In this study, the spark plasma sintering (SPS) is employed to consolidate poorly sinter-able ultra-high temperature ceramic (UHTC) powders due to the fact that the conjoint application of electric current and mechanical pressure during SPS can largely offset the required processing temperature. Zirconium carbide (ZrC) is selected as target material as it broadly represents properties of typical UHTCs. Investigations on SPS of ZrC are concurrently conducted in two correlated regimes: One regime is used to optimize the SPS densification efficiency by manipulating the loading schematics. The other regime is used to produce complex shape carbide components for high temperature applications via SPS. Both theoretical and experimental studies are involved in the achievement of the formulated research objectives.

Consolidation of ZrC has been carried out to form a densification map with determining the optimal processing parameters. The densification of ZrC is studied through the continuum theory of sintering, in which the ZrC power-law creep parameters have been determined through the clarification of electrical and thermal aspects of the employed SPS system. Then the SPS-forging setup is proposed as it is theoretically and experimentally proven to be able to render more densification than the regular SPS. SPS-forging and regular SPS are eventually integrated into a hybrid loading mode SPS regime to combine the advantages of the individual setups to obtain the optimal densification kinetics.

Annular shape ZrC pellets have been fabricated using SPS. Finite element modeling framework is constructed to manifest the thermomechanical interactions during the SPS of annular shape ZrC specimens. The fabrication procedures are practically adapted to produce also annular shape carbide composites with excellent high temperature structural strength being used as alternative SPS tooling components. The applicability of annular shape fuel pellet to accommodate volume swelling under its service conditions is investigated. The irradiation-induced swelling phenomena are analyzed by analytical modeling and finite element simulations, in which the generated fission products are considered to be the sources of the fuel pellet swelling.

The efficiency of Spark Plasma Sintering (SPS) is optimized by means of a temperature-based procedure. The correlations between temperature, porous material structure and tooling geometry are explored. The issue of agglomeration, cause of the formation of hierarchical porous structures in the powder compact, is addressed with an analytical and numerical approach. Densification kinetics, sintering stress, bulk and shear moduli of agglomerated compacts are modeled as functions of the hierarchy characterizing the porosity and the nonlinear viscous rheology of the material. The material nonlinearity is expressed by the strain rate sensitivity parameter, which in turn is dependent on temperature. An empirically-based explicit formulation of strain rate sensitivity as a function of temperature is provided. A subsequent tuning of the strain rate sensitivity allows the optimization of the thermal regime in order to obtain in situ deagglomeration of the SPS specimen or the production of tailored material structures. In order to guarantee that the optimized thermal conditions are experienced by the entire specimen, we study the geometry of the SPS tooling setup. The tooling influence on temperature distributions is assessed in both the radial and the axial directions by means of a combined experimental and finite-element study. The phenomena underlying the presence of thermal gradients are individuated and controlled by tailoring the design of the tooling setup. Novel geometries are proposed for the components of the tooling, capable to annihilate the temperature disparities and thus complete the optimization of the SPS procedure. A comprehensive finite-element framework, in which both agglomerated powder compacts and tooling setup can be reconstructed and improved, is therefore developed.

Spark plasma sintering (SPS) has clear advantages of fast densification and lower grain growth compared with other powder consolidation techniques such as hot pressing (HP). Many researchers studied intrinsic electric current effects during the SPS process, yet the results of these studies are still controversial due to the complexity of the SPS technology.

In this study, we investigate various consolidation methods with and without electric current assistance to achieve the deconvolution of the electric current effects from the temperature effects. The spatial distribution of the electric current passing through the powder during SPS is modeled using the finite element method. The porosity-interparticle neck area geometrical relationship is utilized to estimate the electric current density inside the powder volume subjected to SPS. For the first time, by taking into account the explicit influence of the electric current effect on the SPS densification mechanisms, the governing equations describing hot pressing are modified to enable an SPS-specific constitutive analysis. The densification mechanisms of various ceramic (ZrN) and metallic (W and Mo) powders are determined by the inverse regression of the new SPS constitutive equations and by utilizing the experimental results on powder consolidation with and without the participation of the electric current effect.

We show that the electron backscatter diffraction (EBSD) technique can be a useful tool to demonstrate the densification mechanism of the powders during the sintering. The grain size and grain growth factors are incorporated into the sintering constitutive equation too to elucidate the grain growth effect on the material behavior governing parameters.

Additionally, the known but controversial intrinsic electric current effects such as surface cleaning and retardation of the grain growth are analyzed for the consolidation of the Mo nanopowders using SPS.

Finally, an energy efficient and fast consolidation technique utilizing the highly concentrated electric current flow through a conductive powder by manipulating the electric current trajectory is developed.

Ceramics are known for their wear resistance, hardness, strength, and biocompatibility yet the industrial application of these materials in the orthopedic industry is limited due to the difficulty and cost of manufacturing. Additive Manufacturing (AM) via ink-based powder bed 3D printing provides an exciting opportunity to address these issues in the production of implantable orthopedic components. However, the successful production of mechanically and geometrically compatible bioceramic parts has not been achieved using this coveted technology because the use of advanced sintering technologies together with AM has not been sufficiently investigated. In this work, a multi-faceted comprehensive study was performed which includes experimental testing, detailed characterization, analytical modeling, and finite element simulation of net shape bioceramic manufacturing using Sintering Assisted Additive Manufacturing (SAAM). By integrating the solvent jetting 3D printing method with advanced sintering technologies, high density bioceramic components with tailored geometrical, physical and mechanical properties were produced and provide a novel approach to current bone repair solutions.

In recent times, research efforts have been focused on Additive Manufacturing (AM) technologies, be it high or low energy-based, due to the ability to produce very complex and near-net shaped components. The overall aim of any AM technique is to produce parts that have industrial applicability. However, additively manufactured parts exhibit low service life as a result of part failure during use. These part failures usually arise due to the presence of defects such as excessive porosity, part distortion and property anisotropy. In order to meet the aspirations of using parts manufactured via AM without the fear of catastrophic failure, there is therefore the need for research efforts to be geared towards developing novel approaches to mitigating and eliminating unintentional defects in additively manufactured parts. This body of research work aims to improve on the current understanding of the defect mitigation strategies in additively manufactured parts by contributing towards accelerating new discoveries of innovative approaches in eliminating detrimental defects in high and low energy AM processes. We explored the Binder/Solvent Jetting and the Selective Laser Melting AM technologies when considering low-energy and high-energy based AM processes respectively, with a focus on mitigating excessive porosity and part distortion defects. Our approach to the research work was fundamental in nature, involving both experimental and analytical considerations from both a macroscopic and microscopic point of view and has taken the form of understanding the influence of process parameters on defect formation. We developed novel and innovative approaches to optimizing the AM processes for the production of defects free parts, and further systematically developed empirical relations between process parameters and output variables such as final part density and degree of distortion to help control defect formation from experimental observations and analytical considerations. For example, An empirical equation describing the distortion strain as a function of powder spreading parameters during binder jetting AM is suggested as an approximation of the numerical modeling results, while the results on selective laser melting of metal-matrix-composites show that the filling of the pores between rigid inclusions by the molten matrix is dependent on the laser dwell time, which in turn, depends on the volume fraction of ceramic reinforcement, initial pore sizes between inclusions and materials properties of the matrix phase such as viscosity, surface energy and initial pore sizes between rigid inclusions. Our developed process parameters were used with great success in manufacturing defects free AISI 316L austenitic stainless steel (SS316L) alloy, (SS316L)-WC and functionally graded (SS316L)-WC metal-matrix-composites. Furthermore, a comparative study was conducted to investigate the influence of the various AM processing route on the process-structure-property relationship of processed parts, with results indicting that due to the differences in the kinetics of the various processing routes investigated, different microstructures and hence mechanical properties can be realized.