Том 57, № 6 (2016)

The specific property of nanobubbles with spontaneous spreading over the solid hydrophobic particle substrate adhered to them, which is caused by a high capillary gas pressure in nanobubbles (P_c > 10⁶ N/m²), is considered. The computational principle of bubble spreading curves is considered and parameter X characterizing the intensity is introduced. Dependence X(a) (a is the bubble base diameter) is presented by a bimodal curve, which confirms that the nanobubble spreading is energetically provided by two sequentially acting independent sources. The first source is conditioned by the reduction (approximately by 11%) of the nanobubble curvilinear surface area at the initial spreading stage, and the second source is conditioned by the work of gas expansion caused by the drop of P_c when the bubble is spreading. Parameter X is characterized by a considerably larger slope of dependence X(a) at the first spreading stage compared to the second one. It now turned out that the revealed property, which determines the efficiency of industrial flotation processes in past, finds prospects for application again after its recognition. Since it manifests itself in a limited range of bubble sizes, it is proposed to attribute it to the proper or natural fractal by analogy with the Brownian motion, which manifests itself in a definite range of particle sizes. The influence of the surface activity of flotation reagents on the shape of bubble spreading curves is shown.

The mathematical description of various stages of the deoxidation kinetics of copper with charcoal is performed as applied to the reduction stage of fire refining of blister copper. It is shown that the process rate is controlled by the carbon mass transfer in the melt bulk and the reduction of oxygen contained in metal is performed according to the two-stage scheme. To intensify the copper deoxidation, it is recognized as reasonable to blow-in the finely dispersed coal immediately into the liquid metal bulk with inert or natural gas.

The computational procedures to model the fluorination of the tungsten powder with fluorine and condensation of formed WF₆, which describe the available experimental data satisfactorily, are developed using physicochemical foundations of processes. With their help, the equipment sizes and process parameters of the two-stage fluorination of the tungsten powder with fluorine with the condensation of liquid WF₆ after each stage at 2.5–3.0°C are optimized. The possibility of fabricating WF₆ with productivity of 5.23, 6.53, and 7.83 kg/h in reactors 200, 300, and 360 mm in diameter, respectively, at 300–350°C without the forced cooling of the highest heat-beat first fluorination setup is shown. The completeness of fluorine usage higher than 99.99% is attained in this case, while the amount of harmful gases (F₂, WF₆) leaving the process chain does not exceed their maximal permissible concentrations already in the vent gas volume. Recommendations for the organization of the fluorination production process of the tungsten powder are given.

Mathematical modeling of drawing a billet formed by compacting a titanium sponge is performed in the ABAQUS engineering analysis software package, allowing for the possibility of pore formation during deformation. It is shown that the maximal porosity at small drawing is formed equally due to the variation in occurring pores and the appearance of a new defect; with an increase in the elongation, it is determined mainly by the growth of newly formed continuity defects, the nucleation region of which corresponds to the region of tensile stresses. The volume fraction of newly formed defects substantially affects the total porosity at large elongation and conicity angle, which increases the damage and can become the cause of rod break.

The properties of nanostructured multilayered coatings of the composition (Ti,Al)N–Mo₂N, which were fabricated by the ion-plasma vacuum-arc deposition (arc-PVD), are investigated. The thickness of coating layers is comparable with the grain size, which is about 30–50 nm. The coating hardness reaches 40 GPa with relative plastic deformation work of about 60%. It is established by measuring scratching that the cohesion destruction character of the coating occurs exclusively according to the plastic deformation mechanism, which evidences its high fracture toughness. The local coating attrition to the substrate takes place under a load on the order of 75 N. The coating friction coefficient in testing conditions according to the “pin-on-disc” layout using the Al₂O₃ counterbody under a load of 5 N is 0.35 and 0.50 at temperatures of 20 and 500°C, respectively. The coating is almost unworn because of the formation of MoO₃ oxide (the Magneli phase) operating as the solid lubricant in the friction zone. An increase in the friction coefficient and noticeable wear are observed with the further increase in the testing temperature, which is associated with the sublimation intensification of MoO₃ from the working surfaces and lowering its operational efficiency as the lubricant.

In the present work, stir casting route is used to fabricate the duplex (α–β) brass plate. The machineabilty behavior of the (α–β) brass is analyzed during Electrical Discharge Machining (EDM) using Taguchi method. Experiments were conducted with three machining variables such as current, pulse-on time and voltage. Material removal rates (MRR), Electrode wear rate (EWR) and surface roughness (SR) are chosen as the output parameters. Results of SN ratio analysis showed that peak current was significant factor to affect the all the responses. Analysis of variance (ANOVA) was used to identify the contribution of each parameter.

The substances most suitable for use as oxygen-containing additives for the sodium-thermal fabrication of finely dispersed tantalum powder (K₂Ta₂O₃F₆, K₃TaOF₆, K₂TaOF₅, and KTaOF₄) are selected based on the thermodynamic and experimental investigations of reduction reactions of tantalum compounds with sodium from the melts containing complex oxyfluorides compounds. The application of the mentioned compounds gives the opportunity to fabricate tantalum powders with a specific surface area at a level of 3–5 m²/g, which is 8- to 10-fold higher than for the powders fabricated under the same conditions when reducing K₂TaF₇. It is shown that fabricated tantalum powders can be needed as the initial material for the development of the capacitor powder with a charge of 70000–100000 μFV/g.

Boron carbide is prepared by self-propagating high-temperature synthesis (SHS) in a composition range from 5 to 30 at % carbon. The introduction of an inert (MgO) and active (Mg(ClO₄)₂) additives into the reaction mixture leads to a variation in process parameters such as the temperature and combustion rate. It is established that the unit cell metrics of boron carbide substantially vary depending on the synthesis conditions. The degree of the effect of the SHS mode on the crystal structure rises with an increase in the carbon fraction in the boron carbide structure. This regularity is associated with the ordering diversity of carbon atoms in nonstoichiometric boron carbide. No influence of the synthesis conditions on the unit cell parameters is observed for stoichiometric boron carbide, which is associated with the structure saturation with carbon. It is shown that the variation in the combustion temperature during SHS of boron carbide of the same composition leads to the variability of the structural parameters, thus reflecting the influence of the synthesis conditions on the material crystal structure.

Regularities of dissolution, phase formation, and structure formation during the interaction of double carbides (Ti_1–nMe_n^{IV, V})C with the Ni–25%Mo melt (t = 1450°C, τ = 1 h, vacuum 10^–1 Pa) are investigated for the first time by electron probe microanalysis and scanning electron microscopy. The role of each alloying metal in the composition and microstructure formation of studied compositions is revealed. It is established that Group IV alloying metals (Zr and Hf) almost do not enter the composition of the forming K-phase (carbide Ti_1–nMo_nC_x); therefore, its composition is independent of their concentration in double carbide. In contrast with zirconium and hafnium, Group V alloying metals (V and Nb) actively participate in the formation of the K-phase; however, the dependences of the composition of the K-phase and metallic matrix on the vanadium and niobium content are the opposite in this case. An interpretation of the causes of these distinctions is proposed.

The macrostructure of nickel foam with porosity of 20, 30, 45, and 60 ppi is investigated by X-ray tomography at voltage U = 300 kV, current I = 300 mA, and exposure time t_exp = 354 ms; the number of frames is 2500. It is established that the actual pore parameters deviate from theoretical ones. It is shown that, the higher porosity is, the more uniform pore size is. The X-ray tomography makes it possible to evaluate such characteristics as the wall thickness between the pores, which had not been taken into account previously. It is revealed that the size uniformity of the wall thickness lowers as the sample porosity increases. The results of the X-ray tomography make it possible to recommend it as the method for studying and monitoring foam materials, powders, and various sintered materials, and its data can be used to develop actual three-dimensional models.

The results of studying the microstructure and microhardness of Ni-resist cast iron ChN16D7GKh after laser melt injection by means of introducing titanium particles into the melt are presented. The treatment was performed using a fiber laser with a beam focused into a spot 0.2 mm in diameter with a radiation power of 1 kW and the motion velocity of the laser beam of 10–40 mm/s. Titanium is dissolved in the cast-iron melt, and TiC particles are formed in the structure upon cooling. The coefficient of using the titanium powder increases as the fusion zone size increases and reaches 50% in the best case. A modified layer has a composite structure with a metallic matrix and a comparatively uniform distribution of titanium carbide particles. The microhardness of the modified zone is 600–700 HV. Its further growth is suppressed by the partial removal of carbon from the melt zone in the composition of red fume evolved in the process. Therefore, the Laves phase (TiFe₂) is formed instead of an increase in the TiC content upon increasing the titanium supply. The experimental data on the regularities of the weight loss caused by the substance removal from the melt zone depending on laser melting parameters are presented.

The influence of the tin content on the structure and hardness of Sn–Cu–Co and Sn–Cu–Co–W alloys, which are applied as binders of diamond abrasive tools, is investigated. The binders are prepared by compositional brazing: powdered components are mixed with an organic binder and deposited on a steel base. Sintering is performed at 820–1100°C. The structure of metallic binders is investigated by X-ray diffractometry and electron probe microanalysis. In addition, the microhardness of structural components and macrohardness of binders are measured. It is established that the hardness of metallic binders linearly increases with an increase in the tin content due to an increase in the amount of solid intermetallic phases in their structure. The optimal tin content, which provides high hardness (96–98 HRB) and the absence of lowmelting high-tin intermetallic compounds in their structure, is determined for Sn–Cu–Co–W binders.