Gai Zhao

The effect of the addition of aramid fibers on tribological properties of the polyimide after ultraviolet or atomic oxygen irradiation

The aramid fibers reinforced polyimide composites were fabricated by means of a hot-press molding technique. The mechanical and tribological behaviors of the reinforced composites were studied. The friction and wear properties of the composites, rotating against a GCr15 steel ball, were investigated on a ball-on-disc test rig. To contrast the effects of the ultraviolet radiation and atomic oxygen, experiments without radiation or after ultraviolet or atomic oxygen radiation were conducted. Experimental results revealed that 20% aramid fiber reinforced polyimide exhibited higher modulus. However, the coefficient of friction decreased with the increase of the aramid fiber content. The composites had a lower wear rate value after atomic oxygen radiation. Microstructure of worn surface of the tested composites was inspected by scanning electron microscopy, and X-ray photoelectron spectroscopy study of the composite surface showed that the chemical composition of the surface changed after ultraviolet or atomic oxygen radiation because of the oxidation.

Ultraviolet or atomic irradiation effect on the polyimide composite lubricating coating

Abstract The molybdenum sulfide/lanthanum fluoride/polyimide (MoS2/LaF3/PI) coating was synthesized and irradiated by ultraviolet (UV) or atomic oxygen (AO). The friction and wear behavior were evaluated sliding against GCr15 steel balls (ASTM A295:1998) using a ball-on-disk tribology test rig. This paper aims to discuss these issues. The chemical structure of the irradiated surface was examined by Fourier transform infrared (FTIR) spectroscopy. The worn morphologies of the coatings and counterpart were observed by scanning electron microscopy (SEM) to reveal the wear mechanism. Experimental results indicated that the height and position of the characteristic functional groups and the coefficient of friction of the PI coating changed and the wear rate value decreased after UV or AO irradiation. The effect of LaF3 combined with MoS2 on the tribological properties of the coating and wear mechanism under different irradiated conditions is discussed.

Ultraviolet or Atomic Oxygen Effects on Tribological Properties of the Carbon Fibers/Polyimide Composites

Carbon fibers-reinforced polyimide composites (CF-PI) were fabricated by means of a hot press molding technique. To contrast the effects of ultraviolet and atomic oxygen irradiation under high vacuum on the tribological properties of CF-PI composites, the friction and wear properties of the composites sliding against GCr15 steel ball before and after irradiation were conducted in high vacuum on a ball-on-disk test rig. The experimental results revealed that CF-PI composites exhibited higher modulus and lower coefficient of friction and worn rate value than pure polyimide under high vacuum. However, the coefficient of friction of composites increased and the worn rate value decreased after ultraviolet or atomic oxygen irradiation, which slightly affected the tribological properties of CF-PI composites. The chemical composition of the composites changed after irradiation was inspected by X-ray photoelectron spectroscopy. Microstructure of the worn surfaces of the tested composites was investigated by scanning electron microscopy to reveal the wear mechanism.

UV or AO Irradiation Effects on the Tribological Properties of the Talc/GF/PI Composites

The development of polyimide (PI) composites, particularly for use in aerospace and tribological applications, has gained importance over the past decades. However, there have been scarce studies on the ultraviolet (UV) or atomic oxygen (AO) irradiation characteristics of the polyimide composites, the understanding of which may aid in expanding its application in space environment. To study the irradiated effects of UV and AO on the tribological behaviour of the PI composites, glass fibers (GF)-reinforced PI composites filled with Talc were irradiated by UV or AO in a ground based simulation system. Glass fibers (GF) reinforced PI composites filled with talc were fabricated by means of a hot press molding technique. The volume contents of the talc (10, 20, 30 %) were chosen to study the effects of filler content on the tribological behavior of the composites, while the proportion of the glass fiber was kept at 15 vol %. To contrast the different effects of UV and AO irradiation on the tribological properties of the composites, experiments without irradiation and after UV or AO irradiation were conducted. The specimens of the composites were irradiated with UV for a period of 4h, while AO irradiation chosen for 6h. Dynamic mechanical analysis (DMA) and thermal gravimetric analysis (TGA) measurements were conducted. The friction and wear behaviors of the composites, rotating against GCr15 steel balls, were investigated on a ball-on-disk test rig. at room temperature and at a rotating speed of 0.1256 m/s and a load of 1N. Experimental results revealed that the composites exhibited high modulus and wear rate values with increasing talc content, but low coefficients of friction (COF). After AO irradiation, the COF of the composites increased, but UV irradiation had no obvious effect on the tribological property of the composites. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) study of the composite surface showed that the chemical composition of the surface changed after UV irradiation because of the photooxidation and rough surface or even mountain-like structures were formed after AO erosion. The experiments indicated that the different space environments had an important effect on the tribological properties of the polymer composites. It is expected that this study may help expand the application of the polyimide composites in the field of space.

Friction and Wear Behaviors of the Polydimethylsiloxane Bishydroxyalkyl-Terminated Modified Polyurethane Composites Filled with the Barium Sulfate in Dry Friction and Water Lubrication

The addition of less than 20 wt% of approximate 1 micron barium sulfate (BaSO4) into polyurethane (PU) composites modified by bishydroxyalkyl-terminated polydimethylsiloxane (PDMSBH) resulted in increases in mechanical strength and thermal conductivity and, at the same time, resulted in improvements in the friction and wear properties of the polyurethane composites. These polyurethane composites were suitable for marine use for bearings at high load under dry friction and at fast sliding speed under water lubrication. Characterization with Fourier transform infrared (FTIR) spectroscopy, thermogravimetry analysis (TGA), scanning electron microscope (SEM), and an MRH-3 ring-on-block wear tester indicated that the addition of BaSO4 disrupts the organic phase separation in the polyurethane, resulting in better tribological properties, but there is no special chemical reaction between the particles and polyurethane. Adding too much BaSO4 resulted in higher wear rate because of inorganic–organic phase separation.

Reciprocating friction and wear of polyimide composites filled with solid lubricants

Abstract High-performance engineering polymers are a potential frictional material candidate for mechanical systems with moving parts, especially at high load and speed conditions. In this study, reciprocating friction and wear of aramid fibers/polyimide composites filled with graphite, MoS2 or Polytetrafluoroethylene, respectively, were systematically investigated on a Pin-on-Flat test rig. The experimental setup was simplified into friction materials reciprocating against a phosphor bronze pin to simulate the rotor/stator contact state in ultrasonic motors. A comparative study on friction reduction and wear resistance of polyimide composites indicated that graphite showed the best lubricity with low friction coefficient and wear rate. Experimental results of pressure time average velocity measurements showed that frequencies ranging from 3 to 11 Hz played a significant role on the friction coefficient variations of these porous polyimide composites, whereas increasing pressure from 4 to 6 MPa had little effect on friction reduction. Then, the microstructure of the worn surface of the three different materials was observed by scanning electron microscope to reveal the wear mechanisms. This study is expected to provide a good guidance for porous polyimide composites application in ultrasonic motors.

Properties of POB reinforced PTFE-based friction material for ultrasonic motors

Abstract Polytetrafluoroethylene (PTFE) and its composite coating with various poly-p-oxybenzoyl (POB) proportions was prepared by spray suspensions. The friction and wear behavior were evaluated against a GCr15 steel ball on a ball-on-disc tribometer under dry sliding. The effect of the content of POB on the hydrophobic, mechanical and tribological properties of the PTFE-based coatings and the performances of the corresponding ultrasonic motors (USMs) were studied. Experimental results showed that the optimal content of POB not only increased the hardness, adhesion force and contact angle (CA), but also increased the coefficient of friction and wear resistance of the PTFE coatings. Especially, the wear rate of PTFE coating filled with 15 wt.% POB (3.42×10−4 mm3/N·m) was only a quarter of pure PTFE. The morphologies of the worn surfaces of the PTFE coatings were observed by scanning electron microscopy to discuss the wear mechanism. The mechanical output properties of USMs were the best with filling of 10 wt.% POB into PTFE matrix.

Effects of La2O3 on the microstructure and tribological properties of plasma-sprayed Cr2O3–TiO2 coatings

Cr2O3–TiO2 coatings with different proportions of La2O3 were deposited by atmospheric plasma spraying. The coatings were evaluated by hardness tester, surface roughness tester, SEM and wear tester. The experiment results showed that the addition of La2O3 could improve the microhardness and decrease porosity, wear rate and surface roughness of the coating. The coating containing 2 wt.% La2O3 had the best tribological properties. The dominant wear mechanism is a mixture of abrasive wear and adhesive wear. The microscopic analysis suggests that the addition of La2O3 could refine the microstructure and promote the formation of solid solution powder, and then affect the properties of coatings.

Effect of binary rare earth oxide on the properties of plasma sprayed Al2O3/TiO2 coatings

Purpose This study systematically investigated the effect of the binary rare earth oxide of La2O3 and Sm2O3 on the properties of the Al2O3/TiO2 (AT) coating, including phase transform, wear behavior, etc. Design/methodology/approach AT coatings mixed with different components of binary rare earth oxides of La2O3 and Sm2O3 are prepared by atmospheric plasma spraying. The adhesion strength, micro-hardness, phase transition and tribological behavior of coatings are systematically investigated. Findings The X-ray diffraction (XRD) analysis shows that phase transformation is obvious after spraying, and a-Al2O3 is almost translated into γ-Al2O3 when La2O3 and Sm2O3 are doped together. Meanwhile, solid solution generated between rare earth oxide and Al2O3/TiO2 coatings results in disappearance of TiO2 and rare earth oxide phase. The photos under the scanning electron microscope (SEM) indicate that binary rare earth oxide could increase the melting degree of powder and decrease porosity of coatings.The increasing of Sm2O3 rarely affect micro-hardness and adhesion strength, and the coating with 4 per cent Sm2O3 and 1 per cent La2O3 exhibits the best wear resistance and lowest friction coefficient among all the samples. Originality/value AT coatings mixed with different components of binary rare earth oxide of La2O3 and Sm2O3 are prepared by atmospheric plasma spraying. Binary rare earth oxide could increase the melting degree of powder and decrease porosity of AT coatings.

Effect of Atomic Oxygen Irradiation on the Structural and Tribological Properties of the MoS2/Al2O3/PI Composites

Polyimide (PI) composites have been widely used in a space science due to extraordinary properties, such as excellent mechanical and electrical properties, good thermal stability and chemical inertness, as well as high wear resistance. However, atomic oxygen (AO), as one of the main radiated constituents in low earth orbit, had an important influence on the structrural and tribological properties of the polyimide matrix. To investigate the mechanism of AO erosion on polyimide, MoS2/Al2O3/PI composites were fabricated by means of a hot-press molding technique and irradiated by AO in a ground-based simulation system. The chemical composition change of the irradiated surface was examined by X-ray photoelectron spectroscopy (XPS). Then, the friction and sliding wear behavior against GCr15 steel balls were evaluated in a ground-based simulation facility using ball-on-disk tribology test rig. The worn morphologies and radiated surfaces of the materials were observed by Scanning electron microscope (SEM) to reveal the wear mechanism. Experimental analysis indicated that oxidation induced by AO irradiation and degradation of PI molecular chains on the composite’ surface results in change in chemical composition and formation of “carpet-like” structures. Affected layer, gradually formed during the process of irradiation, plays an important role for wear performance of the materials increasing friction coefficient and wear rate. Incorporation of Al2O3 nanofibers and MoS2 nanoparticles is shown to be favourable for AO resistance, which is helpful for improvement in wear resistance of the PI.