木之下 博

Improving the wear resistance of polvmer-based materials can lead to high energy efficiency' of transport machineries owing to the higher strength-to-weight ratio. Oxidized wood-utilized synthesized copper-based particles (OWCu) was incorporated into epoxv resin to investigate its effect on the wear resistance of OWCu-incorporated epoxy with the steel counterpart using a ball-on-plate tribometer. OWCu was mainly composed of CuO with a small amount of graphitic carbon, and the average partide size was approximately∗ 0.5 um. A higher wear resistance was observed for the OWCu-incorporated epoxv at more than 0.6 mass% compared to the neat epoxv. The mechanism for the improvement of the wear resistance property was discussed, and it was revealed that the surface roughness of the steel counterpart was maintained at a low level for the OWCu-incorporated epoxy', which played a dominant role in reducing the wear of the OWCu-incorporated epoxv. The polishing effect of OWCu could reduce the surface roughness of the steel counterpart in the friction process. In addition, at higher concentrations of more than 5.0 mass%, OWCu adhered to the steel surface, which decreased the wear of the steel surface. OWCu is a promising material to enhance the wear resistance of polvmer-based materials, especially with low elongation.

A substantial quantity of carbon onions in a durable film state is indispensable for its applications. In this study, large area fabrication of closely packed homogeneous carbon onion nanoparticle film using plasma-based ion implantation was demonstrated. Ag film deposited on a Si substrate was used as the implantation target for the hydrocarbon ions accelerated at 20 kV. Nanoparticles with the mean diameter of 7.5 nm were formed at the grain boundary of the Ag film. Carbon onions with the mean diameter of 17.4 nm were synthesized and arranged to a closely packed nanoparticle film with the thickness of around 200 nm by gradual thermal vaporization of the Ag. The closely packed configuration was achieved due to the isolated growth of carbon onion nanoparticle and high uniformity of the diameter. This process can be used in principle large area formation compered to typical ion implantation technique of carbon onion nanoparticle film, which can be applicable for the practical use in mechanical and electrochemical applications.

Graphene oxide (GO) and oxidized wood-derived nanocarbon (oWNC) were dispersed in an epoxy resin to improve the mechanical and tribological properties of an epoxy resin. Mechanical properties of the GO/epoxy (GO/EP) and oWNC/epoxy composites (oWNC/EP) were investigated by tensile tests and nano-indentation. Under dry, water, and oil lubrication conditions, friction coefficients and wear volumes of these composites were measured. GO has single-layered structure. oWNC has chain-like nanostructures, and the nanochains are aggregated. GO and oWNCs have same surface chemical composition due to the same oxidation treatment. Thus, in this study, the structure and morphology dependences of the nanocarbons in mechanical and tribological properties will be appeared when the nanocarbons were used as the additives in the resin. The tensile strength, tensile modulus, and hardness increased by 20%, 63%, and 35% in maximum by the addition of GO and oWNC to the epoxy resin, respectively. Moreover, the wear resistance of GO/EP and oWNC/EP was enhanced compared to that of a neat epoxy. The reduction rates of the wear volume for 5.0 mass% GO/EP and 1.0 mass% oWNC/EP achieved to 91% and 67%, respectively. Comparing GO and oWNC, oWNC effectively reduced the friction coefficient and wear at a lower concentration. The improvement of tribological properties after the addition of the oxidized nanocarbons can be attributed to the increase in the hardness and lubricity of the composites.

Copper based particles were synthesized by a wood-powder-template-based process. Wood powders mixed with an aqueous cooper salt solution were heated under N2 gas flow, and then, the mixture of the charcoal powder and copper particles were heated in air to remove charcoal. Finally, oxidized wood-derived copper particles (OWCu) of less than 1 μm were synthesized. Wood powders acted as the template and limited the sizes of the copper particles. In addition, the tribological properties of OWCu in synthetic oil were investigated using a pin-on-plate reciprocating tribometer. Our results revealed that 0.5 mass% OWCu reduced both the friction coefficient (max 45%) and wear volume (max 39%) of SUJ2 lubricated by PAO4. The best reduction effect was obtained for OWCu synthesized at 400°C under N2 gas flow, which had the smallest average particle size, where the coverage of the OWCu layer on the wear track was the largest. These facts suggest that the particle size reduction of OWCu can increase the coverage and improve the friction coefficient and wear volume reduction effects of the lubricant. Control of the wood-powder-template-based synthesized particles to the optimal size for the roughness of the sliding surface would help achieve better tribological performance.

In this study, a novel method was developed for in situ scanning electron microscopy (SEM) observations of friction interfaces, from a top view, using a Si3N4 thin film (SiN film), which has high electron transmission ability, and a microtribometer. Nanodiamond (ND) aggregates were adsorbed on the back surface of the SiN film. A JIS-SUS304 ball, on which surface ND aggregates were adsorbed and tribofilms were already formed, contacted and slid with the back surface of the SiN film. An SEM electron beam went through the SiN film, and generated secondary electrons from the ball surface contacting the back surface of the SiN film and the adsorbed ND aggregates. Thereafter, the generated secondary electrons from the ball surface penetrated again through the SiN film and reached the SEM electron detector. In other words, the contacting ball surface and the adsorbed ND aggregates were successfully imaged through the SiN film. Energy dispersive X-ray spectroscopy analyses of the friction interfaces were also accomplished. Moreover, in situ SEM observations of friction interfaces under boundary lubrication, using poly-alpha olefin (PAO) oil with graphene oxide aggregates and lithium grease with MoS2 particles, were successfully accomplished. The PAO oil and lithium grease had electron transmission ability, and the friction interfaces were imaged by SEM as seen with an optical microscope.

A study of the effectiveness of graphene oxides (GO) dispersed in water as a lubricant additive between tungsten carbide (WC) pin against stainless steel (SUS304) plate was carried out. A 0.1 wt.% GO was prepared and used as a lubricant under an applied load of 3 N for 20,000 friction cycles of reciprocating tribological testing. The results show that a GO dispersion with pH 3 provided the lowest friction coefficient, which was approximately 0.05. Worn areas on the wear track of the SUS304 flat plate and WC ball surface were also small. The increasing pH obviously affected the tribological properties, where the friction coefficient increased to approximately 0.10-0.20 in the steady state for pH 5, pH 7 and pH 9. Meanwhile, a GO dispersion with pH 10 was not able to provide good tribological properties for the tested materials. The observations on microscopic images revealed the formation of tribofilms on the wear tracks for low pH. The tribofilms caused reduction of the friction force and protected the plates from severe wear during the sliding tests.

Radio frequency-microelectromechanical system (RF-MEMS) switches, which use physical Ohmic contacts, are recently focused for high performance in the high-frequency ranges. Typically, the contact areas of the electrodes in RF-MEMS switches are less than 0.01 mm(2), and they generate just very low normal loads of less than 1 mN. The limited real contact area of the electrodes leads to high electrical contact resistances and wear on the switches. Carbon nanotube (CNT) films, formed with many vertically aligned CNTs on a silicon substrate, are one candidate electrode material for RF-MEMS switches. However, CNT films have a high electrical contact resistance with metals. In this study, precious-metal electroplating (Ag, Au, Pt, Rh, and Cu) on CNT films was performed to decrease the electrical contact resistances of the films and increase their wear resistances. The contact resistances of the electroplated CNT films as a function of normal loads up to 1 mN were measured by phi 2 mm Cu balls. In this study, the Ag-electroplated CNT film with a hydrogen annealing had the lowest electrical contact resistance of 0.10 Omega. Durability experiments of cyclic connection switching were conducted under direct applied voltages of 3 V or 10 V between the films and Cu balls, and with a load of 1 mN for 3 x 10(5) cycles. The electrical contact resistance of the Ag-electroplated CNT film with the hydrogen annealing was stable during this durability experiment.

Metal nanoparticles were formed on graphene oxide by a deposition process with hydrosilane, giving thin layer metal graphene oxide (metal/GO) composites. The particle size and catalytic activity could be controlled by varying the hydrosilane amount. Hydrosilane prevented the aggregation of GO layers by surface functionalization via silane coupling reaction. The metal/GO composites were evaluated as catalysts in hydrosilane oxidation.

Graphene oxide (GO) contains graphitic single-layered sheets, while oxidized wood-based nanocarbon (oWNC) is composed of graphitic nanoshells. Since both materials are thoroughly oxidized with many oxygen-containing functional groups, both GOs and oWNCs form stable dispersions in water. We found that lubrication by dispersions of GOs and oWNCs in water resulted in very low friction coefficients below 0.04 with only slight plate surface wear using a steel plate and tungsten carbide ball. After lubrication, thick tribofilms are formed on the sliding surfaces, which would decrease the friction. In this study, in order to clarify the tribological mechanisms that result in such low friction coefficients, we have performed tribological tests and analyzed the tribofilms formed by dispersions of GO in water using confocal laser scanning microscopy (CLSM) with an optical data storage system, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The GO tribofilm is composed of many carbons and some oxygen, derived from the disordered and aggregated GO sheets. The GO tribofilm have higher hardness and hydrophilic properties than the steel plates, which would provide low friction and wear. It is found that both the GO tribofilm and GOs are necessary to achieve the lowest possible coefficient of friction. It is likely that the oWNC tribofilm has similar friction and wear decreasing effects to that of the GO dispersion.

Although fluorine-containing ionic liquids have excellent lubricity as liquid lubricants for steels, strong tribochemical reactions leading to severe wear are occurred during friction. Recently, it is reported that graphene oxide (GO) dispersed in water and oils exhibit good anti-wear property. In this study, in order to investigate anti-wear effect of GO in lubrication of fluorine-containing ionic liquids for steel, tribological properties of mixtures of GO with concentrations of 0.2, 2, and 10 mass% and an ionic liquid [BMIM][BF4]) were investigated on a steel surface. Under the pure ionic liquid lubrication, a strong tribochemical reaction occurred, resulting in corrosion wear. Under a GO-ionic liquid mixture lubrication with 0.2 mass% GO, wear was much lower than for the pure ionic liquid lubrication. Adding GO would inhibit the strong tribochemical reaction. However, wear was high with the 10 mass% GO-ionic liquid mixture. The GO-ionic liquid mixtures did not greatly decrease the friction coefficient.

We studied the effectiveness of diamond nanoparticles (DNPs) dispersed in water as a lubricant additive between stainless steel plates and sintered tungsten carbide (WC) balls. DNP dispersions with concentrations of 0.01, 0.1 and 1 wt.% were prepared and used as lubricants under a load of 1.88 N, for 240,000 friction cycles. High-friction coefficients of more than 0 3 were observed in an initial period. Then friction coefficients declined and stabilised at values of approximately 0.1. The steady-state friction coefficients were independent of the DNP concentration and lower than that for distilled water. In the initial period, wear of both the plates and ball was obvious. In the steady-state period, additional wear on the plates was a little; however, ball wear scars were clearly observed. The size of the ball wear scars decreased with decreasing the DNP concentration. It is likely that DNPs were embedded mainly in the stainless steel plates, and the embedded DNPs protected the plates and wore the balls in the steady-state period. Compared with the lubrication under distilled water, the friction coefficient and wear of the plate under the lubrication by the 0.01 wt.% DNP dispersion were lower, and the wear of the ball by this lubrication condition was not higher.

Application and tribological properties of graphene oxide (GO) monolayer sheets as additives in water-based lubricants were investigated. The lubricating fluids were applied to a sintered tungsten carbide ball and stainless steel flat plate. It was found that adding GO particles into water improved lubrication and provided a very low friction coefficient of approximately 0.05 with no obvious surface wear after 60,000 cycles of friction testing. GO adsorption occurred on the lubricated surfaces of both the ball and flat plate, suggesting GO sheets may behave as protective coatings. (C) 2013 Elsevier Ltd. All rights reserved.

Pd nanoparticles supported on single layer graphene oxide (Pd-slGO) were prepared by gentle heating of palladium(II) acetate (Pd(OAc)(2)) and GO in ethanol that served as a mild reductant of the Pd precursor. Pd-slGO showed a high catalytic performance (TON and TOF = 237 000) in the Suzuki-Miyaura cross-coupling reaction.

Tribological properties of a vertically aligned carbon nanotube (CNT) film with a thickness of approximately 700 μm under millinewton and micronewton level loads were investigated using a 1 mm radius ball. At the beginning of frictional slidings, friction forces were high, and then decreased and seemed to reach relative steady state values, in both millinewton and micronewton load experiments. After the initial high frictions, top surface CNTs contacting with the counterpart ball on the CNT film were plastically deformed and bundled each other, which were observed to be flat by scanning electron microscopy (SEM). In the relative steady friction states, the friction forces basically increased with increasing loads. Under millinewton level loads, curves of the friction force as a function of the loads seemed to be parabolic. Surprisingly, although no adhesion forces were detected, frictions existed even at zero loads under micronewton loads. Copyright © 2013 Japanese Society of Tribologists.

In this study, carbon based nanomaterials such as onion-like carbon (OLC), carbon nanotube and buckminsterfullerene (or buckyball) (C60) have been investigated as nanocolloidal additives for liquid lubricants. When tested under a range of contact pressures from 0·51 to 1·10 GPa, OLC provided the best overall friction reducing property in synthetic lubricant, although the friction reducing properties of all nanocarbon materials were dependent on the contact pressure. Detailed transmission electron microscopy revealed that the OLC kept its original structure intact and induced the formation during sliding of a thin 'tribofilm' composed of OLC, nanosized wear debris, amorphous carbon and graphitic layers on the wear debris. This tribofilm seems to be responsible for reduced friction under boundary conditions. © 2012 W. S. Maney & Son Ltd.

In order to prevent degradation of diamond surfaces in contact with ferrous metals, a TiN thin film of a few tens of nm thick was synthesized on a diamond surface with a Ti thin buffer layer of approximately 10 nm by a new atom beam method. A diamond surface was simultaneously exposed to pulsed Ti arc plasma and hyperthermal neutral N atom beam generated from an arc plasma gun (APG) and a laser breakdown-type atom beam generator, respectively. Frictional experiment of the TiN thin film was conducted by an in situ scanning electron microscopic (SEM) tribometer using a 1 mm diameter SUS304 pin with an applied force of 0.24 N. The TiN film had a relative high friction coefficient (0.4), but this film showed no notable degradation and relative steady friction. In addition, a TiN coated diamond tip by the new atom beam method showed less wear than that of non-coated diamond tips by three times in the scratching tests of iron with using an atomic force microscope (AFM). (C) 2011 Elsevier B. V. All rights reserved.

Under micronewton loads, the high frictions on carbon nanotube (CNT) films (with a friction coefficient of 1.7) were decreased to less than the noise level (which was roughly a friction coefficient of 0.05) by applying an oscillating motion to a cantilever on which a sliding ball was glued. This large decrease of the friction coefficient has been never observed in previous studies. The applied oscillating motion caused the twisting and bending oscillations of the cantilever, which applied the lateral and normal oscillating forces to the sliding ball. The friction force decreased with increasing the oscillating amplitudes of the twisting and bending forces, and the frequency dependences of the friction force reduction was observed.

In order to investigate a method to increase hydrophilicity on nano-rough carbon surfaces, a nano-rough surface of C(60) film and an atomically flat surface of highly oriented pyrolytic graphite (HOPG) were oxidized by hyperthermal oxygen-atom beam exposure and the hydrophilicities of the surfaces were investigated. Superhydrophilicity were achieved on these exposed carbon surfaces, which had low O/C ratio of approximately 28% and surface roughness (Ra) of approximately 3 nm. The direct oxidations on sp(2) bonded carbon atoms (basal plane) of these two carbon materials by the exposure of hyperthermal O-atom beam would contribute the superhydrophilicity. (C) 2011 Elsevier Ltd. All rights reserved.

A process for bonding a carbon nanotube (CNT) film to a Au film at low temperature was developed with a view to using the CNT films as electrical contact materials in radio-frequency microelectromechanical systems (RF MEMS) switches. First, a CNT film was synthesized on a silicon substrate at 750 °C and subsequently coated with Au at room temperature. Finally, the Au-coated CNT film was bonded at 100 °C to a Au film, which was then deposited on a silicon substrate to act as a transmission line. An apparatus was developed to measure contact resistances and forces under contact conditions mimicking those that will be encountered in actual RF MEMS switches, such as micronewton loads and square-micrometer contact areas. Contact resistance and force of the bonded CNT film sample were simultaneously measured as a function of displacement between the sample and an electrode Au tip and compared with those for a Au film and Au-coated CNT film. The bonded CNT film sample had no adhesion force, but higher contact resistance than the other samples.

Superhydrophobic/superhydrophilic micropatterning on a carbon nanotube (CNT) film has been achieved using a laser plasma-type hyperthermal atom beam facility, which produces a small amount of damage and generates a highly anisotropic beam. Fluorination and oxidation on CNT films by exposure to fluorine-atom and oxygen-atom beams caused superhydrophobic and superhydrophilic surfaces, respectively, while maintaining the as-grown fibrous forms of the CNT films. Micropatterned oxidation on CNT films without using photoresists created superhydrophilic microdots and microchannels. (C) 2010 Elsevier Ltd. All rights reserved.

To investigate the silicon contamination effects on tribology in low earth orbit, diamond-like carbon (DLC) film, Ti plate, and single crystal MoS(2) were exposed to dimethylsilicone (PDMS) in an ultra high vacuum (UHV) facility equipped with a tribometer and X-ray photoelectron spectroscopy (XPS). In-situ tribological tests and surface analyses were conducted without ambient air exposure. The PDMS exposure led to their adsorptions on the DLC and Ti sample surfaces and did not affect the friction property of the DLC samples. The friction of the Ti sample became low due to the PDMS adsorption. A little of PDMS were adsorbed on the MoS(2) sample. The hyperthermal atomic oxygen (AO) irradiation broke C-O and C-Si bonds included in PDMS that adsorbed on the DLC and Ti surfaces. The low friction due to the PDMS adsorption for the Ti sample disappeared due to the hyperthermal AO irradiation. (C) 2009 Elsevier Ltd. All rights reserved.

Microreactors consisting of multiwalled carbon nanotube (CNT) microchannels have been developed. Vertically aligned CNT films with negative pattern shapes of microchannels are grown on silicon oxide films, providing CNT microchannels. Polymethyl methacrylate plates are placed on the CNT microchannels for the flow experiments. Since CNTs are hydrophobic and the silicon oxide film is hydrophilic, fluids can flow in the silicon oxide regions in the CNT microreactors. Fermat's spiral and Y-junction type multimalled CNT microreactors were synthesized. (C) 2009 Elsevier Ltd. All rights reserved.

Fabrication of fullerene C(60) nanorings on a highly oriented pyrolytic graphite (HOPG) was carried out by molecular beam epitaxy (MBE). Morphological characteristics of C(60) were investigated using an atomic force microscope (AFM). An oxidized HOPG, on which circular etch-pits were formed, was used as the template for the fabrication of fullerene nanorings. Evaporated C(60) molecules were selectively adsorbed on the etch-pits edges where the pre-deposited Au islands existed. The diameter of the etch-pits for nanoring template was controlled simply by the oxidation temperature of HOPG. (C) 2008 Elsevier B.V. All rights reserved.

Since the discovery of carbon nanotubes (CNT), a number of tribological researches made because of their high industrial potentials. CNT have cylinder structures with graphene-sheets (the same as graphite, which is used as good solid lubricants), and have high stiffness. A study on CNT films and sliding tips was carried out. CNT films could sustain the tips at the loads and the friction properties. However, the friction coefficient of CNT films were extremely high. Individual CNT stick the asperities of the tip surfaces and were then bended, thus, the extremely high friction would arise from high repulsive forces due to the bending of CNT. Low-friction forces of CNT could be expected if sticks of CNT in the asperities are avoided. This is an abstract of a paper presented at the 2009 World Tribology Congress (Kyoto, Japan 9/6-11/2009).

It has been an impedimental problem, for the laser detonation-type atom beam generator, that a poppet in the pulsed supersonic valve is rapidly eroded by the irradiation of powerful laser light and high temperature plasma. In order to operate the atom beam source for a long duration, a modification was made to hide the poppet from direct irradiation of laser and plasma. The alteration of device configuration resulted in great improvement in endurance of poppet more than 300 000 repetitions. Morphology of a polyimide film exposed to approximately 200 000 pulses of hyperthermal oxygen atom beam showed a shaglike carpet structure, which is a characteristic to that exposed to energetic oxygen atoms. A flux of the oxygen atom beam was estimated to be 5x10(14) atoms/cm(2)/pulse at a location of 30 cm away from the nozzle throat. (C) 2008 American Institute of Physics.

The influences of interactions between C₆₀ thin films and Si substrates were investigated using atomic force microscopy (AFM). It was found that higher friction coefficient was obtained at the C₆₀ domains formed on the H-terminated Si(001) substrate and lower ones on the Si(001)-2×1 substrate. Moreover, lowest friction coefficient was found for the C₆₀ film on the Si(001)-2×1 remained after scratching. Therefore, it is thought that higher interactions between C₆₀ and Si substrate caused lower friction coefficient of C₆₀ thin film. The prepared substrate was examined by reflection high energy electron diffraction (RHEED), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES).

The aim of this study is to probe the influence of water vapor environment on the microtribological properties of a forestlike vertically aligned carbon nanotube (VACNT) film, deposited on a silicon (001) substrate by chemical vapor deposition. Tribological experiments were performed using a gold tip under relative humidity varying from 0 to 100%. Very low adhesion forces and high friction coefficients of 0.6-1.3 resulted. The adhesion and friction forces were independent of humidity, due probably to the high hydrophobicity of VACNT. These tribological characteristics were compared to those of a diamond like carbon (DLC) sample.

In order to measure precisely the circumferential profiles of small cylindrical and spherical objects, we have developed an atomic force microscope that consists of a cantilevered probe, an optical lever system for detecting the probe deflection, and an air bearing sample stage. This rotating atomic force microscope (R-AFM) provided a roundness profile of phi 2mm bearing ball, and the profile was compared with that measured by a commercial roundness instrument. Since the R-AFM uses a sharper tip and a lower applied force than the instrument, the roundness profile obtained by the R-AFM showed higher resolution than that by the commercial instrument. Measurements of circumferential profiles of a phi30mum tungsten wire confirmed that the R-AFM has high accuracy for measuring circumferential profiles of small cylindrical objects. Possible applications of the R-AFM have been demonstrated through obtaining circumferential images of an optical fiber, a human hair, and a spine on an insect's leg.

Usefulness of the application of quartz crystal microbalance (QCM) on the atomic oxygen-induced erosion study of polymeric material is presented. Polyimide film was spin-coated on an Au electrode of QCM sensor crystals and mass change of the polymer film was measured from the frequency shift of the QCM during atomic oxygen beam exposures. Extremely high resolution in mass of QCM measurement is suitable for in-situ mass loss measurement of polymers during AO beam exposure even though its dynamic range of mass measurement is narrow. In this presentation, we are reporting our recent research results on atomic oxygen-induced polymer degradation using QCM; (1) non-linear mass loss phenomenon during atomic oxygen-induced erosion, (2) temperature dependence of polymer erosion due to hyperthermal atomic oxygen beam exposure, and (3) comparison of atomic oxygen flux measurements using Ag-QCM and Kapton witness sample.

In research reporting on the effect of ambient air exposure on atomic oxygen-exposed Kapton-H film, the surface oxygen concentration of atomic oxygen-exposed Kapton-H increased with increasing atomic oxygen fluence and reached a saturated value even without ambient air exposure. These results are contradictory to the previously reported results by Grossman et al. (Grossman, E., Lifshitz, Y., Wolan, J. T., Mount, C. K., and Hoflund, G. B., "In Situ Erosion Study of Kapton Using Novel Hyperthermal Oxygen Atom Source," Journal of Spacecraft and Rockets, Vol. 36, No. 1, 1999, pp. 75-78). From the x-ray photoelectron spectroscopic analysis of the atomic oxygen-exposed Kapton-H with and without ambient air exposures, it was concluded that the atomic oxygen exposure creates a highly oxidized surface and the carbon and oxygen concentrations were affected by the subsequent ambient air exposure. These results clearly indicate that the effect of air exposure should be taken into consideration in the analysis of surface chemistry of materials exposed to low-Earth-orbit space environment.

Erosion phenomenon of polyimide film under the hyperthermal atomic oxygen beam exposure, which is a simulated low Earth orbit space environment, has been investigated. The polyimide film was spin-coated on a sensor crystal of a quartz crystal microbalance, and the mass of the film was measured under the atomic oxygen beam exposure. The spin-coated polyimide film which was exposed to a 4.7 eV atomic oxygen beam showed a mass gain at the beginning of the reaction and then steady-state mass loss followed. The experimental results of the mass change was analysed by the computational model, and the results showed that the carbon abstraction rate at the oxygen-adsorbed sites was two orders higher than that at the unoxidized polyimide surface. The computational results suggested that a large fraction of the carbon abstraction reaction occurred in the oxygen-adsorbed site through a Langmuir-Hinshelwood reaction mechanism.

The oxidation properties of an H-terminated Si(001) surface following the use of a hyperthermal broad atomic oxygen beam have been studied at low temperatures. Effects of sample temperature, flux and energy of the impinging atomic oxygen on the oxidation of silicon were investigated by X-ray photoelectron spectroscopy. It was confirmed that oxide growth at the surface was achieved even at room temperature and the oxide thickness reached a terminal thickness of a few nanometers depending upon the oxidation conditions. The oxidation process was divided into two stages: the fast oxidation stage and the subsequent slow oxidation stage. It was also confirmed that the reaction yield of atomic oxygen with Si increased with increasing translational energy. The characteristic feature of beam oxidation was explained by the effect of energy accommodation from the impinging oxygen atom to the surface silicon atom.

Polyimide surfaces exposed to simulated low Earth orbit space environment, i.e., under hyperthermal atomic oxygen bombardment, were characterized by using atomic force microscopy, X-ray photoelectron spectroscopy, and contact angle measurements. The surface analytical results showed that the roughness and the O/C ratio at the atomic oxygen-exposed polyimide surface increased with increasing atomic oxygen fluence. The advancing and receding contact angles decreased with increasing O/C ratios at the polyimide surfaces. The Lifshitz-van der Waals component, the acid and base parameters of the surface free energy of polyimide films were calculated from the contact angles. The base parameter increased with increasing O/C ratio, whereas the acid parameter and the Lifshitz-van der Waals component did not show a remarkable change. These analytical results agree with the in situ XPS data showing the formation of surface functional groups due to atomic oxygen exposure. It was demonstrated in this study that the polyimide surface in a low Earth orbit space environment may become hydrophilic due to the bombardment by atomic oxygen.

In this paper, die effect of atomic oxygen exposures on the tribological properties of molybdenum disulfide (MoS2) is studied with a laser-induced breakdown type atomic oxygen facility. MoS2 single crystals as well as sputtered films are used as specimens to clarify both fundamental reaction and change in tribological properties Of MoS2. X-ray photoelectron spectroscopy and residual gas analysis show that SO and MoO3 are formed at the MoS2 (0001) surfaces. A significant loss of S is found to be due to volatilization of SO. The atomic oxygen exposure on the MoS2 induces high friction (six times higher than normal value) at the beginning of any sliding action on the sputtered films, but no significant effect on a single crystal. It is demonstrated that high friction with sputtered films is being retained when sufficient atomic oxygen is provided while sliding.

The basal planes of highly oriented pyrolytic graphite (HOPG) surfaces were exposed to atomic oxygen (AO) beam with a translational energy of approximately 5 eV at room temperature. The characterization of the surfaces was carried out using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). The XPS spectra of the hyperthermal AO-exposed surfaces indicated that the oxygen coverage reached the saturated value of 0.94 with the AO fluence of approximately 4 x 10(17) atoms cm(-2). The initial reaction site of the AO on the defect-free HOPG (0001) surface was observed as a protrusion in the STM images. In contrast, a hillock-like structure was formed at the AO-exposed HOPG (0001) surfaces at high AO fluences. The density of protrusions, observed in the initial AO/HOPG reaction, increased in proportion to the AO fluence. The reaction yield of hyperthermal AO with the defect-free HOPG (0001) surface was estimated to be 1.0 x 10(-3). This Value is two orders lower than that determined in the flight experiment aboard the space shuttle where the same chemical reaction was expected. The discrepancy was explained by the high reaction yield of the oxygen-covered prism planes of HOPG presented at the hillock surfaces. (C) 1999 Elsevier Science B.V. All rights reserved.

In-situ erosion rate measurement, residual gas analysis and surface characterization of polyimide films exposed to an energetic atomic oxygen beam were studied. The polyimide films were formed on quartz crystals in order to measure the mass loss rate with a quartz crystal microbalance during atomic oxygen beam exposure. The simultaneous exposure of a 5 eV atomic oxygen beam and ultraviolet light provided a good correlation with the Right data regarding the reaction efficiency of Kapton-H. The results of X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy indicated that the PMDA structure in the polyimide was mainly degradated due to the atomic oxygen exposure. The enhancement of carbon dioxide formation during the simultaneous ultraviolet irradiation was clearly observed. This synergetic effect can be explained by the photochemical reaction of the carbonyl group under the presence of atomic oxygen.

Translational energy dependence of the atomic oxygen reactivity with polyimide films is reported. An ion beam type atomic oxygen source was used to study atomic oxygen reactivity in a wide range of translational energies. Mass change of polyimide films was measured by a frequency shift of the quartz crystal microbalance during atomic oxygen beam exposures. The experimental result showed strong translational energy dependence of mass change of the polyimide film, i.e., mass gain was observed at the translational energies below 30eV whereas mass loss was observed at the higher translational energies. X-ray photoelectron spectra indicated that pyromelliticdianhydride in the polyimide structure was degradated by the 5eV atomic oxygen reaction. Decomposition of oxydianiline was also obvious at higher translational energies. The mass gain observed in this study was explained by the absence of adsorbed oxygen at the polyimide surfaces due primarily to the low flux of the beam, which may lead to the suppression of debonding of polymer backbones.