Hydrogenated Diamond-like Carbon Coatings Research Articles

Structural analysis of hydrogenated diamond-like carbon coatings prepared using magnetron sputtering

In this study, a series of hydrogenated diamond-like carbon (H-DLC) coatings were deposited using an unbalanced magnetron sputtering system. For hydrogenation, acetylene gas was used as the hydrogen source. The typical conditions for preparing DLC coatings were used, with additional acetylene gas being introduced into the depositing chamber. The acetylene flow rate was varied from 0 to 16 sccm at intervals of 4 sccm and finally set at 24 sccm. The chemical composition, microstructure, and phases of the resulting coatings were evaluated. Fourier transform infrared spectrometry, X-ray photoelectron spectroscopy, and Raman analysis were performed, and specific mechanical properties were examined. The results indicated that hydrogenation influenced the H-DLC coatings due to the formation of sp3 bonding. The coating prepared at a flow rate of 8 sccm of acetylene exhibited the highest hardness of 15.4 GPa because it had the highest number of sp3 bonds.

Effect of Coil Current on the Properties of Hydrogenated DLC Coatings Fabricated by Filtered Cathodic Vacuum Arc Technique

We successfully prepared hydrogenated DLC (a-C:H) with a thickness higher than 25 μm on stainless steel using a filtered cathode vacuum arc (FCVA) technique. The structural and mechanical properties of DLC were systematically analyzed using different methods such as x-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, Vickers hardness, nanohardness, and friction and wear tests. The effect of coil current on the arc voltage, ion current, and mechanical properties of resultant films was systematically investigated. The novelty of this study is the fabrication of DLC with Vickers hardness higher than 1500 HV, in the meanwhile with the thickness higher than 30 μm through varying the coil current with FCVA technique. The results indicated that the ion current, deposition rate, friction coefficient, and Vickers hardness of DLC were significantly affected by the magnetic field inside the filtered duct.

Shear-Induced Structural Changes and Origin of Ultralow Friction of Hydrogenated Diamond-like Carbon (DLC) in Dry Environment.

The origins of run-in and ultralow friction states of a sliding contact of hydrogenated diamond-like carbon (H-DLC) and sapphire were studied with an in situ Raman tribometer as well as ex situ analyses of transmission electron microscopy (TEM), Raman spectroscopy, and nanoindentation. Prior to ultralow friction behavior, H-DLC exhibits a run-in period. During the run-in period in dry nitrogen atmosphere, the transfer film was formed and its uniformity and thickness as well as structure were varied. The duration and friction behaviors during the run-in depended on the initial surface state of the H-DLC coatings. A comparative study of pristine and thermally oxidized H-DLC revealed the role of surface oxide layer on run-in friction and transfer film formation. Attainment of the ultralow friction state appeared to correlate with the uniformity and structure of the transfer film evolved during the run-in, rather than its final thickness. TEM cross-section imaging of the wear track and the counter surfaces showed a trace of nanocrystalline graphite and a thin modified surface layer on both rubbing bodies. The comparison of hardness and reduced modulus of the wear tracks and the unworn surfaces as well as the ex situ Raman spectra suggested the densification of the wear track surfaces. Combining the in situ and ex situ analysis results, a comprehensive model was proposed for the formation and structure of the ultralow friction sliding contact of H-DLC.

Effect of ZDDP concentration on the thermal film formation on steel, hydrogenated non-doped and Si-doped DLC

This work focuses on the ZDDP concentration (1, 5 and 20wt%) to form a ZDDP film on surfaces during static thermal tests at 150°C. Silicon-doped and hydrogenated DLC coatings, as well as steel as reference, were studied using Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The results show that, on the three surfaces, the structure of the ZDDP thermal film consists of identical groups of pyrophosphate and zinc oxide, while the sulphuric groups are dissimilar. On the steel surface, the sulphuric part consists of a mixture of organic sulphide and sulphohydryl groups, but on H-DLC and Si-DLC only organic sulphide groups are found; there are no sulphohydryl groups. Moreover, both ATR-FTIR and XPS show that different concentrations of ZDDP do not affect the final chemical structure of the ZDDP thermal film on any of the studied surfaces. In addition, the XPS results show that the thickness of the thermal film is linear with the concentration for the whole range from 1 to 20wt%, supporting also its uniform chemical structure. These thicknesses further show that the reactivity of the ZDDP film is higher on the steel surface than on the DLC coatings.

Effects of molybdenum dithiocarbamate and zinc dialkyl dithiophosphate additives on tribological behaviors of hydrogenated diamond-like carbon coatings

The tribological behaviors of hydrogenated diamond-like carbon (DLC) coatings under varied load conditions lubricated with polyalpha olefin (PAO), molybdenum dithiocarbamate (MoDTC) and zinc dialkyl dithiophosphate (ZDDP) additives were investigated in this paper. Hydrogenated DLC coatings were synthesized through the decomposition of acetylene by the ion source. The tribological performances were measured on a SRV tribometer. The morphologies and chemical structures of the DLC coatings were investigated by the scanning electron microscope (SEM), Raman spectrometer (Raman) and X-ray photoelectron spectroscope (XPS). It was shown that the low friction and high wear were achieved on the hydrogenated DLC coating under MoDTC lubrication, while low wear was found on the hydrogenated DLC coating lubricated by ZDDP. The primary reason was attributed to different tribofilms formed on the contact area and the formation of graphitic layer. Both factors working together leaded to quite different tribological behaviors.

Long-term stability of hydrogenated DLC coatings: Effects of aging on the structural, chemical and mechanical properties

Long-term stability is an essential condition for the commercial use of protective coatings, yet often remains overlooked in the literature. Here we report the effects of long-term environmental aging on the properties of hydrogenated diamond-like carbon (DLC) films. A range of DLC coatings produced by plasma-enhanced chemical vapor deposition were first thoroughly characterized and then stored for three years before the second set of analysis. Raman spectroscopy showed that the films exhibited excellent structural stability during aging, observing no sign of sp3 to sp2 conversion. Similarly, the hardness and smoothness of the DLC coatings remained unchanged, despite the observed relaxation of their intrinsic stress with time. However, X-ray photoelectron spectroscopy analyses provided evidence of aging-induced surface oxidation, which was confirmed by reduced hydrophobicity (water contact angle dropped to 65°). Overall, these findings suggest that DLC possesses a suitable long-term stability when exposed to environmental conditions.

Comparison of Alcohol and Fatty Acid Adsorption on Hydrogenated DLC Coatings Studied by AFM and Tribological Tests

Adsorption on hydrogenated diamond-like carbon (DLC) coatings with respect to boundary lubrication has been studied only very rarely, even though such adsorption has proved to be one of the main boundary-lubricating mechanisms for metal contacts. For this reason, we used atomic force microscopy (AFM) and tribotests to study the ability of hexadecanol and hexadecanoic acid to adsorb onto DLC and affect the tribological properties of DLC coatings, where steel was used as a reference. We have shown that alcohols and fatty acids can adsorb onto the DLC under static conditions. However, under dynamic tribocontact conditions, although alcohol and fatty acid molecules help to decrease the wear of the coatings, they are inefficient in reducing the friction. In all the experiments the fatty acid proved to have superior adsorption abilities compared to the corresponding alcohol. Based on our results and the existing literature, tentative adsorption mechanisms that include an environmental species effect, a temperature effect and a tribochemical effect are discussed for DLC surfaces.

Evaluation of the transformed layer of DLC coatings after sliding in oil using spectroscopic reflectometry

Diamond-like carbon (DLC) coatings continue to attract attention for their low friction and high wear resistance. The transferred or transformed layer, which has a high sp2 carbon content induced by cyclic friction, results in an ultralow friction coefficient in ambient air. However, there are few reports on such layers, especially the transformed layer in oil lubrication, because this transformed layer could be too thin to be measured quantitatively. M. Kano and C. Matta reported the 3nm thickness of a sp2 carbon layer using EFTEM study. The spectroscopic reflectometry technique has been applied to measure the thickness of the transformed layer of the DLC's wear scar after friction under oil boundary lubrication. The purpose of this study was to develop a quantitative method for measuring the thickness of the transformed layer in hydrogen-free DLC (ta-DLC) and two types of hydrogenated DLC coatings. Then, we clarified the friction model of these DLC coatings under oil lubrication as the transformed layer was the solid lubricant film. Consequently, we observed that there were optimum ranges of wavelengths and optical models for measuring the transformed layer of the DLC. The optimum wavelength range was 300–500nm, and the bi-layer model was the best for DLC1, which was the hydrogen-free DLC. For the thicker and un-transmissive coatings, which were hydrogenated DLC coatings, the ranges of wavelengths were 600–800nm, and the optical model was hypothesized to be a single-layer model. From these optical models, we observed that the thicknesses of the transformed layers of the DLC coatings under oil boundary lubrication were approximately 10–200nm in the hydrogen-free DLC and 0–200nm in the hydrogenated DLC. From the analysis of the thickness of the transformed layer and surface roughness, σ⁎, of various DLC coatings, we observed that the friction coefficient of the DLC under oil boundary lubrication was determined by the possibility of breaking the transformed layer. This result strongly suggested that the DLC film was a material that realized self-surface modification. We believe that the results of this study could be developed as a guideline for designs of surfaces with DLC coatings.

Coating Techniques and Tribological Properties of Hydrogenated DLC Coatings Prepared by Plasma Enhanced CVD

Coating Techniques and Tribological Properties of Hydrogenated DLC Coatings Prepared by Plasma Enhanced CVD

Performance of low-friction coatings in helium environments

The aim of this research work was to investigate the possibility of replacing soft PTFE‐based coatings on components operating in helium atmosphere. Focus was on maintaining low friction and improving surface wear resistance. Therefore two commercial DLC coatings (a-C:H and Me-C:H), CrN coating and reference PTFE‐based Rulon and Xylan coatings were included in this investigation. Coatings were deposited on hardened 100Cr6‐bearing steel discs and tested against uncoated steel balls in pin-on-disc contact configuration under dry reciprocating and unidirectional sliding in helium atmosphere. Investigation was concentrated on the effect of running-in, contact pressure, sliding speed and counter-material type and surface treatment on the tribological behaviour of hydrogenated DLC coatings when running‐in in helium.Results show that for PTFE-based self-lubricating coatings low friction is reached through coating removal and formation of thick layer of transferred coating material on the steel counter-surface. As long as coating is not worn through stable coefficient of friction of about 0.2 is maintained. In a similar way DLC coatings can provide even lower friction in helium, at the same time almost eliminating wear of the coated part. However, at the same time hard DLC coatings prolong running-in and provoke wear of the steel counter-surface, which greatly depends on the coating type, counter material and contact conditions. The best tribological behaviour in helium was thus achieved when pairing softer metal doped W-DLC coating with nitrided steel surface.

High temperature tribological behaviour of carbon based (B 4C and DLC) coatings in sliding contact with aluminum

Carbon based coatings, particularly diamond-like carbon (DLC) films are known to resist aluminum adhesion and reduce friction at room temperature. This attractive tribological behaviour is useful for applications such as tool coatings used for aluminum forming and machining. However, for those operations that are performed at elevated temperatures (e.g. hot forming) or that generate frictional heat during contact (e.g. dry machining) the suitable coatings are required to maintain their tribological properties at high temperatures. Candidates for these demanding applications include boron carbide (B 4C) and DLC coatings. An understanding of the mechanisms of friction, wear and adhesion of carbon based coatings against aluminum alloys at high temperatures will help in designing coatings with improved high temperature tribological properties. With this goal in mind, this study focused on B 4C and a hydrogenated DLC coatings sliding against a 319 grade cast aluminum alloy by performing pin-on-disk experiments at temperatures up to 400 °C. Experimental results have shown that the 319 Al/B 4C tribosystem generated coefficient of friction (COF) values ranging between 0.42 and 0.65, in this temperature range. However, increased amounts of aluminum adhesion were detected in the B 4C wear tracks at elevated temperatures. Focused ion beam (FIB) milled cross sections of the wear tracks revealed that the coating failed due to shearing along the columnar grain boundaries of the coating. The 319 Al/DLC tribosystem maintained a low COF (0.15–0.06) from room temperature up to 200 °C. This was followed by an abrupt increase to 0.6 at 400 °C. The deterioration of friction behaviour at T > 200 °C was attributed to the exhaustion of hydrogen and hydroxyl passivants on the carbon transfer layer formed on the Al pin.

DLC coatings for hydraulic applications

Replacement of lubricating oils with water or low-viscosity fluids is highly desirable in many industrial fields, on account of the environmental and economical advantages. Low lubricity of water might be insufficient for proper operation of hydraulic components, and diamond-like carbon(DLC) coatings are very attractive as solid lubricant films. A remote-plasma PACVD process was utilized to deposit hydrogenated DLC coatings (a-C:H) on different substrates. Microindentation measurements show that the coating hardness is around 35 GPa. Tribological behavior was evaluated by block-on-ring tests performed in water and water with alumina. The wear rate was calculated after measuring the wear volume by a laser profilemeter. Morphological and compositional analysis of the wear tracks reveal that coating failure may occur by abrasive wear or delamination, depending on the substrate properties. Hard and smooth substrates give the best results and dispersed alumina particles increase the wear rate.

Tribochemistry of Multiply Alkylated Cyclopentane Oils on Diamond-like Carbon (DLC) Coated Thrust Bearings

Abstract In contrast to typical diamond-like carbon (DLC) coatings, hydrogenated DLC (H-DLC) coatings have demonstrated extremely low friction in vacuum and dry atmospheres. This behavior suggests the coatings’ potential importance for spacecraft applications. However, most studies have concentrated on sliding wear testing, with relatively few concerning their use on ball bearings. We have conducted a preliminary study of H-DLC-coated steel thrust bearings, lubricated with a multiply alkylated cyclopentane (MAC) oil, either unformulated (NSO2001A), or formulated with lead naphthenate (Pbnp) or an aryl phosphate ester mixture (NSO2001). To determine if the additives could provide life enhancement when using H-DLC coatings, post-test analyses of the bearing surfaces were conducted using electron microscopy and Auger electron spectroscopy, and analysis of the residual lubricant was conducted using Fourier-transform infrared spectroscopy. Minimal additive-based tribofilm formation was detected on the surfaces of the H-DLC coatings in the wear tracks relative to that with steels. For example, using Pbnp resulted in considerably thicker Pb-containing tribofilms on the uncoated bearing surfaces compared to the H-DLC-coated surfaces. The results indicate that additives suitable for steels may not be appropriate for H-DLC coatings. The high roughness of the bearings used in this study contributed to local spallation of H-DLC at asperities in the wear track. When tested with NSO2001 or NSO2001A oil, several H-DLC-coated bearings endured for significantly longer than uncoated bearings (>10 Mcycles) tested with the same oils. However, the variation and magnitude of the bearing roughness contributed to statistical uncertainty regarding the endurance enhancement of H-DLC coatings. Future studies are planned with higher quality bearings.

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 MoS 2 by a factor of about two. It is concluded that, in contrast to MoS 2 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.

1012 水素化DLC膜の水環境摺動特性に及ぼす皮膜硬度の影響(基調講演,関東支部 茨城講演会)

1012 水素化DLC膜の水環境摺動特性に及ぼす皮膜硬度の影響(基調講演,関東支部 茨城講演会)

Boundary lubrication mechanisms of carbon coatings by MoDTC and ZDDP additives

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 present a wide range of tribological behavior, including friction coefficients in ultra-high vacuum below 0.02. The engine oil environment which provides similar favourable air free conditions might lead to such low friction levels. In this work, the friction and wear properties of DLC coatings in boundary lubrication conditions have been investigated as a function of the hydrogen content in the carbon coating. Their interaction with ZDDP which is the exclusive antiwear agent in most automotive lubrication blends and friction-modifier additive MoDTC has been studied. Hydrogenated DLC coatings can be better lubricated in the presence of the friction-modifier additive MoDTC through the formation of MoS 2 solid lubricant material than can non-hydrogenated DLC. In contrast, the antiwear additive ZDDP does not significantly affect the wear behavior of DLC coatings. The good tribological performances of the DLC coatings suggest that they can contribute to reduce friction and wear in the engine, and so permit the significant decrease of additive concentration.

Titanium-doped hydrogenated DLC coatings deposited by a novel OMCVD-PIIP technique

Using a novel organometallic CVD-plasma immersion ion processing technique, titanium-doped hydrogenated diamond-like carbon (Ti-h:DLC) coatings were deposited from a metallorganic CVD precursor, tetrakis-dimethylamino titanium. A carrier gas of acetylene was used and the flow rate through the precursor reservoir was varied so the titanium concentration in the deposited coatings could be controlled. Deposited coatings had thickness in the range of 600–1000 nm as determined by surface profilometry. Titanium concentrations in the coatings ranged from 0.4 to approximately 11 at.%. Acetylene flow through the reservoir reached a peak efficiency of titanium throughput at 20 sccm, declining in efficiency as flow was increased to 40 sccm. Hardness values of these coatings ranged from approximately 8–17 GPa, as determined by nanoindentation, decreasing with increasing titanium content. Coefficient of friction values ranged from 0.23 to 0.18, and were lowest for coatings with high titanium content. The coefficient of friction followed the model for adhesive friction. Wear resistance was best for coatings with low titanium content as determined by pin-on-disk methods.