Ion beam deposition of amorphous hydrogenated carbon films on amorphous silicon interlayer: Experiment and simulation

A thick layer of amorphous silicon (a-Si) was deposited on industrial grade crystalline n-Si < 111 > substrate by means of electron beam evaporation. On top of a-Si layer, amorphous hydrogenated carbon (a-C:H) film was grown by direct ion beam deposition from acetylene precursor gas. In order to study on atomic level the a-C:H film growth on amorphous silicon, a theoretical model was developed in a form of reaction rate (kinetic) equations. Numerical simulation using this model has revealed that the ratio of sp³/sp² content in the film is heavily influenced by relaxation rate of the carbon atoms in a sub-surface region of the film that were activated by ion irradiation. The final structure of a-C:H film does not depend much on elemental composition and structure of amorphous Si coating, provided that deposition procedure is not terminated at its initial stage but continues for more than 60 s. It became evident, therefore, that the use of a-Si interlayer with a-C:H films could be particularly beneficial when a need arises to minimize or eliminate the effect of the substrate. As one of such cases, a poor adhesion of amorphous carbon on steel and other ferrous alloys could be mentioned.     

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.     

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.     

Mechanical and high pressure tribological properties of nanocrystalline Ti(N,C) and amorphous C:H nanocomposite coatings

This paper reports on the mechanical and high pressure tribological properties of nanocrystalline (nc-) Ti(N,C)/amorphous (a-) C:H deposited, using low temperature (～ 200 °C) DC reactive magnetron sputtering. The mechanical properties are affected by the nc-Ti(N,C)/a-C:H phase fraction ratio. For increasing C contents (from 31 to 47 at.%) an increase of the a-C:H phase content and a degradation of the nanocrystalline phase occurs leading to a reduction in nanoindentation hardness (H) values (from 15 to 9 GPa) and reduced modulus (Er) values (from 150 to 80 GPa). A strong correlation between H/E ratio and wear performance was exhibited by the coatings. The synthesized coatings survived up to 100 m sliding distance when tested using pin-on-disc sliding configuration at > 4.5 GPa contact pressures and the measured friction coefficient values were similar for all films (μ ～ 0.21–0.25).     

Tribological study of hydrogenated amorphous carbon films with tailored microstructure and composition produced by bias-enhanced plasma chemical vapour deposition

We investigated the mechanical and tribological properties of hydrogenated amorphous carbon (a-C:H) films on silicon substrates by nanoindentation, ball-on-disc tribotesting and scratch testing. The a-C:H films were deposited from an argon/methane gas mixture by bias-enhanced electron cyclotron resonance chemical vapour deposition (ECR-CVD). We found that substrate biasing directly influences the hardness, friction and wear resistance of the a-C:H films. An abrupt change in these properties is observed at a substrate bias of about ?100 V, which is attributed to the bias-controlled transition from polymer- to fullerenelike carbon coatings. Friction coefficients in the range of 0.28–0.39 and wear rates of about 7 × 10^?5 mm³/Nm are derived for the polymeric films when tested against WC–Co balls at atmospheric test conditions. On the other hand, the fullerenelike hydrogenated carbon films produced at ion energies > 100 eV display a nanohardness of about 17 GPa, a strong reduction in the friction coefficient (～ 0.10) and a severe increase in the wear resistance (～ 1 × 10^?7 mm³/Nm). For these films, relative humidity has a detrimental effect on friction but no correlation with the wear rate was found.     

Characterization of thick and soft DLC coatings deposited on plasma nitrided austenitic stainless steel

Thick and soft a-C:H:Si coatings containing more than 45% hydrogen (thickness: 25–27 μm, hardness: 6 GPa, Young's Modulus 38 GPa and low ratio of sp³ bonds) were deposited by PACVD with a DC pulsed discharge on nitrided (duplex sample) and non-nitrided austenitic stainless steel (coated sample). After deposition, the chemical, microstructural and tribological properties were studied. Finally, the adhesion and the atmospheric corrosion resistance of a-C:H:Si coatings were also investigated.In pin-on-disk tests, the friction coefficient using an alumina pin of 6 mm in diameter as counterpart, under 0.59 GPa Hertzian pressure was 0.05 for the coated samples and 0.076 for the duplex samples. These values were more than one order of magnitude smaller than the friction coefficient of the nitrided sample without coating, which was around 0.65. In the coated samples, the wear loss could not be measured. In ball-on-disk tests under dry sliding conditions, the coatings were tested under different Hertzian pressures (1.29, 1.44 and 1.57 GPa) using a steel ball with a diameter of 1.5 mm as counterpart. Using a normal load of 9 N, the a-C:H:Si coating of the coated samples was broken and detached thus leading to a coefficient of friction of around 0.429. However, in contrast to that, the friction coefficient of the duplex samples remained stable and reached as maximum a value of 0.208.In abrasive tests, mass loss was undetectable in both duplex and coated samples. Furthermore it could be seen that the a-C:H:Si film showed only some smaller grooves and no severe damage or deformation. On the contrary, severe damage was observed in the only nitrided sample. With respect to adhesion, the critical load to break the coating was higher in the duplex sample (27 N) than in the only coated sample (16.3 N). By chemical analysis using the salt spray fog test, the duplex sample remained clean, but the coated sample failed and presented film delamination as well as general corrosion.     

Nanoindentation-induced deformation behaviour of tetrahedral amorphous carbon coating deposited by filtered cathodic vacuum arc

The nanoindentation-induced deformation behaviour of a ta-C (tetrahedral amorphous carbon) coating deposited on to a silicon substrate by a filtered vacuum cathodic vapour arc technique was investigated. The 0.17-μm-thick ta-C coating was subjected to nanoindentation with a spherical indenter and the residual indents were examined by cross-sectional transmission electron microscopy. The hard (~ 30 GPa) ta-C coatings exhibited very little localized plastic compression, unlike the softer amorphous carbon coatings deposited by plasma-assisted chemical vapour deposition. However, neither through-thickness cracks nor delamination was observed in the coating for the loads studied. Rather, the silicon substrate exhibited plastic deformation for indentation loads as low as 10 mN and at higher loads it showed evidence of both phase transformation and cracking. These microstructural features were correlated to the observed discontinuities in the load-displacement curves. Further, it was observed that even a very thin coating can modify the primary deformation mechanism from phase transformation in uncoated Si to predominantly plastic deformation in the underlying substrate.     

Si/SiC coated Cf/SiC composites via tape casting and reaction bonding: The effect of carbon content

In this paper, a novel surface modification method for C_f/SiC composites is proposed. Si/SiC coating on C_f/SiC composites is prepared by tape casting and reaction bonding method. The effects of carbon content on the rheological property of the slurries along with the microstructure of the sintered coatings are investigated. The best result has been obtained by infiltrating liquid silicon into a porous green tape with a carbon density of 0.84 g/cm³. In addition, the effect of sintering parameters on the phase composition of the coatings is studied. Dense Si/SiC coating with high density as well as strong bonding onto the substrate is obtained. This Si/SiC coating exhibits an excellent mechanical property with HV hardness of 16.29±0.53 GPa and fracture toughness of 3.01±0.32 MPa m^1/2. Fine surface with roughness (RMS) as low as 2.164 nm is achieved after precision grinding and polishing. This study inspires a novel and effective surface modification method for C_f/SiC composites.     

Microstructure,mechanical and tribological properties of Ti–B–C–N films prepared by reactive magnetron sputtering

Quaternary Ti–B–C–N thin films are deposited on high-speed steel substrates by the reactive magnetron sputtering (RMS) technique. The microstructure, mechanical and tribological properties of Ti–B–C–N films with different carbon contents (from 28.9 at.% to 54.2 at.%) are explored systematically. The microstructure of Ti–B–C–N films deposited by RMS is consisted mainly of Ti(C, N) nano-crystals embedded into an amorphous matrix of a-C/a-CN/a-BN/a-BC. As the carbon content increases, the crystalline size of the films diminishes, but the hardness linearly increases from 14 GPa to 26 GPa. The friction coefficient of the films sliding against steel GCr15 balls in air decreases with the increase of carbon content, which shows that Ti–B–C–N films with both higher hardness and lower friction coefficient can be obtained by means of increasing the carbon concentration in the films.     

Hard silicon carbonitride films obtained by RF-plasma-enhanced chemical vapour deposition using the single-source precursor bis(trimethylsilyl)carbodiimide

Amorphous silicon carbon nitride (Si/C/N) coatings were prepared on steel substrates by RF plasma-enhanced chemical vapour deposition (RF-PECVD) from the single-source precursor bis(trimethylsilyl)carbodiimide (BTSC). The films were characterised by X-ray diffraction (XRD), ellipsometry, FTIR, glow discharge optical emission spectroscopy (GDOES), optical microscopy, AFM, hardness measurements, scratch-, tribological- and corrosion-tests. The results of these studies show that the coatings obtained on the RF-powered electrode (cathode) were black, thick (>20 μm) and hard (21–29 GPa), while those grown on the grounded electrode (anode) were yellow, thin (<4 μm) and soft (∼5 GPa). Coatings on the anode contained around 19 at.% oxygen and exhibited silicon predominantly bonded to oxygen. In contrast, the oxygen content of the films deposited on the cathode was below 2 at.%. Silicon atoms in these coatings are co-ordinated predominantly to nitrogen and carbon. The surface of all coatings was very smooth with a maximum rms roughness between 2 nm and 5 nm for an area of 5 μm × 5 μm. Scratch and tribological tests reveal a brittle nature of the cathode-coatings and rather weak adhesion to the metal substrates. Salt-spray tests indicate an excellent corrosion resistance of the material.     

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.     

Influence of powder size on characteristics of air plasma sprayed silicon coatings

Silicon powders with different medium sizes (114 μm, 79 μm and 31 μm, respectively) were used to fabricate coatings by air plasma spraying. The velocity and temperature of in-flight silicon particles during plasma spraying were determined. The composition and microstructure of the coatings were characterized and some physical properties of the coatings were measured. The obtained results showed that the size of silicon particles had great influence on their velocity and temperature in plasma flame. The oxidation of silicon particles in the spraying process was observed and is higher for particles of smaller sizes. Areas of silicon oxide in micrometer size are embedded and randomly distributed in the coating. The surface roughness and void content of silicon coatings increase with an increase in the particle size of the powders. The microhardness and oxygen content of coatings decrease with an increase in the particle size. However, the size of silicon particles has little impact on the deposition efficiency of silicon under the same deposition conditions.     

One-step synthesis of a multi-functional anti-oxidation protective layer on the surface of carbon/carbon composites

A multi-functional anti-oxidation (AO) protective coating is produced in one-step synthesis on the surface of the carbon/carbon (C/C) composite by a novel electrically induced liquid infiltration (EILI) method. The AO coating involves several protective layers which have different anti-oxidation mechanisms. In this study phosphorus acid is applied as active-site poisoning agents to inhibit oxidations by forming stable glassy complex barriers that decrease oxygen diffusion. Simultaneously silicon carbide (SiC) or SiC/silica layers are formed on the surface of C/C composites that act as physical protection barriers for oxygen penetration. It is proved that under the optimum conditions the acid groups survive the high temperature EILI process. Oxidation tests reveal that formed coatings effectively protect C/C composite from oxidation: average percent of weight losses decrease from 30 to 1 wt.% and from 69 to 5 wt.% for the thermal (1150 K) and catalytic (920 K) oxidation tests, respectively.     

The role of amorphous phase content on the electrical properties of atmospheric plasma sprayed (Ba,Sr)TiO3 coatings

(Ba,Sr)TiO₃ coatings deposited on carbon steel substrates were successfully prepared by an atmospheric plasma spray system. Three sets of samples containing different amount of both crystalline and amorphous phases were deposited and consequently studied in order to determine their electrical properties. The results show a clear correlation existing between the amorphous phase content and coating´s electrical properties. The resistivity increases with increase of amorphous phase content. Relative permittivity for low frequencies decreases and become more stable with frequency tuning when amorphous phase content increased. The maximum relative permittivity value is in the range 75–200 for frequency 1 kHz. The loss factor varies between 0.23 and 0.03 for all studied samples. The loss factor is at the lower limit of these values and frequency much less dependent when the coating contains 15 wt% of amorphous phase and more. The band gap of all samples is between 2.75 eV and 2.90 eV. Microstructure and hardness were evaluated in order to determine basic mechanical properties of deposits.     

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.     

Nanostructure,mechanical and tribological properties of reactive magnetron sputtered TiCx coatings

This study describes the correlation between microstructure, mechanical and tribological properties of TiC_x coatings (with x being in the range of 0–1.4), deposited by reactive magnetron sputtering from a Ti target in Ar/C₂H₂ mixtures at ~ 200 °C. The mechanical and tribological properties were found to strongly depend on the chemical composition and the microstructure present. Very dense structures and high hardness, combined with low wear rates and friction coefficients, were observed for coatings with chemical composition close to TiC. X-ray diffraction and X-ray photoelectron spectroscopy analysis, used to evaluate coating microstructure, composition and relative phase fraction, showed that low carbon contents in the coatings lead to sub-stoichiometric nanocrystalline TiC_x coatings being deposited, whilst higher carbon contents gave rise to dual phase nanocomposite coatings consisting of stoichiometric TiC nanocrystallites and free amorphous carbon. Optimum performance was observed for nanocomposite TiC_1.1 coatings, comprised of nanocrystalline nc-TiC (with an average grain size of ~ 15 nm) separated by 2–3 monolayers of an amorphous a-DLC matrix phase.     

Principles for designing sputtering-based strategies for high-rate synthesis of dense and hard hydrogenated amorphous carbon thin films

In the present study we contribute to the understanding that is required for designing sputtering-based routes for high rate synthesis of hard and dense hydrogenated amorphous carbon (a-C:H) films. We compile and implement a strategy for synthesis of a-C:H thin films that entails coupling a hydrocarbon gas (acetylene) with high density discharges generated by the superposition of high power impulse magnetron sputtering (HiPIMS) and direct current magnetron sputtering (DCMS). Appropriate control of discharge density (by tuning HiPIMS/DCMS power ratio), gas phase composition and energy of the ionized depositing species leads to a route capable of providing ten-fold increase in the deposition rate of a-C film growth compared to HiPIMS Ar discharge (Aijaz et al., 2012). This is achieved without significant incorporation of H (< 10%) and with relatively high hardness (> 25 GPa) and mass density (~ 2.32 g/cm³). Using our experimental data together with Monte-Carlo computer simulations and data from the literature we suggest that: (i) dissociative reactions triggered by the interactions of energetic discharge electrons with hydrocarbon gas molecules is an important additional (to the sputtering cathode) source of film forming species and (ii) film microstructure and film hydrogen content are primarily controlled by interactions of energetic plasma species with surface and sub-surface layers of the growing film.     

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.     

A study of the microstructure and mechanical properties of SiC coatings on spherical particles

We have investigated the effect of the microstructure on the mechanical properties of three nearly stoichiometric SiC coatings (SiC, SiC + C and SiC + Si coating), which were coated onto spherical particles as simulated nuclear fuel particles by fluidized-bed chemical vapour deposition (FBCVD). The mechanical properties of the SiC coatings were studied using micro- and nano-indentation. The microstructure was characterised using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM was also used to elucidate the deformation behaviour under the indentation. The FBCVD SiC coatings studied exhibited a higher hardness than conventional CVD SiC coatings, and SiC coating gave the highest hardness among the three coatings. TEM confirmed that the presence of pores affect the Young's modulus of SiC coatings. The high hardness was attributed to the high density of dislocations and their interactions. The initiation and propagation of micro cracks under the confined shear stress was found to be responsible for the mechanism of plastic deformation. Based on this hardness-related plastic deformation mechanism, the variation of hardness in the three types of SiC coating was due to different grain morphologies.     

Deposition and structural analyses of molybdenum-disulfide (MoS2)–amorphous hydrogenated carbon (a-C:H) composite coatings

In this study we developed composite coatings consisting of amorphous hydrogenated carbon (a-C:H) and molybdenum-disulfide (MoS₂), and clarified their microstructure. In addition, we interpreted the tribological properties of the composite coatings in the viewpoint of a deposition-induced microstructural modification. The coatings were produced by the hybrid deposition technique of RF-generated methane and argon plasma and DC magnetron co-sputtering of MoS₂ target. The deposition parameter investigated in this study was methane flow rate. Structural analyses were performed using a transmission electron microscope (TEM) and an atomic force microscope (AFM). Friction tests were conducted using a ball-on-disk type tribometer. From an electron micrograph, it was confirmed that nano-clusters were embedded into an amorphous carbon host matrix. Surface roughness of the composite coating was ～ 0.25 nm in R_a compared to 5.0 nm in R_a of sputtered MoS₂. The concentration measurements were performed, and the results show that the sulfur and molybdenum concentration ratio, [S]/[Mo], is ～ 0.9, which indicates that the amount of sulfur was reduced due to the discharged plasma. In friction tests, composite coatings showed high friction in a vacuum condition. It was considered that lubricant MoS₂ lamellar structures showing super-low friction in a vacuum condition during friction could not be formed between ball and coating during friction because of the lack of sulfur in embedded clusters.