Vacuum Tribometer Research Articles

Advantages of Using Triboscopic Imaging: Case Studies on Carbon Coatings in Non-Lubricated Friction Conditions.

Triboscopy focuses on the analysis of the temporal evolution of a tribological system, combining local and time-resolved information, most commonly the evolution of friction. In this work, this technique is applied on measurements, which were carried out with a custom-built ultra-high vacuum tribometer in ball-on-disc configuration. Based on these experiments, an extended classification to distinguish different triboscopic features is suggested, depending on the persistence in both track position and time: Uniform, Global, Local, and Sporadic. Further, a filter technique for quantifying triboscopic data regarding this classification is introduced. The new and improved triboscopic techniques are applied to various dry friction measurements of hydrogen-free carbon coatings under varying humidity and pressure. The resulting specific triboscopic features are correlated to wear phenomena, such as counter body coating abrasion, inhomogeneities in the wear track, non-uniform track wear, stick-slip and debris in the contact area, demonstrating the increased analysis and monitoring capabilities when compared to conventional friction curves and wear track images.

Tribological Behaviors of Polycrystalline Cubic Boron Nitride Sliding against Bearing Steel in Vacuum Conditions

In order to understand the surface and interface conditions of polycrystalline cubic boron nitride (PcBN) sliding against bearing steel in vacuum environments, the effects of different loads on the tribological behaviors of PcBN and bearing steel AISI 52100 were studied deeply in a vacuum tribometer. Furthermore, the wear tracks of PcBN and the wear scars of AISI 52100 were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. The results show that the stable coefficient of friction (CoF) of the tribopair experiences a decrease first and then an increase with the increase in loads from 2 N to 15 N. The adhesive layer increases with the increase in loads, and the formation of adhesive layer contributes to the change of CoF and wear rate of counterpart balls. The adhesive layer is formed due to the combination of high contact stress and high temperature. Meanwhile, the exfoliated cubic boron nitride grains are embedded into the adhesive layer as abrasive grains, resulting in abrasive wear. Thus, the main wear mechanisms are adhesive wear and abrasive wear.

Fabrication and high temperature tribological properties of WC reinforced W-Cu composites

In this study, WC doped W-Cu composites with high hardness and excellent tribological properties were prepared using hot pressing sintering with Cu coated W composite powders. Friction and wear behaviors of W-Cu-WC composites and W-Cu composites against Si3N4 balls at room and high temperatures (e.g., 25 ℃, 700 ℃ and 900 ℃) were investigated using a ball-on-disk high temperature tribometer in vacuum. Results showed that the Vickers hardness of WC doped W-Cu composites (245 HV) was relatively higher than that of the W-Cu composites. The specific wear rate of WC doped W-Cu composites was lower than that of the W-Cu composites in the temperature range of 25–900, and that at 700 °C (56.3 × 10−5 mm3/Nm) was an order of magnitude lower than that of the W-Cu composites. The wear resistance of W-Cu composites doped with WC particles was dramatically improved in vacuum. The dominant wear mechanisms of W-Cu composites were abrasive wear, adhesive wear and delamination at various temperatures, whereas the dominant wear mechanisms of W-Cu-WC composites were abrasive wear and delamination at various temperatures. Data availability statementThe data will be made available upon request.

Inducing High-Energy-Barrier Tribochemical Reaction Pathways; Acetic Acid Decomposition on Copper

The surface tribological chemistry of acetic acid on copper is studied using an ultrahigh vacuum tribometer, supplemented by first-principles density functional theory calculations of the surface structure and reaction pathways. Acetic acid forms η2-acetate species on bridge sites at room temperature as identified by reflection–absorption infrared spectroscopy. Rubbing the surface with a tungsten carbide ball reduces the amount of carbon and oxygen in the rubbed region at the same rates to leave some carbon and oxygen on the surface. This is different from the thermal decomposition pathway, where heating to ~ 580 K removes all oxygen, but leave a small amount of carbon on the surface. It is postulated that this arises because sliding along a direction aligned within the plane of the adsorbed acetate species can induce a high-energy-barrier pathway in which the η2-acetate tilts to form an η1-acetate that can react to form a bent CO2δ− species that decomposes to evolve carbon monoxide and deposit atomic oxygen on the surface. Repeated acetic acid dosing and rubbing reduces the total amount of acetic acid that can adsorb on the surface by ~ 50% after ~ 4 cycles, resulting is a stable, low-friction film. At this point, the adsorbed acetic acid is completely tribochemically removed. This suggests that adsorbed acetic acid can form a self-healing film in which any wear of the low-friction film will then allow it to be replenished by shear-induced decomposition of adsorbed acetate species.

Friction and wear behavior of Cu-La2O3 composite sliding against 52100 bearing steel in vacuum

Cu-La2O3 nanocomposite used as sliding electrical contact materials for satellites and spacecraft was fabricated via internal oxidation method followed by powder metallurgy. The Cu-La2O3 composites were subjected to the microstructure analysis, microhardness and tensile test. The tribological properties were performed using a pin-on-disk УТИ TB100 type vacuum tribometer in vacuum, rubbing against 52100 type bearing steel disk at different applied loads varied from 10 to 40 N. Results indicated that the La2O3 particles were uniformly dispersed in the copper matrix, and the size of La2O3 particles was below 50 nm. Besides, the La2O3 reinforcement was effective in improving tensile strength and microhardness. When the La2O3 content was 1.3 wt %, the tensile strength and microhardness of the Cu-La2O3 reached a maximum of 329 MPa and 118 Hv, respectively. The coefficient of friction slightly increased with the increasing La2O3 content. The wear rate of Cu-1.3 wt% La2O3 nanocomposite was 3.3 × 10−3 mg m −1 for 500 m in vacuum, which was reduced by 39% compared with that of pure Cu. When the normal load increased from 10 to 40 N, the wear mechanisms changed from abrasive wear to severe plastic deformation, delamination wear and fatigue wear.

Influence of Surface Treatment on Dry Sliding Wear Behavior of Hydrophobic Silane Coating on AISI 304

Henicosyl-1,1,2,2-tetrahydrododecyltricholrosilane is a hydrophobic silane compound coated on AISI 304 substrate. The surface of the substrate was treated in two ways (silicon oxide and plasma oxide) for adhesion and interface between the inorganic substrate and organic coating. The wear performance of the coating as a comparison to the AISI 304 was investigated using vacuum tribometer and characterized by scanning electron microscope and atomic force microscope. The result showed that the silane compound did improve the wear property of the AISI 304 stainless steel. It also proved that silane can be used as wear resistance agent.

A new ball-on-disk vacuum tribometer with in situ measurement of the wear track by digital holographic microscopy

This contribution presents a new ball-on-disk vacuum tribometer with in situ measurement of the wear track by digital holographic microscopy. This new tribometer allows observation of the evolution of the wear track in situ and in real-time. The method combines a high vacuum high temperature ball-on-disk tribometer with a digital holographic microscope (DHM). The machine was tested and validated by taking DHM images during wear tests at room temperature and in vacuum at 2 · 10−6 of polished 100Cr6 steel disks.We demonstrated that the DHM system is well suited to monitor the evolution of the wear track during sliding. We found that, with an acquisition time of 0.1 ms for the DHM, the maximal linear speed is 10 cm s−1 to have reliable images. We proved, via scanning electron microscope (SEM) pictures, that the lines in the sliding direction in all DHM images exist. We also validated the new tribometer by having an excellent correlation between the images and profiles of the wear track taken by the DHM with the ones from a confocal microscope. Finally, the new tribometer combined with the DHM has four advantages. It can test under vacuum and various atmospheric conditions. The evolution of the wear track is measured in situ and in real-time. Hence, the problem of replacing the sample is avoided. Thanks to the DHM technology, the vertical accuracy of the topographical measurement is 4 nm.

Investigation on the Effect of ECAP Routes on the Wear Behavior of AA2014

ECAP is an effective process to improve the mechanical strength and wear resistance along with mechanical and microstructural properties. AA2014 solutionized at 495 °C and aged at 195 °C was subjected to Equal Channel Angular Pressed (ECAP) through route A and Bc at room temperature. It was well proved that the mechanical strength increased due to ECAP in AA2014. In order to investigate their wear behavior after ECAP, dry sliding wear tests were conducted using vacuum tribometer at nominal loads of 10N and 30N with constant speed of 2 m/s for sliding distance of 2000 m. The co-efficient of friction and loss in volume were decreased after ECAP both in route A and Bc. The dominant wear mechanisms observed were adhesive, delamination and stick slip process. In addition to these wear mechanisms, abrasive wear also appeared along with transfer of iron particles from the counter surface to the AA2014 pin. Presence of black powder and oxide formation were observed using EDAX analysis on wear debris. Routes A and Bc showed similar wear mechanisms and characteristics which were better than in unECAPed specimens.

Dry Sliding Tribological Behaviour of Casted Eutectic Al-Si Alloy with and without Nano Alumina Reinforcement

An investigation on wear behaviour of Al-Si (eutectic) – ENAC48000 have been carried out which are widely employed in the tribological environment as the silicon imparts high wear resistance. Composites with nano alumina (~40 nm) reinforcement of 1%, 2%, 3% (wt %) were prepared using Squeeze casting technique. The samples were polished and grounded well using emery, alumina and diamond paste to study the micro structural morphology using Optical Microscope (OM) followed by SEM. Pin samples were tested for studying the dry sliding wear behaviour using High Temperature Vacuum Tribometer under the load of 10N, 20 N and 30 N at 3.665 m/s. The hardness survey results show a gradually increasing trend in the hardness value with respect to the reinforcement. Regardless of the increasing in the hardness value proportional to the percentage of reinforcement, the wear resistance shows a detrimental pattern as the load and operating temperature increases simultaneously. Abrasion wear mechanism was observed using SEM results on the induced sub surface wear at higher loads. The COF values and mass loss measurement of the composites shows better wear resistance with respect to the reinforcement percentage as the formation of oxide at high temperatures over the surface tends to resist the wear. The results shows promising to employ the composite is low wear working conditions where the thermal distortions due to wear are lesser comparatively.

Tribological properties of plasma-nitrided AISI 4340 steel in vacuum

Tribological properties of nitrided AISI 4340 steel are evaluated using ball-on-disk tribometer in vacuum. Optical microscopy and Vickers hardness tester are used to analyse compound layer thickness and case depth. X-ray diffraction is used to observe the presence of different constituents of nitrided surface before and after the wear tests. The wear mechanisms are investigated by microscopic observations of the worn surface using scanning electron microscopy. At 200 rpm, mild adhesive wear forms the main wear mechanism under the lower load. With increasing the normal load, the dominant wear mechanism changes to mild plastic deformation and abrasive wear. At 600 rpm, the wear mechanisms vary from the mild plastic deformation to severe plastic deformation, abrasive wear and even delamination with increasing the normal load.

Ultrahigh vacuum system for advanced tribology studies: Design principles and applications

The main difficulty in designing of an ultrahigh vacuum (UHV) tribometer combined with tribophysical and tribochemical characterization techniques is to find the critical compromise between the scientific requirements and technical or technological limitations from different subsystems and components. The principal conflicts, their possible solutions and the recommended tribometer configurations are analyzed. The developed methodological principles were applied for designing and construction of two UHV experimental tribological systems: TriDes-2 and Ca3UHV. The advances in the design and development of the vacuum system as well as the UHV force sensor and sample holder are presented and discussed.

Effects of nitriding temperature on microstructures and vacuum tribological properties of plasma-nitrided titanium

Facing the outer space application of titanium and its alloys in drilling pipes and drilling robots, it is necessary to find an optimized surface engineering method to improve their vacuum tribological properties. In order to find an optimized nitriding temperature, a series of plasma nitriding experiments were carried out for commercial titanium (UNS R50400/Gr. 2/TA2/CP3) at various temperatures ranging from 650 to 950°C for 8h to investigate the effects of nitriding temperature on microstructures and mechanical properties. Vacuum tribological properties of the nitrided and untreated samples were examined using a ball-on-disc vacuum tribo-meter. The results show that the wear resistance of titanium tested under vacuum condition is effectively enhanced, due to the formation of the hard nitrided layer. The surface roughness, thickness and hardness of the nitrided layer increase with the increase of nitriding temperature. Nevertheless, the load bearing capacities of the samples nitrided at 900 and 950°C are much lower than those of the samples nitrided at 800 and 850°C. Therefore, the wear volume keeps dropping at the nitriding temperature from 650 to 850°C, whereas, it converts to rise up when the nitriding temperature is above 850°C. In general, it is an optimized process to be nitrided at the temperature of 850°C for 8h to improve the vacuum tribological properties of titanium.

Vacuum Tribological Properties of Titanium with a Nanocrystalline Surface Layer

In order to enhance the tribological properties of titanium (Ti), a gradient structure with a nanocrystalline surface layer was formed on the surface of the UNS R50400 (Gr2/TA2/CP3) Ti discs by means of ultrasonic surface rolling processing (USRP). The microstructure and microhardness of the USRP and untreated Ti were investigated. The tribological properties of the USRP Ti under both vacuum and atmospheric conditions were examined in comparison with the untreated Ti using a ball-on-disc vacuum tribometer. The results showed that an ultrafine-grained (UFG) deformation layer with a thickness of 200 μm was produced on the USRP Ti, and its mean surface grain size was refined to about 38 nm from 45 μm. The surface hardness of the USRP Ti is 310 HV, which is about 2.3 times that of the untreated Ti. Additionally, the USRP Ti possesses a lower friction coefficient and higher wear resistance under vacuum conditions than untreated Ti. Under atmospheric conditions, USRP Ti shows more extensive oxidation wear and milder abrasive wear compared with the untreated Ti. Under vacuum conditions, because of the formation of the UFG deformation layer, USRP Ti exhibits a significant decrease in plastic deformation and adhesive wear compared with the untreated Ti.

Tribological Properties of 1-Alkenes on Copper Foils: Effect of Low-Coordination Surface Sites

It has been suggested from molecular dynamics simulations that low-coordination-number sites are produced at a sliding metal–metal interface, but, because of their instability, they may rapidly relax to increase their coordination number. The possible presence of such low-coordination sites on the surface is tested by exploiting the observation that the desorption temperatures of 1-alkenes on copper increase with both the number of carbons atoms in the 1-alkenes and the surface roughness. Thus, 1-alkenes desorb from a Cu(111) single crystal, with relatively few low-coordination sites, at temperatures between 60 and 100 K lower than from a polycrystalline copper foil. The decrease in friction after impinging a flux of various 1-alkenes on a copper foil, while rubbing in an ultrahigh vacuum tribometer, correlates very well with the corresponding 1-alkene coverages on a copper foil, estimated using the desorption kinetics. This suggests either that rubbing does not result in the formation of lower-coordination sites or that they relax sufficiently rapidly that they do not influence the surface chemistry of 1-alkenes. Surface analyses indicate that shear at the interface causes carbon to diffuse into the subsurface region to form a tribofilm.

Tribological investigations of ionic liquids in ultra‐high vacuum environment

ABSTRACTA tribological investigation of two ionic liquids (ILs) that could be used as potential lubricants for vacuum or space applications is presented. Their tribological behaviour was evaluated using a ball‐on‐flat oscillating reciprocating motion CaTri ultra‐high vacuum (UHV) tribometer and the results were compared against those of a conventional perfluoropolyether (PFPE) vacuum lubricant. The thermal stability of the lubricants was investigated by thermogravimetric analysis before exposing them to UHV environment. The worn steel surfaces were studied by X‐ray photoelectron spectroscopy, optical and confocal microscope as well as scanning electron microscope. The tribological evaluations showed that ILs exhibited better friction reduction and anti‐wear properties than the PFPE lubricant. The excellent tribological properties were attributed to the formation of a surface protective film composed of iron fluoride and iron phosphate, respectively, on the lubricated metal surfaces by a tribochemical reaction with the ILs. Copyright © 2013 John Wiley & Sons, Ltd.

Relating Molecular Structure to Tribological Chemistry: Borate Esters on Copper

The surface and tribological chemistry of 2-propanol and borate esters, isopropoxy tetramethyl dioxaborolane (ITDB), and tetramethyl dioxaborolane (TDB) as gas-phase lubricants for copper surfaces are compared by measuring the friction coefficient and contact resistance in an ultrahigh vacuum tribometer. 2-propanol reacts on copper below room temperature to desorb primarily acetone and causes no friction reduction when used as a gas-phase lubricant. TDB decomposes to deposit BOx species on the surface and produces an initial decrease in friction coefficient that is not sustained. ITDB, which reacts at room temperature to form a tetramethyl dioxaborolide species on the surface, results in an immediate and sustained decrease in friction coefficient when used as a gas-phase lubricant for copper. This indicates that the surface chemistry of the borate esters can be correlated with their tribological properties and illustrates the effect of replacing a hydrogen atom in TDB with a 2-propoxy group in ITDB on both the surface and tribological chemistry. Analysis of the species formed in the wear track after lubrication with ITDB reveals the presence of graphitic or diamond-like carbon and oxygen, and these elements are also found in the subsurface region of the sample. Rubbing the sample in vacuo after reacting with ITDB shows the removal of a boundary film where the coefficient remains constant and the subsequent removal of the subsurface layer in which the contact resistance rises to its original value.

Surface composition variation and high-vacuum performance of DLC/ILs solid–liquid lubricating coatings: Influence of space irradiation

In this paper, we fabricated a DLC/ionic liquid (DLC/ILs) solid–liquid lubricating coating and investigated the effect of atomic oxygen (AO), ultraviolet (UV), proton and electron irradiations on composition, structure, morphology and tribological properties of the DLC/ILs solid–liquid lubricating coatings. A ground-based simulation facility was employed to carry out the irradiation experiments. X-ray photoelectron spectroscope (XPS), Raman spectra, and Fourier Transform Infrared Spectroscopy (FTIR) were used to analyzed the structure and composition changes of DLC film and IL lubricant before and after irradiations. The tribological behavior of the DLC/ILs solid–liquid lubricating coating before and after irradiations was investigated by a vacuum tribometer with the pressure of 10−5Pa. The experimental results revealed that irradiations induced the structural changes, including oxidation, bond break and crosslinking reactions of DLC film and IL lubricant. The damage of proton and AO irradiations to lubricating materials were the most serious, and UV irradiation was the slightest. After irradiations, the friction coefficient of the solid–liquid lubricating coatings decreased (except for AO irradiation), but the disc wear rate increased compared with non-irradiation coatings.

Shear-induced boundary film formation from dialkyl sulfides on copper

The friction of a copper surface, which is exposed to diethyl disulfide (DEDS), dimethyl disulfide (DMDS) or dimethyl trisulfide (DMTS) while rubbing by a tungsten carbide pin, are explored in an ultrahigh vacuum tribometer to investigate the effect of the sulfur-to-carbon stoichiometry on gas-phase lubrication. Surface analyses by Auger spectroscopy of the wear scars after rubbing reveal that the amount of sulfur increases with sulfur content of the gas-phase lubricant, as anticipated. It is found that the friction reduction depends on the normal load, where the friction coefficient tends to decrease more at lower loads, and that the load at which the most friction-reducing tribofilm is formed depends on the stoichiometry of the gas-phase lubricant. DEDS (with a sulfur to carbon ratio of 0.5) only reduces the friction coefficient to a minimum value of ∼0.28 at a normal load of 0.25 N, for DMDS (S/C = 1.0) friction is reduced to ∼0.28 for loads below ∼0.7 N, and DMTS (S/C = 1.5) reduces friction to this value for all loads tested.

Shear-Induced Surface-to-Bulk Transport at Room Temperature in a Sliding Metal–Metal Interface

Predictions from molecular dynamics (MD) simulations, that sliding at a metal–metal interface causes vortices in the near-surface region that transport atoms from the surface into the subsurface region, is tested experimentally. This is accomplished by rubbing a methyl thiolate overlayer grown on a clean copper foil by exposure to dimethyl disulfide at room temperature. Repeatedly rubbing a 1.27 × 10−2 m diameter pin over a thiolate-covered copper surface at an applied load of 0.44 N and sliding speed of 4 × 10−3 m/s in an ultrahigh vacuum tribometer, results in the removal of sulfur from the wear track as measured using spatially resolved Auger spectroscopy. Any remaining surface species, in particular, outside the wear track, are removed by argon ion bombardment. Since sulfur is more thermodynamically stable at the surface, heating the sample causes the sulfur to resegregate to the surface only inside the wear track, thereby directly confirming the predictions from MD simulations.

Reduction in Frictional Force of ZnO Coatings in a Vacuum

Zinc oxide coatings with crystal preferred orientation on stainless steel substrates were deposited by rf magnetron sputtering. The crystal structure of the ZnO coatings was analyzed by X-ray diffraction spectroscopy. The frictional properties of the coatings were measured under atmosphere and in a vacuum using a vacuum tribometer. The frictional properties of the ZnO coatings in a vacuum show irregularities when compared with the normal properties described by the Amonton–Coulomb law. It was revealed that the electric interaction between the probe for the friction test and the coatings changed owing to the piezoelectric effect in a vacuum. A reduction in the frictional coefficient of the ZnO coatings (µ= 0.075) was achieved at a normal load of 0.2 N under vacuum conditions.