Structural analysis of a-C:H and a-C:H:Si films under high-pressure and high-temperature by synchrotron X-ray diffraction

Amorphous carbon films have several outstanding tribology characteristics, including high hardness, surface smoothness, and low friction. Under tribological conditions, their surface is generally exposed to high-temperature and pressure. Although the structure of amorphous carbon films is likely changed by high temperature and pressure, there have been no reports on such structural changes of the films. To obtain information about their structural changes, synchrotron X-ray diffraction was used to analyze two kinds of amorphous carbon films, a-C:H and a-C:H:Si, under high-temperature and high-hydrostatic pressure conditions. Synchrotron X-ray diffraction was applied to films pressurized by a multi-anvil press installed in the PF-AR NE5C beamline at KEK at room temperature and at a high-temperature around 200 °C. The pair distribution functions derived by Fourier transformation of the obtained scattering intensity profiles showed that the sp²/sp³ ratios for both films decreased as the pressure increased and that the sp²/sp³ ratio for the a-C:H film increased as the temperature increased. This indicates that high-pressure creates sp³ stabilization in a-C:H and a-C:H:Si films while high-temperature creates sp² transition in a-C:H film. The sp²/sp³ ratio for the a-C:H:Si film did not change much even at high-temperature due to the high thermal-oxidative stability of a-C:H:Si.     

Properties of a-C:H films grown in inert gas ambient with camphoric carbon precursor of pulsed laser deposition

The influence of the ambient argon gas (Ar) pressure on the properties of the hydrogenated amorphous carbon (a-C:H) films deposited by pulsed laser deposition (PLD) using camphoric carbon (CC) target have been studied. The a-C:H films are deposited with varying Ar pressure range from 0.01 to 0.23 Torr. SEM and AFM show that the particle size of films is decreases, while the roughness increases with higher Ar pressure. The FTIR measurement revealed the presence of hydrogen in the a-C:H films. We found the surface morphology, structural and physical properties structure of a-C:H films are influenced by the presence of inert gas and the ratio of sp² trigonal component to sp³ tetrahedral component is strongly dependent on the inert gas pressure. We suggest that these phenomena are due to the effect of the optimum concentration of the Ar atoms in the C lattice. Improvement of the structural properties of the a-C:H films deposited in inert gas environment using CC target reveals different behaviour than reported earlier.     

Structure and electrical properties of a-C:H thin films deposited by RF sputtering

We investigated the film structure and the electrical properties of hydrogenated amorphous carbon (a-C:H) thin films. a-C:H thin films were prepared by RF magnetron sputtering. Two different RF power sources of 13.56 MHz and 60 MHz were used to deposit the a-C:H films. The bonding hydrogen concentration varied from 1.6 × 10²² cm^? 3 to 8.6 × 10²² cm^? 3. The concentration of incorporated hydrogen atoms varied from 18 to 57 at.%. The optical gap increased from 1.58 eV to 2.56 eV with increasing the hydrogen concentration. The resistivity increased from 10¹³ Ω cm to 10¹⁵ Ω cm with increasing the hydrogen concentration. The permittivity measured at 1 MHz decreased from 5.6 to 2.3 with increasing the hydrogen concentration. These results suggest that the film structure and electrical properties can be controlled by the hydrogen concentration.     

X-ray photoemission spectroscopy of nitrogen-doped UNCD/a-C:H films prepared by pulsed laser deposition

Nitrogen-doped ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) films were deposited by pulsed laser deposition (PLD). Nitrogen contents in the films were controlled by varying a ratio in the inflow amount between nitrogen and hydrogen gases. The film doped with a nitrogen content of 7.9 at.% possessed n-type conduction with an electrical conductivity of 18 Ω^? 1 cm^? 1 at 300 K. X-ray photoemission spectra, which were measured using synchrotron radiation, were decomposed into four component spectra due to sp², sp³ hybridized carbons, C=N and C–N. A full-width at half-maximum of the sp³ peak was 0.91 eV. This small value is specific to UNCD/a-C:H films. The sp²/(sp³ + sp²) value was enhanced from 32 to 40% with an increase in the nitrogen content from 0 to 7.9 at.%. This increment probably originates from the nitrogen incorporation into an a-C:H matrix and grain boundaries of UNCD crystallites. Since an electrical conductivity of a-C:H does not dramatically enhance for this doping amount according to previous reports, we believe that the electrical conductivity enhancement is predominantly due to the nitrogen incorporation into grain boundaries.     

Cathodoluminescence and electron field emission of boron-doped a-C:N films

The optical and electrical properties of so-called carbon nitride films (a-C:N) and boron doped so-called carbon nitride films (a-C:N:B) are studied with cathodoluminescence (CL) spectroscopy and electron field emission measurement. The a-C:N films were first deposited on Si by a filtered cathodic arc plasma system, and then boron ions (∼1 × 10¹⁶ cm⁻²) were implanted into the a-C:N films to form a-C:N:B films by a medium current implanter. The structural and morphological properties of a-C:N and a-C:N:B films were then analyzed using secondary ion mass spectrometer, X-ray photoelectron spectroscopy, FT-IR spectra, Raman spectroscopy and atomic force microscopy. The a-C:N film exhibits luminescence of blue light (∼2.67 eV) and red light (∼1.91 eV), and the a-C:N:B film displays luminescence of blue light (∼2.67 eV) in CL spectra measured at 300 K. Furthermore, the incorporated boron atoms change the electron field emission property, which shows a higher turn on field for the a-C:N:B film (3.6 V/μm) than that for the a-C:N film (2.8 V/μm).     

Hardness of nanocomposite a-C:Si films deposited by magnetron sputtering

Hydrogen-free a-C:Si films with Si concentration from 3 to 70 at.% were prepared by magnetron co-sputtering of pure graphite and silicon at room temperature. Mechanical properties (hardness, intrinsic stress), film composition (EPMA and XPS) and film structure (electron diffraction, Raman spectra) were investigated in dependence on Si concentration, substrate bias and deposition temperature. The film hardness was maximal for ∼ 45 at.% of Si and deposition temperatures 600 and 800 °C. Reflection electron diffraction indicated an amorphous structure of all the films. Raman spectra showed that the films in the range of 35–70 at.% of Si always contain three bands corresponding to the Si, SiC and C clusters. Photoelectron spectra showed dependency of Si–C bond formation on preparation conditions. In the films close to the stoichiometric SiC composition, the surface and sub-surface carbon atoms exhibited dominantly sp³ bonds. Thus, the maximal hardness was observed in nanocomposite a-C:Si films with a small excess of carbon atoms.     

Electrochemical behavior of the Ti–6Al–4V alloy coated with a-C:H films

In this work diamond-like carbon films were deposited on the Ti–6Al–4V alloy, which has been used in aeronautics and biomedical fields, by electrical discharges using a magnetron cathode and a 99.999% graphite target in two different atmospheres, the first one constituted by argon and hydrogen and the second one by argon and methane. Films deposited using the argon/hydrogen mixture were called a-C:H, while films deposited using the argon/methane mixture were called DLC. Raman spectroscopy was used to study the structure of the films. The Raman spectra profile of the a-C:H films is quite different from that of the DLC films. The disorder degree of the graphite crystalline phase in a-C:H films is higher than in DLC films (a-C:H films present small values for the the I_D/I_G ratio). Potentiodynamic corrosion tests in 0.5 mol l⁻¹ NaCl aqueous solution, pH 5.8, at room temperature (≈25 °C) were carried out as for the a-C:H as for the DLC coated surfaces. Comparison between the corrosion parameters of a-C:H and DLC coated surfaces under similar deposition time, showed that DLC coated surfaces present bigger corrosion potential (E_corr) and polarization resistance than those coated with a-C:H films. Electrochemical impedance spectroscopy (EIS) was also used to study the electrochemical behavior of a-C:H and DLC coated surfaces exposed to 0.5 mol l⁻¹ aqueous solution. The EIS results were simulated with equivalent electrical circuit models for porous films. The results of these simulations showed similar tendency to the one observed in the potentiodynamic corrosion tests. The DLC film resistance and the charge transfer resistance (R_ct) for the DLC coated surface/electrolyte interface were bigger than the ones determined for the a-C:H coated surfaces.     

Internal stress of a-C:H(N) films deposited by radio frequency plasma enhanced chemical vapor deposition

Amorphous hydrogenated carbon nitride [a-C:H(N)] films were deposited from the mixture of C₂H₂ and N₂ using the radio frequency plasma enhanced chemical vapor deposition technique. The films were characterized by X-ray photon spectroscopy, infrared, and positron annihilation spectroscopy. The internal stress was measured by substrate bending method. Up to 9.09 at% N was incorporated in the films as the N₂ content in the feed gas was increased from 0 to 75%. N atoms are chemically bonded to C as C–N, CN and CN bond. Positron annihilation spectra shows that density of voids increases with the incorporation of nitrogen in the films. With rising nitrogen content the internal stress in the a-C:H(N) films decrease monotonically, and the rate of decrease in internal stress increase rapidly. The reduction of the average coordination number and the relax of films structure due to the decrease of H content and sp³/sp² ratio in the films, the incorporation of nitrogen atoms, and the increases of void density in a-C:H(N) films are the main factors that induce the reduction of internal stress.     

Water-repellency and surface free energy of a-C:H films prepared by heat-treatment of polymer precursor

Amorphous hydrogenated carbon (a-C:H) films with high water-repellency were prepared on Si substrates by a simple heat-treatment of the poly(phenylcarbyne) polymer at various temperature in Ar atmosphere. The contact-angle (CA) was measured by the sessile-drop technique. The use of CA for different liquid (water, ethylene glycol, and formamide) analysis for the evaluation of surface energy including the dispersion and polar components was described. The influences of surface roughness, chemical composition, and microstructure of carbon films on water CA and surface energy were investigated by atomic force microscopy (AFM), Fourier transforms infrared spectrometry (FTIR), and Raman spectroscopy, respectively. As the results, the a-C:H films exhibit good hydrophobicity and low surface free energy. The CA of a-C:H films slight increases and the surface free energy of a-C:H films reduces with the increase of the heat-treatment temperature. The a-C:H films are hydrophobic for not only pure water but also corrosive liquids, such as acidic and alkali solutions. The roughness of the films has no obvious effect on the CA and the reduction of surface energy with the increase of heat-treatment temperature is related to increase of sp² content in the film.     

Relationship between mechanical properties and coefficient of friction of sputtered a-C/Cu composite thin films

The article reports on properties of a-C films containing different amount of Cu. Films were sputtered by unbalanced magnetron from a graphite target with Cu fixing ring in argon under different deposition conditions. Relationships between the structure, mechanical properties, macrostress σ and coefficient of friction (CoF) μ of a-C/Cu films sputtered on Si substrates were investigated in detail. Besides, a special attention was concentrated on investigation of the effect of a deposition rate a_D of the a-C/Cu film on its hardness H and macrostress σ. Four main issues were found: (1) the addition of Cu into a-C film strongly influences its structure and mechanical properties, i.e. the hardness H, effective Young's modulus E⁎ macrostress σ and CoF, and makes it possible to form electrically conductive films; here E⁎ =  E / (1 − ν²), E is the Young's modulus, and ν is the Poisson's ratio, (2) the hardness H and compressive macrostress σ of the a-C/Cu film decrease with increasing a_D due to decreasing of total energy E_T delivered to the film during its growth, (3) hard a-C/Cu films with low value of CoF (μ ≈ 0.1) can be sputtered at high deposition rates a_D ranging from ~ 10 to ~ 80 nm/min, and (4) CoF decreases with increasing (i) hardness H and (ii) resistance of film to plastic deformation characterized by the ratio H³/E⁎² but only in the case when compressive macrostress σ is low.     

Effect of H2 addition during PECVD on the moisture barrier property and environmental stability of H:SiNx film

A hydrogenated silicon nitride (H:SiN_x) film with enhanced moisture barrier property and environmental stability was developed using plasma-enhanced chemical vapor deposition (PECVD) with the addition of H₂ gas at 100°C. The moisture barrier property and film density of the 100-nm-thick H:SiN_x film were ameliorated by increasing the H₂ gas flow rate during PECVD. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy studies demonstrated that the improved performance was a result of an increase in the amount of Si–N bonds compared to hydrogen-terminated bonds with an increase in the H₂ gas flow rate. It is believed that H₂ gas assisted the formation of aminosilane, which contributed to the condensation of silicon nitride by lowering the activation energy for radicalization reactions of silane and ammonia. After the 85°C/85% RH test, the optimized H:SiN_x film maintained a water vapor transmission rate lower than 5 × 10⁻⁵ g/m²/day owing to the suppression of oxidation. The optimized H:SiN_x film was rarely oxidized owing to the decrease in hydrogen-terminated bonds and increase in the film density. The results indicated that the introduction of H₂ gas during the PECVD process strengthened the environmental stability of the H:SiN_x film.     

Improved adhesion and tribological properties of fast-deposited hard graphite-like hydrogenated amorphous carbon films

Graphite-like hard hydrogenated amorphous carbon (a-C:H) was deposited using an Ar-C₂H₂ expanding thermal plasma chemical vapour deposition (ETP-CVD) process. The relatively high hardness of the fast deposited a-C:H material leads to high compressive stress resulting in poor adhesion between the carbon films and common substrates like silicon, glass and steel. A widespread solution to this problem is the use of an adhesion interlayer. Here we report on the changes in adhesion between the graphite-like a-C:H films and M2 steel substrates when different types of interlayers are used. Insignificant to very small improvements in adhesion were observed when using amorphous silicon oxide (a-SiO_x), amorphous organosilicon (a-SiC_xO_y:H_z) and amorphous hydrogenated silicon carbide (a-SiC_x:H_y) as adhesion layers. However, when sputtered Ti was used as an interlayer, the adhesion increased significantly. The dependence of the adhesive properties on the deposition temperature and interlayer thickness, as well as on the thickness of the a-C:H layer is presented and discussed. The low wear rates measured for the a-C:H/Ti/M2 stack suggest that these films are ideal for tribological applications.     

Thermal grafting of organic monolayers on amorphous carbon and silicon (111) surfaces: A comparative study

Thermally-assisted (160 °C) liquid phase grafting of linear alkene molecules has been performed simultaneously on amorphous carbon (a-C) and hydrogen passivated crystalline silicon Si(111):H surfaces. Atomically flat a-C films with a high sp³ average surface hybridization, sp³ / (sp² + sp³) = 0.62, were grown using pulsed laser deposition (PLD). Quantitative analysis of X-ray photoelectron spectroscopy, X-ray reflectometry and spectroscopic ellipsometry data show the immobilization of a densely packed (> 3 × 10¹⁴ cm^? 2) single layer of organic molecules. In contrast with crystalline Si(111):H and other forms of carbon films, no surface preparation is required for the thermal grafting of alkene molecules on PLD amorphous carbon. The molecular grafted a-C surface is stable against ambient oxidation, in contrast with the grafted crystalline silicon surface.     

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.     

Electric properties of a–C:H/a–C:N:H/aluminum structure

To obtain photovoltaic properties using carbon films without dependence on a Si substrate by drift of carriers, a photovoltaic cell consisted of aluminum (Al) and amorphous carbon films was fabricated. Two types of amorphous carbon were deposited on smooth Al substrates and on n-type Si by radio-frequency chemical vapor deposition. The first layer is nitrogen-doped hydrogenated amorphous carbon (a–C:N:H) film, and the second layer is hydrogenated amorphous carbon (a–C:H) film. Nitrogen atoms in a–C:N:H film were introduced as N–H structure. Both type of films were confirmed to be semiconductors on the basis of the temperature dependence of electroconductivity. The a–C:N:H/n–Si structure exhibited photovoltaic characteristics Furthermore, the a–C:H/a–C:N:H/Al structure cell also exhibited photovoltaic characteristics with an open-circuit voltage and short-circuit current of 5.5 mV and 0.83 μA/cm^− 2, respectively.     

Experimental analysis of the thermal annealing of hard a-C:H films

The a-C:H layers were deposited on silicon substrates in 100 kHz bipolar-pulsed discharges from a fixed mixture of acetylene and argon. Three types of a-C:H material with different hydrogen contents and hardness were obtained by adjusting the pressure during deposition to 2 Pa (hardness ≈ 23 GPa; hydrogen concentration ≈ 19 at.%), 4 Pa (20 GPa; 20 at.%) and 8 Pa (17 GPa; 24 at.%).Annealing was performed in high vacuum at a heating rate of 3 K/min up to a maximum temperature, varied between 200 °C and 900 °C. The annealing process was investigated in situ by mass spectrometric measurement of the effusion products as a function of temperature.After cooling down in high vacuum, ex situ measurements revealed changes in layer thickness (profilometer), hardness (nanoindentation), residual stress (from the curvature of the silicon substrates), elemental composition (elastic recoil detection analysis and Rutherford backscattering), UV/VIS optical properties (variable angle spectroscopic ellipsometry), and bonding (Raman spectroscopy and Fourier transform infrared spectroscopy).The films retained their hardness, level of compressive stress, and elemental composition at least up to 500 °C.The variation of the film thickness with the annealing temperature was systematically analysed. Up to 625 °C, the a-C:H thickness increased by 8.5% without measurable difference between the three layer types nor any influence of the initial a-C:H thickness. With further annealing the increase of the film thickness passed a maximum, the magnitude and temperature-position of which increased with decreasing pressure during deposition. The highest relative film thickness increase of 14% was found for a-C:H deposited at 2 Pa and annealed to 725 °C.Based on the results of the complementary characterisation methods, the effects of annealing in high vacuum on film structure and properties are discussed and fundamental processes, prevailing in characteristic annealing-temperature ranges, are derived.     

Superior mechanical and tribological properties governed by optimized modulation ratio in WC/a-C nano-multilayers

WC/a-C nano-multilayers with different modulation ratio (WC:a-C) ranging from 1:10 to 1.5:1 were deposited by fixing a-C individual layer thickness and tailoring WC individual layer thickness. The effect of modulation ratio on mechanical and tribological performance of WC/a-C nano-multilayers were investigated. Superior mechanical and tribological properties were simultaneously achieved at modulation ratio of 1:1.2. In addition to the improvement of mechanical properties, the improved tribological properties should also be attributed to the friction-induced formation of a WO₃-rich transfer film under an appropriate WC individual layer thickness, which combing the graphitized worn film surface constructed an intrinsically weak-interacting sliding interface (WO₃/C interface). Also, graphitized carbon is an essential coadjutant for the formation of WO₃-rich transfer film.     

Diamond-like carbon films: electron spin resonance (ESR) and Raman spectroscopy

Tetrahedral diamond-like carbon (ta-C) films and hydrogenated a-C:H films were deposited onto Si substrates using filtered cathodic vacuum arc (FCVA) process and direct ion beam deposition from CH₄/C₂H₄ plasma, respectively. Stress of deposited films was varied in the range 2.8–8.5 GPa depending on deposition conditions. Stationary and pulse electron spin resonance (ESR), and Raman spectroscopy techniques were used to analyze sp² related defects in pseudo-gap of undoped as deposited and annealed 20–100 nm thick films.¹ High density of ESR active paramagnetic centers (PC) N_s=(1.0–4.5)×10²¹ cm⁻³ at g=2.0025 was observed in the films. The dependence of ESR line width and line shape vs. deposition conditions and internal film stress were investigated. The several actual mechanisms for ESR line width broadening were considered: spin–spin dipole–dipole and exchange interactions, super-hyperfine interaction (SHFI) with ¹H (for a-C:H), averaging of SHFI due to electron jumps between PC positions with different SHFI values, and broadening due to Mott's electron hopping process. Three types of samples were revealed depending on relative contribution of these mechanisms. Effects of annealing on mechanical and paramagnetic properties of films were studied. An electrical resistance anisotropy at room temperature for ta-C films and g-value anisotropy at low temperature (T<77 K) for both ta-C and a-C:H films were found for the first time. Nature and distribution details of paramagnetic defects in DLC films, anisotropy effects and Raman spectroscopy data are discussed.     

Mechanical and tribological assessment of composite AlCrN or a-C:Ag-based thin films for implant application

The present study focuses on the comparison of cathodic arc deposited AlCrN (ternary coating) and Ag alloyed a-C (amorphous carbon base coating) on chrome nitride (CrN) medical grade 316 LVM stainless steel. The work comprises of morphological, structural, nanomechanical and tribological evaluation in physiological simulated body fluid (SBF) lubrication following conditions pertaining to simulated hip joint. According to the findings, H/E, H³/E² and E_coating/E_substrate significantly effect the nanomechanical and tribological properties of the coatings. While a-C:Ag/CrN exhibited better L_y value compared to AlCrN/CrN due to better surface quality, the later has shown higher L_c2 value during nanoscratch test attributed to lower H³/E² and higher plastic work done. Inspite of lower friction coefficient, a-C:Ag/CrN observed higher wear rate during simulated tribotest attributed to low hardness, separate graphitic structure due to Ag doping and sudden increase of friction coefficient ascribed to severe abrasive delamination of a-C:Ag top layer. The wear mechanism observed under SEM microscopy indicate severe adhesion of Ti6Al4V counterbody on AlCrN/CrN coated surface. The size of wear debris obtained with AlCrN/CrN-Ti6Al4V tribopair was larger in size compared to a-C:Ag/CrN-Ti6Al4V tribopair. Nevertheless, despite inferior surface quality and lower L_y value and larger wear debris size, AlCrN/CrN coating performed better in nanoscratch (at L_c2 value) and demonstrated lower wear in simulated tribotest in physiological SBF condition.     

Mechanical properties of a-C:H multilayer films

Multilayered amorphous hydrogenated carbon (a-C:H) films consisting of alternating sublayers with different mechanical properties have been deposited by an electron cyclotron resonance microwave-plasma chemical vapor deposition (ECR MP-CVD) system and modulating substrate bias voltage. The mechanical properties of the multilayer films were determined using nanoindentation and nanoscratch experiments with reference to single a-C:H layers of which the multilayer structure were composed. In nanoindentation tests, the relationship between the film hardness and indentation depth has been obtained over an indentation depth range of 20–500 nm. Since the films tend to fracture under high load in nanoindentation tests, their critical fracture loads were determined. The critical loads for fracturing the multilayered a-C:H films were higher than those of single a-C:H layers. The nanoscratch tests also showed that the multilayered a-C:H films required a higher critical load for scratching fracture. This study implies that the mechanical properties of a-C:H film can be improved by engineering suitable multilayer structures.