Optimum concentration of reinforcement and solid lubricant in polyamide 12 composites for best tribo-performance in two wear modes

In general polymers are used in the form of composites (fiber reinforced, solid lubricated or both) in tribo-applications, where they may encounter more than one type of wear situations or mechanisms to different extents. The area of investigating the optimum concentration of fillers for best combination of tribo-performance in different wear modes and mechanical strength is sparingly researched. In this paper, research findings on the influence of the contents of short carbon fibers (CF) and PTFE (particulate form) in Polyamide (PA) 12 on friction and wear behavior in two wear modes (adhesive and fretting) have been reported. With increase in contents of CF up to 30% (vol) most of the mechanical properties and tribo-performance improved in adhesive and fretting wear modes. With a view to enhance it further, PTFE was added step by step in the best performing composite (PA+30% CF). This boosted the tribo-performance further, however, at the cost of strength properties. With increase in PTFE percentage (10, 15 and 20% by vol), specific wear rate (K _o) and friction coefficient (μ) both decreased appreciably in adhesive as well as fretting wear modes. The composite consisting of 30% CF and 20% PTFE showed lowest values for μ and K _o rendering it the best tribo-combination for all practical purposes. The abrasive wear behavior of composites was also investigated. However, wear performance in this mode showed exactly opposite trends. Inclusion of fibers or combination of fibers and PTFE proved detrimental.

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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.     

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.     

Fretting Wear Study on Micro-Arc Oxidation TiO<Subscript>2</Subscript> Coating on TC4 Titanium Alloys in Simulated Body Fluid

Tribological properties of TiO₂ coatings synthesized by micro-arc oxidation (MAO) on the surface of TC4 titanium alloys were investigated at the fretting contact against 440C stainless steel in simulated body fluid (SBF). Fretting experiments were carried out by ball-on-flat contact at various loads for 1 h, with an amplitude of 100 μm and a frequency of 5 Hz. Results show that MAO TiO₂ coatings presented good tribological properties with lower friction coefficient in SBF. Less wear volume was observed for MAO TiO₂ coatings compared with that for TC4 alloy. At lower load, the wear mechanism of MAO TiO₂ coatings was dominated to abrasive wear. With an increase of normal load, however, fretting corrosion increased due to chemical reactions with SBF, and therefore, fretting fatigue coexisting with abrasive wear became the predominant mode.     

Etched-pocket, dry-bearing materials

Novel, dry-bearing materials have been fabricated by photo-etching an array of very small, blind holes in metal surfaces and filling them with solid lubricant formulations. Friction and wear were assessed against a tool-steel counterface in reciprocating line-contact conditions. Preliminary experiments demonstrated feasibility with a range of substrates (bronze, steel, cobalt alloy) and lubricants (PTFE + Pb, MoS₂ + polyimide). Detailed studies were made with different copper alloys to examine the effects of substrate hardness and the distribution, size, spacing and depth of the etched pockets. Be---Cu containing PTFE + Pb exhibits wear lives over a range of stresses (Hertzian) to 375 MPa which are appreciably longer than those of a porous bronze composite containing the same lubricant. Wear rates of etched Be----Cu are also much lower than those of a typical, woven PTFE fibre-type of dry-bearing composite. Wear life is not, in general, limited by the depth of the etched pockets and several failure mechanisms have been identified.     

Antiwear Properties of Bonded MoS2 Solid Lubricant Coating under Dual-Rotary Fretting Conditions

Bonded MoS₂ solid lubricant coatings are widely used in tribology for their friction-reducing and antiwear properties. However, such coatings have been rarely investigated in complex fretting conditions, such as dual-rotary fretting (DRF). DRF is a complex fretting wear mode that combines torsional fretting with rotational fretting. In this work, the antiwear properties of bonded MoS₂ solid lubricant coating under dual-rotary fretting conditions were studied. Results indicated that the MoS₂ coating had better friction-reducing and antiwear properties than the substrate for alleviating DRF wear. The coating can greatly influence the fretting regimes and reduce the coefficient of friction. Furthermore, the service life of the coating was strongly dependent on the competition of the two fretting components and was reduced as the rotational fretting component increased.     

Impact of Viscoelasticity on the Tribological Behavior of PTFE Composites for Valve Seals Application

In this article, we studied and explored the impact of viscoelasticity on the friction and wear behavior of pure polytetrafluoroethylene (PTFE), carbon–graphite PTFE composite, and glass fiber–MoS₂ PTFE composite. Tests were carried out using a specific reciprocating tribometer for valve seal application. The worn surfaces of the PTFE composites and the transfer films formed on the counterface were examined with a scanning electron microscope (SEM). Experimental results revealed that the addition of filler materials was effective in reducing the wear volume in all composites studied. In addition, the friction coefficient and wear resistance showed high sensitivity to the viscoelastic behavior of the PTFE seal. SEM investigation showed that the incorporation of particulate fillers into the PTFE matrix could dramatically reduce and stabilize the transfer films to the counterface, so they largely decreased the wear of the PTFE composites.     

Tribological behaviour of new chemically bonded PTFE polyamide compounds

Topic of the study is the formation of new PTFE polyamide materials by reactive extrusion. The new type of formed PTFE polyamide compound shows very good material properties. Recently it has been revealed that carboxylic acid groups exhibit a very high reactivity under polyamide melting conditions. PTFE micro powders functionalized by carboxylic acid groups are the base for the block copolymer formation in polyamide melts under defined process conditions. Such functionalized micro powders are formed from virgin PTFE by electron irradiation in the presence of oxygen. These new PTFE polyamide materials can be processed easily using commercial (common) process equipment like twin-screw extruders and injection molding. Many experimental investigations have been performed under dry sliding friction on PA 6, PA 6.6 and PA 12 compounds with PTFE weight portions between 3.3 and 50%. They show low coefficients of friction and low specific wear rates. The wear resistance of newly developed PTFE polyamide compounds is comparable with commercially available mechanically or physically produced PTFE and PEAK compounds.     

Utility of a fretting device working under free displacement

Relative movements of low amplitudes between two materials in contact are generally reproduced on fretting devices with imposed displacement or imposed tangential force. The damage kinetics observed (cracking, wear) is established under such conditions. In this article, a fretting device working under free displacement is used to characterize the damages generated by seizure and wear. The conditions of seizure are analyzed from the total sliding distance and the discussion is focused on a correlation established with Dupre's work of adhesion. The wear behavior of materials has been characterized from an energetic wear coefficient taking into account the wear volume of contact, the total sliding distance and the dissipated energy.     

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. %.     

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.     

Tribological study of peroxide-cured EPDM rubber filled with electron beam irradiated PTFE powder

The aim of this work is the evaluation of the effects of electron beam modification of Polytetrafluoroethylene (PTFE) powder on the friction and wear properties of PTFE filled Ethylene-Propylene-Diene-Monomer (EPDM) rubber cured by a radical-initiated peroxide system. Friction and wear properties of EPDM vulcanizates were determined with the help of pin-on-disc tribometer in sliding contact with a steel-ball at room temperature without lubrication. PTFE powder was modified in atmospheric conditions with low (20 kGy) and high (500 kGy) irradiation doses using electron beam accelerator. The spectroscopic investigations reveal that the increasing concentration of reactive free radicals and functional groups with irradiation dose has a drastic influence on crosslinking efficiency due to their interference with peroxide radicals in curing process. As a result, non-irradiated and low-irradiated PTFE filled EPDM with higher crosslinking densities showed remarkably enhanced friction and wear properties. On the contrary, 500 kGy PTFE filled EPDM having significantly lower crosslinking density resulted in poor friction and wear characteristics. The apparent crosslinking density measured directly from the curing curves was found to be the most dominating parameter influencing friction, wear as well as the physical properties.     

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.     

Boric Acid Self-Lubrication of B2O3-Filled Polymer Composites

Utility of boric oxide particles in PTFE and epoxy composite materials, in sliding contact with stainless steel, is explored. Boric oxide filler can provide PTFE with a two-decade reduction in wear rate, to 10^?5 mm₃/N-m. With adequate ambient humidity reduced wear rate can be achieved without inducing counterface abrasion, and the friction of PTFE is further reduced slightly. In such environments, boric oxide fillers can also reduce friction coefficient of epoxy from μ>0.7 to as low as μ=0.07. This lubrication mechanism results from replenishment of lubricous boric acid lamellar solid provided to the sliding interface by reaction of boric oxide with ambient water. Maintenance of the lubricating effect depends upon a sufficient rate of boric acid formation, relative to subsequent removal by wear. It is demonstrated that this formation/removal balance is affected by relative humidity and volume fraction of boric oxide filler, as well as normal load and sliding speed.     

Thin layer activation for measuring fretting wear and material transfer in iron alloys

An important difference between fretting wear and macroscopic wear processes lies in the amounts of wear volume. Because of the small relative displacements in fretting wear the wear volumes are much smaller than in (classical) macroscopic wear processes. Classical wear measuring techniques such as weighing and 2D-profilometry often fail to determine the fretting wear volumes. Their sensitivity as well as their accuracy are inadequate. 3D-profilometry partially solves this problem but the obtained wear results do not take eventual material transfer between the test specimens into account. In this work thin layer activation (TLA) is presented as a potentially powerful tool for quantitative fretting studies, especially in relation to material transfer. In contrast with the other mentioned wear measuring technique TLA allows measurements of very small amounts of wear as well as material transfer between the test specimens. Preliminary results are reported for steel specimens. A surface zone of 7 mm² of one test specimen, in which the wearing zone during the fretting experiment is situated, is irradiated with protons from a cyclotron. As a result of a nuclear reaction the iron in the irradiated test specimen forms radionuclides, mainly cobalt-56, in a surface layer of 260 μm depth. The amount of cobalt-56 as compared to iron is negligible so that the physical and chemical properties of the irradiated zone remain unaffected. Measuring the gamma rays of cobalt-56 in the wear debris gives quantitative information about the wear volume and measuring the mating surface gives information on the transfer volume.     

High temperature fretting wear behavior of WC-25Co coatings prepared by D-gun spraying on Ti-Al-Zr titanium alloy

Detonation gun (D-gun) spraying is one of the most promising spraying techniques for producing wear-resistance coatings. A thick layer (about 0.3 mm thickness) of WC-25Co with high hardness was covered on Ti-Al-Zr titanium alloy by D-gun spraying and the fretting wear behavior of WC-25Co coatings was studied experimentally on a high precision hydraulic fretting wear test rig. An experimental layout was designed to perform fretting wear tests at elevated temperatures from room temperature (25 °C) to 400 °C in ambient air. In the tests, a sphere (Si₃N₄ ceramic ball) was designed to rub against a plane (Ti-Al-Zr titanium alloy with or without WC-25Co coatings). It was found that the fretting running regimes of WC-25Co coatings were obviously different from those of Ti-Al-Zr titanium alloy. The mixed fretting regime disappeared in WC-25Co coatings, and the boundaries in the running condition fretting map (RCFM) showed hardly any change as temperature increased. The worn scars were examined using a laser confocal scanning microscope (LCSM), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results showed that the coefficients of friction (COF) of WC-25Co coatings at elevated temperatures were nearly constant in the partial slip regime and very low in the steady state. The fretting damage of the coatings was very slight. In the slip regime, the WC-25Co coatings exhibited a good wear resistance, and the wear volume of the coatings obviously decreased with increasing tested temperature. The fretting wear mechanisms of WC-25Co coatings were delamination, abrasive wear and oxidation wear at elevated temperature. The oxide debris layer formed at higher temperature was denser and thicker on top of WC-25Co coatings, thus providing more surface protection against fretting wear, which played an important role in the low fretting wear of the coatings.     

含硫化铜纳米粒子润滑脂的微动磨损性能研究

在微动磨损试验机上考察了含有硫化铜纳米粒子脲基脂的微动磨损性能。结果表明:硫化铜纳米粒子能显著降低微动磨损体积,随着纳米粒子含量增加,微动磨损量降低;在微动磨损后期纳米粒子的存在还能降低摩擦因数。低负荷下,润滑脂中硫化铜纳米粒子有利于降低微动磨损量;但在高负荷条件下,磨损量迅速增大,说明高负荷下微动磨损方式发生了变化。XPS分析表明,微动磨斑表面膜含有Cu、FeS等物质,说明硫化铜纳米粒子能显著降低微动磨损的原因在于纳米粒子化学性质非常活泼,在微动过程中容易与摩擦表面发生化学反应,形成具有保护性的沉积物膜和化学反应膜。     

Contact fatigue and wear behaviour of bismaleimide polymer subjected to fretting loading under various temperature conditions

Cracking and wear induced by fretting is a critical problem for industrial composite structures. Thermosetting bismaleimide resin is a promising material due to its good mechanical and thermal properties. The effect of temperature regarding fretting cracking and fretting wear is presently investigated. The temperature effect on crack initiation and propagation is quantified combining experiments and modelling. The fretting wear is explained using a friction energy wear approach. A bilinear evolution of wear volume versus the dissipated energy is identified and related to a protective third body layer. These various damage evolutions are compared to the viscoelastic properties of the polymer.     

An investigation of the fretting behaviour of low friction coatings on steel

Three different low friction coatings on steel (electrolytically deposited cadmium, PTFE solid lubricant in epoxy resin and PTFE solid lubricant in polyimide resin) were studied in order to relate their fretting behaviour with mechanical properties. Particular importance was given to adhesion which was measured using a scratch test. Fretting tests were carried out on the steel substrate and on the coatings under the same conditions. The major parameters of the tribological system were identified and then quantified. The values of the parameters obtained for each coating were compared with the corresponding values for uncoated steel. The mechanical characteristics of the coatings and their fretting parameters were plotted using a polar diagram in order to give an overview of the fretting behaviour of each coating. Differences were noted and the corresponding parameters were identified. The influence of the adherence of the coating and of the fretting test load on the lifetime of the coating was determined.     

Reduction in wear of metakaolinite-based geopolymer composite through filling of PTFE

Metakaolinite-based geopolymer composite containing 5-30% (volume fraction) polytetra-fluoroethylene (PTFE) was synthesized using compound activator composed of aqueous NaOH and sodium silicate at room temperature. Flexural strength, compressive strength and elastic modulus of the composite were measured. Tribological behaviour of the composite sliding against AISI-1045 steel was investigated on an MM-200 friction and wear tester. SEM, EDS and XPS analysis were conducted on worn surfaces and wear debris. The results show that mechanical strength of the composite was lower than corresponding geopolymer while the wear model became mild. The friction process was stable and the wear rate was dramatically reducted by 86-99.4%. The improvement of tribological properties of the composite was attributed to form a brown soft thin layer on the worn surface of the composite containing Fe₂O₃ came from tribochemical reaction. EDS analysis on the worn surfaces indicate the content of Fe increase along with the increase of volume content of PTFE in the composite. Furthermore, the counterpart, the steel ring was also protected from terrible wear as occurred when friction with geopolymer without any filling of solid lubricant. There is a brown thin layer mainly composed of Fe₂O₃ on the steel ring.