As-received Powder Research Articles

Turbula mixing characteristics of carbide powders and its influence on laser processing of stainless steel composite coatings

Abstract In this study, the mixing characteristics of feedstock carbides powders for laser synthesis of stainless steel composite coatings were investigated in a Turbula Shaker Mixer T2F. This research was aimed at determining the mixing parameters that give optimal powder homogeneity and determining any metallurgical interactions and/or bonding amongst powder during mixing. Blending/mixing of WC-9Co-4Cr, WC-10Ni and TiC powders mixture was performed in the mixer to prepare WC/TiC feedstock for laser melt injection. The constituent phases and microstructural characteristics of the as-received and feedstock powders were studied by Malvern particle size analyser, an X-ray diffractometer (XRD), a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). From the results obtained, it showed that the feedstock powder particles underwent diffusion and agglomeration. For a longer mixing time of 8 h, homogeneous distribution of the powder was obtained. However, it could be seen that diffusion, shear, segregation, random mixing and partial agglomeration of the powders were prominent. No new phases were formed. Also, the homogenized feedstock substantially improved the uniform distribution of the different carbides in the coatings as less dense powder particles were injected close to the bottom of the coated zone.

Role of nanocrystalline feedstock in the tribological behaviour of alumina coatings deposited by detonation gun

Alumina coatings were prepared using detonation gun from commercial powder (CP) and high energy ball milled powder (MP) with 83nm crystallite size. The sliding wear rate and wear coefficient of friction were lower in the nanocrystalline powder coating. The coating from the as-received commercial powder consisted of thick splats, and a paucity of nanoparticulate inter-splat regions whereas milled powder coating has thinner splat morphology with nanoparticulate inter-splat regions. The nanoparticulate material in the inter-splat regions and the finer microstructure of the MP coating is believed to be responsible for improved wear resistance.

Pressure-less spark plasma sintering effect on non-conventional necking process during the initial stage of sintering of copper and alumina

In the present study, we focus on the characterization of the necking mechanisms during the early stages of pressure-less spark plasma sintering (PL-SPS) compared to conventional sintering (CS) of two different types of powdered materials (Cu and α-Al2O3). SEM observations of the evolution of particle morphology and necks from the as-received powders to sintered ones show the nature of the neck between particles which were either in contact or not. For alumina, no particular necking process (melt or viscous bridge) was observed regardless of the sintering conditions (PL-SPS and CS), even for a very high heating rate 455 °C/min. For copper, this neck morphology is unequivocally not typical of conventional ones, thus, suggesting mass transport by an ejection mechanism. This particular morphology was seen occasionally. In comparison, the conventionally sintered Cu particles presented a smoother surface, with conventional curved necks suggesting the contribution of surface diffusion mechanisms. Based on partial pressure calculations, a direct thermal effect might not explain the observed non-conventional neck for copper. On the other hand, local field enhancement effect and local favourable thermal breakdown voltage conditions are described and discussed in order to support the experimental results.

Characteristic of copper matrix simultaneously reinforced with nano- and micro-sized Al2O3 particles

Abstract The effect of the simultaneous presence of nano- and micro-sized Al 2 O 3 particles on the microstructure and properties of copper matrix was the object of this study. The mixture of inert gas-atomized prealloyed copper powder (with 1 wt.% Al) and 0.6 wt.% commercial Al 2 O 3 powder (serving as micro-sized particles) was used as the starting materials. Strengthening of the copper matrix was performed by treating the powders in the air for up to 20 h in the planetary ball mill. During milling of the prealloyed powder, finely dispersed nano-sized Al 2 O 3 particles were formed in situ by internal oxidation. The approximate size of these particles was between 30 and 60 nm. The highest values of microhardness were reached in compacts processed from 10 h-milled powders. The microhardness of compact obtained from 10 h-milled powder was 3 times higher than the microhardness of compact processed from as-received and non-milled prealloyed powder. At the maximum microhardness the grain size reaches the smallest value as a result of the synergetic effect of nano- and micro-sized Al 2 O 3 particles. Recrystallization, which occurred during prolonged milling, was the main factor influencing the decrease in microhardness. The increase in electrical conductivity of compacts after 15 h of milling is the result of the decrease in microhardness and activated recrystallization processes.

STRUCTURAL AND MORPHOLOGICAL EVALUATION OF NANO-SIZED MoSi2 POWDER PRODUCED BY MECHANICAL MILLING

Nano-sized intermetallic powders have received great attention owing to their property advantages over conventional micro-sized counterparts. In the present study nano-sized MoSi 2 powder has been produced successfully from commercially available MoSi 2 (3 μm) by a mechanical milling process carried out for a period of 100 hours. The effects of milling time on size and morphology of the powders were studied by SEM and TEM and image analyzing system. The results indicate that the as-received micrometric powder with a wide size distribution of irregular shaped morphology changes to a narrow size distribution of nearly equiaxed particles with the progress of attrition milling up to 100 h, reaching an average particle size of 71 nm. Structural evolution of milled samples was characterized by XRD to determine the crystallite size and lattice microstrain using Williamson-Hall method. According to the results, the crystallite size of the powders decreases continuously down to 23 nm with increasing milling time up to 100 h and this size refinement is more rapid at the early stages of the milling process. On the other hand, the lattice strain increases considerably with milling up to 65 h and further milling causes no significant changes of lattice strain.

Porosity of dental phosphate-bonded investments after setting and heating processes

Porosities of set and burnout compacts of phosphate-bonded investments were determined. A gas pycnometer was used to measure the volumes, and hence the densities, of fine powders and porous compacts. Porosities of set and burnout compacts were then obtained from these data for as-received powders and dry set compacts by a numerical simulation method, subsequently leading on to the estimated compositions of conventional and rapid-heating investments used in this study. Excess water content in the hardening investment compact was evaluated as a function of setting time elapsed from the start of mixing. Porosities were about 24-32% for set compacts and 43% for burnout compacts, which well agreed with the numerically computed results. It was concluded that the functional composition of investment powder needed to achieve the optimal porosity as well as process parameters such as water-powder (W/P) ratio and keeping time of mixed investment casting slurry before heat treatment could be determined using the numerical simulation method developed in this study.

Porosity of dental gypsum-bonded investments in setting and heating process

The porosity of gypsum-bonded investments for set and heated compacts was measured and theoretically computed quantitatively, because porosity is an effective factor for determining the strength, setting/heating expansion, and permeability of compacts at casting. A helium gas pycnometer was used to measure the solid volume of fine powders, powder-water mixtures, and porous compacts. The compositions of the conventional cristobalite investment and rapid-heating type investment were estimated from the measured solid densities of the as-received powders and the set investments. The porosity and water content of the set investments were determined from the experimental data. Excess water content in the set investment was calculated in relation to the elapsed time from the start of mixing with water. The experimental porosities of the set and heated investments were about 40% for dry set >compacts and about 50% for fired compacts, which well agreed with the numerically computed estimations, respectively.

In-situ formed γ-Al2O3 nanocrystals repaired and toughened Al2O3 coating prepared by plasma spraying

During process of plasma spraying of alumina with the stable α phase in the as-received powder, metastable γ phase tends to form in the as-deposited coating. In this paper, plasma-sprayed Al2O3 coating was investigated using transmission electron microscopy. In situ formation of γ-Al2O3 nanocrystals was observed in the pre-existing cracks of the as-sprayed Al2O3 coating, which contributes to self-repairing and self-toughening of the Al2O3 coating. Except for the predominant intergranular fracture, the pre-formed γ-Al2O3 nanocrystals would effectively obstruct the crack development in the coating and then the coating fracture toughness is enhanced.

Decomposition of TiH2 studied in situ by synchrotron X-ray and neutron diffraction

Decomposition during constant heating of as-received and pre-oxidized titanium hydride (TiH2) powder under flowing and stationary Ar was studied in situ by synchrotron X-ray and neutron diffraction. Diffraction data were compared to corresponding thermoanalysis traces in order to relate peaks of H2 release with individual phase transformations. We found that every peak of H2 release is correlated with regimes of lattice parameter contraction. Core–shell models to describe the decomposition processes measured were found, and further verified by electron microscopy. The models show that a shell of the hexagonal close-packed hydrogen solid solution (α) around each particle core controls its dehydrogenation for as-received TiH2, while a TiO2 rutile shell controls the outgassing from the pre-oxidized powder.

Microstructure Features of Cold-Sprayed NiCoCrAlTaY Coating and its High Temperature Oxidation Behavior

Superalloy coating was deposited by cold-spraying using a commercial NiCoCrAlTaY powder. The coating microstructure was investigated by scanning electron microscopy and X-ray diffraction to reveal the change of the b-NiAl phase in the as-received powder particle during coating deposition. The oxidation behavior of the cold-sprayed MCrAlY coating and its microstructural evolution during the isothermal treatment were examined. The results show that significant microstructural change occurred to NiCoCrAlTaY superalloy during cold spraying and the thermal exposure. The intensive plastic deformation upon high velocity impact of spray particles results in transformation of b-NiAl to the matrix phase, forming metaltable b-NiAl depletion zones (b-PDZs) which are distributed around the boundaries of deposited particles in the coating. The central part of the deposited particles with limited deformation retained the original phase constitutions of the starting powder. The b-phase with fine grains is re-precipitated uniformly in the areas in b-PDZs in the as-sprayed coating during high temperature exposure. A stable Al2O3 scale was formed on cold-sprayed NiCoCrAlTaY during oxidation possibly due to active b-PDZs on the top surface of the coating.

The Effect of SDBS-Ag<sup>+</sup> Ligand on the Preparation of Flake-Like Silver Particles

The flake-like silver nano-particles were prepared by reducing silver nitrate with hydrazine hydrate in the sodium dodecyl benzene sulfonate (SDBS) and Polyvinyl pyrrolidone (PVP) complex solution. The as-received silver powders were characterized by SEM, EDS, TEM and XRD methods. The effects of SDBS and PVP with Ag+ ligand on the preparation of flake-like shape were analyzed. The results provide evidence that the SDBS-Ag+ ligand play a limiting template role during the silver growth and leading to the formation of flake-like silver consequently.

Influence of Processing Conditions on PVDF Properties

Study of the main properties of PVDF films produced by two processing technologies such as hot pressing from a melt or solution casting was the aim of this paper. All samples were prepared of as-received PVDF powder. First group of samples was prepared by the hot pressing. Second group of samples was prepared by the solution casting method. PVDF powder was dissolved in dimethylformamide. To characterize properties of samples, different experimental methods such as FRA (dielectric spectroscopy), IR-spectroscopy and DSC/TGA analysis were used in this work. It was found that IR-spectra of both studied groups do not change compared to that for virgin PVDF powder. It confirms that molecular structure is practically independent on the processing technology of samples. The only difference has been found that new band centered at <TEX>$1723\;cm^{-1}$</TEX> appears for samples prepared by the hot pressing method. This absorption band is related with formation of C=C bonds in samples prepared by the hot pressing method in contrast both to PVDF powder and samples prepared by the solution casting method.

Pressureless Reaction Sintering and Characterization of TiAl-TiC and Ti3Al-TiC Composites

Dense TiAl- and Ti3Al-based composites reinforced with 10–50 vol. % of TiC particles were fabricated by pressureless reaction sintering of reaction mixtures consisting of commercial titanium aluminide powders (TiAl with traces of Ti3Al and the single phased Ti3Al) blended with the appropriate amount of ceramic reinforcement and sintering agents (Al and Ti powders). The microstructures of the as-sintered composites were studied by scanning electron microscopy and X-ray diffraction analysis. The sintered samples of as-received TiAl and Ti3Al powders were within 10–15 vol. % of the retained porosity, while in samples with 10 vol. % of TiC particles, pressureless sintered without sintering agents, the retained porosity was reduced below 5 vol. %, according to the chemical reactions between TiAl or Ti3Al and TiC. On the contrary, the complete pressureless densification of composite samples with more than 10 vol. of TiC was achieved only by the addition of 5–10 vol. % of free aluminium and 5 vol. % of titanium, particularly involved in final stages of pores closuring. The tensile properties and Vickers hardness of composite samples were measured at room temperature. The improvement in tensile properties (except elongation) and Vickers hardness was found to correlate with the amount of TiC reinforcement in the matrix.

Dissolution properties of BaTiO3 nanoparticles in aqueous suspensions

Dissolution of barium ion from aqueous suspensions of commercial nano-sized barium titanate powders (BaTiO3) has been studied at various pH values, solids loading, different time intervals and different electrolyte concentrations. Zeta potential measurements at various pH values and Fourier transform infrared spectroscopy study were also carried out to know the surface behaviour. Dissolution of Ba2+ depends on the suspension pH and stirring time period. The iso-electric points were found at 3.4 and 12.2 for as-received BaTiO3 powder and 2.3 for the leached BaTiO3. The Ba2+-leached BaTiO3 suspension retards further leaching of Ba2+ ions at different pH values, which favours the achievement of stable suspension.

Characterization of Rapidly Consolidated Titanium Diboride

This study reports on the microstructure and mechanical properties of titanium diboride (TiB2) that was rapidly consolidated via plasma pressure compaction (P2C®). Titanium diboride powder with 2.5 wt % of silicon nitride (Si3N4), as a sintering aid, was rapidly consolidated using P2C® to inhibit grain growth and ensure high strength and ductility of the consolidated material. The consolidated specimens were 93.5% of theoretical density. The microstructure of the consolidated material was characterized using optical microscopy, scanning electron microscopy, and wavelength dispersive spectroscopy. Oxygen concentration in the consolidated sample was 13.55% less than in the as-received powders. The flexure strength, fracture toughness, Young’s modulus, and Vickers hardness of the consolidated specimens were measured at room temperature and were found to be equivalent if not superior to those reported in the literature.

Effects of ball milling and post-annealing on the transparency of spark plasma sintered Lu 2O 3

The effects of ball milling of starting powder and post-annealing of spark plasma sintered (SPSed) Lu 2O 3 on its microstructure and optical property were investigated. When ball-milled powder was used, the SPSed Lu 2O 3 was found to have a larger grain size with wider distribution and lower transparency than in the case using powder without ball milling. After annealing at 1323 K in air, the Lu 2O 3 that was SPSed using ball-milled powder became colorless, had a higher transmittance in the visible spectrum than the case where as-received powder was used, and exhibited transmittances of 71.4% and 81.6% for wavelengths of 550 and 2000 nm, respectively.

Pressureless sintering of ZrC-based ceramics by enhancing powder sinterability

The sinterability of ZrC was enhanced by high-energy ball milling as well as introduction of graphite and SiC as sintering additives. Densification process and microstructure development were investigated for ZrC-based ceramics densified by pressureless sintering. As-received ZrC powder showed poor sinterability. After high-energy ball milling, ZrC powder can be sintered to 98.4% theoretical density at 2100 °C. The obtained ceramic had fine microstructure and fewer entrapped pores. Introduction of 2 wt.% graphite combined with high-energy ball milling lowered the densification temperature of ZrC. The relative density of obtained ceramic reached up to 95% at 1900 °C. Introduced SiC inhibited ZrC grain growth during sintering and consequently avoided the entrapped pores within the grains. The relative density of ZrC–SiC reached up to 96.7% at 2100 °C. ZrC–SiC composite formed an interesting intragranular structure and had high fracture strength at room temperature.

Fabrication and characterization of Ni/ScSZ cermet anodes for IT-SOFCs

In this study the fabrication and characterization of Ni/10ScSZ (Ni/10 mol% Sc 2O 3–90 mol% ZrO 2) and Ni/10Sc1CeSZ (Ni/10 mol% Sc 2O 3–1 mol% CeO 2–89 mol% ZrO 2) cermet anode films was studied and compared. Both 10ScSZ and 10Sc1CeSZ electrolyte powders showed tetragonal and cubic phases at room temperature, respectively. The NiO/10ScSZ and NiO/10Sc1CeSZ composites with 10–60 vol% of Ni content were prepared by mixing as-received commercial powders of NiO, 10ScSZ and 10Sc1CeSZ followed by ink preparation. Samples were sintered for 1 h at temperatures of 1250–1350 °C. All the cermet films were then reduced under a mixture of hydrogen (10%) and nitrogen (90%) at 800 °C for 2 h. The effect of Ni content and sinter temperature on the DC electrical conductivity were investigated, and the results showed a sharp change in conductivity at around 30 vol% Ni, corresponding to continuity/discontinuity of the Ni–Ni contact network, and the conductivity increased as the sinter temperature increased from 1250 to 1350 °C. An acceptable electrical conductivity at 700 °C for these cermet films was obtained at >40 vol% Ni, consistent with behaviour reported for more conventional Ni/YSZ cermets. The effect of sinter temperature on the microstructure and porosity of Ni/10Sc1CeSZ and Ni/10ScSZ cermet films was also investigated. This revealed that the porosity of the cermet films with the same Ni content decreased as the sinter temperature increased and that, for a given sinter temperature, the porosity of the cermet films increased with Ni content. The porosities of 40Ni/60ScCeSZ (40 vol% Ni/60 vol% 10Sc1CeSZ) and 40Ni/60ScSZ (40 vol% Ni/60 vol% 10ScSZ) anodes sintered at 1250, 1300 and 1350 °C for 1 h were in the range of 30–45%. Electrochemical measurement of symmetrical cells using an 8YSZ electrolyte at 700 °C revealed that the lowest electrode polarization resistance of 40Ni/60ScCeSZ and 40Ni/60ScSZ anodes was obtained at sinter temperatures of 1350 °C and 1300 °C respectively. Carbon deposition over 40Ni/60ScCeSZ, 40Ni/60ScSZ and 40Ni/60YSZ catalysts was evaluated at 700 °C for 1 h at S/C = 0.8 and the results showed that the ratio of deposited carbon was lower in the case of Ni/10ScSZ and Ni/10Sc1CeSZ compared to Ni/YSZ (0.35). Overall, Ni/10Sc1CeSZ and Ni/10ScSZ cermets having 40 vol% Ni were found to be optimum, with the 40Ni/60ScCeSZ cermet proving to be better than 40Ni/60ScSZ cermet in terms of both electronic conductivity and electrode polarization resistance, with both materials exhibiting improved tolerance towards carbon deposition compared to Ni/YSZ.

Investigation on rheological behavior of 8 mol% yttria stabilized zirconia (8YSZ) powder using Tiron

In this research, 8 mol% yttria stabilized zirconia (8YSZ) powder were dispersed in de-ionized water by the use of different amounts of Tiron as dispersant. The results of rheological and sedimentation measurements of each suspension were evaluated and the optimum amount of Tiron was selected (0.8% Tiron). Also, various pH were investigated and the best stabilized suspension is achieved at pH 10. Furthermore, the zeta potential of suspension with and without adding dispersant was obtained. The isoelectric point (IEP) of as-received 8YSZ powder was about 8.5 and shifted obviously to acidic region after adding dispersant to the suspension.

Production of Hard Materials with Ubiquitously Fe-B System by Spark Sintering and their Characteristics

The spark sintering technique was utilized to fabricate ubiquitously hard-materials compacts which were 0, 10 and 50vol%Fe added (hereafter called, 100, 90 and 50%FeB, respectively) in as-received powders with mean composition of Fe-19.1wt%B. Rockwell hardness number on C scale was changed 63 to 70 depending on Fe contents, and the value of 100%FeB compact was the same as that of WC-7.8Co. The 100 and 90%FeB compacts showed the fracture strain of 0.32 and 0.41 %, or fracture stress of 212 and 133 MPa, respectively, in three points bending tests at room temperature. The 100%FeB compacts had the same level in fracture stress and increment of 0.1 % in fracture strain at 773 K, compared with those obtained from the bending tests at room temperature. Dynamic friction coefficient of 90 and 100%FeB compacts was lower than that of WC-7.8%Co in pin (SK2) on disk configuration. Same values were shown in both wear loss on specimens (100%FeB or WC-7.8%Co), and SK2, which meant the similar level in these compacts for the cutting performance as the tool materials. The value of flank wear width (VB) was 0.26 mm at cutting distance of 30 m for the work material of Ti-6Al-4V using the cutting tool of 100%FeB, which meant the satisfaction for its standard value of 0.3 mm as the life of tools.