Hi-Tech Ceramics and Composites for Harsh Environments

This lecture traces the main research activity concerning Hi-Tech Ceramics and Composites for severe environments. Harshness characterizes extreme working conditions where thermo-chemical, thermo-physical and thermo-mechanical attacks may concurrently take place. New design concepts make feasible the overcoming of the ceramic's brittleness merging damage tolerance and capacity of withstanding ultra-high temperature regimes in chemically aggressive environments. Added-value materials with self-repairing capability are under development. The R&D activities span from the fundamental understanding of the process-microstructure-property correlations, to the materials functionalization through suitable procedures, up to the realization of technological demonstrators to be validated in relevant environment. The talk shows the last results on a variety of advanced ceramic materials. The materials investigated include damage tolerant ultra-high temperature ceramic matrix composites (UHTCMCs), extremely strong ceramics, super hard and light carbides for structural applications in various harsh environments. First, ultra-high temperature ceramics are considered. This class of composites includes borides and carbides of the 4th to 6th group which possess melting point above 3000°C and a combination of thermo-mechanical properties that make them adequate for application in highly corrosive and ablative hot environments, like component of space vehicles and parts of rocket nozzles. Their major drawbacks include a difficult densification, sensitivity to oxidation, high brittleness and low thermal shock resistance. Here we show that upon suitable incorporation of sintering agents, different types of fibers or suitable powder processing we can manipulate the microstructure to obtain desired properties and stem the above mentioned limits. For example, we obtained ceramics possessing strengths exceeding 1 GPa at 1800°C, when normally their strength at these temperature are around 200 MPa, composites with fracture toughness above 14 MPa m^0.5, with a tremendous improvement compared to conventional bulk UHTCs (typically 3.5 MPa m^0.5), and near Zero-ablative oxidation-resistant UHTC, being thus suitable for reentry and propulsion applications.