Showing papers on "Sintering published in 2001"

Thermal conductivity of dense and porous yttria-stabilized zirconia

Abstract: The thermal conductivity of dense and porous yttria-stabilized zirconia (YSZ) ceramics has been measured as a function of temperature in the range 25 to 1000 °C The dense specimens were either single crystal (8 mol% YSZ) or sintered polycrystalline (3 mol% and 8 mol% YSZ) The porous specimens (3 mol% YSZ) were prepared using the “fugitive” polymer method, where different amounts of polymer spheres (of two different average sizes) were included in the starting powders before sintering This method yielded materials with uniformly distributed porosities with a tight pore-size distributions A theory has been developed to describe the thermal conductivity of dense YSZ as a function of temperature This theory considers the reduction in the intrinsic thermal conductivity due scattering of phonons by point defects (oxygen vacancies and solute) and by the “hopping” of oxygen vacancies It also considers an increase in the effective thermal conductivity at high temperatures due to radiation This theory captures the essential features of the observed thermal conductivity The Maxwell theory has been used to analyze the thermal conductivity of the porous materials An adequate agreement was found between the theory and experiment

Electrophoretic deposition of hydroxyapatite coatings on metal substrates : A nanoparticulate dual-coating approach

Abstract: Hydroxyapatite coatings can be readily deposited on metal substrates by electrophoretic deposition. However, subsequent sintering is highly problematic owing to the fact that temperatures in excess of 1100°C are required for commercial hydroxyapatite powders to achieve high density. Such temperatures damage the metal and induce metal-catalysed decomposition of the hydroxyapatite. Furthermore, the firing shrinkage of the hydroxyapatite coating on a constraining metal substrate leads to severe cracking. The present study has overcome these problems using a novel approach: the use of aged nanoparticulate hydroxyapatite sols (lower sintering temperature) and a dual coating strategy that overcomes the cracking problem. Dual layers of uncalcined hydroxyapatite (HAp) powder were electrophoretically coated on Ti, Ti6Al4V and 316L stainless steel metal substrates, sintered at 875–1000°C, and characterised by SEM and XRD, and interfacial shear strength measurement. Dual coatings on stainless steel had an average high bond strength (about 23 MPa), and dual coatings on titanium and titanium alloy had moderate strengths (about 14 and 11 MPa, respectively), in comparison with the measured shear strength of bone (35 MPa). SEM and XRD demonstrated that the second layer blended seamlessly with the first and filled the cracks in the first. The superior result on stainless steel is attributed to a more appropriate thermal expansion match with hydroxyapatite, the thinner oxide layer, or a combination of these factors.

Microstructure evolution in metal-intermetallic laminate (MIL) composites synthesized by reactive foil sintering in air

Abstract: Metal-intermetallic (aluminide) laminate (MIL) composites have been fabricated in air using dissimilar metal foils. Foils of varying Al thickness were reacted with foils of Ti-3Al-2.5V resulting in microstructures of well-bonded metal-intermetallic layered composites with either Ti or Al residual metal layers alternating with the Al3Ti intermetallic layers. The MIL composites exhibit a very high degree of microstructural design and control. Microstructure characterization by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffractometry (XRD) has been performed, and basic physical properties of the Ti-Al composites have been determined. The Ti-Al reaction has been studied by interrupting the reaction processing, in steps, to observe the microstructural changes. An oxide layer between the Ti and Al foils initially controls the reaction kinetics. After breakdown of the oxide layer, a two-phase Al+Al3Ti layer (∼10 µm thickness) is formed. After formation of the two-phase layer, liquid phases are continuously present, and Al3Ti spherules (∼10 µm diameter) are formed through interfacial tension, solidify (in times of 2 to 4 µs), and are expelled into the liquid. This mechanism allows for a continuous reaction interface and higher reaction rates. Both reaction regimes, diffusion through the oxide, and, subsequently, the intermetallic phase reaction mechanism result in linear kinetics.

Titanium powder sintering for preparation of a porous functionally graded material destined for orthopaedic implants.

Abstract: This work focuses on basic research into a P/M processed, porous-surfaced and functionally graded material (FGM) destined for a permanent skeletal replacement implant with improved structural compatibility. Based on a perpendicular gradient in porosity the Young's modulus of the material is adapted to the elastic properties of bone in order to prevent stress shielding effects and to provide better long-term performance of the implant-bone system. Using coarse Ti particle fractions the sintering process was accelerated by silicon-assisted liquid-phase sintering (LPS) resulting in a substantial improvement of the neck geometry. A novel evaluation for the strength of the sinter contacts was proposed. The Young's modulus of uniform non-graded stacks ranged from 5 to 80 GPa as determined by ultrasound velocity measurements. Thus, the typical range for cortical bone (10-29 GPa) was covered. The magnitude of the Poisson's ratio proved to be distinctly dependent on the porosity. Specimens with porosity gradients were successfully fabricated and characterized using quantitative description of the microstructural geometry and acoustic microscopy.

Microwave sintering and mechanical properties of PM copper steel

Abstract: Microwave processing has gained worldwide acceptance as a novel method for heating and sintering a variety of materials from food to rubber to specialty ceramics, as it offers specific advantages in terms of speed, energy efficiency, process simplicity, novel and improved properties, finer microstructures, and lower environmental hazards. In the present paper, microwave sintering of modulus of rupture (MOR) bar samples of PM copper steel (MPIF FC-0208 composition) and the comparative evaluation of the mechanical properties using both microwave and conventional sintering techniques has been reported.The starting powder characteristics and the processing details of copper steel bar samples sintered in a conventional furnace and in an in house modified commercial microwave oven has been covered at length. In this study, the sintering temperature used typically ranged between 1100 and 1300°C, soaking time ranged from 5 to 20 min, and the atmosphere was controlled using flowing forming gas (mixture of ...

High temperature thermal stabilization of alumina modified by lanthanum species

Abstract: The effects of precursor pretreatment and addition methods of lanthanum species on stabilization of alumina (surface area loss, phase transformations and high temperature interaction with lanthanum species) have been investigated by BET specific surface area measurements (BET), X-ray powder diffraction (XRD), N2 adsorption–desorption isotherms, thermal analysis and X-ray photoelectron spectroscopy (XPS) in the range of 600–1150°C. Although powder La2O3, which is mechanically mixed with γ-Al2O3 or pseudo boehmite, can effectively retard the α phase transformation by solid phase interaction with Al2O3, it does not show a positive effect on retarding the loss of surface area. Compared with the direct impregnation of γ-Al2O3, the gelation of pseudo boehmite by acidification accelerates phase transformations and weakens the stabilizing influence of lanthanum species. At 600°C and for atomic ratio of La/Al up to 0.1 or at 1150°C and La/Al≤0.02, the lanthanum species is highly dispersed in alumina. With the increase of calcination temperature or lanthanum content, lanthanum species is present as dispersed La2O3, LaAlO3 and crystalline La2O3. At T≤1000°C the surface area loss of alumina is mainly attributed to the sintering of particles. The follow-up loss at T>1000°C results from both sintering and phase transformations. The highly dispersed lanthanum species retard both sintering and phase transformations, and their associated surface area loss. However, the formation of LaAlO3 mainly retards the surface area loss resulting from the α phase transformation. Having considered the purely mechanical mixing effect of additive on the surface area loss of alumina, an influence criterion of lanthanum species on retarding the surface area loss whether resulting from sintering or from α phase transformation at high temperature ≥1000°C has been proposed in this paper.

Density prediction of crystalline polymer sintered parts at various powder bed temperatures

Abstract: The effect of powder bed temperature setting on the prediction of density of sintered parts produced by the selective laser sintering (SLS) process is reported. A crystalline polymer, nylon‐12 – commercially named Duraform polyamide – has been used in this work. To study the effect of the powder bed temperature, a two‐dimensional model of the sintering process for crystalline polymers has been developed. Latent heat has been considered in the model. Three powder bed temperature settings, 174, 178 and 182○C, have been applied to study their effect on the sintered parts’ density and size accuracy. This paper only reports on density. Results show that at a powder bed temperature of 182○C, a fully solid density, 970kg/m3, can be obtained at a default energy density of 0.0284J/mm2. By reducing powder bed temperature to 178○C, at the same energy density, density of a sintered part decreases by about 4 per cent.

Influence of casting technique and hot isostatic pressing on the fatigue of an Al-7Si-Mg alloy

Abstract: The influence of the casting filling technique and hot isostatic pressing (hipping) on the fatigue-life distribution of Al-7Si-Mg alloy castings has been studied. To vary the number density and size of oxide-film defects in the castings, test bars were cast using bottom-gated filling systems with and without filtration. Some unfiltered castings were subjected to a hipping treatment of 100 MPa at 500 °C for 6 hours. Test pieces were machined from the castings and were fatigue tested in pull-pull sinusoidal loading, at maximum stresses of 150 and 240 MPa under a stress ratio of R=+0.1. The fatigue lives at any probability of failure and Weibull statistical parameters of the filtered castings were higher than those of the unfiltered and nonhipped castings, illustrating the importance of the casting technique. However, the unfiltered but hipped castings exhibited higher performance. It is proposed that the significant improvement in fatigue life after hipping is due to the deactivation of entrained double oxide-film defects as fatigue-crack initiators.

MgSiN2 Addition as a Means of Increasing the Thermal Conductivity of β‐Silicon Nitride

Abstract: Si3N4 powders with the concurrent addition of Yb2O3 and MgSiN2 were sintered at 1900°C for 2–48 h under 0.9 MPa nitrogen pressure. Microstructure, lattice oxygen content, and thermal conductivity of the sintered specimens were evaluated and compared with Si3N4, Yb2O3, and MgO addition. MgSiN2 addition was effective for improving the thermal conductivity of Si3N4 ceramics, and a material with high thermal conductivity over 140 W·(m·K)−1 could be obtained. For both specimens, lattice oxygen content was decreased with sintering time. However, the thermal conductivity of the MgSiN2-doped specimen was slightly higher than the MgO-doped specimen with the same oxygen content.

Sintering study of 316L stainless steel metal injection molding parts using Taguchi method: final density

Abstract: Sintering is a key step in the metal injection molding process, which affects the final density as well as the mechanical properties of the sintered part. To achieve a high final density, the effects of various sintering factors pertaining to the temperature–time profile and sintering atmosphere have to be characterized and optimized as well. This paper reports the use of Taguchi method in characterizing and optimizing the process factors for sintering water-atomized 316L stainless steel (of average size 6 μm). The effects of four sintering factors: sintering temperature, heating rate, sintering time and sintering atmosphere on the final density were studied. The various factors were assigned to an L9 orthogonal array. It was found that all the chosen sintering factors have significant effects on the final density. Optimum fractional final density of 96.14% of wrought material was achieved with a sintering temperature of 1250°C, a heating rate of 20°C min−1 and isothermal heating at 1250°C for 90 min in a vacuum atmosphere. Confirmatory experiments have produced results that lay within the 90% confidence interval.

High-Surface-Area Alumina Ceramics Fabricated by the Decomposition of Al(OH)3

Abstract: Al 2 O 3 -based porous ceramics with high surface areas were fabricated by adding Al(OH) 3 to the starting powder, followed by pressureless sintering at temperatures >1100°C. Three types of starting powders were used in the present study: pure α-Al 2 O 3 , α-Al 2 O 3 + Al(OH) 3 , and α-Al 2 O 3 + ZrO 2 + Al(OH) 3 . The addition of Al(OH) 3 considerably increased the surface area of the porous Al 2 O 3 , and the addition of ZrO 2 further increased the surface area; a surface area as high as 36.39 m 2 /g was obtained, and the high surface area was retained at higher temperature. The pore size distribution of the specimens with high surface area was bimodal, with one peak at ∼100 nm and the other, which contributed most of the surface area, at ∼10 nm. X-ray analysis showed that in the sample with the fine pores and high surface area, there was θ-Al 2 O 3 phase produced by the decomposition of Al(OH) 3 , presumably because the phase transformation of θ-Al 2 O 3 to α-Al 2 O 3 was incomplete after low-temperature sintering. Moreover, the porous Al 2 O 3 with high surface area retained superior mechanical properties, attributed to the good sinterability of the fine α-Al 2 O 3 powder used in the present study. The sintered specimens could be large and designed to any shape, because pressureless sintering was used for fabrication. The present approach provides a new way of fabricating porous Al 2 O 3 ceramics that could be widely used as catalyst supports in industry, especially for high-temperature catalysis.

Effect of Al and Mn doping on the electrical conductivity of ZnO

Abstract: Samples of Al-doped and Mn-doped ZnO with a doping level up to 1.2 mol% were sintered at temperatures from 1100 to 1400°C in air. dc Electrical conductivities of these samples at room temperature and below were measured, and the effects of the doping type, the doping level, the sintering temperature and time on the electrical conductivity of ZnO were investigated. It was found that Al increased the electrical conductivity of ZnO resulting in a manifestation of a metallic electrical conduction behaviour, and a semiconductor-metal transition occured in the Al-doped ZnO samples. For Mn-doped ZnO samples quenched from the sintering temperatures, the electrical conductivity decreased with the increase in the Mn content, but the samples still showed a semiconductor electrical conduction behaviour. In this way, one could obtain a systematic variation of the ZnO electrical conductivity from the high conductivity, Al-doped case, to the high resistivity, Mn-doped one, spanning over eight orders of magnitude, which is explained in the present communication.

How the trapping of charges can explain the dielectric breakdown performance of alumina ceramics

Abstract: The aim of this work is to bring new elements to the understanding of breakdown phenomena. Different alumina materials are studied in breakdown tests and by absorbed current measurement. The association of these two characterisation methods displays correlations between breakdown strength, the ability to trap or diffuse electrical charges and microstructural parameters. Different materials have been studied with a growing quantity of defects: structural defects in the single crystal, grain boundaries in very pure polycrystalline alumina (99.99%), impurities in less pure polycrystalline alumina (99.8%), interfaces in the case of alumina with a dispersion of zirconia and finally the adding of sintering aids. It appears that materials capable of diffusing injected charges have higher breakdown values than those trapping charges locally. Two other favourable behaviours are characterized: trapping charges at the injection point in order to limit the injection process, and trapping followed by reemission which relaxes the insulator and delays breakdown.

Sintering features of cemented carbides WC–Co processed from fine powders

Abstract: Since the last 30 years, the cemented carbides WC–Co are processed from finer and finer powders. Both densification behaviour and microstructure evolution along the liquid-phase sintering (LPS) are different for materials prepared from powders of micronic or submicronic size. The typical features induced by the use of submicronic particles – large contribution of the solid-state densification, abnormal growth – are presented. The major questions, related to the understanding of densification hindrance and grain growth inhibition, are listed.

High-temperature effects in the fracture mechanical behaviour of silicon carbide liquid-phase sintered with AlN–Y2O3 additives

Abstract: It has been shown that pressureless sintering of SiC to theoretical density is possible with sintering additives from the system AlN–Y2O3. While commonly a combination of oxides is used such as Al2O3–Y2O3 (–SiO2), the oxynitride additives offer the advantage that only a nitrogen atmosphere is required instead of a powder bed for thermochemical stabilisation at the sintering temperature. The thermal decomposition of AlN is suppressed quite effectively when a moderate nitrogen overpressure is applied, resulting in very small mass loss during densification which only depends on the oxygen content of the SiC starting powder. By varying the mass ratio of β-SiC to α-SiC and applying dedicated post-densification heat treatments, a platelet-strengthened microstructure is obtained which shows enhanced fracture toughness. The platelet formation is attributed to a solution / precipitation process with simultaneous phase transformation from β-SiC to α-SiC, followed by anisotropic grain growth of α-SiC. In the present work, recent progress in the mechanical properties of these materials is reported. By means of a simple surface treatment-annealing in air — it is possible to obtain four-point bending strengths in excess of 1 GPa in liquid phase sintered SiC. The strength retention at temperatures around 1200°C is significantly improved.

Influence of impurity content and porosity of plasma-sprayed yttria-stabilized zirconia layers on the sintering behaviour

Abstract: Yttria-stabilized zirconia (YSZ) powders from different manufacturers have been used to prepare atmospheric plasma-sprayed (APS) ceramic coatings with different porosity levels. While the particle morphology of the different powders was similar, the amount of impurities, especially silica, was different, varying between 100 and 1500 ppm. APS coatings were removed from the substrates and the porosity distribution was measured by mercury porosimetry. Typically porosity levels between 10 and 15% have been used. Free-standing coatings were investigated in the dilatometer during long-term (>50 h) annealing at 1200°C. Additionally, the coefficient of thermal expansion was determined from expansion during heating. It turned out that the higher porosity levels, as well as the high impurity levels, led to a significant increase in the sintering rate of the coating during high-temperature annealing. A linear relationship between the total shrinkage after 60 h and the porosity, as well as the silica content, was used to describe the influence of the investigated parameters on the sintering behaviour in a more quantitative way.