Hydrogenated Diamond-like Carbon Research Articles

Wear behaviour of hydrogenated DLC in a pin-on-disc model test under lubrication with different diesel fuel types

The wear behaviour of hydrogenated diamond like-carbon (DLC) coating in DLC/steel tribological contact in a pin-on-disc model test under lubrication with two diesel fuels is presented in this work. The first diesel fuel was standard EN590 that contained ester-based antiwear additives. In contrast to EN590, the second diesel fuel, called GDK650, did not contain antiwear additives. It was experimentally observed that the antiwear additives are detrimental to the DLC. The effects of load, speed and temperature on the DLC and steel counterbody wear were investigated. Steel counterbody wear volume was found to be not affected by pressure, temperature, speed and lubricant, whereas the DLC-coating revealed correlation between the parameters and wear rate. Regarding the results of the tribological tests under both diesel lubrications, new mathematical wear laws were developed.

Anti-reflection coatings for silicon solar cells from hydrogenated diamond like carbon

Aiming towards a specific application as antireflection coatings (ARC) in Si solar cells, the growth of hydrogenated diamond like carbon (HDLC) films, by RF magnetron sputtering, has been optimized through comprehensive optical and structural studies. Various physical properties of the films e.g., (ID/IG) ratio in the Raman spectra, percentage of sp3 hybridization in XPS spectra, H-content in the network, etc., have been correlated with different ARC application properties e.g., transmittance, reflectance, optical band gap, refractive index, surface roughness, etc. The ARC properties have been optimized on unheated substrates, through systematic variations of RF power, gas flow rate, gas pressure and finally controlled introduction of hydrogen to the DLC network at its most favorable plasma parameters. The optimum HDLC films possess (T700)max∼95.8%, (R700)min∼3.87%, (n700)min∼1.62 along with simultaneous (Eg)max∼2.53eV and ∼75.6% of sp3 hybridization in the C-network, corresponding to a bonded H-content of ∼23at.%. Encouraging improvements in the ARC properties over the optimized DLC film were obtained with the controlled addition of hydrogen, and the optimum HDLC films appear quite promising for applications in Si solar cells. Systematic materials development has been performed through comprehensive understanding of the parameter space and its optimization, as elaborately discussed.

Interaction of DLC and B4C coatings with fully formulated oils in boundary lubrication conditions

Diamond-like carbon (DLC) and Boron Carbide (B4C) coatings are promising candidates for sliding–rolling contact parts, especially gears to increase scuffing load capacity and prolong lifetime. Generally, commercially available fully formulated oils are compatible with ferrous surfaces, but normally have not been optimised for coatings. In this study, the tribological properties of tungsten carbide containing DLC (WC-DLC), hydrogenated DLC (H-DLC) and B4C coatings against steel ball with two fully formulated oils in boundary lubrication conditions have been evaluated. They were compared with that of a Steel/Steel tribocouple. Interactions between lubricants and sliding surfaces were also investigated using surface characterisation techniques such as XPS and Raman Spectroscopy. It was found that Steel/H-DLC system provided the lowest friction coefficient for both oils.

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.

Friction and adhesion of fluorine containing hydrophobic hydrogenated diamond-like carbon (F-H-DLC) coating against magnesium alloy AZ91

Tribological tests were performed on hydrogenated diamond-like carbon (H-DLC) and F containing H-DLC (F-H-DLC) coatings sliding against a magnesium alloy (AZ91) in air with relative humidity (RH) ranging between 0% and 75%. Pin-on-disk type friction tests typically exhibited high running-in COF, followed by a steady state regime with a low and constant coefficient of friction (μS). At 32% RH both the F-H-DLC and the H-DLC showed similar friction behavior, but μS of H-DLC increased with an increase in RH while μS of F-H-DLC decreased. Carbonaceous transfer layers were formed on the AZ91 surfaces and incorporated F transferred from F-H-DLC. The transfer layers were passivated by OH molecules, as detected by the Fourier transform infrared and micro-Raman spectroscopy. A first principles interface model that examined the hydrophobic interactions between F and OH terminated diamond (111) surfaces showed that high repulsive electrostatic forces would contribute to low friction in high humidity atmospheres.

Study of protective diamond-like carbon films on GCr15 bearing steel

Hydrogenated diamond-like carbon (H-DLC) were deposited on GCr15 bearing steel by double cathodes pulse glow discharge (DCPGD) plasma vapour deposition at different gas volume ratios. The surface morphology, composition, microstructure, hardness and friction properties of the H-DLC films were observed. It was found that the sp3 bond content and I(D)/I(G) ratio of DLC films were between 58 and 61% and in the range of 0·68–0·88, respectively. When H2∶Ar∶C2H2 gas volume ratio ranges from 4∶1∶1 to 8∶1∶1, the I(D)/I(G) ratio decreases with an increase in the sp3 bond content, the friction coefficient (about 0·1) and wear rate (between 4·5×10−8 and 8·4×10−8 mm3 N−1 m−1) of DLC films were very low. In this study, when H2∶Ar∶C2H2 gas volume ratio is 6∶1∶1, the thickness of the film is 8·13 μm deposited for is 4 h, the hardness reaches a maximum value (22·5 GPa), the friction coefficient and wear rate reduce to a minimum value, 0·094 and 4·5×10−8 mm3 N−1 m−1, respectively.

Tribological evaluation of hydrogenated DLC in Diesel lubricated model test

In this paper, the wear behaviour of hydrogenated diamond like-carbon (DLC) in DLC/steel tribological contact in the skewed cylinders model test under Diesel lubrication was investigated. Standard Diesel EN590 (according to the European norm) was used as a lubricant. The considered DLC was the hydrogenated amorphous (a-C:H, called DLC), deposited on 100Cr6 steel substrates. The effects of load, contact duration, temperature and speed on DLC and steel tribological behaviours in the skewed cylinders model test were investigated. It was experimentally seen that the DLC wear rate increases with increasing sliding distance, contact pressure and temperature. The wear rate of the 100Cr6 counterbody was observed to be independent of the test conditions. Furthermore, the DLC wear rate was found to be higher than that of the 100Cr6 steel counterbody, especially at a high temperature and/or load. This observation is valid only in the case of DLC/100Cr6 tribological contact. The DLC wear was observed to be driven by the graphitisation of the outermost layer, whereas the wear behaviour of the counterbody was dominated by the formation of a tribological induced layer. Based on the mechanism responsible for the DLC wear, a physical motivated new wear law was developed and proposed. This law allows to predict the wear volume as a function of pressure, temperature and contact duration.

Study of depth profile of hydrogen in hydrogenated diamond like carbon thin film using ion beam analysis techniques

The Hydrogenated Diamond Like Carbon (HDLC) thin films are deposited on Silicon substrate at room temperature using asymmetric capacitively coupled RF plasma with varying flow rates of methane. These films are undergone annealing at high vacuum (∼10−7torr) and high temperature (750 and 1050°C) furnace. The as-prepared and annealed HDLC films have been depth profiled for hydrogen using the resonance at 6.44MeV in 1H(19F,αγ)16O nuclear reaction. The as prepared films exhibit non-uniform depth distribution of hydrogen: it decreases with depth. Annealing in vacuum brings about is a significant desorption of hydrogen from the films. Loss of hydrogen, albeit in much lower proportions, is also induced by the bombarding beam. The films also experience a mild loss of carbon, as shown by proton backscattering spectrometry, during high vacuum annealing. The depth profiles of hydrogen in the annealed films are indicative of the prevalence of graphitic carbon near film–substrate interface.

Soft X-ray irradiation effect on the surface and material properties of highly hydrogenated diamond-like carbon thin films

Surface and bulk properties' changes of a hydrogenated diamond-like carbon (H-DLC) film exposed to synchrotron radiation (SR) in the soft X-ray region were investigated by a nano-indenter, an atomic force microscope (AFM), and a combination of Rutherford Backscattering Spectroscopy (RBS) and Elastic Recoil Detection Analysis (ERDA) techniques. The surface of H-DLC films became flat and the hydrogen content of H-DLC films decreased with increasing SR dose. On the other hand, Vickers hardness showed the complicated dependency on the SR dose. It was found that modification processes of H-DLC films by SR exposure included three reactions: flattening on the surface, hydrogen desorption, and etching by SR exposure. • Soft X-ray irradiation effect on the surface and material properties of highly hydrogenated diamond-like carbon thin films was investigated. • The hydrogen content of H-DLC films decreased from » 50% to » 20% by the SR exposure. • The Vickers hardness of the films increased from » 500 HV to » 1600 HV. • The surface roughness and P-V of the H-DLC films decreased at lower SR doses. • The modification processes of H-DLC films by SR exposure include three reactions: flattening on the surface, hydrogen desorption, and etching.

Dry Machining of AA7075 by H-DLC Coated Carbide End Mill

Present investigation evaluates the performance of a hydrogenated diamond like carbon (H-DLC) coating on carbide end mills during machining of an Al-Zn-Mg based alloy i.e. AA7075. Although diamond is known to be the most potential hard coating on carbide tools to deal with the challenges of machining aluminium, DLC coating offers economic solution in dealing with this material at medium cutting speeds. Present study explores the feasibility of using a H-DLC coating, which is eventually softer than non-hydrogenated DLC, in machining this alloy and compares its performance with that of uncoated carbide tools under dry environment. In terms of surface finish and cutting force this H-DLC coated tool substantially outperformed uncoated carbide one in the cutting speed range of 60 to 180 m/min. The coating was able to substantially arrest built up edge (BUE) formation, however, found to be with minor trace of built up layer (BUL).

High temperature tribological behavior of W containing diamond-like carbon (DLC) coating against titanium alloys

The current study has been undertaken to determine friction, adhesion and wear mechanisms of tribological coatings for elevated temperature applications of Ti–6Al–4V alloy. Sliding wear characteristics of diamond-like carbon (DLC) coatings and polycrystalline diamond (PCD) was studied and compared with those of conventional N-based coatings, which exhibited a high coefficient of friction (COF) due to titanium adhesion to the coating surface. Hydrogenated DLC (H-DLC) and W containing DLC (W-DLC) coatings offered low and stable COF values of 0.11–012 at 25°C while the PCD had the lowest COF of 0.05 at 25°C. At temperatures >200°C H-DLC's COF increased rapidly accompanied by high coating wear. At 100°C W-DLC had a COF of 0.06 which increased to 0.46–0.54 between 200°C and 300°C similar to that of H-DLC. However, at 400°C W-DLC's COF decreased to 0.07, and a low COF of 0.08 persisted at temperatures as high as 500°C. The governing mechanisms of the low friction of W-DLC observed at elevated temperatures were revealed by studying the compositions of the coating surfaces and the transfer layers formed on Ti–6Al–4V. Raman spectroscopy indicated that at 25°C the transfer layers were rich in carbon, whereas at 500°C they consisted mainly of tungsten trioxide (WO3) that formed on W-DLC's surface.

Toward Low Friction in High Vacuum for Hydrogenated Diamondlike Carbon by Tailoring Sliding Interface

The high friction of diamondlike carbon (DLC) films in vacuum impedes achieving their application in space environment. Here we show that the vacuum friction coefficients can be lowered below 0.1 by avoiding formation of carbonaceous transfer layers on the counterfaces. First-principles calculations reveal that the low friction coefficients stem from intrinsically weak-interacting sliding interfaces. Conversely, formation of transfer layers and carbonaceous sliding interfaces thus established are invalid for friction reduction for DLC in vacuum. The mating materials are thus highlighted for their crucial roles in transfer-layer formation.

Direct formation of amine functionality on DLC films and surface cyto-compatibility

Hydrogenated diamond-like carbon (H-DLC) films were synthesized on a p-type silicon wafer using radio frequency plasma composed of a mixture of Ar and C2H2 (ratio of 7 to 28). NH3 plasma treatment was carried out to generate surface-terminal amino groups. The treated surfaces were characterized by Raman scattering, atomic force microscopy (AFM), water contact angle, and X-ray photoelectron spectroscopy (XPS). MC3T3-E1 mouse pre-osteoblasts were cultured on the samples, and cell morphology and proliferation were investigated to evaluate the cyto-compatibility. Cell–surface interactions were investigated by fluorescence microscopy in terms of spreading and proliferation. A cell count kit-8 (CCK-8 Beyotime) was employed to determine quantitatively the viable pre-osteoblasts. The formation of the amine functionality led to better cyto-compatibility on the DLC.

Role of Surface Hydrogen Bonds in Determining the Friction Behaviors of Hydrogenated Diamond-like Carbon Films

Friction behaviors of hydrogenated diamond-like carbon (H-DLC) films are investigated on a ball-on-disk type tribometer in dry N2 and dry vacuum. The result shows that the friction behaviors of the H-DLC films are very sensitive to the testing environment, the H-DLC films exhibit a very low friction coefficient of 0.016 in dry N2, and similarly in an inert gas the friction coefficient increases to about 0.063 in dry vacuum. Combining the testing conditions, friction results, SEM and XPS investigation, it is concluded that the friction behaviors of the hydrogenated DLC films are associated with surface hydrogen of the H-DLC films and are dictated by whether or not hydrogen bonds are formed between the transfer films/H-DLC films at the sliding interface. A hydrogen-induced hydrogen bond model is proposed to interpret the friction behaviors of hydrogenated DLC films in different environments.

Structure of hydrogenated diamond like carbon by Micro-Raman spectroscopy

The first-, second- and higher order Raman spectra of hydrogenated diamond like carbon (HDLC) are analyzed considering the effect of partial hydrogenation of hexagonal sp 2 (C C) carbons on the characteristic G and D bands of graphitic materials. That coherency of sp 3 C H and sp 2 C C carbons in HDLC can produce a continuous, non-porous thin film having atomically smooth surface is reported as a new result.

Load Capacity and Durability of H-DLC Coated Hydrodynamic Thrust Bearings

Hydrogenated diamondlike carbon (H-DLC) coatings provide excellent wear resistance and low friction for bearing applications. However, the use of such coatings with aqueous lubricants could pose some difficulties due to the hydrophobic nature of the surface. A thrust bearing tribometer was used to compare performance of hydrophilic and hydrophobic surfaces in hydrodynamic lubrication with a mixture of water and glycerol as the lubricant. Hydrophobic surfaces on both runner and bearing were achieved with the deposition of H-DLC films on titanium alloy surfaces. Hydrophilic surfaces were created through modification of H-DLC surface with covalently bonded heparin. Several possible combinations of hydrophobic and hydrophilic surface conditions were used on the bearing and runner surfaces to provide full-wetting, partial-wetting, and half-wetting conditions. The experimental results confirmed that load support is still possible, when the bearing is half-wetted or partially wetted. However, the full-wetted bearing combination (i.e., Reynolds no-slip boundary condition) provided the highest load support. Introduction of slip at the surface resulted in a lower measured torque. Heparin treatment resulted in a lower than expected static friction and friction in full lubrication regime. The durability of coated surfaces was evaluated in a series of start–stop tests and in impact tests. The results confirmed that the coatings are stable and survive the test regiment that exceeded 50 test cycles; whereas the uncoated titanium alloy bearing surfaces were damaged after ten test cycles.

Covalent Immobilization of Protein onto a functionalized Hydrogenated Diamond-like Carbon Substrate

Hydrogenated diamond-like carbon (HDLC) has an atomically smooth surface that can be deposited on high-surface area substrata and functionalized with reactive chemical groups, providing an ideal substrate for protein immobilization. A synthetic sequence is described involving deposition and hydrogenation of DLC followed by chemical functionalization. These functional groups are reacted with amines on proteins causing covalent immobilization on contact. Raman measurements confirm the presence of these surface functional groups, and Fourier transform infrared spectroscopy (FTIR) confirms covalent protein immobilization. Atomic force microscopy (AFM) of immobilized proteins is reproducible because proteins do not move as a result of interactions with the AFM probe-tip, thus providing an advantage over mica substrata typically used in AFM studies of protein. HDLC offers many of the same technical advantages as oxidized graphene but also allows for coating large surface areas of biomaterials relevant to the fabrication of medical/biosensor devices.

Influence of Microwave Power on the Properties of Hydrogenated Diamond-Like Carbon Films Prepared by ECR Plasma Enhanced DC Magnetron Sputtering

Electron cyclotron resonance (ECR) plasma was applied to enhance the direct current magnetron sputtering to prepare hydrogenated diamond-like carbon (H-DLC) films. For different microwave powers, both argon and hydrogen gas are introduced separately as the ECR working gas to investigate the influence of microwave power on the microstructure and electrical property of the H-DLC films deposited on P-type silicon substrates. A series of characterization methods including the Raman spectrum and atomic force microscopy are used. Results show that, within a certain range, the increase in microwave power affects the properties of the thin films, namely the sp3 ratio, the hardness, the nanoparticle size and the resistivity all increase while the roughness decreases with the increase in microwave power. The maximum of resistivity amounts to 1.1 × 109 Ω · cm. At the same time it is found that the influence of microwave power on the properties of H-DLC films is more pronounced when argon gas is applied as the ECR working gas, compared to hydrogen gas.

Temperature and Water Vapor Pressure Effects on the Friction Coefficient of Hydrogenated Diamondlike Carbon Films

Microtribological measurements of a hydrogenated diamondlike carbon film in controlled gaseous environments show that water vapor plays a significant role in the friction coefficient. These experiments reveal an initial high friction transient behavior that does not reoccur even after extended periods of exposure to low partial pressures of H2O and O2. Experiments varying both water vapor pressure and sample temperature show trends of a decreasing friction coefficient as a function of both the decreasing water vapor pressure and the increasing substrate temperature. Theses trends are examined with regard to first order gas-surface interactions. Model fits give activation energies on the order of 40 kJ/mol, which is consistent with water vapor desorption.

Fretting wear studies on diamond-like carbon coated Ti–6Al–4V

This work is concerned with the performance of diamond-like carbon (DLC) coated Ti–6Al–4V flat samples fretted against alumina ball in two environments—ambient air and simulated body fluid (Ringer solution). Hydrogenated DLC (H-DLC) coatings were prepared by radio frequency plasma enhanced chemical vapour deposition process. Raman scattering measurements were done on the coatings. Fretting wear tests were conducted at different normal loads using a fretting wear test rig working on scotch yoke mechanism. Wear scars on the tested specimens were observed using an optical microscope. Wear volume was calculated based on wear scar diameter measured along parallel and perpendicular to the fretting direction. Tangential force coefficient (TFC) values exhibited by DLC coated samples were lower than those exhibited by uncoated specimens in both ambient air and Ringer solution. Very low TFC values and low wear volume were observed for H-DLC coated samples tested in air at lower loads of 1.9 N and 4.9 N. It was attributed to occurrence of graphitisation and formation of transfer layer on alumina ball.