Friction Modifiers Research Articles

Three-body abrasive wear in the presence of inorganic fullerene-like tungsten disulfide and ionic liquid containing alkanolamines and phosphates

In coiled tubing applications, common constraints e.g., high friction, lateral drag and restricted flow rate increase the demand for friction modifiers. These modifiers also face the inevitable intrusion of abrasive particles in the wellbore that impair the drilling efficiency and accelerate wear of the sliding components. This study shows the tribological effect of sand particles in plain deionized (DI) water, resulting in a high friction environment with the risk of particles embedding in a two-body sliding behavior. To mitigate the drilling system’s performance degradation, two additives, inorganic fullerene-like tungsten disulfide (IF-WS2) and an ionic liquid with alkanolamines named “Base,” were added and their synergic tribological effects with sand were investigated. The presence of sand particles offered a reduction in coefficient of friction (COF) by 22% in the presence of DI water + IF-WS2, and a slight increase in wear. The Base additive was not as efficient as IF-WS2 at the same sand content, resulting in an increase in the COF and wear. In addition to the rolling friction, the gradual exfoliation of the IF-WS2 played an important role in alleviating friction and wear, leading to formation of tungsten-based tribolayer on sand particles.

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

Mechanistic insights into the effect of structural factors on film formation and tribological performance of organic friction modifiers

The influence of molecular structure of organic friction modifiers (OFMs) on their adsorption behaviour on iron oxide surface immersed in PAO oil was investigated using neutron reflectometry (NR). The effects of carbon chain length and levels of unsaturation on film formation was studied using a series of fatty acids with 18–20 carbon atoms in the temperature range between 25 and 100 °C. The results revealed that saturated OFM molecules generate thicker films compared to its unsaturated counterparts. Further, a decrease in film thickness was observed with the degree of unsaturation with the exception of linolenic acid. The film thickness and grafting density of the OFM increased with temperature. A decreased film thickness for the binary additive system comprising GMO and saturated molecules suggested that the adsorption of OFMs on the surface is hindered in the presence of glycerol mono oleate (GMO). A lower boundary friction coefficient and wear exhibited by OFMs with longer chain and linear structure suggest that their tribofilms are more effective in withstanding shear. The tribofilm thickness measured using optical interferometry showed no correlation with the adsorbed layer thickness determined from NR data; this lack of correspondence is attributed to the load and shearing forces experienced by the OFM molecules at the contact surfaces during friction measurements.

Comparative study of different solid lubricants towards friction stability in a non-asbestos disc brake pad

Purpose This study aims to compare the influence of different solid lubricants on the friction stability of a non-asbestos disc brake pad. Design/methodology/approach Three brake pads were developed using three lubricants, namely, non-asbestos brake pad with sulfide mix (NASM), non-asbestos brake pad with bismuth sulfide (NABS) and non-asbestos brake pad with molybdenum disulfide (NAMO). Sulfide mix was indigenously developed by physically mixing friction modifiers, alkaline earth chemicals and various metallic sulfides homogeneously dispersed in graphite medium. The physical, chemical, mechanical and thermal properties of brake pads were characterized as per industrial standards. The tribological performances were studied using the Chase testing machine as SAE-J661-2012. The worn surface of the pads was studied using scanning electron microscope to analyze the dominating wear mechanism. Findings NASM was excellent in fade as well as wear resistance. NABS was better from a wear point of view, but fade resistance was moderate despite its higher cost. NAMO fared average in fade and wear despite its excellent dry lubricating properties. NASM was excellent in terms of fade as well as wear resistance. Originality/value Among the selected metal sulfides, the indigenously developed sulfide mix was better than the other two sulfides, which indicates that the synergetic effect of metal sulfides was always preferable to the individual sulfides.

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.

Elasto-hydrodynamic friction changes on steel surfaces arising from the modified surface energy of the steel due to additive boundary films

The effect of three different film-forming additives mixed in PAO oil on the elasto-hydrodynamic (EHD) friction was investigated and compared to cases with the base oil only. When the boundary films were formed on the surfaces, the organic friction modifier (OFM) decreased the EHL friction by up to 8.7%, the ionic liquid (IL) decreased it by up to 6.4%, while the polymeric organic friction modifier (pOFM) increased it by up to 4.2%. In contrast, if the boundary films were not formed, the EHL friction remained the same as with the base oils only. The oils’ surface tension and viscosity were analysed at 25 °C and 100 °C as potential influencing effects, and the most important parameter for friction changes was found to be the surface energy. The mechanism behind this EHL friction reduction is, therefore, the effect of the boundary films on the poor oil-surface wetting and the consequent boundary slip. This study confirms the poor wetting arising from additive boundary-film formation as a relevant EHL friction-reduction mechanism. This observation is similar to that observed previously for surface coatings, thus suggesting that the boundary films of the surface-energy-reducing additives is a parameter with the potential to reduce the EHL friction.

Tribological Performance and Thermal Stability of Nanorubber-Modified Polybenzoxazine Composites for Non-Asbestos Friction Materials.

Asbestos-free friction composite based on ultrafine full-vulcanized acrylonitrile butadiene rubber particles (UFNBRPs)-modified polybenzoxazine was successfully developed. The UFNBRPs-modified polybenzoxazine friction composite was characterized for chemical, tribological, and mechanical properties as well as thermal stability. The UFNBRPs not only act as a filler to reduce noise in the friction composites due to their suitable viscoelastic behaviors but also play a key role in friction modifiers to enhance friction coefficient and wear resistance in the polybenzoxazine composites. The chemical bonding formation between UFNBRPs and polybenzoxazine can significantly improve friction, mechanical, and thermal properties of the friction composite. The outstanding tribological performance of the friction composite under 100–350 °C, i.e., friction coefficients and wear rates in a range of 0.36–0.43 and 0.13 × 10−4–0.29 × 10−4 mm3/Nm, respectively, was achieved. The high flexural strength and modulus of the friction composite, i.e., 61 MPa and 6.4 GPa, respectively, were obtained. The friction composite also showed high thermal stability, such as 410 °C for degradation temperature and 215 °C for glass transition temperature. The results indicated that the obtained UFNBRPs-modified polybenzoxazine friction composite meets the industrial standard of brake linings and pads for automobiles; therefore, the UFNBRPs-modified polybenzoxazine friction composite can effectively be used as a replacement for asbestos-based friction materials.

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.

The effect of third bodies on wear and friction at the wheel-rail interface

The friction forces between the wheel and rail depend on a number of variables including the third body layer at the wheel–rail interface, the wheel and rail profiles, and the train dynamics. The third body layer significantly influences the damage mechanisms at the wheel-rail interface, especially wear, rolling contact fatigue (RCF), corrugations and other surface defects that then require maintenance. The introduction of additional constituents at the wheel–rail interface in the form of an additive with anti-wear and anti-crack properties can reduce the wear and RCF. In general, such an additive also reduces the friction. However, it is important to avoid the friction coefficient between the wheel tread and the top of the rail falling below 0.3 because the result would be wheel slip and long braking distances. Measuring friction coefficients accurately is still a challenge, as most existing tribometers are unable to replicate the wheel-rail contact conditions, specifically the contact pressure and sliding speed. The present study used a newly designed handheld tribometer that is able to match the typical contact pressure. Results obtained with the handheld tribometer have been compared with values extracted from the traction-force measurement system of a locomotive. The tribometer field measurements have shown that by using a top-of-rail friction modifier (TOR-FM), both the wear and the friction coefficients can be reduced, but also that heavy TOR-FM films may cause unacceptably low friction. Comparing the results of field and laboratory tests confirms that weather and realistic third bodies present on the track have a significant effect on friction and wear.

Diblock polymeric friction modifier (PFM) in the boundary regime: Tribological conditions leading to low friction

The aim of this paper is to investigate the tribological conditions required to obtain low friction with a diblock PIB-PEG polymer friction modifier (PFM) blended in base oil (PAO4 +1% wt PFM) under a severe lubrication regime. Two tribological conditions, rolling/sliding and reciprocating pure sliding, were investigated. A very low friction coefficient (μ ~ 0.035) was obtained at a temperature of 100 °C whatever the tribometer used. ECR measurements, ToF-SIMS characterizations of wear tracks and AFM analysis suggested the presence of an adsorbed polymer film on the rubbing surfaces. ToF-SIMS characterizations showed that the polymer bonds to the steel substrate through polar functions.

Friction Modifiers Optimization on Tribological Properties of Non-asbestos Organic (NAO) Brake Pad by DoE-Taguchi Method

Friction Modifiers Optimization on Tribological Properties of Non-asbestos Organic (NAO) Brake Pad by DoE-Taguchi Method

Tribological performance of organic molybdenum in the presence of organic friction modifier.

The tribological performance of organic molybdenum in the present of organic friction modifier was investigated in this study. Three types of organic friction modifiers were selected, which are Glycerol monooleate, Pentaerythritol and N,N-Dimethylhexadecylamine. The organic molybdenum are MoDTC, MoDDP and molybdenum amide. Friction coefficient and wear were studied in block-on-ring test rig with steel test specimens. Experimental results indicate the Pentaerythritol shows synergistic effect with MoDTC in wide range temperature, while increased the friction coefficient of molybdenum amide in high temperature. N,N-Dimethylhexadecylamine shows synergistic effect with molybdenum amide, while hindered the friction reduction performance of MoDTC in low temperature. The presence of Glycerol monooleate reduced friction coefficient of MoDTC in low temperature, while increased the friction coefficient of molybdenum amide in most situations. All the tested organic friction modifiers improved the friction reduction performance of MoDDP. Most of the tested organic friction modifiers reduced the wear of organic molybdenum. The PT shows the best anti-wear performance with MoDTC. The tribo-chemical products in test specimens lubricated with different lubricant formulas indicate that the presences of Pentaerythritol promotes the production of MoS2 in MoDTC. N,N-Dimethylhexadecylamine promotes the production of MoS2 in molybdenum amide. The side products of MoO1.6S1.6 and Cr/MoS2 of MoDDP in high temperature lead to high friction coefficient.

Influence of Base oil Polarity on the Tribological Performance of Surface-Active Engine Oil Additives

Friction, wear and tribofilm growth of organic friction modifiers (glycerol monooleate and oleamide), anti-wear additive (ZDDP) and binary additive system comprising the organic friction modifiers and ZDDP were studied in polyalphaolefin (PAO) and ester oil. The mechanisms underlying base oil polarity-dependent frictional performance of the OFM and AW additives at high temperature (140 ℃), either singly or in combination, were investigated in the light of chemical composition analysis of the tribofilms post friction measurements using energy dispersive X-ray spectroscopy (EDX), static and dynamic time-of-flight secondary ion mass spectrometry (ToF–SIMS). Depending on the rubbing conditions, the boundary friction coefficient of the binary additive systems was found to be either lower than that of individual additives or to lay between the values for the individual additives. Chemical composition analysis of the tribofilms indicated that the nature of base oil controlled interactions between ZDDP and OFM and consequently adsorption and reactive tribofilm formation in the boundary lubrication layer. Surface roughness and wear scar width measured post tribological tests using 3D surface profiler showed improved wear performance in both PAO and ester-based additive formulations.

Wear behavior of rice straw powder in automotive brake pads

Abstract This paper investigates the use of rice straw powder in a brake pad as a substitute for asbestos which is a carcinogenic with detrimental effects on health. Rice straw powder was used as a novel material in a brake pad. Rice straw powder has a silica content which gives the pad a c eramic-like action. Rice straws were ground after drying in order to produce the powder. Five laboratory varieties were produced, altering the rice straw powder ingredients from 5, 10, 15, 20 and 25 wt.-%, respectively added to other abrasive materials, binder, friction modifiers, solid lubricant, and filler material utilizing conventional techniques. In this study, the friction surface temperature, the wear amount, and the change of the friction coefficient were determined. Additionally, the microstructure specifications of the brake pads were determined using scanning electron microscopy. Experimental results showed that a 15 wt.-% fraction of rice straw powder yielded better wear and thermo-mechanical features as compared with other combinations. The micro-structure shows a uniform distribution of the rice straw powder in the matrix. Hence, rice straw powder can be a possible candidate friction material for producing non-asbestos new brake pad without any detrimental impact.

Tribological behavior of a novel organic molybdenum containing dimercaptothiadiazole as a multifunctional additive in biodegradable base oil

Organic molybdenum friction modifier is an indispensable lubricating material owing to its remarkable tribological behavior, which has been widely used in various fields. Here, a feasible method is provided for the synthesis of thiadiazole organic molybdenum as a multifunctional lubricant additive in the trimethylolpropane trioleate biodegradable base oil. Results show that the coefficient of friction and the wear scar diameter are significantly decreased by ~39% and ~42%, respectively. Moreover, its extreme pressure performance is also improved by more than 50%. The XANES results show that the functional group of the thiadiazole and molybdate in the additive molecule will form a tribofilm rich in sulfate and molybdate to enhance the properties of friction-reducing, anti-wear and load-bearing capacity of the based oil.

Towards a neutron and X-ray reflectometry environment for the study of solid\u2013liquid interfaces under shear

A novel neutron and X-ray reflectometry sample environment is presented for the study of surface-active molecules at solid–liquid interfaces under shear. Neutron reflectometry was successfully used to characterise the iron oxide–dodecane interface at a shear rate of 7.0times {}10^{2}hbox {s}^{-1} using a combination of conventional reflectometry theory coupled with the summation of reflected intensities to describe reflectivity from thicker films. Additionally, the structure adopted by glycerol monooleate (GMO), an Organic Friction Modifier, when adsorbed at the iron oxide–dodecane interface at a shear rate of 7.0times {}10^{2}hbox {s}^{-1} was studied. It was found that GMO forms a surface layer that appears unaltered by the effect of shear, where the thickness of the GMO layer was found to be 24.3^{+9.9}_{-10.2} Å under direct shear at 7.0times {}10^{2}hbox {s}^{-1} and 25.8^{+4.4}_{-5.2} Å when not directly under shear. Finally, a model to analyse X-ray reflectometry data collected with the sample environment is also described and applied to data collected at 3.0times {}10^{3}hbox {s}^{-1}.

Highly Swollen Adsorption Layer Formed by Polymeric Friction Modifier Providing Low Friction at Higher Temperature

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.

Failure of Solid Lubricant W-S-C Coatings under Boundary Lubrication Conditions

A boundary lubrication regime imposes a significant challenge on mechanical systems in terms of both friction and wear. A standard approach to the boundary lubrication regime problems involves coating the contacting parts and lubrication with additivated lubricant. The presented paper explores the application of TMD-based coatings combined with a base oil and an organic-based friction modifier to provide a more environmentally friendly and cheaper solution. An investigation has been performed using a reciprocating tribometer across a range of applied loads, with post-test characterisation performed by a 3D optical profilometer, Raman microscopy, SEM, and TEM. Results show that the coating with higher hardness performs better and that the organic friction modifier increases coefficient of friction but decrease wear in the boundary lubrication regime. Moreover, it was established that without an external source of sulphur TMD-based coatings do not have a satisfactory performance in the boundary lubrication regime.

Spectroscopy analyses of tribofilm and surface microcracks in carburized SCM420 steel under high-pressure rolling contact fatigue

The tribofilm and surface microcracks developed on a carburized SCM420 steel through a two-roller-type pitting test were characterized. A streaky 100- to 200-nm-thick tribofilm was generated on the specimen surface after 1.0 × 105 cycles. The tribofilm was identified to be mainly composed of calcium phosphate Ca3(PO4)2 and iron phosphate Fe3(PO4)2. Microcracks having lengths of a few tens of micrometers were detected on the contact surface without the tribofilm after the test. The number of cycles corresponded to a transition of the tangential force between two rollers decreasing to a stick-slip phenomenon as a steady state. A friction modifier additive (molybdenum dialkyldithiocarbamate) was added to a lubricant to improve rolling contact fatigue (RCF) pitting resistance. This lubricant containing the friction modifier additive exhibited a lifetime that was a factor of 10 greater than the RCF lifetime for the lubricant base oil. Therefore, the tribofilm may affect the tangential force and RCF lifetime. The contact surfaces and subsurface structure, especially in the vicinity of the microcracks, were dynamically reconstructed by severe friction and damage accumulation. Oxygen was detected on the surface of the microcracks.

A novel study of hybrid brake pad composites: new formulation, tribological behaviour and characterisation of microstructure

ABSTRACT In the production of brake pad composites, a well-designed new composition formulation plays a key role in improving performance. The purpose of this study; was to investigate the effectiveness of a specifically developed formulation under brake-test conditions. The composition of the brake lining consists of 18 powder materials. As the friction adjuster, Petro-coke powder was used instead of Cashew, which is cheaper and more readily available. The wear tests were carried out in a special design test device simulated close to the real environment. The results obtained have been compared with commercial brake pads. The friction coefficient values decrease with increasing disc temperatures. Brake tests showed that as a friction modifier, petro-coke provided a more effective stabilizer for the friction coefficient and improved the specific wear rate. Also the study can be utilized effectively in determining the friction coefficient and the specific wear rate of designed brake composites.