lubricant retention capacity of the dimple was the primary cause for the decrease in COF.

Reduced friction and wear enabled by arc-discharge method-prepared 3D graphene as oil additive under variable loads and speeds

A high quality three dimensional graphene nanosheets (3D GNS) with few layers’ thickness was prepared by the arc-discharge method. The 3D GNS is used as oil additive to explore its tribological performance under different working conditions. The results demonstrate that with the help of 3D GNS, the anti-wear and friction-reduction capabilities of base oil are improved in a wide range of speed and load. Especially under low speed and heavy load condition (4.2 mm/s and 1.0 GPa contact pressure), the reduction in coefficient of friction and wear volume loss can be as high as 29.1% and 55%, respectively. Raman spectrum and its mapping together confirmed the existence of protective film consisted by 3D GNS on rubbing surface, which avoids the direct contact of tribo-pairs and thus enhances the tribological performance of oil. In the case of heavier load condition, the lubricating performance of 3D GNS was reduced owing to the deteriorated 3D structure of the GNS. For higher speed, the thicker oil film brings better tribological properties to oil and minute 3D GNS is needed to participate in the friction process. The results are in good agreement with the Stribeck curve prediction. In brief, 3D GNS is helpful for improving the tribological properties of base oil under low speed and heavy load; thus providing a new insight for practical application as lubricant additive.

Analysing the frictional properties of micro dimpled surface created by milling machine under lubricated condition

The work investigated the friction reduction capability of micro-dimples created on a steel plate (disc) using a vertical milling machine under lubrication condition. A L25 orthogonal array was used to determine the combinations of various parameters. Wear tests were carried out using a pin on disc tribometer. SAE 10W30 was used as lubricant which was supplied intermittently between the interface of pin and disc. After the tribo test the optimized dimple parameters exhibited a decrease of 49.76% coefficient friction over the non-dimpled disc. The surfaces were further analysed using a three dimensional profilometer and scanning electron microscope. Artificial neural networks were employed to understand the influence played by each parameter on the coefficient of friction during the tribo test.

Effect of Surface Cleaning on Performance of Organic Friction Modifiers

The performance of surface-active additives, such as friction modifiers, depends on their interactions with surfaces. Their effectiveness thus hinges upon the surface conditions. In this work, the effect of cleaning methods of test substrates on the friction reduction capabilities of different organic friction modifier (OFM) additives was investigated. 52100 steel discs and balls were the test specimens. They were cleaned in five different ways. The cleaned surfaces were characterized by using ellipsometry and atomic force microscopy. The tribological performance of stearic acid (STA), octadecylamine (ODA), and octadecanol (ODO), on these surfaces was then tested.As-received steel surfaces were covered with contaminants that may impede the formation of an OFM surface layer. Cleaning these surfaces with solvents cannot completely remove these contaminants. Cleaning with oxygen or argon plasma results in cleaner surfaces compared to those cleaned by solvents only. The impact of the choice of cleaning method on friction depends on the strength of the interaction between the OFM and the steel surface, which determines the ability of an OFM to displace surface contaminations. Cleaner surfaces result in lower initial friction for STA and ODA. Steady-state friction is also affected but to a lesser extent. This may be because most contaminants that remained in the wear track were mechanically removed during rubbing.

MoDTC Tribochemistry in Steel/Steel and Steel/Diamond-Like-Carbon Systems Lubricated With Model Lubricants and Fully Formulated Engine Oils

This work focuses on the tribochemistry of molybdenum dithiocarbamate (MoDTC) oil additive to improve friction behavior of diamond-like-carbon (DLC) coated systems lubricated in boundary regime. Raman microscopy has been used to investigate surface tribolayers formed on coated (hydrogenated a-C:H and non-hydrogenated ta-C) and steel surfaces when lubricated with model lubricants and commercial engine oils. The effect of the additive package and the type of DLC played a crucial role in the development and composition of the tribolayer and the friction performance. The additive package contained in the fully formulated (FF) oils limited the friction reduction capabilities of MoDTC additive for every material pair. Accelerated a-C:H coating wear related to MoDTC tribochemistry was found. For the first time, it has been shown that a distinctive MoS2-containing tribolayer can be formed on the ta-C surface, leading to a coefficient of friction lower than 0.04. The underlying mechanisms of MoDTC/surface interactions and their effect on friction and wear are discussed.

Dispersion Stability and Tribological Characteristics of TiO2/SiO2 Nanocomposite-Enriched Biobased Lubricant

ABSTRACTThe stable dispersion of nano-additives is highly desirable for the effective lubrication performance of nanolubricants. The compatibility of base oil with selected nano-additives is required for uniform and stable dispersion. This research evaluated the dispersion stability and tribological characteristics of nano-TiO2/SiO2 (average particle size 50 nm) as an additive in a biobased lubricant. The wear protection and friction reducing characteristics of the formulations were evaluated by four-ball extreme pressure tests and piston ring–cylinder liner sliding tests. Surface analysis tools, including scanning electron microscopy, energy-dispersive X-ray spectroscopy, and atomic force microscopy, were used to characterize the worn surfaces. Results showed that the nanolubricants demonstrated appreciable dispersion capability in the absence of a surfactant and an improvement in load-carrying capacity, antiwear behavior, and friction reduction capability.

Development of low-cost deformation-based micro surface texturing system for friction reduction

Surface texturing as a means for enhancing tribological properties of mechanical components has been under intensive investigation over the last two decades. Many methods have been proposed to create surface texture of various patterns and geometries. However, among all these methods, deformation-based micro-surface texturing is least studied. It has many advantages over other methods that could lead to immediate industry application, including high productivity, high geometry fidelity and low cost. In the current work, a simple but effective incremental micro embossing system based on a commercially available press has been developed to create micro surface textures of various shapes and depths with accuracy up to 5 μm. The friction coefficients of the textured surfaces have been tested at different loadings and speeds with various lubricants to demonstrate their friction reduction capability. It has been observed that at high speed conditions, the friction reduction is achieved by the hydrodynamic lift. Interestingly, at the low speed conditions, the micro-surface texture is still capable of reducing the friction, thanks to its lubricant retention and debris entrapment capability. A micro surface textured mechanical face seal demonstrator has been built to further evaluate the micro surface texture created by incremental embossing method. A reduction of 20% in torque friction has been consistently achieved, which is on a par with that of the laser surface texturing method.

Assessing the lubrication performance of vegetable oil-based nano-lubricants for environmentally conscious metal forming processes

In metal forming processes, employing lubricants for providing desirable tribological circumstances at tool-workpiece interface is essential to increase the material formability, prolonging tool life and suppressing energy loss. With increasing the awareness and concerns towards the substantial hazardous effects of conventional metal forming lubricants a step forward is crucial to replace them with benign and compatible lubricants with environment. Despite the fact that vegetable oils have an immense potential to be utilized as green lubricants, but the limited tribological characteristics of them restrict their applications. Hence, the biodegradability and non-toxicity privileges of vegetable oils have been incorporated with friction reduction and extreme pressure capabilities of nanoparticle additives in this study, to develop a high performance lubricant with environmentally friendly feature. The CuO and SiO2 nanoparticles were dispersed in soybean and rapeseed oils in diverse concentrations and their lubrication performance were compared with two conventional metal forming lubricants, CM202A press drawing oil and solid lubricant of zinc phosphate plus sodium soap. The standard ring compression test was utilized to evaluate the efficiency of considered lubricants. The performance comparison was made using two key factors of friction coefficient and deformation load. The necessary friction calibration curves for estimating friction coefficient were provided employing finite element simulation. The acquired results convey that the lubricity of vegetable oil is dependent on its chemical structure. With increasing the concentration of nanoparticles, the friction factor and deformation load reach to a minimum for a certain amount of concentration. According to obtained results, the most efficient concentration of nanoparticles was determined within the range of 0.5–0.7 wt%, which provides 21–31% enhancement. It is demonstrated that the vegetable oil-based nano-lubricants which are enriched with optimum amounts of nanoparticles have the friction reduction capability comparable to conventional lubricants. The outcomes of this study can be fruitful in developing more environmentally conscious metal forming processes.

Friction Reduction Capabilities of Silicate Compounds Used in an Engine Lubricant on Worn Surfaces

Effects of magnesium silicate and alumina dispersed in engine lubricant on friction, wear, and tribosurface characteristics are studied under boundary and mixed lubrication conditions. Magnesium silicate and alumina, henceforth called as friction reducing compounds (FRC), were dispersed in engine lubricant in very low concentration of 0.01% weight/volume. Four-ball wear test rig was used to assess friction coefficient and wear scar diameter of balls lubricated with and without FRC based engine lubricant. Scanning electron microscopy (SEM) equipped with Energy Dispersive X-ray (EDX) was used to analyse the tribosurface properties and elemental distributions on worn surfaces of the balls. Test results revealed that FRC based engine lubricant increases friction coefficient but marginally reduces wear scar diameter of new balls, whereas, test on the worn-out balls running on FRC based engine lubricants shows 46% reduction in friction coefficient compared to the new balls running on engine lubricants without FRC. Investigations on tribosurfaces with respect to morphology and elemental distribution showed the presence of Si and O elements in micropores of the worn surfaces of the balls, indicating role of FRC in friction coefficient reduction and antiwear properties. These FRC based engine lubricants may be used in the in-use engines.

Friction and Wear Properties of Different Types of Graphene Nanosheets as Effective Solid Lubricants

Friction and wear properties of graphene nanosheets prepared by different processes as solid lubricant on silicon dioxide have been comparatively studied via calibrated atomic force microscopy. The effects of normal load, humidity, and velocity on the friction were also investigated. All kinds of graphene nanosheets possess friction-reduction properties on the nanoscale. Mechanically exfoliated graphene nanosheets exhibit ultralubrication and zero wear under high pressure due to perfect graphitic structure and a hydrophobic surface. Defects in chemical vapor deposited graphene nanosheets decrease the antiwear and friction-reduction capability. The graphene oxide nanosheets (GOS) show the weakest friction-reduction properties on account of destroyed graphitic structure and a hydrophilic surface. The reduced graphene oxide nanosheets (RGOS) possess better friction reduction than GOS by virtue of hydrophobic surface properties. Both RGOS and GOS have weak antiwear properties due to the destroyed graphitic structure. Antiwear properties are correlated strongly with the structure, and friction depends mainly on the structure and surface properties.

The effect of nanoparticle additives on lubrication performance in deep drawing process: Evaluation of forming load, friction coefficient and surface quality

The success of a sheet metal forming process is strongly dependent on the performance of provided lubricant at the tool/blank interface. Recent studies have revealed that the tribological properties of lubricating oils can be improved by adding small volume fraction of nanoparticle additives. Nevertheless, the efficiency of Nano-lubricants in actual sheet metal forming processes has not been evaluated so far. In this study, the tribological behavior of Al2O3 nanoparticles dispersed in CM202A press drawing oil was investigated in deep drawing process. Experimental tests were conducted for different concentrations of nanoparticles. The comparison between the formed cups under various lubrication regimes were conducted in terms of surface quality and maximum deformation load. Also, the friction coefficient at die/blank interface was determined inversely by finite element analysis. The results indicated that the friction reduction capability of base oil is enhanced by adding the Al2O3 nanoparticles, which consequently reduces the required forming load. In addition, a noticeable improvement in the surface quality of the formed cups is acquired.

Evidence of slippage breakdown for a superhydrophobic microchannel

A full characterization of the water flow past a silicon superhydrophobic surface with longitudinal micro-grooves enclosed in a microfluidic device is presented. Fluorescence microscopy images of the flow seeded with fluorescent passive tracers were digitally processed to measure both the velocity field and the position and shape of the liquid-air interfaces at the superhydrophobic surface. The simultaneous access to the meniscus and velocity profiles allows us to put under a strict test the no-shear boundary condition at the liquid-air interface. Surprisingly, our measurements show that air pockets in the surface cavities can sustain non-zero interfacial shear stresses, thereby hampering the friction reduction capabilities of the surface. The effects of the meniscus position and shape as well as of the liquid-air interfacial friction on the surface performances are separately assessed and quantified.

The Effect of DLC Coating Thickness on Elstohydrodynamic Friction

The application of surface coatings has been shown to reduce friction in elastohydrodynamic lubrication (EHL), not only in the mixed and boundary regime when asperity interactions occur, but also in the full film regime. Several studies suggest that the full film friction reduction is due to a violation of the no-slip boundary condition and thus slip is taking place between the solid and the liquid. Another hypothesis proposes that the full film friction reduction is due to the low thermal conductivity of diamond-like carbon (DLC) coatings. In this work, two DLC coatings with the same composition, but different thicknesses, are investigated with uncoated steel specimens as a reference, all with the same surface roughness. Friction tests in a ball-on-disk machine show that both coatings reduce friction compared to the uncoated reference case in full film EHL. The thicker coating is significantly more effective at reducing friction than the thinner one at a maximum friction reduction of 41 % compared to 29 % for the thinner coating. Moreover, contact angle measurements, surface energy measurements, and spreading parameter calculations show no statistically significant differences between the two coatings, suggesting that the friction reduction capabilities of coatings in full film EHL cannot be described by solid–liquid interactions alone. The difference in friction reduction between the specimens in this work is mainly attributed to different thermal properties.

The Tribological Properties of Zinc Borate Ultrafine Powder as a Lubricant Additive in Sunflower Oil

This article presents an investigation on the tribological properties of zinc borate ultrafine powder (ZBUP) employed as a lubricant additive in sunflower oil. The stable dispersions of 0.5, 1, and 2 wt% ZBUP in sunflower oil were achieved using an ultrasonic homogenizer. Both a four-ball tester and a pin-on-disc tester were employed to evaluate the antiwear and friction reduction capabilities of ZBUP. Tribofilms with a dark color were generated on the worn surfaces and showed good contrast with the substrate. The worn surface with different morphologies reflected as color alterations on the worn surface were observed when different lubricants were applied. The morphology and elemental analysis of the worn surfaces were studied using atomic force microscopy (AFM) and scanning electronic microscopy (SEM). Mechanical properties of the tribofilms and substrates were studied with a nano-indentation tester. Test results suggest that tribofilms generated on the worn surface have relatively low hardness compared with the steel substrate. The substrates on the worn surfaces lubricated in sunflower oil with the powder demonstrated higher hardness than the substrate lubricated with pure sunflower oil due to the possible tribochemical reaction between the zinc borate additive and substrate. The combination of sunflower oil with 0.5% ZBUP delivered the most balanced performance in friction and wear reduction. This study has demonstrated the possibility of application of this industrially applicable solid lubricant additive (zinc borate) with a decomposable vegetable-based lubricant oil.

Evaluation on tribological design coatings of Al2O3, Ni–P–PTFE and MoS2 on aluminium alloy 7075 under oil lubrication

The current work evaluated the friction and wear properties of tribological design surface coatings on aluminium alloy 7075 under various speed and nominal contact pressure. Hard-anodized Aluminium Oxide (Al2O3), burnished Refractory Metal Sulfide (MoS2) and composite electroless nickel coatings with polytetrafluoroethylene (Ni–P–PTFE) particles were subjected to pin-on-disc sliding test against grey cast iron (GCI) under Mach 5 SL SAE 10W-30 lubrication. The results indicated that Ni–P–PTFE composite coating possessed excellent friction–reduction capability but limited wear resistance due to low mechanical strength. Al2O3 coated sample showed outstanding wear resistance with high friction characteristic leading to high surface contact temperature. Furthermore, MoS2 coating improved the wear resistance of the aluminium alloy.

Tribological behavior of nanodiamond-dispersed lubricants on carbon steels and aluminum alloy

This paper examined the rheological property of nanodiamond-dispersed oil lubricants and their tribological performance with two carbon steels and an aluminum alloy. A commercial lubricant and a base oil with the same viscosity were used as the base stocks. These two oils, with different nanodiamond concentrations, were studied. The superior dispersive performance of the base oil was depicted from its higher zeta potential. The antiwear and friction-reduction capabilities of the various dispersions were explored by conducting a series of tribological pin-on-disk tests. The pin was made of carbon chromium steel. The rotating disks were made of AISI 1045 steel, AISI 1025 steel and 6061-T6 aluminum alloy. Surface topographies of the disks’ wear tracks, friction coefficient, and microhardness of contact surface were compared. The results show that nanodiamond-dispersed lubricants are capable of reducing these three metals’ wear loss. However, the wear mechanisms are different. The dispersions’ antiwear capability for the carbon steels is improved by a mechanism which was characterized not only by the viscosity of the nanodiamond suspensions but also by the promotion of contact surface's toughness due to the embedded nanodiamonds from the lubricant. On the other hand, the wear mechanism of the aluminum alloy is mainly dominated by the viscosity of the nanodiamond suspensions.

Effects of cutting fluid application on tool wear in machining: Interactions with tool-coatings and tool surface features

Minimal Quantity of Lubricant (MQL) application of cutting fluids (CFs), or near-dry machining, is being proposed as an environmentally and economically viable alternative to conventional flooding under conditions where dry cutting is not feasible. However, several issues related to CF application effects on cutting tool wear need further clarification, especially, the interactions of CF application with tool-coatings and chip-breakers, both of which are widely employed in industrial cutting tools, need further study. This paper presents the results of an experimental study into the effects of different CF application methods on tool wear during machining of AISI 1045 steel using flat-faced and grooved, coated carbide cutting tools. The results provide insight into the mechanisms of tool wear in the presence of CFs, as well as the influence of chip-breaking geometric features, and tool-coating systems, on CF action. The wear mode was observed to be dictated by thermal considerations, rather than by any friction reduction capability of different CF application methods, and forced attempts at achieving lubricating action were negatively affecting tool life under some conditions.