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.     

The friction and wear properties of nanometre SiO2 filled polyetheretherketone

Nanometre SiO₂ filled-polyetheretherketone (PEEK) composite blocks with different filler proportions were prepared by compression moulding. Their friction and wear properties were investigated on a block-on-ring machine by running a plain carbon steel (AISI 1045 steel) ring against the composite block. The morphologies of the wear traces and the transfer film were observed by scanning electron microscopy (SEM). It was found that nanometre SiO₂ filled-PEEK exhibited considerably lower friction coefficient and wear rate in comparison with pure PEEK. The lowest wear rate was obtained with the composite containing 7.5 wt.% SiO₂. The SEM pictures of the wear traces indicated that with the frictional couple of carbon steel ring/composite block (fillec with 7.5 wt.% filler), a thin, uniform, and tenacious transfer film was formed on the ring surface. It was inferred that the transfer film contributed largely to the decreased friction coefficient and wear rate of the filled PEEK composites.     

A study on friction and wear characteristics of nanometer Al2O3 /PEEK composites under the dry sliding condition

The friction and wear properties of the polyetheretherketone (PEEK) based composites filled with 5 mass% nanometer or micron Al₂O₃ with or without 10 mass% polytetrafluroethylene (PTFE) against the medium carbon steel (AISI 1045 steel) ring under the dry sliding condition at Amsler wear tester were examined. A constant sliding velocity of 0.42 m s⁻¹ and a load of 196 N were used in all experiments. The average diameter 250 μm PEEK powders, the 15 or 90 nm Al₂O₃ nano-particles or 500 nm Al₂O₃ particles and/or the PTFE fine powders of diameter 50 μm were mechanically mixed in alcohol, and then the block composite specimens were prepared by the heat compression moulding. The homogeneously dispersion of the Al₂O₃ nano-particles in PEEK matrix of the prepared composites was analyzed by the atomic force microscopy (AFM). The wear testing results showed that nanometer and micron Al₂O₃ reduced the wear coefficient of PEEK composites without PTFE effectively, but not reduced the friction coefficient. The filling of 10 mass% PTFE into pure PEEK resulted in a decrease of the friction coefficient and the wear coefficient of the filled composite simultaneously. However, when 10 mass% PTFE was filled into Al₂O₃/ PEEK composites, the friction coefficient was decreased and the wear coefficient increased. The worn scars on the tested composite specimen surfaces and steel ring surfaces were observed by scanning electron microscopy (SEM). A thin, uniform, and tenacious transferred film on the surface of the steel rings against the PEEK composites filled with 5 mass% 15 nm Al₂O₃ particles but without PTFE was formed. The components of the transferred films were detected by energy dispersive spectrometry (EDS). The results indicated that the nanometer Al₂O₃ as the filler, together with PEEK matrix, transferred to the counterpart ring surface during the sliding friction and wear. Therefore, the ability of Al₂O₃ to improve the wear resistant behaviors is closely related to the ability to improve the characteristics of the transfer film.     

A Study on the Sliding Wear of Hybrid PTFE/Kevlar Fabric/Phenolic Composites Filled with Nanoparticles of TiO2 and SiO2

TiO₂ and SiO₂ nanoparticles were introduced into hybrid polytetrafluoroethylene (PTFE)/Kevlar fabric/phenolic composites. The results showed the incorporation of TiO₂ nanoparticles can reduce the wear rate of the fabric/phenolic composite at elevated temperatures, although the wear of hybrid PTFE/Kevlar fabric/phenolic composite did not change much when TiO₂ or SiO₂ nanoparticles were used as filler. The wear behavior was explained in terms of morphology of transfer films and worn surfaces. There was a good correlation between the morphology of transfer film and wear results.     

Tribological Sensitivity of PTFE/Alumina Nanocomposites to a Range of Traditional Surface Finishes

Wear tests were performed with polytetrafluoroethylene (PTFE) + Al ₂ O ₃ nanocomposites on various manufactured surfaces to determine whether or not the wear resistance of these nanocomposites is a strong function of surface preparation. Four different surface finishes of grade 304 stainless steel counterfaces were used: electropolished (R _q = 88 nm), lapped (R _q = 161 nm), wet-sanded (R _q = 390 nm), and dry-sanded (R _q = 578 nm). PTFE + Al ₂ O ₃ nanocomposites made from powders of roughly 2-20 μm PTFE (matrix) and ～44 nm Al ₂ O ₃ (filler) were prepared at filler weight percentages of 0, 1, 5, and 10% and tested on each surface finish. Additionally, 5 wt% 44-nm nanocomposites were compared to identically prepared 5 wt% 80- and 500-nm Al ₂ O ₃ filled PTFE composites on each surface. Friction coefficients were between 0.12 and 0.19 and wear rates decreased from K = 810 × 10^{? 6} mm ³ /(Nm) for the 5 wt% 500-nm alumina-filled PTFE on the dry-sanded surface to K = 0.8 × 10^{? 6} mm ³ /(Nm) for the 5 wt% 80-nm filled composite on the lapped surface. It was found that the minimum wear rate occurred on the lapped counterface for every composite, and the wear rate is a strong function of the transfer film thickness and morphology.     

纳米ZnO填充的PTFE基复合材料摩擦学性能研究

得胜０００型摩擦磨损试验机研究了不同体积含量的纳米氧化锌（ＺｎＯ）填充的ＰＴＦＥ基复合材料在于摩擦条件下与不风对摩时的摩擦学性能,并利用扫描电子微镜（ＳＥＭ）对ＰＴＦＥ及纳米ＺｎＯ／ＰＴＦＥ复合材料的微观结构、磨损表面和转移膜进行了观察和分析。结果表明,纳米ＺｎＯ／ＰＴＦＥ复合材料的摩擦性能与纯ＰＴＦＥ基本相当,但耐磨性明显优于后者,纳米ＺｎＯ在复合材料中的最佳含量为１５ｖｏｌ．％左右。     

Effect of Particle Size on the Wear Resistance of Alumina-Filled PTFE Micro- and Nanocomposites

It was long supposed that the ability of hard particle fillers to reduce the wear rate of unfilled PTFE (typically ～ 10^{? 3} mm ³ /Nm) by an order of magnitude or more was limited to fillers of microscale or greater, as nano-fillers would likely be encapsulated within the large microscale PTFE wear debris rather than disrupting the wear mechanism. Recent studies have demonstrated that nano-fillers can be more effective than microscale fillers in reducing wear rate while maintaining a low coefficient of friction. This study attempts to further elucidate the mechanisms leading to improved wear resistance via a thorough study of the effects of particle size. When filled to a 5% mass fraction, 40- and 80-nm alumina particles reduced the PTFE wear rate to a ～ 10^?7 mm ³ /Nm level, two orders of magnitude better than the ～ 10^?5 mm ³ /Nm level with alumina micro-fillers at sizes ranging from 0.5 to 20 μm. Composites with alumina filler in the form of nanoparticles were less abrasive to the mating steel (stainless 304) countersurfaces than those with microparticles, despite the filler being of the same material. In PTFE containing a mixture of both nano- and micro-fillers, the higher wear rate microcomposite behavior predominated, likely the result of the continued presence of micro-fillers and their abrasion of the countersurface as well as any overlying beneficial transfer films. Despite demonstrating such a large effect on the wear rate, the variation of alumina filler size did not demonstrate any significant effect on the friction coefficient, with values for all composites tested additionally falling near the μ = 0.18 measured for unfilled PTFE at this study's 0.01 m/s sliding speed.     

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.     

Experimental Evaluation of Friction and Wear Properties of Solid Lubricant Coatings on SUS440C Steel in Liquid Nitrogen

The friction and wear properties of the prevailing different solid lubricant coatings (Ion-plated Au, Ion-plated Ag and RF-sputtered PTFE on SUS440C stainless steel) used in the bearings of high-speed cryogenic-turbo-pumps of liquid rocket engines were experimentally evaluated in liquid nitrogen immersed conditions. Also the above experiments were carried out with two newly proposed solid lubricant coatings of sputter-ion-plated MoSTi and a new ion-plated Pb on SUS440C stainless steel. The friction coefficient and wear rates of the coatings of ion-plated Au, ion-plated Ag, RF-sputtered PTFE, the new ion-plated Pb and MoS₂Ti-SIP (with coating thickness of 0.7±0.1 μm) on SUS440C steel against SUS440C stainless steel ball in liquid nitrogen were compared. Worn surfaces were examined microscopically with a microscope and a profilometer for understanding the mechanisms of friction and wear and transfer film lubrication in liquid nitrogen. It is found that the newly proposed solid lubricant coatings are showing promising results for their use in liquid nitrogen immersed conditions. The sputter-ion-plated MoSTi coating on SUS 440C steel shows a minimum value of friction coefficient (μ=0.015) and wear rate (w_c=0.56 × 10⁻⁶ mm³/N m ) in liquid nitrogen.     

Wear and Friction of Filled Polytetrafluoroethylene Compositions in Liquid Nitrogen

Wear and friction behavior of slider materials at cryogenic temperature is important to the development of seals and bearings for missile powerplants. Data were obtained in liquid nitrogen (?320°F) with a series of molded and extruded polytetrafluoroethylene (PTFE) compositions containing various filler materials. A 3/16-in. radius rider specimen (PTFE materials) was caused to slide in a circumferential path on the flat surface of a rotating 2½-in. diameter disk specimen (usually type 304 stainless steel). The sliding velocity was usually 2300 ft per min and the load was 1000 grams.

As compared with reference steels and carbons used in conventional seals and bearings, the filled PTFE compositions gave low wear and friction (friction coefficients from 0.06 to 0.13) in liquid nitrogen. Several extruded compositions have particular promise for seal and bearing materials. An extruded glass-filled material gave wear and friction that was essentially unaffected by sliding velocities to 6000 ft per min.     

High-Temperature Tribological Behavior of Al-20Si-5Fe-2Ni/ZrB2 Composites

Abstract

In this work, Al-20Si-5Fe-2Ni/ZrB₂ composites with 0–20?wt% ZrB₂ were fabricated by spark plasma sintering. The effects of ZrB₂ content on the microstructure, mechanical properties and high-temperature tribological behavior of the composites were investigated. The results indicate that Si, Al₅FeSi, and ZrB₂ particles are uniformly distributed in the aluminum matrix. The density, hardness, and compressive strength increase with increasing ZrB₂ content. The friction coefficient and wear rate are dependent on the ZrB₂ content and test temperature. At a certain temperature, the friction coefficient increases with an increase in ZrB₂ content, whereas the wear rate shows a reverse trend. Due to the improvement in thermal stability and high-temperature softening resistance, the composite shows improved wear resistance and increased transition temperature from mild wear to severe wear.     

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.     

Dry Sliding Friction and Wear Characteristics of Cu-Sn Alloy Containing Molybdenum Disulfide

The wear and sliding friction response of a hybrid copper metal matrix composite reinforced with 10 wt% of tin (Sn) and soft solid lubricant (1, 5, and 7 wt% of MoS₂) fabricated by a powder metallurgy route was investigated. The influence of the percentages of reinforcement, load, sliding speed, and sliding distance on both the wear and friction coefficient were studied. The wear test with an experimental plan of six loads (5–30 N) and five sliding speeds (0.5–2.5 m/s) was conducted on a pin-on-disc machine to record loss in mass due to wear for two total sliding distances of 1,000 and 2,000 m. The results showed that the specific wear rate of the composites increased at room temperature with sliding distance and decreased with load. The wear resistance of the hybrid composite containing 7 wt% MoS₂ was superior to that of the other composites. It was also observed that the specific wear rates of the composites decreased with the addition of MoS₂. The 7 wt% MoS₂ composites exhibited a very low coefficient of friction of 0.35. The hardness of the composite increased as the weight percentage of MoS₂ increased. The wear and friction coefficient were mainly influenced by both the percentage of reinforcement and the load applied. Wear morphology was also studied using scanning electron microscopy and energy-dispersive X-ray analysis.     

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.     

PTFE对纤维增强尼龙66材料摩擦学性能的影响

考察了玻璃纤维（GF）增强尼龙66复合材料的摩擦磨损性能,以及PTFE对复合材料摩擦学性能的影响,利用扫描电镜分析了磨损形貌。结果表明：15％GF增强尼龙复合材料的摩擦学性能改善不明显,而且磨损量高于纯尼龙;加入PTFE在摩擦过程中形成了转移膜,降低了玻璃纤维增强尼龙复合材料的摩擦磨损,改善了其摩擦学性能。     

An investigation of the transfer film characteristics and the tribological behaviors of polyphenylene sulfide composites in sliding against tool steel

The effect of CuS, CuO, CaF₂, and ZnF₂ fillers in particulate sizes of 200–350 mesh on the friction and wear behavior of polyphenylene sulfide (PPS) was investigated. The filler proportion studied was 35 vol.%. The specimens which were prepared by compression molding were tested for friction and wear in a pin-on-disc configuration using hardened tool steel disc as the counterface. The morphologies of the worn surfaces and the transfer films were studied by scanning electron microscopy and the transfer films were analyzed by energy dispersive spectroscopy. The improvement in wear resistance with CuS was considerable and with CuO marginal, while the wear resistance decreased with CaF₂ and ZnF₂. The coefficient of friction increased significantly with CuO and decreased with CaF₂ and was not significantly affected by the other fillers. The changes in the friction and wear behaviors were analyzed in terms of the morphology and the composition of transfer films.     

Tribological Properties of PTFE and PTFE Composites at Different Temperatures

The friction and wear properties of polytetrafluoroethylene (PTFE) and its composites with fillers such as bronze, glass fiber, carbon fiber, carbon, graphite, and polymer were studied at ambient temperature and high temperature. The wear resistance and hardness were enhanced by the fillers. Results showed that the wear resistance of all composites was much higher than that of pure PTFE. Pure PTFE has the lowest friction coefficient at ambient temperature (temperature: 23 ± 2°C, humidity: 50 ± 10%) but highest friction coefficient at high temperature (above 100°C). The PTFE composite filled with bronze showed the best wear resistance at ambient temperature but the poorest wear resistance at high temperature. The carbon-graphite- or polymer-filled PTFE composite showed a lower friction coefficient and moderate wear resistance at both ambient and high temperature.     

Tribological behaviour of PTFE modified cotton polyester composites

Abstract

An attempt on modification of tribological behaviour of cotton polyester composite was done with polytetrafluoroethylene (PTFE). PTFE modified polyester–cotton composites were developed and studied for their friction and sliding wear behaviour at different applied loads. The sliding wear tests of composites were conducted against EN-31 steel counter face. The coefficient of friction μ as well as the sliding wear rate of cotton–polyester composites reduced significantly on addition of PTFE. The reduction in wear rate of PTFE modified polyester–cotton composite has been discussed with the help of SEM observations of worn surfaces and coefficient of friction.     

Tribological behavior of PTFE sliding against steel in sea water

In this paper the tribological behaviors of PTFE against GCr15 steel in air, distilled water, sea water and 3.5 wt.% NaCl solution were comparatively investigated. The influence of sea water composition on the tribological behavior of PTFE was also studied. Results show that the friction process in sea water was relatively stable, the friction coefficient and the wear rate of PTFE were slightly lower and a little larger than those in distilled water, respectively, but both were much lower than those in air and NaCl solution. In aqueous environment, medium affected the tribological behavior of PTFE mainly by corrosion to the counterface, the wear rate of PTFE depended on the corrosion extent of the counterface, and this wear model can be called indirect corrosive wear. In salt solution, green rusts were formed on the counterface and had some lubricating effect. In addition, the results show Mg²⁺ and Ca²⁺ were the key factors for the relatively low friction coefficient and wear rate of PTFE in sea water, because the corrosion of counterface was reduced and the lubricating effect of green rusts was enhanced as a result of the deposition of Mg(OH)₂ and CaCO₃ on the counterface.     

A Novel Sweating-Lubricating Epoxy Composite Coating Filled with Ionic Liquid-Incorporated Hollow Silica Microspheres

Ionic liquids (ILs) are promising high-performance lubricants with remarkable lubrication and antiwear capabilities in tribology. In this article, first solid-state (at room temperature) IL-incorporated SiO₂ hollow microspheres (IL-SiO₂) were synthesized at high temperature, and then a novel epoxy (EP) composite coating filled with IL-SiO₂ hollow microspheres (IL-SiO₂/EP) was successfully fabricated. The effects of IL- SiO₂ on the mechanical and tribological properties of the IL-SiO₂/EP coating were investigated. It was found the IL-SiO₂/EP coating possessed low friction and high wear resistance. When the IL ratio in the silica hollow microspheres was 58%, the flexural strength and hardness of the IL-SiO₂/EP coating surpassed those of the SiO₂/EP coating and pure EP coating. When the IL-SiO₂ content was 15?wt% in the composite, the friction coefficient and wear rate of the IL-SiO₂/EP coating were both minimized and were 46.2 and 81.6% lower than those of the SiO₂/EP coating, respectively. It was indicated that the addition of ILs overcame the heat and electron accumulation of SiO₂/EP coating in the friction process. The solid-state ILs were melted by friction heat and load into the lubricating liquid and released from the hollow silica microspheres to the worn surfaces, which was similar to sweating by the human body. In addition, the liquid-state ILs combined with the debris of EP through chemical bonding to form an internal continuous and steady lubricating protective film, which could directly protect the worn surfaces from abrasion. This IL-SiO₂/EP composite coating can be used to design and fabricate new sweating-lubricating functional coatings.