Tribophyiscal scaling effects in friction and wear of polymers on small‐ and large‐scale tests

Coefficients of friction and wear rates for polyacetal and polytetrafluoroethylene are measured on small‐scale cylinder‐on‐plate tests and large‐scale flat‐on‐flat tests. Three models for extrapolation between small‐ and large‐scale test results, which are based on experimental parameters, are presented: (i) one single mechanical parameter (normal load), (ii) the contact‐pressure–sliding‐velocity model (temperature limit), and (iii) a macroscopic geometry model. The last model is most extensively evaluated and considers thermal effects, sample geometry and contact conditions. After correction for the thermal sliding regime and viscoelastic deformation, the coefficients of friction can be extrapolated while the wear rates are more sensitive to the contact situation. High‐temperature tests on small scale are not representative for high‐load tests on large scale. Besides mechanical effects such as stress concentrations and debris mobility, variations in polymer structure under sliding (formation of rigid amorphous phase) and chemical interactions (wear debris polymerisation) also change with testing scale. The latter effects are explicitly illustrated from spectroscopic measurements on the worn surfaces. Copyright © 2008 John Wiley & Sons, Ltd.     

On the feasibility of using cryo‐treatment as a tool to enhance the abrasive wear behaviour of solid‐lubricated polymeric composites

Cryo‐treatment, a bulk modification technique, is fast emerging as a way with which to improve the wear resistance of metals. This technique has also shown the ability to enhance significantly the abrasive wear performance of some polymers and their short glass‐fibre reinforced composites. In this work, short carbon‐fibre reinforced composites of some heat resistant polymers, such as polyetherimide, polyethersulphone, polyamide 6,6, polyetheretherketone, and polytetrafluoroethylene, were selected to explore the potential of cryo‐treatment. The selected materials were cryogenically treated by cooling them to the temperature of liquid nitrogen. The abrasive wear tests were carried out at ambient temperature in single pass conditions at various loads, on a pin‐on‐disc machine, using silicon carbide paper as a counterface. The investigations revealed that this technique has definite potential to increase the wear performance of carbon‐fibre reinforced composites. An increase in hardness due to cryo‐treatment was thought to be responsible for an observed improvement in wear performance. However, the extent of improvement in the wear performance was not matched by an increase in the hardness value. Scanning electron microscopy proved useful in examining the morphological changes in the composites due to cryo‐treatment.     

Experimental study of the friction and wear behaviour of a polymer disc/primer coating combination used in ball‐joints by means of large‐scale testing

The surfaces of a heavily loaded ball‐joint were initially covered with a sliding spray, and suffer wear. A solution is found by incorporating ultra high molecular weight polyethylene (UHMWPE) discs with a carbon fibre/epoxy reinforced ring as sliding material into the chairs of the structure. The ball side is covered with a zinc phosphate primer coating. For design purposes the local static and dynamic behaviour of the hybrid UHMWPE discs in contact with steel or Zn‐coated counterfaces should be large‐scale tested in terms of their loading capacity, low friction and wear resistance. Also the influence of creep and wear on friction is examined. After the large‐scale verification tests in laboratory, a good correlation is found with a test in the field. Copyright © 2006 John Wiley & Sons, Ltd.     

Load‐carrying capacity of lubricated and dry sliding elements

The factors influencing the load‐carrying capacity of tribological contacts are reviewed. The load‐carrying capacity depends in part on the lubrication regime, and hydrodynamic, hydrostatic, and elastohydrodynamic lubrication are discussed. In some circumstances, lubrication is not possible. The parameters of dry rubbing are described, and approaches to the design and operation of dry bearings are discussed, particularly the choice of suitable non‐metallic materials for use in the wear couple.     

An indirect electric field effect on the friction of boundary‐lubricated contacts

To achieve significant friction variations by means of an externally applied voltage, it is found to be more effective to impose an electric field on the area surrounding a tribopair rather than directly through the contact. Using a new applied electric field setup, the friction behaviour of ceramic‐metal tribopairs, lubricated with an aqueous emulsion, is investigated. The potential affects the surface characteristics during the instant that the surface is out of rubbing contact. Consequently, the nominal frictional behaviour differs from that with no potential effect. The unexcited, excited, and post‐excited friction results for six different ceramic‐metal couples using a pin‐on‐disc tester are presented, and their characteristics are summarised.     

Anti‐friction,anti‐wear and load‐carrying characteristics of environment friendly additive formulation

The extreme pressure (EP), anti‐wear and friction‐reducing characteristics of some of the commercial additive formulation and individual components on which these formulations are based have been studied and compared to characteristics of the components synthesised from naturally available non‐traditional vegetable oils and cashew nut shell liquid that have been refined and partially hydrogenated to improve stability. It has been shown that individual components from sulphurised and phosphosulphurised vegetable oils, esters and hydrogenated cardanol (derived from cashews) have better anti‐wear and friction‐reducing characteristics than the sulphurised olefins and alkylaryl phosphorothioates normally used as EP and anti‐wear additives, while the load‐carrying characteristics of a number of the combination of these derivatives are comparable. It has been shown that all these formulations are rapidly biodegradable and non‐toxic in nature as compared to traditional EP, anti‐wear and friction‐reducing additives, which fill in the category of slightly toxic to harmful. It is possible to formulate energy‐efficient EP gear oils that are fully biodegradable and non‐toxic by a combination of vegetable oil‐based additives of sulphurised vegetable oils, phosphosulphurised methyl recinoleate and phosphosulphurised hydrogenated cardanol amine borate, which meet all the performance characteristics of US steel 224 eg 52100, M‐50 AISI 1010 requirements. Copyright © 2007 John Wiley & Sons, Ltd.     

Effect of solid lubricant and fibrous reinforcement on the abrasive wear of polyamides

Short glass fibre‐reinforced polyamide 6 and polyamide 11 (PA 6 and PA 11) with and without a solid lubricant of polytetrafluoroethylene and metal powders (e.g., bronze) and copper as fillers were formulated and characterised for their compositional and mechanical properties. A tribological evaluation of these composites' abrasive wear was carried out by abrading a polymer pin against 80 grade (175 um) silicon carbide (SiC) paper under single‐pass conditions at various loads. It was observed that the fibre reinforcement reduced the abrasive wear resistance of virgin polymers. A combination of fibre and particulate fillers was further detrimental in this respect. Efforts were made to correlate the wear performance with the ductility factor, fracture toughness, and fracture energy in the case of PA 6 and its composites.     

The friction and wear of different polymers under high‐load conditions

Engineering polymers are increasingly being used as self‐lubricating materials in demanding high‐load applications. In this study, a comparison has been made of the tribological behaviour under high‐load conditions of two grades of polyamide and polyethylene terephthalate as well as polyacetal. Large‐scale testing is used to reduce edge effects. It has been found that the two grades of polyamide and pure polyethylene terephthalate are prone to stick—slip, while internally lubricated polyethylene terephthalate and polyacetal show low friction and wear and no stick—slip.     

Friction and wear behavior of AZ91D magnesium alloy against steel lubricated with N‐ and S‐containing organic borates

The friction reduction and antiwear properties of a range of nitrogen‐ and sulfur‐containing organic borates as oil additives in mineral oil were assessed using a Timken tester with a bearing steel ring against an AZ91D magnesium alloy block. X‐ray photoelectron spectrometry and scanning electron microscopy were employed to examine the boundary film formed on the surface of the magnesium alloy block. The results show that the borate additives were effective at reducing the wear of magnesium. The acting mechanism of borate, which acted as additive in the magnesium‐on‐steel tribological system, was proposed. Copyright © 2006 John Wiley & Sons, Ltd.     

Speed‐ and topography‐dependent boundary friction characteristics of steel

A novel tribometer that undergoes significant changes at ultra‐slow (>5.0 µm s⁻¹) to moderate (<20 cm s⁻¹) sliding speeds was developed in order to study the friction‐speed characteristics of steel lubricated with oil. Three different surface topographies were applied to the specimens, and the friction characteristics with an additive‐free base oil (MO91) and a stearic acid‐formulated oil (StA/MO91) were studied to understand the effects of surface textures on the lubrication performance of an adsorbed molecular layer formed by StA. Friction reduction behaviour of the adsorbed layer observed in the transverse direction was attributed to microscopic hydrodynamic action that maintained the load‐carrying performance of the adsorbed layer. Copyright © 2010 John Wiley & Sons, Ltd.     

Friction and wear characteristics of lead and its compounds filled polytetrafluoroethylene composites under oil lubricated conditions

The friction and wear properties of Pb, PbO, Pb₃O₄, or PbS filled polytetrafluoroethylene (PTFE) composites sliding against GCr15 bearing steel under both dry and liquid paraffin lubricated conditions were studied by using an MHK-500 ring-block wear tester. The worn surfaces and the transfer films of these PTFE composites formed on the surface of GCr15 bearing steel were then investigated by using a scanning electron microscope (SEM) and an optical microscope, respectively. Experimental results show that filling Pb, PbO, Pb₃O₄ or PbS to PTFE can greatly reduce the wear of the PTFE composites, but the wear reducing action of Pb₃O₄ is the most effective. Meanwhile, PbS increases the friction coefficient of the PTFE composite, but Pb and Pb₃O₄ reduce the friction coefficients of the PTFE composites. However, the friction and wear properties of lead or its compounds filled PTFE composites can be greatly improved by lubrication with liquid paraffin, and the friction coefficients of the PTFE composites can be decreased by one order of magnitude. Optical microscope investigation of transfer films shows that Pb, PbO, Pb₃O₄ and PbS enhance the adhesion of the transfer films to the surface of GCr15 bearing steel, so they greatly reduce the wear of the PTFE composites. However, the transfer of the PTFE composites onto the surface of GCr15 bearing steel can be greatly reduced by lubrication with liquid paraffin, but the transfer still takes place. SEM examination of worn surfaces shows that the interaction between liquid paraffin and the PTFE composites creates some cracks on the worn surfaces of the PTFE composites; the creation and development of the cracks reduces the load-carrying capacity of the PTFE composites, and this leads to deterioration of the friction and wear properties of the PTFE composites filled with lead or its compounds under higher loads in liquid paraffin lubrication.     

Load‐carrying capacity of WC/C‐coated gears lubricated with a low‐viscosity oil

The application of low‐friction coatings on the surfaces of gear teeth is primarily motivated by the increasing demands for load‐carrying capacity or the rolling‐contact fatigue life. Despite the costs of manufacturing being higher, better performance is an important consideration for many demanding applications. In the present study, we have investigated the load‐carrying capacity of WC/C‐coated gears that were lubricated with an oil blend that was formulated in‐house. A significantly higher load‐carrying capacity was found for the WC/C‐coated gears lubricated with a tailored, low‐viscosity oil in comparison with the case‐carburised gears lubricated with a conventional gear oil of a similar viscosity. The carburised gears failed due to scuffing on the pinion tip, while coating thinning on the pinion face and coating fracture in the wheel‐root area are the dominant failure mechanisms for WC/C‐coated gears. An approximately 40% lower roughness was found on the coated gear flanks. Copyright © 2015 John Wiley & Sons, Ltd.     

Friction and wear effects on a micro/nano‐scale

In this paper are described tribological effects which can be found in micro‐tribological systems, and in those macro‐systems which can be analysed by micro‐methods, e.g., by atomic force microscopy (AFM) or related methods. Micro‐tribology systems have friction contacts with loads in the micro/nano‐newton range and/or dimensions in the micro/nanometre range. Experiments on the micro/nano‐scale should be easier to explain by theoretical modelling due to their simpler system structure. An example is discussed of adhesion and friction measurements between AFM tips and clean, flat, solid surfaces in ultra‐high vacuum, which shows some of the special aspects of micro/nano‐tribology. Surprising friction characteristics on surfaces with an artificial micro‐structure can be explained by skilled and careful topographical analysis of the friction path with an AFM. In micro‐sensor contacts, ‘single wear events’ can be detected using AFM analysis of the contact region. For ceramic compounds, different friction levels for the components of the material can be found. The problems, difficulties, and dangers of misinterpretation are also discussed.     

Additive influence on wear and friction performance of environmentally adapted lubricants

In this paper, the influence of concentration level and chemical composition of three different additive types on friction and wear coefficient are presented for a synthetic ester base fluid and a mineral base oil. One extreme-pressure (EP), two antiwear (AW) and two yellow metal passivator (Cu-passivators) additives were used. Factorial experimental design was used as the basis for a systematic evaluation of wear rates under mixed and boundary lubrication conditions. A total of 33 different lubricant blends were tested in a Plint and Partner High Frequency Friction Machine. For the synthetic ester, the extreme-pressure (EP) additive, containing phosphorus and nitrogen, was found to be much more effective in reducing wear than either of the two antiwear (AW) additives tested. In fact, the AW and Cu-passivator additives made little or no contribution to the wear protection in most of the cases studied. A synergy effect between the three additive combinations was observed only for the reference mineral oil blend. A significant difference between the antiwear performance of the test lubricants was found. This study suggests that the traditional “AW” and “EP” labels associated with commonly used additives are poor aids when designing of ester based lubricants.     

Anti‐wear performance of organomolybdenum compounds as lubricant additives

A four‐ball tester was used to evaluate the anti‐wear performance of three kinds of organomolybdemun compounds in engine oils and lithium soap grease, i.e. molybdenum dialkyldithiophosphate (MoDDP), molybdenum dialkyldithiocarbamate (MoDTC), and sulphur‐ and phosphorus‐free organomolybdeum (molybdate). The results indicate that a low concentration of MoDDP does not improve the anti‐wear properties of the commercial engine oils, but a high concentration of MoDDP can obviously improve the anti‐wear properties of the engine oils. MoDTC does not improve the anti‐wear properties of the engine oils, but worsens the anti‐wear properties of the oils. Significant improvement of the anti‐wear properties is obtained with molybdate added in the four fully formulated oils. Both MoDDP and molybdate have good anti‐wear properties in the lithium soap grease, especially MoDDP with a high concentration. Copyright © 2006 John Wiley & Sons, Ltd.     

Molecular dynamics simulation of surface energy and ZDDP effects on friction in nano-scale lubricated contacts

Molecular dynamics simulations are used to study the tribological performance of a lubricant mixture containing hexadecane base oil and 5% zinc dithiophosphate (ZDDP) under molecular confinement conditions. The influence of ZDDP additive and the surface-lubricant interaction on the mechanical and thermal interfacial response are studied in detail. Results show that mechanical and thermal slips are reduced by increasing the surface energy. Simulations also demonstrate the migration of ZDDP molecules and their adsorption onto the solid surface resulting in a remarkable suppression of mechanical slip compared to pure hexadecane. Consequently, the effective shear rate is higher and so is the friction.     

A block‐on‐ring tribocorrosion setup for combined electrochemical and friction testing

The combined action of corrosion and wear can cause degradation of equipment, and thereby financial losses related to the renewal or repair of damaged equipment. In the food industry, metal degradation is a major concern since metal release eventually can cause health risks for consumers. This study describes a block‐on‐ring testing facility used to determine sliding wear, and additionally allowing for electrochemical measurements, such as potentiodynamic polarization curves and potentiostatic monitoring of current and potential. To verify the reliability and reproducibility of this block‐on‐ring tribocorrosion setup, the tribological and electrochemical behaviour of stainless steel sliding against a ceramic ring in sulphuric acid has been determined. Furthermore, a case taken from the food industry has been examined. The study shows that results made on the described block‐on‐ring testing facility are reliable and can provide improved information about material properties when the material is exposed to combined chemical and mechanical degradation. Copyright © 2007 John Wiley & Sons, Ltd.     

Influence of cylinder‐on‐plate or block‐on‐ring sliding configurations on friction and wear of pure and filled engineering polymers

Polyamides, polyesters and polyacetals are often used in line contacts under reciprocating or continuous sliding. These contacts are simulated on cylinder‐on‐plate (COP) or block‐on‐ring (BOR) tribotests. Comparative tests for pure, oil‐filled and solid lubricated polymers at 100N and 0.3m/s are presented for relative material classification. Differences are discussed according to the sliding geometries. Thermal effects dominate friction and wear behaviour: the polymer glass transition temperature is exceeded in COP tests while the temperature is lower in BOR tests. Thick and brittle films are observed for pure polymers in BOR tests, promoting higher friction. The test configuration is mainly important for evaluation of internal lubricants. The efficiency of oil‐lubricated polymers is not demonstrated in COP tests, while solid lubricants are not efficient in BOR tests. Deformation restricts the diffusion of oil lubricants in COP tests while solid lubricants are deposited on the polymer surface rather than being incorporated in the transfer film in BOR tests. Copyright © 2007 John Wiley & Sons, Ltd.     

The friction and wear of automotive grey cast iron under dry sliding conditions Part 1-relationships between wear loss and testing parameters

The material investigated in this study is automotive grey cast iron (SAE G-3500) used for the manufacture of cylinder liners in a range of diesel engines. A comprehensive model for the understanding of wear and friction of this type of materials is presented here for the first time. This model takes into account the cumulative contribution of various mechanisms of wear and friction (truncation, adhesion, delamination and ploughing) to the overall phenomenon of wear loss. Quantitative aspects and statistical modelling were employed to develop a mathematical relationship between measurements of friction and wear under varying conditions of load, number of cycles and velocity (rev min⁻¹).     

Misalignment effects on steady‐state and dynamic behaviour of compliant journal bearings lubricated with couple stress fluids

This work concerns the steady‐state and dynamic analysis of misaligned compliant journal bearings considering the effects of couple stresses arising from the lubricant blended with polymer additives. Based on the Stokes micro‐continuum theory, a modified form of the Reynolds equation is derived. The displacement field at the fluid film–bearing liner interface due to pressure forces is determined using the elastic thin liner model. The effects of the misalignment and the couple stress parameters on static and dynamic performances such as pressure distribution, load‐carrying capacity, power loss, side leakage flow, misalignment moment, critical mass and whirl frequency are presented and discussed. Copyright © 2008 John Wiley & Sons, Ltd.