Diamond-like Carbon Coatings Research Articles

Developments and Applications of Diamond-Like Carbon

Carbonous material always promotes the development of human being’s civilization. The diamond-like carbon (DLC) is one of the new kinds of materials. In the past fifty years, DLC has reached the level of semi-industrialization. From the unintentional discovery of DLC to the booming study all over the world, and then to the wide use in various fields, DLC has become a symbol of the era. In order to study the relationship between the structure and properties of DLC, researchers have been working on its classification and puting forward its theoretical model. Preparation technologies of DLC have been improving continuously. Nowadays there are some popular technologies such as the unbalanced magnetron sputtering technology, the electron cyclotron resonance radio frequency technology, the femtosecond laser technology, and the meshed plasma immersion ion deposition technology. The application field of DLC has expanded from simple life products to the field of machinery precision cutting tools, precision molds, medical devices and implants, and the aerospace field. Although DLC has developed a lot in the area of industrial application, there are some problems about its connection between technological parameter and performance. In this paper, the development history, structure, preparation methods, development status and the research prospects of DLC coatings are introduced in detail.

Mechanical properties, chemical analysis and evaluation of antimicrobial response of Si-DLC coatings fabricated on AISI 316 LVM substrate by a multi-target DC-RF magnetron sputtering method for potential biomedical applications

In this study silicon doped diamond-like carbon (Si-DLC) coatings were synthesized on two substrates: silicon and AISI 316LVM stainless steel using a multi-target DC-RF magnetron sputtering method. The Si content in the films ranged between 4 and 16at.%, and was controlled by the electrical power applied in RF regime to Si cathode target.The character of the chemical bonds was revealed by FTIR analysis. With the addition of silicon the hydroxyl absorption (band in the range of 3200–3600cm−1) increased what suggests more hydrophilic character of the coating. There were also observed significant changes in bonding of Si atoms. For low content of dopant, Si-O-Si bond system is predominant, while for the highest content of silicon there is an evidence of the shift to Si-C bonds in close proximity to methyl groups. The Raman spectroscopy revealed that the G peak position is shifted to a lower wavenumber and the ID/IG ratio decreased with increasing Si content, which indicates an increase in the C-sp3 content. Regardless of the coatings’ composition, the improvement of hardness in comparison to pure substrate material (AISI 316 LVM) was observed. Although the reduction of the level of hardness from the level of 10.8GPa for pure DLC to about 9.4GPa for the silicon doped coatings was observed, the concomitant improvement of films adhesion with higher amount of Si was revealed. Although incorporation of the dopant to DLC coatings increases the number of E. coli cells which adhered to the examined surfaces, the microbial colonisation remains on the level of substrate material.The presented results prove the potential of Si-DLC coatings in biomedical applications from the point of view of their mechanical properties.

Hydrogen permeability of diamondlike amorphous carbons

The hydrogen barrier properties of the coatings of nitride and diamondlike amorphous carbon (DLC) were evaluated. Using plasma chemical vapor deposition and sputtering, ZrN, TiAlN, AlCrN, CrN, and DLC coatings were deposited with a thickness of about 3 μm. It was confirmed that all coatings have an impact on decreasing the hydrogen permeation rate. Specifically, by coating stainless steel with a DLC coating, the hydrogen permeation rate was reduced to 1/1000 or lower compared to that without a coating. By providing a buffer layer of Cr-N, the hydrogen barrier function was improved. DLC coatings with high hydrogen content had an especially high hydrogen barrier function. For hydrogen diffusion in coatings, the movement of atoms through hydrogen trap sites is important. There are both sp3 and sp2 bonds in DLC coatings, and excess hydrogen can be found in the interstitial space and the above-mentioned hydrogen trap sites. It is suggested that the hydrogen trap sites in a DLC coating with high hydrogen content are likely already filled with hydrogen atoms, and the movement of new hydrogen atoms could be limited.

Development of diamond-like carbon (DLC) coatings with alternate soft and hard multilayer architecture for enhancing wear performance at high contact stress

Diamond-like carbon (DLC) coatings have attracted overwhelming interest due to a wide range of exceptional mechanical and tribological properties. Unfortunately, they have limited tribological applications where high contact stresses are required due to high residual stresses. We proposed that multilayer DLC coatings may have potential for tribological applications at high stresses. In this study, DLC coatings consisting of alternate soft-layer and hard-layer were deposited onto M2 steel substrates using unbalanced magnetron sputtering technique. The tribological performance at high contact stresses (2.5GPa, 3.2GPa, and 4GPa) was evaluated by a ball-on-disc tribometer. The residual stress, the hardness and reduced modulus are increased with percentage of hard-layer thickness. Multilayer coatings with the soft-top layer improve wear performance compared with single-layer DLC coatings. In contrast, multilayer coatings without the soft-top layer show poor wear performance under high stresses. The soft-top layer reduces the wear volume during run-in period by forming a transfer layer so that decreases the total wear. The 50% hard multilayer coating with the soft-top layer provides the best wear performance at an extreme stress of 4GPa among all coatings in this study because it combines the low magnitude of the residual stress with the acceptable hardness value. Such carbon/carbon multilayer structure offers an alternative to deposit low stress and high wear resistance DLC coatings without introducing other elements.

The investigation of the structures and tribological properties of F-DLC coatings deposited on Ti-6Al-4V alloys

Fluorine doped diamond-like carbon (F-DLC) coatings were deposited on Ti-6Al-4V alloy using a hollow cathode plasma immersion ion implantation deposition system. The aim of this work is to investigate the influence of F content on the surface morphology, structure, mechanical and tribological properties (under air and stroke-physiological saline solution) of the F-DLC coatings. The results reveal that the ratio of H/fE instead of H/E can be well predicting the anti-wear results when the F-DLC coatings have enough adhesion strength, where H is hardness, E is Young's modulus and f is friction coefficient. The F-DLC coatings with high F content exhibit excellent tribological property under air. The low F content F-DLC coatings present super-low friction coefficient and wear rate in stroke-physiological saline solution. Reasons for the tribological property are proposed in combination with the films' morphology and mechanical property.

Tribological Behavior of Cr Doped Diamond-like Carbon Coating in Engine Oil

Tribological Behavior of Cr Doped Diamond-like Carbon Coating in Engine Oil

Thick diamond like carbon coatings deposited by deep oscillation magnetron sputtering

Hydrogenated diamond like carbon (DLC) coatings with thicknesses up to 20μm have been deposited on metal substrates by sputtering a graphite target in argon (Ar) and acetylene (C2H2) mixture using deep oscillation magnetron sputtering (DOMS). The coatings were deposited under moderate peak target current densities (0.47 to 0.6Acm−2) and a low bias voltage of −60V, to achieve dense microstructure but avoid building up of high residual stresses. Higher peak substrate current densities and deposition rates were found when C2H2 was introduced. The effects of the C2H2 flow rate (fC2H2) on the deposition rate, microstructure, adhesion, mechanical and tribological properties of the DLC coatings were studied by means of scanning electron microscopy, Raman spectroscopy, HRC indentation, nanoindentation, dry ball-on-disk test, and block-on-ring test in SAE 10W-30 engine lubricant. The coatings showed low compressive stresses in the range of −0.5GPa to −1.8GPa. The coatings exhibited dense structure, high sp3/sp2 ratio, and excellent wear resistance when the C2H2 flow rate was in the range of 10–20sccm (4%–8% of the total gas flow). The friction and wear behavior of the DOMS DLC coatings both in the ambient air and SAE 10W-30 engine lubricant showed positive results in terms of friction and wear reduction.

Influence of plasma nitriding treatment on the adhesion of DLC films deposited on AISI 4140 steel by PVD magnetron sputtering

Duplex surface treatments composed of diamond like carbon (DLC) coating followed by plasma nitriding have drawn attention for a while. In this study, AISI 4140 steel substrates were plasma nitrided at different treatment temperatures and times. Then, DLC films were deposited on both untreated and plasma nitrided samples using PVD magnetron sputtering. The effect of different plasma nitriding temperatures and times on the structural, mechanical and adhesion properties of DLC coatings was investigated by XRD, SEM, microhardness tester and scratch tester, respectively. It was found that surface hardness, intrinsic stresses, layer thickness values and phase distribution in modified layers and DLC coating were the main factors on adhesion properties of duplex coating system. The surface hardness and residual stress values of AISI 4140 steel substrates significantly increased with both DLC coating and duplex surface treatment (plasma nitriding + DLC coating). Increasing plasma nitriding temperature and time also increased the diffusion depth and the thickness of modified layers. Hard surface layers led to a significant improvement on load bearing capacity of the substrate material. However, it was also determined that the process parameters, which provided lower intrinsic stresses, improved the adhesion properties of the duplex coating system.

Flexible camphor diamond-like carbon coating on polyurethane to prevent Candida albicans biofilm growth

Camphor was incorporated in diamond-like carbon (DLC) films to prevent the Candida albicans yeasts fouling on polyurethane substrates, which is a material commonly used for catheter manufacturing. The camphor:DLC and DLC film for this investigation was produced by plasma enhanced chemical vapor deposition (PECVD), using an apparatus based on the flash evaporation of organic liquid (hexane) containing diluted camphor for camphor:DLC and hexane/methane, mixture for DLC films. The film was deposited at a low temperature of less than 25°C. We obtained very adherent camphor:DLC and DLC films that accompanied the substrate flexibility without delamination. The adherence of camphor:DLC and DLC films on polyurethane segments were evaluated by scratching test and bending polyurethane segments at 180°. The polyurethane samples, with and without camphor:DLC and DLC films were characterized by Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and optical profilometry. Candida albicans biofilm formation on polyurethane, with and without camphor:DLC and DLC, was assessed. The camphor:DLC and DLC films reduced the biofilm growth by 99.0% and 91.0% of Candida albicans, respectively, compared to bare polyurethane. These results open the doors to studies of functionalized DLC coatings with biofilm inhibition properties used in the production of catheters or other biomedical applications.

Effect of running-in for delamination and friction properties of self-mating diamond-like carbon coatings in water

Two running-in processes, i.e., polishing a diamond-like carbon (DLC) coated disk and pre-sliding in ambient air, were suggested as a delamination suppression method for self-mating DLC coatings in water, and the effects of these methods were verified using a ball-on-disk tribometer. An a-C:H-type DLC coating was deposited on SUJ2 bearing steel balls and disks using plasma chemical vapor deposition. It was confirmed that delamination of DLC coatings was suppressed because microcracks were not generated by removing droplets on the DLC coated disk using the polishing process. The friction coefficient in that condition was reduced to about one-fifth of that of the untreated disk. The running-in process of pre-sliding in ambient air also suppressed delamination of DLC coatings. Moreover, the friction coefficient after this process was reduced to half of that with the other polishing process in the boundary lubrication condition. It was found that these effects have a relationship with surface smoothness and hydrophilicity.

Frictional Properties and Characterization of a Diamond-Like Carbon Coating Formed on Orthodontic Stainless Steel

Frictional Properties and Characterization of a Diamond-Like Carbon Coating Formed on Orthodontic Stainless Steel

The effect of two-step heat treatment on hardness, fracture toughness, and wear of different biased diamond-like carbon coatings

A conventional furnace heat treatment critically reduces hardness and fracture toughness after a certain temperature. This study reports the effects of two-step heat treatment on hardness, fracture toughness, and wear of bias varied diamond-like carbon (DLC) coatings deposited without buffer layer on silicon substrates using unbalanced magnetron sputtering system. Pristine specimens were directly heated (1-step) at 300°C in a tube furnace for 30min and took out after 20h furnace cooling. For 2-step 300°C, pristine specimens were heated at 150°C for 30min and furnace cooled for 20h and then re-heated at 300°C followed by the same procedure. The residual stresses in pristine DLC specimens were calculated with Stoney equation. Raman spectroscopy was performed to understand atomic disorder and clustering under bias and heat treatment effects. The reciprocating wear test was performed to understand the effect of bias voltage and heat treatment on wear volume of DLC coatings. Indentation studies were performed to analyze hardness, cracking behavior, and fracture toughness of bias varied pristine to re-heated specimens. 1-step 300°C heat treatment decreases hardness and fracture toughness up to 20% in general, whereas, the 2-step 300°C heat treatment process generally retained pristine values along with the prominent increase of 43% hardness and 9% fracture toughness for specimens which are deposited at a bias of −60V and −140V respectively.

Diamond-like carbon coatings with zirconium-containing interlayers for orthopedic implants

Six types of diamond-like carbon (DLC) coatings with zirconium (Zr)-containing interlayers on titanium alloy (Ti-6Al-4V) were investigated for improving the biotribological performance of orthopedic implants. The coatings consist of three layers: above the substrate a layer stack of 32 alternating Zr and ZrN sublayers (Zr:ZrN), followed by a layer comprised of Zr and DLC (Zr:DLC), and finally a N-doped DLC layer. The Zr:ZrN layer is designed for increasing load carrying capacity and corrosion resistance; the Zr:DLC layer is for gradual transition of stress, thus enhancing layer adhesion; and the N-doped DLC layer is for decreasing friction, squeaking noises and wear. Biotribological experiments were performed in simulated body fluid employing a ball-on-disc contact with a Si3N4 ball and a rotational oscillating motion to mimic hip motion in terms of gait angle, dynamic contact pressures, speed and body temperature. The results showed that the Zr:DLC layer has a substantial influence on eliminating delamination of the DLC from the substrates. The DLC/Si3N4 pairs significantly reduced friction coefficient, squeaking noise and wear of both the Si3N4 balls and the discs compared to those of the Ti-6Al-4V/Si3N4 pair after testing for a duration that is equivalent to one year of hip motion in vivo.

Friction characteristics of magneto-rheological fluid on DLC- and PTFE-coated surfaces

In this study, the friction characteristics of a magneto-rheological fluid are examined with different surface process technologies such as polytetrafluoroethylene coatings and diamond-like carbon coatings by using a reciprocating friction tester. The friction characteristics of the magneto-rheological fluid are also examined with diamond-like carbon coatings at various temperatures. The substrate material (plate) used for coating with diamond-like carbon and polytetrafluoroethylene is aluminum (Al6061), which is widely used in engineering applications. The descending sequence of the coefficients of friction is Al6061 > diamond-like carbon > polytetrafluoroethylene. The surfaces are observed by scanning electron microscopy before and after the experiment. In addition, the chemical compositions of the worn surfaces were analyzed using energy-dispersive X-ray spectroscopy. By comparing the results, the friction characteristics of the magneto-rheological fluid are analyzed based on the different coating methods.

The Effect of Nitrogen Gas Flow Rate on the Cr-Containing DLC (Diamond-Like Carbon) Coating by AEGD Hybrid-CVD Coating Process

The development of electric and hybrid automobiles has gained momentum with the growth of interest in the field of miniaturization of electrode materials. In particular, technologies that improve the electrical property of stainless steel, while maintaining corrosion resistance, are gaining interest in terms of maintaining specific resistivity. The study on metal doping in diamond-like carbon coating is currently in progress to enhance the characteristics of conductivity and corrosion resistance with excellent properties such as corrosion resistance and lubrication coating. It is the process of using Cr arc with DLC coating to actuate AEGD. The change of I(D)/I(G) (Graphite peak (G) and disordered bond peak (D)) ratio and G-peak position in Cr-containing DLC film causes graphitization and thus lowers the basic electric resistance. Simultaneous input of nitrogen gas leads to deposition of CrN by a specific ratio of Cr and N in the DLC coating, and the nitrogen atoms replace hydrogen in bonding to increase the sp3 bond structure in the DLC film, in which CrN is not deposited, to result in specific resistivity of a specific value or less.

Friction and wear performance of bearing ball sliding against diamond-like carbon coatings

We have studied the tribological properties of bearing steel ball (Japan standard, SUJ2) sliding against tetrahedral amorphous carbon (ta-C) coatings and amorphous hydrogenated carbon (a-C:H) coatings. The reciprocating sliding testes are performed with ball-on-plate friction tester in ambient air condition. Analysis of friction coefficient, wear volume and microstructure in wear scar are carried out using optical microscopy, atom force morphology (AFM) and Raman spectroscopy. The results show the SUJ2 on ta-C coating has low friction coefficient (around 0.15) but high wear loss. In contrast, the low wear loss of SUJ2 on a-C:H coating with high (around 0.4) and unsteady friction coefficient. Some Fe2O3, FeO and graphitization have been found on the wear scar of SUJ2 sliding against ta-C coating. Nearly no oxide materials exist on the wear scar of SUJ2 against a-C:H coating. The mechanism and hypothesis of the wear behavior have been investigated according to the measurement results. This study will contribute to proper selection and understand the tribological performance of bearing steels against DLC coatings.

Lubrication of aluminium versus diamond-like carbon contacts with hydrophobin proteins

Hydrogenated diamond-like carbon coatings (a-C:H) and silicon-doped diamond-like carbon coatings (a-C:H(Si)) of 1 μm thickness were deposited on stainless steel substrates by the inductively coupled plasma chemical vapour deposition technique, including a 300 nm-thick SiNx interlayer. Tribological experiments for both types of coating under pure and hydrophobin-containing water lubrication were performed using a pin-on-disc tribometer. To better understand wear behaviour, studies of hardness, surface morphology, water contact angle and chemical bonding were carried out using a nano-indenter, atomic force microscopy, contact angle measurement and X-ray photoelectron spectroscopy, respectively. Silicon doping in the a-C:H coating was found to slightly increase the surface roughness and wettability. The lowest friction coefficient (0.09) was obtained for the a-C:H(Si) coating sliding against an aluminium counterpart in water. The addition of HFBII hydrophobins to water increased both friction and wear of the aluminium counter body, and oxide tribofilm formation was prevented on the aluminium surface.

Friction and Wear Properties of Tetrahedral Si-Containing Hydrogenated Diamond-Like Carbon Coating under Lubricated Condition with Engine-Oil Containing ZnDTP and MoDTC

In this study, we investigated tribological properties of “tetrahedral” Si-containing hydrogenated DLC coating (TMS coating) during sliding against steel or cast-iron lubricated with engine-oil containing MoDTC and ZnDTP additives. TMS coatings derived from only tetramethylsilane were prepared using PACVD with high-bias process. TMS coating had the highly carbon sp3 bonded structure induced by Si (over 20 at.%) with high hydrogen content (30 at.%). TOF-SIMS analysis showed that TMS coating could form the additive-derived tribofilm on both of the non-ferrous coating surface and the ferrous counter surface to promote Mo-sulfide formation effectively, leading a low friction coefficient. The tribo-chemical reactions were related not only to Mo-sulfides formation, but also reduction of Mo-oxides. Mo-oxides could react with DLC material and induce hydrogen evolution and transformation into further weak carbon structure, causing an increasing wear of hydrogenated DLCs. TMS coating showed no significant wear in spite of plenty hydrogen content. Si induced sp3 bond and could act so as not to induce clustering of the sp2 phase. This could inhibit increasing wear of the DLC in the presence of MoDTC. The effectiveness of TMS coating in friction and wear behavior was shown to depend on tribo-chemical reactions and transformation of carbon bond structure.

Development of Hydrogen-Free Diamond-Like Carbon Coating for Piston Rings

The first piston ring application of a hydrogen-free diamond-like carbon (H-free DLC) coating to a mass-production gasoline engine reduces friction. It is found that the friction reduction effect between the cylinder bore and piston is 18%. When H-free DLC is applied to top ring, friction is over 10% reduced. The effect is larger than H-free DLC applied to oil rings. Because DLC as low friction coating is effective in a thin oil film and there are few oil at a large area near piston Top Dead Center (TDC). The friction reduction effect is enhanced when lower viscosity engine oil is used. In this study, the adhesion strength of H-free DLC coating on piston rings is also improved. It is shown that the relation between the adhesion strength and 2 coating parameters; i.e. providing substrate roughness and coating thickness. Adhesion strength is verified by test results using actual parts considered to piston ring environment by gasoline engine. Piston rings with low friction and high reliability are developed.

Electron-beam modification of DLC coatings for biomedical applications

The interest in diamond like carbon (DLC) coatings and their modification for biomedical applications is strongly increasing during the last years. Improvement of surface resistance to scratching and biocompatibility are the main goals of recent developments. Moreover, different capabilities of adjacent areas on surfaces are often required by complex applications.In this study, different deposition methods for diamond-like carbon (DLC) coatings were compared for their suitability in biomedical applications. In addition, DLC coatings modified by non-thermal electron-beams were investigated with regard to their biological interactions. The DLC coatings were deposited using four different methods (plasma-activated chemical vapor deposition, physical vapor deposition by magnetron sputtering, and physical vapor deposition via cathodic arc – both unfiltered and filtered). The different DLC coatings were characterized with regard to their wettability, morphology, roughness, and coating adhesion. The ability of coatings to withstand repeated sterilization cycles was investigated using a specialized test protocol parallel to standard sterilization procedures for medical devices. Biological evaluation was performed using human fibroblasts by assessing metabolic activity and cell counts. Electron-beam modification of the coatings was performed under ambient conditions using a low-energy electron-beam facility (150kV acceleration voltage).All DLC coatings investigated were found to be biocompatible, with the coating deposited by magnetron sputtering showing the best results. In general, electron-beam modification resulted in the hydrophilicity of the different DLC coatings increasing, but no changes in surface morphology or roughness were observed. The cell adhesion to electron-beam-modified DLC coatings was reduced to 30% of the untreated DLC coatings, what can be attributed to changes in the surface chemistry. The long-term stability of the modified DLC coatings was also confirmed. Therefore, electron-beam treatment represents a promising tool for modifying and functionalizing DLC coatings, especially if varying requirements concerning biomedical functionality and cell adhesion are involved.