Tribological behaviour of a PEEK polymer containing solid MoS2 lubricants

The addition of different concentrations (2–10 wt.%) of molybdenum disulfide (MoS₂) to a poly–ether–ether–ketone matrix has been studied in terms of the thermal, mechanical and tribological properties of the materials. The results of dry‐sliding tribological tests, differential scanning calorimetry and scanning electron microscope–energy‐dispersive X‐ray (EDS) analyses show that the concentration of MoS₂ influences the tribological, mechanical and thermal properties. With the highest concentration of MoS₂ (10 wt.%), the coefficient of friction was reduced by as much as 25%, while the maximum reduction in the wear rate was ~20%, which required 5 wt.% of MoS₂. The most important parameter when it comes to achieving an improved tribological behaviour was found to be the combination of a high hardness and a sufficient quantity of transfer film being formed. Copyright © 2015 John Wiley & Sons, Ltd.     

Influence of different friction conditions on the process of cold backward extrusion of steel

Backward extrusion of steel is a deformation process in which very high forces and tool pressures occur. Reduction of such high forces and tool pressures is possible by using an efficient lubricant. This paper describes an experimental investigation of the influence of different lubricants on the stress-strain state within the workpiece volume as well as on the stressing of the die. Best results were obtained when a phosphate-coated specimen was lubricated with MoS₂.     

Synthetic lubricants in cold strip rolling

The embedded-pin technique is used to monitor the variation of the interfacial forces during cold rolling of aluminium slabs. The results confirm earlier conclusions, indicating that the frictional coefficient is a strong function of the process and material parameters. Coefficient of friction values deduced from forward slip measurements are close to the present data, confirming that in spite of several drawbacks of the experimental technique, it can yield very useful results. Lubricants are shown to reduce the loads on the rolling mills in a significant manner.     

Hot backward extrusion comparative analyses by a combined finite element method

Deformation and temperature of hot backward extrusion are complex owing to interaction between deformation and temperature. In this paper, two- and one-way axisymmetric hot backward extrusion problems are analyzed by a combined finite element method, which consists of the volumetrically elastic and deviatorically rigid–plastic finite element method and the heat transfer finite element method. The volumetrically elastic and deviatorically rigid–plastic finite element method is different from the conventional rigid–plastic finite element methods, and has some merits in comparison with the conventional methods. Because contact surfaces between workpiece and tools of the one-way extrusion are different from those of the two-way one, the deformation and temperature of the one-way extrusion are different from those of the two-way one. Contours of effective strain rate, effective strain, temperature, effective stress and hydrostatic stress, as well as plots at different reductions for the two extrusions are obtained successfully. Differences of calculated results for the two extrusions can be clearly seen through comparative analyses.Because the bulk modulus is introduced into the volumetrically elastic and deviatorically rigid–plastic finite element method, influence of temperature on hydrostatic stress can be considered in this paper.     

Recent trends in environmentally friendly lubricants

Many of the lubricants used in the world today cause environmental pollution through total‐loss applications, spillage, evaporation, and in other ways. To reduce this environmental damage, new lubricants that are rapidly biodegradable and ecologically non‐toxic have started to be developed and marketed. This paper discusses the introduction of environmentally friendly lubricants and their constituents, with particular emphasis on their environmental benefits, applications, the limits to their use, their technical performance characteristics, and relative cost aspects; many of these lubricants are based on vegetable oils and esters. Comparisons are also made in the paper with conventional mineral‐oil based materials.     

Comparison studies on degradation mechanisms of perfluoropolyether lubricants and model lubricants

The degradation mechanisms of some perfluoropolyether lubricants, model lubricants and DLC coating were studied in this paper. The degradation fragments from the tests of the PFPE lubricants can be divided into two groups. One group includes the gas fragments containing fluorine atoms, which are generated from the decomposition of the lubricants themselves; while the other group, including H₂, C₂H₃, C₂H₅, and CO₂, is generated from the degradation of the DLC coating on the disk surfaces. The test results from the model lubricants clearly show that the carbon dioxide produced in the tests is generated from the DLC coating, not from the decomposition of the lubricants or model lubricants. The C–O bond is a weak bond in both the lubricant and model lubricant molecules; it is easier to be broken. Because of the polar characteristics of the C–O bond, it is easy to be attacked and broken down by low-energy electrons generated during sliding. Triboelectrical reaction is a dominant degradation mechanism of the lubricants and model lubricants.     

Tribochemistry of aluminium and aluminium alloy systems lubricated with liquids containing alcohol or amine additive types and some other lubricants — a review

The ability of a lubricating oil to reduce wear and prevent damage of interacting solids is a crucial factor controlling lubricant formulation. It is well known that friction produces local high temperatures. Many chemical reactions that are initiated by the friction process itself occur at much lower temperatures than those needed to provide the activation energy. Under boundary lubrication conditions, a clean surface exposed as a result of mechanical activity of the solid surface is extremely reactive, especially in the case of metals. This review mostly relates to the tribochemistry of aluminium, and discusses the tribological characteristics of alcohol‐ and amine‐type liquids used as either additives or lubricants to lubricate aluminium and its alloys under boundary friction conditions. Although tribochemical reactions during sliding are perceived in various ways, here the emphasis is on the negative‐ion‐radical action mechanism (NIRAM) approach. This review addresses the question as to how present knowledge of tribochemistry can be applied to the elucidation of the mechanisms of action by which the boundary lubricant compounds considered reduce aluminium‐on‐aluminium, steel‐on‐aluminium, and aluminium‐on‐steel wear. Also, information and a discussion on the tribological behaviour of other additives and/or lubricants in relation to the friction and wear of aluminium and its alloys are presented. A concise review of the most recent work on the tribochemistry of selected fluorinated alcohols is also included.     

Wear resistant solid lubricant coating made from PTFE and epoxy

A composite coating of polytetrafluoroethylene and epoxy shows 100 × improvements in wear resistance as compared to either of its constituents alone and reduced friction coefficient under testing on a pin-on-disk tribometer. This coating is made by impregnating an expanded PTFE film with epoxy, which provides three unique functions: (1) the epoxy compartmentalizes the PTFE nodes, which is believed to reduce the wear of the PTFE, (2) the epoxy increases the mechanical properties such as elastic modulus and hardness, and (3) the epoxy provides a ready interface to bond the films onto a wide variety of substrates easily and securely. The experimental matrix had normal loads of 1–3 N, sliding speeds from 0.25 to 2.5 m/s, and used a 2.4 mm radius low carbon steel pin in a rotating pin-on-disk tribometer. The skived PTFE films had wear rates on the order of K=10^–3 mm³/Nm and friction coefficients around =0.2. Both the high density films (70 wt%PTFE) and low density films (50 wt% PTFE) had wear rates on the order of K=10^–6 mm³/Nm and friction coefficients around =0.15. The neat epoxy films showed significant scatter in the tribological measurements with wear-rates on the order of K=10^–4 mm³/Nm and friction coefficients around =0.40. The enhanced tribological behavior of these composites is believed to stem from the coatings ability to draw thin PTFE transfer films into the contact from the nodes of PTFE, which act like reservoirs. Nanoindentation mapping of the coatings and the transfer films supports this hypothesis, and accompanies scanning electron microscopy observations of the worn and unworn coatings.     

Molecular tribology of lubricants and additives

Knowledge of the bulk viscosity provides little guidance to predict accurately the interfacial shear strength and effective viscosity of a fluid in a lubricated contact. To quantify these differences between bulk and thin-film viscosity, an instrument was developed to measure the shear of parallel single crystal solids separated by molecularly-thin lubricant films. The effective shear viscosity is enhanced compared to the bulk, relaxation times are prolonged, and nonlinear responses set in at lower shear rates. These effects are more prominent, the thinner the liquid film. Studies with lubricant additives cast doubt on the usefulness of the concept of a friction coefficient for lubricated sliding.     

Influence of solid lubricants and fibre reinforcement on wear behaviour of polyethersulphone

Polyethersulphone (PES), is an amorphous, brittle and high temperature engineering thermoplastic. Two composites of PES containing short glass fibres (GF) and solid lubricants viz. PTFE and MoS₂; and two composites containing short carbon fibre (CF) [30% and 40%] were selected for the present studies. Compositional analysis of selected materials was done with various techniques such as gravimetry, solvent extraction and thermal analysis viz. thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). These materials were studied for adhesive and abrasive wear performance by sliding against a mild steel disc and silicon carbide abrasive paper respectively, under different loads. It was observed that GF reinforcement along with incorporation of solid lubricants (PTFE and MoS₂) enhanced the wear performance of PES by an order of two. In the case of solid lubricants, PTFE proved to be more beneficial than MoS₂. CF reinforcement, however, proved to be the most effective in enhancing wear performance of PES. PES reinforced with 40% CF exhibited a specific wear rate in the order of 10⁻¹⁶m³/Nm which is considered to be very good for the thermoplastic composite. In the case of abrasive wear behaviour, however, incorporation of fibres or solid lubricants deteriorated the performance of the neat matrix. SEM was employed to investigate the wear mechanisms.     

Modeling of double action extrusion—A novel extrusion process for friction characterization at the billet-die bearing interface

The friction in the bearing channel of hot aluminium extrusion was studied by using a novel simulative friction test: double action extrusion (DAE). FEM simulations of DAEs were carried out to evaluate two commonly used friction models. It was found that full-sticking friction represents the experimental results the best. For a further understanding of the DAE, a theoretical model was developed, and a good agreement between the modeling results and experiments was obtained. The theoretical modeling results revealed that the length difference of the extrudates was caused by the combined effects of friction and material rate dependence at elevated temperatures.     

Analysis of hydrodynamic journal bearings having non-Newtonian lubricants

Commercial lubricants, due to the presence of different types of additives, behave like non-Newtonian fluids. The effect of this nonlinear behaviour on the performance characteristics of finite-width journal bearings is investigated using the Eyring model for the shear stress and shear strain rate. the finite element method using Galerkin's technique has been used to solve the momentum equations and the continuity equation in cylindrical coordinates, representing the flow field in the clearance space of a journal bearing system using Newtonian fluids; the non-Newtonian effect is introduced by modifying the viscosity term for the model in each iteration. The results of static performance characteristics for finite-width journal bearings having non-Newtonian lubricants have been obtained.     

Elastohydrodynamic studies of circular journal bearings with non-Newtonian lubricants

A computer-aided study is presented for the static and dynamic performance characteristics of a hydrodynamic journal bearing with a non-Newtonian lubricant. The Navier-Stokes equations in cylindrical coordinates representing the flow field in the clearance space of a journal bearing using Newtonian fluids have been solved by finite element method using Galerkin's technique; the non-Newtonian effect is introduced by modifying the viscosity term for the model in each iteration. Deformation of the bearing shell is obtained by solving the three-dimensional elasticity equations. Using a suitable iterative solution procedure, the converged solutions for the lubricant flow and elastic deformation fields are obtained.     

PTFE-modified lubricants for application in a high vacuum

In order to guarantee reliable and consistent lubrication, with complete freedom from ‘stick-slip’ motion, in a precision sliding guide for operation in vacuum, several lubricants were evaluated in high vacuum tests. The lifetime of the equipment depended on the wear occuring at the sliding interface between a steel shaft and a diamond pin, and the best results were obtained with a combination of certain lubricating oils to which was added a fine-grained PTFE powder dispersed in a fluorine-based carrier.     

Enhancement of biodegradability of lubricants by biodegradation accelerants

The influence of a phosphor–nitrogen modified fatty acid, an amide fatty acid and tributyl phosphate, on biodegradation of a mineral oil and a high‐viscosity polyalphaolefin (PAO‐10) was studied. The results indicate that the phosphorous and nitrogenous compounds as biodegradation accelerants greatly promote biodegradation of unreadily biodegradable mineral oil and PAO‐10, due to the activation of enzymes in the body of microbes and the microbial growth of specific microbes. Copyright © 2008 John Wiley & Sons, Ltd.     

Fuel economy of multigrade gear lubricants

A light‐duty axle efficiency test for evaluating gear lubricants for their fuel economy performance is described. Data collected for an internal reference oil highlight the repeatability of the test with different axles. Comparisons between single‐grade SAE 90 and multigrade gear lubricants were made under a variety of pinion torques and speeds to simulate highway and city driving conditions. Lubricant rheology and its importance in maintaining film strength for adequate bearing and gear lubrication for optimum torque efficiency and axle temperature are discussed.     

Synthetic lubricants in India — an overview

Different synthetic products currently available and which find wide application as lubricants, are described, and their merits and demerits compared with the mineral based lubricants are outlined. Some of the major synthetic lubricants used in Indian industries are identified. The Indian chemical industry's potential to meet some of the requirements of synthetic lubricants is discussed and the need to create additional facilities in India for the manufacture of certain critical products, is described.     

On the rheological behaviour of synthetic fluids thickened by solid additives

A variety of fluid space lubricants, i.e., synthetic oils thickened by solid additives (particles of PTFE and MoS₂), has been developed. These lubricants present a solid-like behaviour under yield stress, and a strong non-Newtonian effect during flow. Measurement of the yield stresses revealed an unexpectedly strong interaction between a synthetic hydrocarbon oil and one type of PTFE particle. Furthermore, the substitution of 1 vol.% PTFE particles by MoS₂ particles introduced a significant change in the yield stress values. This effect is also apparent in viscosity measurements. These lubricants display, through their rheological behaviour, good ability to replenish contacts over a wide range of temperatures, as shown by measurements taken at −20°C and −60°C.     

A mechanism of thermo-oxidative degradation of polyol ester lubricants

Polyol ester lubricants are used in a wide range of temperature conditions, normally from −54 up to +220 °C, and their application can be limited by high temperature oxidation leading to viscosity increase, sludge, and/or formation of deposits. This paper is concerned with the chemical processes which take place during the oxidation of esters, particularly polyol esters, and discusses the selectivity of oxygen attack on ester molecules, and the formation and decomposition of hydroperoxides as well as other products formed as a result of ester oxidation. Proposed mechanisms of polyol ester lubricant oxidation are discussed, and unresolved problems which require additional research efforts are revealed.     

Rolling fatigue tests of three polyglycol lubricants

In this work, the rolling fatigue lives of three polyglycols (PAG-9, PAG-12 and BREOX-B-135X) are determined using IP-300 standard. A four-ball test machine was used and 10% life (L₁₀) and 50% life (L₅₀) were obtained. In addition, the stress-time curves for L₁₀ and L₅₀ were also determined. This work showed that: firstly, all polyglycols were tested under boundary lubrication regime (λ < 1) where in rolling contacts the surface mode of failures prevails; secondly, in oils of the same family, the pressure-viscosity coefficient is relatively constant, therefore an increase in viscosity improve the minimum film thickness with the consequent increase of the λ ratio; and finally, differences in λ ratio for the three polyglycols resulted in different asperity interactions and rolling contact fatigue lives.