Showing papers on "Sintering published in 1995"

Ceramic Processing and Sintering

Abstract: Ceramic fabrication processes - an introductory overview synthesis of powders powder characterization science of colloidal processing sol-gel processing powder consolidation and forming of ceramics sintering of ceramics - fundamentals theory of viscous sintering grain growth and microstructural control liquid-phase sintering problems of sintering densification process variables and densification practice.

Ti(C,N) cermets — Metallurgy and properties

Abstract: An overview of the metallurgical reactions during the vacuum sintering process of powder mixtures for the manufacture of cermets is presented. The relatively complex phase reactions in the multi-component system Ti/Mo/W/Ta/Nb/C,N-Co/Ni are discussed. The liquid binder phase reacts with titanium carbonitride by preferentially dissolving titanium carbide leaving titanium nitride undissolved. The compositions and the amounts of the gas species set free during the sintering process were monitored and led —together with differential thermal analysis — to a better understanding of the mechanisms that govern the sintering behaviour. The properties and the microstructure of cermets depend on the nature and the alloy status of the prematerials. The composition of the prematerials with respect to the carbon-nitrogen ratio, the stoichiometry of the hard phase and the amount and composition of the binder phase have a decisive influence on the properties and the cutting performances of the final products. Optimization of the properties with respect to the desired performance is possible. Examples of the cermet cutting performance in various applications are discussed.

Grain Size Dependence of Hardness in Dense Submicrometer Alumina

Abstract: Pressureless sintered alumina compacts with a submicrometer microstructure exhibit a hardness that approaches or even exceeds the level of advanced hot-pressed composites of Al{sub 2}O{sub 3} + 35 vol% TiC, whereas the strength of both ceramics is approximately the same. The combination of reduced dislocation mobility (due to the small grain size), high density, and density homogeneity are the prerequisites for the surprisingly high hardness. Quasi-conventional powder processing is used to produce these outstanding alumina bodies.

General aspects and limits of conventional ultrafine WC powder manufacture and hard metal production

Abstract: Ultrafine WC/Co hard metals (average WC grain sizes ≤ 0.5 μm) can be successfully and reliably obtained by conventional hard metal manufacturing techniques. In this paper, some of the crucial aspects of conventional powder manufacture, powder milling and liquid phase sintering are discussed. Conventional ultrafine WC powder manufacture is based on the production of tungsten powder by hydrogen reduction of tungsten oxides and subsequent carburization. Alternatively, direct carburization can be carried out. However, inherent to the powder processing techniques used and the particle growth mechanisms involved (oxide precursors used, reduction and carburization history), there exists a lower limit beyond which-finer WC powders cannot be produced. This limit lies in the particle size range of 50–150 nm (0.05–0.15 μm). Powder milling is carried out to obtain an even dispersion of the Co binder in the ultrafine WC matrix. The more uniform the phase distribution (WC, Co, grain growth inhibitor) within the green powder compact, the more uniform will be the material transport during sintering, and hence the uniformity of the WC grain growth/growth inhibition during sintering. Enhanced WC grain growth occurs early in the sintering cycle, even below the temperature at which the liquid phase is formed. This growth can be largely restricted by the addition of VC. However, effective grain growth inhibition has to take place already during this early period of solid-state sintering. The ‘early’ availability of the grain growth inhibitor at the WC/Co interface can, therefore, determine the degree of growth inhibition. Ultrafine hard metals are in particular prone to discontinuous grain growth of the WC. Different reasons for this local growth mode are propounded relating to both the chemical as well as the geometrical departures from uniformity in the green powder compact. While it is still not possible to predict exactly an ultimate WC grain size limit, below which WC grain growth can no longer be restricted, even with proper inhibitor additions, experimental evidence indicates that this average WC grain size limit lies in the range of 200–300 nm. This limit is inherent to the existing conventional processing techniques (powder manufacture, milling, liquid phase sintering) and the WC growth mechanisms involved and can be overcome only by establishing a completely new route in hard metal manufacture.

Effect of Spark Plasma Sintering on Densification and Mechanical Properties of Silicon Carbide

Abstract: Silicon carbide ceramics which 5 mass% Al2O3 and 2 mass% Y2O3 were added to were prepared by the spark plasma sintering (SPS) method under the conditions of 30 MPa and 5 min. Mechanical properties at room temperature were eximined. The SPS brought dense silicon carbide ceramics at a sintering temperature of 1800°C, which was about 200°C lower than that of the hot-pressing process. The silicon carbide obtained by SPS had higher strength and fracture toughness than those obtained by hot-pressing. The results suggest that the inside temperature of the sintered bodies during spark plasma sintering was higher than the measured temperature.

Thermal Instability and Proton Conductivity of Ceramic Hydroxyapatite at High Temperatures

Abstract: In the study of the high-temperature behavior of ceramic hydroxyapatite (HAp), it was found in relation to its ionic conduction properties that HAp underwent partial dehydration of its lattice hydroxide ions. Considering that HAp ceramics are sintered above 1200°C without destruction of the apatitic structure, the dehydration was interpreted as an unstable phenomenon of aging. The evolution of instability of dehydration was reflected in the time-dependent characteristics of conductivity, which exhibited up-and-down change of 103 S·cm−1 above 700°C. The conduction was proved purely protonic by measurements of a hydrogen concentration cell, and it was noted that the protonic conductivity was increased to a high value of 10−3 S·cm−1 at the initial stage of the aging. The aging phenomenon was demonstrated to be reversible in the deuteration of fully aged HAp; the uptake of OD− inside the specimen was confirmed by infrared spectroscopic analysis after exposure to deuterium oxide vapor. Based on those results, a conduction model was proposed consistent with the aging phenomenon. The present study also showed the importance of the supply of H2O vapor to the ambient during sintering, for the lattice hydroxide ions of ceramic HAp were considerably dehydrated in sintering in air at high temperatures.

Electrochemical Synthesis and Sintering of Nanocrystalline Cerium(IV) Oxide Powders

Abstract: Nanocrystalline CeO{sub 2} powders were prepared electrochemically by the cathodic electrogeneration of base, and their sintering behavior was investigated. X-ray diffraction and transmission electron microscopy revealed that the as-prepared powders were crystalline cerium(IV) oxide with the cubic fluorite structure. The lattice parameter of the electrogenerated material was 0.5419 nm. The powders consisted of nonaggregated, faceted particles. The average crystallite size was a function of the solution temperature. It increased from 10 nm at 29 C to 14 nm at 80 C. Consolidated powders were sintered in air at both a constant heating rate of 10 C/min and under isothermal conditions. The temperature at which sintering started (750 C) for nanocrystalline CeO{sub 2} powders was only about 100 C lower than that of coarser-grained powders (850 C). However, the sintering rate was enhanced. The temperature at which shrinkage stopped was 200--300 C lower with the nanoscale powder than with micrometer-sized powders. A sintered specimen with 99.8% of theoretical density and a grain size of about 350 nm was obtained by sintering at 1,300 C for 2 h.

An unusual promotion of the redox behaviour of CeO2-ZrO2 solid solutions upon sintering at high temperatures

Abstract: The reduction behaviour of a high surface area CeO2-ZrO2 solid solution is compared with that of a high surface area CeO2. It is shown that, upon sintering induced by repetitive reduction/oxidation processes, the temperature of reduction of the solid solution decreases from 900 to 700 K. In contrast, the reduction at low temperatures of the CeO2 sample is strongly retarded after such treatments. The role of ZrO2 in promoting the reduction at low temperatures is discussed.

Study of nanostructured WC-Co composites

Abstract: Nanostructured materials, also termed nanocrystalline materials, have emerged upon the horizon in the past few years. In the present research, a preliminary study has been performed regarding sintering, grain growth, and mechanical properties of nanostructured WC-Co. It was found that densification of nanostructured WC-Co powder could be completed in 5 min, or 15 min if grain growth inhibitors are added. Grains grow extremely rapidly, very likely via coalescence, during heating and the first few minutes (< 5) at temperature. After the initial rapid stage, grain growth followed the linear relationship of coarsening. It was also found that samples made from nanostructured powder had better surface crack resistance than samples made from standard submicron powder.

Properties of ultra-fine grain binderless cemented carbide ‘RCCFN’

Abstract: Conventional binderless cemented carbide is known as WC-3% TiC-2% TaC cemented carbide with a mean WC grain diameter of about 2 μm, which does not include a binder phase. This alloy, however, has no binder phase and therefore low strength. The authors investigated the effects of mean diameter of raw WC powder on mechanical characteristics, and found that the smaller the mean diameter of raw WC powder, the lower the temperature at which sintering is possible and the higher the hardness and strength becomes. An investigation was also made on the effects of grain growth suppression additives on the alloy using 0.6 μm diameter WC powder, which offers the highest mechanical characteristics, with the objective of enhancing characteristics through finer grains. Hardness increased with additional amounts of Cr 3 C 2 and VC. Strength peaked at a certain additive amount, with superior values of H R A = 95.5 and transverse-rupture strength = 1.8 GPa. The microstructure was found to be composed of only very fine and uniform carbides; a mirror surface of 7 nm Rtm was obtained by mirror polishing. These characteristics make it possible for this alloy to be used in various optical applications.

The influence of high sintering temperatures on the mechanical properties of hydroxylapatite

Abstract: The kinetics of the thermal decomposition of stoichiometric hydroxylapatite (HA) has been studied up to 1500°C for the purpose of determining the maximum admissible combinations of temperature and time for sintering HA. The influence of the sintering temperature on shrinkage, density and grain growth is then investigated in the temperature range from 1000 to 1450°C. Nearly theoretical density was achieved above 1300°C. A maximum fracture toughness is obtained for the samples sintered at 1300°C whereas hardness increases up to a sintering temperature of 1400°C. These results are discussed in terms of the roles of porosity and grain size.

Thermal stability of a high surface area ceria under reducing atmosphere

Abstract: The influence of a reducing atmosphere on the thermal stability of CeO2 was studied on a high surface area ceria sample (115 m2 g−1) treated 2 h under hydrogen or carbon monoxide at various temperatures between 673 and 1123 K. Comparative experiments were done under air or vacuum, and also in presence of water or carbon dioxide in order to estimate the relative importance of the reduction products. An important decrease of the specific surface area was observed between 850 and 1000 K under air, vacuum, carbon monoxide or water pressure, the residual BET area being below 10 m2 g−1 at 1100 K. Under hydrogen, the same loss of surface was obtained at ca. 150 K lower temperatures. In all cases, the first step was the elimination of the microporosity followed by the growth of the crystallites. The peculiar influence of hydrogen was related to the high concentration of lattice oxygen vacancies created during the reduction of the bulk. In the case of carbon monoxide atmosphere which also reduces the sample, the carbonate species formed during the reduction are eliminated from the bulk at higher temperatures, which explain the better resistance to sintering compared to hydrogen. This was confirmed by a treatment under carbon dioxide atmosphere which was found to preserve the specific surface area better because of the stabilization of the carbonate species on the ceria.

Impedance spectroscopy of grain boundaries in nanophase ZnO

Abstract: Sintered compacts of nanophase ZnO (∼60 nm average grain size, presintered at 600 °C) were made from powders (∼13 nm) prepared by the gas-condensation technique. Impedance spectra were taken as a function of temperature over the range 450–600 °C and as a function of oxygen partial pressure over the range 10−3−1 atm (550 and 600 °C only). The activation energy was determined to be 55 kJ/mole (0.57 eV) and was independent of oxygen partial pressure. The oxygen partial pressure exponent was −1/6. Impedance spectra exhibited nonlinear I-V behavior, with a threshold of approximately 6 V. These results indicate that grain boundaries are governing the electrical properties of the compact. Ramifications for oxygen sensing and for grain boundary defect characterization are discussed.

Evaluation of Sintering of Nanometer-Sized Titania Using Aerosol Method

Abstract: The sintering of titania agglomerates consisting of nanometer primary particles in a heated gas flow is investigated under gas temperatures from room temperature to 1673 K. The test titania agglomerates are produced by thermal decomposition or hydrolysis of TTIP (titanium tetraisopropoxide) vapor. The size changes of the agglomerates of 30–100 nm in diameter are measured using a TDMA (Tandem Differential Mobility Analyzer) system. At a temperature lower than about 1000 K, the agglomerates do not change in size with increasing heating temperature, but a sudden decrease in size is detected at 1000 to 1500 K. From TEM observation, densification of agglomerates accompanying primary particle growth is observed. These experimental results for the rate of reduction in surface area are explained quantitatively by solving the basic equation of sintering under the calculated temperature profile.

Mechanical properties of a partially sintered alumina

Abstract: An experimental investigation was performed to understand the mechanical properties of alumina with moderate amounts of porosity (∼ 20–40%). Young's modulus, strength and fracture toughness showed similar behavior as a function of the degree of sintering. For low sintering temperatures (

An improvement in processing of hydroxyapatite ceramics

Abstract: Hydroxyapatite ceramics have been fabricated via two different processing routes, a conventional processing route and an emulsion-refined route. The conventional precipitation processing of powder precursors for hydroxyapatite ceramics results in the formation of hard particle agglomerates, which degrade both the compaction and densification behaviour of the resultant powder compacts. An emulsion-refinement step has been shown to be effective in “softening” particle agglomerates present in the conventionally processed powder precursor. As a result, the emulsion-refined powder compact exhibits both a higher green density and a higher sintered density than the un-refined powder compact, on sintering at temperatures above 800 °C. The effect of powder agglomeration on densification during both the initial and later stage of sintering is discussed. The attainable sintered density of the conventionally processed material was found to be limited by the presence of hard powder agglomerates, which were not effectively eliminated by the application of a pressing pressure of 200 MPa. These hard powder agglomerates, which form highly densified regions in the sintered ceramic body, commenced densification at around 400 °C which is more than 100 °C lower than the densification onset temperature for the emulsion-refined powder compact, when heated at a rate of 5 °C min−1. The inter-agglomerate voids, manifested by the differential sintering, resulted in the formation of large, crack-like pores, which act as the strength-limiting microstructural defects in the conventionally processed hydroxyapatite. A fracture strength of 170±12.3 MPa was measured for the emulsion-refined material compared to 70±15.4 MPa for the conventionally processed material, when both were sintered at 1100 °C for 2 h.

The sintering of two particles by surface and grain boundary diffusion -- A two-dimensional numerical study

Abstract: We investigate the sintering of two touching circular particles by surface and grain boundary diffusion. Typical examples for the evolution of the shape of the particles, their surface curvatures, and their surface fluxes are given. The sintering kinetics are evaluated as a function of the dihedral angle at the grain boundary-surface junctions and the grain boundary to surface diffusivity ratio. In particular, the growth rates of the neck between the two particles, the growth rate exponents, and the changes in the lengths of the particle pairs are monitored. The times needed to reach certain fractions of the final equilibrium neck sizes are tabulated for typical experimental dihedral angles and diffusivity ratios. Our simulation is based on a rigorous mathematical system modeling the sintering of the two particles, and a rigorous numerical method for solving this system is adopted.

Effect of the Grain Boundary Thermal Expansion Coefficient on the Fracture Toughness in Silicon Nitride

Abstract: The effect of thermal expansion mismatch stress between silicon nitride and different grain boundary phases on the fracture toughness of silicon nitride was investigated. Different sintering aids in the Y-Mg-Si-Al-O-N system produced silicon nitride specimens with very similar microstructures but different grain boundary phase compositions and different values of fracture toughness. The fracture toughness of the silicon nitride increased as the thermal expansion coefficient of the grain boundary phase increased. The presence of tensile residual stress at the grain boundary caused by thermal expansion mismatch between the silicon nitride and the grain boundary phase enhanced crack deflection and grain bridging.

Reaction Sintering of ZnO‐AI2O3

Abstract: The reaction sintering of equimolar mixtures of ZnO and Al{sub 2}O{sub 3} powders was investigated as a function of primary processing parameters such as the temperature, heating rate, green density, and particle size. The powder mixtures were prepared by two different methods. In one method, the ZnO and Al{sub 2}O{sub 3} powders were ball-milled. In the other method, the ZnO powder was chemically precipitated onto the Al{sub 2}O{sub 3} particles dispersed in a solution of zinc chloride. The sintering characteristics of the compacted powders prepared by each method were compared with those for a pre-reacted single-phase powder of zinc aluminate, ZnAl{sub 2}O{sub 4}. The chemical reaction between ZnO and Al{sub 2}O{sub 3} occurred prior to densification of the powder compact and was accompanied by fairly large expansion. The mixing procedure had a significant effect on the densification rate during reaction sintering. The densification rate of the compact formed from the ball-milled powder was strongly inhibited compared to that for the single-phase ZnAl{sub 2}O{sub 4} powder. However, the densification rate of the compact formed from the chemically precipitated mixture was almost identical to that for the ZnAl{sub 2}O{sub 4} powder. The difference in sintering between the ball-milled mixture and the chemicallymore » precipitated mixture is interpreted in terms of differences i the microstructural uniformity of the initial powder compacts resulting from the different preparation procedures.« less

Sintering of fat crystal networks in oil during post-crystallization processes

Abstract: Several foods contain semi-solid fats that consist of solid crystals dispersed in a liquid oil. In oil-continuous margarine, butter, and chocolate, fat crystals determine properties such as consistency, stability against oiling-out, and emulsion stability. Trends toward foods with less fat and/or less saturated fat create a need for understanding and controlling the properties of fat crystal dispersions. Fat crystals form a network in oil due to mutual adhesion. One source of strong adhesion is formation of solid bridges (sintering), which has been studied in this work through sedimentation and rheological experiments. Results indicate that sintering may be created by crystallization of a fat phase with a melting point between that of the oil and the crystal. Generally speaking, β′ crystals were sintered by β′ fat bridges, favored by rapid cooling, and β crystals by β fat bridges, favored by slow cooling. The existence of the same polymorphic form of the crystal and bridge indicated that solid bridges, rather than bridges formed by small crystal nuclei, were formed. A maximum in sintering ability for an optimal sintering fat concentration occurred due to competition between bridge formation and other crystallization processes. Some emulsifiers influenced the sintering process. For example, monooolein made it more pronounced, while technical lecithin had the opposite effect.

Polymer-derived si-based bulk ceramics, part I: preparation, processing and properties

Abstract: The synthesis and processing of dense silicon-based bulk ceramic materials derived from the thermal decomposition of preceramic organosilicon polymers such as polysilazanes and polysilanes was achieved by three different routes A, B, and C. Additive-free silicon carbonitride bodies of the composition Si1.7C1.0N1.6 were produced according to route A, with up to 94% relative density by the pressureless pyrolysis of compacted infusible polysilazane powders at low processing temperatures (1000 °C). The resulting silicon carbonitride was single-phase and amorphous according to X-ray- and TEM-investigations and exhibited a low solid-phase density of 2.33 g/cm3. The maximum room temperature fracture strength of additive-free silicon carbonitride was 370 MPa. Crystallization of the as-synthesized silicon carbonitride bulk samples occurred at temperatures exceeding 1400 °C. In a second process, route B, densification of polysilazane-derived amorphous silicon carbonitride powder was achieved by liquid phase sintering with alumina and yttria as sintering additives in nitrogen atmospheres and at temperatures up to 1900 °C. This process resulted in the formation of dense polycrystalline β-Si3N4/β-SiC-composites. The average room temperature fracture strength and fracture toughness of the gas pressure sintered composite were in the range of 650 MPa and 10.2 MPa√m, respectively. Polycrystalline Si3N4/SiC-composites were obtained in a third process, route C, by the pyrolysis and subsequent sintering of α-Si3N4-powder/polysilane blends. The Si3N4-powder serves as an inert filler reducing the volume shrinkage associated with the polymer-to-ceramic transformation. Dense Si3N4 and Si2N2O bulk ceramics were formed according to route B by the liquid phase sintering of amorphous silicon nitride powder synthesized by the polysilazane pyrolysis under ammonia.

Formation of Nanostructural Materials Induced by Mechanical Processings ( Overview )

Abstract: Mechanical alloying (MA) was firstly developed to synthesize metallic matrix composite by mechanically incorporating preformed oxide and or carbide particles into a metallic matrix. A consecutive compaction process is applied to obtain bulk materials. During MA, powders are repeatedly welded, fractured and rewelded in a high energy mill leading to an intimate mixing on a nano/micro-scale with the possible formation of far from equilibrium phases. The versatility of MA is well known ; high volume, low energy mills can be used to commercially produced dispersion strengthened Al, Ni and other transition metal alloys. Various intermetallics and inorganic compounds (amorphous and/or nanocrystalline) have been synthesized by using higher energy mills which have been specially developed in some cases. Mechanical alloying, it appears, as suggested by T. H. Courtney et al., is the Alladin's lamp of powder processing. All the published works have shown that the reaction and end products of the MA process strongly depend on the milling conditions. As a consequence, it is obvious that an improved understanding of the dynamics of MA process is required to gain a full appreciation of the industrial potential of the technique for synthesizing materials. Recently, M. Abdellaoui and E. Gaffet have shown that the crystal to amorphous phase transition (at least in the case of the model Ni 10 Zr 7 ) only depends on the injected mechanically power, allowing a direct comparison among experiments performed using distinct type of milling apparatus (planetary milling machine, horizontal apparatus). An alternative method has been recently proposed by N. Malhouroux-Gaffet and E. Gaffet, for the solid state synthesis of disilicide powders exhibiting a wide contamination during the direct MA preparation : the mechanically activated annealing process (M2AP). Such a M2AP method has been applied to the synthesis of FeSi 2 , MoSi 2 , WSi 2 compounds. Such a method appears as being a well suitable one for the low temperature synthesis of refractory nanomaterials. Recent applications have been successfully performed to mechanically activated sintering (MAS).

Methods for making WC-containing bodies

Abstract: A method of forming a low level carbon high-density tungsten carbide-containing material includes sintering a preform which contains tungsten carbide powder and has a composition such that the resulting sintered material has at most 6.05 weight percent tungsten-bound carbon based on the total weight of tungsten and tungsten-bound carbon. This low level of carbon may be achieved by, prior to the sintering step, oxidizing the tungsten carbide powder sufficiently to achieve the desired substoichiometric carbon level in the sintered product or by adding a carbon-lowering material selected from the group consisting of tungsten, ditungsten carbide, and tungsten oxide. Optionally, other materials can be present in the preform such as carbon-getter metals and compounds thereof. The carbon-getter metals are those metals of which the carbides thereof are more thermodynamically stable than monotungsten carbide.

Processing and properties of al2o3/sic nanocomposites

Abstract: Summary Alumina/SiC nanocomposites were produced by mechanical mixture of commercial powders. The preparation steps involved the vigorous mixing of the powders and drying under conditions where the homogeneous mixture was kept stable. Pressureless sintering of die-pressed powders achieved reasonable densities (∼97% theoretical density) for 2·5wt% of SiC on sintering at 2073 K. Higher SiC contents strongly reduced the sintered density. The use of a more reactive alumina (finer matrix powder) gave similar results. Hot pressing at 1973 K/1 h/25 MPa produced high-density materials for SiC contents as high as 20 wt%. Transmission and scanning electron microscopy analysis showed that the SiC particles were well distributed and were situated both inside the grains and on the grain boundaries of the alumina matrix. The SiC strongly inhibited grain growth in the matrix in keeping with the Zener model. The bend strength increased as the SiC content increased, a result partly explained by the grain size refinement. The strength improvement of 20% over monolithic was explained in terms of the change to an intergranular fracture mode.