Structure and mechanical properties of oxygen doped diamond-like carbon thin films

In this study, structure and mechanical properties of doped diamond-like carbon (DLC) films with oxygen were investigated. A mixture of methane (CH₄), argon (Ar) and oxygen (O₂) was used as feeding gas, and the RF-PECVD technique was used as a deposition method. The thin films were characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (RS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and a combination of elastic recoil detection analysis and Rutherford backscattering (ERDA-RBS). Nano-indentation tests were performed to measure hardness. Also, the residual stress of the films was calculated by Stoney equation. The XPS and ERDA-RBS results indicated that by increasing the oxygen in the feeding gas up to 5.6 vol.%, the incorporation of oxygen into the films' structure was increased. The ratio of sp² to sp³ sites was changed by the variation of oxygen content in the film structure. The sp²/sp³ ratios are 0.43 and 1.04 for un-doped and doped DLC films with 5.6 vol.% oxygen in the feeding gas, respectively. The Raman spectroscopy (RS) results showed that by increasing the oxygen content in doped DLC films, the amount of sp² CC aromatic bonds was raised and the hydrogen content reduced in the structure. The attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) confirmed the decrease of hydrogen content and the increase the ratio of CC aromatic to olefinic bonds. Hardness and residual stress of the films were raised by increasing the oxygen content within the films' structure. The maximum hardness (19.6 GPa) and residual stress (0.29 GPa) were obtained for doped DLC films, which had the maximum content of oxygen in structure, while the minimum hardness (7.1 GPa) and residual stress (0.16 GPa) were obtained for un-doped DLC films. The increase of sp³ CC bonds between clusters and the decrease of the hydrogen content, with a simultaneous increase of oxygen in the films' structure is the reason for increase of hardness and residual stress.     

Microstructure and mechanical properties of Mo/DLC nanocomposite films

Mo-doped diamond-like carbon (Mo/DLC) films were deposited on stainless steel and Si wafer substrates via unbalanced magnetron sputtering of molybdenum combined with inductively coupled radio frequency (RF) plasma chemical vapor deposition of CH₄/Ar. The effects of Mo doping and sputtering current on the microstructure and mechanical properties of the as-deposited films were investigated by means of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, atomic force microscopy (AFM), and nano-indentation. It was found that Mo doping led to increase in the content of sp² carbon, and hence decreased the hardness and elastic modulus of Mo/DLC films as compared with that of DLC films. The content of Mo in the films increased with the increasing sputtering current, and most of Mo reacted with C atoms to form MoC nanocrystallites at a higher sputtering current. Moreover, the Mo-doped DLC films had greatly decreased internal stress and increased adhesion to the substrate than the DLC film, which could be closely related to the unique nanocomposite structure of the Mo-doped films. Namely, the Mo/DLC film was composed of MoC nanoparticles embedded in the cross-linked amorphous carbon matrix, and such a kind of nanostructure was beneficial to retaining the loss of hardness and elastic modulus.     

Effects of thermal annealing and Si incorporation on bonding structure and fracture properties of diamond-like carbon films

The effects of thermal annealing and Si incorporation on the structure and properties of diamond-like carbon (DLC) films were investigated. As-deposited DLC film (DLC) and Si incorporated DLC film (Si-DLC), both with and without thermal annealing, were analyzed for bonding structure, residual stress, film thickness, elastic modulus and fracture properties using Raman spectroscopy, wafer curvature, nanoindentation, four-point bend fracture testing, and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy clearly showed that thermal annealing of DLC films promotes more sp² bonding character, whereas Si incorporation into the films promotes more sp³ bonding character. Interfacial fracture energies, film hardness and elastic modulus, and residual film stress were all found to vary strongly with the degree of sp³ bonding in the DLC film. These changes in mechanical properties are rationalized in terms of the degree of three dimensional inter-links within the atomic bond network.     

Long-term stability of hydrogenated DLC coatings: Effects of aging on the structural,chemical and mechanical properties

Long-term stability is an essential condition for the commercial use of protective coatings, yet often remains overlooked in the literature. Here we report the effects of long-term environmental aging on the properties of hydrogenated diamond-like carbon (DLC) films. A range of DLC coatings produced by plasma-enhanced chemical vapor deposition were first thoroughly characterized and then stored for three years before the second set of analysis. Raman spectroscopy showed that the films exhibited excellent structural stability during aging, observing no sign of sp³ to sp² conversion. Similarly, the hardness and smoothness of the DLC coatings remained unchanged, despite the observed relaxation of their intrinsic stress with time. However, X-ray photoelectron spectroscopy analyses provided evidence of aging-induced surface oxidation, which was confirmed by reduced hydrophobicity (water contact angle dropped to 65°). Overall, these findings suggest that DLC possesses a suitable long-term stability when exposed to environmental conditions.     

The effect of CNT content on the surface and mechanical properties of CNTs doped diamond like carbon films

The carbon nanotubes (CNTs) doped diamond like carbon films were carried out by spinning coating multi-walled carbon nanotubes (CNTs) on silicon covered with diamond like carbon films via PECVD with C₂H₂ and H₂. The results show that the I_D/I_G and sp²/sp³ ratios are proportional to the CNT contents. For wettability and hydrogen content, the increase of CNT content results in more hydrophobic and less hydrogen for CNT doped DLC films. As for mechanical properties, the hardness and elastic modulus increases linearly with increasing CNT content. The residual stress is reduced for increasing CNT content. As for the surface property, the friction coefficient is reduced for higher CNT content. For CNT doped DLC films, the inclusion of horizontal CNT into DLC films increases the hardness, elastic modulus and reduces the hydrogen content, friction coefficient and residual stress. Like the light element and metal doping, the CNT doping has effects on the surface and mechanical properties on DLC which might be useful to specific application.     

Temperature effect on bonding structures of amorphous carbon containing more than 30at.% silicon

Bonding evolution of amorphous carbon incorporated with Si or a-C(Si) in a thermal process has not been studied. Unhydrogenated a-C(Si) films were deposited by magnetron sputtering to undergo two different thermal processes: i) sputter deposition at substrate temperatures from 100 to 500 °C; ii) room temperature deposition followed by annealing at 200 to 1000 °C. The hardness of the films deposited at high temperature exhibits a monotonic decrease whereas the films deposited at room temperature maintained their hardness until 600 °C. X-ray photoelectron spectroscopy and Raman spectroscopy were used to analyze the composition and bonding structures. It was established that the change in the mechanical property is closely related to the atomic bonding structures, their relative fractions and the evolution (conversion from C–C sp³ → CC sp² or CC sp² → C–Si sp³) as well as clustering of sp² structures.     

Blood compatibility of La2O3 doped diamond-like carbon films

La₂O₃ doped diamond-like carbon films (DLC) with different concentration were deposited by using Radio-Frequency magnetron sputtering. The microstructure and surface properties of DLC films were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and contact angle test. The blood compatibility of the samples was evaluated by tests of platelet adhesion. Results show the sp²-bonded C content increases with increasing of La₂O₃ concentration doped. A remarkable decrease of platelet adhered on the surface of the La₂O₃ doped DLC films was observed comparing to the Chrono flex used in clinical application, suggesting that La₂O₃ doped DLC is able to enhance its blood compatibility. The mechanism of hemocompatibility of doped films was discussed. Our results demonstrate that La₂O₃ doped DLC films are potentially useful biomaterials with good blood compatibility.     

Substrate bias effect on structure of tetrahedral amorphous carbon films by Raman spectroscopy

Diamond-like carbon (DLC) films form a critical protective layer on magnetic hard disks and their reading heads. Now tetrahedral amorphous carbon films (ta–C) thickness of 2 nm are becoming the preferred means due to the highly sp³ content. In this paper, Raman spectra at visible and ultraviolet excitation of ta–C films have been studied as a function of substrate bias voltage. The spectra show that the sp³ content of 70 nm thick DLC films increases with higher substrate bias, while sp³ content of 2 nm ultra-thin films falls almost linearly with bias increment. And this is also consistent with the hardness measurement of 70 nm thick films. We proposed that substrate bias enhances mixing between the carbon films and either the Si films or Al₂O₃TiC substrate such that thin films contain less sp³ fraction. These mixing bonds are longer than C–C bonds, which inducing the hardness decreasing of ultra-thin DLC films with bias. But for 70 nm DLC, the effect of mixing layer can be negligible by compared to bias effect with higher carbon ion energy. So sp³ content will increase for thick films with substrate bias.     

Mechanical stability,corrosion performance and bioresponse of amorphous diamond-like carbon for medical stents and guidewires

Diamond-ike carbon (DLC) coatings have been investigated with respect to biocompatibility, mechanical stability under biofluid exposure, corrosion resistance and the impact of the fabrication or operation of catheter guidewires and stents upon coating integrity. High mechanical tensile and compressive forces, during guidewire winding or stent expansion, pose severe limitations on the use of DLC-coated stainless steel. Doping with silicon and the use of an a-Si:H interlayer can help minimise the risk of adhesion failure or film cracking. The incorporation of Si increased the hydrogen content and the estimated sp³ fraction but reduced the film hardness. Silicon-doped a-C:H coatings exhibit significantly improved corrosion barrier properties, with over two orders of magnitude increase in the charge transfer resistance. Immersion in biofluid, however, reduced the interfacial adhesion strength by up to 75%. Human microvascular endothelial cell attachment was enhanced while platelet attachment was reduced on Si-doped compared to undoped a-C:H. The macrophage response to non-hydrogenated tetragonal (t-aC) carbon show that these coatings stimulate less inflammatory activity than uncoated materials and produce comparable responses to already existing polyurethane coatings.     

Wear-resistant Ti–Al–Ni–C–N coatings produced by magnetron sputtering of SHS-targets in the DC and HIPIMS modes

The hard wear-resistant nanocomposite Ti–Al–Ni–C–N coatings were deposited by direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HIPIMS) in the Ar, Ar+15%N_2, and Ar+25%N₂ atmospheres. The structure of coatings was analyzed using the X-ray diffraction analysis, glow discharge optical emission spectroscopy, and scanning electron microscopy. Mechanical and tribological properties were measured using the nanoindentation and scratch testing as well as by tribological testing using the “pin-on-disc” scheme. Electrochemical corrosion resistance and oxidation resistance of coatings were investigated. The results suggest that the coatings are based on the FCC phases TiCN and Ni₃Al with crystallites size ~3 and ~15 nm, correspondingly. DCMS coatings with optimal composition were characterized by hardness 34 GPa, stable friction coefficient <0.26 and wear rate <5 × 10^-6 mm³N^-1m^-1. Application of HIPIMS mode allowed the increase of adhesion strength, tribological properties and corrosion resistance of coatings.     

Characterization of hydrogen-free diamond-like carbon film deposited on cyclic olefin copolymer

In this study, the feasibility of the diamond-like carbon (DLC) film as a viable component for cyclic olefin copolymer (COC) substrate overcoat was assessed. Featured by its advanced physical and chemical properties such as high hardness, chemical stability, and wide band-gap optical transparency, the hydrogen-free DLC exhibits promising characteristics as the overcoat for flexible substrates or other TFT components. Ultra smooth, DLC thin films were synthesized by using a filter arc deposition (FAD) system and a cathodic arc evaporation (CAE) system. Raman spectroscopy, ESCA, Nano-Indentation, and electron microscopy were used to characterize the electronic, morphological, and microstructure properties of the DLC coatings. Results indicate that the device-quality DLC needs to be synthesized at lower substrate bias potential to retain high sp³/sp² ratio. The bending tests demonstrated a 30-fold improvement of the DLC-protected COC over that of the unprotected COC. Water vapor permeability tests demonstrated a 25-fold improvement of the DLC-protected COC over that of unprotected COC.     

Characteristics and tribological properties in water of Si-DLC coatings

Si-containing diamond-like carbon (Si-DLC) coatings with a Si content ranging between 0 (DLC) and 10 at.% were deposited by thermal electron excited plasma CVD method, and their characteristics and the tribological properties in water environment were investigated. The results showed that doped Si had little effect on the hardness and Young's modulus of the coatings. Increasing Si content reduced the friction and the wear of the mated ball, although the wear of the coatings increased. The wear of the counter ball occurred mainly in the early stage of rubbing. The Raman and XPS analysis revealed that the tribochemical reaction of Si-DLC coating occurred in water, and SiO_x(OH)_y gel was formed on the mated ball surface. It is considered that the tribochemical reaction is also responsible for the tribological properties of the Si-DLC coating and the counter ball, and the reaction may be accelerated by increasing the Si content. Failure-resistant capability is strongly governed by the characteristics of the coating, and can be improved by doping Si. There is an optimum Si content for increasing the failure-resistant capability and it was 6.6 at.% in this work.     

Effects of diamond-like carbon in TPD-Alq3 doped PVK organic light-emitting devices

Effects of an ultrathin (~ 1 nm) diamond-like carbon (DLC) layer in single-layer organic light-emitting devices (OLEDs) that consist of ITO/(TPD-Alq₃ doped PVK)/Al were investigated. DLC layers deposited by using Nd:YAG laser at laser wavelengths of 355 nm were high in sp³ content and resistivity (DLC_UV) while that of 1064 nm laser were lower in sp³ content and resistivity (DLC_IR), as characterized by Raman spectroscopy and resistivity measurements. Although emission were obtained for all the devices, only the device of ITO/DLC_UV/(TPD-Alq₃ doped PVK)/Al exhibited enhanced current density and brightness with lower turn-on voltage as compared to a standard device. Devices of ITO/DLC_IR/(TPD-Alq₃ doped PVK)/Al and ITO/(TPD-Alq₃ doped PVK)/DLC_UV/Al showed poor current and brightness characteristics but failed at higher applied voltage. The enhance performance of device with high resistivity/sp³ DLC film suggests the mechanisms of barrier reduction by sufficiently thin insulating layer which increase the probability of tunneling of carriers at ITO and PVK interface.     

Chemical,mechanical and tribological characterization of ultra-thin and hard amorphous carbon coatings as thin as 3.5 nm: recent developments

Diamond material and its smooth coatings are used for very low wear and relatively low friction. Major limitations of the true diamond coatings are that they need to be deposited at high temperatures, can only be deposited on selected substrates, and require surface finishing. Hard amorphous carbon (a-C), commonly known as diamondlike carbon (DLC), coatings exhibit mechanical, thermal and optical properties close to that of diamond. These can be deposited with a large range of thicknesses by using a variety of deposition processes, on variety of substrates at or near room temperature. The coatings reproduce substrate topography avoiding the need of post finishing. Friction and wear properties of some DLC coatings can be very attractive for tribological applications. The largest industrial application of these coatings is in magnetic storage devices. Recent developments in the chemical, mechanical and tribological characterization of the ultra-thin coatings are reviewed in this paper. The prevailing atomic arrangement in the DLC coatings is amorphous or quasi-amorphous with small diamond (sp³), graphite (sp²) and other unidentifiable micro- or nanocrystallites. The mechanical and tribological properties of the DLC coatings are dependent upon the deposition technique. Thin coatings deposited by filtered cathodic arc, ion beam and ECR-CVD hold a promise for tribological applications. Coatings as thin as 5 nm in thickness provide wear protection.     

Electrochemical characterization of diamond like carbon thin films

Diamond-like carbon (DLC) thin films were deposited from pure graphite target by DC magnetron sputtering method. Experimental parameters, i.e., substrate temperature and negative bias voltage, have been changed to finely tune the chemical bonding property (sp²/sp³) of the as-deposited DLC films. The as-deposited DLC films were characterized as anode materials for Li–ion batteries and special attentions were paid to the effects of sp²/sp³ ratio on the electrochemical properties of the DLC films. The results indicated that a high fraction of sp² bonding in the DLC films is preferred for high lithium storage capacity, flat and low charge voltage plateau, and long cycling retention.     

Formation and characterization of DLC:Cr:Cu multi-layers coating using cathodic arc evaporation

Chromium and copper-doped diamond-like carbon (DLC:Cr:Cu) films were deposited on SKH 51 tool steel. We have prepared multilayers of DLC:Cr and DLC:Cu by cathodic arc evaporation process using chromium (Cr) and copper (Cu) target arc sources to provide Cr and Cu in the DLC. Acetylene reactive gases were also activated at a pressure of 5 mTorr to 25 mTorr and a temperature fixed at 180 °C to provide the DLC. The resulting DLC:Cr:Cu film contained Cr_xCu_y as well as Cr_xC_y nanoparticles vital for the film mechanical properties. The crystal structure was investigated using X-ray diffraction (XRD) and transmission electron microscopy, while the surface morphology and chemical composition were studied by field emission scanning electron microscopy and X-ray photoelectron spectroscopy. The process parameters were compared by studying the various mechanical properties of the films such as microhardness and residual stress. The result of this process enhanced the DLC:Cr:Cu composite coatings for high toughness and lower friction coefficient (0.08). The profiles of sp³/sp² (XPS) ratios corresponded to the change of microhardness profile by varying the pressure of the hydrocarbon gases (C₂H₂).     

Effect of nitrogen concentrations on optical,structural and mechanical properties of self organized a-C:N films

Low friction and surface hardness has become an important aspect to study and understand surface engineering of diamond like coating. Investigation of structural and mechanical properties of nitrogenated amorphous Diamond Like Carbon coating has been done. The cross-sectional microstructures, elemental compositions and various phase constituent of the coated layers under different processing conditions have been characterized. Films are deposited in presence of 5%–20% with the increasing rate of 5% and 40% of N₂ partial pressure along with Ar gas. 20% N₂ pressure shows a critical behavior in Raman spectroscopy and XPS. In this condition the film shows more uniform coating with vertical growth structures as well as brittle behavior. The micro-structural changes on the surface due to migration of N₂ and its related surface properties have been examined. AFM studies clearly show that the percentage change in average roughness decreases to 17% as the film thickness increases at 20% N₂ incorporation. The positive changes in its mechanical properties have been observed by Nano-indentation techniques. Significance of DLC coating in this condition is clearly seen with the increasing sp³compositions by XPS analysis. All results have been correlated and hence critical range of nitrogen partial pressure has been observed between 20%-23% to give similar sp³ fraction and hence properties that of pure DLC films.     

Microstructure and mechanical properties of CeO2 doped diamond-like carbon films

A kind of rare earth oxide, CeO₂, was doped into the diamond-like carbon (DLC) films with thickness of 180–200 nm, using unbalanced magnetron sputtering. All the adhesion strength of CeO₂ doped DLC films is increased, while the residual compressive stress is obviously decreased compared to pure DLC film. Specially, the residual compressive stress of the deposited films are reduced by 90%, when the CeO₂ content is in the range of 5–7%, from a value of about 4.1 GPa to 0.5 GPa. When the CeO₂ content is increased to 10%, the deposited films possess the highest adhesion strength of 85 mN, 37% higher than that of pure DLC film. The nanohardness and elastic modulus exist a transition point at 8% of CeO₂ content within the DLC film. Before this value, nanohardness and elastic modulus of CeO₂ doped DLC films are lower than those of pure DLC film, and after this value, they are higher or adjacent to those of pure DLC film. Auger electron spectroscopy shows a more widened interface of 6% CeO₂ doped DLC film compared to pure DLC film. The enhancement of adhesion strength is mainly attributed to the widening of the film-substrate interface, as well as the decrease of residual compressive stress.     

Mechanical properties of DLC films prepared in acetylene and methane plasmas using electron cyclotron resonance microwave plasma chemical vapor deposition

Diamond-like carbon (DLC) films were deposited on silicon using methane and acetylene plasma induced by electron cyclotron resonance microwave plasma chemical vapor deposition (ECR-MPCVD). The mechanical properties of DLC films were characterized by micro-Raman system, atomic force microscope, tribometer, nano-indenter used for both hardness and nano-scratch test measurements. The mechanical properties of both DLC films, prepared in methane and acetylene plasmas, respectively, strongly depended on the kinetic energy of impinging particles. The deposition at −120 V substrate bias gave rise to DLC films with the best mechanical properties for both methane and acetylene plasmas. The hardness measurements with variable indentation depth showed the characteristic changes in hardness values implying elastic deformations of supporting substrates. The maximum hardness value of DLC^M films was 20 GPa while that of DLC^A films was 28 GPa. However, the hardness dropped when DLC films were prepared at substrate biases more negative than −120 V due to the thermal graphitization. The improvement in DLC properties usually provided the films with smaller hydrogen content and higher density of sp³ bondings. These parameters were engineered through controlling the deposition parameters. Particularly, the bombardment of growing DLC films by energetic ions showed to be extremely important to yield films with lower internal stress.     

In vitro comparison of biocompatibility of CN x7 and DLC coatings

Carbon nitride (CN ^x7 and diamond-like carbon (DLC) coatings were prepared using dc magnetron sputtering at room temperature. The morphology, chemical composition and bonding state of the coatings were characterized by atomic force microscopy (AFM), Auger electron spectroscopy (AES) and Raman spectroscopy. Compared to DLC, CN ^x7 coating exhibited a slight improvement in hardness, coefficient of friction, roughness, and corrosion. The effects of CN ^x7 and DLC coatings on cultures of mouse fibroblasts and human endothelial cells were determined by scanning electron microscopy. The results showed that the coatings caused no adverse effects on the cells. CN ^x7 coating provided a comparable or better surface for the normal cellular attachment, growth, and morphology as compared to DLC. These results support the biocompatibility of both CN ^x7 and DLC and should initiate an interest in the biomedical applications of CN ^x7 coating.