Showing papers on "Sintering published in 1984"

Sinterability of Agglomerated Powders

Abstract: A concept is presented which relates the sinterability of a powder compact to its particle arrangement as defined by the distribution of pore coordination numbers, i.e., the number of touching particles surrounding and defining each void space. Previous thermodynamic arguments suggest that pores will disappear only when their coordination numbers are less than a critical value. The coordination-number distribution of an agglomerated powder is discussed with respect to the size of the multiple-particle packing unit, consolidation forces, and phenomena occurring during sintering. One pertinent conclusion is that the multiple-particle packing units densify and support grain growth as sintering initiates. Grain growth and rearrangement processes decrease the coordination number of remaining pores to allow them to disappear during latter states of sintering. Porosimetry and direct observations of powder compacts of <1 μm Al2O3 heat-treated between 600° and 1600°C support this concept.

The preparation and characterization of alumina membranes with ultra-fine pores

Abstract: Alumina films less than 20μm thick are prepared by a process in which a boehmite sol is successively gelled, dried and calcined. The resulting structure has the unique property that only ultra-fine pores with a narrow pore size distribution are present within large crack-free layers. The microstructure can be influenced by the sintering temperature and the acid concentration of the precursor sol. The minimum modal pore size which has been obtained is 2.5 nm. A consistent model of the microstructure of the membranes and an explanation of its uniformity are presented.

Alumina: Processing, Properties, and Applications

Abstract: 1 Introduction.- 1.1 Scope of the Book.- 1.2 General Remarks on the Use and the Behavior of Ceramic Materials.- 1.3 The History of Alumina.- 1.4 Preceding Summarizing Literature on Alumina.- 2 Physical Properties.- 2.1 Structure.- 2.2 Thermal Properties.- 2.3 Diffusion.- 2.3.1 General Remarks on Diffusion Phenomena in Ceramics.- 2.3.2 Intrinsic Diffusion and Disorder Mechanism.- 2.3.3 Extrinsic Diffusion.- 2.3.4 Problems in Determining the Diffusion Coefficient.- 2.3.5 Diffusion Data.- 2.4 Electrical Conductivity.- 2.5 Sintering and Grain Growth.- 2.5.1 Fundamental Sintering Mechanisms in Al2O3.- 2.5.2 Intermediate and Final-Stage Sintering.- 2.5.3 Influence of Additives.- 2.5.4 Effect of MgO.- 2.5.5 Influence of Atmospheres.- 2.5.6 Grain Growth.- 2.6 Hot Pressing.- 2.7 Segregation.- 3 Mechanical Properties.- 3.1 Elastic Properties.- 3.2 Fracture Strength.- 3.2.1 The Strength of Ceramics.- 3.2.2 Fracture Energy.- 3.2.3 Types of Flaws.- 3.2.4 Strength Data.- 3.2.5 Strength-Grain Size Relationships.- 3.3 Time-Dependent Strength and Subcritical Crack Growth.- 3.3.1 The Model of Time-Dependent Strength.- 3.3.2 Subcritical Crack Growth Data.- 3.3.3 Fatigue Data.- 3.3.4 Mechanisms of Slow Crack Growth in Alumina.- 3.4 Thermal and Mechanical Shock Resistance.- 3.4.1 Thermal Shock Properties.- 3.4.2 Mechanical Shock Properties.- 3.4.3 Crack Healing.- 3.5 Plastic Deformation.- 3.5.1 Slip.- 3.5.2 Twinning.- 3.5.3 Hardness.- 3.5.4 Abrasive Wear.- 3.6 Creep.- 3.6.1 Basic Creep Mechanisms.- 3.6.2 Creep of Pure and MgO-Doped Alumina.- 3.6.3 Effect of Other Dopants.- 3.6.4 High Temperature Failure Mechanisms.- 3.7 Strengthening Mechanisms.- 3.7.1 Second Phase Dispersions.- 3.7.2 Compressive Surface Stresses.- 3.7.3 Transformation Thoughening.- 4 Fabrication.- 4.1 Preparation of Powders.- 4.2 Forming.- 4.2.1 Dry Pressing.- 4.2.2 Hydrostatic Molding.- 4.2.3 Extrusion.- 4.2.4 Injection Molding.- 4.2.5 Hot Pressing.- 4.3 Sintering.- 4.4 Hard Machining.- 4.5 Quality Control.- 4.6 Manufacturing Tolerances.- 4.7 Principles of Design.- 5 Applications.- 5.1 Electronic Applications.- 5.1.1 Metal-to-Ceramic Bonding.- 5.1.2 Spark-Plug Insulators.- 5.1.3 Components and Housings for Electron Tubes.- 5.1.4 Discharge Lamps.- 5.1.5 Microelectronics.- 5.2 Mechanical Engineering Applications.- 5.2.1 Thread Guides for Textile Machines.- 5.2.2 Wire Drawing Step Cones.- 5.2.3 Paper Machine Covers.- 5.2.4 Bearings.- 5.2.5 Cutting Tools.- 5.3 Medical Applications.- 5.3.1 Artificial Joints.- 5.3.2 Dental Implants.- 5.3.3 Maxillary Reconstructions.- 5.4 Armor Applications.- 5.5 Other Applications.- References.

Preferentially binder enriched cemented carbide bodies and method of manufacture

Abstract: Cemented carbide substrates having substantially A or B type porosity and a binder enriched layer near its surface are described. A refractory oxide, nitride, boride, and/or carbide coating is deposited on the binder enriched surface of the substrate. Binder enrichment is achieved by incorporating Group IVB or VB transition elements. These elements can be added as the metal, the metal hydride, nitride or carbonitride.

Influence of Aggregates on Sintering

Abstract: Alumina powders containing controlled amounts of hard aggregates exhibit isothermal shrinkage rates which decrease markedly with increasing aggregate content. Sintered densities achieved after 1- and 24-h anneals decreased linearly with aggregate content. The microstructures of regions near aggregates and within aggregates showed that the compacts sinter differently inside and outside the aggregates.

Sintering Atmosphere Effects on the Ductility of W- Ni- Fe Heavy Metals

Abstract: Residual porosity has a strong negative effect on the ductility of tungsten-nickel-iron heavy metals. This investigation examines the sintering atmosphere role in stabilizing detrimental residual pore structures. Two types of experiments are reported on alloys containing 93, 95, or 97 wt pct W with Ni:Fe ratios of 7:3. The negative effect of prolonged sintering is attributed to pore coarsening involving trapped gas in the pores. Calculated pore growth rates for hydrogen filled pores suggest that pore coarsening involves both ripening and coalescence driven by tungsten grain growth. The effect of the sintering atmosphere is analyzed for final stage pore elimination. It is demonstrated that a change in sintering atmosphere from hydrogen to argon midway through the sintering cycle can aid pore degassing and increase the sintered ductility and strength.

Emergent Process Methods for High-Technology Ceramics

Abstract: The present conference covers colloidal processing of advanced ceramics, novel power-forming and powder-processing methods, the derivation of ceramics by polymer processing, chemical vapor deposition techniques, ion beam deposition methods, the laser and ion beam modification of surfaces, hot isostatic pressing and dynamic compaction, shock conditioning and subsequent densification of ceramics, and very high pressure processing methods. Specific attention is given to the preparation of shaped glasses by the sol-gel method, the synthesis of powders and thin films by laser-induced gas phase reactions, the plasma sintering of ceramics, laser chemical vapor deposition, the microstructure and mechanical properties of ion-implanted ceramics, a computer simulation of dynamic compaction, shock-induced modification of inorganic powders, and diamond anvil technology.

In situ stress measurement of refractory metal silicides during sintering

Abstract: The stress of thin film tantalum silicide, titanium silicide, and tungsten silicide was measured in situ during sintering. These refractory metal silicide films were cosputtered on oxidized silicon, quartz, and sapphire substrates. The films were heated to 910 °C, annealed for 30 min, and cooled down to room temperature. Throughout the heat treatment cycle, the film stress was measured at short intervals by measuring the curvature of the film/substrate couple with a laser beam. It was demonstrated that the intrinsic stress of these silicides is negligible at 910 °C. The final room temperature stress of these silicides, after high temperature sintering, is mainly thermal stress which originates from the difference in thermal expansion coefficients between the films and the substrates. The final stress ranged from 8×108 to 2×109 Pa depending on the silicide and the substrate material. The biaxial elastic moduli were found to be 3.4×1011 Pa for TaSi2.4, 3.9×1011 Pa for TaSi1.4, and 2.2×1011 Pa for TiSi2.4. T...

Crystalline Phase Change in Yttria‐Partially‐Stabilized Zirconia by Low‐Temperature Annealing

Abstract: Changes in the phase composition and microstructure of yttria-partially-stabilized zirconia by low-temperature annealing were investigated at 100° to 500°C using bodies sintered from coprecipitated fine ZrO2-Y2O3 powders at varied temperatures. Tetragonal zirconia on the surfaces of bodies sintered at <1500°C transformed to the monoclinic phase at 100° to 400°C. Transformation behavior was strongly affected by grain size.

Preparation of SiO2 Glass from Model Powder Compacts: II, Sintering

Abstract: The sintering behavior of powder compacts formed from spherical, nearly monosized SiO2 particles was investigated. Highly ordered compacts sintered to high density and translucency at 1000°C. In contrast, less homogeneous samples prepared from flocculated suspensions remained highly porous after sintering under the same conditions. Densification kinetics were determined over the temperature range 900° to 1050°C for ordered compacts. The viscosity at each sintering temperature and the activation energy for viscous flow were determined using available sintering models. Sintering of ordered compacts is divided into several stages. Densification, mercury porosimetry, and electron microscopy results indicate that the first stage is dominated by the shrinkage of three-particle pore channels, whereas the second stage primarily involves the shrinkage of four-particle pore channels.

Fracture Behavior of W-Ni-Fe Heavy Alloys

Abstract: Heavy alloys were liquid phase sintered from compacts of mixed W, Ni, and Fe powders. The alloy compositions ranged from 93 to 97 wt pct W, with the Ni:Fe ratio maintained at 7:3. Sintering was performed under hydrogen in the 1465 to 1485 °C temperature range, giving full density within the first 15 minutes. The room temperature strength and ductility showed major degradation with sintering times in excess of two hours. Tensile tests and bend tests have been performed to isolate the fracture mode and the property determining features. Initial cracking occurs at the tungsten-tungsten grain boundaries and in the tungsten grains. These latter cracks propagate through the structure to give eventual failure. The ductility to failure is shown to be governed by the strength of the tungsten-matrix interface. The maximum elongation depends on the contiguity, which in turn is set by the alloy composition.

Isothermal Sintering of Cordierite-Type Glass Powders

Abstract: Uniaxially pressed pellets of cordierite-type glass powders were sintered in air at 800°, 820°, 840°, and 860°C. There was pronounced anisotropy of shrinkage for these pellets composed of nonspherical crushed glass particles, and the initial axial shrinkage rate was one-half that of the diameter, as though axial particles had twice the radius of diametral ones. The total fractional shrinkage along axes was only 0.12, whereas that of diameters was 0.17. The temperature dependence of sintering followed that of the solid glass viscosity, which is described by the Fulcher equation using independently measured viscosity data. Viscosity decreased approximately an order of magnitude for every 40°C temperature increase. Surface tension calculated from published coefficients for the molar constituents was 0.36 N/m. Linear shrinkage obeyed the Frenkel viscous flow sintering model well, until complete densification was approached, where particles coalesced and thereby lost their identity. A Mackenzie-Shuttleworth analysis, which is based on pore shrinkage, demonstrates the departure of these real powders from ideality. A simple Avrami-type equation permits an empirical fit of data for the entire sintering range when the shrinkage value for 100% densification is known.

Fatigue properties of carbon- and porous-coated Ti-6Al-4V alloy.

Abstract: A porous metal coating applied to a solid substrate implant has been shown, in vivo, to offer advantages over current polymethylmethacrylate cement fixation in orthopedic devices. These advantages may be lost, however, in devices requiring a sintering heat treatment to apply the coating since these treatments may have a detrimental effect on the substrate material mechanical properties. In addition, more biocompatible interface coating materials have come of interest with recent literature reports of metal ion release. These coatings may be of particular use in porous-coated systems since the surface area of implant in contact with the surrounding tissues is greatly increased. This study investigated the effects that both a porous Ti- 6Al -4V alloy coating and a ULTI carbon coating have on the fatigue properties of a Ti- 6Al -4V alloy substrate system. The fatigue properties of uncoated as-received, uncoated sinter heat treated and notched Ti- 6Al -4V material were also investigated. The results of this study revealed endurance limits for Ti- 6Al -4V alloy tested with a rotating beam system of 617 MN/m2 (uncoated as-received), 624 MN/m2 ( ULTI carbon-coated), 377 MN/m2 ( sinter heat treated), 220 MN/m2 (notched) and 138 MN/m2 (porous-coated). No effects on fatigue properties were observed when testing the material in saline compared with air. The slight increase in fatigue strength for the carbon-coated material is thought to be due to the increase in surface hardness resulting from the formation of titanium carbides on the surface. The low-endurance limit of the porous-coated material is due to both the transition from the as-received equiaxed microstructure to a lamellar microstructure upon sintering and to the notch effect created by the porous coating.

Variability of the Sr/Ti Ratio in SrTiO3

Abstract: TiO2 is observed as a second phase in SrTiO3 having Sr/Ti ≤ 0.995 (≥0.5 mol% excess TiO2). The effect of excess TiO2 on the equilibrium electrical conductivity at 1000°C is consistent with a solubility of <0.1 mol% TiO2 with the formation of unassociated oxygen vacancies. More Ti02 is retained in solid solution when samples are quenched from the sintering temperature rather than furnace-cooled. The effect of excess SrO on the equilibrium electrical conductivity also indicates some solid solubility.

Microstructure and mechanical properties of mullite-ZrO2 composites

Abstract: The sintering, microstructure and toughness of mullite/ZrO2 composites with increasing amount of ZrO2 (0 to 20 vol %) have been studied. A very active premullite powder has been used as matrix. TheKIC values increase from 2.1 to 3.2 MN m−3/2 as the volume fraction of zirconia increases from 0 to 0.2. The realtive fraction of tetragonal zirconia decreases as the volume fraction of ZrO2 increases to reach ∼0.1 in the sample with 0.2 volume fraction of ZrO2. The presence of ZrO2 enhances the sintering rate and end-point density of the composites. Finally, the increasing toughness in mullite/ZrO2 composites has been explained by a grain boundary strengthening mechanism produced by a metastable solid solution (∼0.5 wt %) of ZrO2 in mullite.

Modeling of electrical response for semiconducting ferrite

Abstract: The electrical properties of semiconducting ferrite materials are investigated. A simplified model, consisting of bulk material separated by highly resistive grain boundary layers, is proposed. The nature of grain boundary on its electrical behavior is also discussed. Complex impedance technique is used to analyze the electrical response of polycrystalline specimens. The characteristics of the complex impedance dispersion curves for MnZn‐ferrite specimens are affected by the sintering and annealing conditions. All the complex impedance plots differ from idealized semicircles, but can be fitted by circular arcs very well. An experimental equivalent circuit is proposed to explain the observed R‐X dispersion. The tremendous increase in the grain boundary resistivity due to controlled‐atmosphere sintering is ascribed to the preferential oxidation. The decrease in the apparent resistance of the specimens in the high‐frequency region arises from the polarization effect. This is attributed to the special microstructure of such material, namely, semiconducting bulk material with insulating grain boundaries.

Synthesis of sinterable SiC powders by carbothermic reduction of gel-derived precursors; and pyrolysis of polycarbosilane

Abstract: Silicon carbide powders were synthesized by 1) carbothermic reduction of carbon-doped silica gel derived from colloidal silica or methyltrimethoxysilane and 2) pyrolysis of polymeric SiC precursors. Powder characteristics, including sinterability, depended on the type and composition of the precursor. The colloidal silica-sucrose and -pitch SiC powders doped with boron and carbon were sintered to 95% to 96% of theoretical density at 2100C in argon. The microstructures of these sintered SiC bodies consisted of mixtures of elongated grains (up to 50 m) and equiaxed grains (approx. = 10 m) with residual pores (up to 8 m) at grain boundaries or inside grains. 12 references, 7 figures, 2 tables.

Sintering of olivine and olivine-basalt aggregates

Abstract: The sintering behavior of olivine and olivine-basalt aggregates has been examined at temperatures near 1,300° C. Experimental factors contributing to rapid sintering kinetics and high-density, fine-grained specimens include: (i) the uniform dispersion of basalt throughout the specimen, (ii) a very fine, uniform particle size for the olivine powder, (iii) oxygen fugacities near the high PO2 end of the olivine stability field, and (iv) rapid heating to the sintering temperature. Olivine-basalt specimens prepared from olivine particles coated with a synthetic basalt achieve chemical and microstructural equilibrium more rapidly, as well as produce higher density and finer grain-sized aggregates, than do specimens prepared by mechanical mixing of olivine and natural basalt powders. The grain boundary mobility for olivine, measured for olivine-basalt aggregates which have undergone secondary recrystallization, is on the order of 2×10−15 (m/s)/(N/m2) in the temperature range 1,300–1,400° C. Solution-precipitation (pressure-solution) processes make an important contribution to the development of the microstructure in olivine-basalt aggregates.

Abrasive surface coating process for superalloys

Abstract: A combination of sintering, plasma arc spraying, hot isostatic pressing and chemical milling is used to form an abrasive surface on an article. Alumina coated silicon carbide particulates are clad with nickel and sinter bonded to the surface of a superalloy turbine blade tip. An impermeable layer of plasma arc sprayed superalloy matrix is deposited over the particulates and then has its inherent voids eliminated by hot isostatic pressing. The abrasive material so formed on the surface is then machined to expose the particulates. Next, a portion of the matrix is removed so that the machined particulates project into space and are thus best enabled to interact with abradable ceramic air seals in a gas turbine engine. The ceramic particulates are sized so they are larger than the finished thickness of the abrasive and they have small aspect ratios. Thus, a high density spacing can be achieved while at the same time it is insured that matrix adequately surrounds the particles and holds them in place during use.

Metal oxide-coated copper powder

Abstract: Particles of Cu-containing metal having a particle size of 0.5-20 μm in largest dimension bearing a thin, substantially continuous coating of at least one metal oxide having a free energy of formation more negative than -98 kcal/mole and method of making them. The metal oxide-coated particles have improved sintering and shrinking properties which closely match those of green ceramic tape.