The Tribological Properties of Low-friction Hydrogenated Diamond-like Carbon Measured in Ultrahigh Vacuum

In this paper, we investigate the sliding friction and wear behavior of a hydrogenated diamond-like carbon (DLC) film in ultrahigh vacuum (UHV) and under partial pressures of water vapor, oxygen, nitrogen and hydrogen. The initial friction coefficient of the film in UHV was ~0.15, but decreased steadily to values as low as 0.03 after about 30 sliding passes. During longer duration tests, the friction coefficient increased again to values as high as ~0.15 and such an increase in friction coincided with hydrogen desorption from the contacting surfaces (as detected by a mass spectrometer). Heating DLC to temperatures higher than 360 K also caused desorption of hydrogen and a resulting marked increase in friction. The presence of molecular nitrogen, oxygen and hydrogen in the test chamber did not have any noticeable effect on friction, but the presence of thermally dissociated or ionized hydrogen within the close proximity of sliding surfaces had a beneficial effect by restoring the low friction behavior of the DLC films. The introduction of water vapor into test chamber had an adverse effect on friction. The results of this study confirm that hydrogen is key to low friction behavior of hydrogenated DLC films and that the presence of water molecules has an adverse effect on their friction behavior.     

Preparation of carbon films from evaporated fibers and rods at fore-vacuum pressure

Fore-vacuum (pressure ≥ 3 Pa) evaporation of carbon fibers and rods to thin films has been tested and the resulting thicknesses recorded. A modified sputter coater was used as a vacuum evaporator. The quality of the carbon films was evaluated by bright-field and dark-field electron microscopy. Although high-vacuum-evaporated carbon films are superior in quality, low-vacuum-evaporated carbon films were found fully acceptable for routine work in bright field, for both TEM and SEM purposes. Apart from being time-saving, the method presented has the obvious advantage, in SEM preparations, that carbon coating and metal sputtering can be carried out in the same unit without breaking the vacuum.     

Fabrication of sharp tungsten-coated tip for atomic force microscopy by ion-beam sputter deposition

Tungsten (W) is significantly suitable as a tip material for atomic force microscopy (AFM) because its high mechanical stiffness enables the stable detection of tip-sample interaction forces. We have developed W sputter-coating equipment to compensate the drawbacks of conventional Si cantilever tips used in AFM measurements. By employing an ion gun commonly used for sputter cleaning of a cantilever tip, the equipment is capable of depositing conductive W films in the preparation chamber of a general ultrahigh vacuum (UHV)-AFM system without the need for an additional chamber or transfer system. This enables W coating of a cantilever tip immediately after sputter cleaning of the tip apex and just before the use in AFM observations. The W film consists of grain structures, which prevent tip dulling and provide sharpness (<3 nm in radius of curvature at the apex) comparable to that of the original Si tip apex. We demonstrate that in non-contact (NC)-AFM measurement, a W-coated Si tip can clearly resolve the atomic structures of a Ge(001) surface without any artifacts, indicating that, as a force sensor, the fabricated W-coated Si tip is superior to a bare Si tip.     

Nanofriction of silicon oxide surfaces covered with thin water films

A thin water film present on surfaces plays a central role in defining the micro- and nanotribological properties of a system. This paper presents a quantitative examination of the nanotribological effects of thin water films in ultra high vacuum (UHV) on OH-terminated (hydrophilic) and bare (no OH terminations, hydrophobic in vacuum) silicon oxide surfaces. Water film thickness was controlled by varying the water partial pressure in UHV. Friction was measured by scanning force microscopy (SFM) as a function of an external applied load. The surface energy and the shear stress of the nanotribological contact was then approximated by fitting the friction-load curves using the Derjaguin-Muller-Toporov (DMT) model. The surface energy as well as the adhesion force of the OH-terminated hydrophilic sample first decrease and later increase significantly at higher water partial pressures. No such dependence could be deduced from the friction-load curves at varying water pressures for the bare hydrophobic silicon oxide surface. However, at relatively high normal loads (pressures) and water partial pressures the bare hydrophobic silicon oxide is transformed to an OH-terminated surface. This transformation appears to occur only in the area of contact leading to the conclusion that it is friction-induced. This work shows that the chemical composition of the topmost surface layer defines the frictional behavior of the tribosystem.     

Evaluation of the friction of WC/DLC solid lubricating films in vacuum

The accuracy of nanopositioning is to a large extent limited by the friction-caused errors, particularly in vacuum environments. An investigation of the friction behaviour of sp²-bonds dominating diamond like carbon (DLC) coatings and WC_1−x/DLC, WC(N)/DLC multilayer coatings, which are considered to be used in nanopositioning in vacuum, have been performed by a vacuum microtribometer. By using an atomically smooth Si sphere as a counterface, the reciprocating sliding friction was measured at a normal load <5 mN, and running speed at a 1–100 μm/s in ambient air and in ultra high vacuum (UHV) at 10⁻⁷ Pa, and correlated with microstructures and properties of the coatings. When tested in UHV, the coefficient of friction (COF) for pure DLC coatings (thickness: 700 nm) changes significantly between 0.2 and 0.4. Once the thickness of DLC layers is limited to 5 nm by formation of multilayer coatings, the COF in UHV decreases by nearly one order to 0.02–0.05. We suggest that the deformation of DLC films and the transfer films determines COF. Thick DLC coatings can induce more plastic deformation and consumes more energy in sliding resulting in a high COF. Thickening of the transfer film in running leads to a continuous decrease of COF since the deformation of the transfer films turns easier. The low COF of multilayer coatings is mainly due to their confinement of the thickness of DLC films. A consistent velocity-strengthening frictional behaviour of both WC_1−x/DLC and WC(N)/DLC coatings in UHV indicates that the transfer films acting as a thin layer of granular material. Further study of the friction behaviour with the presence of such granular materials might be interesting for the further development of tribological coatings for vacuum applications.     

Structure analysis of sputter-coated and ion-beam sputter-coated films: a comparative study

Gold, platinum and tungsten films were deposited by low energy input (7 mA, 450 V), or high deposition rate (80 mA, 1500 V), diode sputter coating and by ion beam sputter coating. Film structures on Formvar coated grids and on the surface of coated erythrocytes, resin embedded, sectioned, and recorded at high magnification in a TEM were compared using computer-assisted measurements and analysis of film thickness and grain size. The average grain size of the thinnest gold and platinum films was relatively independent of the mode or rate of deposition but as the film thickness increased, significant differences in grain size and film structure were observed. Thick platinum or gold films deposited by low energy input sputter coating contained large grain size and electron transparent cracks; however, more even films with narrower cracks but larger grain size were produced at high deposition rates. Ion beam sputter coated gold had relatively large grain size in 10 nm thick films, but beyond this thickness the grains coalesced to form a continuous film. Platinum films deposited by ion beam sputter coating were even and free of electron transparent cracks and had a very small grain size (1–2 nm), which was relatively independent of the film thickness. Tungsten deposition either by low energy input or ion beam sputter coating resulted in fine grained even films which were free of electron transparent cracks. Such films remained granular in substructure and had a grain size of about 1 nm which was relatively independent of film thickness. Tungsten films produced at high deposition rates were of poorer quality. We conclude that thick diode sputter coated platinum and gold films are best deposited at high deposition rates provided the specimens are not heat sensitive, the improvement in film structure being more significant than the slight increase in grain size. Thick diode or ion beam sputter coated gold films should be suitable for low resolution SEM, and thin discontinuous gold films for medium resolution SEM. Diode sputter coated platinum should be suitable for medium resolution SEM and ion beam sputter coated platinum for medium and some high resolution SEM. 1–5 nm thick tungsten films, deposited by low energy input or ion beam sputter coating should be suitable for high resolution SEM, particularly where contrast is of less importance than resolution.     

An entirely enclosed scanning tunnelling microscope capable of being fully immersed in liquid helium

We present an ultrahigh‐vacuum (UHV)‐sealed high‐stability scanning tunnelling microscope (STM) that can be entirely immersed in liquid helium and readily used in a commercial Dewar or superconducting magnet. The STM head features a horizontal microscanner that can become standalone and ultrastable when the coarse approach inertial motor retracts. Low voltage is enough to operate the STM even at low temperature owing to the powerful motor. It is housed in a tubular chamber of 49 mm outer diameter, which can be pumped via a detachable valve (DV), UHV‐sealed and remain sealed after the DV is removed. The entire so‐sealed chamber can then be inserted into liquid helium, where in situ sample cleavage is done via vacuum bellow. This allows sample cleavage and STM measurements to take place in better UHV with higher cooling power. Quality atomic resolution images of graphite and charge density wave on 1T‐TiSe₂ taken in ambient and 14 K conditions, respectively, are presented.     

真空压铸设备升级改造研究与实践

针对普通真空压铸设备生产过程中出现的产品问题,提出了升级改造压铸机的解决方法并进行实践,为压铸产品质量改善提供一种途径。通过设计并比较不同的方案,经过多次实践实验,确定了升级改造用真空箱结构,并对相应的模具结构设计提出了经济可行的方法,经过试模验证了模具设计的合理性,产品质量更好,实现了有效可靠的压铸机控制系统,为压铸行业设备升级改造提供经济性和可靠性均较高的解决方案。     

UHV friction of tribofilms derived from metal dithiophosphates

The friction‐reduction mechanisms of Modtp and Zndtp were highlighted by submitting tribofilms to friction in ultra‐high vacuum (UHV). The use of an UHV tribometer to understand these phenomena is justified by the fact that the friction coefficient recorded in UHV is close to the friction coefficient obtained in traditional tests in oil. After UHV friction, the transfer films on the pin were analyzed by in situ AES, XPS and AES mapping. Low friction is associated with the transfer to the pin of a sulfur‐rich film. In the case of Modtp, we observe a very thin MoS₂ film. The UHV friction coefficient approaches 0.04. In the case of Zndtp, the transfer film contains ZnS together with some phosphates. Because of the poor capacity of ZnS to reduce friction, the UHV friction coefficient recorded is near 0.15. A global model of the action of dithiophosphates in reducing friction is described on the basis of the hard and soft acids and bases (HSAB) principle. This revised version was published online in July 2006 with corrections to the Cover Date.     

The respective role of oxygen and water vapor on the tribology of hydrogenated diamond-like carbon coatings

The tribological behavior of diamond-like carbon coatings (DLC) strongly depends on the chemical nature of the test environment. The present study proposes to explore the influence of water vapor and oxygen on the friction behavior of a hydrogenated DLC coating exhibiting ultralow friction in ultrahigh vacuum (friction coefficient below 0.01). Using a UHV tribometer, reciprocating pin-on-flat friction tests were performed in progressively increasing or decreasing partial pressures of pure oxygen and pure water vapor. The maximum gaseous pressures of oxygen and water vapor were 60 hPa and 25 hPa (1 hPa = 100 Pa), respectively, the second value corresponding to a relative humidity (RH) of 100% at room temperature. It was found that, for the pressure range explored, oxygen does not change the ultralow friction behavior of DLC observed in UHV. Conversely, water vapor drastically changes the friction coefficient at pressures above 0.5 hPa (RH = 2%), from about 0.01 to more than 0.1. Electron energy loss spectroscopy and in situ Auger electron spectroscopy have been performed to elucidate the friction mechanisms responsible for the tribological behaviors observed with the two different gaseous environments. In all cases no significant oxidation has been observed either inside the wear scars or in the wear debris particles. Ultralow friction is systematically associated with a homogeneous carbon-based transfer film. The higher friction observed at partial pressure of water vapor higher than 0.5 hPa, is associated with a thinner transfer film. Consequently friction seems to be controlled by the transfer film whose kinetics of formation strongly depends on the partial pressure of water vapor. This revised version was published online in August 2006 with corrections to the Cover Date.     

Development and performance characterization of low, medium and high vacuum primary standards

Three primary vacuum standard systems having the highest metrological qualities in which the pressure is deduced directly from the involved physical quantities uniquely with the proper accuracy and precision, fine resolution and reliability have been developed. This vacuum standardization comprises of a Standard Mercury Manometer for low vacuum, a 4-stage Static Expansion Standard for medium vacuum and an orifice flow type Dynamic Expansion Standard for high vacuum. Each system has explicitly been established and precise measurement model equations with multiple variables are given. The performance of these standards has also been checked discretely with internationally recommended and reliably calibrated secondary standards along with the evaluation of different uncertainties that were found to be within the limits.     

Tribological Behavior of Rice Bran Ceramics in a Vacuum Environment

ABSTRACT

In this study, we investigated the friction and wear of rice bran (RB) ceramics—hard porous carbon materials made from rice bran—in a vacuum environment. Sliding friction tests for RB ceramic pin–RB ceramic disk contact were performed using a pin-on-disk-type friction tester installed in a vacuum chamber. The ambient pressure was controlled at 0.02, 0.6, 30, and 10⁵ Pa (i.e., atmospheric pressure). The normal load was 0.49 or 2.94 N, the sliding velocity was 0.01 or 0.1 m/s, and the number of friction cycles was 50,000. The friction coefficient tended to decrease with decreasing ambient pressure for all combinations of normal load and sliding velocity; by contrast, the specific wear rate of the RB ceramic pin and disk specimens tended to increase with decreasing ambient pressure. The friction coefficient exhibited a low value of 0.05 or less at 0.02 Pa. The results suggested that the reduced surface roughness and graphitization of the sliding surface of the RB ceramic pin and disk due to induced friction, as well as the increased ratio between the partial pressure of water vapor and the ambient pressure, are related to the reduction in the friction of RB ceramic–RB ceramic dry sliding contact under vacuum conditions.     

真空吸附装夹系统改进技术

针对数控加工中采用油润真空泵的传统真空吸附装夹系统存在的设备庞大、购置及维护成本高、产品适应性差以及废油废液污染等问题,对传统真空吸附装夹系统进行了改进.通过新型真空发生器的对比选用以及与真空吸附平台的一体化安装,对原真空吸附工作台容积腔进行结构改进,并增加真空度压力监控表等,实现了真空吸附装夹系统的小型化、经济化和实用化改进.经验证,改进后的真空吸附系统吸附压力稳定,产品加工质量合格,满足产品多样化、高效率以及绿色制造需求.     

Superlow Friction of Diamond-Like Carbon Films: A Relation to Viscoplastic Properties

Composition, structure, electrical, optical, mechanical properties and tribological behavior of diamond-like carbon films (DLC) are strongly dependent on the deposition system. Some hydrogenated amorphous carbon films (a-C:H) may exhibit superlow friction properties in ultra-high vacuum (UHV). The present paper compares tribological and mechanical properties of several DLC films prepared under different conditions. Friction coefficients were measured in an analytical ultra-high vacuum tribometer. The mechanical properties were evaluated from force-displacement curves using a nanoindentation instrument. Making use of continuous stiffness mode, Young's modulus and hardness were determined as a function of indentation depth. The measurements were performed at constant strain rates by special control of the load during indentation. We were, thus, able to determine the dependence of hardness on strain rate, characterizing a viscoplastic behavior. Many of the hydrogenated amorphous carbon films studied were significantly viscoplastic. The aim of this paper is to highlight the correlation between superlow friction and viscoplastic behavior.     

Wear debris analysis in rotating Ag-Cu electrical contacts

A copper slip ring was rotated in contact with two current-carrying silver wire brushes in an ultrahigh vacuum system. The copper ring was initially sputter cleaned and the experiments were performed in humidified CO₂ at atmospheric pressure. The wear particles produced at the two contacts were collected for subsequent investigations by scanning electron microscopy and electron probe microanalysis. Currents ranging from 0 to 40 A were used and the slip ring was rotated at 150 rev min^?1. In one zerocurrent experiment a speed of 15 rev min^?1 was used for comparison. The wear particles consisted of pure silver, pure copper and mixed particles with the relative amounts depending on the current through the contact. Some of the characteristic features of the most frequently occurring wear particles are explained and the mechanisms for the generation of the particles are discussed.     

Change in the Composition of Residual Gases in a Vacuum Chamber during Ti Film Deposition

Changes in the spectrum of chemically active residual gases in a vacuum chamber during the deposition of Ti films on its inside walls using magnetron sputter guns are investigated. It was revealed that the decomposition of hydrocarbons in the plasma of operating magnetrons and the interaction of the products of their decay with oxygen influence the gas composition.     

样品室真空度对磁控溅射Ti膜结构的影响

本文利用场发射扫描电子显微镜和能谱仪研究不同样品室真空度条件下磁控溅射Ti沉积膜膜层的氧含量和微观结构。结果发现,当样品室真空度较低时,膜层的氧含量很高,膜层为钛的氧化物;随着真空度的提高,膜层的氧含量降低,Ti的晶粒出现;进一步提高真空度,Ti晶粒更为明显,膜层的截面呈柱状晶结构。     

Rheological and tribological characterisation of six wet lubricants for space

Dry lubricants are often used in space because of their excellent behaviour in a vacuum and their thermal stability. However, mechanisms which are heavily loaded, with a high sliding rate, a long life span, or needing a stable friction coefficient cannot use this kind of product. They can be replaced by wet lubricants specially developed for vacuum applications. Such lubricants have a low vapour pressure in order to minimise outgassing losses and avoid contamination. Since European experience in the field of wet lubrication in space is rather limited, the French Space Agency CNES, with the help of the LMC (Laboratoire de Mécanique des Contacts - Contact Mechanics Laboratory), is currently characterising several wet lubricants for use in space applications. The selected lubricants were tested on a vacuum friction test bench which allows the main contact parameters to be controlled: speed, load, and kinematics. The bench is equipped with vacuum facilities and, since it runs under air, neutral gas, or vacuum, it can be used to study the effects of atmosphere. Variations of physical properties (viscosity) versus temperature (−60°C to 100°) and pressure (from atmospheric pressure to 500 MPa) were studied for all the lubricants tested, and these characteristics take into account in the analysis of the tribological test results. This paper briefly lists the characteristics of a space environment (vacuum, microgravity, etc.) and their consequences for lubrication. It describes the equipment used and the tribological and rheological test results obtained on six wet lubricants (three oils and three greases). It also contains conclusions in terms of recommendations about the use of such lubricants.     

Environmental effects on the friction of hydrogenated DLC films

We have investigated environmental effects on hydrogenated diamond-like carbon (H-DLC) films under various pressures of H₂O, O₂, and N₂ by ultrahigh vacuum (UHV) tribometry. The H-DLC film exhibits an ultralow coefficient of friction (μ = 0.004 in UHV). The μ value increases with increasing pressure of H₂O and O₂. Specifically, μ increases up to 0.07 under 10 Torr of H₂O, and up to 0.03 under 150 Torr of O₂; these are typical H₂O and O₂ contents respectively in ambient air. Our results are consistent with similar environmental effects previously reported. But, we have also discovered that these friction changes are reversible, returning to the ultralow value when UHV is restored. The reversibility of the friction behavior in both environments, coupled with the lack of evidence of tribochemical changes by Auger electron spectroscopy, suggest that the observed friction changes are due to the weakly adsorbed gas molecules that influence the friction property by physically separating the H-DLC interface. Speed-dependent tribometry also supports this argument. In addition, two DLC films with different hydrogen contents and with widely different friction coefficients in UHV are shown to exhibit identical μ values under humid environments, further demonstrating that the frictional properties of these DLC films are essentially determined by the surface layer of adsorbed gas molecules.     

The friction and fragmentation of some refractory solids: The role of protective surface films

This paper describes a study of the friction of refractory solids and the role of naturally occurring surface films. It is shown that when these are worn away in high vacuum so that the films cannot reform, the friction generally rises by a factor of about three. Small amounts of water vapour restore the friction to its lower “atmospheric” value. Beryl is an exception giving a lower friction in vacuum than in air.A separate series of experiments shows that surface cracking is greatly facilitated by an increase in friction because of the interaction between the normal and the tangential forces. Consequently fragmentation occurs very readily when these solids slide together in high vacuum.