A. Erdemir

Tribology of diamond-like carbon films: recent progress and future prospects

During the past two decades, diamond-like carbon (DLC) films have attracted an overwhelming interest from both industry and the research community. These films offer a wide range of exceptional physical, mechanical, biomedical and tribological properties that make them scientifically very fascinating and commercially essential for numerous industrial applications. Mechanically, certain DLC films are extremely hard (as hard as 90 GPa) and resilient, while tribologically they provide some of the lowest known friction and wear coefficients. Their optical and electrical properties are also extraordinary and can be tailored to meet the specific requirements of a given application. Because of their excellent chemical inertness, these films are resistant to corrosive and/or oxidative attacks in acidic and saline media. The combination of such a wide range of outstanding properties in one material is rather uncommon, so DLC can be very useful in meeting the multifunctional application needs of advanced mechanical systems. In fact, these films are now used in numerous industrial applications, including razor blades, magnetic hard discs, critical engine parts, mechanical face seals, scratch-resistant glasses, invasive and implantable medical devices and microelectromechanical systems. DLC films are primarily made of carbon atoms that are extracted or derived from carbon-containing sources, such as solid carbon targets and liquid and gaseous forms of hydrocarbons and fullerenes. Depending on the type of carbon source being used during the film deposition, the type of bonds (i.e. sp 1 ,s p 2 ,s p 3 ) that hold carbon atoms together in DLC may vary a great deal and can affect their mechanical, electrical, optical and tribological properties. Recent systematic studies of DLC films have confirmed that the presence or absence of certain elemental species, such as hydrogen, nitrogen, sulfur, silicon, tungsten, titanium and fluorine, in their microstructure can also play significant roles in their properties. The main goal of this review paper is to highlight the most recent developments in the synthesis, characterization and application of DLC films. We will also discuss the progress made in understanding the fundamental mechanisms that control their very unique friction and wear behaviours. Novel design concepts and the principles of superlubricity in DLC films are also presented. (Some figures in this article are in colour only in the electronic version)

The role of hydrogen in tribological properties of diamond-like carbon films☆

Abstract Extensive research on diamond and diamondlike carbon (DLC) films in our laboratory has further confirmed that hydrogen plays an important role in the tribological properties of these films. Specifically, model experiments in inert gas environments revealed a very close relationship between the hydrogen-to-carbon (H/C) ratios of source gases and the friction and wear coefficients of the resultant DLC films. The friction coefficient of films grown in source gases with very high H/C ratios (e.g. 10) was superlow (0.003), whereas that of hydrogen-free DLC films (with essentially zero H/C ratio) was very high (0.65). The friction coefficients of films grown in source gases with intermediate H/C ratios were between 0.003 and 0.65. Experiments also revealed that the frictional properties of these films were very sensitive to test environments. Specifically, when tested in open air, the friction coefficient of hydrogen-free DLC dropped to 0.25, whereas that of highly-hydrogenated DLC increased to 0.06. Fundamental knowledge combined with surface analytical and tribological studies have led to the conclusion that the type and extent of chemical interactions between carbon–carbon, carbon–hydrogen, and carbon–adsorbate atoms at the sliding-contact interfaces determine the friction and wear properties of DLC films.

Ultrananocrystalline diamond thin films for MEMS and moving mechanical assembly devices

MEMS devices are currently fabricated primarily in silicon because of the available surface machining technology. A major problem with the Si-based MEMS technology is that Si has poor mechanical and tribological properties J.J. Sniegowski, in: B.

Synthesis of diamondlike carbon films with superlow friction and wear properties

In this study, the authors introduce a new diamondlike carbon (DLC) film providing a friction coefficient of 0.001 and wear rates of 10{sup {minus}9} to 10{sup {minus}10} mm{sup 3}/N.m in inert-gas environments (e.g., dry nitrogen and argon). The film was grown on steel and sapphire substrates in a plasma enhanced chemical vapor deposition system that uses using a hydrogen-rich plasma. Employing a combination of surface and structure analytical techniques, they explored the structural chemistry of the resultant DLC films and correlated these findings with the friction and wear mechanisms of the films. The results of tribological tests under a 10-N load (creating initial peak Hertz pressures of 1 and 2.2 GPa on steel and sapphire test pairs, respectively) and at 0.2 to 0.5 m/s sliding velocities indicated that a close correlation exists between the friction and wear coefficients of DLC films and the source gas chemistry. Specifically, films grown in source gases with higher hydrogen-to-carbon ratios had the lowest fiction coefficients and the highest wear resistance. The lowest friction coefficient (0.001) was achieved with a film on sapphire substrates produced in a gas discharge plasma consisting of 25% methane and 75% hydrogen.

Superlow friction behavior of diamond-like carbon coatings: Time and speed effects

The friction behavior of a diamond-like carbon coating was studied in reciprocating sliding contact at speeds from 0.01 to 5 mm/s, in dry nitrogen. “Superlow” friction coefficients of 0.003–0.008 were obtained in continuous sliding at the higher speeds (>1 mm/s). However, friction coefficients rose to values typical of diamond-like carbon in dry and ambient air (0.01–0.1) at lower speeds (<0.5 mm/s) as well as in time-delayed, higher speed tests. The rise of the friction coefficients in both speed and time-delay tests was in good quantitative agreement with gas adsorption kinetics predicted by the Elovich equation for adsorption onto carbon. More generally, superlow friction could be sustained, suppressed, and recovered as a function of exposure time, demonstrating that duty cycle cannot be ignored when predicting performance of superlow friction coatings in devices.

A crystal-chemical approach to lubrication by solid oxides

This paper introduces a new approach to the selection, classification, and mechanistic understanding of lubricious oxides that are used to combat friction and wear at elevated temperatures. Specifically, it describes a crystal-chemical model that enables one to predict the shear rheology or lubricity of an oxide or oxide mixture at elevated temperatures. This model can be used to formulate new alloy compositions or composite oxide structures that can provide low friction at high temperatures. In the case of composite oxides, the model allows one to estimate the solubility limits, chemical reactivity, compound forming tendencies, as well as the lowering of the melting point of one oxide when a second oxide is present. From a tribological standpoint, a prior knowledge of these details is important because they are strongly related to the extent of adhesive interactions, shear rheology, and hence to lubricity of oxides. In light of certain crystal-chemical considerations, general guidelines are provided for the selection of those oxides that can provide low friction at high temperatures. The major goal of this paper is to establish model relationships between relevant crystal-chemical and tribological properties of oxides that can be used as lubricants at high temperatures. Such a model may help eliminate guesswork in high-temperature lubrication and provide a new means to address the difficult lubrication problems experienced at high temperatures.

Macroscale superlubricity enabled by graphene nanoscroll formation

Slip sliding away Many applications would benefit from ultralow friction conditions to minimize wear on the moving parts such as in hard drives or engines. On the very small scale, ultralow friction has been observed with graphite as a lubricant. Berman et al. achieved superlubricity using graphene in combination with crystalline diamond nanoparticles and diamondlike carbon (see the Perspective by Hone and Carpick). Simulations showed that sliding of the graphene patches around the tiny nanodiamond particles led to nanoscrolls with reduced contact area that slide easily against the amorphous diamondlike carbon surface. Science, this issue p. 1118; see also p. 1087 Nanodiamonds wrapped with graphene sheets lead to ultralow friction against a diamondlike carbon surface. [Also see Perspective by Hone and Carpick] Friction and wear remain as the primary modes of mechanical energy dissipation in moving mechanical assemblies; thus, it is desirable to minimize friction in a number of applications. We demonstrate that superlubricity can be realized at engineering scale when graphene is used in combination with nanodiamond particles and diamondlike carbon (DLC). Macroscopic superlubricity originates because graphene patches at a sliding interface wrap around nanodiamonds to form nanoscrolls with reduced contact area that slide against the DLC surface, achieving an incommensurate contact and substantially reduced coefficient of friction (~0.004). Atomistic simulations elucidate the overall mechanism and mesoscopic link bridging the nanoscale mechanics and macroscopic experimental observations.

A tribological investigation of the graphite-to-diamond-like behavior of amorphous carbon films ion beam deposited on ceramic substrates

Abstract A tribological investigation was conducted on the graphite-to-diamond-like behavior of hard carbon films produced on SiC, Si 3 N 4 and ZrO 2 substrates by ion beam deposition. Friction tests were performed on a ball-on-disk machine with pairs of various ceramic balls and disks coated with hard carbon films in dry and humid air, argon and N 2 . The friction coefficients of carbon films sliding against Si 3 N 4 and sapphire balls were in the range 0.02–0.04 in N 2 and argon, but were significantly higher (about 0.15) in humid air. The wear rates of ceramic disks coated with carbon films were unmeasurable, and, depending on the test environment, the wear rates of counterface ceramic balls were reduced by two to four orders of magnitude below those of balls slid against uncoated ceramic disks. Graphite disks were also tested, to obtain friction data that can help us to understand the graphite-to-diamond-like tribological behavior of carbon films. Micro laser Raman spectroscopy and scanning electron microscopy were used to analyze the structure and chemistry of worn surfaces and to elucidate the graphite- and diamond-like tribological behavior of amorphous carbon films.

The Effect of Laser Texturing of Steel Surfaces and Speed-Load Parameters on the Transition of Lubrication Regime from Boundary to Hydrodynamic

Laser surface texturing (LST) is an emerging, effective method for improving the tribological performance of friction units lubricated with oil. In LST technology, a pulsating laser beam is used to create thousands of arranged microdimples on a surface by a material ablation process. These dimples generate hydrodynamic pressure between oil-lubricated parallel sliding surfaces. The impact of LST on lubricating-regime transitions was investigated in this study. Tribological experiments were carried out on pin-on-disk test apparatus at sliding speeds that ranged from 0.15 to 0.75 m/s and nominal contact pressures that ranged from 0.16 to 1.6 MPa. Two types of oil with different viscosities (54.8 cSt and 124.7 cSt at 40°C) were evaluated as lubricants. Electrical resistance between flat-pin and laser-textured disks was used to determine the operating lubrication regime. The test results showed that laser texturing expanded the range of speed-load parameters for hydrodynamic lubrication. LST also reduced the measured friction coefficients of contacts that operated under the hydrodynamic regime. The beneficial effects of laser surface texturing are more pronounced at higher speeds and loads and with higher viscosity oil.

Frictional behavior of diamondlike carbon films in vacuum and under varying water vapor pressure

Frictional behavior of diamondlike carbon films in vacuum and under varying water vapor pressure