Osman Eryilmaz

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.

Synthesis of superlow-friction carbon films from highly hydrogenated methane plasmas

In this study, we investigated the friction and wear performance of diamond-like carbon films (DLC) derived from increasingly hydrogenated methane plasmas. The films were deposited on steel substrates by a plasma-enhanced chemical vapor deposition process at room temperature and the tribological tests were performed in dry nitrogen. Tests results revealed a close correlation between the hydrogen in the source gas plasma and the friction and wear coefficients of the DLC films. Specifically, films grown in plasmas with higher hydrogen-to-carbon ratios had much lower friction coefficients and wear rates than did films derived from source gases with lower hydrogen-to-carbon ratios. The lowest friction coefficient (0.003) was achieved with a film derived from 25% methane, 75% hydrogen, while a relatively high coefficient of 0.015 was found for films derived from pure methane. Similar correlations were observed for wear rates. Films derived from hydrogen-rich plasmas had the least wear, while films derived from pure methane suffered the highest wear. We used a combination of surface analytical methods to characterize the structure and chemistry of the DLC films and worn surfaces.

Effect of source gas chemistry on tribological performance of diamond-like carbon films

Abstract In this study, we investigated the effects of various source gases (methane, ethane, ethylene, acetylene, and methane+hydrogen) on the friction and wear performance of diamond-like carbon (DLC) films produced from the source gases. Specifically, we described the anomalous nature and fundamental friction and wear mechanisms of DLC films derived from gas discharge plasmas with very low to very high hydrogen content. The films were deposited on steel substrates by a plasma-enhanced chemical vapor deposition process at room temperature and the tribological tests were performed in dry nitrogen. The tribological tests revealed a close correlation between the source gas chemistry and the friction and wear coefficients of the DLC films. Specifically, films grown in source gases with higher hydrogen-to-carbon ratios had much lower friction coefficients and wear rates than did films derived from source gases with lower hydrogen-to-carbon ratios. The lowest friction coefficient (0.002) was achieved with a film derived from 25% methane+75% hydrogen, whereas a coefficient of 0.15 was seen in films derived from acetylene. Similar correlations were observed for wear rates. Films derived from hydrogen-rich plasmas had the least wear, whereas films derived from pure acetylene suffered the highest wear. We used a combination of scanning and transmission electron microscopy and Raman spectroscopy to characterize the structural chemistry of the resultant DLC films.

Friction and wear performance of diamond-like carbon films grown in various source gas plasmas

In this study, the authors investigated the effects of various source gases (methane, ethane, ethylene, and acetylene) on the friction and wear performance of diamondlike carbon (DLC) films prepared in a plasma enhanced chemical vapor deposition (PECVD) system. Films were deposited on AISI H13 steel substrates and tested in a pin-on-disk machine against DLC-coated M50 balls in dry nitrogen. They found a close correlation between friction coefficient and source gas composition. Specifically, films grown in source gases with higher hydrogen-to-carbon ratios exhibited lower friction coefficients and higher wear resistance than films grown in source gases with lower hydrogen-to-carbon (H/C) ratios. The lowest friction coefficient (0.014) was achieved with a film derived from methane with an WC ratio of 4, whereas the coefficient of films derived from acetylene (H/C = 1) was of 0.15. Similar correlations were observed for wear rates. Specifically, films derived from gases with lower H/C values were worn out and the substrate material was exposed, whereas films from methane and ethane remained intact and wore at rates that were nearly two orders of magnitude lower than films obtained from acetylene.

Carbon-based tribofilms from lubricating oils

Moving mechanical interfaces are commonly lubricated and separated by a combination of fluid films and solid ‘tribofilms’, which together ensure easy slippage and long wear life. The efficacy of the fluid film is governed by the viscosity of the base oil in the lubricant; the efficacy of the solid tribofilm, which is produced as a result of sliding contact between moving parts, relies upon the effectiveness of the lubricant’s anti-wear additive (typically zinc dialkyldithiophosphate). Minimizing friction and wear continues to be a challenge, and recent efforts have focused on enhancing the anti-friction and anti-wear properties of lubricants by incorporating inorganic nanoparticles and ionic liquids. Here, we describe the in operando formation of carbon-based tribofilms via dissociative extraction from base-oil molecules on catalytically active, sliding nanometre-scale crystalline surfaces, enabling base oils to provide not only the fluid but also the solid tribofilm. We study nanocrystalline catalytic coatings composed of nitrides of either molybdenum or vanadium, containing either copper or nickel catalysts, respectively. Structurally, the resulting tribofilms are similar to diamond-like carbon. Ball-on-disk tests at contact pressures of 1.3 gigapascals reveal that these tribofilms nearly eliminate wear, and provide lower friction than tribofilms formed with zinc dialkyldithiophosphate. Reactive and ab initio molecular-dynamics simulations show that the catalytic action of the coatings facilitates dehydrogenation of linear olefins in the lubricating oil and random scission of their carbon–carbon backbones; the products recombine to nucleate and grow a compact, amorphous lubricating tribofilm.

Self-replenishing solid lubricant films on boron carbide

AbstractThis study introduces a simple annealing procedure that leads to the formation of a self-replenishing solid lubricant film on bulk boron carbide (B4 C) and its coatings. The procedure uses short duration annealing at 800°C for 15 min. Subsequently, the specimens are cooled to room temperature and tested in a pin on disc machine. During annealing, B4 C reacts with oxygen in air and forms a glasslike boron oxide (B2 O3 ) layer on the exposed surface. Eventually, this layer reacts spontaneously with moisture in the surrounding air, thus forming a thin boric acid (H3 BO3 ) film. The friction coefficients of 440C steel and zirconia pins against this film are 0·04–0·06. Electron microscopy and Raman spectroscopy were used to elucidate the formation and self-lubricating mechanisms of this solid boundary film on sliding B4 C surfaces.

Effects of nanoscale surface texture and lubricant molecular structure on boundary lubrication in liquid.

Nanoconfinement effects of boundary lubricants can significantly affect the friction behavior of textured solid interfaces. These effects were studied with nanotextured diamond-like carbon (DLC) surfaces using a reciprocating ball-on-flat tribometer in liquid lubricants with different molecular structures: n-hexadecane and n-pentanol for linear molecular structure and poly(α-olefin) and heptamethylnonane for branched molecular structure. It is well-known that liquid lubricants with linear molecular structures can readily form a long-range ordered structure upon nanoconfinement between flat solid surfaces. This long-range ordering, often called solidification, causes high friction in the boundary lubrication regime. When the solid surface deforms elastically due to the contact pressure and this deformation depth is larger than the surface roughness, even rough surfaces can exhibit the nanoconfinement effects. However, the liquid entrapped in the depressed region of the nanotextured surface would not solidify, which effectively reduces the solidified lubricant area in the contact region and decreases friction. When liquid lubricants are branched, the nanoconfinement-induced solidification does not occur because the molecular structure is not suitable for the long-range ordering. Surface texture, therefore, has an insignificant effect on the boundary lubrication of branched molecules.

Surface Structure of Hydrogenated Diamond-like Carbon: Origin of Run-In Behavior Prior to Superlubricious Interfacial Shear

The oxidized layers at the surface of hydrogenated diamond-like carbon (H-DLC) were studied with X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure, and Raman spectroscopy. The structure of these layers was correlated with the friction and wear behavior observed on H-DLC. H-DLC is well-known for its ultralow friction in inert environments, but the steady superlubricious state is always preceded by a run-in period with a high friction. It was hypothesized that the run-in period is related to the surface oxide layer formed naturally upon exposure of the sample to air. To test this hypothesis, thermal oxide layers were grown, and their structures were analyzed and compared with the native oxide layer on a pristine sample. It was found that the Raman spectra of the surface oxide layers of H-DLC have higher D/G band ratio than the bulk, indicating a larger amount of aromatic clusters compared to the bulk film. Thick oxide layers grown at 300 °C showed a run-in friction behavior that resembled the friction of graphite. The run-in periods were found to become longer when the thickness of the oxide layers increased, indicating that the run-in behavior of H-DLC is attributed to the removal of the surface oxide layers.

The mechanical properties of freestanding near-frictionless carbon films relevant to MEMS

Amorphous, diamond-like carbon films with a mixture of sp2 and sp3 hybridizations have exhibited excellent material properties such as chemical stability, wear resistance and optical transparency resulting in their wide use as protective coatings in numerous applications. The hydrogenated forms of these films, a-C:H, and specifically the near-frictionless carbon (NFC) films developed at Argonne National Laboratory have exhibited the lowest ever recorded friction coefficient, 0.001, and ultra-low wear rates of 10−11−10−10 mm3 N−1 m−1, even under dry sliding conditions and at very high contact pressures (Robertson 2002 Diamond-like amorphous carbon Mater. Sci. Eng. R 37 129). Application of these films to sliding or rotating microelectromechanical systems (MEMS) would open up an entirely new class of commercialized MEMS devices. With this in mind, this paper reports on thin-film mechanical property measurements of the NFC films relevant to MEMS. The membrane deflection experiment was employed to subject microfabricated freestanding films to pure tension and measure mechanical properties such as Youngs modulus, residual stress and fracture strength. Youngs modulus was consistently measured at 35.13 ± 2.29 GPa. The fracture strength varied from 0.12 GPa to 0.90 GPa and the residual stress state was compressive and ranged from 79 MPa to 310 MPa. Width and thickness effects of the membranes were also observed in this work, where fracture strength increased with decreasing membrane width and thickness. Weibull analysis of the fracture strength is also presented in the paper.

Superlubricity of the DLC films-related friction system at elevated temperature

Superlubricity is defined as a sliding regime in which friction or resistance to sliding almost vanishes. While there are a number of superlubricity, providing a high temperature superlubricity remains a challenge. Here we present a high temperature superlubricity achieved from the diamond like carbon (DLC) films friction system. Superlubricity is found about 0.008 for more than 100 000 seconds at the steady state at the temperature of 600 °C due to the formation of the self-generated lubricious composite oxides of γ-Fe2O3 and SiO2 at the contact surfaces through tribochemistry reaction during the running-in process. We propose a superlubricity system based on the repulsive electrostatic forces between the self-generated composite oxides due to high temperature oxidation reaction and the shielding action of hydrogen at the contact surface, which is seem to be a reasonable explanation for super low friction at the elevated temperature.