Friction Of Diamond-like Carbon Films Research Articles

Reducing Friction of Diamond-Like Carbon Film in Sliding through Fluorine Doping

Reducing Friction of Diamond-Like Carbon Film in Sliding through Fluorine Doping

Low Friction of Diamond-Like Carbon Film Due to Liquid Hydrocarbon Generated by Laser Heating in a Nitrogen Gas Environment

In this study, we investigated the low friction mechanism of a diamond-like carbon (DLC) film heated by laser irradiation in a nitrogen gas environment. The friction coefficient decreased from 0.5 to 0.06 when the contact area was heated to 250 °C. Subsequently, we heated the DLC film under non-contact conditions (2 μm distance between DLC film and pin surface) and collected the liquid materials adhered on the pin surface. The adhered materials were revealed to be hydrocarbons with –CH2– main-chains, as observed using Raman spectroscopy. On the other hand, thermal desorption gas chromatography with mass spectrometry using helium as a carrier gas demonstrated that hydrocarbons such as alkenes and alkanes with relatively high molecular weight were generated from heated DLC. This agreement between the results of the two analysis techniques suggested that the hydrocarbons in the liquid phase at room temperature were generated from the DLC film heated to a high temperature over 250 °C in an inert gas atmosphere. In addition, we confirmed the reduction in the friction coefficient after pre-heating a different location, as well as the dependence of the friction coefficient on the sliding velocity. The results suggest that hydrocarbons (e.g., alkenes or alkanes) generated upon DLC heating could possibly act as a lubricant to reduce the friction coefficient.

Quantifying Macroscopic Friction of Diamond-like Carbon Films by Microscopic Adsorption and Removal of Water Molecules.

The adsorption and desorption of water molecules, which affect the physical and chemical properties of the sliding interface, are critical for the friction behaviors of two solid contacts in atmosphere environment. The amount of water adsorbed on the open surface is a function of gas pressure according to an adsorption equation. However, for a confined sliding interface, the variation of surface fraction covered by gas molecules with water vapor pressure and its induced effects on friction have not been figured out. In this work, the macroscopic friction of diamond-like carbon (DLC) films in a water vapor atmosphere is quantified on the basis of microscopic adsorption and removal of water molecules. The studies correlate the fraction of water molecules adsorbed on the interface of self-mated DLC films with water vapor pressure to illustrate the direct relationship between friction coefficient and gas pressure by first-principles calculations and model fitting. The calculated results revealed that chemisorption and physisorption of water molecules occur on the surfaces of hydrogen-free DLC films (ta-C) and hydrogenated DLC films (HCF). Then, the relation between friction and gas pressure was built by employing a fractional coverage model based on the linear adsorption equation and gas removal. The obtained model agrees well with the typical experimental results about the steady-state friction coefficient of both highly hydrogenated DLC film (HCF) and tetrahedral amorphous carbon (ta-C) film during sliding at various water vapor pressures. In addition, it gave the curves of fractional surface coverage as a function of water vapor pressure. These results show that the frictional coefficient of DLC films could be predicted on the basis of fractional surface coverage as well as the intrinsic characters on surface chemistry. We suggest that the model may be thus extended to understand and predict the friction of DLC films under a specific gas pressure at a low load and speed.

Exploring the low friction of diamond-like carbon films in carbon dioxide atmosphere by experiments and first-principles calculations

The friction behavior and the mechanism of DLC films in CO2 atmosphere are rarely explored, which is a significant obstacle for the potential practical application of DLC films in primarily CO2 environment. Here, the experiments and first-principles calculations are performed to simultaneously investigate this theme. We find that DLC films in CO2 atmosphere exhibit astoundingly low friction coefficient compared with in ambient air and vacuum atmospheres. The XPS and Raman spectrums demonstrate the possibly activation of CO2 molecule in the shearing interfaces, which may be critical for the low friction of DLC films in CO2 atmosphere. The calculated results reveal that the lactone groups can easily form during the horizontally chemisorption of CO2 molecule on the DLC surface, which is energetic and is a favorable process under the interfacial stress. Because of the presence of the lone-pairs of the lactone group, the lactone-terminated surfaces appear to be responsible for the low friction of DLC films in CO2 atmosphere. The studies may open up the possibility for DLC films usage in Mars applications.

On the nature of the coefficient of friction of diamond-like carbon films deposited on rubber

In this paper, the nature of the coefficient of friction (CoF) of diamond-like carbon (DLC)-protected rubbers is studied. The relative importance of the viscoelastic and adhesive contributions to the overall friction is evaluated experimentally by modifying the contact load and the adhesive strength between the surface and the counterpart. The results indicate that the increase of CoF during the tribotests under non-lubricated conditions is caused by the increase of the adhesive contribution to friction motivated by the growth of the contact area during the test. In the case of oil lubricating condition, the adhesive force is minimized and the CoF is observed to decrease during the tribotest. This is caused by the reduction of the viscoelastic contribution due to the variation of the shape of the contact area. The role of the microstructure of the DLC film on the efficiency of the oil lubrication is also discussed.

Sp<sup>2</sup> Hybridization Effects on Friction of Diamond-Like Carbon Film (110) Surfaces Studied by First Principles Molecular Dynamics

The influence of sp2hybridization on friction and structure of diamond-like carbon (DLC) films was studied by first-principles molecular dynamics. First-principles calculations have been performed using the pseudo-potential plane wave method. Our results show that: 1) The buffer role of sp2-hybridized state carbon between grain boundaries greatly affected the residual stress of DLC film, and hence the friction decreased steadily with the increased in sp2hybridization content. 2) For thinner films, there was very greatly differences in stress between diamond-like carbon film (110) surfaces in <001> and<1-10> directions. The difference in friction force between soft and hard directions was nonexistent, when the sp2-hybridized state carbon atoms increased to 80%, perhaps because the originally hard C-C and C-H bonds changed, and steadily reject interaction was drive break.

Influences of Film Deposition Condition on Friction of Diamond-Like Carbon Film: A Theoretical Investigation

We theoretically investigated influences of film deposition condition on a friction of diamond-like carbon (DLC) film using our developed molecular dynamics method. The method can deal with chemical reactions on the basis of our original stochastic equation. DLC slab model was firstly constructed, and carbon atoms were deposited onto its surface with various kinetic energies (1, 10 and 30 eV) to mimic physical vapor deposition process. Lower sp3 carbon concentration and density were obtained for the 1 eV case and “subplantation” phenomenon was found for each model. The dynamical friction of the deposited DLC film/iron oxide combination was also simulated. The friction coefficient for the deposited film with kinetic energy of 1 eV was much lower than that of 10 and 30 eV cases due to the friction-induced structural change. The simulations provided us an important insight on DLC film preparation to achieve low friction regime.

Roughness effect of mating ball on friction of diamond-like carbon film and friction mechanism in water and air environment

The dependence of DLC film friction on the roughness of stainless steel mating balls was investigated in water and air environments. It was found that the steel ball surface parameters of Ra and surface density of summits (Dsum) showed a high correlation with the film friction in a water environment but in an air environment, the correlation was low with the friction coefficients scattered across the parameters of roughness. The difference in the frictional behaviors was explained by the presence of lubrication film that formed in the water environment. Furthermore, it was confirmed that the friction behaviour of DLC film in water environment was interpreted on the basis of boundary lubrication theory proposed by Nakahara.

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.

Low temperature internal friction of diamond-like carbon films

Abstract We have studied the internal friction of amorphous diamond-like carbon films prepared by pulsed-laser deposition from 0.4 to 300 K. The low temperature internal friction below 10 K is dominated by the atomic tunneling states for amorphous solids, which is a measure of structure disorder. We have tried to vary the content of sp3 carbon atoms versus sp2 ones by changing laser fleunce, by doping with N and Ar, and by annealing at 500 °C for 20 min. Our results show that the internal friction varies about one order of magnitude from 2×10−5 to 2×10−4, and its value is higher with higher sp3 content when the film quality is generally considered superior. However, it is known that as-deposited diamond-like carbon films with high sp3 content are heavily stressed. Annealing and doping are used to release the stress. We conclude that in addition to tetrahedral bonding, low stress is also important in reducing structure disorder associated with the low energy tunneling states in amorphous solids.

Friction of diamond-like carbon films in different atmospheres

Diamond-like carbon (DLC) films constitute a class of new materials with a wide range of compositions, properties, and performance. In particular, the tribological properties of these films are rather intriguing and can be strongly influenced by the test conditions and environment. In this paper, a series of model experiments are performed in high vacuum and with various added gases to elucidate the influence of different test environments on the tribological behavior of three DLC films. Specifically, the behavior of a hydrogen-free film produced by a cathodic arc process and two highly hydrogenated films produced by plasma-enhanced chemical-vapor deposition were studied. Flats and balls used in these experiments were coated with DLC and tested in a pin-on-disc machine under a load of 1 N and at constant rotational frequency. With a low background pressure, in the 10 −6 Pa range, the highly hydrogenated films exhibited a friction coefficient of less than 0.01, whereas the hydrogen-free film gave a friction coefficient of approximately 0.6. Adding oxygen or hydrogen to the experimental environment changed the friction to some extent. However, admission of water vapor into the test chamber caused large changes: the friction coefficient decreased drastically for the hydrogen-free DLC film, whereas it increased slightly for one of the highly hydrogenated films. These results indicate that water molecules play a prominent role in the frictional behavior of DLC films—most notably for hydrogen-free films but also for highly hydrogenated films.

Effect of film thickness on the stress and adhesion of diamond-like carbon coatings

The important properties of thin films that are used for any applications, especially tribological, are lower internal stress and extremely good adhesive properties. The present investigation examines the effect of film thickness on the compressive stress, adhesive strength (critical load), microstructure and coefficient of friction of diamond-like carbon films prepared under floating conditions (no substrate bias), on silicon substrates. The results show that the compressive stress increases rapidly with thickness in the lower thickness range of up to 50 nm, followed by a slight decrease and thereafter it increases at a slower rate before it delaminates at a thickness of approximately 190 nm. The adhesive strength in terms of critical load shows that the critical load increases with increasing the film thickness up to approximately 110 nm and thereafter it decreases with thickness. The decrease in critical load with thickness, for the films having thickness above 110 nm, is due to the dominant effect of internal stress. The tribological characterisation reveals that the substrate influence on the coefficient of friction is relatively high for the thinnest films (∼<40 nm). The paper also discusses the results of UV and visible Raman spectroscopy as a function of thickness. It is suggested from the characterisation results that appropriate thickness range of the film (prepared under floating condition), is ∼40–110 nm.

Static friction of diamond-like carbon film in MEMS

Abstract As part of the effort to understand the operational characteristics of microelectromechanical systems (MEMS), we have previously studied static friction between the silicon-based materials of interest in MEMS. It is shown that the surface material and gas environment affect the friction significantly. Coating a low-friction material on the contact surfaces is one way to improve the performance of microdynamic devices. Studies on diamond-like carbon (DLC) film confirm its potential as a very low-friction material. This paper presents static friction measurements between DLC/DLC mechanical contacts and compares the results with those of silicon-based materials. The static friction is measured between a 3 mm radius aluminum bullet coated with DLC film and a flat DLC substrate in a ∼ 8 × 10−10 Torr ultra-high-vacuum (UHV) chamber. The coefficient of friction is 0.040 ± 0.005 for DLC/DLC contacts, and is 0.2–0.7 between silicon and silicon-compound contacts measured in the same UHV. Oxygen exposure increases the coefficient of friction by 50% between DLC/DLC, but this value is still an order of magnitude smaller than that between silicon-based materials. As a result, DLC film may be a promising friction-reducing coating material for MEMS.