Tetrahedral Amorphous Carbon Coatings Research Articles

高温環境下における DLC 膜の摩擦特性に及ぼす移着膜と膜構造の影響

Diamond-like carbon (DLC) coatings has found widespread application as a protective coating in many fields because its high hardness, low friction, chemical inertness and other properties. It was reported that tetrahedral amorphous carbon (ta-C) coatings showed lower friction at high temperature than at ordinary temperature. But the mechanism of ta-C coatings' lower friction at high temperature is not clarified. In this study, we clarified the effect of transfer layer and film structure on friction properties at high temperature. We conducted friction test at five temperature environment (23 ~ 400 ℃) and analyzed transfer layer. From the result, friction coefficient of ta-C coatings decrease with test temperature increase and friction coefficient of ta-C coatings become same value at over 200 ℃. And friction coefficient decrease with thickness of graphitized transfer layer calculated by atomic force microscope (AFM) become thicker. We also deposited three kind of sp3 bonding ratio ta-C coatings by changing bias voltage in T-shaped filtered arc deposition (T-FAD) method. And we conducted friction test at 200 ℃. From the result, friction coefficient of fewer sp3 bonding ratio decreased.

Enhanced Diamond-Like Coatings Boost Fuel Efficiency

Abstract A new laser arc method is being used to apply a hard carbon coating to engine components on the production line. This article reports on work to produce hydrogen-free tetrahedral amorphous carbon coatings on an industrial scale with consistent quality.

Dependence between friction of laser interference patterned carbon and the thin film morphology

Tetrahedral amorphous carbon coatings exhibit excellent tribological properties in terms of friction (~0.1) and wear (~10−9mm3/N/m). In this work, the dependence between micro structural changes due to laser structuring and the tribological properties of ta-C are discussed. The laser structuring is made by using a holographic technique called direct laser interference patterning (DLIP). Within this technique an 8ns pulsed UV-laser (wavelength 355nm) is used, to produce cross-like patterns with structural periods ranging from 2μm to 10μm. The influence of the patterns on the frictional behavior is investigated under linear reciprocating sliding conditions with ball on disk method and non-lubricated conditions. It is found that depending on the pattern period the friction is either increased or reduced compared to an unpatterned reference sample. The decrease of the friction coefficient is explained by a reduction of surface contact area and a high hardness of the non-ablated ta-C films. However, the increased friction results from thermally induced changes in the morphology of the ta-C film. This assumption is substantiated by thermal simulation of the DLIP process. Additionally the frictional properties of DLIP processed ta-C- and steel surfaces vs. steel probes with and without a ta-C coating are compared.

Fabrication and characterization of DLC coated microdimples on hip prosthesis heads.

Diamond like carbon (DLC) is applied as a thin film onto substrates to obtain desired surface properties such as increased hardness and corrosion resistance, and decreased friction and wear rate. Microdimple is an advanced surface modification technique enhancing the tribological performance. In this study, DLC coated microdimples were fabricated on hip prosthesis heads and their mechanical, material and surface properties were characterized. An Electro discharge machining (EDM) oriented microdrilling was utilized to fabricate a defined microdimple array (diameter of 300 µm, depth of 70 µm, and pitch of 900 µm) on stainless steel (SS) hip prosthesis heads. The dimpled surfaces were then coated by hydrogenated amorphous carbon (a-C:H) and tetrahedral amorphous carbon (Ta-C) layers by using a magnetron sputtering technology. A preliminary tribology test was conducted on these fabricated surfaces against a ceramic ball in simulated hip joint conditions. It was found that the fabricated dimples were perpendicular to the spherical surfaces and no cutting-tools wear debris was detected inside the individual dimples. The a-C:H and Ta-C coatings increased the hardness at both the dimple edges and the nondimpled region. The tribology test showed a significant reduction in friction coefficient for coated surfaces regardless of microdimple arrays: the lowest friction coefficient was found for the a-C:H samples (µ = 0.084), followed by Ta-C (µ = 0.119), as compared to the SS surface (µ = 0.248).

Tetrahedral Amorphous Carbon Coatings for Friction Reduction of the Valve Train in Internal Combustion Engines

Tetrahedral amorphous carbon (ta‐C) is studied as a tribological coating for the valve train's exhaust camshaft of a combustion engine. The coated camshafts were installed in a non‐fired engine, tested in a computerized component test bench under practice‐relevant conditions and analyzed for their frictional behavior. A notable reduction of the valve train's drive torque on the test bench is demonstrated. Namely, on a roller cam system with ta‐C‐coated camshaft the reduction is about 15% in average within the entire engine‐map. The ta‐C coatings were extensively characterized under laboratory conditions before and after the investigations on the test bench. Mechanistic understanding of the tribological behavior of ta‐C coatings under dry or starving lubricated conditions was achieved by atomistic simulations of the tribological contact. Industrial utilization of these results would lead to a significant increase of the energy efficiency of combustion engines.

Low temperature and large area deposition of nanocrystalline diamond films with distributed antenna array microwave-plasma reactor

Abstract Diamond films grown at low temperature ( 4 concentration in the total gas mixture of H 2 /CH 4 /CO 2 on the morphology and growth rate of the NCD films is reported. The total gas pressure is found to be a critical deposition parameter for which growth rates and crystalline quality both increasing with decreasing the pressure. Under optimized conditions, the process enables deposition of uniform (~ 10%) and high purity NCD films with very low surface roughness (5–10 nm), grain size of 10 to 20 nm at growth rates close to 40 nm/h. Nanotribology tests result in the friction coefficient of the NCD films close to that obtained for the standard tetrahedral amorphous carbon coatings (ta-C) indicating the suitability of this low-temperature diamond coating for mechanical applications such as bearing or micro-tools.

Tribological behavior of tetrahedral amorphous carbon (ta-C) coatings at elevated temperatures

In this study, tribological behavior of tetrahedral amorphous carbon (ta-C) coatings was investigated at elevated temperatures, from which it was found that it could be broadly defined by three distinct temperature regions: Region I at 23 and 100°C; Region II at 200, 300, and 400°C; and Region III at 500°C. A qualitative analysis of the toughness and adhesion of the ta-C coatings within each of these regions was conducted by scratch testing. The formation of a transfer layer on Si3N4 balls in Region II was determined as the cause of a reduced friction and higher wear rate when compared with Region I. In Region III, the generation and propagation of cracks in the ta-C coating resulted in severe delamination.

New application perspective for tetrahedral amorphous carbon coatings

Presently various carbon-based materials are used to achieve different surface functions and coatings for anti-corrosion and electrochemical application. These are in the form of homogeneous composite materials and chemically modified polymers, with many patent pending in this field. However, association of performing anticorrosion, adherence crosslinking and hydrophobic solutions with other surface functions are more often required, including additional simultaneous tribologic, antiwear, electrochemical, optical and optoelectronic properties. Among the wide range of diamond-like and related coating materials, tetrahedral amorphous carbon (ta-C), combines many interesting, simultaneous and superior material properties compared with first generation hard carbon coatings. Especially when ta-C can be modified and adapted for its properties to a specific application by doping with multilayer and gradient configuration, thus offering many new and interesting perspectives of application. We review and discuss common coatings, by comparison and deduction. The corresponding updated and recently developed fundamentals are also considered to show the potential for better performing and up-scaled anti-corrosion and electrochemical solutions.

Direct Laser Interference Patterning of tetrahedral amorphous carbon films for tribological applications

In this work, the influence of surface topography and micro structural changes on the tribological properties of tetrahedral amorphous carbon coatings (ta-C) structured using a holographic technique the direct laser interference pattering (DLIP) is investigated. By utilizing a nanosecond pulsed UV-laser (wavelength 355nm), both ablation and graphitization thresholds were determined as a function of the pulse number. Incubation effects for the ablation threshold (~205mJcm−2) were found to be negligible. However, for the graphitization of the film thresholds varying from 47 to 74mJcm−2 were observed depending on the number of laser pulses utilized (from 1 to 30) and thus obtaining an incubation factor of 1.13. Using two- and three-beam interference setups, dot- and line-like periodic arrays were fabricated. The tribological performance of these patterns was investigated under reciprocating sliding with a ball on disk method under non-lubricated conditions showing that coefficient of friction can be reduced from~0.089 (un-patterned) to~0.055 patterned ta-C (~30% reduction). The results can be explained based on the reduction of surface contact area combined with high hardness as well as the good intrinsic tribological properties of the ta-C films.

Gas permeability reduction in PEEK film: Comparison of tetrahedral amorphous carbon and titanium nanofilm coatings

The reduction of helium gas permeability through polyetheretherketone (PEEK) microfilm (250 μm) coated with titanium (Ti) or tetrahedral amorphous carbon (ta-C) nanofilm was investigated. Two morphologies of PEEK film were used (amorphous and semi-crystalline), and four thicknesses of nanofilm were deposited (5, 20, 50, or 100 nm). Both types of nanofilm coatings reduced the overall permeability of the PEEK film, with the greatest permeability reduction of ∼83% seen for the 100 nm Ti coating on semi-crystalline PEEK. Defects were identified in both types of coating; however the nature of the defects was different. The ta-C coatings exhibited a network of microcracks, while pinholes were present in the Ti coatings. The defects in both coatings were considered the dominant path for gas flow. Overall, Ti performed better than ta-C with regard to permeability reduction, and Ti also demonstrated a correlation between coating thickness and permeability reduction.

Influence of counterbody material on wear of ta-C coatings under fretting conditions at elevated temperatures

The high temperature tribological performance of tetrahedral amorphous carbon coatings has been analyzed at elevated temperatures up to 250 °C in air against three different counterbody materials—steel 100Cr6, α-alumina and silicon nitride. The results show that the counterbody material influences the friction and wear behavior and therefore coating life time strongly. This effect is well known for these coatings at room temperature under dry environmental conditions, equivalent to conditions above 100 °C when water molecules desorb from the surface. However, the sharp difference in tribological performance between silicon nitride on the one hand and alumina and steel on the other hand cannot be understood in this context. Analyzing the friction behavior during the running-in phase, it is evident that only alumina and steel form a stable interface with constant low friction and relatively low wear rates. Silicon nitride forms an unstable interface with fluctuating COF and relatively high wear rates due to its own inherent tendency to tribo-oxidation.

Field emission properties from aligned carbon nanotube films with tetrahedral amorphous carbon coatings

Tetrahedral amorphous carbon (ta-C) film was coated on aligned carbon nanotube (CNT) films via filtered cathodic vacuum arc (FCVA) technique. Field electron emission properties of the CNT films and the ta-C/CNT films were measured in an ultra high vacuum system. The I– V measurements show that, with a thin ta-C film coating, the threshold electric field ( E thr) of CNTs can be significantly decreased from 5.74 V/μm to 2.94 V/μm, while thick ta-C film coating increased the E thr of CNTs to around 8.20 V/μm. In addition, the field emission current density of CNT films reached 14.9 mA/cm 2 at 6 V/μm, while for CNTs film coated with thin ta-C film only 3.1 V/μm of applied electric field is required to reach equal amount of current density. It is suggested that different field emission mechanisms should be responsible for the distinction in field emission features of CNT films with different thickness of ta-C coating.

Filtered Cathodic Vacuum Arc Process Conditions and Properties of Thin Tetrahedral Amorphous Carbon Films

The properties of thin tetrahedral amorphous carbon films were evaluated for various deposition conditions using a filtered cathodic vacuum arc process. The most marked change observed in the properties of the films caused by varying the arc current and substrate bias was the change in the compressive stress of the films. It was found that C+ ion energy values ranging from 55 to 75 eV are optimum for obtaining very hard tetrahedral amorphous carbon coatings.

Surface roughness, mechanical and tribological properties of ultrathin tetrahedral amorphous carbon coatings from atomic force measurements

Atomic force microscope (AFM), lateral force microscope and AFM-based scratch and wear testing techniques were used to evaluate and compare the surface roughness, tribological and mechanical properties of thin (2.7–43 nm) tetrahedral amorphous carbon coatings prepared by pulsed cathodic arc discharge. It was found that surface roughness of ultrathin (2–8 nm) coatings was mainly determined by the roughness of the Si substrate and their average density strongly depended on their thickness. Poor friction, mechanical properties of thinner (2.7–15 nm) coatings can be associated with their low average density. The dense coatings (>15 nm) had lower friction coefficient, better scratch and wear resistance properties that were independent of their thickness. It appears that the over 15-nm coatings studied are feasible for some wear-resistant and tribological applications.

Protection of industrial sensors with ta-C

Tetrahedral amorphous carbon (ta-C) coatings are a very promising approach in order to develop chemically and mechanically resistant protective layers for industrial and medical applications. For this purpose our group has developed thick (50–200 μm), high-quality (sp 3-fraction ∼80%) and hydrogen-free protective ta-C coatings deposited (5 μm/h/20 cm 2) with filtered pulsed arc discharge (FPAD) method. Earlier ta-C coatings have given excellent results in aluminium lathing. For this work ta-C films were tested on paper thickness, density and porosity probes, paper paste viscometer nozzles and support pieces of a water-jet cutter. Tests were performed both in laboratory and industrial environments. In the paper mill the paper sensors and the cutter support pieces were in constant contact with the high-speed (up to 100 km/h) travelling paper web. The ta-C coating reduced significantly the erosion of the sensors and the resin accumulation on the sensor contact surfaces. The viscometer nozzle pieces were tested under pressures that no other material has withstood. In particle erosion tests ta-C coating outperformed WC-Co hard metal, sintered SiC, sintered Al 2O 3, synthetic ruby and TiN coatings (Diamond Rel. Mater. 9 (2000) 1524).

Deposition and properties of tetrahedral amorphous carbon films prepared on magnetic hard disks

The areal density of information stored on the hard disk has doubled every two years. This substantial increase in disk storage has resulted from the application of giant magnetoresistance heads, new thin film media, and better electronic recording channels. However, such an increase cannot be easily attained without reducing the separation between the magnetic read-write head and magnetic recording medium surfaces. This can be achieved by using a thinner protective overcoat. In this study, ultrathin tetrahedral amorphous carbon (ta-C) films were deposited on magnetic hard disks (CoCrTa/Cr/NiP/Al–Mg) by a magnetic filtered cathodic arc deposition under different substrate bias voltages. The obtained films exhibited smoother surfaces than the uncoated disks as indicated by the atomic force microscopic measurement. The Raman spectra acquired showed a single asymmetric Lorentzian curve shape. Tetrahedral amorphous carbon coatings were subjected to an accelerated corrosion test in vapors of concentrated hydrochloric acid for 24 h. The corrosion test demonstrated a reduced density of corrosion sites when compared to conventional diamond-like carbon films. Similarly, the scratch resistance of the ta-C coated disks, investigated by a nanoindenter, showed significant improvement in comparison to uncoated disks.

Preparation Of Superhard Functionally Graded Tetrahedral Amorphous Carbon Coatings By Pulsed Laser Deposition

AbstractThe internal compressive stress as large as 10 GPa has been the major stumbling block for preparation of relatively thick superhard tetrahedral amorphous carbon (Ta-C) films. We have successfully deposited Ta-C films as thick as 1000 nm by mechanical doping to reduce and alleviate the level of internal compressive stresses. In this paper, we reported the preparation of functionally graded Ta-C coatings by pulsed laser deposition. The thickness of the Ta-C films with significantly improved adhesion was measured to be up to 1500 nm. The concentration of foreign atoms such as silver, copper, silicon and titanium was decreased away from the coating/substrate interface, and the surface layer was pure Ta-C. Nanoindentation measurements were performed on the coatings. Nanohardness as high as 65 GPa and Young‘s modulus as large as 600 GPa were obtained for the functionally graded Ta-C films. Micro-Raman measurements and microstructural analysis by transmission electron microscopy was carried out to acquire information about the bonding environment and atomic structure of the coatings as a function of foreign atoms. The results were discussed in combination with theoretical models associated with the prediction of elastic properties of amorphous carbon networks.

Load-carrying capacity evaluation of coating/substrate systems for hydrogen-free and hydrogenated diamond-like carbon films

Diamond-like carbon (DLC) coatings have shown excellent tribological properties in laboratory tests. The coatings have also been introduced to several practical applications. However, the functional reliability of the coatings is often weakened by adhesion and load-carrying capacity related problems. In this study the load-carrying capacity of the coating/substrate system has been evaluated. The DLC coatings were deposited on stainless steel, alumina and cemented carbide with two different deposition techniques: the tetrahedral amorphous carbon (ta-C) coatings were deposited by a pulsed vacuum arc discharge deposition method and the hydrogenated carbon (a-C:H) films by radio frequency (r.f.) plasma deposition method. The load-carrying capacity of the coated systems was evaluated using a scratch test, Rockwell C-indentation test and ball-on-disc test. The effect of substrate material, substrate hardness, coating type and coating thickness was studied. An increase in substrate hardness increased the load-carrying capacity for the coated systems, as expected. The two coating types exhibited different performance under load due to their different physical and mechanical properties. For the load-carrying capacity evaluations the ball-on-disc configuration was found to be most suitable.

Tetrahedral amorphous carbon (ta-C) coatings for processing tools to reduce contamination in the analysis of trace elements

The possibility of reducing contamination in trace element analysis by coating processing tools with tetrahedral amorphous carbon (ta-C) coating has been studied. The 0.5 μm thick ta-C coating was deposited on pressing tool pellets made of Mo, W and WC-Co hard metal using the plasma accelerator method. The method was optimised to get high adhesion and high sp3 fraction (approx. 80%) to ensure minimal wear. The contamination of sugar test samples pressed with uncoated pellets was clearly detected with particle induced X-ray emission method (PIXE) for all the pellet materials used. In the samples pressed with ta-C coated pellets contamination from the pellets was below the detection limit, i.e. < 0.1–1 ppm. Furthermore, ta-C coatings withstood the pressing and normal cleaning procedures without any delamination or other kind of damage. The results obtained should be applicable to other processing tools as well.

Electron Emission Through Tetrahedral Amorphous Carbon Coatings on Mo and Si Emitters

ABSTRACTThe field emission properties of molybdenum and silicon emitter coated with tetrahedral amorphous carbon (or amorphous diamond) were studied. The tetrahedral amorphous carbon was deposited by laser ablation and showed a uniformly coated columnar structure over the entire emitter. In general, current conditioning improved stability and increased the current density. Coatings of ta-C on Mo emitters with and without nitrogen incorporated both yielded significantly higher emissivity than uncoated emitters. Nitrogen incorporation reduced the effective workfunction and the sp3/sp2 ratio. However similar depositions on Si emitters reduced the emissivity, and may be attributable to the residual oxide at the ta-C/Si interface. Annealing in a hydrogen atmosphere enhanced emissivity from both ta-C/Mo and ta-C/Si emitters. In general, thick coatings lowered the emissivity and the slope of the I-V curves. A temperature dependence of emission was observed only in the low field regions.