Electrochemical corrosion and bioactivity of titanium–hydroxyapatite composites prepared by spark plasma sintering

The corrosion resistance and bioactivity of titanium–hydroxyapatite (Ti–HA) composites prepared by mechanical alloying (MA) followed by spark plasma sintering were investigated. Potentiodynamic polarization tests showed that Ti composites containing 0–10% HA exhibited higher corrosion resistance than commercially pure Ti. An increase in HA concentration (20–30%) decreased corrosion resistance owing to the craterlike defects induced by ceramic particle detachment. Increasing MA time significantly reduced defect density and improved corrosion resistance. Bioactivity test on Ti HA composites in simulated body fluid revealed the growth of a nano-HA layer, which indicates the high potential of Ti–HA composites for application in biomedical implants.     

Microstructure and mechanical properties of ZrB2–SiC composites prepared by gelcasting and pressureless sintering

ZrB₂–SiC ceramic composites were prepared through water-based gelcasting and pressureless sintering. Effects of the pressureless sintering temperature (1500–2000 °C), heating rate (5–15 °C/min) and soaking time (0.5–2 h) on the relative density, microstructure and mechanical properties of the ZrB₂–SiC composites were investigated in detail. A sintering temperature of 2000 °C, a heating rate of 5 °C/min and a soaking time of 2 h were found to be the optimal pressureless sintering procedure. The relative density, flexural strength and fracture toughness of the ZrB₂–SiC composite prepared under the optimum condition were 97.8%, 403.1 ± 27.8 MPa and 4.05 ± 0.42 MPa·m^1/2, respectively.     

An investigation on the in situ synthesis–sintering and mechanical properties of MoSi2–xSiC composites prepared by spark plasma sintering

Composites of MoSi₂–x wt.% SiC (x = 5, 10, 15, 20) prepared using spark plasma sintering. The effect of temperature on the in-situ synthesis–sintering was investigated between 1100 °C and 1500 °C. X-ray diffraction patterns showed that at 1100 °C the reactions were incomplete and elementary diffraction peaks of Mo, Si and C still exist. With an increase in temperature from 1100 to 1300 °C the reactions were performed completely. The study showed that the sintering ability at higher temperature at the presence of enough mechanical pressure was better because the heat released from the reaction between Mo, Si and C causes higher temperature than the melting point of Si (1410 °C). Consequently the silicon would melt during the heating process. The molten Si can strengthen the interconnections and it has higher diffusion rate. Therefore, due to the liquid phase sintering and at the presence of mechanical pressure, the sintering ability at higher temperature is better than lower temperature. Scanning electron microscopy showed that with the addition of carbon, there was no silica phase in the microstructure of the synthesized samples, due to the formation of SiC. Therefore, it can be noted that the addition of carbon leads to better mechanical properties due to elimination of silica phase.     

A study of the wear resistance of a TiC–SiC composite ceramic material prepared by spark plasma sintering

The effect of the silicon carbide content and the spark plasma sintering temperature on the porosity and wear resistance of a composite material is determined. An increase in the silicon carbide content leads to an increase in the porosity from 4 to 12%. The temperature effect on the porosity becomes apparent above 1200°C. The material consists of a matrix represented by titanium carbide grains with a size of 1–3 μm and silicon carbide grains with a size of 10–30 μm. It is found that the TiC–20%SiC compound exhibits the highest wear resistance, which is 12 times higher than that of hot-pressed silicon carbide.     

Synthesis and characterization of Al2O3–TiC nano-composite by spark plasma sintering

Al₂O₃–10TiC composite was synthesized by high energy ball milling followed by spark plasma sintering (SPS) process. Microstructure of the sintered composite samples reveals homogeneous distribution of the TiC particles in Al₂O₃ matrix. Effect of sintering temperature on the microstructure and mechanical properties was studied. The sample sintered at 1500 °C shows a measured density of 99.97% of their theoretical density and hardness of 1892 Hv with very high scratch resistance. These results demonstrate that powder metallurgy combined with spark plasma sintering is a suitable method for the production of Al₂O₃–10TiC composites.     

Fe–B–C composites produced using spark plasma sintering

Fe–B–C composites were produced using iron and boron carbide powders. The powders were mixed to produce various compositions, ranging from 1 vol.% Fe to 80.1 vol.% Fe. Spark plasma sintering (SPS) was used to densify the composite powder green compacts. The sintering temperatures used ranged from 900 °C for the composites with a high iron content to 2000 °C for those with a high boron carbide content. It was evident that during the sintering process the iron reacted with the boron carbide. XRD analysis showed the presence of FeB, Fe₂B, Fe₃C, Fe₃(B_0.6C_0.4), Fe₂₃(B,C)₆ and residual carbon as reaction products. The composites were found to have hardness values between 9.8 and 33.1 GPa with the higher hardness being associated with the higher boron carbide contents. The fracture toughness values determined were in the range of 2.8–5.3 MPa m^0.5. With increasing iron content from 1 to 5 vol.%, it is evident that the FeB formed begins to embrittle the material rather than increase the fracture toughness as a result of the high residual stresses between the B₄C and FeB phases.     

Heat treatment process of new NdFeB magnet prepared by spark plasma sintering

1　INTRODUCTIONTraditionallysinteredNdFeBmagnetcanmeettherequirementofdimensional precisionthroughpost machining ,whichusuallyresultsin 4 5 %ofma terialloss.Post machiningnotonlywastestherareearthresources ,butalsoincreasesthe productioncost .Moreover ,itisdifficulttopreparehomogeneousworkpieceswithlargedimensionandcomplicatedshapeduetosomeuncontrollablefactorsinthecon ventionalsintering process .Ontheotherhand ,al thoughthebondedNdFeBmagnethasbetterforma bilityanddimensionalprecision ,…     

Influence of powder characteristics on structure and properties of Ni3Al fabricated by spark plasma sintering

Three kinds of Ni and Al powder mixtures with nominal composition Ni75Al25 were employed to prepare Ni3Al alloys by spark plasma sintering（SPS） process. The raw powders include fine powder, coarse powder and mechanically-alloyed fine powder. The effects of powder characteristics and mechanical alloying on structure and properties of sintered body were investigated by scanning electron microscopy（SEM）, X-ray diffraction（XRD）, bending test and Vickers hardness measurements. For all mixture powders near fully dense Ni3Al alloys （relative density〉99.5%） are obtained after sintering at 1150℃ for 5 min under 40 MPa. However a small fraction of Ni can be reserved for alloy from coarse powders. The results reveal that grain size is correlated with particle character of raw powder. Ni3Al alloy made from mechanically-alloyed fine powder has finer and more homogenous microstructure. The hardness of all alloys is similar varying from HV470 to 490. Ni3Al alloy made from mechanically-alloyed fine powder exhibites higher bending strength （1 070 MPa） than others.     

Microstructure,mechanical properties and oxidation behavior of powder compacts of the Nb–Si–B system prepared by spark plasma sintering

Powder compacts with eight different compositions in the Nb–Si–B system were prepared by spark plasma sintering and their microstructure, mechanical properties and oxidation behavior were investigated. Oxidation resistance of Nb₅Si₃B₂ compacts is better than that of Nb₅Si₃ compacts, but extremely poorer than that of NbSi₂ compacts. However, since the oxidation experiment was of short duration, details of the oxidation behavior of Nb–Si–B compacts have yet to be investigated. Compacts were prepared via the following three different routes: (i) elemental powders were mixed in a rotational ball mill and then consolidated by spark plasma sintering method, (ii) elemental powders were mechanically alloyed and consolidated similarly to (i) and (iii) pulverizing compacts prepared via route (i) and consolidated once again similarly to (i). Compacts prepared via the third route exhibit more homogeneous microstructures and contain a smaller amount of SiO₂ than compacts prepared via the other two routes. Compacts with compositions around the line of Nb₅Si₃–Nb₅Si₃B₂–NbB₂ exhibit a high hardness at room temperature and a high compressive strength at high temperatures in comparison to those with compositions away from the line. The strength of compacts containing NbSi₂ decreases with increasing the volume fraction of NbSi₂ phase.     

Reactive spark plasma sintering of TiB2–SiC–TiN novel composite

The effects of adding SiC as a reinforcement and TiN as an additive on TiB₂-based composites fabricated by the spark plasma sintering (SPS) technique were investigated. SPS was implemented at the sintering conditions of 1900 °C temperature, 7 min holding time and 40 MPa pressure. Adding these two secondary phases had noticeable effects on the microstructure of TiB₂-based composites. A relative densities of 99.9% was obtained for TiB₂–SiC–TiN composite. Detection of in-situ formed phases and investigation on them were done using SEM, XRD, EDS and thermodynamic assessment. These evaluations proved the formation of in-situ phases of TiC, BN nano-platelets, TiSi and B₄C in the TiB₂-based composite codoped with SiC and TiN. Formation of these in-situ phases had fascinating effects on the sinterability and ultimate microstructure of titanium diboride.     

Effect of short carbon fiber addition on pressureless densification and mechanical properties of ZrB2–SiC–Csf nanocomposite

In the present paper, ZrB₂–SiC–C_sf composites were produced by pressureless sintering method. Carbon fiber and SiC nanoparticles with different weight percentages were added to the milled ZrB₂ powder. The mixed powders were formed by hot pressing and cold isostatic press (CIP) and after the pyrolysis, were sintered at 2100 °C and 2150 °C. In order to compare the microstructure and mechanical properties of samples scanning electron microscopy (SEM) equipped with EDS spectroscopy, XRD analysis, hardness and toughness tests were used. The results show that with the increase in weight percentage of carbon fiber, the porosity increases but the hardness, fracture toughness and density decrease. On the other hand, with the increase in weight percentage of SiC nano-particles, the porosity decreases and fracture toughness, hardness and density increase. The results indicate that in an optimal percentage of both additives, the hardness and toughness increase. Additionally, with the increase in sintering temperature, the values of hardness and fracture toughness increase and porosity decreases.     

A comparative study of spark plasma sintering and hybrid spark plasma sintering of 93W–4.9Ni–2.1Fe heavy alloy

Mixed 93W–4.9Ni–2.1Fe powders were sintered via the spark plasma sintering (SPS) and hybrid spark plasma sintering (HSPS) techniques with 30 mm and 60 mm samples in both conditions. After SPS and HSPS, the 30 mm and 60 mm alloys (except 60 mm-SPS) had a relative density (> 99.2%) close to the theoretical density. Phase, microstructure and mechanical properties evolution of W–Ni–Fe alloy during SPS and HSPS were studied. The microstructural evolution of the 60 mm alloys varied from the edge of the sample to the core of the sample. Results show that the grain size and the hardness vary considerable from the edge to the core of sintered sample of 60 mm sintered using conventional SPS compared to hybrid SPS. Similarly, the hardness also increased from the edge to the core. Furthermore, the 60 mm-HSPS alloy exhibited improved bending strength of 1115 MPa when compared to that of 60 mm-SPS, 920 MPa. The intergranular fracture along the W/W grain boundary is the main fracture modes of W–Ni–Fe, however in the 60 mm-SPS alloy peeling of the grains was also observed which diminished the properties. The mechanical properties of SPS and HSPS 93W–4.9Ni–2.1Fe heavy alloys are dependent on the microstructural parameters such as tungsten grain size and overall homogeneity.     

Microstructure and mechanical properties of bulk nanocrystalline Al–Fe alloy processed by mechanical alloying and spark plasma sintering

Nanocrystalline Al–5 at.% Fe alloy powders produced by mechanical alloying were consolidated by spark plasma sintering. The sintered sample showed high strength >1000 MPa with a large plastic strain of 15% at room temperature and 500 MPa at 350 °C. Microstructure characterizations by transmission electron microscopy and atom probe tomography revealed that the sintered samples are composed of α-Al and Al₆Fe nanocrystalline regions with 90 nm in diameter and a minor fraction of Al₁₃Fe₄ phase and coarsened 0.5–1 μm α-Al grains. This bimodally grained feature is attributed to the relatively large plastic strain for the strength level of 1000 MPa at room temperature.     

Development of in situ NiAl–Al2O3 nanocomposite by reactive milling and spark plasma sintering

NiAl–10 vol.% Al₂O₃ in situ nanocomposite has been synthesized by reactive milling and subsequent spark plasma sintering. The synthesized nanocomposites have ～96% of theoretical density after sintering at 1000 °C for 5 min. Microstructural analysis of consolidated samples using TEM has revealed the presence of α-Al₂O₃ particles of 10–12 nm size in NiAl matrix of submicron grain size. Consolidated NiAl–10 vol.% Al₂O₃ nanocomposite has shown very high hardness of 772 HV_0.3 and compressive strength of 2456 MPa with ～14% plastic strain. The high hardness and compressive yield strength are attributed to the presence of nanocrystalline α-Al₂O₃ particles and the appreciable plastic strain is attributed to the submicron grains of NiAl.     

Characterization of phase transformation and microstructure of nano hard phase Ti（C,N）-based cermet by spark plasma sintering

By means of optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM), the process of densification, the characterization of phase transformation and the microstructure for spark plasma sintering (SPS) nano hard phase Ti(C, N)-based cermet were investigated. It is found that the spark plasma sintering (SPS) enables the nano hard phase Ti(C,N)-based cermet to densify rapidly, however, the full densification of the sintered samples can not be obtained. The rate of phase transformation is significantly quick.When being sintered at 1 200℃ for 8 min, Mo2C is completely dissolved, and TiN dissolves into TiC entirely and disappears. Above 1200℃, Ti(C,N) begins to decompose and the atoms of C and N separate from Ti(C,N) resul-ting in the generation of N2 and the graphite. Due to the denitrification and the graphitization, the density and the hardness of sintered samples are rather low. The distribution of grain size of the sample sintered at 1350℃ covers a wide range of 90-500 nm, and most of the grain size are about 200 nm. The hard phase is not of typical core-rim structure. Oxides on the surface of particles can not be fully removed and present in sample as titanium oxide TiO2.Graphite exists in band-like shape.     

Mechanical properties and microstructure of Fe3Al intermetallics fabricated by mechanical alloying and spark plasma sintering

Fabrication technology and mechanical properties of the Fe3Al based alloys were studied by spark plasma sintering from elemental powders （Fe-30Al, volume fraction, %） and mechanically alloying powders. The mechanically alloying powders were processed by the high-energy ball milling the elemental mixture powders with the milling time of 5, 8 and 10 min, respectively. The spark plasma sintering process was performed under the pressure of 50 MPa at 1 050 ℃ for 5 min. The phase identification by X-ray diffraction presents the Fe reacts with Al completely during the processing time. The samples are nearly full density （e.g. the relative density of sinter of raw powder is 99%）. The microstructure was observed by TEM. The mechanical properties were tested by three-point bending at room temperature in air. The results show that the mechanical properties are better （e.g. bend strength of 1 500 MPa ） than those of the ordinary Fe3Al casting.