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.     

Characteristics and surface energy of silicon-doped diamond-like carbon films fabricated by plasma immersion ion implantation and deposition

Diamond-like carbon (DLC) films doped with different silicon contents up to 11.48 at.% were fabricated by plasma immersion ion implantation and deposition (PIII-D) using a silicon cathodic arc plasma source. The surface chemical compositions and bonding configurations were determined by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results reveal that the sp³ configuration including Si–C bonds increases with higher silicon content, and oxygen incorporates more readily into the silicon and carbon interlinks on the surface of the more heavily silicon-doped DLC films. Contact angle measurements and calculations show that the Si-DLC films with higher silicon contents tend to be more hydrophilic and possess higher surface energy. The surface states obtained by silicon alloying and oxygen incorporation indicate increased silicon oxycarbide bonding states and sp³ bonding states on the surface, and it can be accounted for by the increased surface energy particularly the polar contribution.     

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.     

Effect of carbonitriding temperature process on the adhesion properties of diamond like-carbon coatings deposited by PECVD on austenitic stainless steel

The deposition of adherent coatings such as diamond-like carbon (DLC) on substrates of iron-based materials is difficult to obtain for two reasons: high residual compressive stress occurs in the inner film formation, and the mismatch of thermal expansion coefficient between steel and DLC film generates delamination effects. In order to determine the carbonitriding temperature prior to film deposition, the steel substrate and the DLC films were analyzed for their microstructure and mechanical properties of adhesion as a function of temperature. The technique used to deposit the coating was DC-pulsed plasma enhanced chemical vapor deposition. The delamination distances and the critical load of the film were obtained by scratch testing. The surface analysis by X-ray diffraction indicated the formation of nitride phases on the steel. Raman spectroscopy showed the fraction of sp³ carbon bonds in DLC films. Hardness profiling was used to verify the extent of the interface modified by carbonitriding along the cross section. For this, the steel sample with the appropriate surface modification to have high adhesion of the DLC film was used.     

Influence of plasma ionization on the elastic modulus and tribology behavior of carbon films deposited by the HiPIMS technique

This work reveals the influence of discharge current on carbon-ion energies of plasma, elastic modulus, and friction coefficient at the nano- and macroscale of carbon films deposited via high-power impulse magnetron sputtering. Three applied discharge current conditions in deposition processes were employed to obtain three-carbon films of interest. The number of carbon ions with their energies was obtained via mimic tests of the deposition process using three similar discharge currents through a quadrupole mass spectrometer detector based on the time-averaged ion energy distribution function. The bonding structure of the films was evaluated using Raman spectroscopy, fitting the Diamond and Graphite peaks to obtain a semiquantitative analysis. The elastic modulus of the carbon films was determined from atomic force acoustic microscopy measurements avoiding the influence of the substrates. The friction coefficient was analyzed at the nanoscale via atomic force microscopy and at the macroscale via tribometry. Significant alterations were observed in the number and energy level of the carbon ions with the variation of discharge current. These alterations significantly influenced the bonding properties, elastic modulus, and tribology behavior. A higher elastic modulus and higher sp³ bond content were observed for the film with a lower number of carbon ions and less energy.     

Changes in chemical bonding of diamond-like carbon films by atomic-hydrogen exposure

We have deposited unhydrogenated diamond-like carbon (DLC) films on Si substrate by pulsed laser deposition using KrF excimer laser, and investigated the effects of atomic-hydrogen exposure on the structure and chemical bonding of the DLC films by photoelectron spectroscopy (PES) using synchrotron radiation and Raman spectroscopy. The fraction of sp³ bonds at the film surface, as evaluated from C1s spectra, increased at a substrate temperature of 400 °C by atomic-hydrogen exposure, whereas the sp³ fraction decreased at 700 °C with increasing exposure time. It was found that the sp³ fraction was higher at the surfaces than the subsurfaces of the films exposed to atomic hydrogen at both the temperatures. The Raman spectrum of the film exposed to atomic hydrogen at 400 °C showed that the clustering of sp² carbon atoms progressed inside the film near the surface even at such a low temperature as 400 °C.     

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.     

A method of determining the elastic properties of diamond-like carbon films

The elastic properties of diamond-like carbon (DLC) films were measured by a simple method using DLC bridges which are free from the mechanical constraints of the substrate. The DLC films were deposited on a Si wafer by radio frequency (RF) glow discharge at a deposition pressure of 1.33 Pa. Because of the high residual compressive stress of the film, the bridge exhibited a sinusoidal displacement on removing the substrate constraint. By measuring the amplitude with a known bridge length, we could determine the strain of the film which occurred by stress relaxation. Combined with independent stress measurement using the laser reflection method, this method allows the calculation of the biaxial elastic modulus, E/(1−ν), where E is the elastic modulus and ν is Poisson's ratio of the DLC film. The biaxial elastic modulus increased from 10 to 150 GPa with increasing negative bias voltage from 100 to 550 V. By comparing the biaxial elastic modulus with the plane–strain modulus, E/(1−ν²), measured by nano-indentation, we could further determine the elastic modulus and Poisson's ratio, independently. The elastic modulus, E, ranged from 16 to 133 GPa in this range of the negative bias voltage. However, large errors were incorporated in the calculation of Poisson's ratio due to the pile up of errors in the measurements of the elastic properties and the residual compressive stress.     

Effect of substrate bias voltage on the properties of diamond-like carbon thin films deposited by microwave surface wave plasma CVD

Diamond-like carbon (DLC) thin films were deposited on silicon and ITO substrates with applying different negative bias voltage by microwave surface wave plasma chemical vapor deposition (MW SWP-CVD) system. The influence of negative bias voltage on optical and structural properties of the DLC film were investigated using X-ray photoelectron spectroscopy, UV/VIS/NIR spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy. Optical band gap of the films decreased from 2.4 to 1.7 with increasing negative bias voltage (0 to − 200 V). The absorption peaks of sp³ CH and sp² CH bonding structure were observed in FT-IR spectra, showing that the sp²/sp³ ration increases with increasing negative bias voltage. The analysis of Raman spectra corresponds that the films were DLC in nature.     

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.     

Bonding defects and optical band gaps of DLC films deposited by microwave surface-wave plasma CVD

The effects of CH₄ / C₂H₄ flow ratio and annealing temperature on the defect states and optical properties of diamond-like carbon (DLC) films deposited by novel microwave surface-wave plasma chemical vapour deposition (MW SWP CVD) are studied through UV/VIS/NIR measurements, atomic force microscopy, Raman spectroscopy and electron spin resonance analysis. The optical band gap of DLC has been tailored between a relatively narrow range, 2.65–2.5 eV by manipulating CH₄ / C₂H₄ flow ratio and a wide range, 2.5–0.95 by thermal annealing. The ESR spin density varied between 10¹⁹ to 10¹⁷ spins/cm³ depending on the CH₄ / C₂H₄ flow ratio (1 : 3 to 3 : 1). The defect density increased with increasing annealing temperature. Also, there is a strong dependence of spin density on the optical band gap of the annealed-DLC films, and this dependency has been qualitatively understood from Raman spectra of the films as a result of structural changes due to sp³/sp² carbon bonding network. The surfaces of the films are found to be very smooth and uniform (RMS roughness < 0.5 nm).     

Effects of post-deposition annealing on different DLC films

Amorphous hydrogenated carbon films have been deposited by plasma-enhanced chemical vapour deposition at different process pressures and substrate temperatures, resulting in film properties ranging from polymer-like to diamond-like. The deposition parameter combinations were chosen by experimental design to enable the determination of both deposition pressure and deposition temperature effects on the annealing behaviour. The deposited films have then been annealed in vacuum at successively higher temperatures. Changes in optical band gap, internal stress, film thickness and infrared (IR) absorption spectra have been recorded to assess the influence of deposition parameters on the thermal stability of diamond-like carbon (DLC) and to identify temperature-induced modifications in the bonding structure. The results show a large variation in thermal stability between the different DLC films; samples deposited at low pressure display the greatest stability. The internal stress of the films starts decreasing at very low annealing temperature (<100 °C for the majority of samples), in most cases long before any decrease in the optical band gap can be detected. This is explained by a movement of hydrogen from sp² to sp³ sites as detected by IR measurements.     

Residual stress relief of hard a-C films though buckling

To characterize the adhesive failure mode in amorphous-carbon (a-C) films, and to explore the effects of stress relief mechanism on the mechanical properties of the films, the microstructure and the morphologies of the buckled and peeled a-C films were characterized by various techniques, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectrum and X-ray photoelectron spectroscopy (XPS). Results indicated that there is obvious buckling between the stress relieved a-C films and Si substrates, and the development of the buckling blister was derived from the residual compressive stress. The as-deposited a-C films voluntarily buckled along the film growth direction above Si substrate when film thickness reached a certain size, and became more and more remarkable, resulting in eventual peeling. These buckling and peeling processes can relieve the residual stress of the a-C films by eliminating the mechanical restriction of Si substrates. The corresponding sp² hybridization transformation and the reconfiguring graphitic phase were detected in the stress relived a-C films, which can induce buckling and spalling in the a-C films.     

Thermal annealing behaviour of alloyed DLC films on steel: Determination and modelling of mechanical properties

A shortcoming of diamond-like carbon (DLC) films is the poor stability of their microstructure and properties at elevated temperatures. In this study, the effect of annealing on the stability of DLC films alloyed with silicon and deposited on steel is investigated. A comprehensive study of the mechanical properties is carried out by a novel method combining normal indentations with micro- and macroindentors assisted by finite element calculations of the indentation. The mechanical properties of the layers are correlated to structural changes in the film and to interface reactions.While it has become a common practice to determine hardness and the Young's modulus of thin films by nanoindentation and to calculate residual stresses from the bending of the film/substrate system, evaluation of the interface toughness, which is a measure of adhesion, and of the film rupture strength is less straightforward. Here, Hertzian-type ring cracks are generated in the film by nanoindentation of the film/substrate system with spherical diamond tips. From the critical load for crack generation the film rupture strength is deduced using finite element calculations. Similarly, Rockwell C hardness tests in combination with calculations are performed to measure the interface toughness.Applying these methods to DLC films on steel, it has been found that the Young's modulus decreases with increasing silicon content and the residual stress drops below 1 GPa. The rupture strength approaches its theoretical limit of E/10. Annealing at 500 °C reduces the adhesion energy significantly. The variation of mechanical properties can be attributed to structural changes in the film as investigated by Raman spectroscopy.     

Electrochemical characteristics of lithium metal anodes with diamond like carbon film coating layer

To overcome the poor electrochemical characteristics of lithium metal anodes due to the dendrite formations, diamond like carbon (DLC) films were deposited onto the surface of lithium metal by radio frequency-plasma enhanced chemical vapor deposition (CVD) technique using acetylene gas as carbon precursor. The substrate temperature was selected as the main experimental parameter to control the bonding characteristic (sp²/sp³ ratio) of the films. The presence of diamond like structures was confirmed by Raman and Fourier transform infra red spectroscopy. The DLC coated lithium metal was then characterized as an anode material for lithium secondary batteries. The results showed that the DLC coated lithium metal anodes exhibited better electrochemical characteristics in terms of higher specific capacity and smaller interfacial impedance. These improved characteristics were attributed to the presence of DLC film coating which might suppress the dendrite's formation by protecting the lithium metal surface from the direct contact with the electrolyte.     

Characterization of the mechanical properties of diamond-like carbon films

A recently suggested method to measure the elastic modulus of diamond-like carbon (DLC) films was reviewed. This method used a DLC bridge or free overhang which is free from the mechanical constraint of the substrate. Because of the high residual compressive stress of the DLC film, the bridge or the overhang exhibited a sinusoidal displacement on removing the mechanical constraint. Measuring the amplitude and wavelength of the sinusoidal displacement made it possible to measure the strain of the film which occurred by stress relaxation. Combined with independent stress measurement using the laser reflection method, this method allowed the calculation of the biaxial elastic modulus of the DLC film. This method was successfully applied to obtain the elastic properties of various DLC films from polymeric hydrogenated amorphous carbon (a-C:H) to hard tetrahedral amorphous carbon (ta-C) films. Since the substrate is completely removed from the measurement system, this method is insensitive to the mechanical properties of substrate. The mechanical properties of very thin DLC films could be thus measured and then can reveal the structural evolution of a-C:H films during the initial stages of deposition.     

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.     

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.     

Characteristics of carbon films prepared by plasma-based ion implantation

A series of carbon films have been prepared by plasma-based ion implantation (PBII) with C on pure Al and Si. Emphasis has been placed on the effect of implanting voltage on the characteristics of these films. The structures of the films were analyzed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The morphologies were observed by atomic force microscope (AFM). Surface hardness and electrical resistivity were also measured. The results indicate that the characteristics of these films are strongly dependent on the implanting voltage. An implanting voltage threshold value ranging from 3 to 5 kV starts to form a C-substrate transition layer owing to C⁺ ions implanted into the substrate. The transition layer exhibits a gradual change in composition and structure and effectively connects the carbon film and the substrate. Also, an implanting voltage threshold value ranging from 5 to 10 kV starts to form diamond-like carbon (DLC) films. An increasing voltage causes the resultant DLC films to be smoother and more compact. Moreover, Raman spectrum, chemical state of C1s, surface hardness and electrical resistivity all prove an optimum voltage of approximately 30 kV corresponding to the lowest ratio of sp²/sp³.     

Thermal stability of microhardness and internal stress of hard a-C films with predominantly sp2 bonds

Annealing in vacuum at temperatures up to 820 °C was used to study the thermal stability of mechanical properties of magnetron-sputtered thick (approx. 1.5 μm) a-C films. A predominance of sp² bonds was characteristic for all these films. The microhardness, internal stress, electron diffraction, Raman and optical spectra of three sets of films with different initial microhardness (H≈50, 20 and 10 GPa, respectively) were compared. Annealing of the hardest film up to 500 °C led to an increase in microhardness accompanied by a decrease in internal stress. Internal stress did not relax completely for hard films, even after annealing up to 820 °C, and the microhardness remained rather high (∼40 GPa). Both the high internal stress and the specific film nanostructure are responsible for the high microhardness of these sp²-bonded films.