Bulletin PNRPU. Mechanical engineering, materials science

Heat capacity is one of the most important physical properties of solids, which characterizes the change in the state of matter with temperature. The study of heat capacity is one of the main methods for studying structural and phase transformations in alloys. From the temperature dependence of the heat capacity, it is possible to determine other physical characteristics of a solid: temperature and type of phase transformation, Debye temperature, vacancy formation energy, coefficient of electronic heat capacity, etc. Experimental measurement of heat capacity for different temperature ranges - from extremely low to high - is the main method for determining the thermodynamic properties of substances. In this work, the heat capacity of the zinc alloy Zn55Al with zirconium was determined in the "cooling" mode from the known heat capacity of a reference copper sample. For this, polynomials describing their cooling rates were obtained by processing the curves of the cooling rate of samples made of zinc alloy Zn55Al with zirconium and the standard. Further, according to the experimentally found values of the cooling rates of the samples from the alloys and the standard, knowing their masses, the polynomials of the temperature dependence of the heat capacity of the alloys were established, which are described by a four-term equation. Using integrals of specific heat capacity, models of the temperature dependence of changes in enthalpy, entropy and Gibbs energy were established. The obtained dependences show that with an increase in temperature, the heat capacity, enthalpy, and entropy of alloys increase, while the Gibbs energy decreases. At the same time, zirconium additives increase the heat capacity, enthalpy and entropy of the initial Zn55Al alloy up to a temperature of 350K, then the additive decreases the heat capacity. In this case, the value of the Gibbs energy decreases.

In this work, the characteristics of synthesizing processes of cobalt-based nanopowders (Co(OH)2, Co3O4, Co) by chemical-metallurgy method were studied. Co(OH)2 nanopowder was synthesized by chemical precipitation from aqueous solutions of cobalt nitrate Co(NO3)2 (10 wt. %) and alkali NaOH (10 wt. %) under conditions of continuous stirring, temperature control T=25°C and pH=9. The synthesized Co(OH)2 precipitate was washed with distilled water using a Buchner funnel until the dissolved salt ions with a pH value were completely washed out over the precipitate was 7. Co3O4 and Co nanopowders were obtained by thermal decomposition and hydrogen reduction of Co(OH)2 hydroxide, respectively, in a tubular furnace “SNOL 0.2/1250”. The study of the crystal structure and phase composition of the powder samples was carried out by the method of XRD phase analysis. The specific surface area of the powder samples was determined using the BET method with adsorption of nitrogen in low temperature. The average particle size D was calculated from the measurement of the specific surface area. The size characteristics and morphology of the powder particles were studied by scanning electron microscopy. It has been established that the optimal temperatures for thermal decomposition and reduction processes are 180 and 280°C respectively, the holding time of the processes is about two hours. The obtained nanoparticles Co(OH)2 and Co3O4 mainly have an acicular shape, with a diameter up to a few tens of nm and the length up to 200-300 nm. Co nanoparticles mainly have a spherical shape, the size of which is also up until several tens of nm. They are in a sintered state, each of them is connected to several neighboring particles by isthmuses.

Composite materials titanium-aluminum and titanium-foam aluminum are increasingly used in industry. Of the number of methods used to obtain composite materials titanium-aluminum and titanium-foam aluminum, the most universal and simple is liquid phase. The essence of the liquid-phase method is the formation of a composite by the interaction of molten aluminum and a solid titanium phase. methods of forming composite materials are liquid phase. In liquid-phase methods, as a result of the reaction diffusion of titanium and aluminum, a transitional intermetallic layer is formed at the phase boundary of the composite, the thickness and composition of which is determined by the temperature of the phases during the formation of the composite material, the time of their high-temperature interaction, and their chemical composition. The mechanical and operational properties of titanium and composite material will largely be determined by the parameters, composition and properties of this transition layer. Therefore, the aim of the work is to study the processes of interaction of titanium with liquid aluminum and the influence of these processes on its properties under the conditions of formation of composite materials. To achieve this goal, experimental studies of the formation of an intermetallic layer on titanium during liquid-phase aluminization of titanium were carried out. The alitizing temperature during the experiments varied in the range of 750-950 °С, and the alimination time was in the range of 3-60 s. Metallographic studies and X-ray microanalysis of aluminized titanium samples made it possible to establish the effect of temperature and exposure time of titanium samples on the thickness, chemical and phase composition of the intermetallic transition layer. The mechanical tests showed the influence of the temperature of the aluminum melt during interaction with titanium on the strength properties of titanium-aluminum bimetal.

Zr1- x Al x N coatings are formed by pulsed magnetron sputtering in the range of technological parameters: pressure of the gas mixture P = 0.25…1.0 Pa and nitrogen content in the gas mixture N2 = 5…40 %. The phase and elemental composition, as well as tribological, heat-resistant, crack-resistant and adhesion properties were investigated for P = 0.75 Pa and N2 = 5…15 %. With a different combination of parameters, an X-ray amorphous coating is formed. In the investigated range, three-phase coatings Zr1- x Al x N are formed based on the phases: c -Zr3AlN, w -Zr3AlN, δ-Zr3N4. Phases h -ZrN0.28 and w-AlN are optional. Depending on the nitrogen content in the gas mixture, the Zr1- x Al x N coating is formed in three different states. Stoichiometric three-phase coating Zr1- х Al х N (20 at. % Al, 20 at. % Ti, 60 at. % N) based on c -Zr3AlN, w -Zr3AlN, δ-Zr3N4 phases is formed at N2 = 15 %. Maximum crack resistance K cr = S ref / S po = 0.1, microhardness H = 24 GPa, adhesion strength and ability to elastic recovery, as well as minimum friction force F fr = 4.1 N and friction coefficient µ = 0.06 corresponds to nanostructured coating Zr1- x Al x N with the maximum content of the w -Zr3AlN phase ( Vw -Zr3AlN = 27.56 %) and Al (55.44 at. %), minimum thermal stresses and surface defects. In the case of deposition of an X-ray amorphous three-phase coating Zr1- x Al x N, its microhardness sharply decreases with a significant deterioration of tribological properties. A decrease in the proportion of the thermally stable phase w -Zr3AlN in the Zr1- х Al х N coating has a greater effect on the deterioration of its tribological, heat-resistant, crack-resistant and adhesive properties. Stoichiometric three-phase coating Zr1- х Al х N (20 at. % Al, 20 at. % Ti, 60 at. % N) based on c -Zr3AlN, w -Zr3AlN, δ-Zr3N4 phases, formed at N2 = 15 %, has a minimum crack resistance. Maximum crack resistance K cr = = S coating peeling / S ∑ = 0.1, microhardness H = 24 GPa, adhesion strength and ability to elastic recovery, as well as minimum friction force F fr = 4.1 N and friction coefficient µ = 0.06 corresponds to nanostructured coating Zr1- x Al x N with the maximum content of the w -Zr3AlN phase ( = 27.56 %) and Al (55.44 at. %), minimum thermal stresses and surface defects. In the case of deposition of an X-ray amorphous three-phase coating Zr1- x Al x N, its microhardness sharply decreases with a significant deterioration of tribological properties. A decrease in the proportion of the thermally stable phase w -Zr3AlN in the Zr1- x Al x N coating has a greater effect on the deterioration of its tribological, heat-resistant, crack-resistant and adhesive properties.

The article examines the study of the processing modes influence on such indicators of the quality of the surface layer of parts made of aluminum alloys, such as surface roughness and residual stresses. During the experimental studies, we used 3M elastic polymer-abrasive wheels of the FS-WL, DB-WL, CF-FB brands. As a result of experimental studies, it was found that the transverse roughness in the parameter Ra increases with increasing deformation of the circle. This is due to the fact that with an increase in deformation, the vertical component of the force grows, and, consequently, the depth of penetration of single grains into the processed material increases. As the cutting speed increases, the Ra transverse roughness also increases. This is due to the fact that with increasing speed, the centrifugal component of the impact force of the abrasive grain on the treated surface increases. In the course of statistical processing of the experimental data, it was proved that the transverse roughness does not depend on the longitudinal feed. An analysis of variance of the experimental results proved that the longitudinal roughness in terms of the Ra parameter for all used elastic polymer-abrasive wheels does not depend on the specified processing modes. As a result of the research carried out, an empirical dependence was obtained, which allows predicting the expected roughness when designing a technological process for manufacturing a part. The study of the formation of residual stresses in the surface layer of parts made of aluminum alloy V95PchT2 during processing with elastic polymer-abrasive wheels. As a result, it was found that when the samples obtained by cylindrical and face milling with elastic polymer-abrasive wheels are processed, the residual stresses are completely re-formed. Considering that this processing process takes place on a very thin softened surface layer, it has been proven that compressive residual stresses arise in the processed material at a shallow depth of occurrence, which has a positive effect on the operational properties of parts. The analysis of the state of the surface layer was also carried out using metallographic and electron microscopy. Based on these studies, it was concluded that the cases of darkening of the workpiece made of aluminum alloy cannot prevent the introduction of surface cleaning with polymer-abrasive tools in the aviation industry, since all particles present on the surface are easily removed during preparation for anodizing.