Non-hydrogenated Diamond-like Carbon Research Articles

Tribological behavior of DLC film models in base oils: Analysis of influence of sp3/sp2 ratio and hydrogen content

DLC coatings have been proven in formulated oils used in the automotive industry. The development of electric cars and its consequences on the future of the internal combustion engine makes it increasingly necessary that these coatings take their place in other applications where lubrication is generally of lower quality such as for example strips cold rolling where DLC shows significant potential both for production and finishing cold rolling. As a result, the interest of the behavior of these coatings in base oils takes on a new dimension. In this study, we explore the influence of the composition and the sp3/sp2 ratio on tribological behavior of DLC coatings in mineral and synthetic base oils. Hydrogenated DLC films deposited by conventional PECVD with various levels of hydrogen measured by Elastic Recoil Detection Analysis (ERDA) and non-hydrogenated DLC films deposited by PLD in the nanosecond mode (ns-PLD) and by cathodic arc evaporation, thus allowing to have different sp3/sp2 ratios confirmed by Multiwavelength Raman spectrometry, are tested and compared. The tests are performed on a ball-on-flat device with coated balls to better evaluate wear level. The behavior of highly hydrogenated DLC is of particular interest; with a very high level of durability in both base oils and friction level which comparatively varies between low and very low depending on the base oil.

OPTIMIZATION OF DIAMOND-LIKE CARBON COATINGS FOR MECHANICAL AND TRIBOLOGICAL APPLICATIONS. REVIEW

Diamond-like carbon (DLC) coatings are used to improve the quality of industrial products due to a wide range of exceptional physical, mechanical and tribological properties. Problems that limit the scope of application of hard non-hydrogenated DLC coatings with properties closest to those of diamond are considered. The scientific literature is reviewed devoted to improving the characteristics of DLC-based wear-resistant coatings. The approaches currently used to reduce internal compressive stresses, improve adhesion to the substrate, and increase the strength of coatings are analyzed. Aspects of optimization of deposition parameters, doping, constructing the gradient and multilayer structure are considered. Based on the generalization of the results of the reviewed publications, a conclusion is made about the advantages of using coatings with a multilayer structure. A promising design undoped multilayer DLC coatings with high protective properties is discussed.

Progression of galling during punching of AA5754 Al sheets with DLC-coated and uncoated steel tools

Galling is a commonly observed type of sliding contact-induced damage that occurs as a result of material transfer from the aluminum alloys to the tool surfaces during forming and shaping of aluminum sheets. Progression of galling on uncoated AISI M2 steel tools and tools coated by non-hydrogenated diamond-like carbon (NH-DLC) with amorphous carbon (a-C) structure was investigated during the punching of AA5754-O sheets. Experiments were done either by lubricating aluminum sheets with mineral oil-based lubricant (LUB1), a method commonly used in industry, or by lubricating both the sheet and the punch prior to the initial punching stroke (LUB2). Galling rates were determined using the galling volume per punching stroke (between 20th and 125th strokes) measured using white light interferometry (WLI). Galling rates were lower for LUB2 punching compared to LUB1. In punching with NH-DLC-coated tools using LUB1, the coating was removed from the punch surface after 30 strokes, whereas for LUB2 punching, a negligible amount of aluminum transfer was detected by scanning electron microscopy (SEM) and WLI after 500 strokes, and the coating remained intact.

From pulsed-DCMS and HiPIMS to microwave plasma-assisted sputtering: Their influence on the properties of diamond-like carbon films

The fabrication of high-hardness non-hydrogenated diamond-like carbon (DLC) via standard magnetron sputtering (MS) is often hindered by the low sputtering yields and ionisation rates of carbon, therefore investigations into pulsed alternatives of MS, else sputtered species post-ionisation methods, are of particular interest. This work focuses on investigating the influence of pulsed-direct current MS (pDCMS), high power impulse magnetron sputtering (HiPIMS) and their microwave plasma-assisted (MA-pDCMS, MA-HiPIMS) variants on the properties of the fabricated DLC films. Two setups were used for the pDCMS- and HiPIMS-based methods, respectively. The films were characterised using Raman spectroscopy, nanoindentation, X-ray reflectometry and scanning electron microscopy, where the pDCMS-produced films were additionally characterised by film-stress measurements. Moreover, in situ time-resolved Langmuir probe plasma analysis was performed under HiPIMS and MA-HiPIMS conditions to analyse the influence of the magnetron and microwave plasmas on one another. For both DCMS- and HiPIMS-based procedures, it was found that the addition of microwave plasma did not facilitate attaining hardnesses beyond 30 GPa, however, it did enable modifying the morphology of the films. Furthermore, this study shows the potential of synchronised sputtering with substrate biasing, as well as the importance of microwave plasma source positioning in relation to the substrate. • DLC films produced via pulsed magnetron sputtering and microwave plasma-assisted methods. • In situ plasma diagnostics were used to study magnetron-microwave plasma interactions. • Maximum DLC film hardness did not significantly exceed 30 GPa. • Microwave plasma caused DLC film surface morphology modifications. • Substrate biasing and microwave plasma source positioning affect DLC film tunability.

The influence of Cr and Ni doping on the microstructure of oxygen containing diamond-like carbon films

Non-hydrogenated diamond-like carbon (DLC) films doped with metals and oxygen were deposited by direct current magnetron sputtering. The influence of chromium and nickel on the surface morphology, elemental composition, bonding structure, adhesion force, optical transmittance and nanohardness of the films was characterized by atomic force microscopy (AFM), energy dispersive X-ray spectroscopy (EDX), multi-wavelength Raman spectroscopy, UV–VIS–NIR spectrophotometry and nanoindenter. The surface roughness was reduced with the addition of Cr (7.4 at. %) or Ni (8.9 at. %) into DLC films. The EDX measurements indicated that the addition of Cr increased the oxygen content by ~37%, while Ni reduced it by ~50%, compared to the oxygen doped DLC film. The microRaman analysis indicated that the G band shifted to a higher wavenumbers range, became narrower and the sp3/sp2 sites fraction ratio decreased for the metals doped DLC films. The nanohardness values of the sputtered doped DLC films varied in the range of 4.5–7.6 GPa. It was observed that the Ni-DLC film showed the highest adhesion force value and was optically opaque.

Nanoindentation of Amorphous Carbon: a combined experimental and simulation approach

Amorphous carbon (aC), in particular diamond-like carbon (DLC), is one of the most studied and promising coating materials, but many of the atomic-scale mechanisms involved in their plastic deformation process are not fully understood. The mechanical response of non-hydrogenated DLC films with different sp3 concentrations is investigated using nanoindentation experiments, ab-initio simulations, and classical molecular dynamics simulations. Experimental characterization is carried out with Raman spectroscopy and Electron Energy Loss Spectroscopy (EELS). Ab-initio and classical simulations show good agreement for sp1, sp2 and sp3 content of in-silico samples. Elastic modulus and hardness of DLC films increase with sp3 content, for sp3 between 10% and 55%, and excellent agreement is obtained between experiments and simulations. Simulated strain distributions are shown to be highly anisotropic, unlike continuum-scale predictions for the use of a perfectly spherical indenter in an amorphous solid. MD simulations also reveal two different plasticity modes depending on the sp3 level of the indented sample. For films with sp3 concentrations less than 40%, plasticity is mainly mediated by the sp2 to sp3 transition. For larger sp3 concentrations, DLC plastic deformation is attributed to densification due to bond rearrangement. All in all, our work offers a comprehensive study of DLC, revealing unexpected plastic deformation mechanisms that had not been considered before. Our study might help to the fundamental understanding of amorphous carbon coatings for both scientific and technological purposes.

Effect of sp3 Content on Adhesion and Tribological Properties of Non-Hydrogenated DLC Films.

Non-hydrogenated diamond-like carbon (DLC) films with various ratios of sp3/sp2 were prepared on cemented carbide YG8 with DC magnetron sputtering technology. A pure graphite target was selected as the carbon source. Before DLC deposition, a surface etching pretreatment was carried out by mid-frequency magnetron sputtering method, using Ti atoms to improve adhesion strength. The ratios of sp3/sp2 were adjusted by bias voltages. In order to investigate the effect of the ratio of sp3/sp2 on adhesion and tribological properties, Raman spectra, XPS spectra, adhesion scratch test and ball-on-disk dry friction tests were applied. The results indicated that the ratio of sp3/sp2 fluctuated with bias voltage, increasing in the range of 0.74 to 0.98. The adhesion strength decreased from 31.5 to 18.4 N with the increasing ratio of sp3/sp2, while the friction coefficient rose in DLC-Si3N4 and dropped in DLC-Ti6Al4V. For DLC-Ti6Al4V, the oxidation of Ti6Al4V had a greater influence than graphitization of DLC. The hard oxides of Ti6Al4V broke the graphite transfer layer leading to a high friction coefficient. The wear rate was approximately linearly related to bias voltage. The coefficients of the linear regression equation were influenced by different friction materials. The adhesion strength and the friction coefficient were fitted as a function of the ratio of sp3/sp2.

氧气氛环境对无氢类金刚石膜红外性能的影响

氧气氛环境对无氢类金刚石膜红外性能的影响

Infrared and mechanical properties of the oxygen-doped non-hydrogenated diamond-like carbon film prepared by pulsed laser deposition

The non-hydrogenated diamond-like carbon films were prepared by the femtosecond laser in the oxygen atmosphere, and the influence of the oxygen atmosphere on the microstructural, surface, infrared and mechanical properties of the oxygen-doped diamond-like carbon films were researched. The oxygen atmosphere was propitious to reduce the granules on the surfaces but increase the surface roughness of the diamond-like carbon DLC films. During the pulsed laser deposition, the carbon and oxygen atoms were inclined to form the CO bond, CO bond and CC hybridized bond, when the oxygen pressure was lower than 5 m Torr. Although the CO and CO bonds led one additional absorption band and two additional absorption peaks in the infrared spectra of the oxygen-doped diamond-like carbon films, their transmission in the medium infrared band increased. Nano-hardness of the DLC films was reduced when the oxygen pressure increased dominantly because the CC sp3 bonds with higher atomic binding energy were replaced the CO bonds with lower atomic binding energy.

Low friction and wear behaviour of non-hydrogenated DLC (a-C) sliding against fluorinated tetrahedral amorphous carbon (ta-C-F) at elevated temperatures

The low coefficient of friction (COF) of sp2 rich non-hydrogenated diamond-like carbon (a-C) coatings make them suitable for tribological applications including machining of lightweight alloy castings and for certain components in internal combustion engines. Retaining the low COF of a-C at temperatures >100 °C is however a challenge that has limited further industrial applications of these coatings. This study examines the high temperature stability of a non-hydrogenated a-C coating (H < 2 at.%) sliding against a sp3 rich fluorinated tetrahedral amorphous carbon (ta-C-F) and tool steel counterfaces using ball-on-disk type experiments. It was shown that a-C coated balls run against ta-C-F maintained a low steady state COF that varied between 0.1 and 0.2 up to 300 °C while a-C sliding against uncoated tool steel resulted in high COF values accompanied with high wear rates of a-C coating. Transfer layers were formed on the a-C coatings’ contact surfaces sliding against ta-C-F and these layers consisting of fluorine incorporating carbonaceous material remained stable up to 300 °C. The results suggested that a low COF in this tribological system can be achieved at elevated temperatures as a result of passivation of the surface carbon atoms.

Experimental Study on Surface Element Composition and Hydrophobic Properties of DLC Coating

Precision rubber mold is an extremely important basic process equipment in industrial production, however, there are some classic problems, such as viscose, coke accumulation and difficult demoulding, can not be effectively resolved. In order to solve this problem, in this paper, a new process of preparing high performance DLC coated precision rubber mold is proposed by using the excellent anti-sticking properties, self-cleaning properties and hydrophobic properties of DLC coating. In the experiment, steel ( S35C ) substrate with the same material as tire mold is used. Hydrophobic of DLC coating surface mainly depends on the surface element composition and the roughness. All the substrates are exactly processed, so as to ensure that the roughness has minimal impact on hydrophobic. Plasma enhanced chemical vapor deposition (PECVD) technology is employed to prepare fluorine doped diamond-like carbon (F-DLC) coating and hydrogenated diamond-like carbon (a-C:H) coating. Arc ion plating (AIP) technology is applied to produce Non-hydrogenated diamond-like carbon (a-C) coating. The coating is deposited on the surface of steel ( S35C ). Then, the roughness, the element composition and the hydrophobicity of the prepared coating are tested. Experimental results demonstrate that the water contact angle of a-C coating only 73.8 degrees, water contact angle of a-C:H coating up to 80.16 degrees, however, water contact angle of F-DLC coating could reach to 89.01 degrees, meanwhile, the surface energy can be reduced to 22.62 mN/m. To some extent, the hydrophobicity of the coating enhances with the increase of H content, and the content of weak polarization group CF2 becomes stable with the incorporation of F element. As a consequence, the surface of coating becomes smoother and the wettability of coating and water becomes poorer, therefore the performance of hydrophobic of the coating is further enhanced.

Friction reduction mechanisms in cast iron sliding against DLC: Effect of biofuel (E85) diluted engine oil

Parasitic friction losses during piston ring-cylinder liner interactions in an internal combustion (IC) engine consume large portions of the available fuel energy. This work investigates whether the tribological performance of grey cast iron (CI) cylinder liner would be improved using a piston ring coated with non-hydrogenated diamond-like carbon (NH-DLC) in comparison to uncoated CI and steel rings. Coefficient of friction (COF) values and volumetric wear losses of CI were determined using ball-on-disk type tests as a function of sliding distance. CI tested against itself during boundary lubricated sliding with synthetic engine oil showed a COF of 0.14 and a volumetric wear of 11.0×10−4mm3 after 6×105 cycles, whereas the use of NH-DLC coated counterface resulted in a COF of 0.11 and lesser wear of 0.5×10−4mm3. Dilution of the engine oil by ethanol containing E85 biofuel was beneficial as COF was further reduced to 0.08 for CI tested against NH-DLC while maintaining low wear of 0.2×10−4mm3. According to TEM and XPS analyses, an oil residue layer (ORL) formed on the CI contact surfaces as a result of sliding-induced degradation of zinc-dialkyldithiophospahte (ZDDP) additive in the oil. The ORL, which consisted of nano-crystalline particles of sulphides and phosphates of zinc (anti-wear components) embedded in an amorphous carbon matrix, was responsible for maintaining low wear. Ethanol dilution of the synthetic oil facilitated the formation of ORL. TEM/EELS studies of the NH-DLC counterface provided evidence for OH adsorption and passivation of carbon bonds at the surface that reduced the friction. It is anticipated that piston rings coated with NH-DLC run against CI liners would show low friction and wear during boundary lubricated sliding, an effect that could be enhanced when ethanol diluted oil is used.

Friction Behaviour of Multilayered Graphene against Steel

ABSTRACTFrictional behaviour of multilayered graphene was studied in air with different relative humidity (RH) levels (10–52% RH). Pin-on-disk type sliding tests were performed and the running-in and steady state coefficient of friction (COF) values were measured against M2 tool steel counterface. On increasing the RH, multilayered graphene showed a reduction in steady state COF from 0.11 at 10% RH to 0.08 at 52% RH. The low steady state COF values observed in graphene could be attributed to the transfer layer formed on the M2 tool steel counterface. A sliding-induced structural change was observed in graphene transfer layers which could have facilitated the graphitic transfer layer formation. The multilayered graphene showed a lower steady state COF values at all RH compared to non-hydrogenated diamond-like carbon (NH-DLC) which recorded a steady state COF of 0.47 at 10% RH and 0.25 at 52% RH.

Frictional properties of DLC films in low-pressure hydrogen conditions

The effects of hydrogen gas on the frictional behavior of diamond-like carbon (DLC) films were investigated by varying the hydrogen pressure during a friction test. Hydrogenated DLC (a-C:H) and non-hydrogenated DLC (ta-C) coatings have been tested. The ta-C/ta-C tribopair exhibited a linear relationship between the hydrogen pressure and the average friction coefficients. On the other hand, the a-C:H/a-C:H tribopair exhibited an extremely low friction coefficient of less than 0.01 when the hydrogen pressure reached 5000Pa. In addition, the friction coefficients from the tests at increased and decreased hydrogen pressures exhibited loop-like behavior. It is considered that the hydrogen pressure and the hydrogen content of the DLC film has an effect on the tribofilms and that the tribofilms played an important role in the loop-like frictional behavior. The evaluation of the hydrogen content of the DLC films was carried out by the G-peak method. The value of logarithm of (N/S) can be used as a measure of the hydrogen content, where the intensity of the fitted G-peak; S, the maximum intensity of the G-peak; and N, the intensity of the photoluminescence background. The behavior of logarithm of (N/S) from the Raman spectra for various hydrogen pressures indicated an interesting relationship between the average friction coefficient and the hydrogen content of tribofilm on a-C:H ball. The relationship showed that the average friction coefficient decreases as a linear relationship with the increase in hydrogen content. These results show that the hydrogen pressure and tribofilms also play an important role in the superlow friction of the DLC films.

Preparation and Characterization of Sulfurized Tungsten Doped Non-hydrogenated Diamond-Like Carbon Films

A tungsten doped non-hydrogenated diamond-like carbon (W-DLC) film was prepared by ion beam assisted deposition, and then W-DLC film was sulfurized by low temperature ion sulfurization. The structural analyses were performed on the scanning electron microscope, optical profilometer, Raman spectroscope and transmission electron microscope equipped with energy dispersive spectroscope. The results showed that the low temperature ion sulfurization treatment increase ID/IG ratio from 3.5 to 4.0 and 3.4 to 3.8 for sulfurized DLC and W-DLC films, respectively. The microstructures of sulfurized DLC film changed from amorphous to nanocrystalline/amorphous structure after low temperature ion sulfuration. The sulfurized W-DLC film was composed of nanocrystallites β-WC1−X, WS2 and FeS, which uniformly dispersed in the amorphous DLC matrix.

アルミニウム合金に対するＤＬＣコーテッド工具のドライ切削性能

The purpose of this study was to clear the conditions for good performance in dry cutting of aluminum alloys with non-hydrogenated diamond-like carbon (DLC) coated tool. The intermittent and continuous cutting tests were conducted under dry condition to 4 types of aluminum alloys. Consequently, it was found out that DLC coated tool showed good performance in intermittent cutting of A5052 and A7075 (Wrought aluminum alloys), but it didn't show good performance in intermittent cutting of AC2A and AC8A (Aluminum castings) and in continuous cutting of A5052 and AC2A. It was thought that the disappearance of DLC coating on the tool rake face caused adhesion in cutting of aluminum castings and continuous cutting of wrought aluminum alloys.

Sliding properties of Zr-DLC coatings: The effect of tribolayer formation

Amorphous carbon-based coatings (DLC) exhibit excellent mechanical and tribological properties such as high hardness, high elastic modulus, low friction and low wear. Reduced friction is often related to the formation of a low-friction tribolayer, which is formed during sliding and transferred to the counterpart. Here, we investigate the sliding of hydrogenated and non-hydrogenated DLC coatings alloyed with zirconium; pure DLC films are used as reference. The coatings were deposited by magnetron sputtering in Ar (non-hydrogenated) and Ar/methane atmosphere (hydrogenated) onto steel substrates and silicon wafers. The total thickness of the coatings was around 1.5μm including a complex Ti/TiN/TiCN adhesion-improving interlayer with a thickness of 450nm. All deposited coatings were amorphous, Zr/C ratio was approx. 0.05. The hardness was in the range of 9–13GPa. Tribological tests were carried out in humid air at room temperature, at 100°C and in nitrogen environment using pin-on-disk. Intermittent tribological test analysis has been performed to understand running-in behaviour. The worn surfaces and wear debris were analysed by Raman spectroscopy. Coatings alloyed with Zr showed lower friction and wear at room temperature compared to pure DLC. In general, Zr-doped hydrogenated coating outperformed the non-hydrogenated one when tested in an inert nitrogen atmosphere or at elevated temperature (100°C), exhibiting almost super-low friction (μ=0.03 in the steady-state regime) due to the formation of a homogenous, thick and stable tribolayer.

Tribological comparison of different C-based coatings in lubricated and unlubricated conditions

The use of carbon-based coatings (hydrogenated and non-hydrogenated DLC, doped and alloyed-DLC) is of wide interest due to its applications in mechanical components submitted to friction and wear including sliding parts in automotive engines. A tribological comparative analysis using a reciprocating (SRV) tester in lubricated and unlubricated conditions with a 4-stroke motor oil has been carried out on six currently relevant state-of-the-art coatings (namely WC/a-C, TiBC/a-C and TiC/a-C:H nanocomposites, Ti-doped DLC, BCN film and a crystalline monolithic TiC film as reference). The quantification of the fraction of the sp2-bonded matrix has been done by fitting of C 1s XPS peak and the mechanical properties evaluated by nanoindentation. The comparative analysis has allowed us to identify the capabilities of each system depending on the testing conditions and the possible synergies as a function of the chemical composition and film nature. Under lubricated harsh conditions (max. contact pressure 1.7GPa) only coatings displaying hardness superior to 20GPa could stand the sliding motion without failure. At lower contact pressures, a significant fraction of sp2 carbon (≥75%) is advantageous for reducing wear in boundary lubrication. WC/a-C, BCN and Ti-DLC films showed the best tribological response in dry sliding conditions. This fundamental information would be of relevance for assisting engineers in selecting best partnership for lubrication systems.

Tribological behavior and machining performance of non-hydrogenated diamond-like carbon coating tested against Ti–6Al–4V: Effect of surface passivation by ethanol

The role of an ethanol environment on the tribological behavior of non-hydrogenated DLC (NH-DLC) coatings sliding against Ti–6Al–4V was investigated. Pin-on-disk tests were performed under different test atmospheres consisting of dry argon (0% RH) and ambient air (40% RH) and also while the samples were submerged in water (H2O) and ethanol (C2H5OH). The running-in friction of the NH-DLC coatings under each condition was studied and the results were used to rationalize the drilling behavior of Ti–6Al–4V by NH-DLC coated tools. The highest running-in coefficient of friction (μR=0.55) was observed under a dry argon atmosphere. In ambient air, the μR was 0.42, while a reduction in μR to 0.21 was recorded when the tests were carried out in H2O. The observed reductions in μR were attributed to OH passivation of the NH-DLC surfaces. Ethanol was effective in providing OH passivation, as evident from the lowest μR of 0.15. Consistent with the friction measurements, a 45% reduction in drilling torque was obtained when 10% ethanol was blended with the metal removal fluid (MRF) consisting of water–oil (1:3) mixture compared to an ethanol free MRF.

The effect of oil temperature and additive concentration on the wear of non-hydrogenated DLC coating

Diamond-like carbon (DLC) coatings are amorphous hard carbon films which offer excellent mechanical and tribological properties. Recently, these coatings have been applied to engine and powertrain components in passenger cars, including those under boundary lubricated conditions. Tribological performance of DLC coatings in oil lubricated conditions and their interaction with various lubricant additives are very complex due to the variety of DLC coatings and lubricants. In this study, the effects of oil temperature and additive concentration on the wear and friction performance of non-hydrogenated tetrahedral amorphous carbon (ta-C DLC) coating have been investigated. Tribological tests were conducted using a pin-on-disc tribotester with ta-C DLC coated steel pins sliding against non-coated steel discs. Tribological tests were also carried out between DLC coated pin against germanium disc in base oil for clarification of wear mechanism. The ta-C coating gave very low boundary friction with base oil at 50°C but the coating experienced limited durability with increased wear rate at higher temperature against steel disc. Organic friction modifier glycerol-mono-oleate was able to preserve low friction behavior of the coating at higher temperature with enhanced durability. The anti-wear additive Zinc Dialkyldithiophosphate (ZnDTP) provided excellent wear protection for the ta-C coating due to the formation of thick pad-like tribofilm on steel counterpart at all temperature ranges but rubbing in ZnDTP additivated oil lead to higher friction coefficient. The results obtained at high temperature show the significant and beneficial influence of oil additives on the wear performance of the coating. Based on the obtained results and analysis, the wear performance of non-hydrogenated DLC was found to have a clear dependence on the concentration of the lubricant additives, test temperature and counter material.