Development and research of a laser photo-acoustic SF<Subscript>6</Subscript> gas analyzer

An analysis of various optical schemes for the development of a laser SF₆ gas analyzer based on a CO₂ laser operating in free-running mode and a resonant photo-acoustic detector (PAD) is presented. The use of a sealed gas-filled cell to normalize PAD signals on the absorbed power in the cell is suggested. Compensation for the influence of the tuning of the CO₂ laser wavelength near 10.6 μm on measured SF₆ concentration is possible. The results of experimental studies of a laser photo-acoustic SF₆ gas analyzer at various concentrations, including in the air flow, are presented. It is shown experimentally that the relative measurement error of the SF₆ concentration due to the instability of the laser radiation wavelength does not exceed 5% in the range from ~80 ppb to 40 ppm. The limit of the sensitivity of the developed gas analyzer was ~1 ppb SF₆.     

Reducing the Energy Consumption of a Laser Photo-Acoustic SF<Subscript>6</Subscript> Gas Analyzer

Experimental studies of the operating modes of a laser photo-acoustic SF₆ gas analyzer that were aimed at reducing its energy consumption were carried out. It was shown in the experiments that an average power of CO₂ laser radiation of at least 100 mW is required for the assured detection of low SF₆ concentrations (less than 100 ppb). To reduce the energy consumption of the gas analyzer, it is proposed to decrease the repetition frequency of CO₂ laser pulses by several times and operate on subharmonics of the resonance frequency of the photo-acoustic detector. The experimental results made it possible to reduce the energy consumption of the gas analyzer to ~15 V A and use a Li-ion battery from a laptop to power it. The duration of the continuous operation of the gas analyzer on one battery charge was at least 6 h.     

On the possibility of improving antifriction properties of MoS<Subscript>2</Subscript> coatings by alloying

The paper presents an explanation of the improved antifriction properties of MoS₂ in vacuum compared to their properties in air. It is shown that the effect of superlow friction upon intensive irradiation results from the formation of a “two-dimensional gas” consisting of sulfur atoms knocked out of their positions. The possibility of the alloying of MoS₂ by elements which do not react with sulfur is analyzed. The alloying of MoS₂ coatings by an excess number of sulfur atoms to realize the effect of superlow friction in vacuum and air is substantiated.     

A Dispersion Interferometer Based on a CO<Subscript>2</Subscript> Laser

A double-pass dispersion interferometer based on a 9.6-μm CO₂ laser with a sensitivity of 〈 n _e l〉_min ∼ 1 × 10¹³ cm⁻² and a temporal resolution of ∼50 μ s, designed to measure linear plasma density, is described. A ZnGeP₂ nonlinear crystal is used as the frequency doubler. The main advantages of the interferometer are its compactness and a low sensitivity to vibrations of optical elements. The interferometer requires no special vibration isolation. Its main components are arranged compactly on an optical bench outside the apparatus, except for a window for radiation injection and a retroreflector; these are mounted on the wall of the experimental facility's vacuum chamber. The advantages of the dispersion interferometer have been demonstrated in an experiment with a gas-dynamic trap. __________ Translated from Pribory i Tekhnika Eksperimenta, No. 5, 2005, pp. 96–106. Original Russian Text Copyright ? 2005 by Solomakhin, Bagryanskii, Voskoboinikov, Zubarev, Kvashnin, Lizunov, Maksimov, Khil'chenko.     

Microstructure,chemical composition,wear, and corrosion resistance of FeB–Fe<Subscript>2</Subscript>B–Fe<Subscript>3</Subscript>B surface layers produced on Vanadis-6 steel using CO<Subscript>2</Subscript> laser

The paper presents the study results of laser modification of FeB–Fe₂B surface layers produced on Vanadis-6 steel using pack cementation method. Microstructure, x-ray phase analysis, chemical composition study using wave dispersive spectrometry method, microhardness, corrosion resistance as well as surface condition, roughness, and wear resistance were investigated. The diffusion boronizing processes were performed at 900 °C for 5 h in the EKabor® powder mixture. The boronized layers had a dual-phase microstructure composed of two types of iron borides, FeB and Fe₂B, and their microhardness ranged from 1800 to 1400 HV. The laser surface modification was carried out on specimens after diffusion boronizing process using CO₂ laser with a nominal power of 2600 W. Laser beam power used in this experiment was equal to 1040 W and was constant. While the three values of scanning speed were used: 19, 48, and 75 mm/s. During laser modification, the multiple tracks were made where distance between of axis tracks was equal to 0.5 mm. As a result of this process, microstructure consisted of remelted zone, heat-affected zone, and substrate was obtained. In remelted zone, the boron-martensite eutectic was observed. Boronized layers after laser modification were characterized by the mild gradient of microhardness from surface to the substrate and their value was dependent on the scanning speed used and was between 1700 and 1100 HV. Corrosion resistance tests revealed reducing the current of corrosion in case of laser modification process. Wear resistance of laser modified specimens was improved in comparison to diffusion boronized layers.     

A stabilized pulsed repetition-rate CO<Subscript>2</Subscript> laser for a laser-plasma generator of multiply charged ions

The design of a laser oscillator with a stabilized frequency composition and a stabilized intensity of output radiation is described. The oscillator’s basic component is a TEA CO₂ module pumped by a self-maintained discharge and operating in a repetition-rate mode at a frequency of up to 3 Hz. A circuit for the formation of a self-maintained homogeneous discharge in the working volume of a CO₂ + N₂ + He mixture at atmospheric pressure is the basic component of the gas-discharge module. This circuit is based on the generation of a high-voltage pulse with a special profile, which provides high-reliability excitation of a discharge and pulse-to-pulse reproducibility of the discharge characteristics. The use of a hybrid circuit in the optical oscillator allows selection of a single longitudinal mode in the output radiation spectrum, thus eliminating undesirable interference phenomena, which lead to instability of the instantaneous values of the radiation intensity. During the development of the oscillator, the optimal operating parameters of the hybrid circuit were obtained, which ensure the high quality and reproducibility of the spatiotemporal and energy parameters of laser radiation.     

Nanotribology and lubrication mechanisms of inorganic fullerene-like MoS<Subscript>2</Subscript> nanoparticles investigated using lateral force microscopy (LFM)

Inorganic fullerene-like (IF) MoS₂ nanoparticles were produced by arc discharge in water, and their tribological properties were investigated using a lateral force microscope in dry nitrogen and humid air. Two types of tips – Si and Si₃N₄ tips were used in this work. The sharp Si tip produced a much higher contact stress than the blunt Si₃N₄ tip. The measurement of lateral forces using a Si₃N₄ tip resulted in almost no wear, while the measurement made using a Si tip resulted in MoS₂ transfer due to the high contact stress. For comparison, measurements were also made on MoS₂ films grown by pulsed laser deposition (PLD). The experimental results demonstrated that IF-MoS₂ nanoparticles had significantly lower friction than the MoS₂ films prepared by PLD. Variation of the test environment from dry to wet did not affect the tribological performance of the IF material as much as it did PLD films due to the chemical inert structure of the IF-MoS₂ nanoparticles. The multi-wall-encapsulated structure of inorganic fullerenes has a nearly isotropic geometry. They can supply a slippery surface in all orientations, though only the basal planes of 2H–MoS₂ crystals are optimum for lubrication. Therefore, the inorganic fullerenes do not have to be oriented by rubbing as does most layer-structured solid lubricants. However, the lack of reactive edge planes impedes bonding of the lubricant to the surface. The lubrication mechanisms of IF-MoS₂ nanoparticles are discussed in detail.     

Microstructure and Tribological Properties of a HfB<Subscript>2</Subscript>-Containing Ni-Based Composite Coating Produced on a Pure Ti Substrate by Laser Cladding

A HfB₂-containing Ni-based composite coating was fabricated on Ti substrates by laser cladding, and its microstructure and tribological properties were evaluated during sliding against an AISI-52100 steel ball at different normal loads and sliding speeds. The morphologies of the worn surfaces were analyzed by scanning electron microscopy (SEM) and three-dimensional non-contact surface mapping. The results show that wear resistance of the pure Ti substrate and NiCrBSi coating greatly increased after laser cladding of the HfB₂-containing composite coating due to the formation of hard phases in the composite coating. The pure Ti substrate sliding against the AISI-52100 counterpart ball at room temperature displayed predominantly adhesive wear, abrasive wear, and severe plastic deformation, while the HfB₂-containing composite coating showed only mild abrasive wear and adhesive wear under the same conditions.     

A Raman Spectroscopic Study of MoS<Subscript>2</Subscript> and MoO<Subscript>3</Subscript>: Applications to Tribological Systems

Molybdenum disulfide (MoS₂) and molybdenum trioxide are investigated using Raman spectroscopy with emphasis on the application to tribological systems. The Raman vibrational modes were investigated for excitation wavelengths at 632.8 and 488 nm using both micro-crystalline MoS₂ powder and natural MoS₂ crystals. Differences are noted in the Raman spectra for these two different wavelengths, which are attributed to resonance effects due to overlap of the 632.8 nm source with electronic absorption bands. In addition, significant laser intensity effects are found that result in laser-induced transformation of MoS₂ to MoO₃. Finally, the transformation to molybdenum trioxide is explored as a function of temperature and atmosphere, revealing an apparent transformation at 375 K in the presence of oxygen. Overall, Raman spectroscopy is an useful tool for tribological study of MoS₂ coatings, including the role of molybdenum trioxide transformations, although careful attention must be given to the laser excitation parameters (both wavelength and intensity) when interpreting Raman spectra.     

Wear Behaviour of In Situ Al/TiB<Subscript>2</Subscript> Composite: Influence of the Microstructural Instability

In this present work, the in situ Al (A380)/5 wt%TiB₂ composites were fabricated through salt–melt reaction using halide salts such as potassium hexafluorotitanate (K₂TiF₆) and potassium tetra fluoroborate (KBF₄) salts as precursors. The composites were produced at four different melt temperatures (700, 750, 800, 850 °C). The formation of particle was confirmed from XRD results. The wear behaviour of Al/5 wt% TiB₂ composite was investigated by varying the wear test parameters such as sliding temperature (25, 100, 150, 200 °C), applied load (10, 20, 30, 40 N), sliding velocity (0.4, 0.7, 1, 1.3 m/s). The microstructure of Al/5 wt% TiB₂ composite was correlated with the wear characteristics of the composites. The wear resistance of Al/5 wt% TiB₂ composite was significantly improved due to the presence of TiB₂ particle in Al matrix material. The composite produced at melt temperature 800 °C showed a higher wear resistance at applied load: 10 N, sliding temperature: 25 °C and sliding velocity: 0.7 m/s. The wear mechanism for each of the tested condition was identified from the worn surfaces using scanning electron microscopy (SEM). ANOVA test was carried out to find out significant factor for the wear resistance of composite. The checking of adequacy of experimental value for the wear behaviour of composite for different testing condition was analysed by residual plots using statistical software.     

Data recording system of a dispersion interferometer based on a CO<Subscript>2</Subscript> laser

The data recording system of a multichannel double-pass dispersion interferometer based on a CO₂ laser is described. This system has been designed to record the linear density of plasmas in a real-time mode with a time discreteness of 4 μs and resolution 〈N _e L〉 ～ 0.34 × 10¹³ cm^?2 (N _e is the electron component of the plasma density, and L is the plasma size in the wave propagation direction) in the range of linear density variations of up to 10¹⁷ cm^?2. The system is built from unified recording modules that use fast ADCs to record the shape of photodetector and modulator signals and FPGA-based digital units of dataflow processing to form results of measurements. The single-channel recording module of the dispersion interferometer has been tested under actual experimental conditions of the GDL gas-dynamic trap and the TEXTOR tokamak (Julich, Germany).     

Wear Behavior of Composites Based on ZA-27 Alloy Reinforced by Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Particles Under Dry Sliding Condition

In present study, the effect of Al₂O₃ particle reinforcement on the sliding behavior of ZA-27 alloy composites was investigated. The composites with 3, 5, and 10 wt% of Al₂O₃ particles were produced by the compocasting procedure. Tribological properties of unreinforced alloy and composite were studied, using block-on-disk tribometer under unlubricated sliding conditions at different specific loads and sliding speeds. The worn surfaces of samples were examined by the scanning electron microscopy (SEM). The test results revealed that those composite specimens exhibited significantly lower wear rate than the ZA-27 matrix alloy specimens in all combinations of applied loads and sliding speeds. The difference in the wear resistance of composite with respect to the matrix alloy, increased with the increase of the applied load/sliding speed and Al₂O₃ particle content. The highest degree of improvement of the ZA-27 alloy tribological behavior corresponded with change of the Al₂O₃ particles content from 3 to 5 wt%. At low sliding speed, moderate lower wear rate of the composites over that of the matrix alloy was noticed. This has been attributed to micro cracking tendency of the composites. Significantly reduced wear rate, experienced by the composite over that of the matrix alloy at the higher sliding speeds and loads, could be explained due to enhanced compatibility of matrix alloy with dispersoid phase and greater thermal stability of the composite in view of the presence of the dispersoid. Level of wear rate of tested ZA-27/Al₂O₃ samples pointed to the process of mild wear, which was primarily controlled by the formation and destruction of mechanical mixed layers (MMLs).     

Surface nanostructuring of steel 35 by electrospark machining with electrodes based on tungsten carbide and added Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanopowder

The formation of an alloyed layer on steel 35 by VK8 hard alloy with added Al₂O₃ increases its wear resistance. Effective conditions for this process are determined. The formation of regular tungsten-carbide nanostructure is observed.     

Characteristics of a pulse–periodic CO<Subscript>2</Subscript> laser for applications in the field of laser-produced plasma

The design of a pulse–periodic СО₂ laser oscillator that operates at a high level of the specific energy deposition into a self-sustained discharge is described. The laser is intended for generating pulses with a high-density radiation flux in a laser-plasma generator of multiply charged ions at the Institute of Theoretical and Experimental Physics (ITEP). The results of investigations of the spatiotemporal and energy characteristics of laser output radiation in a wide range of the discharge excitation level and the mixture composition are presented. The optimal conditions are determined under which the oscillator provides an output energy of >10 J in a pulse with a duration of ~28 ns and a record specific peak radiation power of 190 MW per liter of the active volume of a CO₂: N₂: He mixture. The high quality of the spatial characteristics was confirmed in measurements of the radial energy-density distribution in the far-field zone, whose characteristic size is close to the diffraction limit.     

Registration of picosecond CO<Subscript>2</Subscript> laser pulses by means of two-stage parametric transformation in nonlinear crystals

A technique for registering the temporal structure of picosecond pulses of CO₂ laser radiation with an energy of 1.5–4.5 μJ at a wavelength of 10.27 μm using two-stage parametric transformation of IR radiation frequency into visible light under pumping of nonlinear crystals by Nd:YAG-laser radiation in a Q-switched mode is described. A GaSe nonlinear crystal was used at the first stage of transformation (10.27 μm + 1.064 μm → 0.964 μm). Radiation was further transformed (1.064 μm + 0.960 μm → 0.506 μm) by using the same pumping in an α-HIO₃ nonlinear crystal. For the first time, no additional optical elements were present between the stages of the frequency transformer in the proposed optical scheme. The transformed radiation was registered with a Hamamatsu Temporal Disperser C1587 streak camera in a region of the photocathode maximum spectral sensitivity of ～0.5 μm with a temporal resolution of up to 2 ps. The minimum recorded pulse duration of the CO₂ laser was ～45 ps.     

Modeling the formation of silicon nanoclusters during annealing SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> layers

The mechanism of formation of silicon nanoclusters in layers of nonstoichiometric composition is studied by Monte Carlo simulation. Interest in silicon nanoclusters (Si-nc) coated with an oxide layer is due to their applications in modern optoelectronics and nanoelectronics. A lattice Monte Carlo model is proposed to study atomic processes in the Si-SiO₂ system. The formation of silicon nanoclusters during annealing of single SiO layers and SiO₂-SiO-SiO₂ layered structures is studied. Along with the diffusive motion of particles, the model takes into account the formation and collapse of mobile molecules of silicon monoxide. It is shown that accounting for transport of silicon under high-temperature annealing due to the motion of SiO accelerates the formation of Si-nc. Dependences of the size of nanoclusters on temperature, annealing time, and the composition of the SiO _x layer are obtained. It is found that annealing of silica films containing layers of nonstoichiometric composition can lead to the formation of silicon nanoclusters or cavities.     

Determining the coefficient of transverse turbulent diffusion in a flow of the active medium of a CO<Subscript>2</Subscript> laser with fast axial gas circulation

A method for determining the coefficient of transverse turbulent diffusion in a flow of the active medium of a CO₂ laser with fast axial gas circulation is described. The method is based on the phase conjugation during degenerate intracavity four-wave mixing.__________Translated from Pribory i Tekhnika Eksperimenta, No. 2, 2005, pp. 114–119.Original Russian Text Copyright © 2005 by Buyarov, Galushkin, Dubrov, Zavalov.     

Tribological Performance of Fe<Subscript>67</Subscript>B<Subscript>33</Subscript> Alloy Prepared by a Self-Propagating High-Temperature Synthesis Technique

A bulk Fe₆₇B₃₃ alloy was prepared by a self-propagating high-temperature synthesis technique that is convenient, low in cost, and capable of being scaled up for tailoring the bulk materials. The Fe₆₇B₃₃ alloy is composed of dendrites with the t-Fe₂B phase and eutectic matrix with the α-Fe and t-Fe₂B phases. The content of the dendrite t-Fe₂B is above 80 vol.%. The compressive fractured strength and Vickers microhardness are 3400 MPa and 12.4 GPa, respectively. The tribological performance of the Fe₆₇B₃₃ alloy is investigated under dry sliding and water lubricant against Si₃N₄ ceramic ball. The wear rates of the Fe₆₇B₃₃ alloy are of the magnitude of 10⁻⁵ to 10⁻⁴ mm³/m under water lubricant. It is lower than that of the Fe₆₇B₃₃ alloy under dry sliding (10⁻⁴ mm³/m). But both the friction coefficients are almost identical. Oxide layers form in both environments via different tribochemical mechanisms, which led to significant differences in wear behavior.     

Synergistic Effect of Nano-MoS<Subscript>2</Subscript> and Anatase Nano-TiO<Subscript>2</Subscript> on the Lubrication Properties of MoS<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> Nano-Clusters

A MoS₃ precursor deposited on anatase nano-TiO₂ is heated at 450 °C in an H₂ atmosphere to synthesize MoS₂/TiO₂ nano-clusters. The nano-clusters are then characterized, and their tribological properties are evaluated. MoS₂ is found to be composed of layered structures with 1–10 nm thicknesses, 10–30 nm lengths, and 0.63–0.66 nm layer distances. The MoS₂ sizes in the MoS₂/TiO₂ nano-clusters are smaller and their layer distances are larger than those of pure nano-MoS₂. The MoS₂/TiO₂ nano-clusters also present a lower average friction coefficient than pure nano-MoS₂, but the anti-wear properties of both the nano-clusters and pure nano-MoS₂ are similar. X-ray photoelectron spectroscopy indicates that nano-TiO₂ and the element Mo are transferred to the friction surface from the MoS₂/TiO₂ nano-clusters through a tribochemical reaction. This produces a lubrication film containing TiO₂, MoO₃, and other chemicals. The nano-MoS₂ changes in size and layer distance when combined with nano-TiO₂, producing a synergistic effect. This may further be explained using a micro-cooperation model between MoS₂ nano-platelets and TiO₂ solid nanoparticles.     

Direct visualization of sliding-induced tribofilm on Au/MoS<Subscript>2</Subscript> nanocomposite coatings by c-AFM

The nanoscale lubrication mechanism of nanocomposite Au/MoS₂ solid lubricant coatings has been studied by conductive atomic force microscopy (c-AFM). A direct visualization of the lubricating process suggests tribomechanical formation of a MoS₂ tribofilm to be a key mechanism. The sliding-induced tribofilm formation was visualized by a reduction in local friction and conductivity in nanoscale AFM images. The tribofilm was found to possess considerable crystallinity and orientation, which was not observed in the as-deposited coatings. The observed mechanism is broadly applicable to a range of nanocomposite metal/MoS₂ coatings.