Tribological behavior of vegetable oil-based lubricants with nanoparticles of oxides in boundary lubrication conditions

This work studied the development of vegetable based lubricants and the addition of oxides nanoparticles (ZnO and CuO) as additive for extreme pressure (EP), exploring the EP and oil base influence in tribological behavior. The results showed that with the addition of nanoparticles to conventional lubricant, the tribological properties can be significantly improved. A smoother and more compact tribofilm has formed on the worn surface, which is responsible for the further reduced friction and wear. Also, lubricants developed from modified vegetable oil can replace mineral oil, improving the tribological and environmental characteristics. However, the addition of nanoparticles in vegetable base lubricants is not beneficial to wear reduction.     

Tribological behavior of tungsten-doped DLC coating under oil lubrication

Diamond-like carbon (DLC) protective films have received considerable attention in recent years. Beside tools and dies, diamond-like and related coatings are starting to find application in some highly stressed mechanical components, including bearings and gears. Several different gear tests show an increase in carrying load capacity or prolonged lifetime with tungsten carbide-doped DLC (W-DLC)-coated gears. The lubricants have been mostly commercially available gear oils that were primarily developed or tailored for ferrous materials and are probably far from optimal for amorphous W-DLC layers.In the present work, the influence of lubricant chemistry on the friction and wear in W-DLC-coated contacts was investigated using a reciprocating test device. The first six test lubricants were conventional non-inhibited API/ATIEL base stocks followed by selected test blends in combination with different surface-active single additives. Experimental results in coated contact have shown that for friction reduction performance, additives are not necessary, while wear resistance is related with the chemistry of the lubricant. It may be concluded at this stage that different mechanisms of friction-induced interaction between lubricants and W-DLC-coated layers are possible.     

Influence of different surface modification technologies on friction of conformal tribopair in mixed and boundary lubrication regimes

Moving machine assemblies are generally designed to operate in full film lubrication regime to ensure high efficiency and durability of components. However, it is not always possible to ensure this owing to changes in operating conditions such as load, speed, and temperature. The overall frictional losses in machines are dependent on the operating lubrication regimes (boundary, mixed or full-film). The present work is thus aimed at investigating the role of different surface modification technologies on friction of a sliding bearing/roller tribopair both in boundary and mixed lubrication regimes. A special test rig comprising of two bearings was built for the experimental studies. Tribological tests were conducted in a wide speed range to enable studies in boundary and mixed lubrication regimes. The influence of application of different surface modification technologies on both the sliding bearing and the roller surfaces on friction has been studied. The rollers used in these studies were provided with five different coatings (hard DLCs and a soft self-lubricating coating). Additionally, two uncoated rollers having different surface roughness were also studied. Uncoated bearings were used in all tribopairs except two. These two bearings were coated with DLC and phosphate coatings respectively and uncoated rollers were the mating counterparts. Friction measurements were made on the new as well as the previously run-in surfaces. It was found that the rollers with self-lubricating coating resulted in lowest boundary friction closely followed by the rollers with the hardest DLC coatings. The DLC coating applied on to the bearing showed lower boundary friction after running-in. Mixed friction has been found to be mainly dependent on the surface topography characteristics of both the original and the run-in surfaces of bearings and rollers. The harder DLC coatings and the phosphated bearing showed the lowest mixed friction due to an efficient running-in of the bearing surface.     

A computer model of mixed lubrication in point contacts

A numerical model of mixed lubrication is presented in this paper. The idea introduced here is that asperity contact may be viewed as a result of a continuous decrease in film thickness, so that the transition between contact and non-contact is continuous and the same mathematical model should work for both regions. The pressure over the thin films is assumed to obey the Reynolds equation, and the solution of the equation, under the condition of h→0, is expected to be the same as that predicted by the theory of elasticity. To achieve convergent and stable solutions, the left-hand side terms of the Reynolds equation are switched off when the local film thickness approaches zero, leading to a reduced Reynolds equation. Pressure distributions over the entire computation domain are thus obtained through solving a unified equation system without identifying hydrodynamic or asperity contact regions. Computations were conducted for several example cases and results show that convergent solutions are achievable on different types of roughness, over a wide range of λ ratios (0.01 to infinity), and for different slide-to-roll ratios (0.0–2.0).     

Tribological behaviour of steel‐alumina sliding pairs under boundary lubrication conditions

The tribological behaviour of oil‐lubricated steel‐alumina sliding pairs was investigated using a ball‐on‐disc tribometer at room temperature. Commercial bearing balls of 10 mm diameter were mated to 99.7% Al₂O₃ discs, and additive‐free mineral oil was fed into the contact area. The sliding speed and the applied normal load were varied, and the initial surface roughness of the Al₂O₃ disc was altered using different polishing and grinding procedures. The results showed that the surface roughness of the ceramic discs dominated the tribological behaviour under the given experimental conditions. The sliding speed as well as the normal load showed less effect on the friction behaviour, but the amount of wear depended strongly on the normal load. From the results it was concluded that improvement of the surface roughness and optimised surface machining of the ceramic material can be essential for improving the tribological performance for boundary‐lubricated steel‐ceramic sliding pairs.     

A microwedge model of sliding contact in boundary/mixed lubrication

A refined friction model of sliding contact in boundary/mixed lubrication regime is developed. In addition to the well-known asperity flattening and roughening effects, significant deformation of asperities can be incited by the elastic microwedges on the tool surface. A model of asperity deformation, which includes the effects of smoothing, roughening, and microwedge, is proposed for processes where smooth tool and rough workpiece are used. A finite element formulation incorporating the microwedge effect in the Reynolds equation for lubricant flow is also derived. Numerical results showed that the inclusion of microwedge in the analysis provides a good agreement with the experimental measurements, especially with many interesting phenomena that can be neither explained nor predicted by the other friction models.     

边界润滑抗磨与降噪的机理分析

采用特殊配制的润滑油实现边界润滑,可以降低高达125dB(A)以上的钢轨矫直噪声和110dB(A)以上的车削噪 声,可以极大地延长恶劣接触状态下矫直辊模拟试件的使用寿命,由此提出表面微凸体油膜弹性支承物理模型。分析该 模型抗磨与降噪机理,并指出今后开展这一新概念研究的方向与意义。     

The influence of thin boundary films on real surface roughness in thin film, mixed EHD contact

The behaviour of thin viscous boundary films in the rough surface rolling–sliding point contact operated under thin film lubrication conditions have been observed by thin film colorimetric interferometry. Changes in film thickness distribution within the lubricated contact between steel ball and glass disc were studies with both mineral base oil and mineral oil formulated with non-functionalized polyalkylmethacrylate (PAMA). Recent studies by other researchers showed that this polymer-containing viscosity index improver exhibits some friction-reducing capabilities even though it forms only very thin boundary films on rubbing surfaces. Results obtained in the current study proved that thin viscous boundary films formed on rubbing surfaces can reduce asperities interactions of rubbing surfaces under very thin film conditions. Even though these boundary films do not separate rubbing surfaces completely, they still can provide some protection of contacting bodies against excessive friction and wear.     

The investigation of contact ratio in mixed lubrication

In order to describe the mixed lubrication in nano-scale which is constituted from dry friction, boundary lubrication, and thin film lubrication, a contact ratio between surfaces of a glass disk and a steel ball in a pure rolling process has been measured by the technique of Relative Optical Interference Intensity (ROII) with a resolution of 0.5 nm in the vertical direction and 1 μm in the horizontal direction. The relationships between the contact ratio and its influence factors have been investigated. Experimental results indicate that the contact ratio in the static state is related to the combined surface roughness, maximum Hertz pressure, and the combined elastic module of tribo-pair in an exponential function. The decrease of rolling speed or lubricant viscosity, and the increase of the pressure will enhance the dynamic contact ratio which is the contact ratio measured in the rolling process. The addition of polar additives into basic oil will reduce the contact ratio. The contact ratio between rough surfaces is larger than that between smooth surfaces in the higher speed region. However, the former becomes smaller than the later after speed decreases below a critical value. A formula for calculating the dynamic contact ratio is given in the end of the paper.     

Wear of aluminium bronze on steel under conditions of boundary lubrication

An investigation was conducted to examine the wear of aluminium bronze against tool steel when lubricated under a boundary lubrication regime by aviation kerosene, both with and without the inclusion of a commercially available boundary additive. The objective was to isolate the mechanisms of boundary film formation, protection and wear of the metal surfaces, through mechanical measurements and analyses of surfaces and debris.A model is proposed to explain these mechanisms in terms of material transfer and subsequent competing reactions between dissolved oxygen and long chain hydrocarbon acid molecules, and the metal surface.     

The behaviour of friction modifiers under boundary and mixed EHD conditions

The behaviour of a range of model and commercial friction modifiers (FMs) has been evaluated under elastohydrodynamic (EHD) and boundary lubrication conditions. Using a series of long‐chain carboxylic acids, it has been shown that measured boundary friction coefficients (BFCs) decrease with increasing chain length, unsaturation level, temperature, and concentration. Base oil polarity was found to have no effect under these conditions. Commercial oleate esters in synthetic base fluids gave lower BFCs than nitrogen‐containing compounds under the same conditions. This difference was observed over a range of concentrations and temperatures. The friction performance of formulated oils under mixed and full‐film EHD conditions was found to be dependent on FM, base oil, and detergent type. Under boundary conditions, friction was found to vary with FM type, but the effect of changing the base oil and the detergent system was negligible.     

Friction and adhesion in boundary lubrication measured by microtribometers

The measuring and modelling of friction are critically important for the motion control in nanopositioning, particularly when bearings are employed to cover the wide working distances. Since the positioning system usually operates at very low speed to achieve fine positioning, the boundary lubrication is the dominant regime. A detailed characterization of the friction of boundary lubrication formed by Poly–α–Olefin (PAO) with and without surfactant and a suspension of MoS₂ in base oil has been performed in reciprocating sliding tests by steel/steel point contacts, and correlated with adhesion measurements by silicon/silicon point contacts. A microtribometer based on laser interferometers and glass springs, which can resolve 100 nN force in a speed range of 1–1000 μm/s was employed to detect the minute changes in forces. We find that a simple linear function instead of a logarithmic function is possible to describe the relationship between the friction force and operating speed for all the lubricants tested, though the gradients are quite different and under the influence of normal load. Comparing to PAO+surfactant and MoS₂ suspension, PAO shows a much higher load-dependent coefficient of friction. This result is further confirmed by the repulsion force measurements, which shows a higher increase of contact pressure with the increase of normal load for PAO.     

Normal vibration of a plain bearing working under boundary lubrication conditions

During sliding in a plain-bearing system under boundary lubrication conditions, the impact of the existing microasperities causes the system to vibrate. The resulting spectrum depends, among other things, on the type of material and the micro- and macrogeometries of the sliding surfaces. Running-in of plain bearings can be monitored by studying the changes in the respective spectra of vibrations     

Reduced description of mixed lubrication

Modelling friction and wear between rough and lubricated surfaces is an important problem of modern technologies. The complexity of the problem is in the necessity of simultaneously solving for elastic deformations of contacting bodies and fluid flow between the bodies. We show that the problem can be considerably reduced in the case when the lubrication layer is so thin that the main contribution to the contact interaction comes from a small part of microcontacts with a distance much smaller than the average distance between the bodies. In this case, it is possible to model the dynamics of lubrication by non-conservative forces between surface elements depending on both the distance and the relative velocity. The possibility of further reduction in the problem by using a one-dimensional system is discussed.     

Tribological performance of PTFE- and PEEK-based coatings under oil-less compressor conditions

Due to favorable tribological performance, PTFE- and PEEK-based polymeric coatings have received interest in air-conditioning and refrigeration compressor applications, as a potential solution to supplement and potentially replace conventional oil lubricants. The literature in this area is somewhat scarce, especially on the tribological performance of PTFE- and PEEK-based polymeric coatings under aggressive conditions simulating compressor operation. In this work, several PTFE-, PEEK-, resin- and fluorocarbon-based polymeric coatings, coated on gray cast iron were tribologically evaluated using a specialized tribometer under compressor specific conditions, that included oscillatory motion (simulating piston-type compressors) and unidirectional motion (simulating swash plate compressor operation). The coatings showed good to excellent tribological performance, and in general PTFE-based coatings exhibited better friction and wear behavior than the rest of the coatings, including PEEK-based coatings. Long-term durability experiments also showed the superiority and suitability of PTFE/Pyrrolidone coating for potential use in oil-less compressors (where oil-less conditions refer to operation in the absence of any liquid lubricant).     

Molecular orbital indexes criteria for friction modifiers in boundary lubrication

Friction modifiers, due to their adsorption onto metal surfaces, play an important role in boundary lubrication. The molecular orbital indexes were used as the criteria to study the interaction between lubricant polar end groups and metal surfaces. By comparing the net electric charge of bonding atoms, the highest occupied molecular orbital energy (E_HOMO) and the lowest unoccupied molecular orbital energy (E_LUMO) of interactive molecules, one could find that the hydrogen bond strength between alcohol and oxide metal surface which is hydroxylated was stronger than that between a hydroxylated oxide metal surface and ester. On the other hand, the interaction between naked aluminium atoms and ester is stronger than that between naked aluminium atoms and alcohol. Thus, lubricants consisting of alcohol and ester show a combination friction-reducing effect because each of the two components has its own advantages in interaction with the hydroxylated aluminium oxide and naked aluminium atoms on the lubricated interface.     

Effect of high-pressure rheology of lubricating oils on boundary lubrication

High-pressure rheology of lubricating oil was determined using different experiments, and the phase diagram was drawn. The four-ball wear tests were used to evaluate anti-wear characteristics of oils in boundary lubrication condition. The bridged ring compound oils showed the minimum wear scar in the four-ball wear tests. The diameter of wear scar decreases with increasing the elastohydrodynamic film-forming capability. Next, we considered the molecular packing parameter T_VE−T at the four-ball wear test. The T_VE−T values of bridged ring compound oils were in the range 250-360 and oils were elastic-plastic solid. It is concluded that the solidified oil film under boundary lubrication conditions has the anti-wear action.     

Wear resistance by copolymers with controlled structure under boundary lubrication conditions

Lubricants are of paramount importance in protecting metallic contact surfaces and reducing friction. The viscosity of lubricating oil can be engineered by introducing long linear polymers, such as poly(lauryl methacrylate) (PLMA). In particular, the formation of adsorption films by using polymers with hydroxy or amino side groups has attracted much attention in recent years. In this study, copolymers with controlled structure were synthesised by SARA ATRP, which can be used in large scale production. A comparison of friction and wear under boundary lubrication was conducted using both statistical and block copolymers with low Ð. Friction test results using a reciprocating sliding machine (SRV) showed that the block copolymers were less likely to desorb from the metal surface than the statistical copolymers. In addition, the wear evaluation after the SRV test showed that the block copolymer had less wear and less wear debris.     

On the effort to discriminate the principal function of tribofilm on friction under the boundary lubrication condition

Friction coefficient under boundary lubrication conditions is affected by many factors. In order to study the friction characteristic of tribofilm, friction coefficient of tribofilm formed on steel disk under the boundary lubrication conditions was separately measured under dry condition using a laboratory-made micro-tribometer to eliminate the hydrodynamic lubrication action of fluid. This paper introduces the approaching method to understand the friction characteristic of tribofilm. In this paper, sliding speed dependency of friction exerted by tribofilm was observed that could not be interpreted by the classical boundary lubrication mechanisms. The additives having long alkyl chain showed strong sliding speed dependency of friction.     

Comparative study of the tribological properties of various modified mild steels under boundary lubrication condition

AISI1045 steel was modified by laser heat-treatment and conventional heat treatment. The friction and wear behaviors of the steel specimens after various surface modifications sliding against SAE52100 steel under the lubrication of liquid paraffin containing sulphurized olefin were comparatively investigated on an Optimol SRV oscillating friction and wear tester. The worn surface morphologies of the modified steel specimens were analyzed using a scanning electron microscope. The elemental compositions and chemical states of some typical elements on the worn surfaces of the modified steel specimens were analyzed with an energy dispersive X-ray analyzer and X-ray photoelectron spectroscope, respectively. It was found that the laser heat-treated specimen showed the highest hardness and best wear-resistance. The laser heat-treated and conventionally heat treated AISI1045 steel specimens sliding against SAE52100 steel under the lubrication of liquid paraffin containing sulphurized olefin registered smaller friction coefficients than under the lubrication of liquid paraffin alone. This was partly attributed to the increased hardness of the modified specimens. The tribochemical reaction between the steel and the active elements in the additive was involved in the sliding of the modified steel specimens against SAE52100 steel ball under the boundary lubricating condition, with the formation of a surface protective film composed of various tribochemical products. This also contributed to improve the friction and wear behavior of the modified steel specimens. The steel specimens subject to different surface modifications showed differences in the wear mechanisms under the boundary lubricating condition as well. Namely, the tempered steel specimen was mainly characterized by plastic deformation and pitting, the quenched specimen by grooves and delaminating, and the laser heat-treated one by polishing and mild adhesion.