Mixed Lubrication Regime Research Articles

Applying load-sharing method to the sliding contact in the presence of nano-lubricants

The main goal of this study is to present a model to investigate the effect of nano-particles’ weight fraction on the friction coefficient of rough contact in the mixed-lubrication regime. Experimental testing involves pin-on-disk measurements of the friction coefficient with CuO nano-particles added to engine oil. Theoretical analyses involve developing a method for treating an EHL line contact with provision for surface roughness that takes into account the load-carrying capacity of surface asperities, lubricant, and nano-particles. Results show that theoretical and experimental results for friction coefficients are in good agreement. A parametric study is conducted to investigate effect of load, the geometry of the nano-particles, and their mechanical properties as well as their weight fraction on the friction coefficient.

Friction properties of micro/nanogroove patterns in lubricating conditions

Tribological properties can be improved by surface texturing; however, most of the conducted studies used microscale patterns. This study evaluated the lubrication properties of four different micro/nanogroove layouts (i.e., one straight layout and three different types of V-shaped layouts) created using the nanostripe fabrication technology. For each layout, microgrooves were arrayed at a pitch of 4 μm or 6 μm, and nanogrooves were formed on the ridge between these microgrooves. Friction tests were conducted at different loads and speeds using a single-crystal SiC disk as the counter specimen. We showed that patterns with 6 μm pitch microgrooves had a lower friction coefficient than those with 4 μm pitch microgrooves. The layouts with a higher ratio of nanogrooved area obtained a lower friction coefficient in high load and low speed conditions that are considered as boundary or mixed lubrication regimes.

A Comprehensive Numerical Study on Friction Reduction and Wear Resistance by Surface Coating on Cam/Tappet Pairs under Different Conditions

As a vital component in the valve train of internal combustion engines (ICEs), the cam/tappet pair undergoes high mechanical and thermal loads and usually works in a mixed and boundary lubrication regime. This leads to considerable friction loss and severe surface wear. Currently, the applications of diamond-like carbon (DLC) coatings for automotive components are becoming a promising strategy to reduce the friction and lower the wear. However, the practical performance of the coating is related to many factors, including friction coefficient, thermal properties, load conditions, and surface topography. In order to investigate these factors and successively improve the fuel efficiency and durability of the cam/tappet pair, a comprehensive multi-physics analytical model considering the mechanical, thermal and tribological properties of DLC coatings is established in this paper. Simulations are carried out for the coated as well as the uncoated cam/tappet conjunctions with different roughness at various ambient temperatures. The results show that both the fluid and asperity contact friction for the coated cam/tappet conjunction are significantly reduced due to their favourable characteristics. As a result, the friction loss of the coated cam/tappet pair is noticeably lower by almost 40% than that of the uncoated, despite a slightly higher asperity contact. In addition, the wear resistance of DLC coatings is also impressive, although the wear condition becomes progressively more severe with the increasing ambient temperature. Moreover, the roughness has complex effects on the friction and wear under different conditions.

PH Dependence of Tribochemical Wear of Silicon Nitride Sliding against Polycrystalline Diamond in Alkaline Solutions

The tribological behaviors of silicon nitride sliding upon sintered polycrystalline diamond (PCD) were explored via altering pH levels. High reductions in the friction and wear are achieved when sodium hydroxide solution is introduced at the contact interfaces. This reduction is found to depend on the pH value of the alkaline solution. The friction coefficient and wear loss of silicon nitride vary with pH level by more than half an order of magnitude and achieves the lowest values at pH 13. Moreover, the tribological behaviors reveal three stages versus the pH level, for which different tribochemical reactions occur due to the dissimilar hydroxyl concentrations. The dissolution of reaction products, such as Si(OH)4 and dimers (OH)3Si-O-C(OH)3, might lead to an increased proportion of hydrodynamic lubrication in the mixed lubrication regime and hence yield reduced friction and wear.

Experimental and numerical prediction of wear coefficient in non-conformal lubricated rectangular contact using continuum damage mechanics

Friction and wear play an important role in the tribological performance of any tribo-system. These two parameters are themselves functions of a variety of different factors such as material properties, surface roughness, applied load, sliding speed, and lubricant properties. In this research, experimental and theoretical investigation on the friction coefficient in line contact is conducted. The theoretical part is based on the load-sharing concept and the friction coefficient is obtained assuming non-Newtonian behaviour for the lubricant. The wear coefficient is then predicted using continuum damage mechanics (CDM) approach in which three times the number of cycles for a wear fragment to form is assumed to be equal to the inverse of the wear coefficient. The experimental part is conducted using a pin-on-disk test rig with disks made from ST37 and pin made of AISI 52100 bearing steel. The geometry of the pin is changed to make sure that a rectangular contact occurs. The predicted values for friction coefficient and wear coefficient are compared to experimentally determined values. It is shown that the presented CDM-based approach has the capability to be used for predicting the behaviour of a tribo-system in the mixed lubrication regime.

Evaluation of 1,3-diketone as a novel friction modifier for lubricating oils

With the growing demand in both lubrication performance and environmental protection, it is eager to seek novel lubricant additives of good oil miscibility and high tribological efficiency but without harmful elements such as sulfur and phosphorus. As a recently developed material with superlubricity (μ < 0.01), 1,3-diketone EPND (1-(4-ethyl phenyl) nonane-1,3-dione) could modify the contacts of steel surfaces through the tribochemical reaction with iron and also exploit strong intermolecular forces to control the near surface fluid interactions. Owing to its oil-soluble nature, EPND also shows the potential functioning as an additive in lubricating oils. In this study, EPND was introduced into both pure base oils and fully-formulated oils to evaluate its performance as a novel friction modifier. The results suggest that with small influences on the viscosity and oxidation stability in the blended oil, EPND leads to an effective friction reduction in both boundary and mixed lubrication regimes. Moreover, the addition of EPND enhances the oil extreme pressure capacity as well. This work indicates that 1,3-diketone is a promising friction modifier even in the presence of some other commercial additives.

Two fatty acid anion-based ionic liquids - part II: Effectiveness as an additive to a polyol ester

This work is about the use of two ionic liquids obtained from fatty acids (FAILs) as an additive at 0.5 and 2 wt% to a polyol ester (POE) base oil. Tribological tests under two different configurations (reciprocating and sliding/rolling “ball-on-disc”) were performed at different temperatures and loads. Worn surface was measured and analyzed after reciprocating experiments by confocal and scanning electron microscopies and by X-ray photoelectron spectroscopy (XPS). Main results showed that these FAILs at low concentration did not change the viscosity of the POE and hence the tribological behaviors of the mixtures and the POE under elastohydrodynamic lubrication (EHL) in rolling/sliding tests were similar, but using mixtures resulted in lower friction regarding the POE under mixed lubrication regime; the friction behavior in reciprocating tests was similar with the use of all mixtures and the POE at 25 °C for the two loads used, while the mixtures outperformed the antiwear behavior of the POE; the antifriction behavior of the mixtures under both loads at 100 °C was slightly worse than that of the POE and all mixtures had better antiwear performance than the POE.

A tribometer and methodology for wear and friction testing of porous journal bearings at elevated temperatures

PurposeThis paper aims on a methodology to overcome the fact that conventional service life testing of porous journal bearings (PJBs) requires long test times and is not economical.Design/methodology/approachThis paper sketches out a pathway to strongly accelerated life tests for PJBs enabled by high load and elevated temperature, which saves months or even years of testing. The testing time is not only reduced to a few weeks, but the results are also statistically secured via a multiple test rig construction of a custom-made tribometer.FindingsAn exemplary bearing-lubricant combination is tested in the mixed lubrication regime, where the coefficient of friction is monitored during the test.Originality/valueA Weibull curve is fitted to the experimental results to show the survival probability of the combination over time.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-08-2019-0351/

Scuffing Resistance and Starved Lubrication Behavior in Helicopter Gear Steels Coated with Nanocomposite Surface Coatings with and without a Hard Sublayer

Helicopter drivetrain components depend on oil lubrication to maintain elastohydrodynamic lubrication between sliding surfaces and are susceptible to a loss of lubrication event. A loss of lubrication event starves the mechanical interfaces of oil, pushing them from elastohydrodynamic toward mixed and boundary lubrication regimes. In those regimes, scuffing initiation and thermal runaway occur, resulting in a loss of torque transmission. Lubricious nanocomposite coatings made of a molybdenum disulfide and graphitic carbon are studied in combination with a titanium carbon nitride (TiCN) hard sublayer to improve scuffing resistance. Two experimental methods using a rotating ball-on-disc tribometer were conducted to evaluate the scuffing performance of the coatings. The first method is a load capacity experiment that incrementally increases load until either a maximum stress of 2.38 GPa or scuffing occurs. The second method is a loss of lubrication experiment that maintains a constant speed and contact stress with a running-in period before the lubricant supply is turned off. The elapsed time between the initiated scuff and the oil supply removal is the key experimental result. Experimental results show the time to scuffing initiation after loss of lubrication supply is extended with the application of a nanocomposite coating on a hard sublayer above the application of a hard coating alone and the uncoated medium superfinish baseline. Additional experiments were performed to study the effect of surface roughness and nanocomposite coating composition, but conclusions were limited due to a weaker adhesion strength of the TiCN sublayer applied with a lower deposition temperature to maintain steel substrate hardness.

Tribochemical Interaction of Multicomponent Aluminum Alloys During Sliding Friction with Steel

In this work, aluminum multicomponent alloys were studied after friction with steel in a mixed lubrication regime. The resulting secondary structures on the friction surface were investigated by scanning electron microscopy (SEM), energy dispersive analysis (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction analysis (XRD). In addition to the mass transfer of steel counterbody particles, phase transformations and new chemical compounds formed as a result of interaction with the lubricant were revealed. The release of elements, mainly magnesium and to a lesser extent zinc, from a solid solution of aluminum alloy was also observed, which indicates the occurrence of a non-spontaneous reaction with a negative entropy production.

Two fatty acid anion-based ionic liquids - part I: Physicochemical properties and tribological behavior as neat lubricants

Two new ionic liquids ([N 6,6,6,6 ][C 8:0 ] and [N 6,6,6,6 ][C 16:0 ]) synthetized from fatty acids through a salt metathesis reaction were both characterized from a physicochemical and tribological point of view. Density, viscosity, corrosion activity and thermal stability tests were performed before making tribological tests under ball-on-disc both unidirectional (with fixed and variable slide-to-roll ratio) and reciprocating configuration. The main results of this work were the following: the longer the length of the alkyl chain of the anion the lower both viscosity and density and higher both viscosity index and thermal stability; these ionic liquids (ILs) are not corrosive; the [N 6,6,6,6 ][C 16:0 ] behaved better changing from elastohydrodynamic to mixed lubrication regime at lower speeds; their antiwear behavior at high load seems depend on their viscosity (for tests made at room temperature) and thermal stability and IL-surface interaction (for tests made at high temperature). • Two fatty acids anion-based ionic liquids were obtained and tribologically studied. • These fatty acids anion-based ionic liquids are not corrosive. • The anion alkyl chain length determines viscosity, thermal and tribological behavior.

Effect of surface texture on the improvement of tribological performance of ring‐deformed liner conjunction under different oil supplies

AbstractIn this work, a tribological performance analysis is numerically conducted for a surface‐textured ring‐deformed liner conjunction. In the analysis, three different distribution patterns of surface texture are designed and machined on the surface of ring. To evaluate the frictional properties of the textured rings under mixed lubrication regime, a transient lubrication model considering the oil film thermal effect is developed. Meanwhile, to evaluate the oil film consumption and the transition of lubrication condition in an engine cycle, an oil transport model is built to determine the boundary positions of lubrication zone. Based on the models, the oil consumption, MOFT (minimum oil film thickness), friction force, power loss, and FMEP (friction mean effective pressure) value of the textured ring are studied under different oil supplies. The frictional characteristics of the rings with various distribution patterns of surface texture are also analyzed and compared. The results show that the edge‐textured ring can improve the tribological performance significantly, and the improvement varies with the oil supply.

Performance of Poly Alpha Olefin Nanolubricant

The viscosity and tribological behavior of nanofluids formed by dispersed nano-diamond particles within Poly-Alpha-Olefin (PAO6) lubricant is studied at different concentrations. The variation of coefficient of friction with nanoparticle concentration is measured using pin-on-disc tribometry under boundary, mixed, and hydrodynamic regimes of lubrication. A multi-scale multi-physics thermo-mixed lubrication model is developed to provide fundamental understanding of the measured tribometric results. The analytical approach combines continuum contact mechanics, thermal-mixed lubrication comprising the interaction of rough surfaces, as well as a thermal network heat transfer model. In particular, Einstein’s viscosity model for dispersed hard particles together with Vogel’s viscosity-temperature dependence model for fluid viscosity containing nanoparticles represent new contributions to knowledge. This integrated numerical-experimental study of nanofluid thermal and tribological assessment has not hitherto been reported in literature. It is shown that improved heat transfer capability of nanofluids is particularly effective in the reduction of friction under a mixed regime of lubrication.

Energy-Based Modelling of Adhesive Wear in the Mixed Lubrication Regime

Adhesive wear in dry contacts is often described using the Archard or Fleischer model. Both provide equations for the determination of a wear volume, taking the load, the sliding path and a set of material parameters into account. While the Fleischer model is based on energetic approaches, the Archard formulation uses an empirical factor—the wear coefficient—describing the intensity of wear. Today, a numerical determination of the wear coefficient is already possible and both approaches can be deduced to a local accumulation of friction energy. The aim of this work is to enhance existing energy-based wear models into the mixed lubrication regime. Therefore, the pressure distribution within the contact area will be determined numerically taking real surface topographies into account. The emerging contact area will be divided into one part of solid and a second part of elastohydrodynamically lubricated (EHL) contacts. Based on the resulting pressure and shear stress distribution, the formation of micro cracks within the loaded volume will be described. Determining a critical number of load cycles for each asperity, a locally resolved wear coefficient will be derived and the local wear depth calculated.

Modeling Mixed Lubrication Friction for Sheet Metal Forming Applications

Accurate modeling of friction is important in Finite element (FE) analyses of forming processes. Friction behavior depends on various parameters such as local contact load, surface topographies of sheet metal and tool, their material properties and the lubrication condition. In a typical deep drawing process, mixed lubrication condition is common, meaning that a lubricant can influence the coefficient of friction. Friction in the mixed lubrication regime is governed by the direct asperity contact as well as the hydrodynamic pressure developed by the lubricant. Local hydrodynamic pressure is also influenced by surface topography in addition to the lubricant amount and other process parameters. Direct numerical implementation of a measured surface topography in FE simulations is impractical due to the enormous computational effort. In this study, the overall frictional behavior in mixed lubrication regime is determined with the main objective to incorporate real measured surface topography in an efficient manner. An average Reynolds equation is solved on global FE domain of the forming simulation to determine lubricant pressure. A coupled friction model combining the effects of lubricant pressure and direct asperity contact is implemented in the forming simulations.

Experimental characterization of the inter-ply shear behavior of dry and prepreg woven fabrics: Significance of mixed lubrication mode during thermoset composites processing

A custom-built test device was developed to measure the inter-ply shear resistance of dry carbon woven fabrics as well as prepregs at different temperatures, consolidation pressures, and forming rates typical to those used in autoclave processing. A quantitative understanding of the nature of the shear forces encountered during fabric consolidation was established, suggesting the strong dependence of these forces on the process conditions along with the dominance of a mixed lubrication regime. Among the most influential state variables, the resin viscosity and distribution of the resin on the prepreg surface, as well as the surface roughness of the reinforcing fibers could be identified.

EXPERIMENTAL STUDY ON STEADY DYNAMIC FRICTION OF MWCNTs MIXED LUBRICANTS

This paper investigates the steady sliding behavior of multiwalled carbon nanotubes (MWCNTs) mixed lubricants at the metal–metal interface in direct shear tests. Slide-free-slide (SFS) experiments were performed to understand dynamic frictional stress of the sliding surfaces above the critical velocity. The experimental observations show that the dynamic stress decreases with increase in sliding velocity, but the same increases with normal stress. Further, in the case of change in concentration of the MWCNTs in the lubricant, the dynamic stress increases initially and then decreases to a minimum value and then further increases with addition of more nanoparticles in the lubricant. Dynamic stress and corresponding critical velocity are found to be minimum about 1.6% (wt./vol.) of MWCNTs concentration. Magnitude of cohesion as well as the coefficient of friction were also determined experimentally with the Coulomb friction law. The friction results are discussed in terms of the scaling laws in three regimes, namely viscous, rolling and sliding. These laws are justified on the basis of the mixed lubrication regime of Stribeck curve. Surface morphology of the test specimens before and after the experiment was also examined using SEM and EDS tests. No evidence of surface damage owing to motion of the nanoparticles was observed.

Enhanced tribological performance of alumina composites reinforced with acid-treated carbon nanotubes under water lubrication

Alumina composites reinforced with acid-treated multiwalled carbon nanotubes (MWCNTs) are prepared, and the tribological properties of the composite are investigated under water-lubricated conditions using a ball-on-disk technique against an alumina ball. The coefficient of friction (COF) in the steady-state region of the composite with acid-treated MWCNTs is found to be 0.06, which is 32–37% lower than that of the monolithic alumina and the composite with pristine MWCNTs. Microstructural observations and in-depth Raman spectroscopy analyses of the disk samples and counterparts before and after the friction tests reveal that the MWCNTs located in pits on the disk surface pull-out and that the resultant MWCNT fragments adhere to the friction surface of the counterparts. The enhanced tribological performance shown in the composite with acid-treated MWCNTs is mainly because the negatively charged functional groups adsorbed on the MWCNTs participate in the hydrogen-bond network of the hydrodynamic lubrication film and lead to acceleration of the transformation from a boundary lubrication regime to a mixed lubrication regime.

Investigations of the Friction Losses of Different Engine Concepts. Part 2: Sub-Assembly Resolved Friction Loss Comparison of Three Engines

In this work, friction loss investigations and comparisons of three different four-cylinder engines for passenger car applications are presented, using a recently developed combined approach. By merging extensive experimental with reliable and predictive journal bearing simulation results, a sub-assembly-resolved friction loss analysis of the piston group, crankshaft journal bearings and valve train is conducted for all three engines. The engines have been chosen individually based on their specific power output and crank train geometry. The measurement program covers a wide range of corresponding engine operation points (identical speed, load and thermal boundary conditions). In addition, the investigations are carried out for different engine media supply temperatures ranging from 70 ∘ C to 110 ∘ C for a comprehensive consideration of the friction losses at reduced lubricant viscosity. For reasons of comparability, all investigations conducted in this work have been carried out using the same modern SAE 5W30 lubricant. This is done to exclude influences from different lubricant properties which may have significant effects on the tribological behaviour of the engines’ sub-assemblies. While the diesel engine showed a friction reduction potential over the entire engine operation range when increasing the engine media supply temperatures, the gasoline engines showed a different behaviour. For the gasoline engines, disadvantages arise especially at low engine speeds. By using the developed combined approach, it was possible to assign mixed lubrication regimes at the valve train systems and at the piston groups.

Improved mixed elastohydrodynamic lubrication of hypoid gears by the optimization of manufacture parameters

Extensive wear appears in the case of dry contacts, or when the lubrication of the contacting surfaces is not appropriate. The aim of this research is to improve the mixed elastohydrodynamic lubrication in hypoid gears by the optimization of manufacture parameters for tooth surface processing.A full numerical analysis of the thermal mixed EHL in hypoid gears is applied. The equation system and the numerical procedure are unified for a full coverage of all the lubrication regions including the full film, mixed, and boundary lubrication. In the hydrodynamically lubricated areas the calculation method employed is based on the simultaneous solution of the Reynolds, elasticity, energy, and Laplace's equations. In the asperity contact areas the Reynolds equation is reduced to an expression equivalent to the mathematical description of dry contact problem. The real geometry and kinematics of the gear pair based on the manufacturing procedure is applied, thus the exact geometrical separation of the mating tooth surfaces is included in the oil film shape, and the real velocities of these surfaces are used in the Reynolds and energy equations. The transient nature of gear tooth mesh is included. The oil viscosity variation with respect to pressure and temperature and the density variation with respect to pressure are included. The non-Newtonian behaviour of the lubricant is considered. Using this model, the pressures, film thickness, temperatures, and power losses in the mixed lubrication regime are predicted. By using the developed method, the influence of the manufacturing parameters on the conditions of mixed elastohydrodynamic lubrication is investigated. On the basis of the obtained results recommendations are formulated to improve the mixed EHL and the efficiency of face-milled hypoid gears.