Tribology of selectively irradiated PTFE surfaces

Selectively irradiated polytetrafluoroethylene (PTFE) surfaces combining the low friction and non-abrasive attributes of an unirradiated polymer with the enhanced wear resistance of wholly irradiated PTFE are demonstrated. Augmented wear resistance, similar to that of filled PTFE composites, is obtained without the accompanying counterface abrasion typical of hard particulate fillers. Friction of these ‘composite’ irradiated/unirradiated surfaces can be less than that of unirradiated PTFE, as irradiated regions limit transfer morphology to only thin oriented films. Spatially distributed irradiated and unirradiated surface regions are patterned by masking 225 keV incident electrons. Under the given contact conditions (6.5 MPa nominal pressure against polished stainless steel) such self-lubricating, wear-resistant composite surfaces had lifetimes of several kilometers sliding distance before giving way to rapid wear of the underlying unmodified PTFE. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of transfer film structure, composition and bonding on the tribological behavior of polyphenylene sulfide filled with nano particles of TiO2, ZnO, CuO and SiC

The tribological behavior of polyphenylene sulfide (PPS) filled with inorganic nano particles was studied. The fillers investigated were TiO₂, ZnO, CuO and SiC whose sizes varied from 30 to 50 nm. The polymer composites were compression molded with varying proportions of these fillers. Wear and friction tests were performed in a pin-on-disk configuration at a sliding speed of 1.0 m/s, nominal pressure of 0.65 MPa, and counterface roughness of 0.10 μm Ra. The polymer composite pins slid against hardened tool steel counterfaces. The transfer films of the composite materials formed on the counterfaces during sliding were studied by optical microscopy and X-ray photoelectron spectroscopy (XPS) and the adhesion between the transfer film and counterface was measured in terms of the peel strength. It was found that the wear rate of PPS decreased when TiO₂ and CuO were used as the fillers but increased with ZnO and SiC fillers. The optimum wear resistance was obtained with 2 vol.% CuO or TiO₂. These filled composites had the coefficients of friction lower than that of the unfilled PPS. The wear behavior of the composites is explained in terms of the topography of transfer film and adhesion of transfer film to the counterface as observed from peel strength studies. There is a good correlation observed between the transfer film-counterface bond strength and wear resistance.     

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.     

Transfer film evolution and its role in promoting ultra-low wear of a PTFE nanocomposite

Polytetrafluoroethylene (PTFE) is an important solid lubricant with an unusually high wear rate. For a half-century, fillers have been used to reduce PTFE wear by >100× with >10% loading through hypothesized mechanisms involving mechanical load support, crack arresting, and transfer film adhesion. More recently it was discovered that specific nanoparticles provide a unique nanoscale reinforcement mechanism enabling unprecedented wear reductions of 10,000× with as little as 0.1% nano-fillers. Although the mechanisms responsible for this dramatic improvement remain unclear, there is substantial evidence that the transfer film plays a critical role. This paper uses interrupted microscopy measurements to investigate the evolution of transfer film development for an ultra-low wear PTFE nanocomposite. The run-in wear rates were similar to those of more traditional PTFE composites and transfer films consisted of large plate-like debris. Although the run-in wear rate and debris size decreased monotonically with distance, the run-in transfer films were removed each cycle. Detectible debris vanished and wear rates approached zero at an abrupt transition. During this ultra-low wear transition period, nanoscale and oxidized fragments of PTFE were transferred to the counterface. Most of these fragments persisted for the duration of the test and initiated the transfer film by progressively scavenging trace material from the bulk, growing into small islands, and merging with neighboring islands. The results of this study reflect a complex interplay involving elements of transfer film adhesion, chemistry, debris morphology, and mechanics.     

Tribological studies of glass fabric-reinforced polyamide composites filled with CuO and PTFE

The effect of glass fabric reinforcement on the friction and wear behavior of nylon was investigated for varying fabric proportions. The effect of the addition of particulate CuO filler, and the filler and PTFE to the glass fabric-reinforced composites on the tribological properties of the composites was also studied. The composites with different proportions of fiber, filler and PTFE were compression molded and tested for friction and wear in a pin-on-disk configuration. It was found that the proportion of glass fabric for optimum wear resistance was about 20% by volume. The addition of CuO filler did not contribute to improvement in the wear resistance of the fabric-reinforced composites because a good transfer film did not develop in the presence of fibers and the composites became fragile. PTFE was very effective in reducing the wear of filled and fabric-reinforced composites. The friction and wear behavior of these composites has been interpreted in terms of their ability to form transfer film on the steel counterface and the changes occurring on the pin surface. The composition for maximum wear resistance was 25 vol.% CuO-11.3 vol.% glass fabric-10 vol.% PTFE-nylon.     

PTFE复合材料的摩擦磨损性能研究

用机械共混、冷压成型烧结的方法制备了纳米SiO2/石墨/玻璃纤维/PTFE复合材料试样。用MM-200型磨损试验机测试了在干摩擦条件下不同载荷时各试样的摩擦磨损性能;用扫描电镜对磨损后试件表面进行观察和分析。研究结果表明：纳米SiO2和玻璃纤维有效提高了PTFE的承载能力,石墨的加入起到了减小摩擦的作用;在本试验条件下,在摩擦过程中三元混合填充PTFE复合材料在偶件表面形成了转移膜,减少了复合材料与偶件的直接接触,因而表现出优异的抗磨性。     

Investigation of transfer phenomenon by X-ray photoelectron spectroscopy and tribological properties of polymers sliding against polymers

Tribological properties of polymeric materials were investigated with various polymer-polymer combinations by means of a pin-on-disk wear testing apparatus. The specific wear rate was related to the cohesive energy density of the polymeric materials, but hat of a given polymer slider was dependent on the mated polymeric materials, and a high wear rate was observed in the sliding against a polymer counterface with higher cohesive energy than that of the slider. By means of X-ray photoelectron spectroscopy it was confirmed that the polymer transfer did occur on the mated polymer, and the degree of covering of the track with transfer films could be estimated. The covering ratio with transfer films was dependent on the polymer-polymer combinations. Poly(tetrafluoroethylene) (PTFE) transfer film on various polymers was very effective in reducing friction irrespective of the covering with the transfer films. With the other polymers, high density poly(ethylene) (HDPE), poly(propylene) (PP), and polyacetal (PAc), the transfer film was less effective in reducing friction than PTFE transfers and friction in the steady state dependent on the polymer-polymer combinations.     

Dry sliding wear behaviour of polyamide 66 and polycarbonate composites

A study has been made of the reciprocating dry sliding wear behaviour of polyamide 66 and polycarbonate containing glass fibres, ultra high molecular weight polyethylene (UHMWPE) and polytetrafluoroethylene (PTFE/2% Si oil). Studies have been conducted at sliding loads of 2 kg and 10 kg at an average velocity of 0.33 m s⁻¹ against a hardened stainless steel counterface with a surface roughness of 0.3 μm.It has been shown that additions of 10–15% of filler/reinforcement lead to greatly improved sliding wear behaviour. PTFE/2% Si oil filled polyamide 66 has been shown to have the best overall wear performance whilst the high glass filled variants of polyamide 66 and polycarbonate have the best combination of wear resistance and mechanical strength. These findings are discussed with reference to composite constitution and properties, thermal effects and counterface interactions. Explanations are advanced to account for the differences in behaviour inter alia the composite materials.     

The role of transfer film and back transfer behavior on the tribological performance of polyoxymethylene in sliding

The role of transfer films formed during the sliding of polymer composites against steel counterfaces was studied in terms of the tribological behaviors of the composites. The composites were prepared by compression molding and sliding tests were run in pin-on-disk sliding configuration. The counterface was made of tool steel hardened to 55–60 HRC and finished to a surface roughness of 0.09–0.10 μm Ra. Wear tests were run for 6 hrs at the sliding speed of 1.0 m/s and contact pressure of 0.65 MPa. Transfer films formed on the counterfaces during sliding were investigated using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The results showed that as the transfer film became smooth and uniform, the wear rate decreased. The examination of worn surfaces using Energy Dispersive Spectroscopy (EDS: dot mapping mode) showed the back-transfer of the steel counterface material to the polymer pin surface. This behavior is believed to strengthen the polymer pin surface during sliding thereby contributing to the decrease in wear rate. This paper was recommended for publication in revised form by Associate Editor Jae Cheon Lee Minhaeng Cho received his B.S. and M.S. degrees in Mechanical Engineering from Chung-Ang University, Seoul in 1993 and 1995, respectively. He received his M.S. degree in Materials Science and Engineering from Oregon State University in 2000, and his Ph.D. in Mechanical Engineering from Iowa State University in 2004. Dr. Cho is currently an Assistant Professor at the School of Mechanical Engineering at Chung-Ang University in Seoul, Korea. His research interests are in the area of tribology, surface phenomena, and functional surfaces such as laser surface texturing and ultra-thin coatings.     

Friction and wear of polyphenylene sulfide composites filled with micro and nano CuO particles in water-lubricated sliding

The tribological behavior of polyphenylene sulfide (PPS) composites filled with micro and nano CuO particles in water-lubricated sliding condition were studied. Pin-on-disk sliding tests were performed against a steel counterface of surface roughness 0.09–0.11 μm. The lubrication regimes were established from friction data corresponding to various combinations of loads and sliding speeds. Later experiments were performed using the sliding speed of 0.5 m/s and contact pressure of 1.95 MPa, which corresponded to boundary lubrication regime. Micro CuO particles as the filler were effective in reducing the wear of PPS but nano CuO particles did not reduce wear. The steady state wear rate of PPS-30 vol.% micro CuO composite was about 10% of that of unfilled PPS and the coefficient of friction in this case was the lowest. The examination of the topography of worn pin surfaces of nano CuO-filled PPS by SEM revealed grooving features indicating three-body abrasion. The transfer films formed on the counterfaces during sliding were studied by optical microscopy and AFM. The wear behavior of the composites in water-lubricated sliding is explained using the characteristics of worn pin surfaces and transfer films on the counterface.     

Friction and wear behavior of self-lubricating and heavily loaded metal–PTFE composites

A new 16 MnNb steel–PTFE composite (A) containing 60% area proportion of PTFE composite was developed. Another type of common solid lubricant embedded C86300 bronze–PTFE composite (B) containing 35% area proportion of PTFE composite was also selected for a comparative investigation under similar testing conditions. Friction and wear experiments were performed in an oscillating sliding tribotester at an oscillating frequency of 0.13 Hz, contact mean pressures from 15 to 80 MPa and counterface roughness of 0.10 μm R_a. The composites A and B slid against a 38CrMoAlA steel shaft. Results showed that the composite A exhibited low coefficient of dry friction and long wear life as compared to that of the composite B. It was found that the surface of PTFE composite was higher than that of steel backing at the intervals of testing. This was because modulus of the elasticity of PTFE composite was much lower than that of 16 MnNb steel backing; under a same load condition the elastic deformation amount of PTFE composite was much bigger than that of steel backing. Thus, the composite A provided sufficient lubrication during the whole tests.     

Pre-Polishing the Metal Counterface of Metal–UHMWPE Wear Pair with Filler-Filled UHMWPE Composites to Generate Counterface Changes for an Effective Reduction in Pure UHMWPE Wear

Ultra-high molecular weight polyethylene (UHMWPE) is well known for high-wear-resistance applications. Its long-chained easy sliding molecules and semi-crystalline structures enable the polymer’s great wear resistance. UHMWPE composites made for higher wear resistance study have been analyzed in this paper. Pure UHMWPE, 1 wt% CNT UHMWPE, 1 wt% PEEK UHMWPE, 1 wt% alumina (nano)–UHMWPE composites were made to be tested against metal disk on pin-on-disk tribometer. The metal disk surface conditions were found to have significant influence on the UHMWPE–polymer wear than the composite itself. This result indicates a simple and industrial applicable method that involves transfer film on the counterface to reduce polymer wear for metal–polymer wear pair applications.     

Friction and wear properties of metal powder filled PTFE composites under oil lubricated conditions

Four kinds of polytetrafluoroethylene (PTFE)-based composite, pure PTFE, PTFE+30vol.%Cu, PTFE+30vol.%Pb and PTFE+30vol.%Ni composite, were prepared. The friction and wear properties of these metal powder filled PTFE composites sliding against GCr15 bearing steel under both dry and lubricated conditions were studied using an MHK-500 ring-block wear tester. The worn surfaces of the PTFE composites and the transfer films formed on the surface of GCr15 bearing steel were examined using scanning electron microscopy (SEM) and optical microscopy respectively. Experimental results show that the friction and wear properties of the PTFE composites can be greatly improved by liquid paraffin lubrication. The wear of these PTFE composites can be decreased by at least 1 to 2 orders of magnitude compared with that under dry friction conditions, while the friction coefficients can be decreased by 1 order of magnitude, SEM and optical microscopy investigations of the rubbing surfaces show that metal fillers of Cu, Pb and Ni not only raise the load carrying capacity of the PTFE composites, but also promote transfer of the PTFE composites onto the counterfaces, so they greatly reduce the wear of the PTFE composites. However, the transfer of these PTFE composites onto the counterfaces can be greatly reduced by liquid paraffin lubrication, but transfer still takes place.     

Dry sliding wear behaviour of PTFE filled with glass and bronze particles

Abstract

The wear behaviour of polytetrafluroethylene (PTFE) filled with 25% glass and 40% bronze particles was studied on a pin on disc test rig. Solid lubricant composite materials were prepared by compression moulding technique. The wear parameters considered for the study were applied load, sliding speed and sliding distance. The experimental results indicate that the weight loss increases with increasing load, sliding speed and sliding distance, as expected. Sliding distance has more effect on weight loss followed by applied load. The 40% bronze+PTFE composite exhibits better wear resistance compared to other types. The dominant interactive wear mechanisms during sliding of PTFE and its composites are discussed in this paper.     

The tribological properties of PTFE filled with thermally treated nano-attapulgite

The nano-attapulgite powder was treated by heating at 100, 200, 300, 400, 500, 600, 700 and 800 °C for 2 h in a muffle furnace. PTFE composites were prepared by compression molding PTFE and thermally treated nano-attapulgite. The friction and wear tests were performed on a block-on-ring wear tester. Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectrometer (EDS) and Differential Scanning Calorimetry (DSC) were utilized to investigate material microstructures and examine modes of failure. Experimental results showed that under all experimental conditions there was no significant change in coefficient of friction, but the wear rate of PTFE composites was orders of magnitude less than that of pure PTFE under same experimental conditions. Moreover, thermally treated attapulgite was superior to untreated attapulgite in enhancing the wear resistance of PTFE. In addition, the wear resistance increased monotonically with increasing treated attapulgite concentration. Hardness analysis revealed the hardness of PTFE composites increased with increasing content of treated attapulgite. Investigation of transfer film and analysis of debris for PTFE and its composites showed that thermally treated nano-attapulgite filled to PTFE could facilitate formation of transfer film on the steel ring surface and inhibit breakage of PTFE molecular chain. The composites with higher heat absorption capacity exhibited improved wear resistance. Furthermore, the steel ring counterface abrasion was not found.     

A study of the thermal,dynamic mechanical,and tribological properties of polyphenylene sulfide composites reinforced with carbon nanofibers

The thermal, dynamic mechanical, and tribological properties of polyphenylene sulfide (PPS) composites reinforced with carbon nanofiber (CNF) were studied. Dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC) were used to study the viscoelastic properties and thermal transitions. In order to study the tribological properties, friction and wear tests in a pin-on-disk configuration were performed. The changes in melting point, crystallization temperature, and glass transition temperature were found to be small as a result of reinforcement. Steady state wear rates of the reinforced composites sliding against the counterface of roughness 0.13–0.15 μm Ra were significantly lower than that of the unreinforced PPS. When the composites were tested against the smoother counterface of 0.06–0.11 μm Ra, the wear rates were higher. The coefficient of friction in all the cases was not practically affected by the presence of CNF. The transfer films formed on the counterface during sliding were examined by optical microscopy and atomic force microscopy (AFM). The variation of wear is discussed in terms of the texture and topography of transfer film.     

The influence of metal surface composition on the tribological properties of filled PTFE/steel couples

In this paper, the friction and wear behaviors of filled PTFE/steel couples are studied. The stationary specimen was made of filled PTFE. The rotating specimen, made of steels, underwent various treatments. The experimental results showed that the frictional coefficient and wear rate were low when the surface of the steel specimen contained nitrogen. The structure of the transfer film was examined by X-ray photoelectron spectroscopy (XPS). The transfer film containing nitrogen on the steel surface was thick. The authors consider that the hypothesis of the adhesion of the n–p–n structure may explain the characteristics of the different sliding couples in this paper and might be an important way to improve the friction and wear properties of the filled PTFE/metal sliding couples.     

Lubrication of carbon fibre-reinforced polymers : Part II—Organic fluids

A variety of organic fluids, including mineral oils and synthetic lubricants, are shown to be effective boundary lubricants for carbon fibre-reinforced polymers sliding against stainless steel. The wear rates of the composites in some fluids are extremely low, and scanning electron microscopy and pyrolysis-gas chromatography show that low wear is associated with the development of films on the steel counterface. These films appear to be mixtures of lubricant and resin from the composite matrix. The wear rates of epoxy/Type II carbon fibre composites in aircraft hydraulic fluids are generally much less than those of many metal combinations, and the composites therefore show promise as replacements for metals in lubricated systems.     

Wear behavior of graphite-fiber-reinforced glass

Composites of borosilicate glass reinforced with high strength, high modulus and ultrahigh modulus (UHM) graphite fibers were tested for wear against a cast iron counterface in a pin-on-disk configuration at sliding velocities of 0.24, 0.60 and 0.96 m s^?1 under a load of 5 MPa.Discontinuous fiber-reinforced composites generally exhibited higher wear rates and friction coefficients than the continuous fiber-reinforced systems. Continuous UHM fiber-reinforced pins exhibited the lowest wear rate. It is shown that the wear rate depends on the strength and dimensions of the graphite filaments.Two types of wear, mild and severe, were observed at low and high sliding velocities. Mild wear is characterized by a smooth thin stable film on both the pin and the counterface with minimal pitting and plastic deformation of the cast iron. Severe wear is characterized by rough pitted surfaces on the pin and the counterface accompanied by a thick film on the counterface. A proposed mechanism of wear of unidirectional graphite-fiber-reinforced composites which incorporates the transition between mild and severe wear can explain the phenomenological observations.     

Influence of counterface roughness on the friction and wear of polytetrafluorethylene- and polyacetal-based composites

The friction of various composites is generally little dependent upon the roughness. When the composites contain glass or carbon fibres, their wear rates are also little dependent upon the roughness. However, the wear rates of PTFE incorporating MoS₂, graphite or bronze and polyacetal incorporating PTFE increase rapidly as the roughness increases beyond a certain critical value characteristic of each of these composites. The roughness-dependency of the wear rate is markedly affected by the transfer during sliding.