Doped Diamond-like Carbon Coatings Research Articles

Doping effects on the tribological performance of diamond-like carbon coatings: A review

Diamond-like carbon (DLC) coatings have garnered considerable interest due to their unique features, which include wear resistance with a low friction coefficient, high hardness, biocompatibility, and chemical inertness. Their tribological performance is highly dependent on tribological environment, structure, and mechanical properties of the coating. A major problem that can adversely impact the tribological properties of the DLC coating is insufficient adhesion to the substrate. This issue caused by mismatch of the substrate and coating thermal expansion coefficient (TEC) and high amount of internal stress in DLC coating. The limitations of DLC coatings for tribological performance can be overcome by incorporating different types of metallic and non-metallic elements into the coating structure. Recently, various efforts have been made to address this issue by doping certain elements into the DLC coating, including fluorine, nitrogen, silicon, and metals. This review discusses new advancements in doped DLC coatings designed to improve their tribological behavior in different specific applications such as high relative humidity, elevated temperature, and bio-implant.

Tribological Behavior of Doped DLC Coatings in the Presence of Ionic Liquid Additive under Different Lubrication Regimes

Diamond-like carbon (DLC) coatings are widely used in industries that require high durability and wear resistance, and low friction. The unique characteristics of DLC coatings allow for the possibility of creating adsorption sites for lubricant additives through the doping process. In this study, the combined use of europium-doped diamond-like carbon (Eu-DLC), gadolinium-doped diamond-like carbon (Gd-DLC), and pure DLC coatings and an ionic liquid (IL) additive, namely, trihexyltetradecylphosphonium bis (2-ethylhexyl) phosphate [P66614] [DEHP], with a 1 wt.% concentration in polyalphaolefin (PAO) 8 as a base lubricant was investigated. Higher hardness, higher thin-film adhesion, a higher ratio of hardness to elastic modulus, and a higher plastic deformation resistance factor were achieved with the Gd-DLC coating. The CoF of the Gd-DLC coating paired with the IL was superior compared to the other pairs in all lubrication regimes, and the pure DLC coating had a better performance than the Eu-DLC coating. The wear could not be quantified due to the low wear on the surface of the DLC coatings. The friction reduction demonstrates that tribological systems combining Gd-DLC thin films with an IL can be a potential candidate for future research and development efforts to reduce friction and increase the efficiency of moving parts in internal combustion engines, for instance.

TCAD study of DLC coatings for large-area high-power diodes

The most relevant transport features of doped diamond-like carbon (DLC) films have been implemented in a TCAD setup to provide a theoretical tool to assess the reliability expectations for high-voltage device passivation. Starting from the band structure and boundary conditions of a metal-insulator-semiconductor (MIS) device, trap states in the bandgap have been used to determine the characteristics of differently doped DLC layers against experiments. The role of the DLC as a passivation layer on top of the bevel termination of a high-voltage diode has been studied and compared with experiments. The breakdown voltage is significantly influenced by the properties of the DLC as clearly explained by the TCAD simulation results.

Silver doped diamond-like carbon antibacterial and corrosion resistance coatings on titanium

The present work is focused on the micro-structural, antibacterial potential and corrosion resistance of silver (Ag) doped diamond-like carbon (DLC) coatings deposited by Thermionic Vacuum Arc on titanium substrates. The Ag-DLC coatings have spherical nanoparticulates in the film, with a size of either ≈50 nm or ≈20 nm, depending on the deposition time. The incorporation of Ag in the DLC coatings was clearly evident, as observed in both energy dispersive X-ray spectroscopy and in structural analysis (X-ray diffraction), while X-ray photoelectron spectroscopy confirmed their composition as well as the presence of Ag and diamond-like carbon. Such coatings show an increase in the wettability, which can be related to the increase in the surface free energy. Comparing their corrosion resistance to bare (uncoated) titanium, an increased electrochemical stability is observed. Subsequently, the silver release profiles in phosphate buffer saline, for a 10 days period, were investigated; the release is initially fast in the first 12 h followed by saturation at 66 h. These coatings were further investigated for antibacterial efficacy against Staphylococcus aureus (model Gram positive bacteria) which confirms that the silver doped DLC coatings exhibit an excellent antibacterial activity.

Diamond like carbon coatings doped by Si fabricated by a multi-target DC-RF magnetron sputtering method - Mechanical properties, chemical analysis and biological evaluation

Doped diamond-like carbon coatings (DLC) are a promising solution for improvement of the performance of medical implants. Among various additives that can alter either mechanical or biological properties of DLC, silicon is a possible choice.This work focuses on the improvement of mechanical and biological properties by silicon incorporation into a DLC matrix. The successful deposition of Si-DLC coatings was demonstrated by FTIR spectra revealing the presence of Si-C, Si-O-Si and Si-Mex bonds (where Me – methyl group). The amount of dopant ranged from ∼ 1.2 at.% to∼ ∼ 21.9 at.%. Increasing the Si content resulted in the increase of hydrophilic character of the doped DLC coatings. Incorporation of silicon also increased the films' hardness, by 40% in the case of the most highly-doped coating. The biological evaluation with use of human osteosarcoma cells (Saos-2) revealed the possibility of tailoring the proliferation of mammalian cells by varying the amount of silicon dopant in such coatings. All of the examined coatings reduced the microbial colonisation of E. coli bacteria, compared to uncoated substrate.The most highly-doped coating was shown to be the most promising material among the examined samples for biomedical use, due to exhibiting highest hardness and critical force as well as low number of adhered bacteria.

Influence of zinc dialkyldithiophosphate tribofilm formation on the tribological performance of self-mated diamond-like carbon contacts under boundary lubrication

Diamond-like carbon (DLC) coatings offer excellent mechanical and tribological properties that make them suitable protective coatings for various industrial applications. In recent years, several engine and power train components in passenger cars, which work under boundary lubricated conditions, have been coated with DLC coatings. Since conventional lubricants and lubricant additives are formulated for metal surfaces, there are still controversial questions concerning chemical reactivity between DLC surfaces and common lubricant additives owing to the chemical inertness of DLC coatings. In this work, we present the influence of zinc dialkyldithiophosphate (ZnDTP) anti-wear additives on the tribological performance of various self-mated DLC coatings under boundary lubrication conditions. The effects of hydrogen, doping elements, and surface morphology on the reactivity of DLC coatings to form a ZnDTP-derived tribofilm were investigated by atomic force microscopy, field emission scanning electron microscopy and X-ray photoelectron spectroscopy. The results confirmed that ZnDTP-derived pad-like or patchy tribofilm forms on the surfaces depending on the DLC coating. It is seen that hydrogen content and doping elements increase pad-like tribofilm formation. Doped DLC coatings are found to give better wear resistance than non-doped DLC coatings. Furthermore, the addition of ZnDTP additives to the base oil significantly improves the wear resistance of hydrogenated DLC, silicon-doped hydrogenated DLC, and chromium-doped hydrogenated DLC. Hydrogen-free tetrahedral amorphous DLC coatings provide the lowest friction coefficient both in PAO (poly-alpha-olefin) and PAO+ZnDTP oils.

Anti-bacterial property of Si and F doped diamond-like carbon coatings

In this study, the anti-bacterial performance of modified diamond-like carbon (DLC) coatings with Si and F was investigated and compared with standard DLC coatings and stainless steel using Klebsiella pneumoniae as test bacterium. The experimental results indicated that the F and Si doped DLC coatings reduced the surface energy and improved anti-bacterial ability of DLC coatings. The extended DLVO theory was used to calculate the interaction energy between the coatings and the bacterium. The anti-bacterial mechanism was explained using the extended theory.

Fouling behaviour of milk and whey protein isolate solution on doped diamond-like carbon modified surfaces

Doped diamond-like carbon (DLC) coated stainless steel surfaces were studied for their fouling behaviour with milk and whey protein isolate (WPI) solution at both laboratory and pilot-scale. Stainless steel (316 SS 2B) surfaces were modified with three different doped DLC coatings (DLC-1, DLC-2 and DLC-3) using plasma assisted chemical vapour deposition. At the laboratory-scale, the DLC-1 coated surface showed a statistically significant reduction in the mass of milk fouling deposits. However, at the pilot-scale, none of the modified surfaces offered significant benefit in fouling mitigation over the control stainless steel surface. Subsequently, in the laboratory-scale trials with a whey protein isolate (WPI) solution, the mass of fouling deposits for all doped DLC modified surfaces were about 35–40% lower in comparison to their controls when fouled for 120min. However, these benefits were reduced to less than 15% with longer fouling duration. Thus, the results obtained in this study found no commercial benefit of modified surfaces in fouling mitigation when fouled for longer times with skim milk, whole milk or WPI solution.

Anti-sticking behavior of DLC-coated silicon micro-molds

Pure carbon- (C), nitrogen- (N) and titanium- (Ti) doped diamond-like carbon (DLC) coatings were deposited on silicon (Si) micro-molds by dc magnetron sputtering deposition to improve the tribological performance of the micro-molds. The coated and uncoated Si molds were used in injection molding for the fabrication of secondary metal-molds, which were used for the replication of micro-fluidic devices. The bonding structure, surface roughness, surface energy, critical load and friction coefficient of the DLC coatings were characterized with micro-Raman spectroscopy, atomic force microscopy (AFM), contact angle, microscratch and ball-on-disc sliding wear tests, respectively. It was observed that the doping conditions had significant effects on Raman peak positions, mechanical and tribological properties of the coatings. The G peak shifted toward a lower position with N and Ti doping. The DLC coating deposited with 1 sccm N2 flow rate showed the lowest G peak position and the smoothest surface. The surface energies of the pure carbon and Ti-doped DLC coatings were lower than that of the N-doped DLC, which was more significant at a higher N2 flow rate. In terms of adhesion and friction coefficient, it was observed that the Ti-doped DLC coating had the best performance. Ti incorporated in the DLC coating decreased the residual stress of the coating, which improved the adhesive strength of the coating with the Si substrate.

Characterisation of Transfer Layer and Wear Debris on Various Counterparts Sliding Against Undoped and Doped DLC Coatings

Tribological behaviour of undoped DLC (diamond-like carbon) and titanium-doped DLC coatings, referred here as TiC:H, were investigated. Tribological properties of frictional contacts with DLC coating significantly depend on the type of counterpart and generally are conditioned on the processes of abrasive wear, and the oxidation of counterpart and coating, and to smaller extent on the process of coating graphitisation. On the other hand, tribological properties of frictional contacts with TiC:H coating are mainly conditioned on the processes of graphitisation and coating oxidation, which already at the initial stage leads to the formation of a stable transfer layer consisting mainly of graphite-like carbon and titanium oxides, i.e. products of low shear strength playing the part of a solid lubricant.

Tribological performance of titanium doped and pure DLC coatings combined with a synthetic bio-lubricant

Thermal degradation of environmentally friendly lubricants prevents the spread of its utilisation in industrial applications. This process can be promoted by frictional heating occurred during accidental contacts of moving parts or star-up and shut-down operations. The use of low friction coatings, like diamond-like carbon (DLC), can offer a solution to these problems. Their low friction properties, high wear resistance and excellent corrosion resistance can prevent the occurrence of such local heat spikes, which will protect the lubricant and hence prolong the lifetime of the tribological system. In this work, a synthetic bio-lubricant has been evaluated and compared with a mineral oil. Combinations with pure and Ti doped DLC coatings were taken into account. In order to have a proper evaluation of the tribosystem a wide range of conditions have been considered in high frequency reciprocating and unidirectional tests. The Stribeck curve at variable sliding speeds and loads was obtained. In steel/steel contacts friction is clearly lower when synthetic oil is used compared to a mineral based oil, which is not always true with DLC/DLC contacts. As a result of the tribotesting, the best combination of materials was chosen in order to be validated in a real system (mechanical component in a machine tool), where results confirmed our expectations.

Tribological reactions between oil additives and DLC coatings for automotive applications

In the past, the development of modern engines and transmissions would have been impossible without advanced lubricant additive chemistry and proper lubricant formulation. Introduction of diamond and diamond-like carbon (DLC) coatings, especially metal doped DLC coatings, opens further possibilities in improving performance of engine and transmission components, which cannot longer be achieved only by lubricant design. However, the lack of knowledge on tribological behaviour of DLC coatings when lubricated with oils originally designed for metallic surfaces limit their use in practical applications. The results of the present investigation clearly show the existence of tribochemical reaction between metal doped DLC coatings (W-DLC) and S-based EP additives, while for a-C:H coatings additives had no influence. Furthermore, tribological behaviour of W-DLC coatings was found to be EP additive concentration and contact temperature dependant.

Preparation and mechanical properties of composite diamond-like carbon thin films

We have investigated mechanical properties of diamond-like carbon (DLC) thin films, particularly the internal compressive stress and ways to alleviate it. Foreign atoms such as copper, titanium, and silicon were incorporated into the DLC films during pulsed laser deposition. The chemical composition of the doped films was determined using Rutherford backscattering spectrometry (RBS) and x-ray photoelectron spectroscopy (XPS). Optical microscopy of the doped films showed that DLC films containing Cu exhibit much less particulate density as compared to the films containing Ti and Si. Visible Raman spectroscopy was used to characterize the films. The effect of dopants on the Raman spectrum was analyzed in terms of peak shape and position. Optical microscopy of the pure DLC of a certain thickness showed severe buckling. The mechanisms of adhesion associated with DLC coatings were discussed. Qualitative scratch tests on the specimens showed that pure DLC films have relatively poor adhesion due to a large compressive stress, while the doped DLC films exhibit much improved adhesion. Wear tests show improved wear resistance in the doped DLC coatings. Nanoindentation results suggest that pure DLC has an average hardness above 40 GPa and effective Young’s modulus above 200 GPa. The doped DLC films showed slightly decreased hardness and Young’s modulus as compared to pure DLC films. These results can be rationalized by analyzing the internal stress reduction as derived from Raman G-peak shift to lower wavenumbers. A preliminary interpretation of the stress reduction mechanism is discussed.

Mechanical properties of diamond-like carbon composite thin films prepared by pulsed laser deposition

We have investigated the mechanical properties of diamond-like carbon (DLC) thin films that contain foreign atoms. The DLC films were prepared by pulsed laser deposition. A novel target design was adopted to incorporate foreign atoms into the DLC films during film deposition. Copper, titanium and silicon are chosen as the dopants. The chemical composition of the doped films was determined using Rutherford backscattering spectrometry, X-ray photoelectron spectroscopy and calibrated extrapolation. Experimental results of both visible and UV Raman are presented and discussed in terms of peak shape and position. The effect of dopants on the Raman spectrum is also analyzed. Optical microscopy of the pure DLC of a certain thickness showed severe buckling. A brief review of the theoretical background of adhesion is given and the possible mechanisms of adhesion that may work in DLC coatings are discussed. Qualitative scratch tests on the specimens show that pure DLC has quite poor adhesion due to the large compressive stress, while the doped DLC films exhibit much improved adhesion. Wear tests show improved wear resistance in the doped DLC coatings. Nanoindentation results give an average hardness above 40 GPa and effective Young's modulus above 200 GPa for pure DLC. The copper doped DLC films showed slightly decreased hardness and Young's modulus as compared to pure DLC films. Ti and Si can reduce the hardness and Young's modulus more than Cu. All these can be understood by analyzing the internal stress reduction as derived from Raman G-peak shift to lower wavenumbers. A preliminary model of the stress reduction mechanism is discussed.

Dry machining and increase of endurance of machine parts with improved doped DLC coatings on steel, ceramics and aluminium

In order to machine without cooling lubricants, hard coatings having very low dry friction values are necessary as layers on the surfaces of machining tools. They keep friction temperatures low at the very locus where they come into existence so that less heat than normal has to be dissipated. Diamond-like carbon (DLC) coatings serve this purpose best. After a short description of the plasma chemical vapour deposition process, the properties of doped DLC are given followed by three examples of applications concerning machining tools and sliding bearings for chemical pumps.

Ion beam assisted deposition of diamond-like nanocomposite films in an acetylene atmosphere

In recent years, doped diamond-like carbon (DLC) coatings have been shown to solve some intrinsic application difficulties of DLC, esp. reducing internal stresses and improving thermal stability. Diamond-like nanocomposite (DLN), by incorporating an amorphous Si-O network into the DLC, is a special class of modified DLC coatings, which have many excellent properties and wide applications. In this paper, a diamond-like nanocomposite film was synthesized by an ion beam assisted deposition method in an acetylene atmosphere. The composition and microstructure of the DLN film were investigated by various spectroscopic analyses, including Fourier transform infrared absorption spectroscopy, Rutherford backscattering spectroscopy. Raman scattering spectroscopy, X-ray photoelectron spectroscopy, and ultraviolet-visible absorption spectroscopy.

Tribological performance and tribochemistry of nanocrystalline WC/amorphous diamond-like carbon composites

The tribological performance and tribochemistry of vacuum deposited nanocrystalline WC/amorphous diamond-like carbon (DLC) composite coatings were investigated. Coating chemistry was varied by regulation of carbon content between 30 and 90at.%. Deposition temperature was used to adjust the degree of β-WC crystallinity from near amorphous with 1–3nm sized grains to nanocrystalline with 5–10nm sized grains, which were embedded in unhydrogenated DLC matrix. The microstructure critically influenced both the hardness and tribological properties of WC/DLC composites. Coatings with poorly crystallized WC were softer (15GPa) and exhibited lower friction and higher wear rates in comparison to harder (26GPa) composites with larger WC nanocrystals. The hardness of WC/DLC composites was at least twice as high as the hardness of metal doped DLC coatings reported in the literature. Micro Raman and X-ray photoelectron analysis was performed on transfer films formed on the surface of steel balls in sliding against WC/DLC composites in humid air and dry nitrogen, which found both oxides and graphite carbon, providing self-lubrication. The high hardness and self-lubricating properties of WC composites resulted in low wear rates and friction coefficients of about 0.2. Relationships among coating chemistry, structure, hardness, friction, wear rates, as well as, tribofilm composition and structure are discussed.