Effect of porosity and density on the mechanical and microstructural properties of sintered 316L stainless steel implant materials

In this study, the microstructure and mechanical properties of sintered AISI 316L stainless steel implant materials produced by powder metallurgy (P/M) method were investigated as a function of porosity amount. AISI 316L stainless steel powders were cold-pressed with 800 MPa pressure and sintered at 1200 °C, 1250 °C and 1300 °C for 30 min in a nitrogen atmosphere. The mechanical properties of the 316L implant samples were determined by tensile, fatigue and microhardness tests. Metallographic studies such as pore formation, and fractured surface analyses were performed by Scanning Electron Microscopy (SEM) and Light Optical Microscopy (LOM). The results of this study indicate that, irregular pore formation tendencies increase with an increase in porosity (%). Furthermore, an increase in porosity was shown to decrease the mechanical properties of sintered AISI 316L stainless steel. Sintering temperature is important parameter in decreasing the porosity of P/M materials.     

Mechanical properties of hydroxyapatite–zirconia compacts sintered by two different sintering methods

Microwave sintering is traditionally employed to reduce the sintering temperature required to densify powder compacts. The effect of microwave heating on hydroxyapatite (HA)–zirconia (ZrO₂) green bodies has been investigated in order to understand how microwave energy may affect the physical and mechanical properties of the resultant densified composites. Laboratory synthesised nano-sized HA and a commercial nano-sized ZrO₂ powder have been ball milled to create mixtures containing 0–5 wt% ZrO₂ loadings. Compacts were microwave sintered at either 700, 1000 or 1200°C with a 1 h hold time. Comparative firings were also performed in a resistive element furnace using the same heating profile in order to assess the differences between conventional and microwave heating on the physical, mechanical and microstructural properties of the composites. Samples sintered at 700°C show little sign of densification with open porosities of approximately 50%. Composites conventionally sintered at 1000°C were between 65 and 75% dense, whereas the samples microwave sintered at this temperature were between 55 and 65% dense. Samples sintered at 1200°C showed the greatest degree of densification (>80%) with a corresponding reduction in open porosities. TCP generation occurred as a consequence of sintering at 1200°C, even with 0 wt% ZrO₂, and increased degradation of the HA phase to form significant amounts of TCP occurred with increasing additions of ZrO₂, along with increasing open porosity. Nanosized ZrO₂ prevents the densification of the HA matrix by effectively pinning grain boundaries and this effect is more pronounced in the MS materials. Similar strengths are achieved between the microwave and conventionally sintered samples. Greater amount of open porosity and pore interconnectivity are seen in the MS samples, which are considered to be useful for biomedical applications as they can promote osteo-integration.     

Preparation and sintering properties in air of silver-coated copper powders and pastes

In this paper, uniform and well dispersed silver-coated copper powders were prepared by replacing reaction at first. The structure and properties of the bimetallic powders were investigated by XRD, SEM and TGA. It was found that anti-oxidization of the silver-coated copper powders increased with the increase of the silver content slightly and a dense coating surface was observed at Ag content of 53.9 wt%. Furthermore, the pastes with relatively low silver content which were prepared from silver-coated copper powders, displayed high conductivity similar to pure silver even after sintering in air. And their sintering properties were also investigated at different temperature in air atmosphere. The film exhibits good electrical properties at sintering temperature between 800 and 900 °C. When the paste from silver-coated copper powder with Ag content of 53.9 wt% was printed on Al₂O₃ substrate and sintered at 800 °C in air, the sheet resistance of the film is 0.036 Ω/□ only.     

Nanostructured and near defect-free ceramics by low-temperature pressureless sintering of nanosized Y-TZP powders

Nanosized (∼6 nm) Y-TZP (3 mol% Y₂O₃) powders have been produced by chemical co-precipitation (Y-inorganic + Zr-organic precursors) and thorough isopropanol-washing step, after calcining in air at 450 °C. The nanocrystalline Y-TZP powders consisted of spherical soft agglomerates (∼100 nm in size) which were easily broken down during compaction resulting in a very uniform green microstructure with a narrow pore size distribution (average pore size less than 6.5 nm) and no detectable compacting defects. In spite of the relatively low green density (43% theoretical), Y-TZP powder compacts sintered to near theoretical density in the very low-temperature range of 1000 °C for 80–100 h to 1070 °C for 2 h, maintaining a grain size in the nanoscale (< 100 nm) and the sintered bodies were nearly defect-free. Hardly any grain growth took place up to 1000 °C; it was very rapid above this temperature. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of sintering parameters on the microstructure and properties of strontium modified PZT ceramics prepared using spray-dried powders

PZT powders of the composition Pb_0.94Sr_0.06 (Zr_0.53Ti_0.47)O₃, prepared by spray drying and calcining techniques, were processed to sintered ceramics by conventional cold pressing and sintering at various temperatures and periods between 1000 to 1250°C for 0.5 to 12h. Sintered ceramics were evaluated for their microstructure and electromechanical properties. Highly dense ceramics having bulk density of the order of 97% of the theoretical value could be obtained after sintering at a considerably lower temperature of 1000°C in comparison to the 1300°C generally required for powders prepared by conventional ceramic processing. However, the increase in sintering temperature of reactive spray-dried powders causes the entrapment of closed pores as a result of exaggerated grain growth and subsequent pore coarsening thereby leading to a decrease in the bulk density of the ceramics. It has been observed that minor variations in the sintering parameters influence the porosity, grain size and electromechanical properties. Values of the dielectric constant, piezoelectric strain coefficient and electromechanic coupling factor increase with the increase in grain size and decrease with the increase in porosity of the sintered ceramic whereas the dielectric dissipation factor decreases with the increase in sintering temperature.     

Processing and electrochemical characterization of Ni2Si intermetallic compound produced by vacuum sintering

In this study, a powder metallurgy approach for fabrication of Ni₂Si intermetallic compound was utilized. In this regard, mechanically activated Ni-33 at.% Si powders were used as feedstock. The powders were investigated by differential scanning calorimetry to study phase transformations that occurred during heating in the calorimeter. It was shown that the milled powders reacted after heating up to 850 °C. In order to fabricate bulk Ni-33 at.% Si, the as-milled powders were cold-pressed and subsequently sintered at 900 °C for 1 h in a vacuum furnace. X-ray diffraction results indicated that the phase composition of sintered materials consisted of only δ-Ni₂Si intermetallic. Consolidated samples exhibit 12% porosity and microhardness up to 773 HV_0.05. Further investigations showed the corrosion rate of sintered compound is higher than that of Ni–Si alloys reported in the literature. Here, anodic dissolution near the surface pores seems to be corrosion mechanism which resulted in increase in surface porosity up to 27% after corrosion.     

Enhanced pyroelectric properties of lead free KNN/P(VDF-TrFE) composite film by optimizing KNN sintering temperature

The effect of potassium-sodium niobate (KNN) powder sintering temperature on the structure and properties of the KNN/{poly(vinylidenefluoride-co-trifluoroethylene 70:30) [P(VDF-TrFE) 70:30]} composite thick films have been studied in this paper. KNN powders were sintered by solid-state reaction at different temperatures ranging from 750 to 900 °C. Then the KNN powders were used to fabricate composite thick films by casting the KNN/P(VDF-TrFE) suspension on to ITO substrates. The pyroelectric and dielectric properties of the composite thick films have been investigated systematically. It was found that sample made up of KNN ceramic powders sintered at 850 °C show optimal properties for pyroelectric appliance. The highest pyroelectric coefficient was 63 μCm^?2/K and the highest detectivity figure-of-merit was 4.94 μPa^1/2.     

Fabrication and material properties of powder injection molded Fe sintered bodies using nano Fe powder

Injection molded Fe sintered bodies were fabricated using two kinds of Fe powders which have an average size of 50 nm and a diameter of 3 μm, respectively. Using Fe powder of about 50 nm in diameter, comparatively dense bodies (94–97%) were obtained even at a low sintering temperature (600 °C–700 °C), whereas the Fe sintered body (1100 °C) using about 3 μm Fe powder showed relatively low density (about 93%). In the sintered bodies using 50 nm Fe powders, the grain size increased as the sintering temperature increased, but the values of hardness decreased. In the sample sintered at 650 °C, the values of relative density and grain size were 96% and 0.97 μm, respectively. The minimum value of wear depth was obtained due to the combination of fine grain size and comparatively high relative density.     

Study of the sintering properties of plasma synthesized ultrafine SiC powders

A study on the sintering of ultrafine SiC powders synthesized from elemental Si and CH₄ using radio frequency (r.f.) induction plasma technology is reported. The powder had a particle size in the range of 40 to 80 nm and was composed of a mixture of α and β-SiC. It was subjected to pressureless sintering in an induction furnace in the presence of different sintering aids. With the addition of B₄C (2.0 wt% B) by mechanical mixing, the powders could only be partially densified, with the highest value of 84.5% of theoretical density being achieved at 2170 °C for 30 min. Through the use of “in-flight” boron doping of the powder during the plasma synthesis step (1.65 wt % B), the ultrafine powder obtained could be densified to above 90% of its theoretical density at 2050 °C for 30 min. The addition of oxide sintering aids (7.0 wt % Al₂O₃ + 3.0 wt % Y₂O₃) by mehanical mixing produced sintered pellets of 95% of theoretical density at 2000 °C for 75 min. The Vicker’s microhardness of the sintered pellets in this case was as high as 31.2 GPa. In order to improve our understanding of the basic phenomena involved, extensive microstructural (scanning electron energy microscopy: SEM), physical (shrinkage, weight loss, porosity, hardness) as well as chemical analysis (prompt gamma neutron activation analysis (PGNAA), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA)) was carried out. This helped establish a relationship between the properties of the as-synthesized powder and their sintering properties. The influences of sintering temperature, sintering time, additive concentration, and powder purity on the densification behaviour of the plasma-synthesized powders was investigated. The results were compared with data obtained using commercial powder. This revised version was published online in November 2006 with corrections to the Cover Date.     

Nanocrystalline hydroxyapatite doped with magnesium and zinc: Synthesis and characterization

During recent years, there have been efforts in developing nanocrystalline bioceramics, to enhance their mechanical and biological properties for use in tissue engineering applications. In this research, we made an attempt to synthesize nanocrystalline bioactive hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAp) ceramic powder in the lower-end of nano-range (2–10 nm), using a simple low-temperature sol–gel technique and studied its densification behavior. We further studied the effects of metal ion dopants during synthesis on powder morphology, and the properties of the sintered structures. Calcium nitrate and triethyl phosphite were used as precursors for calcium and phosphorous, respectively, for sol–gel synthesis. Calculated quantities of magnesium oxide and zinc oxide were incorporated as dopants into amorphous dried powder, prior to calcination at 250–550 °C. The synthesized powders were analyzed for their phases using X-ray diffraction technique and characterized for powder morphology and particle size using transmission electron microscopy (TEM). TEM analysis showed that the average particle size of the synthesized powders were in the range of 2–10 nm. The synthesized nano-powders were uniaxially compacted and then sintered at 1250 °C and 1300 °C for 6 h, separately, in air. A maximum average sintered density of 3.29 g/cm³ was achieved in structures sintered at 1300 °C, developed from nano-powder doped with magnesium. Vickers hardness testing was performed to determine the hardness of the sintered structures. Uniaxial compression tests were performed to evaluate the mechanical properties. Bioactivity and biodegradation behavior of the sintered structures were assessed in simulated body fluid (SBF) and maintained in a dynamic state.     

Preparation and characterization of NASICON with a new sol–gel process

NASICON powders with the composition of Na₃Zr₂Si₂PO₁₂ were synthesized by using a sol–gel method. In the course of synthesis, a different material of oxalic acid was used to modify the synthesis process. The resulted precursors were sintered at the temperatures ranging from 700 to 1000 °C to get NASIOCN powders. X-ray diffractometer (XRD), IR and Raman spectra were employed to characterize the sintered products. Also, the ionic conductivity measurement conducted in the temperatures of 150–300 °C was used to evaluate their electronic properties. Furthermore, CO₂ sensor was prepared based on the pressed NASICON bulk. The relationship between its EMF response and the target gas concentration was checked. The experiment results showed that the NASICON material sintered at 900 °C possessed better properties in comparison with those sintered at other temperatures.     

Magnetic and microwave-absorbing properties of SrAl<Subscript>4</Subscript>Fe<Subscript>8</Subscript>O<Subscript>19</Subscript> powders synthesized by coprecipitation and citric- combustion methods

Al-substituted M-type hexaferrite is a highly anisotropic ferromagnetic material. In the present study, the coprecipitation and the citric-combustion methods of synthesis for SrAl₄Fe₈O₁₉ powders were explored and their microstructure, magnetic properties, and microwave absorptivity examined. X-ray diffraction (XRD), scanning electron microscopy (SEM), a vibrating sample magnetometer, and a vector network analyser were used to characterize the powders. The XRD analyses indicated that the pure SrAl₄Fe₈O₁₉ powder was synthesized at 900°C and 1000°C for 3 h by coprecipitation, but only at 1000°C for the citric-combustion processes. The SEM analysis revealed that the coprecipitation process yielded a powder with a smaller particle size, near single-domain structure, uniform grain morphology, and smaller shape anisotropy than the citric-combustion process. The synthesis technique also significantly affected the magnetic properties and microwave-absorptivity. Conversely, calcining temperature and calcining time had less of an effect. The grain size was found to be a key factor affecting the property of the powder. The powders synthesized by coprecipitation method at calcining temperature of 900°C exhibited the largest magnetization, largest coercivity, and best microwave absorptivity.     

Hot-sprayed yttrium iron garnet

Hot spraying (evaporative decomposition of solution) of sulfates of yttrium and iron at 1000°C and above produced powders which could be sintered to 99% X-ray density with homogeneous micro-structure and controlled grain size. The powder obtained from the hot-spray reactor was well gelated. However, no detectable YIG phase was obtained in the powder. The only constituents were YFeO₃ and Fe₂O₃. This is attributed to the fast nucleation of YFeO₃ phase in the reactor. The calcination studies of these powders showed that the conversion to YIG phase began above 900°C. The calcined powder with more than 90% conversion to YIG phase was still very fine and reactive. It could be sintered to high density product with average grain size ranging from 2 μ upwards.     

Microstructure and mechanical properties of spark plasma sintered austenitic ODS steel

In this work, austenitic oxide dispersion strengthened (AODS) steel of composition Fe–16Cr–16Ni–1.5 W–0.21Ti–0.3Y₂O₃ (wt. %) was fabricated using two–stage ball milling followed by consolidation through spark plasma sintering (SPS). In the first–stage, mechanical alloying (MA) of ferritic powder and nano sized Y₂O₃ was carried out. This was followed by the addition of Ni in second–stage milling. SPS of the milled powder was carried out at 900, 950, 1000 and 1050 °C to explore the role of SPS temperature on density, microstructure as well as mechanical properties of the consolidated samples. A relative density of ～ 99% was obtained for samples sintered at 950 and 1000 °C. The as–sintered samples were subsequently solution annealed at 1075 °C for 2 h and water quenched. X–ray diffraction studies confirmed the presence of austenite in the consolidated and solution annealed samples. Electron back scatter diffraction analysis of solution annealed samples sintered at all the temperatures revealed a bimodal microstructure. The average grain size of 1.07 ± 0.72 µm was obtained for solution annealed samples sintered at 1000 °C. Yield strength and elongation of the same was measured as 851 MPa and 18%, respectively at room temperature. These values are the best combination of strength to elongation achieved on AODS alloys processed using MA and SPS, which makes this AODS steel much promising for high temperature applications.     

Synthesis of boron suboxide (B6O) with alkaline earth metal oxide materials with improved properties

The pure boron suboxide (B₆O) powder admixed with 1.08?vol% of MgO, CaO, and CaCO₃ (corresponding to 1.5, 1.4, and 0.7?wt%, respectively) were synthesized by hot-pressing and the mechanical properties were evaluated. Pure B₆O powders were sintered at a temperature of 1900°C, while the B₆O-alkaline earth metal oxide admixed powders were sintered at a temperature of 1850°C. All sintering was done under an applied load of 80?MPa in an argon environment. The microstructures and phase analyses were studied by scanning electron microscopy and X-ray diffraction techniques, respectively. The addition of the alkaline earth metal oxide additives resulted in the formation of boride and borate phases, depending on the oxide used. The mechanical properties of the hot-pressed materials were characterized based on their density, hardness, and fracture toughness. Between 95 and 98% of the theoretical densities were achieved and the result indicate a good combination of hardness (between 31.6 and 32.1?GPa) and fracture toughness (between 6.1 and 6.8?MPa.m^0.5) in the B₆O-alkaline earth metal oxide materials. The introduction of the additive enhances the fracture toughness of the pure B₆O-materials, while the fracture mode observed in the sintered materials is mainly transgranular.     

Synthesis of lanthanum titanate (La2Ti2O7) for high temperature sensor applications

Lanthanum titanate (La₂Ti₂O₇) with perovskite-like layered structure is a candidate material for high temperature sensor application due to its high curie temperature (T_c?=?1461 °C) and linearity of temperature vs. electrical resistance. La₂Ti₂O₇ (LTO) was synthesized by solid state reaction using constituent powders at 1250 °C for 2 h. The LTO samples prepared in the form of circular pellets were sintered in temperature ranges (1350 to 1400 °C for 2 h). The sintered density was found highest at 1400 °C for LTO samples (>?97.24% Th.). Moreover, the sintered LTO samples were characterized for their ferroelectric properties as well as DC electrical resistivity (ρ) measured in the temperature range of 100 to 900 °C. The electrical resistivity was decreased from 10¹³ to 10⁶ Ω cm linearly with the increase in temperature from 100 to 900 °C. Hence, LTO is a promising sensor material for high temperature applications.

     

Role of dispersion conditions on grindability of yttria stabilized zirconia (YSZ) powders

A precursor for zirconia - 8 mole% yttria (YSZ-ZrO₂-8 m% Y₂O₃) powder was prepared by coprecipitation and the calcination temperature was fixed as 900°C from TG-DTA and XRD studies. The calcined powder could be dry ground only to a mean particle size (D₅₀) of 6 Μm containing substantial amount of coarse agglomerates in the size range 10–100 Μm. The dispersion conditions for its wet grinding were evaluated through zeta-potential and viscosity studies. The zeta-potential variation with pH of the aqueous suspensions of the powder exhibited maximum numerical values at 3 and 11 pH, exhibiting the ideal pHs for dispersion stability through electrostatic columbic repulsion mechanism. Slurries of dry ground powders with solid concentration in the range 15–30 vol.% exhibited pseudo-plastic flow characteristics, indicating presence of flocculates. With progress of grinding, the increase in viscosity of the slurries became less significant with decreasing solid concentration. Even though the particle size of the ground slurries decreased with decreasing solid content, there was little change in it for slurries with solid content < 20 vol.%. Grinding conditions for formation of sinter-active powders of YSZ with sub-micron size (D ₅₀ ∼ 0.7 Μm free of agglomerates of size > 5 Μm) were established. Compacts from this powder could be sintered at 1400°C to translucent bodies with 99% theoretical density.     

Mechanical and microstructural properties of cordierite-bonded porous SiC ceramics processed by infiltration technique using various pore formers

Cordierite-bonded porous SiC ceramics were prepared by air sintering of cordierite sol infiltrated porous powder compacts of SiC with graphite and polymer microbeads as pore-forming agents. The effect of sintering temperature, type of pore former and its morphology on microstructure, mechanical strength, phase composition, porosity and pore size distribution pattern of porous SiC ceramics were investigated. Depending on type and size of pore former, the average pore diameter, porosities and flexural strength of the final ceramics sintered at 1400 °C varied in the range of ~ 7.6 to 10.1 µm, 34–49 vol% and 34–15 MPa, respectively. The strength–porosity relationship was explained by the minimum solid area (MSA) model. After mechanical stress was applied to the porous SiC ceramics, microstructures of fracture surface appeared without affecting dense struts of thickness ~ 2 to 10 µm showing restriction in crack propagation through interfacial zone of SiC particles. The effect of corrosion on oxide bond phases was investigated in strong acid and basic salt medium at 90 °C. The residual mechanical strength, SEM micrographs and EDX analyses were conducted on the corroded samples and explained the corrosion mechanisms.     

Sintering of grey cast iron powder recycled via jet milling

Porous grey cast iron powder metallurgy parts were made from grey cast iron powder manufactured via target jet milling of machining scraps. The powders were used in the as-milled state without any further physical or heat treatment.Sintering was conducted at 1025, 1100 and 1175 °C in an argon atmosphere and the effect of sintering temperature on microstructure, sintered density and apparent hardness of the grey cast iron specimens pressed to 5.8 g/cm³ was investigated.Although diffusion processes were partially activated at 1025 °C, it was determined that a temperature of 1175 °C proved to be the ideal temperature for solid state sintering of grey cast iron parts. The hardness value and sintered density for the specimens sintered at 1175 °C were found to be 96 BHN and 6.1 g/cm³ (around 15% porosity) respectively, all of which lends itself to promising properties for making self-lubricating bearings and parts with sliding properties.     

Powder metallurgy of the porous Ti-13Nb-13Zr alloy of different powder grain size

The objective of the present project was to determine the effects of powder granulation (fraction of grain size) for the Ti-13Nb-13Zr alloy, produced by powder metallurgy, on its porosity, grain cohesion, compressive strength, and Young`s modulus. Two powder fractions, 45–105 µm, and 106–250 µm were applied. The 50 mass pct of NH<sub>4</sub>HCO<sub>3</sub> was added as a space holder. The specimens were in compaction stage uniaxially pressed at pressure 625 MPa for 120 s. The brown bodies were sintered at a temperature 1150°C for 3.5 h. The well-joined grains were observed for both powder granulations. The increase in powder granulation resulted in an increase of porosity from 51% to 59%, and it was only 30% with no space holder used. The compressive strength increased with decreased porosity from 57 to 236 MPa. Young`s modulus was measured as 4.8 GPa for finer powder and 0.9 GPa for coarser powder. It is evident from the results obtained that the applied process parameters, the space holder and its fraction, and the use powder granulation between 45 and 105 µm bring out the porous material fulfilling mechanical and biological requirements specific of load-bearing titanium implants.