The unique features of the Spark plasma sintering (SPS) were used to investigate the sintering and deformation behaviour of titanium and titanium–titanium diboride composites, and to control the sintering and grain growth of ferroelectric Na0.5K0.5NbO3 (NKN) and of hydroxyapatite (HAp). In the SPS the samples experience a temperature different from that recorded by the thermocouple (pyrometer) used and this temperature difference has been estimated for Ti and NKN.   Sintering and deformation of titanium was investigated. Increasing heating rate and/or pressure shifted the sintering to lower temperatures, and the sintering and deformation rates changed when the α→β phase transition temperature was passed. Fully dense Ti/TiB2 composites were prepared. The Ti/TiB2 composites could be deformed at high temperatures, but the hardness decreased due to the formation of TiB.    The kinetic windows within which it is possible to obtain fully dense NKN and HAp ceramics and simultaneously avoid grain growth are defined. Materials have a threshold temperature above which rapid and abnormal grain growth takes place. The abnormal grain growth of NKN is due to a small shift in the stoichiometry, which in turn impairs the ferroelectric properties. Fully transparent HAp nanoceramics was prepared, and between 900 and 1050 oC elongated grains are formed, while above 1050 oC abnormal grain growth takes place.NKN samples containing grains of the sizes 0.35–0.6 µm yielded optimum ferroelectric properties, i.e. a high remanent polarization (Pr = 30 µC/cm2) and high piezoelectric constant (d33= 160 pC/N). The ferroelectric domain structure was studied, and all grains exhibited a multi-domain type of structure. / At the time of doctoral defense the following articles were unpublished and had a status as follows: Article 4: Manuscript; Article 5 : Manuscript

Currently, there is a high demand for lightweight aerospace materials, driven by the desire to provide enhanced fuel efficiency by reducing vehicular weight. Aluminum alloys are attractive due to their excellent mechanical properties and high strength to weight ratios. Powder metallurgy (PM), which converts metal powder into a high performance product, presents an alternative to traditional forming techniques, which are often unable to provide adequate dimensional tolerances. The challenge is to determine if aluminum PM alloys and technologies can be successfully employed within aerospace applications. This research focuses on the PM processing technologies (die compaction, cold isostatic pressing (CIP), and spark plasma sintering (SPS)) of two alloys, PM2024 and PM7075. Processing parameters were assessed using attributes such as density, hardness, and tensile properties. Both powders showed comparable densities and tensile properties to their wrought equivalents. Ultimately, the groundwork was laid for future research into these alloys and their processing methods.

Spark plasma sintering (SPS), also known as direct current sintering (DCS) is an advanced sintering technique that and uses a continuous pulsed direct current to rapidly process materials through Joule heating and offers significant advantages and versatility over conventional sintering methods. The technique features in energy saving owing to high heating rates and is very suitable for consolidation as well as diffusion bonding of electrical conductive advanced ceramic materials such as ultra high temperature ceramics (UHTCs). However, cooling rate in SPS also plays an important role as it directly influences the generation of residual stress especially for specimens consist of dissimilar phases such as composites and laminates primarily due to CTE mismatch. Therefore, in order to produce high quality materials, a zirconium diboride with addition of silicon carbide (ZrB2-SiC) ultra high temperature ceramic composite is selected to investigate the effect of cooling rate in SPS on microstructure and mechanical properties. After being densified at the target temperature, ZrB2-25vol%SiC specimens are cooled from 1800°C using controlled cooling rates of 10 °C/minute to ~225.5 °C/minute (free cooling). A time dependent finite element analysis (FEA) model is used to simulate the temperature gradients across the specimens at dwell times and during the cooling processes. The residual stress within the specimens are experimentally verified using X-ray diffraction (XRD) and Raman spectrometry, and found maximum residual stress within the specimen cooled at 225.5 °C/minute. Peak Hardness and moderate elastic modulus is found for specimen sintered at 1800 °C and cooled at 100 °C/minute, which make this temperature and cooling rate appropriate conditions for future fabrication of UHTCs with similar thermal and electrical properties. These materials are of great interest for their excellent high-temperature capabilities, wear and corrosion resistance, and are regarded as material candidates for engineering applications in extreme environments. Therefore, development of an effective joining technique is important since near-net shape fabrication is challenging, and joints formed by brazing or conventional solid-state diffusion bonding limit the mechanical strength and high temperature applications of the base materials. Using SPS we have rapidly and successfully joined ZrB2 to hafnium diboride (HfB2) at 1750 and 1800 °C within a minute through electric current assisted solid-state diffusion bonding. The electric current enables localized Joule heating as well as plastic deformation of the mating surface asperities, and enhances the elemental interdiffusion process at the HfB2/ZrB2 interfaces owing to electromigration, which leads to the formation of ZrxHf1-xB2 solid solution. A series of characterization and analytical techniques including scanning electron microscopy (SEM), wavelength dispersive spectroscopy (WDS), electron backscatter diffraction (EBSD), and scanning transmission electron microscopy (S/TEM) are employed to study the microstructure and chemical composition at of the HfB2/ZrB2 interfaces. Apart from enhanced diffusion as a result of electromigration, the applied electric current can also be use to promote plastic deformation in ZrB2, which does not go through gross plastic deformation due to its extremely high melting point and brittle nature even when elevated temperature and pressure are applied. Through “electroplastic effect” (an effect based on electromigration) the mobility and multiplication of the existing dislocations in ZrB2 is enhanced, and a “metal-like” primary recrystallization phenomenon in the ZrB2 is observed meaning the material has experienced a sufficient amount of plastic deformation and reached the critical dislocation density and configuration for nucleation of “strain-free” grains. The average grain size of the recrystallized grain is only ½ of its original value. These findings suggest great potentials in microstructural tailoring and grain refinement of conductive advanced ceramics using SPS, and provide promising ideas for future fabrications and applications.

Tantalum carbide (TaC) and hafnium carbide (HfC) possess extremely high melting points, around 3900 oC, which are the highest among the known materials. TaC and HfC exhibit superior oxidation resistance under oxygen deficient and rich environments, respectively. A versatile material can be expected by forming solid solutions of TaC and HfC. However, the synthesis of fully dense solid solution carbide is a challenge due to their intrinsic covalent bonding which makes sintering challenging. The aim of the present work is to synthesize full dense TaC-HfC solid solutions by spark plasma sintering with five compositions: pure HfC, HfC-20 vol.% TaC (T20H80), HfC- 50 vol.% TaC (T50H50), HfC- 80 vol.% TaC (T80H20), and pure TaC. To evaluate the oxidation behavior of the solid solutions carbides in an environment that simulates the various applications, an oxygen rich, plasma assisted flow experiment was developed. While exposed to the plasma flow, samples were exposed to a temperature of approximately 2800 oC with a gas flow speed greater than 300 m/s. Density measurements confirm near full density was achieved for all compositions, with the highest density measured in the HfC-contained samples, all consolidated without sintering aids. Confirmation of solid solution was completed using x-ray diffraction, which had an excellent match with the theoretical values computed using Vegard’s Law, which confirmed the formation of the solid solutions. The solid solution samples showed much improved oxidation resistance compared to the pure carbide samples, and the T50H50 samples exhibited the best oxidation resistance of all samples. The thickness of the oxide scales in T50H50 was reduced more than 90% compared to the pure TaC samples, and more than 85% compared to the pure HfC samples after 5 min oxidation tests. A new Ta2Hf6O17 phase was found to be responsible for the improved oxidation performance. Additionally, the structure of HfO2 scaffold filled with molten Ta2O5 was also beneficial to the oxidation resistance by limiting the availability of oxygen.

The quasicrystals are characterized by unusual rotational symmetries that are not observed in the crystalline materials, which is the cause of their interesting material properties. Because of that a particular attention was paid to quasicrystalline structures in the literature research. The research also contains a description of electron beam technology, spark plasma sintering method and introduction to the problematics of wear. As the default materials for the experimental part were selected Titanium Grade 2 powder and Cristome A5 powder which was partially composed of quasicrystalline phase. The first series of samples was sintered only from powder Cristome A5. The second series was sintered from the mixture of 80 % Titanium Grade 2 powder and 20 % Cristome A5 powder. For the compaction of samples spark plasma sintering technology was selected. Samples were then systematically modified by electron beam and subjected to pin on disc tests. Samples modified at 750 °C had the best wear resistance. Samples modified at 1150 °C contained increased amount of quasicrystalline phase.

The diploma thesis deals with the preparation of the composite material based on the NiSi system using powder metallurgy supplemented by the sintering with the usage of SPS method (spark plasma sintering). Theoretical part contains general information about the mechanical-chemical process and sintering, while materials and methods used for experimental observation are explained in a separate chapter. Experimental part explains the procedure of the experiment and selected parameters of individual processes. In the experiment, two powder mixtures were created in order to form the NiSi phase in the maximum possible amount of powder material. After successfully reaching presence of the NiSi phase in the range of 87 to 89 wt. %, both mixtures were used to produce sintered samples at temperatures from 700 to 900 °C. Experiments showed the best results for sample number 2, which was sintered at 900 °C for 4 minutes. Resulting porosity was 0.9 % and hardness reached a maximum value of 718 HV 1. However, all sintered samples show cracks at room temperature associated with increased brittleness of the material.

Ceramic materials are in general characterized by high hardness, high modulus of elasticity, excellent abrasion resistance, etc. These properties make ceramics among others useful in optically transparent applications. An ideal form of optically transparent ceramic material is monocrystalline. However, the monocrystalline fabrication is expensive and/or time consuming. From this point of view polycrystalline ceramics is preferred. But the polycrystalline transparent ceramics fabrication is fraught with complications such as porosity, inappropriate grain size and insufficient purity. These circumstances could be solved by using sintering additives. This master’s thesis compiles literature research summarizing modern technologies of advanced ceramics sintering and ceramic polycrystalline microstructure dependence on its optical properties. The experimental part of this thesis focuses on the fabrication parameters of polycrystalline advanced ceramics based on Al2O3MgO and evaluation of their optical properties. Polycrystalline magnesium-aluminate spinel with sintering additive contents 0; 0.3 and 0.6 weight % LiOH was fabricated by optimalisation of Spark Plasma Sintering cycle. Fully dense ceramic samples of polycrystalline magnesium-aluminate spinel with favourable optical properties in visible spectrum radiation were achieved. Real In-line Transmission RIT and Total Forward Transmittance TFT were analysed. RIT exceeded 84 % at wavelength of 633 nm and TFT exceeded 83 % at wavelength above 860 nm. The decisive factors in terms of the optical properties of ceramics sintered with sintering additives were the amount of time-spending at high temperatures and the purity of ceramic powders.

Porous ceramic materials are an interesting group of materials due to a wide range of physical properties, low density, and good permeability. Production of a monolith with a shape stability that would also have a high specific surface area and high porosity is a common problem with porous ceramics. The goal of this work was to maintain the high specific surface area and to produce a monolith with a shape stability. Two forms of porous silica nanofibers (as prepared and milled) were used and partially sintered using the Spark Plasma Sintering method (SPS). Different sintering times and temperatures for SPS were tested. The findings revealed that the best SPS conditions were as follows: temperature: 600 °C, sintering time: 5 minutes, pressure: 3 MPa, and the heating rate: 144 °C/min. These sintering conditions resulted in a stable silica based machinable monolith made from fibers or milled fibers. The monoliths have the specific surface area of up to 470 m^2/g and porosity of 72 %, or the specific surface area of up to 422 m^2/g and porosity of 69 % for as prepared fibers and milled fibers, respectively.

Ce travail de thèse porte sur l’élaboration et l’étude des propriétés physiques de céramiques à base d’alumine optiquement transparentes et luminescentes pour lesquelles des applications sont envisagées dans le domaine des matériaux résistants aux rayonnements ionisants. L’enjeu de ce travail a consisté à obtenir un matériau qui présente simultanément une densité élevée et des tailles de grains à l’échelle nanométrique afin de conférer respectivement à la céramique des propriétés de transparence et une résistance aux radiations ionisantes par la capture et la recombinaison des charges induites au niveau des joints de grains.Des céramiques de composition Al2O3 et MgAl2O4 ont été consolidées par Spark Plasma Sintering (SPS) à partir d’alumines ultra-poreuses (UPA) d’une part, et par frittage réactif d’un mélange de précurseur Al2O3 : MgO dans un ratio 1 : 1 d’autre part. Les UPA ont été élaborés par un procédé original mis en place au LSPM. Elles ont ensuite été imprégnées par une solution de nitrate de magnésium puis calcinée à basse température afin d’obtenir le « Précurseur Nanostructuré » (PN) pour la phase spinelle MgAl2O4. Les PNs synthétisés, tout comme les UPA, ont ensuite été consolidées par SPS. Les paramètres de frittage ont été optimisés de manière à obtenir des céramiques possédant les propriétés microstructurales et physiques visées. L’effet de Ta2O5 comme inhibiteur de croissance des grains a été éprouvé sur la microstructure des céramiques élaborées. Les propriétés de transmittance ainsi que de luminescence, avant et après irradiation, des matériaux les plus prometteurs ont été mesurées et corrélées à leurs caractéristiques structurales. / This thesis deals with the elaboration and study of the physical properties of optically transparent and luminescent alumina-based ceramics for which applications are foreseen in the field of ionizing radiation resistant materials. The challenge of this work was to obtain a material that simultaneously has a high density and grain size at the nanoscale in order to give the ceramic transparency properties and resistance to ionizing radiation by capturing and recombination of induced charges at the grain boundaries. Ceramics of composition Al2O3 and MgAl2O4 were consolidated by Spark Plasma Sintering (SPS) from ultra-porous aluminas (UPA) on the one hand, and by reactive sintering of a mixture of Al2O3: MgO precursor in a ratio of 1 : 1 on the other hand. The UPAs were developed using an original process implemented at the LSPM. They were then impregnated with a solution of magnesium nitrate and then calcined at low temperature to obtain the "Nanostructured Precursor" (NP) for the spinel phase MgAl2O4. The synthesized NPs, like the UPAs, were then consolidated by SPS. The sintering parameters have been optimized to obtain ceramics with the desired microstructural and physical properties. The effect of Ta2O5 as a grain growth inhibitor has been tested on the microstructure of elaborated ceramics. The transmittance and luminescence properties, before and after irradiation, of the most promising materials were measured and correlated with their structural characteristics.

<p><a>Zirconium oxide (zirconia, ZrO₂) is one of the essential structural ceramics for industrial applications due to its superb strength and fracture toughness. ZrO₂ has three main polymorphs: cubic, tetragonal, and monoclinic phase, depending on temperature, type, and concentration of dopants. Stabilized zirconia with metastable tetragonal phase can transform into monoclinic phase with ~ 4% volume expansion under an applied external stress. The tetragonal-to-monoclinic transformation can hinder crack propagations by generating a compressive stress field near crack field, thereby enhancing fracture toughness. In addition, other deformation mechanisms such as dislocation activities, crack deflection, and ferroelastic domain switching can further enhance its deformability. Bulk ZrO₂ is typically prepared by sintering at high temperatures over a long period of time. Recently, field-assisted sintering techniques such as flash sintering and spark plasma sintering have been applied to effectively sinter ZrO₂. These techniques can significantly decrease sintering temperature and time, and more importantly introduce a large number of defects in the sintered fine grains.</a></p> <p>The miniaturization of sample dimension can alter the mechanical properties of materials by increasing the surface-to-volume ratio and decreasing the likelihood of retaining process-induced flaws. The knowledge of mechanical properties of ZrO₂ at micro and nanoscale is critical in that superelasticity and shape memory effect of ZrO₂ can be utilized for applications of actuation, energy-damping, and energy-harvesting at small scale. Here, we performed <i>in-situ</i> microcompression tests at various temperatures inside a scanning electron microscope to examine and compare the mechanical properties of ZrO₂ prepared by flash sintering, spark plasma sintering, plasma spray, and thermal spray. Detailed microstructural analyses were conducted by transmission electron microscopy. The unique microstructures in ZrO₂ prepared by field-assisted sintering largely improved their plasticity. Temperature and processing technique-dependent underlying deformation mechanisms and fracture behavior of ZrO₂ are discussed.</p>