Plasma Sintering Research Articles

Correlation between properties of zirconia mullite and Y2O3 stabilised zirconia mullite derived by various processing techniques

Zirconia mullite (MUZ) and Y2O3–MUZ composites were synthesised by plasma melting, plasma sintering and conventional sintering techniques. Mixtures of tetragonal and cubic phases have been observed in all the processed Y2O3–MUZ composites. This observation infers that though the available temperature in plasma sintered and conventionally sintered MUZ specimens is not sufficient for the transformation of monoclinic to cubic zirconia, the addition of Y2O3 favours this transformation and also stabilises the cubic zirconia phase at room temperature. The optical absorption studies completely reflect the improvement in optical band gap of Y2O3–MUZ composites to prove it as a suitable candidate for refractory applications. The bulk density, hardness and higher crystalline nature of plasma sintered composites indicate that better quality of products can be synthesised by direct current extended arc thermal plasma heating technique within a short duration of time.

Characterization and Sintering Behaviors of Fe Based Nanopowders Prepared by Arc Discharge Process

The formation of Fe nanopowders by Plasma Arc discharge process and sintering behavior of Fe nanopowder has been investigated. The effect of processing parameters on the size and microstructure and their pressureless sintering behaviors have been analyzed. Also the addition effect of micron powder to nanopowder on the compaction and sintering was studied. The prepared Fe nanpowder showed nearly spherical shape with the average size of 100 nm. The surface oxide layer of nanopowder played an important role on the initial densification of the green compact. The hardness and sintered density of nano/micron bimodal powder was similar to those of pure nanopowder.

Microstructural and mechanical characterization of iron samples sintered in DC plasma

Samples of unalloyed iron were sintered in an abnormal glow discharge using a confined anode–cathode configuration. In this geometry the samples were placed on the holder, which acted as the discharge anode, while the cathode surrounded the sample undergoing bombardment by ions and energetic neutral atoms/molecules, leading to heating and sputtering. For comparison, another set of samples was sintered in a resistive furnace. The presence of large grains was observed for plasma-sintered unalloyed iron, which could be attributed to a large amount of atomic hydrogen in the electric discharge. These grains influence the mechanical properties of the sintered product. Three-point bending tests revealed lower yield strength and, quite surprisingly, ductility, compared to samples sintered in a furnace under the same temperature and time conditions. The decrease in ductility is associated with intergranular crack propagation in plasma-sintered samples. Such embrittlement could be attributed to the absorption of hydrogen during plasma sintering or higher impurity content at grain boundaries.

Extended arc thermal plasma assisted sintering of Al-Zr oxide composites

An inexpensive and simple sintering technique using an extended arc thermal plasma assisted heating (EATPAH) source has been designed and fabricated for rapid sintering of Al x-Zr100- x ( X = 0, 10, 20,…, 100) oxide composites. High heat and mass transfer kinetics of thermal plasma assisted heating allows quick densification within a few minutes when compared with conventional heating source that takes tens of hours for identical densification. Sintering parameters such as sintering time, plasma power and plasmagen gas flowrate, and initial green density of composites are critically optimized. The green powder compacts sintered at a higher heating rate produced a sample with higher density and fine grain microstructure, whereas abnormal grain growth and lower density result at lower heating rate. This reveals that rapid sintering by EATPAH is effective for promoting the physical densification of the sintered end product. Hardness of the sintered products is studied to understand the density and shrinkage behaviour during plasma sintering. It is observed that highly dense sintered product with greater hardness and homogeneous phase distribution can be prepared within a few minutes by low cost EATPAH technique.

Thermoelectric properties of nonstoichiometric Ag1－xPb18SbTe20 materials

Ag1-xPb18SbTe20(x＝0，0.25，0.5，0.75) samples were prepared by melt-quench-spark plasma sintering(SPS). As the Ag content decreased, a second phase Sb2Te3 occurred in samples and the carrier concentration increased to 5×1018cm-3 and then ceased to increase as Ag content decreased further. The carrier mobility decreased first, and then increased, as the Ag content decreased. As temperature increased, the carrier scattering mechanism shifted gradually from ionized impurity scattering to acoustic phonon scattering. As the Ag content decreased, the electric conductivity increased while the Seebeck coefficient decreased, and the thermal conductivity show an increasing trend.

Sintering Behavior of NbC-Reinforced Steel

Powdered steel reinforced by NbC dispersed particles was sintered both in resistive furnace at 1180°C or in plasma reactor at 850°C (reference temperature) using heating rates that ranged from 10 to 100°C/min. Fe3P was used as liquid phase sintering additive. The microstructure of the resulting materials was visualized by scanning electronic microscopy. Distinctive microstructural features were observed as a function of the heating source and heating rate. Plasma sintering at rates ~ 30°C/min revealed different microstructural features comparing edge and sample bulk. Homogeneous mixtures of Fe and NbC could be sintered in resistive furnace and plasma reactor using relatively low heating rates. Plasma sintering at 800°C for 1 h (heating rate of 10°C/min) resulted in relative densities of ~ 91% of the theoretical density of the composite. Sintering in resistive furnace for 1150°C resulted in relative densities ~ 94%.

Comparison between carbonization of wood charcoal with Al-triisopropoxide and alumina

A comparison was made between the catalytic carbonization of biomass carbon suspended in Al-triisopropoxide and in biomass carbon mixed with 40 μm sized Al 2O 3 particles. Both types of samples were plasma sintered during 5 min under an argon pressure of 50 MPa at temperatures up to 2200 °C. Plate-like catalytic graphitization develops by formation and dissociation of plate-like Al 4C 3. Plasma sintering under the proper CO partial pressure and heat treatment temperature is instrumental in forcing the Al 2O 3 to react with the carbon, forming first Al 4C 3 and subsequently graphite. The difference between Al-triisopropoxide and Al 2O 3 is a matter of intensity of the graphite reaction versus the size of the graphite patches.

The Energy Balance during Plasma Sintering

High heating rate during plasma sintering of metallic powders, mainly the supersolidus, can lead to better properties of the sintered material. In this work, powders with different thermal conductivities (Cu and Ni) were plasma sintered using both, planar discharge and hollow cathode discharge (HCD) configuration. The thermal evolution was studied using three thermocouples inserted inside the samples at different depths. Thermal gradient and the heating rate were measured as a function of the source power, discharge configuration, pressure and thermal conductivity of the material. We have observed that the thermal gradient changed greatly with the discharge configuration. The thermal difference between most profound and most shallow inserted thermocouple in Cu samples was around 30 °C for both planar and HCD discharge configuration. Ni samples and the planar configuration gave thermal difference of 70°C while for HCD configuration thermal difference went superior 140 °C. For this last case, heating rate of 300 °C/min was obtained.

Plasma Sintering of Fe-NbC Composites

Ferrous alloys have been reinforced by particle dispersion using hard ceramics such as niobium carbide, tantalum carbide, or titanium carbide. The resulting composites have a number of potential applications in the ceramic and textile fields where abrasion plays an important role. Their cost-efficient production allows them to exceed the capabilities of conventional non-reinforced tool steels. The present study presents some of the results obtained sintering Fe-NbC composites either in a resistive oven or in a d.c. plasma furnace. Final densities of approximately 95% TD were obtained in the latter atmosphere at temperatures as low as 750°C. Dilatometric studies carried out in resistive furnace indicated that this composite did not show significant densification below 1240°C in the presence of a liquid phase formed by the addition of small amounts of Fe 3 P. The differences between conventional and plasma sintering are discussed herein along with the effect on the microstructure of the composite.

Plasma Dewaxing and Sintering of Compacted Unalloyed Iron

This work reports on the use of plasma technology in lubricant removal and sintering of compacted PM components. Unalloyed iron samples were treated in a hydrogen and argon discharge at a pressure of 400 Pa (3 Torr), varying the temperature and soaking time. The discharge was generated in a confined cathode-anode configuration with the sample placed on the anode. Samples were characterized by mass loss and the results were correlated with the plasma parameters. Approximately 80% of zinc stearate was removed using a H 2 discharge at 350 °C during 5 minutes reaching 93% as the time was increased to 30 minutes. After dewaxing, sintering was carried out with the sample placed on a support, which worked as the cathode of a 80% Ar + 20% H2 discharge at a pressure of 1330 Pa (10 Torr) for 30 minutes. Characterization of the sintered samples showed that plasma technology resulted in an efficient process.

Effects of Sn deposition and plasma sintering on the magnetic properties of Sm 2Fe 17 nitride compound

Metal bonded Sm 2Fe 17 nitride magnets with Sn adhesion were fabricated. A chemical process was used to deposit Sn on the surface of the powder grains, and a plasma DC technique was used to sinter the magnets. The sample exhibited H c =4.6 kOe, M s =145 emu/g and M r =92.6 emu/g . Structural measurements also indicate that no chemical reaction between the nitride and the bonding metal occurred.

Use of mechanical alloying and plasma sintering in fabrication of FGM: H. Kimura and S. Kobayashi (National Defence Academy, Yokosuka, Japan), J Japan Soc Powder and Powder Metallurgy, Vol 39, No 4, 1992, 287–290. (In Japanese)

Use of mechanical alloying and plasma sintering in fabrication of FGM: H. Kimura and S. Kobayashi (National Defence Academy, Yokosuka, Japan), J Japan Soc Powder and Powder Metallurgy, Vol 39, No 4, 1992, 287–290. (In Japanese)

Study of Plasma Sintering Temperature for Y<sub>2</sub>O<sub>3</sub> Doped AlN

The temperature measurement of Y2O3 doped AlN during plasma sintering was studied. The temperature of specimen, Ts was measured using a (W-5%Re)-(W-26%Re) thermocouple buried in the specimen. Ts below 1100°C could be measured directly in-situ using a low pass filter. Ts above 1100°C could not be measured due to interference of plasma noise. Hence, Ts was measured by extrapolation of a cooling curve after extinguishment of plasma. The extrapolation curve was determined by simulating the cooling curve. The thermal conductivity and emissivity of the specimen could be also estimated from simulation of the cooling curves. The measurement of Ts and simulation of the cooling curve showed that Ts was elevated from 1800° to 2200°C by the change of surface morphology after AlN evaporation, but emissivity was kept constant in spite of the change in morphology. It was found that rapid re-heating and re-densification after AlN evaporation occurred by the increase in heating from plasma, which was enhanced by Y2O3 powder deposited on the specimen surface.

Time Dependence of AlN Densification by Plasma Sintering

Pure and 3wt%-Y2O3 doped AlN powder compacts were sintered in an r.f. inductively coupled plasma under reduced pressure. The effects of the r.f. output power and gas pressure on their densification were investigated. AlN powder compacts doped with 3wt%-Y2O3 showed rapid densification to the theoretical density, while pure AlN densified only to 68% of the theoretical density under the experimental conditions studied. Time dependence of the densification of 3wt%-Y2O3 doped AlN was found to be divided into three stages; (1) rapid densification region occurring during insertion to plasma, (2) plateau region of constant density, (3) re-densification region with evaporation of AlN. Direct measurements of the specimen temperature showed that the densification at the first stage proceeded during rapid heating up to 1800°C, and that the re-densification at the third stage occurred as a result of elevation of the specimen temperature from 1800°C to >2000°C.

Rapidly sintered particulate ceramic matrix composites

The effect of rapid rate furnace sintering and HCD-plasma sintering on density, microstructure, and mechanical properties of three Al 2O 3-5% vol. metal carbide composites and Al 2O 3-10% vol. ZrO 2 was studied and the results are compared to conventionally sintered composites. Significantly higher densities were achieved in Al 2O 3-5% vol. SiC composites by plasma sintering as well as rapid rate furnace sintering, which also improved the attendant mechanical properties. High fractional densities and similar mechanical properties were obtained in Al 2O 3-5% vol. TiC and Al 2O 3-10% vol. ZrO 2 composites produced both through rapid rate and conventional sintering. The finest microstructure in particulate—CMCs was achieved by plasma sintering.

Sintering of SiC by Low Temperature Plasma

SiC powder compacts were sintered in inductively coupled plasma (ICP) at 4MHz. Boron and carbon were added to SiC powder as sintering aids in the present experiments. The plasma used here was low temperature Ar plasma working at low gaseous pressure=0.2-6.5kPa, since it was easy for samples to be inserted into plasma. Parameters influencing the densification were gaseous pressure, gas flow rate, sintering duration and r.f. out-put power. SiC was found to be extensively densified under the experimental conditions: gaseous pressure=1.0kPa, gas flow rate=70-270 SCCM, r.f. power=12kW, duration=2min. The weight loss during sintering was a problem to be solved in the present low temperature plasma. Morphological studies showed that abnormal grain growth could not be observed. A crystallographic change was observed from 15R and 3C to 6H after plasma sintering.

Temperature‐Gradient‐Driven Diffusion in Rapid‐Rate Sintering

The vacancy diffusion flux, driven by the neck growth curvature (1 /ρ) which contributes to conventional sintering of a powder compact, is theoretically compared with that arising from thermal diffusion, driven by the temperature gradient (grad T/T) during the final stage of sintering. Under conditions experienced in ultrarapid sintering, the thermally driven vacancy flux can exceed the curvature‐driven flux by a factor of 1000. This effect may explain the phenomenon of enhanced densification rates observed in plasma sintering.

Application of Plasma to Processing for Ceramics

The thermal plasma processing for ceramics was surveyed with a stress on plasma spraying, plasma CVD and plasma sintering. The development of the low pressure plasma process and RF plasma process in plasma spraying is worth of special mention. These processes will be applied to ceramics coating in near future. Researches on thermal CVD are being carried out very actively for synthesis of high purity ultra fine ceramic powders such as silicon nitride and silicon carbide. This process has a very high potential as a means for rapid growth of ceramic films on various substrates. Low pressure RF inductive plasma and high power microwave plasma have characteristics quite different from RF glow discharge plasma. It is very interesting that these plasmas are applied to synthesize diamond and cubic boron nitride films. Rapid sintering of ceramic powders in plasma atmosphere is one of the most important applications of plasma to materials processing, but many problems remain to be solved in the sintering process.

Application of plasma to processing for ceramics

The thermal plasma processing for ceramics was surveyed with a stress on plasma spraying, plasma CVD and plasma sintering. The development of the low pressure plasma process and RF plasma process in plasma spraying is worth of special mention. These processes will be applied to ceramics coating in near future. Researches on thermal CVD are being carried out very actively for synthesis of high purity ultra fine ceramic powders such as silicon nitride and silicon carbide. This process has a very high potential as a means for rapid growth of ceramic films on various substrates. Low pressure RF inductive plasma and high power microwave plasma have characteristics quite different from RF glow discharge plasma. It is very interesting that these plasmas are applied to synthesize diamond and cubic boron nitride films. Rapid sintering of ceramic powders in plasma atmosphere is one of the most important applications of plasma to materials processing, but many problems remain to be solved in the sintering process.