Papers by Keyword: Nanostructured Powder

Abstract: Spark erosion of WC-8Co carbide pieces in oil resulted in a powder consisting of nanostructured spherical microparticles formed by rapid crystallization of the melt. These particles consist of rounded WC grains with an average diameter of about 0.18 μm, surrounded by cobalt. The process productivity, specific energy consumption, microstructure, particle size distribution, chemical and phase compositions of the obtained powder are determined. It was found that as a result of oil pyrolysis, free carbon is formed (3.4 %), which makes this powder unsuitable for the production of carbide products from it. A technique has been developed and the process of controlled removal of excess carbon by annealing the obtained powder in a CO₂ atmosphere at a temperature of 1000 °C has been studied. As a result of annealing for 120 minutes, the carbon content decreases to the required value (5.6 %). Studies of the phase composition and microstructure showed that the obtained particles consist of elongated WC grains, the average diameter of which increased to 0.43 μm.

Abstract: Bone reconstruction biomaterials are topics of interest in dentistry, orthopedics, scientific, commercial. The most popular bone repairing and reconstruction biomaterials are calcium phosphates. The demand for biomaterials is associated with the chemical and crystallographic characteristics of the human bone apatite. The wet synthesis method is common in the production of nanostructured powders of hydrated calcium phosphates, providing nanoparticles with sizes less than 50nm. This study aimed to synthesize and characterize hydrated calcium phosphate powders in the molar ratio of Ca/P = 1.67. After calcination at temperature 900°C/2h, these powders provide nanostructured hydroxyapatite matrix. The characterization studies were performed with Scanning Electron Microscopy, X-rays diffraction and Infrared Spectroscopy by Fourier Transform. The results show that the synthesis method provides hydrated calcium phosphate powders formed by aggregated and agglomerated nanoparticles. The thermal treatment of hydrated calcium phosphate powder led to formation of hydroxyapatite matrix.

Abstract: In the present research, nano-crystalline Fe­₇₀Si₃₀ alloy powders were prepared by mechanical alloying using Fe and Si powder as starting materials. Powder samples were taken from the mill at selected time intervals (5, 20, 40, 50, 60, 70h) for the structural properties analysis. The phase composition, morphology and magnetic properties of nano-structured alloy powders were analyzed by XRD, FESEM and VSM techniques, respectively. The evolution of non-equilibrium solid solution Fe (Si) during milling was accompanied by refinement of crystallite size. Dissolution of Si atoms into Fe matrix was also studied. The quantitative analysis of phase composition was carried out using Maud software which is based on the Rietveld method combined with Fourier analysis. XRD results showed a coexistence of BCC, DO₃ and FeSi structural phases for 40h milled sample, quantity of DO₃ and FeSi phases decreased for prolonged milling time. These phases disappeared in 60h milled sample and BCC FeSi phase with a mean crystallite and particle sizes of 20 and 70 nm were formed. The results also indicated that by mechanical alloying of the FeSi system for 60h, it was possible to extend the solubility of the BCC FeSi structure to 30 at% Si.

Abstract: Nanostructured Fe₆₅Co₃₅ alloy powders were fabricated by mechanical alloying in an attritor mill with different milling times. The milling process carried out in speed of 350 rpm, with 20:1 ball to powder weight ratio and under argon protective atmosphere. A continuous cooling system applied to avoid increasing temperature during the milling. The effect of milling time on structural and magnetic properties investigated by X-ray diffraction, scanning electron microscopy and vibration sample magnetometer. According to the obtained results, nanostructured Fe₆₅Co₃₅ solid solution powders resulted with an average particle size of 400 nm and crystallite size of 6.8 nm by milling for 20 hours. With increasing the milling time, the lattice parameter decreased and the lattice strain increased for Fe₆₅Co₃₅ powders. The maximum saturation magnetization with 1311 emu/cc value and the minimum coercivity with 22 Oe value occurs after milling for 15 hours.

Abstract: Nanocrystalline Fe72Al28 alloy samples were prepared by the mechanical alloying process using planetary high-energy ball mill. The alloy formation and different physical properties were investigated as a function of milling time, t, (in the 0-24 h range) by means of the X-ray diffraction (XRD) technique, scanning electron microscopy (SEM), energy dispersive X-ray (EDAX) and Mössbauer spectroscopy. The complete formation of bcc-FeAl solid solution is observed after 4 h of milling. The lattice parameter, a (nm), quickly increases within the first hours of milling and reaches a maximum value of 0.291 nm at 12 h of milling time; then a (nm) decreases to a value of 0.2885 nm for 24 h. The grain size decreases from 55 to 10 nm, while the strain increases from 0.18% to 0.88%. Grain morphologies at different formation stages were observed by SEM. The Mössbauer spectra show different behaviors with the increase of milling time. Indeed, after 4 h, the Mössbauer spectrum shows the presence of a singlet and sextet. The singlet indicates the presence of paramagnetic phase characteristic of A2 disordered structure and the sextet with a mean hyperfine field, , of 21 T is indicative of ordered DO3 structure. After 8 h of milling, the paramagnetic phase disappears leading to the appearance of a sextet, with a mean hyperfine field, Hhf, equal to 24.18 T which is characteristic of DO3’ structure. For the higher milling time i.e. 24 h, the Mössbauer spectrum was analyzed with two components. The first one with equal to 29.9 T is still indicative of ordered DO3, however, the second with a value of 10.25 T is characteristic of the fine domain B2 ordered structure.

Abstract: In order to synthesize WC-Co nanopowders through an integrated mechanical and thermal activation process, WO₃-Co₂O₃-C nanopowders need to be obtained first. It is critical how to obtain the WO₃-Co₂O₃-C nanopowders efficiently. The effect of processing parameters on the grain size during high-energy-milling of WO₃-Co₂O₃-C mixed powders was studied via X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that the grain size of reactants can be effectively decreased with increasing the milling time, rotation speed, and charge ratio. After a certain time milling, both WO₃ and C powders achieve nano-level in grain size and mixed homogeneously. The appropriate milling parameters for fabricating nanosized WO₃+C+Co₂O₃ powders are suggested to be 4 to 8 hours of milling time, 400 RPM of rotation speed, and 40:1 to 60:1 of charge ratio.

Abstract: Thermal sprayed WC-Co coatings are used extensively to enhance the wear resistance of a wide range of engineering components. In this paper, erosive resistance of plasma atmospheric sprayed WC-12Co coatings has been evaluated. Solid particle erosion tests were conducted on these coatings at different angles of impact with silica and alumina abrasives of size 250 µm. Coatings have been deposited by using micrometric and nanometric agglomerated powders, employing H2 and He as plasmogen gas. In order to determine the erosion regime (ductile or brittle), the influence of impact angle on the erosion rate has been studied. Optical microscope and FESEM have been used to analyze the eroded surface. The influence of the plasmogen gas and the powder employed on the erosive behaviour of the coating has been evaluated. An attempt to connect the erosive behaviour with mechanical properties and microstructure has been made. Hardness has been determined by means of several measurements of Vickers microhardness; fracture toughness has been estimated through indentation method. Identification of phases has been made by means of X Ray diffraction.

Abstract: We review the current development status of Mo-Si-B alloys consisting of Mo solid solution and the intermetallic phases Mo3Si and Mo5SiB2 which could take advantage of the beneficial oxidation resistance of the silicide phases and of the outstanding mechanical properties of molybdenum. For adequate low temperature toughness a continuous Mo solid solution matrix should be established in the microstructure. Besides, wrought processing of such alloys at elevated temperatures requires the presence of an ultra-fine grained (UFG) microstructure. Both the prerequisites can be fulfilled using mechanical alloying (MA) as the crucial processing step which even yields nanostructured supersaturated powders after milling. However, values for the ductile-to-brittle transition temperature (DBTT) close to room temperature are unlikely due to grain boundary embrittlement by Si segregation. The possibility of reducing this segregation tendency by various micro-alloying additions will be demonstrated. Finally, the high temperature deformation behaviour of these UFG materials will be comparatively assessed against state-of-the-art Nickelbase single-crystalline superalloys.

Abstract: The high-energy milling uses the mechanical energy to activate chemical reactions by developing structural changes in the powder particles. High-energy milling with an acceleration of 28g was applied for the mechanical activation of the aluminium and silicon nitrides mixture with yttria additive. The activated powders showed the significant damage of the crystal structure and limited formation of a solid solution. Sintering of the activated precursor demonstrated higher ability for densification and started at 300 °C lower temperature in comparison to the standard mixture. The phase evolution during sintering was dependent on the starting composition and degree of powder activation.

Abstract: Spark Plasma Sintering (SPS) of nanostructured FeMo powder produces samples with satisfactory density, however the final grain size critically depends on the sintering temperature. Two groups (sets A and B) of samples have been examined by means of internal friction (IF) and dynamic modulus measurements carried out in successive test runs on the same samples to assess their structural stability. Set A and B had been sintered at 1113 and 1128 K and had an average grain size of 100 nm and 1 µm, respectively. TEM and XRD have been performed on the samples in as-prepared condition and after IF measurements cycles. The samples with smaller grains are more stable and substantially are not affected by grain coarsening which, on the contrary, occurs in those with grains of larger size. The heating up to 923 K during the tests diminishes dislocation density in both the groups. An anomalous trend of resonance frequency during the first test run in samples of set A has been ascribed to the formation of small cracks relaxing internal stresses.