G.R. Fenske

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.

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.

Tribological properties of nanocrystalline diamond films

In this paper, the authors present the friction and wear properties of nanocrystalline diamond (NCD) films grown in A-fullerene (C{sub 60}) and Ar-CH{sub 4} microwave plasmas. Specifically, they address the fundamental tribological issues posed by these films during sliding against Si{sub 3}N{sub 4} counterfaces in ambient air and inert gases. Grain sizes of the films grown by the new method are very small (10--30 nm) and are much smoother (20-40 nm, root mean square) than those of films grown by the conventional H{sub 2}-CH{sub 4} microwave-assisted chemical-vapor-deposition (CVD) process. Transmission electron microscopy (TEM) revealed that the grain boundaries of these films are very sharp and free of nondiamond phases. The microcrystalline diamond (MCD) films grown by most conventional methods consist of large grains and a rough surface finish, which can cause severe abrasion during sliding against other materials. The friction coefficients of films grown by the new method (i.e., in Ar-C{sub 60} and Ar-CH{sub 4} plasmas) are comparable to those of natural diamond, and wear damage on counterface materials is minimal. Fundamental tribological studies indicate that these films may undergo phase transformation during long-duration, high-speed and/or high-load sliding tests and that the transformation products trapped at the sliding interfaces can intermittently dominate friction and wear performance. Using results from a combination of TEM, electron diffraction, Raman spectroscopy, and electron energy loss spectroscopy (EELS), they describe the structural chemistry of the debris particles trapped at the sliding interfaces and elucidate their possible effects on friction and wear of NCD films in dry N{sub 2}. Finally, they suggest a few potential applications in which NCD films can improve performance and service lives.

Tribological characteristics of DLC films and duplex plasma nitriding/DLC coating treatments

Abstract An innovative approach to improving tribological behavior of surfaces and meeting long-term durability requirements of engineering devices is to design and develop novel systems incorporating multilayers and/or duplex diffusion/plasma coating treatments. In the present work, the wear and friction characteristics of diamond-like carbon (DLC) films and composite surface layers were studied by conducting pin-on-disc experiments. M 50 steel and Ti-6A1-4V alloy were used as substrate materials. The composite layers consisted of a N-diffusion zone obtained by ion nitriding, followed by a 500 A vapor-deposited Si bond layer and a 0.4 µm DLC film. The purpose of the bond layer was to enhance adhesion between the substrate and the DLC films. An ion-beam method was used for the deposition of the DLC films. The pin-on-disc results showed that for both materials the DLC coating produced a reduction in the coefficient of friction of about one order of magnitude. The reduction in the coefficient of friction was found to be consistent with the formation of a carbon-rich transfer film on the contact surfaces. Wear scar profiling and weight loss calculations showed that the wear resistance of the DLC-coated materials was dramatically improved. Comparisons between duplex N-diffusion layer/DLC coating and single DLC coating on Ti-6A1-4V alloy substrates showed that the duplex treatments possessed a significantly higher wear resistance. Nitriding was found to cause substrate hardening that reduces subsurface deformation, thus improving coating support and extending considerably DLC film lifetime.

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.

Formation of ultralow friction surface films on boron carbide

In this letter, we describe the formation and ultralow friction mechanisms of a surface film on boron carbide (B4C). This film results from sequential reactions between B4C and oxygen and between the resulting boron oxide (B2O3) and moisture; it can afford friction coefficients of 0.03 to 0.05 to sliding steel surfaces. At temperatures above 600 °C, B4C undergoes oxidation and forms a layer of boron oxide (B2O3) in the upper surface. During cooling to room temperature, the B2O3 reacts with moisture in the air to form a secondary film, boric acid (H3BO3). The sliding friction coefficient of 440C steel balls against this film is 0.04, compared to 0.7 against the bare B4C surfaces. Mechanistically, we propose that the ultralow friction behavior of the heat‐treated B4C surface is due mainly to the layered‐crystal structure of the H3BO3 film that forms on the sliding surface.

Characterization of transfer layers forming on surfaces sliding against diamond-like carbon

Abstract Metallic and ceramic surfaces may become covered with a carbon-rich transfer layer during sliding against diamond-like carbon (DLC) films. The presence of such layers at sliding interfaces may dominate the long-term friction and wear performance of these films. In this study, we use Raman and infrared spectroscopies to characterize the chemical structure of such transfer layers forming on the surface of magnesia/partially-stabilized ZrO2 (MgOPSZ) balls. The DLC film (≈ 2 μm thick) was prepared by ion-beam deposition at room temperature, with methane used as the source gas. Tribological tests were performed on a ball-on-disk machine in open air at room temperature (≈22 ± 1°C) and humidity of 30–50%. Sliding velocity ranged from 1 to 6 m s−1 and the tests were continued until the DLC films were effectively worn through. The results showed that the friction coefficients of DLC against MgOPSZ were initially 0.08–0.12 but decreased to 0.05–0.06 after about 200 000–500 000 sliding passes, depending on velocity. They remained constant at 0.05 for the duration of the tests, which was 1.5 million cycles at 6 m s−1 but > 4 million cycles at 1 m s−1. The low friction coefficients observed in each test coincided with the formation of a carbon-rich transfer layer on the rubbing surfaces of MgOPSZ balls. Micro-laser-Raman and Fourier transformed infrared (FTIR) spectroscopies confirmed that these carbon-rich transfer layers had a disordered graphitic structure.

Physical and tribological properties of diamond films grown in argon-carbon plasmas

Nanocrystalline diamond films have been deposited using a microwave plasma consisting of argon, 2--10% hydrogen and a carbon precursor such as C{sub 60} or CH{sub 4}. It was found that it is possible to grow the diamond phase with both carbon precursors, although the hydrogen concentration in the plasma was 1--2 orders of magnitude lower than normally required in the absence of the argon. Auger electron spectroscopy, x-ray diffraction measurements and transmission electron microscopy indicate the films are predominantly composed of diamond. Surface roughness, as determined by atomic force microscopy and scanning electron microscopy indicate the nanocrystalline films grown in low hydrogen content plasmas grow exceptionally smooth (30--50 nm) to thicknesses of 10 {mu}m. The smooth nanocrystalline films result in low friction coefficients ({mu}=0.04--0.06) and low average wear rates as determined by pin-on-disk measurements.

Effect of source gas and deposition method on friction and wear performance of diamondlike carbon films

Abstract We investigated the tribological performance of diamondlike carbon (DLC) films derived from methane, acetylene, and hydrogen + methane source gases in a magnetron sputtering system and an ion-beam-deposition system. Films have been deposited on AISI 440C bearing-steel substrates and were tested in a pin-on-disk machine under a wide range of conditions in open air and in dry nitrogen. We found that the films grown in a hydrogen + methane plasma resulted in significantly lower friction coefficients (i.e. 0.01) and ball wear rates during tests in dry nitrogen (N 2 ). The friction coefficients for the methane-derived films were also low (0.02 in dry N 2 but the friction coefficients of the acetylene-produced films were 2- to 10-times higher than those recorded for the methane- and hydrogen + methanegrown films, especially in dry N 2 . The methane- and hydrogen + methane-grown films also resulted in very small wear losses on counterface balls, regardless of the test environments. Raman spectroscopy and electron microscopy were used to elucidate the structural chemistry of each film and correlate the findings with tribological performance.