Initial Powder Composition Research Articles

An ultrasonic study of the effect of uniaxial green compaction pressure on the elastic anisotropy of porous copper fabricated using the pore-former route

Porous copper is potentially very attractive for various functional applications such as filters and heat exchanger materials. Within the scope of this study, porous copper samples with dual pore morphologies were fabricated following the space holder technique using a 1:2 mixture ratio of K2CO3 to NaCl as pore formers. The initial compaction pressure required for green body fabrication was systematically varied between 120 and 180 MPa, and the influence of this pressure over porosity, pore morphology, volume shrinkage, and longitudinal elastic constants was systematically studied. Three longitudinal elastic constants of the porous Cu samples were determined following the non-destructive ultrasonic phase spectroscopy method through the measurement of the phase shift vs. frequency spectra along the three orthogonal directions in each sample. When the applied pressure is increased from 120 to 160 MPa and the Cu:PF mixture ratios are 70:30 and 60:40, respectively, the elastic constant values in the green compaction direction (C11) decrease from 34.2 GPa to 7.5 GPa and 23.7 GPa–17.9 GPa. However, for Cu:PF mixture ratios of 70:30 and 60:40, under 180 MPa green compaction pressure C11 increases to 30.9 GPa and 21.2 GPa, respectively. As the applied green compaction pressure increased from 120 to 160 MPa, the samples' average elastic anisotropy ratio initially increased from 0.29 to 0.59 and then decreased to 0.55 at 180 MPa pressure. While the initial increase in anisotropy is due to increased pore flattening with increasing green compaction pressure, the decrease in anisotropy ratio in the samples fabricated under 180 MPa green compaction pressure is due to enhanced sample densification. Contour maps for different properties have been proposed to identify the optimum combination of initial powder composition and green compaction pressure for the intended end application.

ZIRCONIUM DIBORIDE WASTE RECYCLING BY GRINDING AND RECURRENT SPARK PLASMA SINTERING

An economically viable and environmentally friendly method is proposed for recycling the wastes from electrical discharge cutting of the blanks obtained by the spark plasma sintering of a powdered composition based on zirconium diboride with the addition of silicon carbide, lanthanum and yttrium oxides. The wastes are ground in a flow-type centrifugal mill TsEM-7-1, and the product of milling is further consolidated by spark plasma sintering. The microstructure and phase composition of the sintered materials made from initial powder composition and the powder obtained by grinding the ceramic wastes from final machining of finished products are investigated. It is shown that the largest pieces in the wastes that cannot be ground in the mill are to be crushed preliminarily to enhance the yield of ground powder. The absence of differences in the phase composition of initial and processed powder mixtures and blanks made of them has been revealed, which demonstrates the possibility of repeated use of the wastes for spark plasma sintering of the materials formed from ceramic powders based on zirconium diboride, for the purpose of decreasing the consumption of initial components.

Exchange Bias Demonstrated in Bulk Nanocomposites Processed by High-Pressure Torsion.

Ferromagnetic (Fe or Fe20Ni80) and antiferromagnetic (NiO) phases were deformed by high-pressure torsion, a severe plastic deformation technique, to manufacture bulk-sized nanocomposites and demonstrate an exchange bias, which has been reported predominantly for bilayer thin films. High-pressure torsion deformation at elevated temperatures proved to be the key to obtaining homogeneous bulk nanocomposites. X-ray diffraction investigations detected nanocrystallinity of the ferromagnetic and antiferromagnetic phases. Furthermore, an additional phase was identified by X-ray diffraction, which formed during deformation at elevated temperatures through the reduction of NiO by Fe. Depending on the initial powder composition of Fe50NiO50 or Fe10Ni40NiO50 the new phase was magnetite or maghemite, respectively. Magnetometry measurements demonstrated an exchange bias in high-pressure torsion-processed bulk nanocomposites. Additionally, the tailoring of magnetic parameters was demonstrated by the application of different strains or post-process annealing. A correlation between the amount of applied strain and exchange bias was found. The increase of exchange bias through applied strain was related to the microstructural refinement of the nanocomposite. The nanocrystalline maghemite was considered to have a crucial impact on the observed changes of exchange bias through applied strain.

Steam and air oxidation behaviour of thermal spray chromium carbide-based composite coatings

Thermal spray chromium carbide-based composites have been widely used industrially because of their superior oxidation resistance at elevated temperatures compared to other metal matrix composite coatings. In this work, the serviceability of HVOF sprayed coatings from an atomized chromium carbide-based feedstock (Cr23C6-40NiCr) was compared with those from a conventional Cr3C2-25NiCr agglomerated and sintered feedstock. Coatings were exposed to 540 °C and 610 °C for 168 h in both air and steam environments. These conditions are indicative of industrial power plant working conditions to assess their real-time performance. Quantitative Rietveld analysis of the Cr23C6-40NiCr coating XRD patterns indicated a comparable concentration of Cr23C6 in both the powder and coating, implying minimal carbide dissolution. However, the Cr3C2 content in the conventional Cr3C2-25NiCr coating was markedly reduced compared to the initial powder composition, indicating that carbide dissolution occurred to a greater extent. Air oxidation led to the formation of coarse surface oxide, due to the initial formation of Ni-based oxides before a continuous Cr2O3 layer could develop underneath. A notably finer and more uniform oxidized surface composed only of Cr2O3 was formed during steam treatment across both coating types. More importantly, the oxide scale was intact and crack-free in Cr23C6-40NiCr compared to Cr3C2-25NiCr. Moreover, despite Cr23C6 being the main carbide phase and the high binder content (40%NiCr), the microhardness of the Cr23C6-40NiCr coating was comparable to that of the conventional Cr3C2-25NiCr coating, both in the as-sprayed and heat exposed states.

Chemically Bound Resorbable Ceramics as an Antibiotic Delivery System in the Treatment of Purulent–Septic Inflammation of Bone Tissue

Local drug delivery systems are an effective approach in the treatment of purulent–septic inflammation of bone tissue. Chemically bonded multiphase ceramics based on calcium-deficient carbonate-substituted hydroxyapatite combine resorbability, osteoconductivity, and the possibility of volumetric incorporation of antibiotics. Macroporosity is regulated by the concentration of polyethylene glycol granules introduced into the initial powder composition, followed by their extraction. The selected conditions for the consolidation of the ceramic matrix and the extraction of PEG granules retain the activity of vancomycin, which is confirmed by the results of microbiological studies. The concentration of vancomycin and the porosity affect the local concentration and release of the antibiotic. The incorporation method provides a prolonged release of the antibiotic for up to 31 days. In vivo experiments with bone implantation have shown that chemically bound macroporous ceramics with incorporated vancomycin are a therapeutically effective carrier of the substance during the healing of bone defects in conditions of surrounding purulent–septic inflammation, and can be considered as a carrier for local antibacterial therapy, at the site of implantation.

Effect of process parameters and grain refinement on hot tearing susceptibility of high strength aluminum alloy 2139 in laser powder bed fusion

High strength aluminum alloys, especially those that are age-hardenable, such as 2xxx series, 6xxx series, and 7xxx series, are widely used as structural materials in transport and aerospace industries due to their good mechanical properties. However, additive manufacturing of high strength aluminum alloys is challenging due to their susceptibility to hot tearing. In this work, a systematic study has been conducted in an attempt to eliminate hot tearing in laser powder bed fusion (PBF-LB/M) manufacturing of Al2139 alloy through selecting process parameters and the addition of an AlTiB grain refiner. It was found that hot tearing in Al2139 during PBF-LB/M can be reduced or eliminated by increasing volumetric energy density. Furthermore, grain refinement by AlTiB addition shows a clear effect in reducing hot tearing, even though the refined grains remain predominantly columnar rather than equiaxed. The effect of increasing volumetric energy density on hot tearing of Al2139 during PBF-LB/M was analyzed by thermomechanical finite-element simulation, which showed that the reduction in hot tearing with increasing energy density is associated with a decrease in the thermal residual stress. However, it was also shown that there can be a substantive loss of Mg due to evaporation at high energy densities. From a hot tearing model based on solidification thermodynamics, the loss of Mg on hot tearing in Al2139 was estimated to reduce hot tearing by up to 10%, compared to the initial powder composition. With the selected PBF-LB/M parameters, crack-free tensile specimens were fabricated, with and without the addition of AlTiB. Compared with their wrought or cast counterparts, the PBF-LB/M Al2139 and Al2139-AlTiB specimens show lower yield strength but better ductility, which can be attributed to the loss of Mg during PBF-LB/M.

Керметные плазменные покрытия с карбидами TiC и TiCrC

Four plasma cermet coatings with similar compositions based on TiC carbide and NiCrMo matrices with additional Cr3C2, WC carbides, and carbon were studied. The average oxygen content for four cermets increases from 0.51 % for powders to 0.86 % for coatings with a minimum plasma power and does not increase with its increase. The maximum average nitrogen content in the coatings, 0.34 %, is determined by the nitrogen content in the plasma. When spraying coatings, the loss of carbon, relative to the initial powder composition, is 2.79 – 3.76 %, less than in the manufacture of powders for spraying 4.3 – 6.6 %. Carbides Cr3C2, WC, matrix elements and the content of additional carbon determine the formation of the annular zone around TiC carbide, increase the total carbide content in the coating from 60 to 74 – 83% and the microhardness of the coating is 18 GPa with an indenter load of 200 gf.

Production of α-Fe <sub>2</sub>O <sub>3</sub> powder material with multilevel gradient porosity

The paper studies methods for obtaining a multilevel gradient porous material by the layer-by-layer sintering of distributed α-Fe2O3 nanopowders and submicron powders. Nanopowders with an average particle size of 12 nm were obtained by the coprecipitation method, and submicron powders, which are hollow spheres, were obtained using the spray pyrolysis method. Powders were consolidated by sintering in a muffle furnace, hot pressing, and spark plasma sintering (SPS) at various temperatures, loads, and holding times. It was shown that muffle furnace sintering and hot pressing methods cannot provide a compact of enough strength due to the different activity of nanopowders and submicron powders. Powder materials were obtained by spark plasma sintering when holding at 700, 750, 800, and 900 °С for 3 min. It was found that a series of samples obtained by SPS at 750 °С has sufficient strength and open porosity of 20 % with a total porosity of 37 %. Rising temperature in this method leads to an increase in the particle size in the nanopowder volume to a micron size and partial destruction of hollow submicron spheres. It was found during the study that the phase composition of samples obtained is identical to the phase composition of initial powders. However, for a series of samples obtained by hot pressing and SPS in the nanopowder volume, there is a directed growth of crystals towards the highest electrical and thermal conductivity [001] along the punch axis. This is due to the temperature gradient between the powder volume and punches and the lowest value of the plane surface energy (110), which includes direction [001].

Formation of structure and properties of hot-deformed powder steels microalloyed with sodium and calcium during thermal and thermomechanical treatment

One of the main problems limiting further growth in the production of parts by the hot forging of porous performs (HFPP) is that the obtained materials are prone to brittle fracture due to the poor quality of interparticle jointing formed during hot deformation, as well as the presence of impurities in the composition of initial powders. The paper studies the possibility of increasing the mechanical properties and endurance performance of hot-deformed powder steels by doping them with sodium or calcium microadditives and using thermomechanical treatment. Sodium bicarbonate and calcium carbonate were used for microalloying. Carbon was added as pencil graphite powder. The temperature of heating porous preforms before hot forging and the carbon content in steels were varied; the content of microalloying additives was, wt.%: 0.2 for sodium, and 0.3 for calcium. Mechanical properties as well as contact and low-cycle fatigue life were tested on 5 × 10 × 55 mm and 10 × 10 × 55 mm prismatic specimens, as well as ∅ 26 × 6 mm cylindrical specimens. In comparison with carburizing and thermal treatment, thermomechanical treatment improves the impact strength and endurance performance of hot-deformed powder steels with Na or Ca microadditives under the contact and low-cycle fatigue loading, and the hot repressing temperature of porous preforms is reduced without compromising the mechanical properties of powder steels obtained. It may be associated with the formation of a more fine-grained structure and higher microstresses of the crystal lattice. The cooling down of preform surface layers during hot forging process operations creates conditions for ausforming in them.

Structure and Percolation Thresholds in Magnesia Stabilized ZrO2-MgO Porous Composite

This work reports the influence of porous ZrO2-MgO ceramic composition on its structure and mechanical properties. The bimodal pore structure were formed by introduction of organic pore forming particles into the initial powder composition and due to the voids between ceramic powder particles. It was shown, that the mechanical parameters of the porous ZrO2-MgO composite are substantially defined by the value of the microstresses, the MgO content, and, apparently, the formation of a percolation structure. Thus, it was shown that by varying the composition and the size of the pore-forming particles, it is possible to achieve a high degree of identity with the structure and properties of natural bone tissue.

Obtaining Functional Intermetallic Ni–Ti Coatings by Heterophase Transfer Combined with Laser Treatment

The results of the investigation of the intermetallic Ni–Ti system coating obtained by the complex method of cold gas dynamic spraying and laser treatment are presented. The dependence of the coating charge composition on the initial powder composition is shown. Its structure, porosity, and microhardness are studied, and also the results of X-ray phase analysis confirming the presence of the intermetallic phase are presented. The microhardness is increased by more than a factor of 5 to 632 HV, and the porosity is decreased by a factor of 25 to 0.2%. Investigation of the substrate–coating boundary shows the presence of mutual diffusion.

Dependence of the structure and characteristics of a Russian alternative for AISI 304 stainless steel powder on the parameters of their laser cladding on substrates from low-carbon and structural steels

The work studies X8 CrNiSiMg 17-8-6 steel powder, a Russian alternative for AISI 304 steel, cladded by ytterbium fiber laser on two types of substrates: heat-resistant steel and low-carbon steel. The results demonstrate the effect of laser radiation power on the macrostructure of single cladded beads and their fusion with the substrate. The selected regimes allow forming solid cladded layers. The paper also studies the microstructure of metallographic sections of cladded specimens, the phase composition of cladded layers and initial powder. The deliverables include the data on wear resistance, resistance to intercrystalline corrosion and strength of cladded layer fusion with the substrate.

Experimental Optimization of Ni-GDC Microstructure with Various Phase Fractions for Solid Oxide Fuel Cell Anode

The design of electrode microstructure is important to enhance performance and stability of solid oxide fuel cells (SOFCs). The most common material used for the fuel electrode is nickel-yttria stabilized zirconia (Ni-YSZ) composite, in which electron, ion and gas conduct in the metallic Ni phase, the ceramic YSZ phase and the pores, respectively. The electrochemical reaction therefore takes place at the triple-phase-boundary (TPB), hence the increase in TPB density improves electrode performance. Recently, the ceramic materials, which exhibits mixed ionic-electronic conductivity (MIEC) properties, are considered to be beneficial solution for the SOFC electrodes. They can greatly enhance the available reaction sites by introducing surface reactions on the double phase boundaries (DPB) between MIEC phase and pore. One of the promising materials is considered to be gadolinium doped ceria (GDC). GDC exhibits higher ionic conductivity compared to YSZ and has MIEC property in the reducing atmosphere. In the case of Ni-GDC composites, the electrochemical performance is supported by both DPB and TPB reactions. In general, the active DPB density increases with the share of GDC phase in the composite. On the other hand, the active TPB density achieves maximum value when the share of both solid phases is balanced. The optimal electrode design has to be determined by the ratio between DPB and TPB reactions and their kinetics. In the present study, electrolyte supported SOFC with Ni-Gd0.1Ce0.9O2-δ anodes with various phase fractions were fabricated and their polarization characteristics were experimentally investigated. The ratio of Ni and GDC was controlled by the initial powder composition, and the porosity was varied by isostatically pressing the anode after screen printing. Initially, NiO (AGC Seimi) and GDC (ShinEtsu) powders were mixed to achieve the target volume fractions of Ni:GDC = 70:30, 60:40, 50:50, 40:60, 30:70 vol%. The anodes were then screen-printed on the YSZ discs and calcinated at 1350oC. In order to modify the porosity of the anodes, some of the cells undergoes isostatic pressing treatment at 200 MPa for 30 minutes prior to the calcination. The LSCF (lanthanum strontium cobalt ferrite by Fuel Cell Materials) cathode with GDC (ShinEtsu) barrier layer were screen-printed and sintered on the other side of the electrolyte. Current – voltage curves and electrochemical impedance spectra were measured at 500 – 800 oC. Additionally, the anodes with optimal phase composition were discharged for 100 hrs. 3D microstructural analysis was conducted with the focused ion beam scanning electron microscopy (FIB-SEM). The microstructural parameters, such as phase volume fractions, particle intercept length, TPB and DPB densities and tortuosity factors were quantified. By evaluating the connectivity of each phase, the total and active TPB and DBP densities were calculated. Quantified microstructural parameters were correlated with the polarization characteristics. Neither the DPB reactions nor the TPB reactions could individually explain the measured polarization resistances of the composite anodes. In the case of screen-printed anode samples without isostatic pressing, the high porosity (ca. 52% for all of the samples) lead to low connectivity of the solid phases. The optimal composition of the cermet was found to be Ni:GDC = 0.4:0.6 vol%. Introducing isostatic pressing, improvements in both ohmic and polarization resistances are observed. The decrease in ohmic resistance is mainly attributed to the improved connectivity between the electrode and the electrolyte. The polarization resistance was influenced by the changes in active DPB and TPB densities. As the porosity is reduced and connectivities of the solid phases are enhanced by the isostatic pressing, not only the nominal values of active DPB and TPB densities increased, but also the ratio between them has changed. This effect is most significant for Ni:GDC = 0.3:0.7 vol% sample, which exhibits the smallest polarization resistance among all the investigated electrodes. The discharge testing showed the improved stability of the cells with the decreased porosity. It is therefore concluded that not only the ratio between Ni and GDC but also porosity is a very important parameter for the performance and stability of the anode. Figure 1

Preparation, microstructure and density of nanocrystalline powders of nonstoichiometric vanadium carbide VCy

Nanocrystalline powders of VCy with different stoichiometry and the average particle size were produced by high-temperature solid-phase synthesis and subsequent milling. The crystal structure, phase composition, microstructure and particle morphology of initial and milled VCy powders, their specific surface area and density were studied using XRD, SEM, BET method and gas pycnometry. The anisotropy of microstrains in nanocrystalline VCy powders was experimentally studied by XRD. It was shown that the inclusion of microstrain anisotropy and inhomogeneous broadening in the analysis make it possible to more accurately describe experimental data on diffraction reflection broadening. It was found that microstrains mainly contribute to the broadening of diffraction reflections of VCy nanopowders produced by milling. The experimental dependence of the pycnometric density of VCy powders on the specific surface area was established and an expression describing it was obtained. It was shown that the main reason for pycnometric density reduction of the powders with increasing specific surface area is oxygen adsorption on the surface of carbide particles.

The effect of mechanical activation on synthesis and properties of MgAl2O4 ceramics

Abstract Magnesium aluminate, MgAl2O4 and other alumina-based spinels are ceramics with high hardness, high melting point and mechanical strenght. Spinels can also be used as dielectrics in microwave applications. The goal of this study was to examine the effects of mechanical activation and sintering temperatures on physico-chemical properties of spinel. MgAl2O4 was produced by solid state reaction between MgO and α-Al2O3. The starting powders were mixed by ball milling to homogenize without significant particle size reduction. Mechanical activation of mixed powders was performed in a high-energy planetary ball mill in air for 1 h. Powders were compacted at 300 MPa. Heat treatments were performed in air, at temperatures ranging from 1200 to 1600 °C with 2 h dwell time, to determine the amount of spinel formation as a function of temperature. Phase composition and microstructure of initial powders and heated samples were determined by means of X-ray diffraction, particle size analysis, and scanning electron microscopy. The influences of milling and consolidation parameters were studied by electrical measurements and mechanical characterization. The main conclusion of this study was that mechanical activation for 60 min initiated a mechano-chemical reaction, resulted in spinel formation at much lower temperatures than within non-activated powders, and indicated that final sintering stage started at much lower temperatures for activated powders. Changes in microstructure parameters, as a consequence of mechanical treatment and subsequent heating of investigated powder mixtures, strongly affect electrical and mechanical properties of the final ceramics.

Оптимизация процесса газотермического напыления полимерных покрытий, модифицированных наноалмазами

С использованием метода многомерного проектного синтеза разработана экспериментально-статистическая математическая модель процесса газотермического напыления композиционного полимер-наноалмазного покрытия для различных видов наноразмерного модификатора (очищенные наноалмазы детонационного синтеза (НАДС) и алмазосодержащая шихта (ША)), в которой в качестве управляющих параметров приняты концентрация модификатора в исходной полимер-наноалмазной порошковой композиции (C), давление горючей смеси (Р), дистанция наплыления (l) и толщина покрытия (h), а критериями оптимизации являются адгезионная прочность (σсц) и коррозионная стойкость (К). На основе полученных уравнений регрессии рассчитаны оптимальные параметры процессов газотермического нанесения композиционных полимер-наноалмазных покрытий на полиамидной матрице с различными видами наноалмазного модификатора, удовлетворяющих заданным показателям качества (σсц = 10,0–15,0 МПа, К = 35–40 сут), в следующих границах: для покрытия «полимер-НАДС»: C = 0,15–0,21 масс.%, Р = 0,15–0,25 МПа, l = 83–113 мм, h = 97–111 мкм; для покрытия «полимер-ША-А»: С = 0,18–0,26 масс.%, Р = 0,20–0,30 МПа, l = 71–101 мм, h = 110–134 мкм.

Influence of the powder grain size on dielectric and piezoelectric properties of lead-free KxNa1-xNbO3 piezoelectric ceramics

The dependence of dielectric and piezoelectric properties of lead-free piezoceramics on fractional composition of initial powders has been investigated. The material on the basis of solid solutions of potassium and sodium niobates doped with strontium is considered. Piezoceramic samples with different porosity values (from 4% to 36%) were obtained using powders of piezoceramic material of different granulometric composition. For these samples, the dielectric constant decreased monotonically with increasing porosity from 600 to 300, while the value d33 (80–90pC/H) varied slightly.

Obtaining the functional intermetallic Ni–Ti coatings by heterophase transfer combined with laser treatment

The results of the investigation of the intermetallic Ni–Ti system coating obtained by complex method of cold gasodynamic spraying and laser treatment are presented. The dependence of the coating charge composition on the initial powder composition is shown. Its structure, porosity, and microhardness are studied, and also the results of X-ray phase analysis confirming the presence of intermetallic phase are presented. The microhardness increased more than 5 times to 632 HV, and porosity decreased 25 times up to 0.2%. Investigation of the substrate-coating boundary showed the presence of mutual diffusion.

The influence of process parameters on structure and abrasive wear resistance of laser clad WC-NiCrBSi coatings

In laser cladding, the final structure of coatings and thus their functional properties are dependent on process parameters used and the initial powder composition. This study focuses on laser cladding of WC-NiCrBSi coatings. As the powder used is a blend of 60% tungsten carbide spherical particles and 40% NiCrBSi matrix, the carbides’ morphology and distribution in the final coating would be dependent on laser cladding parameters. Coatings were cladded using a solid state laser and by varying key process parameters: process speed, powder feed rate and laser power. Cladded coatings are evaluated in terms of their microstructure, hardness and abrasive wear resistance tested by dry sand rubber wheel test ASTM G-65. It was found that the amount of dissolved WC carbides increases with increasing heat input and both Fe dilution and carbide dissolution affect the coatings´ properties. This shows that the selection of process parameters is crucial, as they significantly affect the final coatings´ wear resistance. In the overlay regions, the non-homogeneity of the coating’s microstructure was responsible for significant selective abrasive wear behavior.

Microstructure characterization of cemented carbide fabricated by selective laser melting process

This article presents detailed microstructure characterization of cemented carbide fabricated by selective laser melting process. Nickel-coated tungsten carbide powders were used for this study; the selective laser melting process induced complex microstructure consisting of partially-melted carbide, carbide precipitates and dendritic microstructure. Four phases were identified, i.e. MC, M2C, FCC and η in the as-built specimen. CALPHAD phase diagram was utilized to describe possible phase formation sequence during selective laser melting process, which involved melting, solidification and repeated heating. Although the composition of initial powder was very simple (spherical W2C-WC and Ni-coating), selective laser melting allowed the decomposition of initial carbides, subsequent formation of dendritic structure and precipitation of additional carbides. To the best of authors' knowledge, for the first time, detailed microstructure characterization of cemented carbide fabricated by selective laser melting is presented.