Modification of T15K6 hard-alloy surface layers with titanium and zirconium coatings under the action of compression plasma flows

The effect of compression plasma flows (CPFs) with energy densities of 22 and 27 J/cm² on the T15K6 hard-alloy surface with predeposited titanium and zirconium coatings is investigated. It has been ascertained that tungsten nitride (WN) is formed in the surface layers of the systems under study after the action of CPFs in gaseous nitrogen. In the case of the deposition of a zirconium coating, the (Ti, W)C solid solution is alloyed with zirconium. Treatment by CPFs leads to the alloying of the coating and the hard-alloy substrate, generating two types of modified-surface morphology. These types differ in the number of microcraters and pores per unit of surface area. The combined action with energy densities of 22 and 27 J/cm² on a hard alloy enhances its microhardness by a factor of more than 2.6 and 2.2, respectively.     

Bulk periodic structures formation on monocrystalline silicon surface under the action of compression plasma flows

The results of numerical simulation of monocrystalline silicon melting and crystallization under the action of compression plasma flow generated by quasistationary plasma accelerators with regard to phase transformations based on Kolmogorov equation are presented. Temporal and spatial characteristics of melting and crystallization processes for pulses of various forms are discussed. Based on data received and estimates made, the conclusion on substantial role of thermoelectric instability in bulk periodic structures formation was made.     

Elemental composition of the surface layer of a heat-resistant nickel alloy doped with zirconium atoms under the action of compression plasma flows

The element distribution over the surface layer in a heat-resistant nickel alloy, which is doped with zirconium under the action of compression plasma flows generated by a compact magnetoplasma compressor, is investigated. An analysis is performed using the helium ion backscattering method (the ion energy is 6MeV) and energy-dispersive X-ray microanalysis. It is established that the doping element concentration decreases due to an increase in the absorbed-energy density and the number of pulses. The segregation of zirconium atoms is observed in the surface layer with a thickness determined by the conditions of the action. This process is related mainly to the formation of zirconium oxide (oxynitride).     

Change in the relief of a target surface treated by compression plasma flows

The change in the surface relief of a steel-3 target (GOST 380) treated by compression plasma flows is experimentally studied. The energy density absorbed by the target varies in the range of 10–35 J/cm² and the pulse duration is 100 μs. It is shown experimentally and numerically that the development of KelvinHelmholtz instability strongly affects the formation of the target surface treated with compression plasma flows: a large-scale wave-like relief with characteristic sizes of 200 × 1000 μm is formed on the target surface and, as a result, the roughness of the surface increases. However, the microrelief at the scale of individual elements is smoothed to a maximum roughness of about 0.5 μm.     

Adsorption compression in surface layers

A recently elucidated aspect of adsorption, compression in confined phases, is discussed. Grand Canonical Monte Carlo simulations were performed for the adsorption of Lennard–Jones molecules and new details of intermolecular interactions in adsorbed layers are analysed. It is shown that a strong attraction to a surface can cause adsorption compression not only in the first layer, but also in higher layers. Compression of the first layer creates a pattern of active sites; the second layer tends to be commensurate with this pattern and has density higher than that of a ‘free’ layer. This pattern propagates to higher layers. However, there is a wide range of chemical potentials where the first layer is compressed and the second layer is not yet formed. It was found that transition to adsorption compression results in oscillations of the isosteric heat of adsorption. These oscillations are determined by a combination of (a) changes in adsorbed layers’ structure and (b) exchange of molecules between layers. In particular, at high affinity to adsorbent, the adsorption isotherm for the first layer has a slight maximum because an increase of the chemical potential causes molecules to leave the compressed first layer and move to the second layer. For this reason, the isosteric heat of adsorption decreases and can become negative. Analysis of adsorption compression mechanisms in the context of theory and emerging experimental results indicates that the significance of this phenomenon is not limited to fundamental aspects of adsorption and capillarity. These mechanisms play a crucial role in various applications, such as heterogeneous catalysis, membrane separations, and self-assembly on surfaces. Results are discussed in a broader context of theory, experiments and previous simulations.     

Interaction between oppositely directed compression plasma flows

The dynamics of the interaction of two oppositely directed plasma flows generated by miniature gas-discharge magnetoplasma compressors is studied. The maximum plasma electron temperature and density in the region where the plasmas interact are found to be 4.5 eV and 1.4⋅10¹⁷ cm^–3. Exposure of samples to this kind of plasma near the interaction region leads to an energy flux at the surface of 2–8 J/cm², which is sufficient for modification of the surface properties of various materials.     

Formation of relief on a metallic target surface under the action of compression-plasma flows

The results of experimental and theoretical investigations of relief formation on the surface of a steel target (grade St 3 steel, GOST (State Standard) 380) during treatment by compression-plasma flows are represented. The density of energy absorbed by the target varied in the range from 15 to 25 J/cm², the pulse duration was 100 μs, and the pulse number was N = 1, 3, 5, 7. The experiment revealed the expansion of boundaries of the central area (the area on which the plasma flow is incident normally to the surface) with increasing pulse number. This is explained by the more uniform surface treatment at a greater pulse number. It is shown that to describe relief formation in the central area there is a need to take into account the pressure of the plasma flow on the target surface, apart from surface tension forces and energy dissipation due to viscosity.     

Effects of plasma surface layers on the efficiency of plasma flowswitching

The plasma flow switch is a possible fast opening-switch for Z-pinches on inductive-store pulsed-power machines. This paper presents results of two-dimensional, radiation-magnetohydrodynamic models for a well-characterized plasma flow switch operating into a load region at current levels of 5-6 MA. The calculations predict that motion of the switch plasma along coaxial electrodes leads to the formation along the inner electrode of a boundary layer that can interfere with switching. The simulations describe the effect of the boundary layer on switching and show that it may be removed by means of a boundary-layer trap. Predictive calculations were used to eliminate the effects of the boundary layer and led to an improvement in the switch's performance. Comparison with experiments lends credence to the model     

Melting and crystallization dynamics of single-crystal silicon exposed to compression plasma flows

The melting and crystallization of single-crystal silicon wafers exposed to compression plasma flows generated by quasi-stationary plasma accelerators are studied by numerical simulation. The results include the phase transition kinetics described on the basis of the Kolmogorov equation. The space-time characteristics of the melting and crystallization processes for variously shaped plasma pulses are discussed. Based on the experimental data and estimates, it is concluded that thermoelectric instability plays an essential role in the formation of three-dimensional periodic structures.     

Structural and phase changes in single-crystalline silicon treated with compression plasma flows

The structural-phase changes in p-type single-crystalline silicon treated with compression plasma flows (CPFs) with an energy density of 5–12 J/cm² are investigated by the X-ray diffraction method depending on the crystallographic orientation of the silicon and the plasma energy density. In addition, the conductivity type on the treated silicon surface is determined by means of measuring the sign of the thermopower.; the surface morphology, by scanning electron microscopy; and the open-circuit voltage, upon illumination of the treated silicon surface (AM1.5 spectrum). It is found that treatment with CPFs results in the occurrence of the photovoltaic effect conditioned by the formation of an n-type modified surface layer. Depending on the crystallographic orientation, the modified layer either remains single crystalline (for the initial orientation (111)) or is subjected to amorphization (for the initial orientation (100)). At an energy density of ～8–9 J/cm² the action of CPFs leads to texture formation on the silicon surface.     

Fluctuating boundary layers in combined convection flows along a vertical surface

The effect of buoyancy force on laminar boundary layer in two dimensional flow and heat transfer along a semi-infinite vertical surface, when the velocity of the on-coming stream oscillates in magnitude about a steady mean, is analysed. Two separate solutions valid for low and high frequency ranges are developed. It is found that for low frequency oscillations the phase angles of oscillatory components of skin friction and the rate of heat transfer increase as the Grashof number increases. For very high frequencies, the velocity field is of shear-wave type unaffected by the mean flow; the phase lead in the skin friction is, then, /4 and the rate of heat transfer fluctuation indicates a phase lag of /2 over that of the free stream oscillations.The authors are thankful to referee for many useful suggestions.     

The structure of titanium nickelide surface layers formed by pulsed electron-beam melting

X-ray structure analysis technique has been developed for a material heterogeneous over the depth. The structure of the TiNi intermetallide subjected to surface modification by an electron beam has been investigated. The following three layers differentiated in the microcrystalline structure have been singled out: (i) the external sharply textured layer 1–3 μm in thickness with a texture in the [100] B2 direction and a lattice periodicity parameter reduced by 1% in the direction of the normal, (ii) the intermediate gradient-stressed layer 10–15 μm in thickness with the texture of the initial ingot, and (iii) the original material. The crystal lattice of the surface as well as of the intermediate layer is in a state stretched along the surface and compressed along the normal to it. This lattice distortion is maximal in the surface layer and decreases towards the bulk of the material.     

Shaping dynamics and structure of large-scale surface plasma formations under single-pulse laser action on various materials

It is established that, in the case of laser action on materials in large illuminated spots in atmospheric-pressure air for a laser radiation power density q≤2 MW/cm² (λ=1.315 μm), an evaporative regime with plane scattering of the laser-produced erosion plasma is realized while, for q≈5–17 MW/cm², the plasma front is transferred to the air, leading to plasma screening of the target and shaping of a subsonic radiation wave in the air. As the duration of the laser pulses increases (τ∼160 μsec), in spite of the large illuminated spots (S∼150 cm²) the evaporative regime of the laser action with plane plasma scattering goes over into a regime with jet outlow and formation of a quasistationary shock wave. Deceased. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 261–268, March–April, 1998.     

Effect of damaged surface layers on the parameters from plasma reflectivity spectra

This paper is an extension of the previous work, on surface characterization by the analysis of i.r. plasma reflectivity spectra. It is now shown that the apparent variations in the effective mass value, due to the presence of a damaged surface layer, can be approximately correlated with the nature of the surface. It has also been shown that the observation of a smaller optical relaxation time by the plasma reflectivity method compared to the corresponding d.c. relaxation time has nothing to do with the surface effects. Instead, the presence of a damaged surface layer always results in an increased estimate of relaxation time.     

Slow variations of the surface potential of CdS layers under illumination

Observations have been made of slow changes in the surface potential of CdS layers as a result of illumination, the kinetics and certain other regularities of this phenomenon have been investigated, and a phenomenological theory for the phenomen is presented. It is shown that the disruption of the adsorption equilibrium upon illumination does not play an important role in the mechanism of the surface photoemf, which is determined mainly by changes in the charge on fixed surface traps.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 79–84, December, 1970.     

Controlled removal of overpainting and painting layers under the action of UV laser radiation

Laser material removal applied to selective overpaintings and subsequent painting layers detachment was studied in order to select the best cleaning practice of painted artworks. The ablation depth as a function of incident laser fluence/intensity and irradiation pulse number was considered as a reference parameter. We have measured the ablation depth with both a contact microprofilometer and a white light interferometer as a function of laser irradiation parameters. The measurements have evidenced that the ablation depth in our experiments varied between 2 and 100 μm making possible selective removal of painting.     

Modification of surface oxide layers of titanium targets for increasing lifetime of neutron tubes

The peculiarities of interaction of hydrogen ions with a titanium target and its surface oxide layer were studied. Two ways of modification of the surface oxide layers of titanium targets for increasing the lifetime of neutron tubes were proposed: (1) deposition of an yttrium oxide barrier layer on the target surface; (2) implementation of neutron tube work regime in which the target is irradiated with ions with energies lower than 1000 eV between high-energy ion irradiation pulses.     

Titanium segregation on the surface of 12Kh18N10T steel under the action of an electron beam

Structural and phase modifications on the surface of 12Kh18N10T steel exposed to a continuous electron beam are studied by scanning electron microscopy. It is found that steel-beam interaction causes considerable evolution of titanium from the steel. The temperature distribution upon the thermal action of electrons is calculated. The depth distribution of the alloy components is measured.