Study on preparation and property of porous tungsten via tape-casting

A novel method of preparing porous tungsten via tape-casting is developed in this study. Micron-sized bulk porous tungsten with an open, biporous structure with large pores of 3–6 μm and with small pores of ~ 1 μm was successfully fabricated. The morphology of large pore depends on NaCl space-holder, and the uniform porous structure can be attributed to dispersant and binder added in tape-casting slurry which keeps tungsten powders decentralized, make the slurry stable and ordered. Compared with conventional process, the sintering temperature is reduced by at least 300 °C with the help of exothermic carburization of tungsten where carbon is introduced in the process of removing organics. W₂C phase was in situ generated on the surface of W particle and became the boundary between W grains. Furthermore, tape-casting samples show typical compressive properties of brittle porous material with higher compressive strength, which is attributed to the hard phase (W₂C) and uniform porous structure.     

Pore formation mechanism and characterization of porous NiTi shape memory alloys synthesized by capsule-free hot isostatic pressing

Porous NiTi alloys with different porosities were fabricated by capsule-free hot isostatic pressing (CF-HIP) with ammonium acid carbonate (NH₄HCO₃) as a space-holder. The microstructure and porosity of porous NiTi produced with different NH₄HCO₃ contents and sintering temperatures were determined. Two different creep expansion models are used to explain the pore expansion mechanism during the sintering process, which involves slow and continuous reduction of the argon pressure at high temperatures. When the NH₄HCO₃ content is 30 wt.% and the sintering temperature is 1050 °C, an ideal porous NiTi alloy with 48 vol.% porosity and circular pores (50–800 μm) is obtained. Compression tests indicate that the porous NiTi alloys with 21–48% porosity possess not only lower Young’s moduli of 6–11 GPa (close to that of human bones) but also higher compression strength and excellent superelasticity. Cell cultures reveal that the porous NiTi prepared here has no apparent cytotoxicity. The porous materials are thus promising biomaterials in hard tissue replacements.     

Structural and mechanical characteristics of porous 316L stainless steel fabricated by indirect selective laser sintering

Selective laser sintering (SLS) technique is capable of rapidly fabricating customized implants with porous structure. A simple encapsulation process was developed to coat 316L stainless steel (316L SS) powder with ethylene-vinyl acetate copolymer (EVA). Subsequently, porous 316L SS was prepared by SLS preforming of EVA-coated metal powders, debinding and sintering in hydrogen atmosphere. The effects of processing parameters on pore characteristics and mechanical properties were analyzed. The results indicate that the porosity of green body mainly depends on laser energy density, while the pore features and mechanical properties of sintered specimens are largely dominated by sintering temperature. After sintering at 1100–1300 °C, the average pore size and porosity are 160–35 μm and 58–28%, respectively. In addition, the elastic modulus and compressive yield strength are 1.58–6.64 GPa and 15.5–52.8 MPa, respectively. It is revealed that the pore structural parameters and mechanical properties of the as-sintered porous 316L SS can be controlled readily to match with those of cancellous bone by modification of SLS processing parameters and subsequent sintering temperature.     

Formation of porous metallic glass compacts by electro-discharge sintering

A single pulse of 0.1–0.9 kJ/0.45 g atomized amorphous Cu₅₄Zr₂₂Ti₁₈Ni₆ powders in size range of 90–150 μm was applied to fabricate porous metallic glass compacts using electro-discharge sintering (EDS) with 3 and 4 mm in diameter. The structural and thermal analysis of the samples indicated that formation of the porous metallic glass compacts occurs only when low electrical input energy was induced on the amorphous powders. Furthermore, the critical input energy inducing crystallization of the amorphous phase during EDS is strongly dependent on the size of the sample.     

Formation of TiAl–Ti2AlC in situ composites by combustion synthesis

Preparation of TiAl–Ti₂AlC in situ composites with a broad range of composition was conducted by self-propagating high-temperature synthesis (SHS) with compressed samples from the mixture of elemental powders. When compared with SHS formation of monolithic TiAl, the addition of carbon particles to the Ti–Al powder mixture enhances the sustainability of the reaction. It was found that no prior heating was required for the samples prepared to produce the composites containing more than 20 mol% Ti₂AlC, in contrast to the need of preheating at 200 °C for single-phase TiAl formation. This is attributed to the fact that formation of Ti₂AlC is more exothermic than that of TiAl. As a result, the combustion temperature and combustion wave velocity increase with the content of Ti₂AlC formed in the TiAl–Ti₂AlC composite, and approach the values associated with formation of single-phase Ti₂AlC when considerable amounts of Ti₂AlC are yielded. The XRD analysis of the end products confirms formation of TiAl–Ti₂AlC in situ composites. Moreover, simultaneous formation of Ti₂AlC promotes the phase evolution of the aluminide compounds. That is, the secondary aluminide phase, Ti₃Al, was no longer detected in the TiAl–matrix composites containing Ti₂AlC of 30 mol% or above.     

Influence of nanocrystalline Ni–Ti reaction agent on self-propagating high-temperature synthesized porous NiTi

Nanocrystalline Ni–Ti was used in self-propagating high-temperature synthesis (SHS) to fabricate porous NiTi. The SHS of porous NiTi using elemental powders was also prepared for comparison. Results showed that the main phase was NiTi with unreacted Ni when using elemental powders, which is detrimental to medical use. A large amount of Ti₂Ni secondary phase was also detected. By employing mechanically alloyed nanocrystalline Ni–Ti as a reaction agent, the secondary intermetallic phase (i.e. Ti₂Ni) was significantly reduced and the unreacted Ni was eliminated. The addition of 25 wt% nanocrystalline Ni–Ti reaction agent produced porous NiTi with an average porosity of 52–55 vol% and a general pore size of 100–600 μm under preheating temperatures of 200 and 300 °C. This general pore size in the range of 100–600 μm is beneficial to biomedical application for osseointegration. By further increase of the reaction agent to 50 wt% in the reactant, a porous NiTi part was produced at ambient temperature (i.e. no preheating was necessary) and a dense part was formed at preheated temperature of 200 °C due to the large amount of energies in the nanocrystalline reaction agent. This revealed that the use of nanocrystalline reaction agent effectively lowered the activation barriers for combustion synthesis reaction.     

Processing and properties of sintered reaction-bonded silicon nitride with Y2O3–MgSiN2: Effects of Si powder and Li2O addition

The effects of Si powder and Li₂O addition on the processing, thermal conductivity and mechanical properties of sintered reaction-bonded silicon nitride (SRBSN) with Y₂O₃–MgSiN₂ sintering aids were studied. Addition of Li₂O provides a less-viscous liquid phase that results in a more uniform and finer pore structure in RBSN with the coarser Si powders, but the pore structure plays a less important role in the densification of RBSN. The thermal conductivity of SRBSN without porosity decreases with increased Al impurity content and also decreases with the Li₂O addition regardless of the Si purity. The impurest coarse Si powder produces the lowest thermal conductivity (93 W m⁻¹ K⁻¹) but the highest four-point bending strength (∼700 MPa) and a higher fracture toughness (∼10 MPa m^1/2). However, the purer fine Si powder produces the highest thermal conductivity (119 W m⁻¹ K⁻¹) and highest toughness (∼11 MPa m^1/2) but the lowest strength (∼500 MPa).     

Effect of starch filler content and sintering temperature on the processing of porous 3Y–ZrO2 ceramics

A direct casting process was used to produce porous 3Y–ZrO₂ ceramics using starch as a fugitive filler and binder. The compositions with low additions of starch had higher porosity than the volume fraction of starch initially in the green body (X_st), whereas, the compositions with high amounts of starch produced lower porosity than the predicted value. The well ordered structure consisted of spherical pores of 8–10 μm diameter, retained from the original starch particles, connected by channels. The interconnection between pores was dependent on the volume fraction of starch incorporated, as well as on the sintering temperature. Pore interconnection was observed for all the compositions sintered at 1000–1300 °C. Increasing the sintering temperature to 1400–1500 °C produced a marked dependence of the open to total porosity ratio on X_st. For a high porosity material, a bimodal channel size distribution was found at 1400 and 1500 °C. The primary pore channel diameter was 0.7 μm and the secondary one was close to 4 μm. As the sintering temperature increased, the volume of the connecting channels decreased; at 1500 °C only a minor volume of the larger channels was found.     

Microstructure and some properties of TiAl-Ti2AlC composites produced by reactive processing

The TiAl–Ti₂AlC composites with and without impurities, Ni, Cl and P, were prepared by combustion reaction from the elemental powders and cast after arc melting. The resulting composites had about 18 vol% Ti₂AlC in the lamellar matrix consisting of γ-TiAl and Ti₃Al (α₂). In the homogenized specimens, the α₂ phase decomposed to γ-TiAl and Ti₂AlC. The composite material had a high strength both at ambient and elevated (1173 K) temperatures; about 800 and 400 MPa, respectively, with an ambient temperature ductility of 0.7% at bending test. The fracture toughness test also proved that the homogenized composite has higher toughness than the as cast one. The toughness value reached to 17.8 MPa m^1/2. The zigzag cracks propagated in the homogenized composite and the reinforcement Ti₂AlC particles and the finely precipitated Ti₂AlC particles were main obstacles to the crack propagation. The composite with impurities showed a marginal improvement in the oxidation resistance over the composites without impurities.     

Fabrication and thermophysical properties of porous TiB2 ceramics fabricated by reactive spark plasma sintering

Titanium oxide (TiO₂) and boron carbide (B₄C) were added to TiB₂ raw powders to prepare porous TiB₂ ceramics by reactive spark plasma sintering, and the gas escape (such as CO and B₂O₃) resulted in higher porosity. X-ray Diffraction results indicated that the reduction reaction was completed after the reactive spark plasma sintering process. The porosity could be controlled by changing the ratio of synthesized TiB₂ to raw TiB₂ powders. The porosity of porous TiB₂ ceramics with 20 wt.% and 40 wt.% synthsized TiB₂ ceramics are 18.5% and 22.2%, respectively. The thermal diffusivity of the porous TiB₂ ceramics decreased with the porosity due to the low diffusivity behavior of gas and vacuum in pores, and the thermal conductivity for porous TiB₂ ceramics decreased as the temperature increased throughout the measured temperature range. The results here pointed to a potential method for fabricating porous TiB₂ ceramics with controllable thermophysical properties.     

Effect of tool rotating rate on foaming properties of porous aluminum fabricated by using friction stir processing

A1050 porous aluminum is fabricated by the FSP route and the effect of the tool rotating rate on the porosity and morphology of the pores is investigated. To fabricate high-porosity porous aluminum with a uniform pore size distribution, a certain amount of stirring action is necessary; however, excessive stirring action is ineffective. A sufficiently uniform mixture is realized by traversing the FSP tool two times at a tool rotating rate exceeding 2200 rpm. The results indicate the minimum necessary amount of stirring action and will provide a guideline for improving productivity. Also, to improve the morphology of pores, optimizing the amount of Al₂O₃ is effective. Closed-cell porous aluminum with a porosity of about 80% was successfully fabricated by 2-pass FSP at 2200 rpm with the addition of 7 mass% Al₂O₃, a holding temperature of 998 K and a holding time of 10 min.     

Processing and characterization of sintered reaction bonded Si3N4 ceramics

The present contribution reports the influence of nitridation and sintering conditions on the densification, microstructure, mechanical and thermal conductivity properties of sintered reaction bonded Si₃N₄ (SRBSN) mixed with 3.5% Y₂O₃-1.5% MgO. The nitridation of samples was carried out at 1450 and 1500 °C for different time schedules (2.5, 8 and 16 h) in order to increase β Si₃N₄ phase and subsequently sintering was performed at various temperatures (1850, 1900 and 1950 °C) for 10 h to enhance densification and properties of SRBSN ceramics. It was observed that the density of the samples slightly decreased and β Si₃N₄ phase significantly increased to 87% with increasing nitridation temperature and time. The density of gas pressure sintered (GPS) samples increased with increasing sintering temperature, almost full density was measured for all the samples at the respective sintering temperature (except those samples which were given nitridation at 1500 °C for 16 h). The microstructure of SRBSN samples were characterized by bimodal microstructure with equiaxed and rod like elongated grains and average grain size of SRBSN samples varied between 1.62 and 2.43 μm and aspect ratio of grains varied from 3.78 to 6.88 with varying the sintering temperature. Depending on the sintering density and microstructure, the SRBSN samples exhibited hardness (16.69 to 19.47 GPa), fracture toughness (7.02 to 9.20 MPa·m^1/2) and thermal conductivity (77.32 to 98.52 W/m·K). The coarsening of grain size and aspect ratio negatively affected hardness and fracture toughness, on the contrary the thermal conductivity increased. Among all samples, the SRBSN (which was subjected to nitridation at 1500 °C for 16 h; GPS at 1950 °C for 10 h) measured with good combination of hardness: 17.32 GPa, fracture toughness: 8.36 MPa·m^1/2and thermal conductivity: 98.52 W/m·K.     

Effect of Fe3O4, Fe and Cu doping on magnetic properties and behaviour in physiological solution of biological hydroxyapatite/glass composites

The effect of Fe, Fe₃O₄ and Cu additives on magnetic properties and behaviour in physiological solution of glass-reinforced biological hydroxyapatite composites has been investigated. 1 wt% of additive was admixed before the final sintering of composites at 500 or 780 °C. The relative green and sintered densities of the samples were analyzed for the influence of additives and sintering temperature on the composite properties. The morphology of composites exhibited a porous structure with a pore size of 0.1–200.0 μm for OK 015 and 0.2–600.0 μm for OK 6 samples. It has been found that the magnetic properties of the doped composites depend on their compositions, the nature of additive and sintering conditions. The influence of the additive phases on the degradation of the composites in physiological solution was studied in vitro. It has been shown that Fe₃O₄-doped OK 015, which has ferromagnetic properties, can be the most suitable material for targeted delivery of drugs.     

Structural characterization and electrochemical behavior of a laser-sintered porous Ti–10Mo alloy

The structural and electrochemical properties of a selective laser sintered porous Ti–10Mo alloy were investigated. As the sintering temperature increases from 1100 to 1400 °C, the porosity and pore size decrease from 63% to 28% and from 178 to 56 μm, respectively. The alloy comprises major α and minor β phases at room temperature. With the decrease of porosity, the corrosion potential shifts towards positive direction and corrosion current density reduces, while the passive current density decreases and passivation range widens. The lower porosity, smaller pore size and slightly increased β phase fraction cause a superior corrosion resistance.     

Nanoindentation of porous bulk and thin films of La0.6Sr0.4Co0.2Fe0.8O3−δ

In this paper we show how reliable measurements on porous ceramic films can be made by appropriate nanoindentation experiments and analysis. Room-temperature mechanical properties of the mixed-conducting perovskite material La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF6428) were investigated by nanoindentation of porous bulk samples and porous films sintered at temperatures from 900 to 1200 °C. A spherical indenter was used so that the contact area was much greater than the scale of the porous microstructure. The elastic modulus of the bulk samples was found to increase from 33.8 to 174.3 GPa and hardness from 0.64 to 5.32 GPa as the porosity decreased from 45% to 5% after sintering at 900–1200 °C. Densification under the indenter was found to have little influence on the measured elastic modulus. The residual porosity in the “dense” sample was found to account for the discrepancy between the elastic moduli measured by indentation and by impulse excitation. Crack-free LSCF6428 films of acceptable surface roughness for indentation were also prepared by sintering at 900–1200 °C. Reliable measurements of the true properties of the films were obtained by data extrapolation provided that the ratio of indentation depth to film thickness was in the range 0.1–0.2. The elastic moduli of the films and bulk materials were approximately equal for a given porosity. The 3-D microstructures of films before and after indentation were characterized using focused ion beam/scanning electron microscopy tomography. Finite-element modelling of the elastic deformation of the actual microstructures showed excellent agreement with the nanoindentation results.     

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.     

Effect of high-temperature aging on the thermal conductivity of nanocrystalline tetragonal yttria-stabilized zirconia

The thermal conductivity of yttria-stabilized zirconia (YSZ) thermal barrier coatings increases with high-temperature aging. This common observation has been attributed to the densification of the coatings as porosity sinters out and pores and cracks spheroidize to minimize their surface energy. We show that the thermal conductivity of fully-dense 3 mol.% Y₂O₃ stabilized zirconia (3YSZ) also increases with high-temperature aging, indicating that densification and pore shape changes alone are not responsible for all the observed increase in thermal conductivity of coatings. Instead, there are also increases due to a combination of phase separation and grain growth. The increase in thermal conductivity can be described by a Larson–Miller parameter. It is also found that the increase in thermal conductivity with aging is greatest when measured at room temperature and decreases with increasing measurement temperature. Measured at 1000 °C, the thermal conductivity of zirconia is almost temperature independent and the changes in thermal conductivity with aging are less than 15%, even after aging for 50 h at 1400 °C.     

Influence of SPS parameters on the density and mechanical properties of sintered Ti3SiC2 powders

The densification of Ti₃SiC₂ MAX phase was performed by the Spark Plasma Sintering (SPS) technique. The SPS parameters, such as sintering temperature, pressure and soaking time, were optimized to obtain fully densified samples which were characterized to obtain the best mechanical properties. The sintering temperature was varied from 1070 to 1300 °C, the soaking time from 1 to 10 min and the applied pressure from 60 to 180 MPa. The best full densified samples were sintered at 1300 °C applying 60 MPa for 7 min. Ti_xC_y and TiSi₂ secondary phases were found in samples densified at 1200, 1250 and 1300 °C, due to decomposition of Ti₃SiC₂. These secondary phases, detected by XRD patterns, were confirmed by microhardness testing, FESEM observations and EDAX analyses.     

Microstructural evolution during high-temperature oxidation of Ti2AlC ceramics

Microstructural development during high-temperature oxidation of Ti₂AlC below 1300 °C involves gradual formation of an outer discontinuous TiO₂ layer and an inner dense and continuous α-Al₂O₃ layer. After heating at 1400 °C, an outer layer of mixed TiO₂ and Al₂TiO₅ phases and a cracked α-Al₂O₃ inner layer were formed. After heating to 1200 °C and cooling to room temperature, two types of planar defect were identified in surface TiO₂ grains: twins with (2 0 0) twin planes, and stacking faults bounded by partial dislocations. Formation of planar defects released the thermal stresses that had generated in TiO₂ grains due to thermal expansion mismatch of the phases (TiO₂, α-Al₂O₃ and Al₂TiO₅) in the oxide scale. After heating to 1400 °C and cooling to room temperature, crack propagation in TiO₂ grains resulted from the thermal expansion mismatch of the phases in the oxide scale, the high anisotropy of thermal expansion in Al₂TiO₅ and the volume changes associated with the reactions during Ti₂AlC oxidation. An atomistic oxidation mechanism is proposed, in which the growth of oxide scale is caused by inward diffusion of O^2? and outward diffusion of Al³⁺ and Ti⁴⁺. The weakly bound Al leaves the Al atom plane in the layered structure of Ti₂AlC, and diffuses outward to form a protective inner α-Al₂O₃ layer between 1100 and 1300 °C. However, the α-Al₂O₃ layer becomes cracked at 1400 °C, providing channels for rapid ingress of oxygen to the body, leading to severe oxidation.     

Fabrication of W–20 wt.%Cu alloys by powder injection molding

W–20 wt.% Cu balls were fabricated by powder injection molding using a binder system consisted of paraffin wax, high density polyethylene, ethylene vinyl acetate and stearic acid. By optimizing the injection molding parameters, defect-free green parts were obtained. A two-step debinding process was employed to extract the binders in the molded samples. All soluble ingredients of the binders in the green parts were extracted during solvent debinding, and the residual binders can be removed in thermal debinding. The debound W–Cu samples were sintered in H₂ atmosphere at temperatures ranging 1050–1150 °C for 2 h. It was shown that relative density of the sintered W–Cu samples increases from 87.37% of the theoretical to 95.58% as sintering temperature rises from 1050 °C to 1150 °C. Microstructures of the molded, the debound and the sintered W–Cu samples were observed by scanning electron microscope, and the sintered W–Cu balls have fine and homogeneous microstructures. Maximum compressive strength of W–Cu balls with 8.5 mm diameter reaches 58 kN.