Effect of atomic oxygen irradiation on the structural and tribological characteristics of polytetrafluoroethylene and its composites

Based on the ground‐based simulation facility, the effects of atomic oxygen (AO) irradiation on the structural and tribological properties of pure polytetrafluoroethylene (PTFE) and carbon fiber and MoS₂‐filled PTFE composites were studied by scanning electron microscopy, X‐ray photoelectron spectroscopy, and a ball‐on‐disc tribometer. The results shown that AO irradiation had significant effects on the structural and tribological properties of pure PTFE, in which the surface morphologies, mass loss, friction coefficient, and wear rate had been changed greatly after AO irradiation. However, it was noticeable that the addition of carbon fiber and MoS₂ filler to PTFE could improve the AO resist capacity and tribological properties of PTFE composites significantly. Copyright © 2016 John Wiley & Sons, Ltd.     

Effects of atomic oxygen irradiation on structural and tribological properties of polyimide/Al2O3 composites

To understand the effects of atomic oxygen (AO) irradiation on the structural and tribological behaviors of polymer composites, polyimide/Al₂O₃ composites were irradiated with AO in a ground‐based simulation facility. The structural changes were characterized by X‐ray photoelectron spectroscopy and attenuated total‐reflection FTIR, whereas the tribological changes were evaluated by friction and wear tests as well as scanning electron microscopy analysis of the worn surfaces. It was found that AO irradiation induced the oxidation and degradation of polyimide molecular chains, which increased the O concentration and decreased the C concentration in the composite surfaces. The destruction action of AO changed the surface chemical structure and morphology of the samples. Friction and wear tests indicated that AO irradiation decreased the friction coefficient but increased the wear rate of both pure and Al₂O₃ filled polyimides. In terms of the tribological properties, appropriate content of Al₂O₃ might be favorable for the improvement of tribological properties in AO environment. Copyright © 2011 John Wiley & Sons, Ltd.     

Structural and tribological properties of polytetrafluoroethylene composites in atomic oxygen environment

Effects of atomic oxygen (AO) irradiation on the structural and tribological behaviors of glass fiber (GF) and MoS₂ filled polytetrafluoroethylene (PTFE) composites were investigated in a ground‐based simulation facility, in which the average energy of AO was about 5 eV and the flux was 5.0 × 10¹⁵ cm^–2 s^–1. The structural changes were characterized by XPS and attenuated total‐reflection Fourier transform infrared spectroscopy, and the tribological changes were evaluated by friction and wear tests and SEM analysis of the worn surfaces. It was found that AO irradiation induced the degradation of PTFE molecular chains, and the primary erosion mechanism is collisionally induced rather than chemically induced. The addition of MoS₂ filler significantly increased the AO resistance of PTFE composites. Friction and wear tests indicated that GF and MoS₂ improved the tribological properties of materials before and after AO irradiation. Short GF and MoS₂ exhibited a good synergistic effect for improving the AO resistant and tribological properties of PTFE material. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of solid lubricant particles on the tribological properties of PTFE composites lubricated with natural seawater

An orthogonal test was used to design different mixture ratios of molybdenum disulfide(MoS₂), graphite, and SiO₂ particles, which were filled with polytetrafluoroethylene (PTFE) composite. MoS₂-, graphite-, and SiO₂-modified PTFE was obtained by pressing and sintering, and the processing parameters were determined using progressive studies and experiments. The friction and wear properties of different PTFE composites lubricated with natural seawater were analyzed using an MMU-5G wear tester. A laser scanning confocal microscope was employed to examine the morphological characteristics of the worn surface. Moreover, the influence of particle proportions on the tribological property of composites was analyzed. Results show that the addition of SiO₂, MoS₂, and graphite can increase the bearing capacity, improve the wear resistance, reduce the friction coefficient, and increase the self-lubricating ability of the PTFE matrix.     

Morphology of polytetrafluoroethylene before and after irradiation

Supramolecular structure and morphology of as-polymerized, sintered, and gamma-irradiated suspension PTFE were studied with scanning electron microscopy. Irradiation was performed both below and above melting point of crystal phase. Fibrillar supramolecular structure of as-polymerized PTFE is preserved after its sintering. In contrast to as-polymerized PTFE, in the sintered polymer some segments of fibrils form lamellae of thickness 100-300 nm and length up to several microns, with fibrils arranged perpendicularly to a lamella. Irradiation below the melting point (20 and 200 °C) does not change quantitatively PTFE morphology. In both cases and also in the case of pristine PTFE, dense and loose (porous) regions are present in its morphology. Dense regions are packages of irregular shape and consist of densely packaged fibrils. Loose regions consist of individual ribbons and fibrillar lamellae. Irradiation at 200 °C increases greatly the width of lamellae. PTFE structure rearrranges drastically under irradiation above the melting point. New morphology units, spherulites of size about 50 μm, are formed, the spherulites consisting of radially extending fibrils, and porosity decreases substantially. Formation of spherulites is ascribed to radiation-induced chain scission and decrease in molecular mass and viscosity of polymer.     

Cross-linking of polytetrafluoroethylene during room-temperature irradiation

Exposure of polytetrafluoroethylene (PTFE) to α-radiation was investigated to determine the physical and chemical effects, as well as to compare and contrast the damage mechanisms with other radiation types (β, γ, or thermal neutron). A number of techniques were used to investigate the chemical and physical changes in PTFE after exposure to α-radiation. These techniques include: Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and fluorescence spectroscopy. Similar to other radiation types at low doses, the primary damage mechanism for the exposure of PTFE to α-radiation appears to be chain scission. Increased doses result in a change-over of the damage mechanism to cross-linking. This result is not observed for any radiation type other than α when irradiation is performed at room temperature. Finally, at high doses, PTFE undergoes mass-loss (via small-fluorocarbon species evolution) and defluorination. The amount and type of damage versus sample depth was also investigated. Other types of radiation yield damage at depths on the order of mm to cm into PTFE due to low linear energy transfer (LET) and the correspondingly large penetration depths. By contrast, the α-radiation employed in this study was shown to only induce damage to a depth of approximately 26 μm, except at very high doses.     

Effect of electron irradiation on nitrofurans and their metabolites

Research on the degradation of aqueous furazolidone, nitrofurantoine and semicarbazide (SC) solutions, and 3-amino-2-oxazolidinone (AOZ) residues in tissues of chicken and crucian under electron beam irradiation have been carried out. Results showed that about 75% furazolidone and 70% nitrofurantoine degraded at 6 kGy dose, and SC with the initial concentration of 667 μg/L degraded by 94% at 12 kGy dose. While AOZ in the crucian and chicken degraded by 22.5% and 20.7%, respectively, after being irradiated at 12 kGy. The degradation conditions were investigated to provide a reference to improve irradiation techniques.     

Modification of perfluorinated polymers by high-energy irradiation

This review surveys about the possibilities for the modification of perfluorinated polymers using high-energy irradiation: degradation, functionalization, branching, and cross-linking. The reaction mechanisms for the different reaction conditions are discussed. Electron irradiation of polytetrafluoroethylene (PTFE) with a very high dose leads to a complete degradation of the macromolecules to low-molecular products. In the presence of oxygen perfluorocarboxylic acids and in an inert atmosphere, mixtures of perfluorinated olefins and paraffins can be obtained. Virgin PTFE is disintegrated by high-energy irradiation in air with a lower dose into a micropowder modified with COOH groups. This powder can be homogeneously incorporated in other polymers. So, the special properties of PTFE can be made effective in these polymers. Micropowders functionalized with COOH groups and polyamides (PA) form by reactive extrusion PTFE-PA blockcopolymers which can be used as slide bearing materials. The copolymers poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) and poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether) (PFA) irradiated in air show a significantly higher degree of COOH functionalization compared with PTFE. Irradiation of molten PTFE in an inert atmosphere leads to formation of different kinds of double bonds, CF₃ side groups, long-chain branches as well as cross-links. Irradiation of PFA in vacuum results in the generation of COF and COOH groups; in molten state also branches and cross-links are formed.The focus of the present paper is on the work that has been carried out at the Institute of Polymer Research Dresden.     

Effects of UV irradiation on tribological properties of nano‐TiO2 thin films

One‐layer and two‐layer nano‐TiO₂ thin films were prepared on the surface of common glass by sol–gel processing. Water contact angle, surface morphology, tribological properties of the films before and after ultraviolet (UV) irradiation were investigated using DSA100 drop shape analyzer, scanning probe microscopy (SPM), SEM and universal micro‐materials tester (second generation) (UMT‐2MT) friction and wear tester, respectively. The stored films markedly resumed their hydrophilicity after UV irradiation. But UV irradiation worsened tribological properties of the films. After the film was irradiated by UV, the friction coefficient between the film and GCr15 steel ball increased about 10–50% and its wear life shortened about 20–90%. Abrasive wear, brittle break and adherence wear are the failure mechanisms of nano‐TiO₂ thin films. It was believed that UV irradiation increased surface energy of the film and then aggravated adherence wear of the film at initial stage of friction process leading to severe brittle fracture and abrasive wear. Copyright © 2009 John Wiley & Sons, Ltd.     

Characterization and tribological investigation of self-assembled trimethoxysilyl-functionalized poly(phthalazinone ether ketone)thin films on glass substrates

<正>Trimethoxysilyl-functionalized PPEK(PKGS) films had been designed to serve as wear resistant coatings for silicon surfaces. These surface films were formed by a dip-coating technique applied to self-assembled monolayers(SAMs).The formation and wetting behavior of PKGS films were characterized by means of contact angle measurement.The friction coefficient of the film prepared is very low(about 0.1),and the anti-wear behavior is good,with a lack of failure after sliding over 1800 s.     

Effect of pyrolytic carbon content on microstructure and tribological properties of C/C-SiC brake composites fabricated by isothermal chemical vapor infiltration

Four types of C/C-SiC composites were prepared by isothermal chemical vapor infiltration (ICVI) of SiC on C/C preforms of different densities. Tribological properties of the composites were evaluated by using MM-1000 testing machine. The results indicated that the friction behaviors of the composites are a strong function of the content of pyrolytic carbon (in matrix). Moreover, friction film was formed on the surface and increased with pyrolytic carbon content. Debris originated of particulate and film-type have been observed after friction testing with ratio and size determined by the content of pyrolytic carbon.     

Effects of photoaging on the tribological properties of engineering plastics

Ball‐on‐disk tests were performed to investigate the effects of photoaging on the tribological properties of engineering plastics under dry sliding. Results exhibit that different molecular structures of plastics result in different aging morphologies, and ultra high molecular weight polyethylene has better resistance to photoaging than polyformaldehyde (POM). However, POM has better wear resistance before and after the photoaging. Marked increase of the width and depth of wear tracks is observed after aging. Analysis of worn surface has revealed that photoaging significantly affects the wear mechanism of ultra high molecular weight polyethylene and POM. Although the major wear mechanism before the aging of materials is identified as plastic deformation and adhesive wear, it becomes quite complicated when the materials experienced photoaging. Such complex wear mechanism hence deteriorates the tribological performance of the engineering plastics. Copyright © 2014 John Wiley & Sons, Ltd.     

Investigate on mechanical and tribological properties of solution styrene butadiene and butadiene rubber composites

Solution styrene butadiene and butadiene rubber (SSBR‐BR) composites reinforced with different contents of SiO₂‐graphene have been fabricated firstly. The mechanical properties of the rubber composites were comparatively investigated using tensile tests; experimental results showed that, as an overall trend, the tensile and tear strength increased with increasing contents of SiO₂‐graphene. Most importantly, under the condition of simulating practical working condition, the tribological behavior of SSBR‐BR composites with different contents of SiO₂‐graphene was explored via a universal ring‐plate frictional tester in detail. Combined with the surface roughness of the counterparts, the wear mechanisms were discussed for SSBR‐BR composites under the cement and asphalt counterparts. Finally, several wear mechanisms under different actual working conditions were proposed.     

Physical and tribological properties of a new polycarbonate-organoclay nanocomposite

A new polycarbonate (LS2) nanocomposite containing a 3 wt% proportion of the organically modified montmorillonite bentone 2010 (B 2010) has been prepared by extrusion and injection moulding, and its tribological properties determined under a pin-on-disc configuration against stainless steel. The nanocomposite (LS2 + 3% B 2010) presents a 88% reduction in friction and up to two orders of magnitude reduction in wear rate with respect to the base polymer. The new nanocomposite has been characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD), and its thermal and dynamic mechanical properties have been determined by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) techniques. The nanocomposite shows a uniform dispersion of the nanoclay as pointed out by two different statistical methods. The good tribological performance of the new nanocomposite is attributed to this uniform microstructure and to the increase in the nanoclay stacking distance.     

Comparative study of the effects of nano‐sized and micro‐sized CF and PTFE on the thermal and tribological properties of PEEK composites

The tribological characteristics of PEEK composites fretting against GCr 15 steel were investigated by a SRV‐IV oscillating reciprocating ball‐on‐disk tribometer. In order to clarify the effect of type and size of fillers on the properties of PEEK composites, nano‐sized and micro‐sized CF and PTFE fillers were added to the PEEK matrix. The thermal conductivity, hardness, and fretting wear properties of PEEK composites reinforced by CF or PTFE were comparatively studied. The results showed that the type and size of the fillers have an important effect on both the friction coefficient and wear rate, by affecting their thermal conductivity, hardness, as well as the surface areas of their transfer films. In comparison, the effect on improving the tribological properties of micro‐sized CF was superior to that of nano‐sized CF, while the effect of nano‐sized PTFE was superior to that of micro‐sized PTFE. Considering the acceptable friction coefficient and wear rate of the composite under the fretting wear test, it seemed that 4% nCF, 20% mCF, 2% nPTFE and 10% mPTFE were desired additive proportions. And it also can be found that during the fretting wear test, the abrasive and adhesive wear resulted in accumulative debris at the contacting surface. The transfer films made of debris were formed on the counterfaces.     

Influence of electron beam irradiation on physicochemical properties of poly(trimethylene carbonate)

Electron beam (EB) irradiation of poly(trimethylene carbonate) (PTMC), an amorphous, biodegradable polymer used in the field of biomaterials, results in predominant cross-linking and finally in the formation of gel fraction, thus enabling modification of physicochemical properties of this material without significant changes in its chemical structure. PTMC films (M_w: 167-553 kg mol⁻¹) were irradiated with different doses using an electron accelerator. Irradiation with a standard sterilization dose of 25 kGy caused neither significant changes in the chemical composition of the polymer nor significant deterioration of its mechanical properties. Changes in viscosity-, number-, weight-, and z-average molecular weights of PTMC for doses lower than the gelation dose (D_g) as well as gel-sol analysis and swelling tests for doses above D_g indicate domination of cross-linking over degradation. EB irradiation can be considered as an effective tool for increasing the average molecular weight of PTMC and sterilization of PTMC-based biomaterials.     

Proton transfer and coordination properties of aromatic α-hydroxy hydrazones

The prototropic properties of two nitro derivatives of aromatic α-hydroxy hydrazones [2,4-dinitro-N-phenyl-N′(2-hydroxy-1-phenylmethylene hydrazine) and 2,4-dinitro-N-phenyl-N′(2-hydroxy-1-naphthylmethylene hydrazine] have been analysed through electronic absorption, ¹H and ¹³C NMR and semiempirical molecular orbital methods. The metal binding properties of both compounds and of the unsubstituted hydrazones were evaluated from potentiometric and spectrophotometric data in dioxane-water mixtures. On the basis of the spectral analysis and PM3 semiempirical calculations, the structures of the formed complexes were proposed.     

Effect of maleic anhydride‐grafted polytetrafluoroethylene on the tensile and tribological properties of blending polytetrafluoroethylene with polyamide‐6

Blending polytetrafluoroethylene (PTFE) to polyamide‐6 (PA6) with and without maleic anhydride‐grafted polytetrafluoroethylene (PTFE‐g‐MA) was produced in a corotating twin screw extruder, where PTFE acts as the polymer matrix and PA6 as the dispersed phase. The effect of PTFE‐g‐MA on the tensile properties and tribological propertiesof PTFE/PA6 polymer blends is studied. Results show that the structural stability and morphology of the blends were greatly improved by PTFE‐g‐PA6 grafted copolymers, which were formed by the in situ reaction of anhydride groups with the amino end groups of PA6 during reactive extrusion forming an imidic linkage. The presence of PTFE‐g‐PA6 in the PTFE continuous phase improves the interfacial adhesion, as a result of the creation of an interphase that was formed by the interaction between the formed PTFE‐g‐PA6 copolymer in situ and both phases. Compared with thePTFE/PA6 without PTFE‐g‐MA, the PTFE/PA6 with PTFE‐g‐MAhad the lowest friction coefficient and wear under given applied load and reciprocating sliding frequency. The interfacial compatibility of the composite prevented the rubbing‐off of PA6, accordingly improved the friction and wear properties of the composite. Copyright © 2009 John Wiley & Sons, Ltd.     

The degradation of DGEBA/DICY under 100 keV proton irradiation

The degradation process of diglycidyl ether of bisphenol A (DGEBA)/ dicyandimide (DICY) solidified system under 100 keV proton irradiation was investigated. It was found that the proton irradiation results in mass loss, which the maximum is approximately 15.5 μg/cm², and change in surface morphology of DGEBA/DICY. The analyses of FT-IR and XPS showed that, the proton irradiation induces the debonding of the weak groups such as -CH₃, C-O, leading to formation of stable carbon-rich structure by recombination of the occurred free radicals, and chemical reaction between the free radicals with participation of proton. The degradation of DGEBA/DICY exhibits exponential variation with the proton fluence, which the degradation rate is high at the initial stage of irradiation, and becomes slow trending to constant after the proton fluence reaches 6 × 10¹⁵ /cm².     

Calorimetric and diffractometric study of the room-temperature transitions of polytetrafluoroethylene from aqueous suspension polymerization

The room-temperature transitions of native (never-melted) polytetrafluoroethylene (PTFE) obtained by aqueous suspension polymerization have been studied through differential scanning calorimetry (DSC) and wide-angle x-ray scattering (WAXS). Two crystal-crystal transitions are observed, instead of the four shown for native PTFE from aqueous emulsion polymerization. One crystalline component seems to be present, which may correspond to the low-transient, already evidenced for PTFE from aqueous emulsion polymerization.