The influence of polymer composition on the wear of the metal surface during fretting of steel on polymer

The effects of the nature of the polymer on the amount of metal wear during fretting of steel on polymers in laboratory air have been studied under a range of loads (130–330 g), amplitudes (3–10 μm) and frequencies (30–60 Hz).A number of polymers can cause damage to the metal, which takes the form of adhesive transfer of α-Fe₂O₃ particles to the polymer surface. The amount of metal wear depends on the polymer counterface and, under a given set of experimental conditions, increases in the order polytetrafluoroethylene (PTFE) and polyethylene, polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), polysulphone, Polyvinylchloride (PVC), polymethylmethacrylate (PMMA), polycarbonate, nylon 66. No metal wear occurs when the counterface is PTFE and only occurs with polyethylene when the amplitude is greater than 7 μm. These differences are explained in terms of the adhesive properties of the polymers, as determined by their surface energetics. Wear of the polymer during fretting takes the form of fibre formation for polysulphone, PVC, polycarbonate, nylon 66 and, to a small extent, polyethylene, while it takes the form of a transfer of a polymer film to the metal for PTFE and PVDF. No polymer wear occurs for PCTFE or PMMA.     

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.     

On the effects of friction on free volume holes in PTFE

Since the macroscopic properties of materials are relevant to their micro structure, the detection of microscopic defects or their changes in size and concentration is an interesting topic in materials science. The positron annihilation lifetime (PAL) method used here is established as a unique technique that can provide information about the size and concentration of free volume holes directly without significantly intervening in the bulk properties of the specimens. To our knowledge, this is the first time that PAL techniques have been used to study the wear mechanism of macromolecular tribomaterials. Here we present the results of a study in which the correlation between the fretting wear behaviour of polytetrafluorethylene (PTFE) and the changes in its free volume size and concentration in amorphous-crystal interface regions was found to be δω =A.δτ₃, where δω is the wear loss of PTFE under fretting wear conditions, A is a constant, and δτ₃ represents the relative change in the third lifetime components after the friction test. The results indicate that fretting mainly influences the characteristics of atomic-scale free volume holes in the amorphous-crystal region in PTFE, and under high-load fretting wear conditions the ductile polymer PTFE possesses a self-curing function for its defect-free micro volume holes.     

Low-Friction MoS x Coatings Resistant to Wear in Ambient Air of Low and High Relative Humidity

The investigation of the tribological performance of MoS₂-based coatings in air of high humidity is critical for the future use of such low-friction and high-wear-resistant coatings in ambient air. Sulfur-deficient MoS _x coatings with a basal plane (x = 1.3) and a random (x = 1.8) crystallographic orientation were produced by planar magnetron sputtering. The coefficient of friction and the wear loss of MoS_x coatings in comparison with TiN and amorphous TiB₂ coatings were investigated in bi-directional sliding fretting tests performed in ambient air of different relative humidity. The wear rate expressed as a volumetric loss per unit of dissipated energy was determined. From these results, the best friction and wear performance was achieved with basal-plane-oriented MoS _x coatings tested at a relative humidity in the range of 10-50%. A coefficent of friction of 0.06-0.08 and a wear rate of 4 × 10³ m³J^-1, at a normal load of 1 N and a fretting frequency of 10 Hz, were recorded for that type of MoS _x coatings.     

Abrasive wear performance of carbon fabric reinforced polyetherimide composites: Influence of content and orientation of fabric

Dry abrasive wear performance of five plain weave carbon fabric (CF) reinforced Polyetherimide (PEI) composites, developed with increasing CF contents (in the step of 10 vol%) is reported in this paper. It was observed that composite reinforced with 65 vol% CF (IP₆₅) exhibited the best tensile and shear strength and closely followed the leader (IP₇₅) in flexural strength. IP₆₅ when abraded against silicon carbide paper showed highest wear resistance (W_R) and lowest friction coefficient (μ) among all composites. The composites IP₈₅ and IP₄₀ containing highest and lowest amount of CF respectively showed least enhancement in strength properties and poorest wear performance. Parallel studies on the influence of fabric orientation with respect to the sliding plane and direction, on W_R showed that when CF was oriented parallel to the sliding plane, it had poorest wear resistance. The performance improved for the composites when fabric was oriented normal to the plane. The parallel or anti-parallel orientation of warp or weft fibers with respect to sliding direction showed marginal changes in friction and wear performance. Wear mechanisms were suggested and strongly supported by worn surface analysis using SEM.Efforts were also done to investigate the wear-property correlation. It was observed that the W_R was directly proportional to the product of interlaminar shear strength (I_s) and elastic modulus (E). Fairly good linearity was shown for specific wear rate (K₀) as a function of factor (μP/I_sE) where μ is coefficient of friction and P is the normal pressure (N/mm²).     

Influence of intergranular metallic nanoparticles on the fretting wear mechanisms of Fe–Cr–Al2O3 nanocomposites rubbing on Ti–6Al–4V

This paper presents a study concerning the influence of the amount of metallic nanoparticles on the wear behaviour of Fe_0.5–Cr_0.5–alumina nanocomposites rubbing on Ti–6Al–4V in fretting. Due to the elaboration process (metal–oxide nanopowder prepared by selective reduction in hydrogen of oxide solid solution and densified by spark plasma sintering), these materials generally own two sorts of nanoparticles: the intragranulars (size: ) located within the alumina grains and the intergranulars (size: ) located at the grain boundaries. This paper focuses on the role of each sort of nanoparticles with respect to the wear of the nanocomposite. Results show that the presence of intergranular nanoparticles is crucial for improving the wear resistance of nanocomposites whereas the intragranulars rather improve the mechanical properties of matrix grains. The lowest wear rate of the nanocomposite is obtained when the amount of intergranulars is about 3.5 wt%. Finally, the fretting wear mechanism of nanocomposites and the mechanism enabling to prevent it by using nanoparticles are both identified and discussed.     

The influence of water vapour in air on the friction behaviour of pure metals during fretting

An investigation was conducted to determine the effect of water vapour content in air on the frictional behaviour during fretting of pure metals: iron, aluminium, copper, silver, chromium, titanium and nickel. The fretting experiments were carried out under various humidity levels, ranging from dry air to 50% relative humidity at 23°C. During the experiment the frictional force between fretting surfaces was measured. Pure metals, except iron, were found to have a maximum value of the coefficient of friction during the steady-fretting stage (μ_s) at a specific humidity (RH_max). Iron showed a rapid decrease in μ_s with increasing humidity at RH_max. Each pure metal also exhibited maximum fretting wear at RH_max. The value of μ_s at RH_max for each metal was strongly related to the heat of formation of the lower metal oxide, indicating that the adhesive contact area was larger at RH_max for the fretting of metals with less chemical activity. At high humidity levels water vapour generally reduced the coefficient of friction, μ_s.     

Modeling adhesive wear resistance of Al-Si-Mg-/SiCp PM compacts fabricated by hot pressing process, by means of ANN

In this study, modeling adhesive wear resistance of Al-Si-Mg/SiC_p MMC compacts were performed by ANN, using a back-propagation neural network that uses gradient descent learning algorithm. Powder compacts were fabricated by PM hot pressing process with 5–10–20% SiC_p fractions and contents of specimens (N1, N2, N3 andN4) were given in Table 1. The wear tests were carried out under 10, 20 and 30 N variable loads, while disk rotation speed 90 rpm kept unchanged. Adhesive wear looses were measured and recorded for 250, 500, 1,000 and 1,500 m distances. Microstructure examination at wear surface was investigated by optical microscopy and EDS for metallographic evaluations. In neural networks training module, SiC_p reinforcement fractions (wt), loads and wear distances (m) were used as input, lost mass (g) of specimens were recorded as outputs. Then, the neural network was trained using the prepared training set (also known as learning set). At the end of the training process, the test data were used to check the system accuracy. As a result ANN was found successful in modeling of adhesive wear behavior and lost mass values of Al/SiC_p PM compacts.

Influence of content of carbon fabric on the low amplitude oscillating wear performance of polyetherimide composites

Tribo-potential of bi-directionally (BD) reinforced polymer composites is not yet adequately explored especially in low amplitude oscillating wear (LAOW)/fretting wear mode. Hence five composites of Polyetherimide (PEI) containing carbon fabric (plain weave) in the range 40–85 by vol% were developed by impregnation technique followed by compression molding. These composites along with unfilled PEI were evaluated for their LAOW performance on SRV Optimol tester under different loads using ball-on-plate configuration. The performance was compared with that of composite evaluated in earlier work but developed with different processing technique (hand lay up). With increase in load, specific wear rates of all the composites increased while friction coefficient (μ) decreased. It was concluded that carbon fabric inclusion in all amounts proved significantly beneficial for improving friction and wear performance and limiting load of PEI. Very high and very low amount of CF (85 and 40 vol%) proved least beneficial from strength and tribo-performance point of view. Composites with moderate amount of CF (65 and 55 vol%) proved most promising with almost similar potential in reducing μ and wear rate of PEI. Overall CF in the range of 55–65 vol% appeared to be the optimum range for tailoring the strength properties along with tribo-performance in fretting wear mode. The impregnation technique proved significantly better than the hand lay up technique for enhancement in strength and tribo-performance. SEM studies on worn surface proved helpful in understanding wear mechanisms.

Influence of weave of carbon fabric on abrasive wear performance of polyetherimide composites

Three composites of Polyetherimide (PEI) reinforced with carbon fabric (CF) of three weaves viz. plain, twill and satin-4 H were developed keeping the amount of fabric constant (55% by vol.). Studies on mechanical properties confirmed that the twill weave composite (T) showed the highest strength, modulus (both tensile and flexural) and interlaminar shear strength (ILSS) followed by satin (S) and plain weave (P) composites. The performance order, however, was reverse in the case of toughness and elongation to break. Specific wear rate in a single-pass, unidirectional and un-lubricated abrasive wear mode against SiC paper showed strong influence of weave in mild wear condition (load 10 N). Composite S showed the highest wear resistance (W _R) followed by composites T and P. With increase in load, the difference in performance diminished to the extent that at 40 N, it was almost similar for all the three composites. This was correlated with the difference in the length of the fibers between crossover points which, in turn, allowed the microdisplacement of fibers in the composites during abrasion. This was supported by the SEM.

Influence of PTFE content in PEEK-PTFE blends on mechanical properties and tribo-performance in various wear modes

Few papers are available on the optimum composition of PEEK-PTFE blends for the best possible combination of mechanical and tribological properties in the adhesive wear mode. Nothing is reported in this context on low amplitude oscillating/fretting wear mode. Moreover, the influence of increasing amounts of PTFE in the blend on abrasive wear behaviour along with a correlation with strength properties is not reported. Hence, in this work, five injection-moulded blends of PEEK with PTFE (in the range of 0-30 wt.%) were evaluated on a pin-on-disc configuration on an SRV Optimol Tester for their tribo-behaviour in the low amplitude oscillating wear mode. The data in the abrasive wear mode were generated by abrading a pin loaded against an abrasive paper fitted on the rotating disc. Data on neat PTFE were also included for comparison. It was observed that inclusion of PTFE affected the adhesive wear and low amplitude oscillating wear (LAOW) in a beneficial way. With an increase in PTFE contents, coefficient of friction in both the wear modes (adhesive and low amplitude oscillating) decreased but the trends in wear performance differed. In the adhesive wear mode, the specific wear rate showed minima for 7.5% PTFE inclusion followed by a slow increase for further PTFE addition. In the case of LAOW mode, on the other hand, the wear rate continuously decreased for the selected compositions. The 30% PTFE blend showed excellent combination of μ, wear rate and limiting pressure-velocity (PV) values. Unfilled PEEK proved to be fairly good wear-resistant material but exhibited high μ, a stick-slip tendency and a low PV limit value. Abrasive wear performance of the blends on the other hand, deteriorated with increasing amount of PTFE. Fairly good correlation was observed between the wear rate and product of H and S (H-hardness and S-ultimate tensile strength) rather than Ratner-Lancaster plot (product of S and e, where e is elongation to break).Thus, with increase in PTFE contents, though adhesive and LAOW performance increased substantially, it was at the cost of deterioration in all mechanical properties (except impact strength) and abrasive wear performance.     

Fretting wear behaviors of nanometer Al2O3 and SiO2 reinforced PEEK composites

The influence of diameter and content of Al₂O₃ particles on the tribological behaviors under fretting wear mode was investigated. The surface of PEEK composite and steel ball were examined by SEM and EDS, to identify the topography of wear scar and analyze the distribution of chemical elements in the friction counterparts, respectively. It can be found that the filling of Al₂O₃ powder improves the fretting wear resistance of PEEK composite. With the increase of Al₂O₃ diameter, the area of wear scar on specimen increases first and decreases afterward. However, the wear of composites increases monotonically with increasing Al₂O₃ content. Although the filling of 10 wt.% and 200 nm PTFE powder in PEEK makes the lowest wear of all specimens, no synergistic effect was found when Al₂O₃ and PTFE were filled into PEEK composite together. For the friction pair of PEEK composite and steel ball, abrasive wear and adhesive wear dominate the fretting wear mechanism during fretting. Thermal effect plays a very important role during fretting; thus the property of temperature resistance for polymer material would affect the wear degree on the surface of wear scar.     

Fretting fatigue strength estimation considering the fretting wear process

In fretting fatigue process the wear of contact surfaces near contact edges occur in accordance with the reciprocal micro-slippages on these contact surfaces. These fretting wear change the contact pressure near the contact edges. To estimate the fretting fatigue strength and life it is indispensable to analyze the accurate contact pressure distributions near the contact edges in each fretting fatigue process.So, in this paper we present the estimation methods of fretting wear process and fretting fatigue life using this wear process. Firstly the fretting-wear process was estimated using contact pressure and relative slippage as follows:

W=K×P×S,

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.     

Use of Au Nanoparticle-Filled PTFE Films to Produce Low-Friction and Low-Wear Surface Coatings

Polytetrafluoroethylene (PTFE) possesses exceptional lubricating properties; however, its uses are limited due to its high susceptibility to wear. In an effort to overcome this shortcoming, a great deal of focus is placed on creating PTFE composites that exploit the strengths of PTFE and also reduce or eliminate its weaknesses. This investigation explores the use of Au nanoparticle-filled PTFE films to produce low-friction and low-wear surface coatings. PTFE + Au nanoparticle composite films were produced by dip coating stainless steel substrates into a mixture of colloidal PTFE and Au nanoparticles. Tribological tests showed that the composite film has a wear life that is twice that of pure PTFE and possesses an average coefficient of friction that is up to 50 % lower. PTFE suffers delamination as a result of poor adhesion of the film to the substrate and tearing resulting from a dominant adhesive wear mode. PTFE + Au, on the other hand, shows no sign of delamination or adhesive wear. This change in wear mode caused by the addition of Au nanoparticles significantly increases the wear resistance and durability of PTFE.     

Wear-Resistant PTFE/SiO2 Nanoparticle Composite Films

Polytetrafluoroethylene (PTFE) is a polymer that is well known for its exceptional tribological properties and, as such, it is commonly used to reduce the coefficient of friction between surfaces. In recent years it has also been established that by incorporating nanoparticle fillers in PTFE, it is possible to extend the polymer's life by reducing its wear rate. Although much study has been placed on bulk PTFE, very little study has been focused on thin films. This article demonstrates that SiO ₂ nanoparticles can be used as a filler to significantly reduce the wear of PTFE thin films while also maintaining a low coefficient of friction. The wear resistance and coefficient of friction of PTFE/SiO ₂ composite films on stainless steel substrates were tested using a linear reciprocating tribometer and compared to pure PTFE films and bare stainless steel to evaluate the benefit of incorporating the SiO ₂ filler in the film. The composite films showed a significant improvement in wear resistance when compared to pure PTFE films. The coefficient of friction for the composite film remained low and stable during a 50 g normal load friction test for a duration of approximately 300 cycles, whereas that of PTFE showed an increasing trend at onset. In addition, of 1.7 and 3.3 wt% SiO ₂ concentrations in solution, 3.3 wt% SiO ₂ showed better performance, with a much higher wear resistance than that of 1.7% SiO ₂ after being subjected to a 1,000-cycle abrasive wear test.     

Tribological behaviour of polymeric coatings. Part I. Aramid particle‐reinforced epoxy nanocomposite systems

An epoxy‐based nanocomposite containing a low concentration of nanometric TiO₂ (4 vol. %), graphite powder (7.21 vol. %), and 2–14 vol. % aramid particles was developed as a coating material. The mechanical and tribological performance of the composites was investigated. The epoxy filled only with TiO₂ possessed significantly improved impact strength and flexural properties, whereas the further incorporation of graphite and aramid particles had a deleterious effect on most of the mechanical properties, except the modulus. The tribological behaviour of the composites was tested in sliding and fretting modes. Under sliding conditions, the addition of nanometric TiO₂ alone significantly improved the wear resistance and decreased the coefficient of friction compared to the neat epoxy. The sliding wear and friction behaviour was further enhanced with the incorporation of graphite and aramid particles. Contrary to the sliding wear behaviour, the fretting wear and friction behaviour was worse for the epoxy filled only with TiO₂, but was significantly enhanced by the incorporation of graphite and aramid particles. The optimum aramid particle content for sliding and fretting wear of the epoxy‐based nanocomposite was determined as 10 vol. %.     

表面改性硅灰石纤维填充PTFE复合材料的摩擦学性能

用KH550硅烷偶联剂表面改性的硅灰石纤维(WF)填充PTFE,在MPX-2000型磨损试验机上研究复合材料的摩擦磨损性能,并与经典的炭纤维(CF)填充PTFE复合材料进行比较。采用SEM对磨损面和对偶面进行分析。结果表明:较高载荷(200和300 N)下复合材料摩擦因数随WF含量变化的幅度不大,较稳定地维持在较低值;细小尺寸WF填充PTFE复合材料的耐磨性能较好,在WF质量分数为10%时,复合材料的磨损量只有相同含量CF填充PT-FE复合材料的81%;细小尺寸WF填充PTFE复合材料的磨损面较为平整,存在轻微黏着磨损,其对偶面转移膜平整光滑、结构致密;而CF/PTFE复合材料磨损面存在许多裸露和碎断的CF,犁削和磨粒磨损是主要的磨损形式。     

Tribological Behavior of Carbon-Nanotube-Filled PTFE Composites

Carbon nanotube/polytetrafluoroethylene (CNT/PTFE) composites with different volume fractions were prepared and their friction and wear properties were investigated using a ring-on-block under dry conditions. It was found that CNTs signifi-cantly increased the wear resistance of PTFE composites and decreased their coefficient of friction. PTFE composites with 15–20 vol.% CNTs exhibited very high wear resistance. The significant improvements in the tribological properties of CNT/PTFE composites are attributed to the super-strong mechanical properties and the very high aspect ratio of CNTs. The CNTs greatly reinforce the structure of the PTFE-based composites and thereby greatly reduce the adhesive and plough wear of CNT/PTFE composites. The CNTs are released from the composite during sliding and transferred to the interface of the friction couples. They thus serve as spacers, preventing direct contact between the mating surfaces and thereby reducing both wear rate and friction coefficient.     

The Leiden MEMS Tribometer: Real Time Dynamic Friction Loop Measurements With an On-Chip Tribometer

On-chip MEMS tribometer devices until now have been much less sophisticated for dynamically sensing frictional forces than their FFM (friction force microscope) counterparts. In this article, we present a MEMS-based tribometer that can be used to measure dynamically, on-chip and in-situ, the frictional properties of MEMS-scale contact geometries. The device provides the first FFM-like friction loops with contacting MEMS sidewall surfaces. Depending on the normal load two regimes of operation are identified. At low and intermediate loads, the frictional behaviour reflects wear-less relative motion of the silicon oxide surfaces of the MEMS device and we observe repeatable, irregular stick-slip behaviour, related to the surface roughness. At very high loads, wear causes changes in the topography of the contacting surfaces.