Powder Metallurgy and Metal Ceramics最新文献

The features peculiar to the solid-state synthesis of MoSi₂ through vacuum heat treatment of a powder mixture of molybdenum and silicon nitride, as a precursor, in the temperature range 1000–1400°C were examined. X-ray diffraction established that the solid-state interaction began at 1100°C and progressed through the reaction diffusion of highly active silicon, resulting from the decomposition of Si₃N₄, into molybdenum to form lower Mo₃Si and Mo₅Si₃ silicide phases. In the temperature range 1100–1300°C, the redistribution of phases occurred: the contents of the starting molybdenum and β-Si₃N₄ components in the reaction mixtures gradually decreased, while the contents of lower molybdenum silicides increased. Molybdenum disilicide formed in situ at 1400°C via successive development of lower silicide phases. The final product contained Mo₅Si₃. This was attributed to a deficiency of silicon as it evaporated at a temperature above 1200°C. This led to the conclusion that the addition of 20 wt.% excess silicon nitride was necessary to produce a homogeneous MoSi₂ phase and up to 40 wt.% excess silicon nitride to produce a two-phase MoSi₂–Si₃N₄ composite powder. The elevated temperature in the synthesis of MoSi₂ compared to conventional synthesis from simple elements was explained by the slow formation of active silicon in the Si₃N₄ dissociation process. Based on the features observed in the solid-state vacuum interaction within the powder mixture of molybdenum and silicon nitride, as a precursor, a method was proposed for producing MoSi₂–Si₃N₄ composite powders, involving the introduction of 30 and 40 wt.% excess Si₃N₄ powder. The synthesis resulted in agglomerated composite powders with a homogeneous distribution of the MoSi₂ and β -Si₃N₄ phases. The MoSi₂ phase exhibited a capsular structure with a smooth surface. The synthesized composite powders are intended for the fabrication of components and parts with high oxidation resistance and corrosion resistance at elevated temperatures.

High-entropy coatings produced by electron-beam deposition of multicomponent Al‒Ni‒Co‒Fe‒Cr‒Ti‒B_x (x = 0, 0.25, 0.5, and 1 mol) powder mixtures onto steel substrates in vacuum were examined. The effect of boron content on the phase composition, structure, and strength properties of the AlNiCoFeCrTiB_x coatings was studied employing X-ray diffraction, microstructural analysis, and micromechanical tests. The AlNiCoFeCrTi and AlNiCoFeCrTiB_0.25 coatings showed a typical dendritic and interdendritic structure and consisted of two substitutional solid solutions with a body-centered cubic (bcc) structure, differing in lattice parameters. An increase in the boron content to 0.5 mol changed the phase composition and led to the formation of in-situ titanium diboride TiB₂ as fine inclusions and chromium boride Cr₂B as elongated inclusions in the coatings besides the two bcc solid solutions (bcc1 and bcc2). When 1 mol of boron was added, the coatings remained four-phase, while the amount and sizes of TiB₂ and Cr₂B inclusions increased. Moreover, with 1 mol of boron, the ratio between the bcc1 and bcc2 phases increased toward bcc2 because of the removal of chromium and titanium atoms. Mechanical tests showed that the microhardness and yield stress of the AlNiCoFeCrTiB_x coatings produced by electron-beam deposition increased by 1.6 times when boron content raised to 1 mol: from 8.8 and 2.4 GPa for the AlNiCoFeCrTi coatings to 14.2 and 4 GPa for the AlNiCoFeCrTiB coatings. The significant enhancement in the strength indicators (hardness and yield stress) of the high-entropy coatings with greater boron content could be attributed to the solid-solution strengthening effect of interstitial boron atoms and to the strengthening effect of boride phase inclusions.

Mechanical alloys Mg + 10 wt.% Ti and Mg + 5 wt.% Ti + 5 wt.% TiC (MAs) were synthesized by reactive mechanical alloying (RMA). Thermal stability and hydrogen desorption kinetics of the nanosized MgH₂ phase in the obtained MAs were examined by means of thermal desorption spectroscopy at a hydrogen pressure of 0.1 MPa. The stabilizing effect of Ti on the nanocrystalline structure and growth of the crystallites (grains) of the MgH₂ phase during the cycling was also evaluated. It has been established that the complex doping by Ti and TiC leads to a significant improvement in the desorption of hydrogen in the nanosized MgH₂ phase of MAs. The role of Ti and TiC as alloying elements in improving the hydrogen desorption kinetics of MAs was studied. The catalytic effect of adding 5 wt.% Ti + 5 wt.% TiC to magnesium in improving the kinetics of hydrogen desorption is significantly lower than the catalytic effect of adding 10 wt.% Ti. Due to such alloying, the decrease in the thermodynamic stability of MgH₂ is not established. The average rate of the reaction does not depend on the hydrogen concentration and is equal to the rate constant k = = k₀ exp(–Ea/RT) (the Arrhenius equation). The tested materials showed high potential as hydrogen storage alloys, especially for stationary applications.

High-temperature electrochemical synthesis (HTES) in molten salts is highly promising among the up-to-date methods for the production of carbide powders. Ultrafine composite powders of tungsten carbides (WC|C, WC|C|Pt, W₂C|WC, and W₂C|W) were synthesized using the HTES method in electrolytic baths with different chemical compositions under various synthesis conditions (cathode current density, CO₂ pressure in the electrolyzer, temperature, and cathode material). Composite powders (up to 3 wt.% free carbon) with a WC particle size of 20–30 nm were prepared using Na, K|Cl (1 : 1)–Na₂W₂O₇ (6.4 wt.%)–CO₂ (1.25 MPa) and Na, K|Cl (1 : 1)–Na₂WO₄ (12.0 wt.%)–NaPO₃ (0.7 wt. %)–CO₂ (1.25 MPa) electrolytic baths at a temperature of 750°C. When the CO₂ pressure was reduced to 0.75 MPa, composite W₂C|WC powders formed at the cathode. The ratio of carbide phases in the composites depended on the initial concentration of tungsten salts in the electrolyte and on the CO₂ gas pressure in the electrolyzer. The addition of Li₂CO₃ (4.5 wt.%) to the electrolytic salt mixture decreased the tungsten carbide particles to 10 nm, changed their morphology, and increased the free carbon content in the composite up to 5 wt.%. The specific surface area of the powder increased by a factor of 4 to 7 (from 20–35 to 140 m²/g). The resulting products were modified with fine platinum particles through the use of platinum cathodes. The HTES method demonstrated its potential for producing tungsten carbide powders with the properties allowing their use as electrocatalysts in the hydrogen evolution reaction. For the WC|C composite powders synthesized in the Na, K|Cl–Na₂W₂O₇–Li₂CO₃–CO₂ system, the hydrogen evolution potential was –0.02 V relative to the normal hydrogen electrode, the overpotential η at a current density of 10 mA/cm² was –110 mV, the exchange current was 7.0 ⋅ 10^–4 A/cm², and the Tafel slope was –85 mV/dec.

The powders of hBN, SiC, and B₄C were employed as the raw ingredients to prepare the B₄C–SiC–hBN composite ceramics by a vacuum high-pressure sintering method with hBN and SiC contents in the range of 0, 10, and 20 wt.%, respectively. A pin-on-disc testing equipment was used to assess the tribological properties of B₄C–SiC–hBN composite ceramics with various hBN and SiC content when sliding against AISI 347 steel immersed in the emulsion. The experiment’s findings indicate that the sliding COF of the B₄C/AISI 347 steel pair marginally drops as the sliding distance increases. Besides, the sliding COF of the B₄C–10 wt.% SiC–20 wt.% hBN/AISI 347 steel pair rapidly declines. By tribopairs of B₄C–SiC–hBN composite ceramics against AISI 347 steel under the condition of lubrication by water-based emulsion, the steady-state friction may move into a state of mix lubrication as the hBN concentration rises, improving the tribological performance. The steady-state COF considerably decreases to 0.01 from 0.386 as the hBN and SiC concentration is increased to 20 wt.% and 10 wt.% from zero, showing a decreasing trend for both the B₄C–SiC–hBN pin and AISI 347 steel disc samples’ COWs. The steady-state friction of tribopairs of B₄C–SiC–hBN composite ceramics against AISI 347 steel may enter a state of mixed lubrication in the emulsion. The wear resistance of composite ceramics was improved by the addition of hBN and SiC particles because of their lubricating and reinforcing effects. These findings offer valuable insights into the design and development of advanced composite ceramics for various industrial applications that require improved tribological properties.

The paper examines the effect of doping elements on the structurization and properties of a new antifriction composite produced from grinding waste of the R2AM9K5 high-speed tool steel and CaF₂ solid lubricant. The composite is intended for operation at loads of 2.0–3.0 MPa and high rotation speeds (5,000–7,000 rpm) in contact joints of high-speed printing machines. The production process imparted a heterophase structure to the antifriction composite. The composite consists of a metal pearlite–carbide and carbonitride matrix and CaF₂ solid lubricant particles being evenly distributed in it. Valuable Mo, Cr, W, V, N, and Co doping elements contained in the R2AM9K5 steel waste particles promote the formation of strengthening phases in the composite’s metal matrix. In combination with CaF₂ solid lubricant, these strengthening phases impart high antifriction properties to the material under high-speed friction at speeds up to 7,000 rpm and loads of 2.0–3.0 MPa. Comparative tests of the new R2AM9K5 steel + (4.0−8.0)% CaF₂ composite demonstrated significant advantages in the antifriction properties over cast brass, currently used for units of modern rotary printing machines and can perform effectively only under continuous liquid lubrication. The R2AM9K5 steel waste composite containing CaF₂ solid lubricant permanently forms a protective antifriction film on the contact surfaces in the friction process, which was confirmed by electron microscopy studies. Under these friction conditions, the film is continuous, uniform, and smooth and is constantly restored on its worn areas, leading to self-lubrication. When the rotation speed increases up to 8,000 rpm, the composite antifriction properties decrease as the film on the contact surfaces becomes discontinuous. The research allowed operating limits to be determined for applying the new composite and proved the effectiveness of industrial grinding waste in developing high-quality structural materials through a reasoned choice of secondary raw materials, considering the nature of doping elements present in them.

An important process task is to expand the range of raw materials applied for the synthesis of dysprosium titanate (neutron-absorbing material for VVER-1000 reactors) and to establish and optimize methods for producing powders and pellets with characteristics that meet specific technical requirements, primarily those for the density and structural and phase compositions of the materials. The influence of MoO₃ doping additions and TiO₂ powders with different particle sizes on the density and phase composition of dysprosium titanate (Dy₂O₃ · TiO₂) pellets was experimentally studied. Titanium oxide powders of two grades were used: powder with spherical 10–30 μm particles (OSCh 7-3 grade as per TU 6-09-3811–79) and powder with an average particle size of ~63 nm (China Rare Metal Material Co., China). The nanosized TiO₂ powder intensified the sintering of the pellets, which achieved a density of 6.9 g/cm³ and acquired a single-phase hexagonal structure at 1650°C. The coarse TiO₂ powder did not promote high density of the sintered pellets, nor did it facilitate complete synthesis of dysprosium titanate since a significant amount of intermediate dysprosium dititanate (Dy₂Ti₂O₇) and initial dysprosium oxide (Dy₂O₃) remained in the synthesized material. The introduction of MoO₃ intensified the sintering of pellets, increased the pellet density up to 6.7 g/cm³, and led to a single-phase cubic structure of pyrochlore type, regardless of the TiO₂ powder grade. The simultaneous use of the nanosized TiO₂ powder and MoO₃ doping addition increased the density of the sintered dysprosium titanate pellets to 7.1 g/cm³ and promoted a single-phase structure of pyrochlore type.

The use of refrigeration technology is widespread in national security, industrial and agricultural production, biomedicine, and everyday life. High efficiency, environmental friendliness, and low cost make solid-state refrigeration based on electrocaloric effect (ECE) a promising refrigeration technology. Lead-free ferroelectric ceramics (1–x)Ba(Zr_0.2Ti_0.8)O₃–x(Ba_0.7Ca_0.3)TiO₃ (BZT–BCT) are promising materials for electrocaloric refrigeration in the field. In this paper, Sm-doped 0.5BZT–0.5BCT ceramic was fabricated by the conventional solid-state reaction method. The effect of Sm-doping contents (0, 1.0, 2.0, 2.5, and 3.0 mol.%) on the phase structures, dielectric properties, ferroelectricity, and electrocaloric properties of 0.5BZT–0.5BCT ceramics was systematically examined. The results indicate that all ceramics have a pure perovskite structure with no other secondary phase available. High relative densities are observed in all lead-free ferroelectric ceramics and all of the samples show transgranular fracture with no clear grain boundaries seen. The ceramics’ ferroelectric hysteresis loops become thinner as the Sm doping content increases. At that, remanent polarization Pr decreases, indicating that more polar nanoregions (PNRs) are formed in BZT–BCT lead-free ceramics through Sm doping. The increase in Sm doping content resulted in a change in the dielectric permittivity and electrocaloric temperature that first increased and then decreased. The maximum dielectric permittivity is 5,518 when the doping content of Sm is 2.5 mol.% and the maximum electrocaloric temperature change ΔT_max of 0.109 K at 4 kV/mm was obtained when Sm doping content was 2 mol.%. The results show that an appropriate Sm doping is favorable for improving the dielectric, ferroelectric, and electrothermal properties of lead-free ceramics 0.5BZT–0.5BCT.

Aluminum metal matrix composites (AMMCs) are a new modern group of composite materials that are becoming more popular in industrial progress. As a solid welding method to fabricate metal matrix composites, accumulative press bonding (APB) is one of the most capable processes. One of the major disadvantages of the APB process is the weak bonding strength. This study utilizes tin (Sn) particles as filler metal to enhance the bonding strength of aluminum laminates. Thus, AA1060 bars with different content of Sn particles (interlayer filler material) were manufactured at various pressing temperatures and APB steps. The peeling test was used to evaluate the bonding strength. It was found that by increasing the number of APB steps, Sn content, and pressing temperature, better bonds of higher strength and quality were generated. The bonding strength was improved to 424 N for a sample fabricated with 15 wt.% of Sn particles at 300°C. Scanning electron microscopy (SEM) was used to examine the peeling surface of Al/Sn composite samples.