Papers by Keyword: Spark Plasma Sintering (SPS)

Abstract: To clarify the influence of internal pulsed current upon the sintering behavior of powder materials during spark plasma sintering processing, simultaneous measurement of internal current using magnetic probe was carried out. Magnetic probe is installed to the side of the sintering ZnO powder material through the carbon graphite sintering die, and detects the magnetic field generated by internal current which flow through the specimen. By magnetic probe measurement, the internal current that flows through the specimen during SPS process was several hundred ampere, and the ratio of the internal current to the total current was found to be dependent on the electrical conductivity, diameter of powder material and the progress of SPS process. The measurement and estimation of an internal pulsed current using a magnetic probe in the specimen is very useful for in situ observation of the sintering behavior during the SPS process.

Abstract: Capabilities of synthesizing new structural and functional materials by SPS processing were indicated by exemplifying the synthesis of nano-structured alumina with high bending strength or high transparency, Al/diamond composites with high thermal conductivity and zirconia(3Y)/ SUS410L FGM. In the synthesis of alumina, the bending strength of more than 720MPa was attained by choosing suitable SPS conditions. It was also indicated that SPS processing could easily synthesize Al/diamond composites with high thermal conductivity of more than 400W/(m･K), suggesting elaborate control of interface between Al and diamond in SPS consolidation. Further, zirconia(3Y)/SUS410L FGM could easily be fabricated by SPS. Mechanical weakness in the zirconia(3Y)-rich layers of the FGM was shown from the analysis of stress state based on Raman scattering method. It is suggested that the designing of the layer staking in FGM based on the Raman scattering analysis is effective for the improvement of the weakness in the FGM.

Abstract: Air-atomised pure aluminium powder with additions of 10 at.% of AgO, PtO2 or PdO was mechanically alloyed (MAed) by using a vibrational ball mill, and MAed powders were consolidated into bulk materials by a spark plasma sintering (SPS) process. Mechano-chemical reactions among pure Al, precious metal oxide and stearic acid, added as a process control agent, during the mechanical alloying (MA) process and subsequent heat treatments were investigated by X-ray diffraction. The mechanical properties of MAed powders obtained under various heat treatment conditions and those of the SPS materials were evaluated by hardness tests. Mechano-chemical reactions occurred in Al/precious metal oxide composite powders during 36 ks of the MA process to form AlAg2, Pt and Al3Pd2 for the Al-AgO, Al-PtO2 and Al-PdO systems, respectively. Further solid-state reactions in MAed powders have been observed after heating at 373 K to 873 K for 7.2 ks. The hardness of MAed powders initially increased significantly after heating at 373 K and then generally decreased with increasing heating temperatures. The full density was obtained for the SPS materials under the conditions of an applied pressure of 49 MPa at 873 K for 3.6 ks. All the SPS materials exhibited hardness values of over 200 HV in the as-fabricated state.

Abstract: Ultrafine grained WC cemented carbides were manufactured successfully by reaction sintering with Co-Al intermetallic compounds as binder from mixed powders of a nominal composition of WC-8Co-2Al (wt%). The purpose of adding aluminum to the composition is to promote the mechanical properties of hardmetals by the in situ formation of Co-Al intermetallic phase. To manufacture bulk ultrafine WC hardmetals, ultrafine and even nanocrystalline WC powders were prepared as precursors from the mixed commercial powders by mechanical milling. Experimental results show that the transverse rupture strength and hardness of sintered hardmetal reached the maxima, being 1747.8 MPa and 90.2 HRA respectively, when the milling time expands to 55 h. It is affirmed that mechanical milling weakens the thermostability of WC-8Co-2Al powders, and facilitates the formation of Co-Al intermetallic compounds. However, overlong milling results in decreasing of properties and densities of sintered hardmetals, because the powder particles are severely refined and markedly agglomerated.

Abstract: In this study, Spark Plasma Sintering has been used to sinter pure iron with an initial crystallite size around 100 nm. The process parameters for sintering pure iron have been optimized in order to obtain fully dense materials and avoid excessive grain growth. Archimede's method has been used to calculate the relative density of the sintered samples. It appears that almost fully dense materials can be obtained (95%). X Ray diffraction applied to the sintered samples shows the presence of iron and of the wustite oxide FeO (around 6% wt) formed during the sintering process. Peak enlargement measurements show that the grain size after sintering is around 200nm. This is confirmed by TEM observations showing a dual distribution of grain size. Finally, mechanical characterization has been carried out. The sintered compact exhibits a very high hardness of about 400 Hv. Compression test reveals a very high maximal stress of about 1.2 GPa and that the ductility in compression is non negligible. Using the Hall and Petch law, the calculated grain size should be around 450 nm which is in accordance with direct observations.

Abstract: Porous aluminum alloy has been developed by powder metallurgy route using Spark plasma sintering (SPS) technique. Sintered material was produced by SPS system after getting the mixture of Al-12Si alloy and titanium hydride powders. Porous materials are prepared under various process conditions, and the pore morphology was investigated. Compression test is performed at crosshead speed of 1mm/min, 10mm/min and 100mm/min with no lubricant. The compression strength, σC i.e. plateau stress was estimated 12MPa at the density of porous materials, 0.7 Mg/m3. Densification strain εD from compression curve is around 0.6. These properties depend on pore morphology of porous materials, and it is possible to control the morphology under specific condition with this process. Plateau stress and absorbed energy of heat treated porous Al-Si alloy were estimated by measurement of a first peak stress and calculated an area up to 0.5 strain from compressive stress-starin curves. Young’s modulus is measured by starin gauge method under compression test. Porous aluminum alloy filled mold die is also produced successfully.

Abstract: High plastic Ti66Nb13Cu8Ni6.8Al6.2 composites with in situ precipitated ductile -Ti phase were firstly synthesized by mechanical alloying and subsequent consolidation by spark plasma sintering with crystallization. Microstructure analysis indicated that all composites contain soft (Cu, Ni)-Ti2 regions and hard -Ti regions, but the two regions have different scale and distribution. The synthesized composites exhibit high fracture strength of 2415 MPa and large plasticity as high as ~31.8%. The large plastic deformability was well explained based on the distinctive microstructure by a developed “hard-soft model”.

Abstract: Mo-Ti3SiC2 layered material was prepared by spark plasma sintering. Mixed Ti, Si, graphite and Al powder with molar ratio of 3Ti:1Si:2C:0.2Al was put into a graphite mould and pressed with a pressure of about 0.5 MPa, then, Mo powder was put on top of the mixed powder. Experimental results showed that Mo-Ti3SiC2 layered material could be fabricated successfully by sintering the above powder mixture at 1300°C for 20 minutes under a pressure of 50 MPa in vacuum. The surface and interfaces of the layered composite were tight and clear without any observable crack. In order to study the thermal stability at elevated temperature, the fabricated Mo-Ti3SiC2 layered composite was heat treated at 800°C for 5, 10, 20 and 40 hours. After 40 hours of annealing, the intermediate layers formed between the Mo and Ti3SiC2 matrix grew thicker. The interfaces are clean and tight with no obvious formation of voids and new phases. The initial 10 hours of annealing is the fast growing period, after that, the growth rate slowed down significantly.

Abstract: Origins of high Q are considered on intrinsic as high symmetry, ordering structure and high density crystal structure. It was concluded that the high symmetry brings high Q instead of ordering comparing some cases as follows: As if ordering ratio of Ba(Zn1/3Ta2/3)O3 (BZT) is high of about 80%, Q values are distributed from low to high Q. Disordered BZT ceramics with high density obtained for short sintering time by spark plasma sintering (SPS) showed high Q. Ba(Zn1/3Nb2/3)O3 (BZN) with order-disorder transition showed high Q at disorder form sintered over the transition temperature. And, the disordered BZN with high Q annealed at lower temperature changed to order structure without improvement of Q.

Abstract: This paper has examined some recent findings concerning the processing of fully dense hetero-nanostructured materials (i.e. consisting of nano, ultrafine and micrometric grains) which can be produced by using the interplay between heavy deformation and recrystallization. By plastic deformation of bulk materials, an improved strength/ductility balance can be obtained directly by imparting high strain deformation (by ECAE) until the occurrence of recrystallization. Using a powder metallurgy route, the strong potential of electric field assisted sintering (ECAS) for producing multi-scale microstructures when a milled powder is used is also demonstrated. In this case, in addition to modify the classic processing parameters (time/temperature of ECAS), altering the nature of the milled powder - by Y2O3 addition during the milling stage - is also a good way to delay the onset of recrystallization and, thereby, increase the fraction of ultrafine grains.