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 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.     

Proton irradiation effects on the structural and tribological properties of polytetrafluoroethylene

Polytetrafluoroethylene (PTFE) was irradiated with protons in a ground-based simulation facility to study the effects of proton irradiation on the structural and tribological properties of PTFE. The structural changes were characterized by X-ray photoelectron spectroscopy (XPS) and attenuated total-reflection FTIR (ATR-FTIR), while the tribological properties were evaluated by friction and wear tests. It was found that proton irradiation induced the degradation of PTFE molecular chains, resulting in the increase of C concentration and the decrease in F concentration on the sample surfaces, and the surface chemical structure and morphology of the samples changed, which affected the friction coefficient and decreased the wear rate of the specimens as the friction and wear tests revealed.     

The effect of the addition of talc on tribological properties of aramid fiber‐reinforced polyimide composites under high vacuum,ultraviolet or atomic oxygen environment

The aramid fibers‐reinforced polyimide composites filled with talc were fabricated by means of a hot press molding technique, and mechanical and tribological behaviors were comparatively investigated. Experimental results showed that the elastic modulus of the composites increased with an increase of the talc, but the impact intensity and loss factor decreased. Besides, the coefficient of friction decreased with the increase of the talc content. To contrast the effects of the ultrahigh vacuum (VC), ultraviolet (UV) or atomic oxygen (AO) on the composites, experiments without irradiation or after UV or AO irradiation were conducted. Scanning electron microscopy and X‐ray photoelectron spectroscopy (XPS) analysis showed that UV or AO irradiation can change the surface structure and chemical composition of the polymer because of the photooxidation and chemical erosion. Copyright © 2012 John Wiley & Sons, Ltd.     

Friction and wear behavior of PI and PTFE composites for ultrasonic motors

High‐performance polymer‐based frictional materials have become increasingly important to improve the mechanical output properties of ultrasonic motors. This study discussed the friction and wear behavior of 2 dominating frictional materials of polymer composites for ultrasonic motors, polyimide (PI), and polytetrafluoroethylene (PTFE) filled by aramid fibers (AF) and molybdenum disulfide (MoS₂). To explore the wear mechanisms, the tribo‐pair contact stress was theoretically characterized, and the interface temperature rise was numerically predicted. The predictions showed that the flash temperature on asperity tips could reach the glass transition temperature of the polymer materials. The experimental results indicated that the contact stress and sliding speed have a small effect on the friction of the PI composite but influence considerably the friction of the PTFE composite. A higher contact stress brings about a higher specific wear rate, but a higher sliding speed reduces the wear rate. Compared with AF/MoS₂/PTFE, the AF/MoS₂/PI has much better tribological performance under high loads and speeds.     

The effect of interface electrostatic interaction based on acid–base theory on friction behavior

Fluorine‐containing amorphous carbon films [fluoring‐containing diamond‐like carbon (F‐DLC)] were fabricated on Si wafer by direct current plasma enhanced chemical vapor deposition (dc‐PECVD) technique using CF₄ and Ar as gas sources, confirmed by XPS and Raman analyses. The friction tests were carried out on a rotating ball‐on‐disk apparatus in high vacuum atmosphere (≤5.0 × 10^?4 Pa) at the load of 0.5 N selecting glass (mainly containing silicon–oxygen tetrahedron structure) and Al₂O₃ with the same hardness and surface roughness as the counterpart balls. The results indicate that glass/F‐DLC results in lower friction coefficient of 0.14 than that of the Al₂O₃/F‐DLC (0.20). At the same time, no wear was occurred, and the transfer layer was not formed on the counterpart ball for glass/F‐DLC, while the wear of Al₂O₃/F‐DLC is slightly larger than that of glass/F‐DLC. However, just like the glass ball, there is no formation of transfer layer on the Al₂O₃ ball surface. Furthermore, the chemical state of fluorine in the film after friction, which mainly existed in the form of the C–CF and C–F bonds, did not change compared with the F‐DLC film, while the fluorine content has changed significantly. As a result, it is assumed that interface electrostatic interaction based on acid–base theory plays an extremely important role in the process of friction. Copyright © 2017 John Wiley & Sons, Ltd.     

The mechanical properties and tribological behavior of epoxy resin composites modified by different shape nanofillers

Mechanical properties and tribological behavior of epoxy resin (EP) and EP nanocomposites containing different shape nanofillers, such as spherical silica (SiO₂), layered organo‐modified montmorillonite (oMMT) and oMMT‐SiO₂ composites, were investigated. The SiO₂‐oMMT composites were prepared by in situ deposition method and coupling agent modification, and transmission electron microscopy (TEM) analysis shows that spherical SiO₂ is self‐assembled on the surface of oMMT, which forms a novel layered‐spherical nanostructure. The mechanical properties test results show that oMMT obviously improves the strength of EP and SiO₂ enhances its toughness, but oMMT‐SiO₂ exhibits a synergistic effect on toughening and reinforcing EP simultaneously. A pin‐on‐disc rig was used to test friction and wear loss of pure EP and EP nanocomposites. The tribological test results prove that these nanofillers with different shapes play different roles for improving the wear resistance of EP nanocomposites. Morphologies of the worn surfaces were studied further by scanning electron microscopy (SEM) observations, and it was clarified that the EP and EP nanocomposites undergo similar wear mechanisms. Copyright © 2006 John Wiley & Sons, Ltd.     

Effect of nano‐modified SiO2/Al2O3 mixed‐matrix micro‐composite fillers on thermal,mechanical, and tribological properties of epoxy polymers

Thermo‐mechanically durable industrial polymer nanocomposites have great demand as structural components. In this work, highly competent filler design is processed via nano‐modified of micronic SiO₂/Al₂O₃ particulate ceramics and studied its influence on the rheology, glass transition temperature, composite microstructure, thermal conductivity, mechanical strength, micro hardness, and tribology properties. Composites were fabricated with different proportions of nano‐modified micro‐composite fillers in epoxy matrix at as much possible filler loadings. Results revealed that nano‐modified SiO₂/Al₂O₃ micro‐composite fillers enhanced inter‐particle network and offer benefits like homogeneous microstructures and increased thermal conductivity. Epoxy composites attained thermal conductivity of 0.8 W/mK at 46% filler loading. Mechanical strength and bulk hardness were reached to higher values on the incorporation of nano‐modified fillers. Tribology study revealed an increased specific wear rate and decreased friction coefficient in such fillers. The study is significant in a way that the design of nano‐modified mixed‐matrix micro‐composite fillers are effective where a high loading is much easier, which is critical for achieving desired thermal and mechanical properties for any engineering applications. Copyright © 2016 John Wiley & Sons, Ltd.     

Achieving ultra-low friction of a-C:H film grown on 9Cr18Mo steel for industrial application via programmable high power pulse magnetron sputtering

Owing to the high hardness and hydrogen passivation of carbon bonds, hydrogenated diamond-like carbon (a-C:H) film has shown promising potential to achieve ultra-low friction and wear on steel surfaces. Here, a-C:H film was successfully deposited on 9Cr18Mo steel via programmable high power pulse magnetron sputtering and potential application for industrial was evaluated. The a-C:H films against different mating materials of GCr15 steel balls, Al₂O₃, Si₃N₄, ZrO₂, and a-C:H-coated GCr15 balls all showed ultra-low friction under a normal load of 5 N in a dry ambient air environment. Among them, self-mating tribo-system a-C:H films on steel surfaces and a-C:H-coated steel balls achieve best friction performance; the principal reason is that both contacting surfaces coated with a-C:H film have the lower electron affinities compared with other tribo-systems. However, the differences of coefficient of friction (COF) for uncoated-GCr15, Al₂O₃, ZrO₂, Si₃N₄, and a-C:H(GCr15) balls can be attributed to different sizes of clustering in wear debris. This work provides new insights on synthesis and industry application of the a-C:H films with ultra-low friction properties.     

Effect of SiC/nano‐CeO2 on wear resistance and microstructures of Ti3Al/γ‐Ni matrix laser‐cladded composite coating on Ti–6Al–4V alloy

The Ti–6Al–4V alloy is an important aviation material, but has a poor resistance to slide wear. Laser cladding of the Al₃Ti + Ni/Cr/C + TiB₂/Al₂O₃ + SiC/nano‐CeO₂ preplaced powders on the Ti–6Al–4V alloy can form the Ti₃Al/γ‐Ni matrix composite coating, which improves the wear resistance of the substrate. In this study, the Al₃Ti + Ni/Cr/C + TiB₂/Al₂O₃ + SiC/nano‐CeO₂ laser‐cladded coating was researched by means of X‐ray diffraction, scanning electron microscopy, and energy dispersive spectrometry. The experimental results indicate that under the action of SiC/nano‐CeO₂, this composite coating exhibited a fine microstructure. Furthermore, the proper content of nano‐CeO₂ decreased the crack tendency. The results above indicated that, it is feasible to improve the tribological property of the Al₃Ti + Ni/Cr/C + TiB₂/Al₂O₃ laser‐cladded coating by adding of SiC/nano‐CeO₂. Copyright © 2011 John Wiley & Sons, Ltd.     

Structure–tribological property relationship of nanoparticles and short carbon fibers reinforced PEEK hybrid composites

In a previous work, the roles of low‐loading, that is, 1 vol %, nano‐SiO₂ particles on the tribological behavior of short carbon fibers (SCFs)/polytetrafluoroethylene (PTFE)/graphite filled polyetheretherketone (PEEK) were studied. In the present work, the effects of nanoparticle content, varying from 1 to 4 vol %, on the structure and the tribological performance of the composite was investigated. The polished cross sections of the composites were inspected using a scanning electron microscope (SEM). The incorporated nanoparticles significantly reduce the friction coefficients of the composite. With low pressure‐sliding velocity (pv) factors, nanoparticle agglomerates seem to exert an abrasive effect on SCF, and thereby lead to high wear rates. Under such conditions, an increase in nanoparticle content decreases the wear resistance. With high pv factors, the nanoparticles remarkably improve the wear resistance of the composite and the nanoparticle contents do not play an important role on the wear resistance. The worn surfaces, transfer films and wear debris of the composites were analyzed. The tribological mechanisms were discussed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 801–811, 2010     

Study of mechanical and tribological properties of nylon 66‐titanium dioxide microcomposite

Nylon 66 microcomposites with various weight percentage of titanium dioxide (TiO₂) were prepared by a twin screw extruder and investigated for mechanical and tribological properties. Mechanical properties of the composite such as tensile strength/modulus, flexural strength/modulus, impact, and compressive strength first showed an increase up to 6 wt% TiO₂ followed by a decrease at higher filler loading. The value of heat deflection temperature increased with the increase in wt% of TiO₂. Sliding wear tests were performed on pin‐on‐disk equipment under different loads, sliding velocity, and sliding distance combinations. It was found that micro‐TiO₂‐Nylon 66 composite exhibited reduced wear and coefficient of friction up to 6 wt% TiO₂. Micro‐TiO₂ at 2 wt% was most effective in improving the tribological properties of plain nylon 66. The worn surfaces were examined by scanning electron microscopy to understand the wear mechanism. The optimal combination from 2 wt% to 6 wt% micro‐TiO₂‐Nylon 66 can be used depending upon the application requiring improvement in tribological or mechanical properties, respectively.     

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.     

Enhanced tribology properties of ZnO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite nanoparticles as liquid lubricating additives

Monodispersed and hydrophobic ZnO/Al₂O₃ composite nanoparticles are prepared by a nonhydrolytic sol–gel method. ZnCl₂ and AlCl₃ are dissolved in acetone and used as precursors. Oleic acid is adopted as an oxygen donor. The tribology properties of the prepared ZnO/Al₂O₃ composite nanoparticles are studied by the four-ball friction and thrust ring friction test. It is demonstrated that the average friction coefficient and the wear scar diameter are reduced by 37.5 and 26.2%, respectively, in comparison with pure lubricating oil. Moreover, the ZnO/Al₂O₃ composite nanoparticles bear the merits of ZnO and Al₂O₃ when used as lubricant additives, exhibiting both excellent antifriction and antiwear behaviors simultaneously. The ZnO/Al₂O₃ composite nanoparticles improve the lubrication effect not only by turning the sliding friction into rolling friction, but also forming a hard Al₂O₃ protective film onto the thrust-ring surface containing ZnO/Al₂O₃ nanoparticles, which have much potentiality in industrial applications.     

Enhanced dielectric performance and energy storage of PVDF‐HFP‐based composites induced by surface charged Al2O3

In order to enhance dielectric properties and energy storage density of poly(vinylidene fluoride‐hexafluoro propylene) (PVDF‐HFP), surface charged gas‐phase Al₂O₃ nanoparticles (GP‐Al₂O₃, with positive surface charges, ε’ ≈ 10) are selected as fillers to fabricate PVDF‐HFP‐based composites via simple physical blending and hot‐molding techniques. The results show that GP‐Al₂O₃ are dispersed homogeneously in the PVDF‐HFP matrix and the existence of nanoscale interface layer (matrix‐filler) is investigated by SAXS. The dielectric constant of the composites filled with 10 wt % GP‐Al₂O₃ is 100.5 at 1 Hz, which is 5.6 times higher than that of pure PVDF‐HFP. The maximum energy storage density of the composite is 4.06 J cm^?3 at an electrical field of 900 kV mm^?1 with GP‐Al₂O₃ content of 1 wt %. Experimental results show that GP‐Al₂O₃ could induce uniform fillers’ distribution and increase the concentration of electroactive β‐phase as well as enhance interfacial polarization in the matrix, which resulted in enhancements of dielectric constant and energy storage density of the PVDF‐HFP composites. This work demonstrates that surface charged inorganic‐oxide nanoparticles exhibit promising potential in fabricating ferroelectric polymer composites with relatively high dielectric constant and energy storage. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 574–583     

The effect of surface modification with nitric acid on the mechanical and tribological properties of carbon fiber‐reinforced thermoplastic polyimide composite

Polyacrylamideacrylate (PAN)‐based carbon fibers were submitted to nitric acid oxidation treatments to improve the interfacial adhesion of the carbon fiber (CF)‐reinforced polyimide (CF/PI) composite. The carbon fiber surfaces were characterized by X‐ray photoelectron spectroscopy (XPS). Nitric acid oxidation not only affects the oxygen concentration but also produces an appreciable change in the nature of the chemical functions, namely the conversion of hydroxy‐type oxygen into carboxyl functions. Nitric acid oxidation treatment modifies the element constituting the fiber, the nitrogen concentration being about 1.2 times higher at the fiber external surface compared to the untreated one. The mechanical and tribological properties of the polymide (PI) composites reinforced by the carbon fibers treated with nitric acid oxidation were investigated. Results showed that the tensile strength of the CF/PI composites improved remarkably due to nitric acid treatment along with enhancement in friction and wear performance. Copyright © 2009 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.     

Effect of Y2O3 on microstructural characteristics and wear resistance of cobalt‐based composite coatings produced on TA15 titanium alloy surface by laser cladding

The effects of Y₂O₃ on the microstructure, phase composition of the coatings, microhardness and wear resistance of cobalt‐based composite coatings prepared by laser cladding were investigated. The TA15 titanium alloy was selected as substrate which the cobalt‐based composite powder with different content of Y₂O₃ was cladded on. The microstructure of the coatings was observed by scanning electron microscope (SEM) and metallurgical microscope. The phase structure of the coatings was determined by X‐ray diffraction (XRD), and the microhardness and wear resistance of the coatings were measured by hardness tester and wear testing machine. The results show that the rare earth oxide Y₂O₃ can refine and purify the microstructure of the coatings, reduce the porosities and cracks and improve compactness of the coatings. Moreover the addition of Y₂O₃ improves the microhardness of the coatings and reduces the friction coefficient, thus improving the wear property of the coatings. And the wear resistance of the coating with Y₂O₃ has improved about 50 times; the highest value of microhardness in the coating is HV1181.1. And 0.8 wt% content of Y₂O₃ in the coating is the best choice for improving the microhardness and wear resistance of the coating. It is feasible to improve the microstructure and tribological properties of laser cladding coatings by adding of Y₂O₃. Copyright © 2014 John Wiley & Sons, Ltd.     

Study on the friction and wear behavior of surface‐modified carbon nanotube filled carbon fabric composites

The functionalization of multi‐walled carbon nanotubes (MWNTs) was achieved by grafting furfuryl amine (FA) onto the surfaces of MWNTs. Furthermore, the functional MWNTs were incorporated into carbon fabric composites and the tribological properties of the resulting composites were investigated systematically on a model ring‐on‐block test rig. Friction and wear tests revealed that the modified MWNTs filled carbon fabric composite has the highest wear resistance under all different sliding conditions. Fourier transform infrared spectroscopy (FTIR), X‐ray photoelectron spectroscopy (XPS), and thermal gravimetric analysis (TGA) revealed that MWNTs were successfully functionalized and the modification led to an improvement in the dispersion of MWNTs, which played an important role on the enhanced tribological properties of carbon fabric composites. It can also be found that the friction and wear behavior of MWNTs filled carbon fabric composites are closely related with the sliding conditions such as sliding speed, load, and lubrication conditions. Copyright © 2010 John Wiley & Sons, Ltd.     

Selection of solvent for the mechanical activation process: A molecular dynamics simulation and experiment study

Herein, molecular dynamics simulations and experiments were carried to select solvent for the mechanical activation process. The interaction between solvent and each component of the Al‐PTFE mechanical activated energy composites has been studied by means of molecular dynamics. Then, the status of Al and PTFE in solvent was analyzed, and the microstructure of the composite was also studied combined with experiment. At last, the mechanical activated energy composites were prepared with n‐hexane as the solvent. The results show that the adsorption of PTFE, toluene, and hexane on the (0 0 1), (0 1 0), and (100) surface of Al₂O₃ is stable. Al₂O₃ (0 0 1) surface and Al₂O₃ (0 1 0) surface interact with 3 substances mainly via the electrostatic force. Al₂O₃ (1 0 0) surface interacts with 3 substances mainly via the van der Waals force. The binding energies of toluene‐Al₂O₃ and hexane‐Al₂O₃ are larger than PTFE‐Al₂O₃. PTFE cannot adsorb on the surface of aluminum in the existence of n‐hexane and toluene. n‐Hexane can make PTFE disperse uniformly, and a considerable part of these PTFE will be coated on the surface of Al after n‐hexane is removed. The uniformity of Al/PTFE mechanical activated energetic composites prepared in n‐hexane is good, and it increases with the milling time.