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.     

Manufacturing and mechanical characterization of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/β-TCP/TiO<Subscript>2</Subscript> biocomposite as a potential bone substitute

This paper focuses on the mechanical characterization of a bioceramic based on commercial alumina (Al₂O₃) mixed with synthesized tricalcium phosphate (β-TCP) and commercial titania powder (TiO₂). The effect of β-TCP and TiO₂ addition on the mechanical performance was investigated. After a sintering process at 1600 °C for 1 h, various mechanical properties of the samples have been studied, such as compressive strength, flexural strength, tensile strength, elastic modulus, and fracture toughness. The measurements of the elastic modulus (E) and the tensile strength (σ _t ) were conducted using the modified Brazilian test while the compressive strength (σ _c ) was determined through a compression test. Also, semi-circular bending (SCB) specimens were used to evaluate the flexural strength (σ _f ) and the opening mode fracture toughness (K _IC). From the main results, it was found that the best mechanical performance is obtained with the addition of 10 wt.% TCP and 5 wt.% TiO₂. Alumina/10 wt.% tricalcium phosphate/5 wt.% titania composites displayed the highest values of mechanical properties and a good combination of compressive strength (σ _c ?≈?352 MPa), flexural strength (σ _f ?≈?98 MPa), tensile strength (σ _t ?≈?86.65 MPa), and fracture toughness (K _IC?≈?13 MPa m^1/2).     

Tribological Properties of Organic Functionalized ZrB<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Epoxy Composites

The tribological behaviors of epoxy composites filled with organic functionalized ZrB₂–Al₂O₃ were environmentally investigated and compared with those with as-received fillers under both dry and oil sliding conditions in this work. The worn surfaces and the transfer films on the counterparts were characterized by scanning electronic microscope (SEM), and the frictional temperature rising was investigated by infrared thermometer. The results demonstrated that the coefficient of friction (CoF), the wear rate, as well as the frictional temperature rise of the epoxy composites were all decreased due to the introduction of ZrB₂–Al₂O₃ fillers. And with the increase in filler content, similar variation tendencies of CoF and wear rate of epoxy composites were observed under the different sliding conditions. Besides, the organic functionalization of ZrB₂–Al₂O₃ fillers, which made the epoxy composites exhibit lower CoF and wear rate than those with as-received fillers, lowered the frictional temperature as well. In comparison, the epoxy composites filled with 5 vol% modified fillers presented better tribological properties, suggesting a stronger interfacial bonding between modified fillers and epoxy matrix. The dominant wear mechanisms of filled composites under dry and oil sliding conditions could be inferred as the combination of adhesive wear and abrasive wear and the fatigue wear, respectively, on the basis of SEM images of worn surfaces.     

A photo-thermally stimulated exoemission technique used to study the properties of HTSC ceramics based on YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript>

The method of photo-thermally stimulated exoelectron emission (PTSE) is used to study the processes in the surface layers of HTSC ceramics based on YBa₂Cu₃O_7–. A correlation is observed between variations of the exoemission current and the transition to the superconducting state. The experimentally detected hysteresis in the PTSE intensity of YBa₂Cu₃O_7– ceramics, when thermally cycled in the range T=80–160 K, is limited by two processes. Below T=90 K, the chemisorption has a significant effect on the physical phenomena under study, thus impeding the search for new PTSE regularities in HTSC ceramics. At the same time, exoemission properties of the ceramics considerably change in the range 80–160 K. It is also shown that the superconducting transition and the thermal absorption phenomena for an YBa₂Cu₃O_6.8 specimen are split in the studied temperature range, which allows the PTSE technique to detect the superconducting state and to observe the transition dynamics.Translated from Defektoskopiya, Vol. 40, No. 12, 2004, pp. 54–59.Original Russian Text Copyright © 2004 by Syurdo, Kortov, Milman, Slesarev, Mikhailovich.This revised version was published online in April 2005 with a corrected cover date.     

Spectrometry analysis of fumes of (U<Subscript>0.80</Subscript>Pu<Subscript>0.20</Subscript>)O<Subscript>2</Subscript> hybrid nuclear fuel samples heated to a temperature of 2000°C

The results of the spectrometry analysis of a gaseous cloud that forms over (U_0.80Pu_0.20)O₂ hybrid uranium-plutonium fuel samples for reactors, which were heated to a temperature of >2000°C, are described. Samples were heated using a laser facility designed for conducting out-of-pile experiments with the nuclear fuel. The obtained results will make it possible to empirically forecast the isotopic composition of volley emissions from the reactor core during accidents on the 1000-MW power reactor with the (U_0.80Pu_0.20)O₂ hybrid fuel.     

Environmental Effects on the Tribology and Microstructure of MoS<Subscript>2</Subscript>–Sb<Subscript>2</Subscript>O<Subscript>3</Subscript>–C Films

The tribology of molybdenum disulfide (MoS₂)–Sb₂O₃–C films was tested under a variety of environmental conditions (ambient 50% RH, 10⁻⁷ Torr vacuum, 150 Torr oxygen, and 8 Torr water) and correlated with the composition of the surface composition expressed while sliding. High friction and low friction modes of behavior were detected. The lowest coefficient of friction, 0.06, was achieved under vacuum, while sliding in 8 Torr water and ambient conditions both yielded the highest value of 0.15. Water vapor was determined to be the environmental species responsible for high friction performance. XPS evaluations revealed a preferential expression of MoS₂ at the surface of wear tracks produced under vacuum and an increase in Sb₂O₃ concentration in wear tracks produced in ambient air (50% RH). In addition, wear tracks produced by sliding in vacuum exhibited the lowest surface roughness as compared to those produced in other environments, consistent with the picture of low friction originating from well-ordered MoS₂ layers produced through sliding in vacuum.     

Assessment of critical level of shear stresses in tribocouples of Аl<Subscript>2</Subscript>O<Subscript>3</Subscript> and metal details of ITER blanket modules

The shear strength of plasma-sprayed electroinsulating Аl₂O₃ coating has been determined for blanket modules of a thermonuclear reactor in a friction couple with 316L(N)-IG steel and BrAZhNMts9-4-4-1 bronze. An analysis of the experimental and calculated data on the influence of shear stresses on the electric insulation of blanket modules has revealed the need to decrease the coefficient of friction to permissible level [f] = 0.28 in the range of contact pressures Р = 3–500 MPa.     

Fabrication In Situ TiB<Subscript>2</Subscript>–TiC–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Multiple Ceramic Particles Reinforced Fe-Based Composite Coatings by Gas Tungsten Arc Welding

A Fe-based composite coating reinforced by multiple TiB₂–TiC–Al₂O₃ ceramic particles was developed by gas tungsten arc welding (GTAW) melting process. Mixture of aluminum (Al), boron carbide (B₄C), and titanium dioxide (TiO₂) powders was used as precursors, and as a consequence TiB₂–TiC–Al₂O₃ multiple ceramic particles were in situ synthesized during GTAW melting process. Microstructural investigations showed that TiB₂ particles exhibit a blocky morphology, TiC particles are of flower-like shape, and the Al₂O₃ particles exist as small black dots and located in the core of reinforced particles. The hardness and wear resistance of the coatings increased drastically in comparison with that of the substrate.     

Microstructural and Tribological Properties of A356 Al–Si Alloy Reinforced with Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Particles

In the present study, the effect of the Al₂O₃ particles (average size of 12 μm, 3 and 10 wt.%) reinforcement on the microstructure and tribological properties of Al–Si alloy (A356) was investigated. Composites were produced by applying compocasting process. Tribological properties of unreinforced alloy and composites were studied, using pin-on-disc tribometer, under dry sliding conditions at different specific loads and sliding speed of 1 m/s. Microhardness measurements, optical microscope and scanning electron microscope were used for microstructural characterization and investigation of worn surfaces and wear debris. During compocasting of A356 alloy, a transformation from a typical dendritic primary α phase to a non-dendritic rosette-like structure occurred. Composites exhibited better wear resistance compared with unreinforced alloy. Presence of 3 wt.% Al₂O₃ particles in the composite material affected the wear resistance only at specific loads up to 1 MPa. The wear rate of composite with 10 wt.% Al₂O₃ particles was nearly two order of the magnitude lower than the wear rate of the matrix alloy. Dominant wear mechanism for all materials was adhesion, with others mechanisms: oxidation, abrasion and delamination as minor ones.     

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.     

Formation of silicon nanocrystals in Si—SiO<Subscript>2</Subscript>—<Emphasis Type="Italic">α</Emphasis>-Si—SiO<Subscript>2</Subscript> heterostructures during high-temperature annealing: Experiment and simulation

Experiments and simulations are performed to study the formation of silicon nanocrystals (Si-NCs) in multilayer structures with alternating ultrathin layers of SiO₂ and amorphous hydrogenized silicon (α-Si:H) during high-temperature annealing. The effect of annealing on the transformation of the structure of the α-Si:H layers is studied by methods of high-resolution transmission electron microscopy, Raman spectroscopy, and photoluminescence spectroscopy. The conditions and kinetics of Si-NC formation are analyzed by the Monte Carlo technique. The type of the resultant crystalline silicon clusters is found to depend on the thickness and porosity of the original amorphous silicon layer located between SiO₂ layers. It is shown that an increase in the thickness of the α-Si layer in the case of low porosity leads to the formation of a percolation silicon cluster instead of individual Si nanocrystals.     

A small Raman laser based on a KGd(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> : Nd<Superscript>3+</Superscript> Crystal (λ=1.538 µm) with passive resonator <Emphasis Type="Italic">Q</Emphasis> switching

Generation properties of a Raman laser based on a KGd(WO₄)₂ : Nd³⁺ crystal and operating in a mode of passive resonator Q switching by a YAG : V³⁺ crystal at a wavelength safe for human eyes is studied. A small laser with a mass of 35 g, operating over a wide temperature range at a wavelength of 1.54 µm and an output energy of >6 mJ, has been created on the basis of these studies.__________Translated from Pribory i Tekhnika Eksperimenta, No. 2, 2005, pp. 112–113.Original Russian Text Copyright © 2005 by Ustimenko, Zabotin.     

A two-channel two-frequency low-temperature photodetector at wavelengths of 119 and 220 μm of an H<Subscript>2</Subscript>O laser

The design and characteristics of a compact device consisting of four cooled photodetectors of radiations at two frequencies (2.53 and 1.36 THz) are presented. The photodetectors are based on two pairs of Ge:Sb and n-type InSb single crystals and equipped with preamplifiers. The photodetector is intended for operating jointly with an H₂O laser, which operates in a mode of generation of orthogonally polarized waves, and can be used in interferometers with insufficiently stable lengths of optical paths, e.g., during investigation of plasma in high-power pulsed facilities. The photodetector is manufactured in the form of an insert into a commercial portable helium Dewar flask with a 12-mm-diameter neck.     

Dynamics of Growth of the Native Oxide of Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te

The growth of the native oxide of the Cd_xHg_1?xTe (MCT) compound is studied by methods of laser and spectral ellipsometry. It is found that a non-absorbing oxide film is formed from the very beginning in the case of MCT oxidation with hydrogen peroxide vapors, whereas oxidation with atmospheric oxygen leads to the formation of absorbing layers on the surface at the first stages of the process. When the oxide film thickness reaches 1–2 nm, the oxidation rate drastically decreases. If MCT samples that were stored for a long time (for years) in air at room temperature are heated at T = 200 °C, the optical thickness of the oxide film decreases.     

In Situ Studies of TiC<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Hard Coating Tribology

TiC_1−x N _x hard coatings present time-dependent tribological behavior with an initial running-in period (500–2000 cycles) marked by an elevated friction coefficient, followed by >10000 cycles with low-friction and wear at room temperature (RT) in ambient air. The mechanisms behind this behavior are not completely understood. Tribological tests performed at RT and at different relative humidity (RH) levels revealed that a minimum value between 15 and 25% RH is needed to trigger the low-friction regime at a sliding speed of 100 mm s⁻¹. By in situ observations of transfer film growth, it could be observed that third body material is formed during this running-in period by plowing of the coating and shearing of the removed material. The appearance and thickening of the transfer film marks the beginning of the steady-state low-friction regime where the velocity is accommodated by interfacial sliding. At this stage in the tribological test, the recorded Raman spectra indicated the presence of C–H bonds in the wear track. Use of in situ analytical tools during wear tests provided insights with respect to tribological phenomena that were not available by conventional, post-mortem analysis methods.     

Dry Sliding Wear Behavior of (Cr,Fe)<Subscript>7</Subscript>C<Subscript>3</Subscript>-γ(Cr,Fe) Metal Matrix Composite (MMC) Coatings: The Influence of High Volume Fraction (Cr,Fe)<Subscript>7</Subscript>C<Subscript>3</Subscript> Carbide

The high volume fraction (Cr, Fe)₇C₃ carbide particle-reinforced metal matrix composite (MMC) coatings with different Cr/C ratios were produced by flux-cored arc welding (FCAW). The in situ synthesized effectiveness of (Cr, Fe)₇C₃ carbide in the coatings was studied by the aid of CALPHAD and differential scanning calorimeter. The microstructure of the coatings was observed by optical microscope and field emission scanning electron microscope, and their phases were determined by the X-ray diffraction. Meanwhile, the hardness and wear resistance of the coatings were measured. The results show that the (Cr, Fe)₇C₃ carbide can be in situ synthesized in the coatings. With decreased Cr/C ratio, the in situ synthesized effectiveness of (Cr, Fe)₇C₃ carbide is improved and the mass fraction of the (Cr, Fe)₇C₃ carbide is increased. The microstructure of the coatings consists of in situ synthesized (Cr, Fe)₇C₃ carbide and eutectic (Cr, Fe)₇C₃/ γ (Cr, Fe) matrix. The hexagonal-rod-form (Cr, Fe)₇C₃ carbide can be fractured and segregated from austenite-matrix under a relatively high load force (50 N), and transforms the wear state from two-body-abrasion to three-body-abrasion, which facilitates the coating be seriously abraded and even adhered. The wear resistance of the MMC coatings can be effectively improved by the formation of high volume fraction of (Cr, Fe)₇C₃ carbide.     

Dependences of the optical characteristics of Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N films on the substrate composition and polarity

Optical properties of Ga- and N-polar triple nitrides Al _x Ga_1−x N with molar fractions of aluminum from 0 to 0.6 are studied by a nondestructive contactless method of spectroscopic ellipsometry. Correlation dependences of the shift of the fundamental absorption edge and the behavior of the real and imaginary parts of the pseudodielectric function on the composition x and polarity of the Al _x Ga_1−x N layers are revealed. It is verified that the polarity of the layers grown by molecular beam epitaxy is defined by the formation of the AlN nucleating layer.     

Using the φ-<Emphasis Type="Italic">S</Emphasis><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nomogram in calculating the dynamics of a braked wheel

The work deals with the investigation of parameters having effects on road holding and response of wheeled vehicles. A new procedure for evaluating forces in the tire contact area by means of the φ-S _x nomogram accounting for lateral effects on the wheel is put forth. The procedure may be used in assessment of the road holding ability of a vehicle during vehicle design, in selection of a functioning algorithm for the antilock braking systems of vehicles, and also in technical expertise on motor vehicle accidents.     

Impact of Wire-EDM on Tribological Characteristics of ZrO<Subscript>2</Subscript>-based Composites in Dry Sliding Contact with WC–Co-Cemented Carbide

In the present investigation, experiments were conducted by unidirectional sliding of pins made of FCC metals (Pb, Al, and Cu) with significantly different hardness values against the steel plates of various surface textures and roughness using an inclined pin-on-plate sliding apparatus in ambient conditions under both the dry and lubricated conditions. For a given material pair, it was observed that transfer layer formation and the coefficient of friction along with its two components, namely adhesion and plowing, are controlled by the surface texture of the harder mating surfaces and are less dependent of surface roughness (R _a) of the harder mating surfaces. The effect of surface texture on the friction was attributed to the variation of the plowing component of friction for different surfaces. It was also observed that the variation of plowing friction as a function of hardness depends on surface textures. More specifically, the plowing friction varies with hardness of the soft materials for a given type of surface texture and it is independent of hardness of soft materials for other type of surface texture. These variations could be attributed to the extent of plane strain conditions taking place at the asperity level during sliding. It was also observed that among the surface roughness parameters, the mean slope of the profile, Δ _a, correlated best with the friction. Furthermore, dimensionless quantifiable roughness parameters were formulated to describe the degree of plowing taking place at the asperity level.     

Effect of Grain Boundary on the Wear Behaviour of NiTi Shape Memory Alloys When <Emphasis Type="Italic">M</Emphasis><Subscript>f</Subscript> &lt; <Emphasis Type="Italic">T</Emphasis> &lt; <Emphasis Type="Italic">A</Emphasis><Subscript>f</Subscript>

Wear behaviour of NiTi SMA is closely corresponds to deformation mechanisms associated with different plastic strain accumulation process. Plastic strain accumulation is achieved by dislocation motion; however, grain boundary acts as a strong barrier. In this work, wear behaviour of single-crystalline and polycrystalline NiTi SMAs was studied to understand the effect of grain boundary on the plastic strain accumulation in the wear process. Wear tests were conducted at M_f < T < A_f, where phase boundary exists between martensitic and austenitic phases. Tests were conducted under ball-on-disc sliding wear mode, and alumina (Al₂O₃) counter-body was used. For single-crystalline NiTi SMA, transition wear occurred even when the applied load was relatively low (i.e., 100 mN). For polycrystalline NiTi SMA, with increasing applied load and wear cycles, the wear has shifted from near-zero wear stage to severe wear stage; no transition behaviour was observed. Significant differences in the wear process were discussed with respect to deformation mechanisms associated with dislocation motion in the single-crystalline and polycrystalline NiTi SMAs.