Tribological Properties of Diamond-Like Carbon Films in Low and High Moist Air

Diamond-like carbon (DLC) film was deposited on Si wafer by a plasma CVD deposition system using benzene. Tribological properties of the DLC film were evaluated using a ball-on-disk tribo-meter in low (RH 1720 %) and high humidity (RH 9095 %) conditions in air. The effect of sliding speed (4.2 mm/s to 25 mm/s) and load (1.06 N to 3.08 N) on friction and wear was investigated. The friction behavior of the DLC film was obviously different in low and high humidity. When tested under low humidity conditions, the friction coefficient decreased significantly with increasing speed, and increased with load. However, under high humidity conditions, the friction coefficient increased with the speed and decreased with increasing load. The wear of the DLC film was little influenced by the sliding speed, normal load and humidity; a level of 10^-8 mm³/Nm could be obtained in all tests. The formation of a uniform transfer layer would be the main factor which controlled the friction coefficient of the DLC films. Unlike the friction, the wear resistance of the DLC film is not so easy to discuss and may be affected mainly by the tribo-chemical reaction in all the test conditions.     

Tribochemical effects on the friction and wear behaviors of diamond-like carbon film under high relative humidity condition

Friction and wear behaviors of diamond-like carbon (DLC) film in humid N₂ (RH-100%) sliding against different counterpart ball (Si₃N₄ ball, Al₂O₃ ball and steel ball) were investigated. It was found that the friction and wear behaviors of DLC film were dependent on the friction-induced tribochemical interactions in the presence of the DLC film, water molecules and counterpart balls. When sliding against Si₃N₄ ball, a tribochemical film that mainly consisted of silica gel was formed on the worn surface due to the oxidation and hydrolysis of the Si₃N₄ ball, and resulted in the lowest friction coefficient and wear rate of the DLC film. The degradation of the DLC film catalyzed by Al₂O₃ ball caused the highest wear rate of DLC film when sliding against Al₂O₃ ball, while the tribochemical reactions between DLC film and steel ball led to the highest friction coefficient when sliding against steel ball.     

Tribological characterisation of hard coatings with and without DLC top layer in fretting tests

The potential of coatings to protect components against wear and to reduce friction has led to a large variety of protective coatings. In order to check the success of coating modifications and to find solutions for different purposes, initial tests with laboratory tribometers are usually done to give information about the performance of a coating. Different Ti‐based coatings (TiN, Ti(C,N), and TiAlN) and NiP were tested in comparison to coatings with an additional diamond‐like carbon (DLC) top coating. Tests were done in laboratory air at room temperature with oscillating sliding (gross slip fretting) with a ball‐on‐disc arrangement against a ceramic ball (Al₂O₃). Special attention was paid to possible effects of moisture (relative humidity). The coefficient of friction was measured on line, and the volumetric wear at the disc was determined after the test from microscopic measurements of the wear scar and additional profiles. The friction and wear behaviour is quite different for the different coatings and depends more or less on the relative humidity. The DLC coating on top of the other coatings reduces friction and wear considerably. In normal and in moist air the coefficient of wear of the DLC top‐layer coating is significantly less than 10⁻⁶ mm³/Nm and the coefficient of friction is below 0.1. In dry air, however, there is a certain tendency to high wear and high friction. Copyright © 2006 John Wiley & Sons, Ltd.     

Study of tribological performance of ECR-CVD diamond-like carbon coatings on steel substrates: Part 1. The effect of processing parameters and operating conditions

Diamond-like carbon (DLC) coatings were prepared on AISI 440C steel substrates at room temperature by electron cyclotron resonance chemical vapor deposition (ECR-CVD) process in C₂H₂/Ar plasma. Using the designed Ti/TiN/TiCN/TiC interfacial transition layers, relatively thick DLC coatings (1-2 μm) were successfully prepared on the steel substrates. The friction and wear performance of the DLC coatings was evaluated by ball-on-disk tribometry using a steel counterbody at various normal loads (1-10 N) and sliding speeds (2-15 cm/s). By optimizing the deposition parameters such as negative bias voltage, DLC coatings with hardness up to 30 GPa and friction coefficients lower than 0.15 against the 100Cr6 steel ball could be obtained. The friction coefficient was maintained for 100,000 cycles (∼2.2 km) of dry sliding in ambient environments. In addition, the specific wear rates of the coatings were found to be extremely low (∼10⁻⁸ mm³/Nm); at the same time, the ball wear rates were one order of magnitude lower. The influences of the processing parameters and the sliding conditions were determined, and the frictional behavior of the coatings was discussed. It has been found that higher normal loads or sliding speeds reduced the wear rates of the coatings. Therefore, it is feasible to prepare hard and highly adherent DLC coatings with low friction coefficient and low wear rate on engineering steel substrates by the ECR-CVD process. The excellent tribological performance of DLC coatings enables their industrial applications as wear-resistant solid lubricants on sliding parts.     

Friction and wear of carbon nanohorn-containing polyimide composites

Polyimide (PI)-based composites containing single-wall carbon nanohorn aggregate (NH) were fabricated using the spark plasma sintering (SPS) process. For comparison, composites with carbon nanotube (NT) and traditional graphite (Gr) were also fabricated. The NH was produced using CO₂ laser vaporization and a graphite target and the NT was produced by a chemical synthesis method. We evaluated the friction and wear properties of the PI-based composites with a reciprocating friction tester in air using an AISI 304 mating ball. NH drastically decreased the wear of PI-based composites; the specific wear rate of composite with NH of only 5 wt% was of the order of 10⁻⁸ mm³/Nm, which was two orders of magnitude less than that of PI alone. The wear reduction ability of NT seemed to be slightly inferior to that of NH, although it was considerably better than that of Gr. NH and NT lowered the friction of composites. The friction coefficient of composite with 10 wt% NH was less than 0.25, although it was slightly higher than that of composite with 10 wt% Gr. There was no clear difference in the friction reduction effect of NH and NT. The further addition of Gr to composites with NH or NT rather deteriorated the antiwear property of composites, although the friction coefficient was slightly reduced. The transferred materials existed on the friction surface of the mating ball, sliding against composites with three types of carbon filler. These transferred materials seemed to correlate with the low friction and wear properties of composites.     

Genesis of superlow friction and wear in diamondlike carbon films

Diamondlike carbon (DLC) films offer enormous possibilities for applications that require low friction and high wear resistance. The range of physical, chemical, mechanical, electrical, and optical properties offered by these films is also exceptional and can meet the increasingly multifunctional application needs of machine elements, microelectronics, and biological systems. Since the early 1990s, carbon has been used as a precursor in our laboratory for the design and synthesis of superhard and low-friction carbon films. As a result of systematic studies over the years, in 1997, we developed a new class of DLC films that provide friction and wear coefficients of 0.001–0.005 and 10⁻¹⁰–10⁻⁹ mm³/N m, respectively, in inert-gas or vacuum test environments. This paper will mainly concentrate on the tribology of these superlow-friction carbon films and provide an update on our understanding of the fundamental tribological mechanisms of such films. It will also expand on the effects of hydrogen within the films and gaseous species within the test environments on friction and wear.     

Tribological Performance of Diamond and Diamondlike Carbon Films at Elevated Temperatures

In this study, the authors investigated the tribological performance of diamond and diamondlike carbon (DLC) films as a function of temperature. Both films were deposited on silicon carbide (SiC) by microwave plasma chemical vapor deposition and ion-beam deposition processes. Tribological tests were performed on a reciprocating wear machine in open air (20 to 30% relative humidity) and under a 10 N load using SiC pins. For the test conditions explored, the steady-state friction coefficients of test pairs without a diamond or DLC film were 0.7 to 0.9 and the average wear rates of pins were 10^?5 to 10^?7 mm³/N·m, depending on ambient temperature. DLC films reduced the steady-slate friction coefficients of the test pairs by factors of three to five and the wear rates of pins by two to three orders of magnitude. Low friction coefficients were also obtained with the diamond films, but wear rates of the counterface pins were high due to the very abrasive nature of these films. The wear of SiC disks coated with either diamond or DLC films was virtually unmeasurable while the wear of uncoated disks was substantial. Test results showed that the DLC films could afford low friction up to about 300° C. At higher temperatures, the DLC films graphitized and were removed from the surface. The diamond films could withstand much higher tempera-lures, but their tribological behavior degraded. Raman spectroscopy and scanning electron microscopy were used to elucidate the friction and wear mechanisms of both films at high temperatures.     

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.     

Wear resistant solid lubricant coating made from PTFE and epoxy

A composite coating of polytetrafluoroethylene and epoxy shows 100 × improvements in wear resistance as compared to either of its constituents alone and reduced friction coefficient under testing on a pin-on-disk tribometer. This coating is made by impregnating an expanded PTFE film with epoxy, which provides three unique functions: (1) the epoxy compartmentalizes the PTFE nodes, which is believed to reduce the wear of the PTFE, (2) the epoxy increases the mechanical properties such as elastic modulus and hardness, and (3) the epoxy provides a ready interface to bond the films onto a wide variety of substrates easily and securely. The experimental matrix had normal loads of 1–3 N, sliding speeds from 0.25 to 2.5 m/s, and used a 2.4 mm radius low carbon steel pin in a rotating pin-on-disk tribometer. The skived PTFE films had wear rates on the order of K=10^–3 mm³/Nm and friction coefficients around =0.2. Both the high density films (70 wt%PTFE) and low density films (50 wt% PTFE) had wear rates on the order of K=10^–6 mm³/Nm and friction coefficients around =0.15. The neat epoxy films showed significant scatter in the tribological measurements with wear-rates on the order of K=10^–4 mm³/Nm and friction coefficients around =0.40. The enhanced tribological behavior of these composites is believed to stem from the coatings ability to draw thin PTFE transfer films into the contact from the nodes of PTFE, which act like reservoirs. Nanoindentation mapping of the coatings and the transfer films supports this hypothesis, and accompanies scanning electron microscopy observations of the worn and unworn coatings.     

CVD diamond, DLC, and c‐BN coatings for solid film lubrication

The main criteria for judging coating performance were coefficient of friction and wear rate, which had to be less than 0.1 and 10^-6 mm³/(N.m), respectively. Carbon‐ and nitrogen‐ion‐implanted, fine‐grain, chemical‐vapor‐deposited (CVD) diamond and diamondlike carbon (DLC) ion beam deposited on fine‐grain CVD diamond met the criteria regardless of environment (vacuum, nitrogen, and air). This revised version was published online in July 2006 with corrections to the Cover Date.     

Friction and wear of diamond-containing polyimide composites in water and air

Polyimide-based composites containing fine diamond powder were fabricated using spark plasma sintering. The based material was polyimide (PI) containing a small amount of polytetrafluoroethylene (PTFE). Two types of diamond powder were used: one synthesized by statically high pressure, i.e., high-pressure diamond (HD), and the other synthesized by shock compression, i.e., shock-compression diamond (SD). We evaluated their tribological properties using a reciprocating friction tester in water and air using an Al₂O₃ mating ball. Adding HD to the polyimide-PTFE-based material decreased the composite's friction in water, but the effect of this addition in air was negligible. The specific wear rate of composites with different HD content was similar to that of the based material alone in water, while the wear of composites decreased with the addition of diamond in air. The effect of diamond powder size on friction and wear of composites was generally low in both water and air. The addition of SD decreased the friction coefficient of composites, but SD content only negligibly affected the friction in water and air. The specific wear rate was minimal at SD content of 5 vol.%, when diamond content was varied. Wear was almost independent of diamond powder size. SD reduced composite friction and wear better than HD; regardless of environment, its friction coefficient was less than 0.1 and the specific wear rate was in the level of 10⁻⁷ mm³/N m in both water and air.     

Study of tribological performance of ECR-CVD diamond-like carbon coatings on steel substrates: Part 2. The analysis of wear mechanism

This paper describes the tribological performance of diamond-like carbon (DLC) coatings deposited on AISI 440C steel substrates by electron cyclotron resonance chemical vapor deposition (ECR-CVD) process. A variety of analytic techniques were used to characterize the coatings, such as Raman spectroscopy, atomic force microscopy (AFM) and nano-indentation. The sliding wear and friction experiments were carried out by the conventional ball-on-disk tribometry against 100Cr6 steel counterbody at various normal loads (1-10 N) and sliding speeds (2-15 cm/s). All the wear tests were conducted under dry sliding condition in ambient air for a total rotation cycle of 1 × 10⁵ (sliding distance ∼2.2 km). Surfaces of the coatings and the steel balls were examined before and after the sliding wear tests. The DLC coatings that had been tested all showed relatively low values of friction coefficient, in the range of 0.1-0.2 at a steady-state stage, and low specific wear rates (on the order of 10⁻⁸ mm³/Nm). It was found that higher normal loads or sliding speeds reduced the wear rates of the coatings. Plastic deformation became more evident on the coating surface during the sliding wear test at higher contact stresses. The friction-induced transformation of the coating surface into a graphite-like phase was revealed by micro-Raman analysis, and the flash temperature of the contact asperities was estimated. It was suggested that the structural transformation taking place within the wear tracks was mainly due to the formation of compact wear debris layer rather than the frictional heating effect. On the other hand, an adherent transfer layer (tribolayer) was formed on the counterface, which was closely related to the steady-state friction during sliding and the wear mechanisms. Fundamental knowledge combined with the present tribological study led to the conclusion that adhesive wear along with abrasion was probably the dominant wear mechanism for the DLC/steel sliding systems. Additionally, fatigue processes might also be involved in the wear of the coatings.     

Tribological Performance of Boronized,Nitrided, and Normalized AISI 4140 Steel against Hydrogenated Diamond-Like Carbon-Coated AISI D2 Steel

In the current work, AISI 4140 steel was pack-boronized at 950°C for 3 h and gas-nitrided at 550°C for 72 h. All specimens used in this work were prepared from the same steel bar. A 3-µm-thick diamond-like carbon (DLC) coating (a-C:H) was deposited on the AISI D2 high-carbon, high-chromium, cold-worked tool steel by a plasma-assisted chemical vapor deposition technique. Normalized, boronized, and nitrided steel pins were tested against DLC-coated AISI D2 steel at various normal loads (15, 30, 60, and 80 N) for 1,000 and 3,000 m sliding distance in ambient air. Specific wear rate of all pins decreased with increasing load, and a similar trend was observed for the coefficient of friction (COF). Microscopic and energy-dispersive spectroscopic (EDS) analysis confirmed the role of the transfer layer for a low COF with increasing load. At all loads, the specific wear rate of boronized pins was lower than that of the nitrided and normalized pin specimens. Boronized pins showed a specific wear rate in the range of 0.27 × 10^?8 to 0.44 × 10^?8 mm³/Nm and the COF was about 0.1.     

Effect of Particle Size on the Wear Resistance of Alumina-Filled PTFE Micro- and Nanocomposites

It was long supposed that the ability of hard particle fillers to reduce the wear rate of unfilled PTFE (typically ～ 10^{? 3} mm ³ /Nm) by an order of magnitude or more was limited to fillers of microscale or greater, as nano-fillers would likely be encapsulated within the large microscale PTFE wear debris rather than disrupting the wear mechanism. Recent studies have demonstrated that nano-fillers can be more effective than microscale fillers in reducing wear rate while maintaining a low coefficient of friction. This study attempts to further elucidate the mechanisms leading to improved wear resistance via a thorough study of the effects of particle size. When filled to a 5% mass fraction, 40- and 80-nm alumina particles reduced the PTFE wear rate to a ～ 10^?7 mm ³ /Nm level, two orders of magnitude better than the ～ 10^?5 mm ³ /Nm level with alumina micro-fillers at sizes ranging from 0.5 to 20 μm. Composites with alumina filler in the form of nanoparticles were less abrasive to the mating steel (stainless 304) countersurfaces than those with microparticles, despite the filler being of the same material. In PTFE containing a mixture of both nano- and micro-fillers, the higher wear rate microcomposite behavior predominated, likely the result of the continued presence of micro-fillers and their abrasion of the countersurface as well as any overlying beneficial transfer films. Despite demonstrating such a large effect on the wear rate, the variation of alumina filler size did not demonstrate any significant effect on the friction coefficient, with values for all composites tested additionally falling near the μ = 0.18 measured for unfilled PTFE at this study's 0.01 m/s sliding speed.     

箔片空气轴承表面钛掺杂DLC膜的制备与表征

采用中频非平衡磁控溅射方法在箔片空气轴承的主轴材料40Cr钢、支承元件铍青铜箔片及硅片上制备了钛掺杂的DLC膜,并对膜的结构、摩擦磨损性能、结合强度以及内应力等进行了表征.结果表明:所制备的DLC膜含有较多的sp2键,与基体结合力强,两种轴承材料上沉积DLC膜之间的摩擦配副的减摩抗磨效果较好,摩擦因数在0.06～0.0...     

DLC solid lubricant coatings on ball bearings for space applications

The environment of space offers special challenges for the lubrication of components in sliding and rolling mechanisms. Hydrogenated diamond-like carbon (DLC) films are being studied as solid lubricant coatings to simultaneously fulfil specifications regarding wear resistance and low friction behaviour under ambient atmosphere and in vacuum.In this paper, the tribological behaviour of highly hydrogenated DLC coatings (50 at% hydrogen) is assessed. Coating composition was optimised on flat AISI 52100 steel substrates based on ball-on-disc tribotest results in air, vacuum and dry nitrogen environments. The developed DLC coatings can be tailored to yield ultra-low friction values in vacuum (μ=0.008). The average friction coefficient range obtained in humid air, dry nitrogen and vacuum for the range of applied loads were, respectively, 0.22 to 0.27, 0.02 to 0.03, and 0.007 to 0.013.New in this work is that optimised DLC coatings were applied to ball bearings for space applications. The torque and life tests of coated pairs of angular contact bearings in air revealed that relatively high bearing torques are generated which increase with time, but the amount of coating wear generated during in-air operation appears relatively light. In vacuum, low torques are generated after a prolonged running-in period. Low-torque life exceeds that observed for MoS₂ by a factor of about two. It is concluded that, in contrast to MoS₂ coated bearings, DLC-coated bearings for space applications might therefore be capable of undergoing in-air ground testing without too much disruption of the subsequent in-space performance.     

Reciprocating Sliding Friction and Wear Property of Fe<Subscript>3</Subscript>Si Based Alloys Containing Cu in Water Lubrication

Fe₃Si, Fe₃Si alloys containing Cu were fabricated by arc melting followed by hot-pressing. The friction and wear behaviors of Fe₃Si based alloys with and without Cu addition against Si₃N₄ ball in water-lubrication were investigated. The friction coefficient and the wear rates of Fe₃Si based alloys decreased as the load increased. The wear rate of Fe₃Si was higher than that of AISI 304. The addition of Cu can significantly improve the friction and wear properties of Fe₃Si based alloys and substantially reduce the wear rates of Si₃N₄ ball. The wear rate of Fe₃Si–10%Cu was 2.56 × 10⁻⁶ mm³ N⁻¹ m⁻¹ at load of 20 N and decreased to 1.64 × 10⁻⁶ mm³ N⁻¹ m⁻¹ at load of 90 N. The wear rate of Si₃N₄ ball against Fe₃Si–10%Cu was 1.41 × 10⁻⁶ mm³ N⁻¹ m⁻¹, while the wear rate of Si₃N₄ ball against AISI 304 was 5.20 × 10⁻⁶ mm³ N⁻¹ m⁻¹ at load of 90 N. The wear mechanism was dominated by micro-ploughing. The combination of mechanical action (i.e., shear, smear and transference of Cu) and tribochemical reaction of Si₃N₄ with water was responsible for the improved tribological behavior of Fe₃Si alloys containing Cu under high loads.     

Improved mixed and boundary lubrication with glycerol-diamond technology

Abstract

The fuel economy and reduction of harmful elements in lubricants are becoming important issues in the automotive industry. An approach to respond to these requirements is the potential use of low friction coatings in engine components exposed to boundary lubrication conditions. Diamond-like carbon (DLC) coatings extensively studied as ultralow friction films to protect the surfaces of ductile metals for space applications are expected to fulfil this part. The main purpose of this work is to investigate the friction and wear properties of glycerol lubricated DLC coatings under boundary lubrication conditions. The DLC material consists of tetrahedral hydrogen free amorphous diamond-like carbon (denoted as ta-C) as shown by the time of flight secondary ion mass spectroscopy (ToF-SIMS) analyses and the nanoindentation measurements. The friction coefficient below 0&middot.01, called superlubricity, and no measurable wear were obtained by sliding the ta-C/ta-C friction pair in the presence of pure glycerol as a lubricant at 353 K. The mechanism by which glycerol is able to reduce the friction in the millirange was revealed by ToF SIMS analyses inside and outside wear scars formed by friction experiments using deuterated glycerol and ¹³C glycerol.     

Effect of hydrogenated DLC coating hardness on the tribological properties under water lubrication

Hydrogenated diamond-like carbon (DLC) coatings were deposited using unbalanced magnetron sputtering (UBM) equipment with different hardnesses. Effects of coating hardness on tribological properties were investigated with tribo-tests under water lubrication. Results showed that the wear volume increased rapidly during the initial running-in process, but remained nearly constant after the running-in process. The ball wear rate increased as the hardness of the DLC coating increased when metals (stainless steel and brass) were used as counter parts. In contrast, the UHMWPE ball wear rate was independent of the DLC hardness. TEM analysis and nano-indentation measurements were conducted of the transfer layer on the counter bodies’ contact surfaces. The transfer layer consisted mainly of Fe, O and C. The low friction of DLC coating is attributed to this low hardness transfer layer, which acts as a boundary-lubricating layer with low shear strength.     

RSM base study of the effect of deposition temperature and hydrogen flow on the wear behavior of DLC films

A statistical study of the effects of deposition temperature and hydrogen flow on the wear behavior of DLC films was examined using the RSM method based on a central composite design. DLC films were deposited on the nitrocarburized AISI 4140 steel by the pulsed DC PACVD method at temperature range of 60–120 °C and in an atmosphere of hydrogen range of 0–40 sccm. Results indicated that a combination of relatively high deposition temperature and low hydrogen flow or low deposition temperature and high hydrogen flow produce DLC film with low wear rate and low friction coefficient.