Diamondlike nanocomposite a-C:H:Cr Coatings: Structure,mechanical, and tribological properties

The results from a comprehensive investigation of the structure, phase and chemical composition, microhardness, and nanomechanical and tribological properties of chromium-doped coatings of hydrogenised amorphous carbon a-C:H:Cr are presented. The coatings are deposited via reactive magnetron sputtering in an Ar + C₂H₂ + N₂ gas mixture at various volume concentrations of nitrogen and acetylene. It is found that the carbon in the coatings is formed as disordered mixtures of domains with tetrahedral (sp ³) and hexagonal (sp ²) carbon coordinations. In addition, the doping metal in the coating consists of nanosized inclusions of metallic chromium and its carbide and nitride phases. Additional nitrogen doping resulting in the formation of chromium nitride is shown to improve the micromechanical and tribological properties of the obtained coatings.     

Deposition of silicon–carbon coatings from the plasma of a non-self-sustained arc discharge with a heated cathode

Amorphous hydrogenated carbon doped with silicon oxide (a-C:H:Si:O), which is referred to as silicon–carbon coatings in this work, consists of thin amorphous films, which are used as commercial solid lubricants due to their higher stability under extreme environmental conditions as compared to amorphous hydrogenated carbon. The deposition of silicon–carbon coatings from the plasma of a non-self-sustained arc discharge with a heated cathode is considered. Silicon–carbon coatings are deposited using polyphenul methylsiloxane as a precursor at a flow rate of 0.05 mL/min in an argon atmosphere at a pressure of 0.1 Pa. A high-frequency power supply is used to apply a high-frequency bias voltage to a substrate during deposition. After deposition, the mechanical properties of the coatings are studied. The maximum hardness of the coating is 20 GPa at a minimum friction coefficient of 0.16 and a wear rate of 1.3 × 10^–5 mm³ N^–1 m^–1. Energy dispersive analysis shows that the coatings contain a significant content of carbon and oxygen (about 80 and 15%, respectively) and a low content of silicon (about 5%).     

Oxidation of vanadium nitride and titanium nitride coatings

The oxidation of vanadium nitride (VN) and titanium nitride (TiN) coatings in ultra-high vacuum has been investigated in situ by X-ray photoelectron spectroscopy. On the VN coatings mixed oxide layers containing V³⁺ and V⁴⁺ species form at elevated temperatures (?600°C) and at high oxygen exposures, which cover completely the VN surface. Under similar oxidation conditions the TiN surface oxidises partially to a mixture of TiO₂ and Ti oxynitride (TiO_xN_y) phases. This oxidation behaviour has been correlated to the tribological properties of the VN and TiN coatings investigated recently.     

Study and simulation of the growth of solid lubricant MoSe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> coatings during pulsed laser deposition

The chemical composition and tribological properties of the thin-film diselenide molybdenum coatings deposited by pulsed laser deposition in vacuum and a rarefied inert gas (argon) atmosphere are studied. Upon deposition in a gas at a pressure of ∼2 Pa, stoichiometric coatings with improved antifriction properties as compared vacuum-deposited coatings form. However, a too strong increase in the argon pressure (to ∼10 Pa) degrades the tribological properties of the coating. Structure formation in the MoSe _x coatings grown by pulsed laser deposition on an unheated substrate is investigated. Deposition in vacuum or argon at a pressure of 2 Pa leads to formation of rather smooth coatings with a dense amorphous structure containing molybdenum nanoinclusions. Deposition at a high argon pressure results in a developed surface relief and a loose coating structure. A mathematical model is developed using the kinetic Monte Carlo method in order to describe structure formation in the coatings that grow during physical deposition of an atomic flux. A comparative analysis demonstrates satisfactory agreement between the simulated and experimentally studied structures in the coatings created by pulsed laser deposition at various gas pressures.     

Photoionization experiments in dense argon gas

At densities above 50 Amagats, light of frequency 2.818 x 10⁴cm^-1 ionizes efficiently argon gas containing less than 1 part per million of impurities. For a given density we find for the number of free charges produced a square law dependence on light intensity. At constant light intensity the number of free charges produced varies approximately like the sixth power of density in the range 200–500 Amagats.     

Oriented growth of CoPt nanoparticles by pulsed laser deposition

The magnetic nanoparticles of Fe/FeCo/FePt, in the past, in a PLD system were grown by us using argon ambient gas pressure of about 0.1–75.0 mbar, as the ambient gas pressure can be used to tune the energy of the incident plasma plume species, the expansion volume, the growth duration, etc. which can control the particle size. In present paper, we report the direct synthesis of small-sized nanoparticles even when no ambient gas was used, with the experiments being done in higher vacuum of about 10^?5 mbar in PLD chamber. The deposition rate under vacuum condition is significantly higher than the deposition rate at high ambient pressure. The study of inplane and outplane magnetic properties, along with XRD results, confirmed that the as-deposited CoPt nanoparticles thin film has oriented growth. The as-deposited CoPt nanoparticles are in magnetically soft fcc phase and a post deposition annealing at 600°C resulted in phase transition to magnetically hard fct phase.     

Emission characteristics of a barrier discharge in an Ar-H2O mixture

We have experimentally studied the UV radiation of a low-temperature barrier discharge plasma in an Ar-H₂O mixture. The spectral interval 300–400 nm has been examined in detail. Addition of argon with a pressure of 24 kPa to a barrier discharge in water vapor at a pressure of ～0.1 kPa leads to a ninefold increase in the UV radiation power of excited hydroxyl molecules. An increase in the duration of the UV radiation pulse of the mixture in the longitudinal discharge decay has been achieved for the first time, which may be direct evidence of energy transfer from metastable argon atoms to water molecules. An estimate of the upper boundary of the dissociative excitation rate constant of hydroxyl molecules OH*(A ²Σ⁺) upon interaction of metastable argon atoms with water molecules has been obtained.     

The effect of alloying on the structure and peculiarities of tribological behavior of vacuum-deposited diamond-like coatings

In this work, we report the results of a study of the atomic and crystalline structures, phase and chemical compositions and tribological properties of DLC coatings deposited by plasma-assisted (PA) CVD (a-C:H:Si and a-C:H:Si:Mo) and magnetron reactive sputtering (a-C:H:Cr). The a-C:H:Si:Mo coatings revealed the formation of ultra-dispersed inclusions of either molybdenum carbides or silicides, whereas no such inclusions were found in the a-C:H:Si compound. The a-C:H:Cr coatings that were deposited in an acetylene–nitrogen gas mixture exhibited a nanocomposite structure composed of chromium, its carbide and nitride phases. The tribological tests showed that the DLC coatings with silicon and silicon–molybdenum have a high friction coefficient and a low working performance, while the chromium-containing coatings have high levels of their mechanical and tribological characteristics, making them promising materials for operation under high contact pressures.     

AlNxOy thin films deposited by DC reactive magnetron sputtering

AlN_xO_y thin films were produced by DC reactive magnetron sputtering, using an atmosphere of argon and a reactive gas mixture of nitrogen and oxygen, for a wide range of partial pressures of reactive gas. During the deposition, the discharge current was kept constant and the discharge parameters were monitored. The deposition rate, chemical composition, morphology, structure and electrical resistivity of the coatings are strongly correlated with discharge parameters. Varying the reactive gas mixture partial pressure, the film properties change gradually from metallic-like films, for low reactive gas partial pressures, to stoichiometric amorphous Al₂O₃ insulator films, at high pressures. For intermediate reactive gas pressures, sub-stoichiometric AlN_xO_y films were obtained, with the electrical resistivity of the films increasing with the non-metallic/metallic ratio.     

Surface nitriding of Ti–6Al–4V alloy with a high power CO2 laser

Surface nitriding of a Ti–6Al–4V alloy by laser melting in a flow of nitrogen gas has been investigated, with the aim of increasing surface hardness and hence improving related properties such as wear and erosion resistance. The effect of the scanning speed, nitrogen dilution, and nitrogen flow rate on microstructure, microhardness, and cracking of the nitrided layers was studied. Optical, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction (XRD) were used to reveal the microstructure and to identify the phases formed. It is shown that smooth, deep, and crack-free nitride layers of a surface hardness ranging between 500 and 800 HV can be obtained by controlling the processing parameters. Cracks are present in the sample processed at slow scanning speed and high laser power. Dilution of the nitrogen gas with argon gas leads to a crack-free nitride layer at the expense of a reduction in surface hardness. Slow scanning speeds lead to the formation of a deep and hard surface layer, and increasing the nitrogen flow rate results in a rough surface with a slight increase in hardness.     

Synthesis of boron nitride nanostructures from catalyst of iron compounds via thermal chemical vapor deposition technique

For the first time, patterned growth of boron nitride nanostructures (BNNs) is achieved by thermal chemical vapor deposition (TCVD) technique at 1150 °C using a mixture of FeS/Fe₂O₃ catalyst supported in alumina nanostructured, boron amorphous and ammonia (NH₃) as reagent gas. This innovative catalyst was synthesized in our laboratory and systematically characterized. The materials were characterized by X-ray diffraction (XRD), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), Thermogravimetric analysis (TGA), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The X-ray diffraction profile of the synthesized catalyst indicates the coexistence of three different crystal structures showing the presence of a cubic structure of iron oxide and iron sulfide besides the gamma alumina (γ) phase. The results show that boron nitride bamboo-like nanotubes (BNNTs) and hexagonal boron nitride (h-BN) nanosheets were successfully synthesized. Furthermore, the important contribution of this work is the manufacture of BNNs from FeS/Fe₂O₃ mixture.     

Ti-B-C-N纳米复合薄膜结构及力学性能研究

利用反应磁控溅射方法在单晶硅和高速钢(W18Cr4V)基片上制备出不同C含量Ti-B-C-N纳米复合薄膜. 使用X射线衍射和高分辨透射电子显微镜研究了Ti-B-C-N纳米复合薄膜的组织和微观结构,用纳米压痕仪测试了它们的硬度和弹性模量. 结果表明,利用往真空室通入C₂H₂气体的方法制备得到的Ti-B-C-N纳米复合薄膜中,在所研究成分范围内只发现TiN基的纳米晶. 当C₂H₂流量较小时,C元素的加入可以促进Ti-B-C 关键词： Ti-B-C-N薄膜 磁控溅射 微观结构 力学性能     

Quadrupole oscillations in a quantum degenerate Bose–Fermi mixture

We study the collective dynamics in a degenerate Bose–Fermi mixture of ¹⁷⁴Yb and ¹⁷³Yb atoms. We excite collective oscillations by a sudden reduction of the trapping confinement and observe low m=0 quadrupole oscillations of condensates in ¹⁷⁴Yb. First the oscillations in ¹⁷⁴Yb atoms alone are investigated, and they are well described by the time-dependent Gross–Pitaevskii equation in the Thomas–Fermi approximation. Using the same procedure the quadrupole oscillations are excited for a ¹⁷⁴Yb–¹⁷³Yb Bose–Fermi mixture. In comparing data taken with and without fermionic ¹⁷³Yb atoms, the oscillation frequency of the quadrupole mode in the condensate decreases with the presence of ¹⁷³Yb atoms.     

Fluid synthesis and structure of a new polymorphic modification of boron nitride

A new previously unknown phase of boron nitride with a hardness of 0.41–0.63 GPa has been pre-pared by the supercritical fluid synthesis. The presence of a new phase is confirmed by the X-ray spectra and IR absorption spectra, where new reflections and bands are distinguished. The fundamental reflection of the X-ray diffraction pattern is d = 0.286–0.291 nm, and the characteristic band in the infrared absorption spectrum is observed at 704 cm^?1. The X-ray diffraction pattern and the experimental and theoretical infrared absorption spectra show that a new synthesized boron nitride phase can be a cluster crystal (space group 211) with a simple cubic lattice. Cage clusters of a fullerene-like morphology B₂₄N₂₄ with point symmetry O are arranged in lattice sites.     

Emission characteristics of pulse-periodic barrier-discharge plasma in a mixture of krypton with argon and liquid freon vapor

Radiation of a nanosecond barrier discharge in a mixture of krypton, argon, and carbon-tetrachloride vapor is studied in the spectral range of 150–300 nm. The plasma radiation spectra and the dependences of the intensities of the 258 nm Cl₂(D′ → A′), 222 nm KrCl(B → X), and 175 nm ArCl(B → X) bands on the partial pressure of liquid freon vapor, argon, and krypton, as well as on the discharge excitation conditions, are studied. The optimal compositions of gas mixtures for creating a broadband UV-VUV emitter based on the band system of argon chloride, krypton chloride, and chlorine molecule are determined.     

Spectroscopic studies of the sulphur afterglow

The afterglow emission spectrum of sulphur and argon mixture is found to consist of (a) the main band system of S₂(B³Σ^-_u?X³Σ^-_g) in the region 3200–6600 Å and (b) the atomic spectrum of argon in the 7500–9000 Å region. Although B?X bands of S₂ obtained by ordinary excitation extends from 2829 to 7100 Å, the lower wavelength limit of these bands from the afterglow is only 3200Å. It is proposed that the S₂ molecules are formed in the B³Σ^-_u state through inverse predissociation when two S atoms approach each other along the potential curve of the predissociating electronic state 1_u.     

Measurements of the spatially resolved electron temperature and plasma potential in ferrite-core side type Ar/He inductively-coupled plasmas

Spatial distributions of the effective electron temperature (T_eff) and plasma potential were studied from the measurement of an electron energy probability function in a side type ferrite-core inductively coupled plasma with an argon–helium mixture. As the helium gas was diluted at the fixed total gas pressure of 5 mTorr in an argon discharge, the distribution of the plasma density and plasma potential changed from a concave to a flat profile, and finally became a convex profile, while all spatial profiles of T_eff were hollow shapes with helium dilution in the argon discharge. This evolution of the plasma potential with helium gas could be explained by the increased energy relaxation length (λ_ε), indicating the transition of electron kinetics from local to non-local kinetics.     

Laser/sol–gel synthesis: a novel method for depositing nanostructured TiN coatings in non-vacuum conditions

In this work results of experiments on the in situ production of titanium nitride by the reaction of titania sol–gel with a nitrogenous admixture under laser irradiation are reported. A diode laser beam at different powers and traverse speeds was applied to the mixture placed on EN43 mild steel and 316L stainless steel substrates. Composite coatings of titanium nitride and titanium oxide with a hardness of 17–21 GPa have been achieved by this new method. Surface morphology and microstructure of the deposited coatings and substrate surface layers were examined using optical microscopy, scanning electron microscopy, and field-emission gun scanning electron microscopy. Chemical composition was determined by energy-dispersive X-ray analysis. The phases were identified by X-ray diffraction. Results of microhardness and nanohardness at the top surface were evaluated. PACS 81.15.Fg; 81.20.Fw; 81.05.-t     

Structure and properties of Ti-B-N,Ti-Cr-B-(N), and Cr-B-(N) coatings deposited by magnetron sputtering of targets prepared by self-propagating high-temperature synthesis

Transmission and scanning electron microscopy, x-ray phase analysis, x-ray photoelectron spectroscopy, and atomic-force microscopy were used to study the structure and surface topography of Ti-B-N, Ti-Cr-B-(N), and Cr-B-(N) thin films. Physical, mechanical, and tribological characteristics of coatings were comparatively analyzed, including determination of the hardness, elastic modulus, elastic recovery, critical load, friction coefficient, and wear rate. It was shown that Ti-B-N and Ti-Cr-B-N coatings are superior to conventional TiN-and Ti-C-N-based coatings in terms of their physicomechanical and tribological properties. Ti-B-N and Ti-Cr-B-N coatings deposited under optimum conditions were characterized, accordingly, by a hardness of 31–34 and 40–47 GPa, an average elastic modulus of 378 and 506 GPa, a friction coefficient of 0.49–0.60 and 0.45–0.52, a dry-wear rate of (3.4–4.6) × 10^?7 and (6.0–6.8) × 10^?7 mm³ N^?1 m^?1, and a largest critical load of 50 and 22 N. Features in the determination of the physicomechanical properties of films during nanoindentation and their wear properties are discussed.     

Corrosion and oxidation properties of NiCr coatings sprayed in presence of gas shroud system

The oxidation of a NiCr bond coat during air plasma spraying was controlled by designing a gas shroud system attached to the plasma torch nozzle. Two nozzles, termed as “normal” and “high-speed” nozzles examined the effect of nozzle internal design on the microstructure and phase structure of coatings. X-ray diffraction and SEM morphologies showed that the shroud system reduced the oxidation of NiCr particles during the spray process. Compared with conventional air plasma spraying, the argon gas shroud reduced the coating hardness because the volume fraction of partially melted particles increased. The high-speed nozzle reduced the oxidation and hardness of NiCr coatings due to the increase of partially melted particles in the coatings.