Titanium foam through powder metallurgy route using acicular urea particles as space holder

In the present work, Ti foam has been synthesized employing powder metallurgy route. Irregular titanium powder particles were used as the matrix and acicular urea particles as the space holder. The distribution of the urea particles in the matrix of the compacted mass was observed to be fairly uniform. Pore morphology and compressive behavior of the resulting foam have been studied. The processed foam consisted of acicular porous regions of size up to 500 μm. The porous regions contained a large number of micro-pores along with the occasional presence of coarse pores, the latter thought to be unhealed portions of the original acicular pores. The foam delineated a distinct plateau region with plateau stress of 275 MPa and energy absorption capacity of 55 MJ/m³.     

Synthesis of aligned porous poly(ε-caprolactone) (PCL)/hydroxyapatite (HA) composite microspheres

We synthesized poly(ε-caprolactone) (PCL)/hydroxyapatite (HA) composite microspheres with an aligned porous structure and evaluated their potential applications in bone tissue engineering. A range of HA particles (0, 5, 10 and 20 wt.% in relation to the PCL polymer) were added to a PCL solution in order to improve the biocompatibility of the porous PCL/HA composite microspheres. All the synthesized microspheres showed that the HA particles were distributed well in the PCL matrix, while preserving their aligned porous structure. The average size of the PCL/HA composite microspheres increased from 62 ± 7 to 179 ± 95 μm with increasing HA content from 0 to 20 wt.%. The incorporation of the HA particles to the PCL polymer led to a considerable improvement in in vitro bioactivity, which was assessed by immersing the PCL/HA composite microspheres in simulated body fluid (SBF). A number of apatite crystals could be precipitated on the surface of the aligned porous PCL/HA composite microspheres after soaking in the SBF for 7 days.     

Preparation and properties of porous Ti–10Mo alloy by selective laser sintering

In this study, porous Ti–10Mo alloy was prepared from a mixture of titanium, molybdenum and epoxy resin powders by selective laser sintering preforming, debinding and sintering at 1200 °C under a pure argon atmosphere. The influence of sintering process on the porous, microstructural and mechanical properties of the porous alloy was discussed. The results indicate that the pore characteristic parameters and mechanical properties mainly depend on the holding time at 1200 °C, except that the maximum strain keeps at about 45%. The matrix microstructure is dominated by α phase with a small quantity of β phase at room temperature. As the holding time lengthens from 2 to 6 h, the average pore size and the porosity decrease from 180 to 50 μm and from 70 to 40%, respectively. Meanwhile, the Young's modulus and the compressive yield strength increase in the ranges of 10–20 GPa and 180–260 MPa, respectively. Both the porous structure and the mechanical properties of the porous Ti–10Mo alloy can be adjusted to match with those of natural bone.     

Characterization of Ti-HA composite fabricated by mechanical alloying

Titanium (Ti) and its alloys possess suitable mechanical characteristics for utilization in orthopedic implants. However, their poor integrity with native tissues is a major challenge in their clinical application. Composite structures of Ti and hydroxyapatite (HA) can be used to promote the bone ingrowth and integration of the implant with the surrounding tissue. Here, we report the fabrication of Ti-HA nanocomposite powders using a high energy planetary ball mill. We investigate the effects of fabrication parameters including HA content (10–30% w/w), milling time (20 and 50 h), and HA particle size (50 nm and 15 μm) on the characteristics of the fabricated composites. In particular, we determine the samples hardness, sintering density, surface roughness and topography for different conditions. The results show that the addition of HA to Ti decreases the sintering density and enhances the surface hardness. Also, we observe a direct relationship between HA concentration in the Ti matrix and the surface roughness.     

Fabrication of porous titanium scaffold with controlled porous structure and net-shape using magnesium as spacer

This paper reports a new approach to fabricating biocompatible porous titanium with controlled pore structure and net-shape. The method is based on using sacrificial Mg particles as space holders to produce compacts that are mechanically stable and machinable. Using magnesium granules and Ti powder, Ti/Mg compacts with transverse rupture strength (~ 85 MPa) sufficient for machining were fabricated by warm compaction, and a complex-shape Ti scaffold was eventually produced by removal of Mg granules from the net-shape compact. The pores with the average size of 132–262 μm were well distributed and interconnected. Due to anisotropy and alignment of the pores the compressive strength varied with the direction of compression. In the case of pores aligned with the direction of compression, the compressive strength values (59–280 MPa) high enough for applications in load bearing implants were achieved. To verify the possibility of controlled net-shape, conventional machining process was performed on Ti/Mg compact. Compact with screw shape and porous Ti scaffold with hemispherical cup shape were fabricated by the results. Finally, it was demonstrated by cell tests using MC3T3-E1 cell line that the porous Ti scaffolds fabricated by this technique are biocompatible.     

Studies on the processing of nickel base porous wicks for capillary pumped loop for thermal management of spacecrafts

Nickel base sintered porous wicks have potential application as capillary structure in two-phase heat transfer loops of a heat dissipation system, like capillary pumped loop (CPL) and loop heat pipe (LHP). A porous wick is located inside the evaporator of the CPL system and it transports working fluid in the loop by capillary action.In the present work, experimental trials were carried out to achieve porous wick having high porosity with interconnected pores of average size less than 5 μm, higher aspect ratio (L/D >10) and permeability better than 10 m-Darcy (10^?14 m²). A carbonyl nickel powder (2–7 μm) was used as raw material. Loose carbonyl nickel powders (2–7 μm) were sintered in graphite mould under hydrogen atmosphere at different temperatures in order to optimize porosity, pore size and permeability of the sintered wick. For mechanical and physical properties characterization, samples were cut from sintered rod using EDM wire cut to avoid pore closure. Profile making on the sintered rod is also done by wire EDM. Microstructural characterization as well as the effect of W-EDM on the surface pores was done using Scanning Electron Microscopy (SEM). Surface profile making through W-EDM had shown encouraging result. After optimization of process parameters cylindrical wick (L/D ratio: 10) with porosity of 64%, average pore size of 5 μm and a permeability of 70 m-Darcy could be realized.The present paper explain the details of processing of cylindrical shaped porous wicks through sintering technique and effect of EDM on surface pores characteristic.     

Dynamic freeze casting for the production of porous titanium (Ti) scaffolds

This paper proposes dynamic freeze casting as a new manufacturing technique for producing porous Ti scaffolds with a uniform porous structure and good ductility. In this method, Ti/camphene slurries with various initial Ti contents (15, 20, and 25 vol.%) were frozen at 44 °C for 12 h in rotation, which allowed for the extensive growth of camphene crystals and the uniform construction of walls made of Ti particles. All the fabricated samples showed spherical-like pores surrounded by dense Ti walls that were uniformly formed after sintering at 1300 °C for 2 h in a vacuum. The porosity decreased from 71 to 52 vol.% with an increase in Ti content from 15 to 25 vol.%, whereas the pore size decreased from 362 to 95 μm. On the other hand, the compressive strength and stiffness increased considerably from 57 ± 4 to 183 ± 6 MPa and from 1.3 ± 0.5 to 5.0 ± 0.8 GPa, respectively, due to the decrease in the porosity of the samples.     

The effect of nano-sized stainless steel powder addition on mechanical and physical properties of micropowder injection molded part

Micropowder injection molding (μPIM) is a new technology that has potential in the mass production of microcomponents. A bulk material of nanoparticles possesses completely different properties from those of large-sized particles. The main objective of this study is to study the effects of nano-sized powder addition on the μPIM process of powder-polymer mixtures for the fabrication of miniature parts. The binder systems consist of polyethylene glycol (PEG), polymethyl methacrylate (PMMA), and stearic acid (SA) with different powder loading blended with powders. The results indicate that increasing the nanopowder content to 30 wt.% increased the powder loading and decreased the injection and sintering temperatures. The sintered parts had densities of 96% of the theoretical value. High physical and mechanical properties of the sintered specimen were achieved with the 30 wt.% nano-sized powder sintered at 1200 °C at a heating rate of 5 °C/min under vacuum atmosphere. A significant reduction of the surface roughness of the sintered parts using the nano–microhybrid powder (S_a = 0.365 μm) was observed compared with the sintered parts with only micropowder (S_a = 1.002 μm). Using nanopowders, the hardness also increased from 182 HV to 221 HV with a linear shrinkage of approximately 9%, which is less than that of the micropowders (18%).     

Effect of grain size on mechanical,surface and biological properties of microwave sintered hydroxyapatite

Hydroxyapatite (HA) compacts having average grain sizes of 168 ± 0.086 nm, 1.48 ± 0.627 μm and 5.01 ± 1.02 μm are processed from synthesized HA powder by microwave sintering at varying sintering temperature for different times. Superior mechanical and biological properties are shown by nano-grain HA compacts as compared to their micron grained counterparts. Compressive strength, indentation hardness, and indentation fracture toughness are increased with the decrease in HA grain size. The highest surface energy and maximum wettability are exhibited by nano-grain HA. HA compacts are assessed for cell–material interaction by SEM, MTT and immunochemistry assays using human osteoblast cell line for 1, 5 and 11 days. MTT assays showed higher number of living cells and faster proliferation on nano-grain HA surface. Osteoblast cells on nano-grain HA surface expressed significantly higher amount of vinculin and alkaline phosphatase (ALP) protein markers for cell adhesion and differentiation respectively. This study shows the effect of grain size on physical, mechanical and in vitro biological properties of microwave sintered HA compacts.     

Dilatometry of attrition milled nanocrystalline titanium powders

The sintering behavior of nanosized titanium powders was investigated by dilatometry. The nanosized Ti powders (40 nm) were produced by the attrition milling of micron sized Ti powders (12 μm) in Ar atmosphere. Sintering was carried out in Ar atmosphere in the temperature range of 450–1250 °C for nanosized Ti and 650–1250 °C for micron sized Ti by heating at 10 °C/min, up to the sintering temperature followed by isothermal holding for 1 h. The nanosized Ti powders exhibited a lower sintering onset temperature, larger shrinkage, larger shrinkage rate, and lower activation energy for sintering as compared to the micron sized Ti powders. The sintered micron sized Ti specimens exhibited both intraagglomerate and interagglomerate porosity while the nanosized Ti specimens exhibited well densified agglomerates (almost no interagglomerate porosity) and large intraagglomerate porosity. In nanosized Ti grain growth was found to take place beyond 700 °C and reached a maximum of 66 nm in samples sintered at 1100 °C.     

Mechanical properties of sinter-hardened Cr–Si–Ni–Mo based steel foam

Highly porous sinter-hardenable Cr–Si–Ni–Mo based steel foam for automotive applications was produced by space holder method. Steel powders were mixed with binder (polyvinylalcohol) and space holder (carbamide), and compacted. Carbamide in the green compacts was removed by water leaching at room temperature. The green specimens were then sintered at temperatures between 1100 °C and 1250 °C for sintering times of 15, 30 and 45 min. In addition, the steel foams were sinter-hardened to enhance mechanical properties. Sinter-hardening combines sintering and heat treatment in one step by increasing the post-sintering cooling rate. This reduces the cost of operation and makes powder metallurgy more competitive. Effects of sinter-hardening process parameters on compressive strength, Young’s modulus, hardness and energy absorption of the steel foams were investigated.     

Preparation of porous scaffold from hydroxyapatite powders

Hydroxyapatite (HA) powder was prepared by wet chemical method. The hydroxyapatite phase was stable up to 1250 °C without decomposition to beta-tricalcium phosphate. Interconnected porous hydroxyapatite scaffold resembling trabecular bone structure was developed from polymeric replica sponge method. The prepared scaffold has 60 vol.% porosity having a major fraction of ~ 50–125 μm pore diameter. The pore content, pore morphology, pore interconnectivity of scaffold and their compressive strength were dependent on the solid loading and binder content. In-vitro bioactivity and bioresorbability confirmed the feasibility of the developed scaffolds.     

Phase transformation in Ti–48Al–6Nb porous alloys and its influence on pore properties

Ti–48Al–6Nb porous alloys were synthesized by the powder metallurgy (PM) method, and the associated phase transformation and pore parameter were investigated in order to reveal the pore-formation mechanism. The present results indicate that the Nb–Al and Ti–Al phase transformations contribute to the pore-formation. It was found that the five-step phase transformations for the Ti–48Al–6Nb porous alloys occur as follows: (1) Ti + Al → TiAl₃ at 600–700 °C; (2) Nb + Al → NbAl₃ at 700–900 °C; (3) TiAl₃ + Ti → TiAl at 900–1100 °C; (4) TiAl + Ti → Ti₃Al/TiAl at 1100–1350 °C; (5) NbAl₃ + Nb → Nb₂Al and the Ti₃Al turns to the major phase at 1350 °C. These phase transformations made the pore-diameter increasing continuously from 1.71 μm to 12.10 μm and also made the pore volume distributing widely. At the second step of 700–900 °C, the Nb–Al phase transformation leads to 5% more volume expansion compared to the Ti–Al based porous alloys. Meanwhile, the porosity and total pore area initially increase and then decrease at this step, but they increase intensely at the final step, which is needed as a catalytic carrier.     

Processing and mechanical properties of porous Ti–7.5Mo alloy

Titanium (Ti) and its alloys continue to be utilized extensively for skeletal repair and dental implants. Most metallic implant materials including pure Ti and Ti alloys used today are in their solid forms and are often much stiffer than human bone. However, the elastic modulus of Ti and Ti alloys can be reduced through the introduction of a porous structure, which may also provide new bone tissue integration and vascularization abilities. In the present study, porous Ti–7.5Mo alloy scaffolds made from ball-milled alloy particles and sintered at 1100 °C for 10, 15 and 20 h respectively were successfully prepared through a space-holder sintering method. In the sintered Ti–7.5Mo, no obvious diffraction peaks of elemental Mo remained after the sintering, and a duplex α + β microstructure was confirmed from the XRD pattern. The samples made from BM15 (the alloy particles ball-milled for 15 h) had higher relative density, compressive strength and elastic modulus performance than those from BM3 and BM30 (the alloy particles ball-milled for 3 and 30 h, respectively) when they were sintered under the same conditions. Moreover, the longer sintering time lead to the higher relative density and the greater compressive strength and modulus of the sample. In this work, the strength and modulus of the sintered porous Ti–7.5Mo conforms to the basic mechanical property requirement of cancellous bones.     

Novel,simple and reproducible method for preparation of composite hierarchal porous structure scaffolds

Demand to develop a simple and adaptable method for preparation the hierarchical porous scaffolds for bone tissue regeneration is ever increasing. This study presents a novel and reproducible method for preparing the scaffolds with pores structure spanning from nano, micro to macro scale. A macroporous Sr-Hardystonite (Sr–Ca₂ZnSi₂O₇, Sr–HT) scaffold with the average pore size of ~ 1200 μm and porosity of ~ 95% was prepared using polymer sponge method. The struts of the scaffold were coated with a viscous paste consisted of salt (NaCl) particles and polycaprolactone (PCL) to provide a layer with thickness of ~ 300–800 μm. A hierarchical porous scaffold was obtained with macro, micro and nanopores in the range of 400–900 μm, 1–120 μm and 40–290 nm, after salt leaching process. These scales could be easily adjusted based on the starting foam physical characteristics, salt particle size, viscosity of the paste and salt/PCL weight ratio.     

Study on the genotoxicity of HA/ZrO2 composite particles in vitro

To evaluate the genotoxicity of the HA/ZrO₂ composite particles by using the micronucleus test (MNT) in vitro. HA/ZrO₂ composite particles prepared by sintering at high temperature and pressure, that used powder of HA and ZrO₂ of different proportions, were compared with pure HA particles and pure ZrO₂ particles. The effect of the composite particles on cell proliferation of rabbit mesenchymal stem cells, and its the genotoxicity to rabbit mesenchymal stem cells were detected by MNT method. The MTT test showed that both pure HA particles and composite particles which contained HA promoted cell proliferation of rabbit mesenchymal stem cells, while pure ZrO₂ particles did not, and there was a significant difference (P < 0.05). The MNT test showed no significant difference between the HA group and the negative control group (P > 0.05), but a significant difference between the HA group and the positive control group (P < 0.05). The difference between the ZrO₂ group and the negative control group was significant (P < 0.01), while the difference between the ZrO₂ group and the positive control group was insignificant (P > 0.05). The genotoxicity of the HA/ZrO₂ composite particle increased with a higher proportion of ZrO₂ and an increase in the concentration of the composite, and the 30 wt.% HA/70% ZrO₂ composite with 200 μg/mL concentration showed significant genotoxicity (P < 0.01).     

Synthesis of porous titanium implants by environmental-electro-discharge-sintering process

Porous Ti implants with various porosities were first fabricated by environmental-electro-discharging-sintering (EEDS) of atomized spherical Ti powders. Powders in two size range (50–100 and 200–250 μm) were settled by vibration into a quarts tube and subjected to a high voltage and high density current pulse. A single pulse of 0.75–2.0 kJ/0.7 g-powder, using 150, 300 and 450 μF capacitors, was applied to produce fully porous and porous-surfaced Ti implant compacts. The solid core was automatically formed in the center of the compact after discharge and porous layer consisted of particles connected in three dimensions by necks. The solid core and neck sizes increased with an increase in input energy and capacitance. On the other hand, pore volume decreased with increased capacitance and input energy due to the formation of a solid core. Capacitance and input energy are the only controllable discharge parameters even though the heat generated during a discharge is the unique parameter that determines the porosity of compact. It was shown that EEDS of spherical Ti powders can efficiently produce fully porous and porous-surfaced Ti implants with various porosities in short times (<400 μs) by manipulating the discharging conditions such as input energy and capacitance including powder size.     

Electrophoretic bilayer deposition of zirconia and reinforced bioglass system on Ti6Al4V for implant applications: An in vitro investigation

The physical, chemical and biological properties of the bioglass reinforced yttria-stabilized composite layer on Ti6Al4V titanium substrates were investigated. The Ti6Al4V substrate was deposited with yttria stabilized zirconia — YSZ as the base layer of thickness ≈ 4–5 μm, to inhibit metal ion leach out from the substrate and bioglass zirconia reinforced composite as the second layer of thickness ≈ 15 μm, which would react with surrounding bone tissue to enhance bone formation and implant fixation. The deposition of these two layers on the substrate was carried out using the most viable electrophoretic deposition (EPD) technique. Biocompatible yttria-stabilized zirconia (YSZ) in the form of nano-particles and sol gel derived bioglass in the form of micro-particles were chosen as precursors for coating. The coatings were vacuum sintered at 900 °C for 3 h. The biocompatibility and corrosion resistance property were studied in osteoblast cell culture and in simulated body fluid (SBF) respectively. Analysis showed that the zirconia reinforced bioglass bilayer system promoted significant bioactivity, and it exhibited a better corrosion resistance property and elevated mechanical strength under load bearing conditions in comparison with the monolayer YSZ coating on Ti6Al4V implant surface.     

Martensitic transformation behaviors of rapidly solidified Ti–Ni–Mo powders

For the fabrication of bulk near-net-shape shape memory alloys and porous metallic biomaterials, consolidation of Ti–Ni–Mo alloy powders is more useful than that of elemental powders of Ti, Ni and Mo. Ti₅₀Ni_49.9Mo_0.1 shape memory alloy powders were prepared by gas atomization, and transformation temperatures and microstructures of those powders were investigated as a function of powder size. XRD analysis showed that the B2–R–B19 martensitic transformation occurred in powders smaller than 150 μm. According to DSC analysis of the as-atomized powders, the B2–R transformation temperature (T_R) of the 25–50 μm powders was 18.4 °C. The T_R decreased with increasing powder size, however, the difference in T_R between 25–50 μm powders and 100–150 μm powders is only 1 °C. Evaluation of powder microstructures was based on SEM examination of the surface and the polished and etched powder cross sections and the typical images of the rapidly solidified powders showed cellular morphology. Porous cylindrical foams of 10 mm diameter and 1.5 mm length were fabricated by spark plasma sintering (SPS) at 800 °C and 5 MPa. Finally these porous TiNi alloy samples are heat-treated for 1 h at 850 °C, and then quenched in ice water. The bulk samples have 23% porosity and 4.6 g/cm³ density and their T_R is 17.8 °C.     

Effects of SiC interphase by chemical vapor deposition on the properties of C/ZrC composite prepared via precursor infiltration and pyrolysis route

Silicon carbide (SiC) interphase was introduced by chemical vapor deposition (CVD) process to prevent carbon fiber degradation and improve fiber–matrix interface bonding of C/ZrC composite prepared via precursor infiltration and pyrolysis (PIP) process. Moderate thickness of SiC interphase in fiber bundles could increase the density of the composite, but when the thickness of SiC interphase was over 0.5 μm, more close pores formed and the density of the composite decreased. The SiC interphase could protect carbon fiber effectively from carbo-thermal reduction, but could not enhance the mechanical properties of C/ZrC composite. The flexural strength and fracture toughness of C/ZrC composites with 0.05 μm thickness SiC layer were 252 MPa and 13.6 MPa m^1/2, and for those with 0.5 μm thickness SiC layer 240 MPa and 12.8 MPa m^1/2, both close to the value of the composite without SiC interphase (254 MPa and 14.5 MPa m^1/2), while those with 0.7 μm thickness SiC layer were only 191 MPa and 10.8 MPa m^1/2, respectively. Moderate content of SiC interphase could improve the ablation property of C/ZrC composites; however excessive content of SiC interphase would decrease the ablation property.