Eugene A. Olevsky

Consolidation enhancement in spark-plasma sintering: Impact of high heating rates

Spark-plasma sintering (SPS) provides accelerated densification and, in many cases, limited grain growth compared to regular hot pressing and sintering. Possible mechanisms of this enhancement of the consolidation in SPS versus conventional techniques of powder processing are identified. The consolidation enhancing factors are categorized with respect to their thermal and nonthermal nature. This paper analyses the influence of a major factor of thermal nature: high heating rates. The interplay of three mechanisms of material transport during SPS is considered: surface diffusion, grain-boundary diffusion, and power-law creep. It is shown that high heating rates reduce the duration of densification-noncontributing surface diffusion, this favors powder systems’ sinterability and the densification is intensified by grain-boundary diffusion. Modeling indicates that, besides the acceleration of densification, high heating rates diminish grain growth. The impacts of high heating rates are dependent on particle s...

Mechanical properties and the laminate structure of Arapaima gigas scales

The Arapaima gigas scales play an important role in protecting this large Amazon basin fish against predators such as the piranha. They have a laminate composite structure composed of an external mineralized layer and internal lamellae with thickness of 50-60 μm each and composed of collagen fibers with ~1 μm diameter. The alignment of collagen fibers is consistent in each individual layer but varies from layer to layer, forming a non-orthogonal plywood structure, known as Bouligand stacking. X-ray diffraction revealed that the external surface of the scale contains calcium-deficient hydroxyapatite. EDS results confirm that the percentage of calcium is higher in the external layer. The micro-indentation hardness of the external layer (550 MPa) is considerably higher than that of the internal layer (200 MPa), consistent with its higher degree of mineralization. Tensile testing of the scales carried out in the dry and wet conditions shows that the strength and stiffness are hydration dependent. As is the case of most biological materials, the elastic modulus of the scale is strain-rate dependent. The strain-rate dependence of the elastic modulus, as expressed by the Ramberg-Osgood equation, is equal to 0.26, approximately ten times higher than that of bone. This is attributed to the higher fraction of collagen in the scales and to the high degree of hydration (30% H(2)O). Deproteinization of the scale reveals the structure of the mineral component consisting of an interconnected network of platelets with a thickness of ~50 nm and diameter of ~500 nm.

SHOCK CONSOLIDATION: MICROSTRUCTURALLY-BASED ANALYSIS AND COMPUTATIONAL MODELING

The most important microstructural processes involved in shock consolidation are identified and discussed; the energy dissipated by a shock wave as it traverses a powder is assessed. The basic microstructural phenomena are illustrated for a metal (nickel-based superalloy), an intermetallic compound (rapidly solidified Ti3Al), and ceramics (silicon carbide). Interparticle melting, vorticity, voids, and particle fracture are observed and the plastic deformation patterns are identified. Various energy dissipation processes are estimated: plastic deformation, interparticle friction, microkinetic energy, and defect generation. An analytical expression is developed for the energy requirement to shock consolidate a powder as a function of strength, size, porosity, and temperature, based on a prescribed interparticle melting layer. This formulation enables the prediction of pressures required to shock consolidate materials; results of calculations for the superalloy and silicon carbide as a function of particle size and porosity are represented. The fracture of ceramic particles under shock compression is discussed. Tensile stresses are generated during compaction that may lead to fracture. It is shown that the activation of flaws occurs at tensile reflected pulses that are a decreasing fraction of the compressive pulse, as the powder strength increases. These analytical results are compared to numerical solutions obtained by modeling the compaction of a discrete set of particles with an Eulerian finite element program. These results confirm the increasing difficulty encountered in shock consolidating harder materials, and point out three possible solutions: (a) reduction of initial particle size; (b) reduction of shock energy; (c) post-shock thermal treatment. Two possible and potentially fruitful approaches are to shock densify (collapse voids with minimum bonding) powders and to apply post-shock thermal treatments, and to shock consolidate nanosized powders. The latter method requires high shock energy and careful minimization of the shock reflections.

Effect of gravity on dimensional change during sintering—I. Shrinkage anisotropy

The effects of gravity on sintering shrinkage and dimensional uniformity are analyzed using a continuum theory of sintering. Shape change caused by gravity during sintering is described both analytically and numerically. For a cylindrical sample shape, analytical approximations to characterize gravity-induced nonuniformities in shrinkage and compact aspect ratio are shown. As an illustration of the concept, analysis is performed for the solid-phase sintering of a copper powder cylindrical specimen to replicate earlier experiments reported by Lenel et al. (Trans. Am. Inst. Min. Engrs, 1963, 227, 640) and Exner (Rev. Powder. Metall. Phys. Ceram. Soc., 1968, 51, 604). The intensity of shrinkage anisotropy is compared for viscous and diffusional mechanisms of sintering. An algorithm is introduced for minimization of gravity-induced shrinkage anisotropy, suggesting an asymptotic approach to the peak temperature for best dimensional uniformity.

Combustion synthesis/densification of an Al2O3–TiB2 composite

The self-propagating gasless combustion reaction 3TiO2+ 3B2O3+ 10Al 5Al2O2+ 3TiB2 was used to produce an Al2O3 –TiB2 composite, which was densified by uniaxial loading immediately following completion of reaction. The densification was enabled by the high temperatures produced by the combustion reaction ( 2000°C) which rendered the reaction product ( 70% porosity) plastic. The microstructure was characterized by columnar TiB2 grains with a diameter of 1–2 m and length of 5–10 m embedded in equiaxed A12O3 (grain size 50m); the TiB2 phase tended to agglomerate in clusters. A few of the TiB2 grains exhibited dislocations, while the A12O3 was annealed. This indicates that recovery processes took place after the plastic deformation involved in densification. Several constitutive models (corresponding both to rigid-plastic and power-law creep material behavior) were used to describe the mechanical response of the porous and ductile ceramic product and compared to the experimental results, with satisfactory agreement for power-law creep models. These constitutive models have a temperature-dependent term that incorporates the effect of specimen cooling, that occurs concurrently with densification; thus, it was possible to obtain a flow stress dependence of temperature which is in reasonable agreement with values interpolated from literature experimental results.

Effect of gravity on dimensional change during sintering—II. Shape distortion

Abstract Phenomena of shape distortion caused by gravity under solid- and liquid-phase sintering are described numerically using the finite-element method based upon a continuum theory of sintering. Finite-element analysis is performed for the solid-phase sintering of a copper powder cylindrical specimen to replicate earlier experiments reported by Lenel et al ( Trans. Am. Inst. Min. Engrs , 1963, 227 , 640). The results of the finite-element calculations are also compared with experimental data on liquid-phase sintering of a W–Ni–Fe powder system using a quantitative criterion of shape distortion. It is demonstrated that shape distortion accumulated during solid-phase sintering should be taken into account when considering gravity-induced configuration—dimensional changes under liquid-phase sintering. The finite-element analysis of shape distortion under liquid-phase sintering reproduces an “elephant foot” shape—a final shape observed in sintering practice.

Instability of sintering of porous bodies

Sintering models are discussed and used to analyze flow instabilities that may arise during preliminary compaction of powders. These instabilities can be at the origin of heterogeneities in the densification. The material is modeled as a viscoplastic thermal sensitive porous material. The modeling includes the limit case of a linear viscous material. The eAects of sintering conditions (temperature and pressure in the case of pressure sintering) and the eAects of material characteristics such as porosity, heat capacity, theoretical density, surface tension, particle size and creep parameters on stability of sintering are investigated. The heat release associated with the plastic flow is shown to sometimes have an important role. Stability criteria are derived and applied to the analysis of sintering and hot isostatic pressing, using various sintering models. These stability criteria can be used to optimize the densification process; one can control, for example, temperature so as to avoid any instability. Stability maps enabling an optimization of temperature‐pressure regime in hot isostatic pressing are built for sample metal (nickel) powder. # 2000 Elsevier Science Ltd. All rights reserved.

Localized Overheating Phenomena and Optimization of Spark-Plasma Sintering Tooling Design

The present paper shows the application of a three-dimensional coupled electrical, thermal, mechanical finite element macro-scale modeling framework of Spark Plasma Sintering (SPS) to an actual problem of SPS tooling overheating, encountered during SPS experimentation. The overheating phenomenon is analyzed by varying the geometry of the tooling that exhibits the problem, namely by modeling various tooling configurations involving sequences of disk-shape spacers with step-wise increasing radii. The analysis is conducted by means of finite element simulations, intended to obtain temperature spatial distributions in the graphite press-forms, including punches, dies, and spacers; to identify the temperature peaks and their respective timing, and to propose a more suitable SPS tooling configuration with the avoidance of the overheating as a final aim. Electric currents-based Joule heating, heat transfer, mechanical conditions, and densification are imbedded in the model, utilizing the finite-element software COMSOL™, which possesses a distinguishing ability of coupling multiple physics. Thereby the implementation of a finite element method applicable to a broad range of SPS procedures is carried out, together with the more specific optimization of the SPS tooling design when dealing with excessive heating phenomena.

Modelling of anisotropic sintering in crystalline ceramics

We present a model that describes anisotropic shrinkage during sintering in a powder compact of aligned, elongated particles by deriving the anisotropic sintering stress and the anisotropic generalized viscosity as a function of material and geometric parameters. The powder compact consists of elongated particles, which are perfectly aligned and simply packed with elliptical pores at all the quadra-junctions between the particles. The model considers mass transport by grain boundary diffusion and surface diffusion. Shrinkage rates are calculated for a variety of geometries and are compared to kinetic Monte Carlo simulations.

Densification mechanisms of spark plasma sintering: multi-step pressure dilatometry

The effects of electrical current and mechanical pressure on the densification of spherical copper powder during spark plasma sintering (SPS) are examined. A novel multi-step pressure dilatometry method is introduced to compare the constitutive behavior of the copper powder under nearly equivalent current-insulated and current-assisted SPS process conditions. The strain rate sensitivity agrees with that predicted for a dislocation climb-controlled creep densification mechanism for both processing setups. Accelerated densification rate leading to a higher final relative density is observed for the current-assisted SPS.