Understanding ceramics for extreme environments: the importance of nano-scale investigations

Advanced ceramic materials are the most appropriate to find application in extreme environments in view of their exceptional combination of unordinary properties. The definition of extreme environment is very broad, but in the field of materials science it can be summarized as the set of applications in which materials with properties bordering on what is currently available are required. It was decided to divide the possible applications on the basis of the thermal regime: low, medium, and high, as the requirements that the materials must satisfy are completely different. With regard to the low temperature regime, B4C-TiB2 composites for ballistic applications, such as body armors, have been studied due to their lightness, impact resistance and high hardness. The influence of the processing and of small WC additions on the microstructure has been thoroughly analyzed to ultimately correlate a set of mechanical properties to the fine and overall microstructure assembly. The dense ceramics were typified by development of a core/shell structure of the boride grains, with the shell comprising a (Ti,W)B2 solid solution with d variable amount of W guest cation depending on the processing route. The concentrations of tungsten in the solid solution have been correlated to nano-hardness and hence to the theoretical strength by means of a series of nanoindentations and explained an increased plastic behavior for high W substitutions, beneficial to retain fracture strength. Then, moving to the intermediate temperature regime, encountered especially by cutting tools for applications such as mining and machining, even in corrosive environments like sea waters, in which, in addition to the high hardness, resistance to wear and corrosion are required, a binderless tungsten carbide ceramic containing 5 vol% silicon carbide was hot pressed to full density at 1820°C. The formation of a transient W-C-Si-O liquid which facilitated the oxide removal at relatively low temperature, was conducive to the development of a microstructure with bimodal grain size in the form of polygons or rods. This microstructural asset led to outstanding mechanical properties from room to elevated temperature. For the first time, WC-materials displayed strength over 1 GPa up to 1500°C and fracture toughness from 7 to 15 MPa??m. Subsequently, materials for the high- and ultra-high temperature regime were studied, as possible candidate materials for aerospace applications, such as hypersonic aircraft nozzles in which materials with high melting temperature, resistance to ablation and damage coupled with a relatively low density are of vital importance. The most suitable materials are the ultra-high temperature ceramics (UHTCs), including borides and carbides of the transition metals in group IV and V. The core-shell structures formed upon addition of an external guest cation in the boride matrix were studied in depth using both SEM and TEM. These structures revealed the precipitation of nano-inclusions of different nature, size and shape depending on the added cation, which were partially responsible for improvements in the mechanical properties of the ceramics especially in the high-temperature range. This finding represents a starting point towards the production of new hierarchical nanocomposites for extreme environment and a first step towards the understanding of the incredible properties of high entropy ceramics. In order to reduce the density of bulk UHTCs below 6 g/cm3, the addition of TiB2 to a ZrB2 matrix was studied. The further addition equal to 5% of third compound containing Hf, V, Nb, Cr was introduced in order to overcome the poorer oxidation resistance that characterizes the TiB2 phase. In this way solid solutions with three elements have been obtained. They have been investigated from the microstructural point of view and, from these first preliminary analyses, have shown a different formation behavior as compared to the simple binary solutions. The last part of this dissertation is focused on the microstructural characterization of a recently born class of materials, known as ultra-high temperature ceramic matrix composites (UHTCMCs ) obtained by slurry infiltration or reactive melt infiltration to understand: 1) the role of Y2O3 during densification, on the microstructure evolution and on the mechanical properties of a slurry infiltrated carbon fiber preform by a ZrB2-SiC slurry; 2) the mechanism of formation of the particular microstructure resulted from reactive infiltration and low temperature sintering of a ZrB2-Zr2Cu-B mixture in to a Cf preform.