Journal of the American Ceramic Society

Kang, Suk‐Joong L.; Bordia, Rajendra; Bouvard, Didier; Olevsky, Eugene

doi: 10.1111/j.1551-2916.2012.05312.xpmid: N/A

Luo, Jian; Kang, S.‐J.

doi: 10.1111/j.1551-2916.2011.05059.xpmid: N/A

This article critically assesses the current status and future directions for the development of interfacial phase diagrams for applications in activated sintering and other fields. The origin of solid‐state activated sintering is attributed to the enhanced mass transport in sintering‐aid‐based, nanoscale, quasi‐liquid, interfacial films that are stabilized below the bulk solidus line. Interfacial thermodynamic models have been developed via extending a phenomenological premelting theory and incorporating the computational thermodynamic (CalPhaD) methods. A primitive type of interfacial phase diagrams, λ‐diagrams, have been computed, and these diagrams have been validated by experiments and proven useful. More rigorous interfacial phase diagrams with well‐defined transition lines and critical points may also be constructed. A long‐range scientific goal is proposed to develop interfacial phase diagrams as a new materials science tool. Future studies should be conducted in several areas to achieve this goal, and special efforts should be made to predict the complex interfacial phase behaviors in multicomponent ceramic materials. Potential broad applications are envisaged.

Cardona, Cristina G.; Tikare, Veena; Patterson, Burton R.; Olevsky, Eugene; Bordia, R.

doi: 10.1111/j.1551-2916.2012.05164.xpmid: N/A

Microstructural evolution during sintering can be simulated using the Potts kinetic Monte Carlo model. This model simulates detailed evolution of the powder particles, pore shapes, neck growth, and other microstructural features with sufficient resolution over a sufficiently large compact so that interfacial energies and curvatures of a statistically representative sample of surfaces in a complex compact can be obtained from the simulations. In this work, we present a technique based on measuring curvature of surfaces to obtain sintering stress of sintering powder compacts with arbitrarily complex geometries of powder size and powder shape distributions. The method is applied to three distinct powder compacts with very different sintering behavior to obtain sintering stress for each of these cases. The sintering stress for the three simulated cases were distinct and dependent on the geometric microstructural details of the powder compacts.

Maximenko, Andrey; Kuzmov, Andrey; Grigoryev, Evgeny; Olevsky, Eugene; Bordia, R.

doi: 10.1111/j.1551-2916.2012.05083.xpmid: N/A

A new multi‐scale numerical approach for the modeling of sintering of macroscopically inhomogeneous materials is put forward. The new approach does not require the formulation of material constitutive equations: it specifies material properties through the definition of macroscopic unit cells. As a result, the influence of any number of material structure parameters on sintering kinetics and on specimen distortion can be investigated. The method is based on the consideration of the sintered body macroscopic behavior in parallel with an online analysis of the mesoscopic evolution of the unit cell structure. The developed modeling approach provides the information on the sintering progress at both macro‐ and meso‐scale levels. The examples of diffusion sintering of ceramic composites and of viscous sintering of a bi‐layer porous specimen containing voids of anisotropic shapes are considered.

Wakai, Fumihiro; Bordia, Rajendra K.; Blendell, J.

doi: 10.1111/j.1551-2916.2012.05211.xpmid: N/A

The microstructures of sintering bodies become anisotropic in constrained sintering of a thin film on rigid substrate, as well as in stress‐assisted densifications such as sinter forging and hot pressing. The deformation and the stress states in the body have been described by general constitutive equations using sintering stress tensor and viscosity tensor. These macroscopic quantities were determined from local microstructure and microscopic kinetics for the special case of face‐centered tetragonal structure. The simulation results were applied to the cases of sinter forging and constrained sintering and qualitatively compared to the experimental results available in the literature. This model is able to predict both the evolution of the anisotropic microstructure during sintering, and also the effect of the local micro‐structure on anisotropic shrinkage.

Bruchon, Julien; Pino‐Muñoz, Daniel; Valdivieso, François; Drapier, Sylvain; Bouvard, D.

doi: 10.1111/j.1551-2916.2012.05073.xpmid: N/A

This article proposes a numerical strategy to simulate the mass transport by surface and lattice diffusion into a granular packing. This strategy is based on two cornerstones. First, the developed approach is based on a Eulerian description of the problem: the grains are described by using a Level‐Set function, and can evolve through a fixed mesh, with respect to the physical laws. In this way, the mesh does not experience large distortions and topological changes, such as the formation of necks or of closed porosity, are implicitly taken into account by the Level‐Set method. Second, the computation of the mechanical state into the grains is directly performed when considering the lattice diffusion route. Hence, a mechanical problem, coupling the grain elastic behavior to the fluid behavior of the surrounding phase, is established and solved by finite element. The diffusion flux is then related to the gradient of the pressure field. The results obtained with this numerical strategy are compared with success to the usual geometrical models for two spherical grains. The possibilities of the numerical approach are shown by presenting the changes occurring by lattice diffusion into a granular packing.

Olevsky, Eugene A.; Bradbury, William L.; Haines, Christopher D.; Martin, Darold G.; Kapoor, Deepak; Kang, S.‐J.

doi: 10.1111/j.1551-2916.2012.05203.xpmid: N/A

Scalability experiments on the spark plasma sintering (SPS) of similarly shaped alumina specimens of the four different sizes are conducted. The utilized experimental methodology, based on the principle of rigorous proportionality of all the specimen and tooling dimensions, employs two different SPS devices of different scales. The processed specimens are characterized in terms of relative density and grain‐pore structure.

Olevsky, Eugene A.; Garcia‐Cardona, Cristina; Bradbury, William L.; Haines, Christopher D.; Martin, Darold G.; Kapoor, Deepak; Kang, S.‐J.

doi: 10.1111/j.1551-2916.2012.05096.xpmid: N/A

A comprehensive three‐dimensional fully coupled thermo‐electro‐mechanical finite element framework is developed for modeling spark plasma sintering (SPS). The finite element model is applied to the simulation of spark plasma processing with four different tooling sizes and various temperature regimes. The comparison of modeling and experimental results shows that the model is reliable for qualitative predictions of the densification behavior and of the grain growth in powder specimens subjected to SPS with a given temperature regime. The conducted modeling indicates the possibility of changing the heating pattern of the specimen (warmer central areas of the specimen's volume and cooler outside areas or vice versa) depending on the size of the tooling. High heating rates and large specimen sizes elevate the temperature and, in turn, material structure gradients during SPS processing. The obtained results suggest that the industrial implementation of SPS techniques should be based on the predictive capability of reliable modeling approaches.

Grasso, Salvatore; Hu, Chunfeng; Maizza, Giovanni; Sakka, Yoshio; Olevsky, E.

doi: 10.1111/j.1551-2916.2011.05009.xpmid: N/A

A new method was developed to fully consolidate binderless tungsten carbide and diamond powders by means of spark plasma sintering (SPS) in current‐control mode (CCm). Below 900°C, the 2 cm diameter sample was slowly heated by a dc current of 1000 A. Above 900°C the imposed current was suddenly raised to 4000 A. The combination of the relatively high heating rate of 2000°C/min and the relatively short holding time of 1.5 min (above 1300°C) was successful to fabricate fully dense binderless WC/diamond composite. No graphitization of diamond was detected after ultrafast sintering as confirmed by optical and SEM microstructure observations, XRD and Raman analysis. The sample showed very high wear resistance in comparison to fully dense monolithic binderless WC compacts. The developed method, unlike previously published works, did not require any diamond coating to prevent diamond graphitization.

Lee, Hyun‐Wook; Moon, San; Choi, Chang‐Hak; Kim, Do Kyung; Kang, S.‐J.

doi: 10.1111/j.1551-2916.2012.05085.xpmid: N/A

A facile synthetic strategy was implemented to obtain nanosized barium titanate (BaTiO3) powders with tetragonal structure. The nanoparticles were synthesized using solvothermal process employing diethanolamine and triethanolamine to suppress the particle growth and the as‐prepared nanopowders were characterized using X‐ray diffraction, scanning electron microscopy, and high‐resolution dispersive Raman spectroscopy. It was found that the particle size can be easily tuned by adjusting the experimental parameters while retaining the tetragonality. The average diameters of the particles prepared with and without the organic amines were found to be 80 and 100 nm, respectively. All the synthesized BaTiO3 nanopowders exhibit a narrow size distribution with a uniform morphology. Rietveld refinement of the XRD patterns and Raman spectra revealed that the synthesized BaTiO3 nanopowders have tetragonal asymmetry dominant structures. A slight decrease in the tetragonality of the prepared powders with decrease in particle size is attributed to the presence of cubic shell layer and inner defects. The tetragonal‐dominant structure was also confirmed by normalizing the peak area of the Raman spectra.