Structure and elastic recovery of Cr-C:H films deposited by a reactive magnetron sputtering technique

Cr-containing hydrogenated amorphous carbon (Cr-C:H) films were deposited on silicon substrates using a DC reactive magnetron sputtering with Cr target in an Ar and C₂H₂ gas mixture. The composition, bond structure, mechanical hardness and elastic recovery of the films were characterized using energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and nano-indentation. The film tribological behavior was also studied by a ball-on-disc tribo-tester. The results showed that the films deposited at low C₂H₂ flow rate (<10 sccm) presented a feature of composite Cr-C:H structure, which consisted of hard brittle chromium carbide phases and amorphous hydrocarbon phase, and thus led to the observed low elastic recovery and poor wear resistance of the films. However, the film deposited at high C₂H₂ flow rate (40 sccm) was found to present a typical feature of polymer-like a-C:H structure containing a large amount of sp³ C-H bonds. As a result, the film revealed a high elastic recovery, and thus exhibited an excellent wear resistance.     

Structure and hardness of a-C:H films prepared by middle frequency plasma chemical vapor deposition

a-C:H films were prepared by middle frequency plasma chemical vapor deposition (MF-PCVD) on silicon substrates from two hydrocarbon source gases, CH₄ and a mixture of C₂H₂ + H₂, at varying bias voltage amplitudes. Raman spectroscopy shows that the structure of the a-C:H films deposited from these two precursors is different. For the films deposited from CH₄, the G peak position around 1520 cm⁻¹ and the small intensity ratio of D peak to G peak (I(D)/I(G)) indicate that the C-C sp³ fraction in this film is about 20 at.%. These films are diamond-like a-C:H films. For the films deposited from C₂H₂ + H₂, the Raman results indicate that their structure is close to graphite-like amorphous carbon. The hardness and elastic modulus of the films deposited from CH₄ increase with increasing bias voltage, while a decrease of hardness and elastic modulus of the films deposited from a mixture of C₂H₂ + H₂ with increasing bias voltage is observed.     

Chemical reactivity of PVD-coated WC-Co tools with steel

The chemical reactivity of CrN, ZrN, TiC_xN_1−x and naCo^® PVD coatings on a WC-Co cemented carbide substrate with steel has been evaluated by means of the static interaction couples technique. Diffusion experiments with coated and uncoated tools were carried out at 900, 1100 and 1300 °C in order to establish the maximum temperature at which the substrate-coating-workpiece combinations are chemically stable. Computational equilibrium thermodynamics was used to identify the interaction products formed at elevated temperature and the chemical solubility of the different coating materials into iron. A metallic (Fe, Co) fcc solid solution was identified at the steel side of the interface from 1100 °C on for all the coated tools and from 900 °C for the uncoated carbide. In addition to this interaction product, the ?-carbide was identified at 1300 °C on the WC-Co side of the interface. Both of the experimental findings and thermodynamic equilibrium solubility calculations demonstrated that the PVD-coated WC-Co tools exhibit a lower chemical reactivity with respect to the uncoated tools.     

Bias voltage effect on the structure and property of chromium copper-diamond-like carbon multilayer films fabricated by cathodic arc plasma

Chromium copper-diamond-like carbon (Cr:Cu)-DLC films were deposited onto silicon and by cathodic arc evaporation process using chromium (Cr) and copper (Cu) target arc sources to provide Cr and Cu in the Me-DLC. Acetylene reactive gases were the carbon source and activated at 180 °C at 13 mTorr, and a substrate bias voltage was varied from −50 V to −200 V to provide the (Cr:Cu)-DLC structure. The structure, interface, and chemical bonding state of the produced film were analyzed by transmission electron microscope (TEM), IR Fourier transform (FTIR) spectra, and X-ray photoelectron spectroscopy (XPS). The results showed that the Cr-containing a-C:H/Cu coatings exhibited an amorphous layer of DLC:Cr layer and a crystalline layer of Cu multilayer structure. The profiles of sp³/sp² (XPS) ratios corresponded to the change of microhardness profile by varying the pressure of the negative DC bias voltage. These (Cr:Cu)-DLC coatings are promising materials for soft substrate protective coatings.     

Electrochemical corrosion behaviors of a-C:H and a-C:NX:H films

The a-C:H and a-C:N_X:H films were deposited onto silicon wafers using radio frequency (rf) plasma enhanced chemical vapor deposition (PECVD) and pulsed-dc glow discharge plasma CVD, respectively. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize chemical nature and bond types of the films. The results demonstrated that the a-C:H film prepared by rf-CVD (rf C:H) has lower I_D/I_G ratio, indicating smaller sp² cluster size in an amorphous carbon matrix. The nitrogen concentrations of 2.9 at.% and 7.9 at.% correspond to carbon nitride films prepared with rf and pulse power, respectively.Electrochemical corrosion performances of the carbon films were investigated by potentiodynamic polarization test. The electrolyte used in this work was a 0.89% NaCl solution. The corrosion test showed that the rf C:H film exhibited excellent anti-corrosion performance with a corrosion rate of 2 nA cm⁻², while the carbon nitride films prepared by rf technique and pulse technique showed a corrosion rate of 6 nA cm⁻² and 235 nA cm⁻², respectively. It is reasonable to conclude that the smaller sp² cluster size of rf C:H film restrained the electron transfer velocity and then avoids detriment from the exchange of electrons.     

Analysis of EIS characteristics of CO2 corrosion of well tube steels with corrosion scales

The electrochemical impedance spectroscopy (EIS) was used to study the characteristics of CO₂ corrosion of N80 and 4Cr steels with corrosion scales. The results indicated that CO₂ corrosion scale on tube steel could prevent the rate of mass transfer remarkably, corrosion rate was controlled by ions diffusion in corrosion scale, which led to finite length diffusion impedance occurred in electrochemical impedance spectra. Additionally, pitting of N80 steel could lead to additional capacitive reactance in impedance spectrum. The ion diffusion coefficient in corrosion scale and porosity of corrosion scale could be calculated by Warburg impedance coefficient, the results shown that the value of H⁺ diffusion coefficient in N80 and 4Cr corrosion scale is (3.46 and 1.76) × 10⁻¹⁰ m² s⁻¹, respectively. The protective ability of 4Cr corrosion scale was better than that of N80 corrosion scale.     

Establishment of equivalent circuits from electrochemical impedance spectroscopy study of corrosion inhibition of steel by pyrazine in sulphuric acidic solution

This paper describes the use of the electrochemical impedance spectroscopy technique (EIS) in order to study the corrosion inhibition process of steel in 0.5 M H₂SO₄ solution at the open circuit potential (OCP). Diethyl pyrazine-2,3-dicarboxylate (Prz) as a non-ionic surfactant (NS) inhibitor has been examined. The Nyquist diagrams consisted of a capacitive semicircle at high frequencies followed by a well-defined inductive loop at low frequency values. The impedance measurements were interpreted according to suitable equivalent circuits. The results obtained showed that the Prz is a good inhibitor. The inhibition efficiency increases with an increase in the surfactant concentration to attain 80% at the 5 × 10⁻³M. Prz is adsorbed on the steel surface according to a Langmuir isotherm adsorption model.     

Galvanomagnetic properties of air-stable and highly conductive potassium-intercalated graphite sheet

Magnetoresistance and Hall coefficient of air-stable potassium-intercalated graphite sheets (hereafter abbreviated as K-PGS) were determined at room temperature. The magnitude of the magnetoresistance and the absolute value of Hall coefficient of K-PGS decreased with increasing potassium content of K-PGS, n_K/n_C. Two-carrier model was used for calculating carrier density and mobility. The electron density increased with increasing n_K/n_C: 3.07×10²⁰ cm⁻³ (n_K/n_C=0.005), 5.67×10²⁰ cm⁻³ (n_K/n_C=0.008) and 6.40×10²⁰ cm⁻³ (n_K/n_C=0.011). The value of the electron density of K-PGS with n_K/n_C=0.011 (nominal composition KC₉₁) was about 80% of the reported value, 7.8×10²⁰ cm⁻³, for KC₄₈ (n_K/n_C=0.021) prepared from HOPG (highly oriented pyrolytic graphite). The mobility decreased with increasing n_K/n_C: 2.11×10³ cm² V⁻¹ s⁻¹ (n_K/n_C=0.005), 1.42×10³ cm² V⁻¹ s⁻¹ (n_K/n_C=0.008) and 1.34×10³ cm² V⁻¹ s⁻¹ (n_K/n_C=0.011). The value of the mobility of K-PGS with n_K/n_C=0.011 was about 60% of the reported value (2300 cm² V⁻¹ s⁻¹) for KC₄₈ prepared from HOPG.     

Mechanical, tribological, and electrochemical behavior of Cr1−xAlxN coatings deposited by r.f. reactive magnetron co-sputtering method

Chromium aluminum nitride (Cr_1−xAl_xN) coatings were deposited onto AISI H13 steel and silicon substrates by r.f. reactive magnetron co-sputtering in (Ar/N₂) gas mixture from chromium and aluminum targets. Properties of deposited Cr_1−xAl_xN coatings such as compositional, structural, morphological, electrochemical, mechanical and tribological, were investigated as functions of aluminum content. X-ray diffraction patterns of Cr_1−xAl_xN coatings with different atomic concentrations of aluminum (0.51 < x < 0.69) showed the presence and evolution of (1 1 1), (2 0 0), and (1 0 2) crystallographic orientations associated to the Cr_1−xAl_xN cubic and w-AlN phases, respectively. The rate of corrosion of the steel coated with Cr_1−xAl_xN varied with the applied power; however, always being clearly lower when compared to the uncoated substrate. The behavior of the protective effect of the Cr_1−xAl_xN coatings is based on the substitution of Cr for Al, when the power applied to the aluminum target increases. The mechanical properties were also sensitive to the power applied, leading to a maximum in hardness and a reduced elastic modulus of 30 and 303 GPa at 350 W and a monotonic decrease to 11 and 212 GPa at 450 W, respectively. Finally, the friction coefficient measured by pin-on disk revealed values between 0.45 and 0.70 in humid atmosphere.     

Synthesis and electrochemical characteristics of Ta-N thin films fabricated by cathodic arc deposition

Ta-N thin films were deposited on AISI 317L stainless steel (SS) substrates by cathodic arc deposition (CAD) at substrate biases of −50 and −200 V. The as-deposited films were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray analysis (EDX). The results show that stoichiometric TaN with hexagonal lattice (3 0 0) preferred orientation was achieved at the bias of −200 V. On the other hand, Ta-rich Ta-N thin film deposited at −50 V shows amorphous nature. According to the XPS result, Ta element in the films surface exist in bonded state, including the Ta-N bonds characterized by the doublet (Ta 4f_7/2 = 23.7 eV and Ta 4f_5/2 = 25.7 eV). Electrochemical properties of the Ta-N coated stainless steel systems were investigated using potentiodynamic polarization and electrochemical impedance spectroscope (EIS) in Hank's solution at 37 °C. For the Ta-N coated samples, the corrosion current (i_corr) is two or three orders of magnitude lower than that of the uncoated ones, indicating a significantly improved corrosion resistance. Growth defects in the Ta-N thin films produced by CAD, however, play a key role in the corrosion process, especially the localised corrosion. Using the polarization fitting and the EIS modelling, we compared the polarization resistance (R_p) and the porosity (P) of the Ta-N coatings deposited at different biases. It seems that Ta-N film with comparatively lower bias (−50 V) shows better corrosion behavior in artifical physiological solution. That may be attributed to the effect of ion bombarding, which can be modulated by the substrate bias.     

Kinetics for the reaction of phenyl radical with phenylacetylene and styrene

The kinetics for the reactions of C₆H₅ with phenylacetylene and styrene have been measured by CRDS in the temperature range 297-409 K under an Ar pressure of 3.6 Torr. The total rate constants can be given by the following Arrhenius expressions (in units of cm³ mol⁻¹ s⁻¹): k₁(C₆H₅ + C₆H₅C₂H) = 10^13.0±0.1exp [−(2430 ± 150)/RT] and k₂(C₆H₅ + C₆H₅C₂H₃) = 10^13.3±0.1 exp [−(2570 ± 180)/T]. Additional DFT and MP2 calculations have been carried out to assist our interpretation of the measured kinetic data. The addition of C₆H₅ to the terminal CH_x (x = 1 or 2) sites is predicted to be the dominant channel for both reactions. The calculated bimolecular rate constants are in reasonable agreement with experimental values for the temperature range studied.     

Magnetic and electrical properties of amorphous Ge1−xCrx thin films grown by low temperature vapor deposition

Amorphous Ge_1−xCr_x thin films are deposited on (1 0 0)Si by using a thermal evaporator. Amorphous phase is obtained when Cr concentration is lower than 30.7 at%. The electrical resistivities are 1.89×10⁻³–0.96×10² Ω cm at 300 K, and decrease with Cr concentration. The Ge_1−xCr_x thin films are p-type. The hole concentrations are 5×10¹⁶–7×10²¹ cm⁻³ at 300 K, and increase with Cr concentration. Magnetizations are 7.60–1.57 emu/cm³ at 5 K in the applied field of 2 T. The magnetizations decrease with Cr concentration and temperature. Magnetization characteristics show that the Ge_1−xCr_x thin films are paramagnetic.     

Ethylene adsorption on the Pt-Cu bimetallic catalysts. Density functional theory cluster study

The adsorption of ethylene on Cu₁₂Pt₂ clusters has been studied within the density functional theory (DFT) approach to understand the high ethylene selectivity of Cu-rich Pt-Cu catalyst particles in the reaction of hydrogen-assisted 1,2-dichloroethane dechlorination. The structural parameters for Cu₁₂Pt₂ clusters with D_4h, D_2d, and C_3v symmetry have been calculated. The relative stability of the isomeric Cu₁₂Pt₂ clusters follows the order: C_3v > D_2d > D_4h. Each isomer has an active site for ethylene adsorption that consists of a single Pt atom surrounded by Cu atoms. The interaction of ethylene with the active site yields a π-C₂H₄ adsorption complex. The strongest π-C₂H₄ complex forms with the cluster of C_3v symmetry; the bonding energy, ΔE_π(C₂H₄), is −15.6 kcal mol⁻¹. The bonding energies for the π-C₂H₄ complex with Cu₁₄ and Pt₁₄ clusters are −6.5 and −18.8 kcal mol⁻¹, respectively.The addition of Pt to Cu modifies the valence spd-band of the cluster as compared to a Cu₁₄ cluster. The DOS near the Fermi level increases when C₂H₄ adsorbs on the Cu₁₂Pt₂ cluster. As well, the center of the d-band shifts toward lower binding energies. Ethylene adsorption also induces a number of states below the d-band. These states correspond to those of gas-phase C₂H₄.The vibrational frequencies of C₂H₄ adsorbed on the clusters of D_4h and C_3v symmetry have been calculated. The phonon vibrations occur below 250 cm⁻¹. The intense bands around 200 cm⁻¹ are attributed to stretching vibrations of the Pt-Cu bonds normal to the cluster surface. The stretching vibrations of the Pt-C bonds depend on the local structure of the active site: ν_s(Pt-C) = 268 cm⁻¹ and ν_as(Pt-C) = 357 cm⁻¹ for the cluster of the D_4h symmetry; ν_s(Pt-C) = 335 cm⁻¹ and ν_as(Pt-C) = 397 cm⁻¹ for the cluster of the C_3v symmetry. Bands in the range of 800-3100 cm⁻¹ are attributed to vibrations of the adsorbed C₂H₄ molecule. The signature frequencies of the π-C₂H₄ adsorption complex are the δ_s(CH₂) deformation vibration at ∼1200 cm⁻¹ and the ν(C-C) stretching vibration at ∼1500 cm⁻¹. These vibration are absent for di-σ-C₂H₄ adsorption complexes.     

Formation of α-Si1−xCx:H and nc-SiC films grown by HWCVD under different process pressure

In this work, hydrogenated amorphous silicon carbide (α-Si_1−xC_x:H) and nanocrystalline SiC (nc-SiC) thin films were deposited by hot wire CVD (HWCVD) using SiH₄/C₂H₂/H₂ gas mixtures. It was found that the films prepared under low gas pressure were α-Si_1−xC_x:H and those prepared under high gas pressure were nc-3C-SiC. The α-Si_1−xC_x:H films showed enhanced density of C-H_n and Si-C bonds with increasing C₂H₂ fraction, which induced an increase in optical gap from 1.8 to 3.0 eV. For the deposition process of nc-SiC, the E_g opt of the deposited films varied from 1.9 eV to 2.5 eV as the filament temperature increased from 1700 to 2100 °C. The deposition rate decreased rapidly from 5.74 nm/min to 0.8 nm/min with increasing T_F.     

EPR and optical absorption study of Cr-doped ammonium oxalate monohydrate single crystals

The electron paramagnetic resonance (EPR) study of the Cr³⁺-doped ammonium oxalate monohydrate (AOM) single crystal is done at room temperature. Two magnetically inequivalent sites for chromium are observed. The hyperfine structure for Cr⁵³ isotope is also obtained. The spin Hamiltonian parameters are evaluated as: D=(309±2)×10⁻⁴ cm⁻¹, E=(103±2)×10⁻⁴ cm⁻¹, g=1.9820±0.0002, A=(161±2)×10⁻⁴ cm⁻¹ for site I and D=(309±2)×10⁻⁴ cm⁻¹, E=(103±2)×10⁻⁴ cm⁻¹, g=1.9791±0.0002, A=(160±2)×10⁻⁴ cm⁻¹ for site II, respectively. On the basis of EPR data the site symmetry of Cr³⁺ doped single crystal is discussed. The optical absorption spectra are recorded in 195-925 nm wavelength range at room temperature. The energy values of different orbital levels are determined. On the basis of EPR and optical data, the nature of bonding in the crystal is discussed. The values of different parameters are B=803, C=3531, D_q=2208 cm⁻¹, h=0.59 and k=0.21, where B and C are Racah parameters, D_q is crystal field parameter and h and k are nephelauxetic parameters, respectively.     

Thermal decomposition of toluene: Overall rate and branching ratio

The two-channel thermal decomposition of toluene, C₆H₅CH₃ → C₆H₅CH₂ + H (1) and C₆H₅CH₃ → C₆H₅ + CH₃ (2), was investigated in shock tube experiments over the temperature range of 1400-1780 K at a pressure of 1.5 (±0.1) bar. Rate coefficients for reactions (1) and (2) were determined by monitoring benzyl radical (C₆H₅CH₂) absorption at 266 nm during the decomposition of toluene diluted in argon and modeling the temporal behavior of the benzyl concentration with a kinetic model. The first-order rate coefficients determined at a pressure of 1.5 bar are expressed by k₁(T) = 2.09 × 10¹⁵ exp (−87510 [cal/mol]/RT) [s⁻¹] and k₂(T) = 2.66 × 10¹⁶ exp (−97880 [cal/mol]/RT) [s⁻¹]. The resulting branching ratio, k₁/(k₁ + k₂), ranges from 0.8 at 1350 K to 0.6 at 1800 K.     

Chemical and morphological difference between TiN/DLC and a-C:H/DLC grown by pulsed vacuum arc techniques

In order to improve the adherence of DLC films, interlayers of amorphous hydrogenated carbon (a-C:H) and titanium nitride (TiN) were deposited by means of the pulsed vacuum arc technique. Bilayers were obtained by using a carbon target of 99.98% of purity in mixtures of (Ar + CH₄) and (Ar + H₂) for producing a-C and DLC, respectively and a target of titanium of 99.999% in a mixture of (Ar + N₂) for growing TiN. After the deposition, chemical and morphological differences between TiN/DLC and a-C:H/DLC bilayers grown on silicon and stainless steel 304 were studied using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), and scanning probe microscopy (SPM) techniques. XPS analysis showed a difference in sp³/(sp²+sp³) bonds ratio for each bilayer, being 0.67 for TiN/DLC and 0.45 for a-C:H/DLC bilayers. sp³ and sp² bonds were also observed by the FTIR technique. SPM images, in atomic force microscopy (AFM) and lateral force microscopy (LFM) modes were carried out for illustrating the comparison between TiN/DLC and a-C/DLC morphologic characteristics. Roughness and grain size were studied as a function of the H₂ concentration for both bilayers.     

Pressure and temperature dependence of the reaction of vinyl radical with alkenes II: Measured rates and predicted product distributions for vinyl + propene

This work reports measurements of the absolute rate coefficients and Rice-Ramsperger-Kassel-Markus (RRKM) master equation (ME) simulations of the C₂H₃ + C₃H₆ reaction. Direct kinetic studies were performed over a temperature range of 300-700 K and pressures of 15, 25, and 100 Torr. Vinyl radicals were generated by laser photolysis of vinyl iodide at 266 nm, and time-resolved absorption spectroscopy was used to probe vinyl radicals through absorption at 423.2 nm. A weighted modified Arrhenius fit to the experimental rate constant is k₁ = (1.3 ± 0.2) × 10⁻¹² cm³ molecule⁻¹ s⁻¹(T/1000)^1.6 exp[−(1510 ± 80/T)]. Fifteen stationary points and 48 transition states on the C₅H₉ potential energy surface (PES) were calculated using the G3 method in Gaussian 03. RRKM/ME simulations were performed using VariFlex on a simplified PES to predict pressure dependent rate coefficients and branching fractions for the major channels. For temperatures between 350 and 700 K, the calculated rate coefficient agrees with the experimental rate coefficient within 20%. At low temperatures, the primary products are the initial adducts 4-penten-2-yl and 2-methyl-3-buten-1-yl. At higher temperatures, the dominant products are 1,3-butadiene + methyl, allyl + ethene, and 1,3-pentadiene + H. Although C₂H₃ + C₃H₆ → allyl + ethene is thermodynamically favored, the simulations predict that it does not become the dominant product until 1700 K.     

The high-resolution, jet-cooled infrared spectrum of pentafluoroethane

The jet-cooled spectrum of pentafluoroethane (C₂HF₅) has been recorded between 1100 and 1325 cm⁻¹ at a resolution of 0.0022 cm⁻¹. A rotational temperature of approximately 10 K was achieved by expanding 50 Torr of C₂HF₅ in 500 Torr of helium. Transitions belonging to five different fundamental vibrations have been assigned and fit to a Watson Hamiltonian: the ν₃ band at 1309.880494(189) cm⁻¹, ν₄ at 1200.734645(67) cm⁻¹, ν₅ at 1142.78147(33) cm⁻¹, ν₁₃ at 1223.334098(115) cm⁻¹, and ν₁₄ at 1147.394185(163) cm⁻¹. The fit of the ν₄ band has an rms deviation of 0.000436 cm⁻¹ compared to the uncertainty in the experimental line position of 0.0002 cm⁻¹. Satisfactory fits were achieved for the other four bands (ν₃, ν₅, ν₁₃, ν₁₄) at this cold temperature, with most of the centrifugal distortion constants fixed at the ground state values. Joint fits with previous work were attempted for the ν₄ and ν₁₃, successfully in the former case and unsuccessfully in the latter.     

Electrochemical investigation of the surface energy: Effect of the HF concentration on electroless silver deposition onto p-Si (1 1 1)

Electroless silver deposition onto p-silicon (1 1 1) from 0.005 mol l⁻¹ AgNO₃ solutions with different HF concentration was investigated by using an electrochemical direct current polarization method and open circuit potential-time (Ocp-t) technique. The fact that three-dimensional (3D) growth of silver onto silicon is favored with increasing the HF concentration was ascribed to the drop of the surface energy and approved by electrochemical direct current polarization, Ocp-t technique and atomic force microscopy (AFM). The drop slope of open-circuit potential, K_{−ΔE(OCP)/t}, was educed from the mixed-potential theory. K_{−ΔE(OCP)/t} as well as the deposition rate determined by an inductively coupled plasma atomic emission spectrometry (ICP-AES), increased with the HF concentration, yet was not a linear function. Results were explained by the stress generation and relaxation mechanisms.