This lecture traces the main research activity carried out at CNR concerning Ultra high temperature Ceramics and Composites for application in harsh environments.
Harshness characterizes extreme service 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. The activities span from fundamental understanding of the process-microstructure-property correlations, to materials functionalization through suitable procedures, up to the realization of technological demonstrators to be validated in relevant environment.
The talk will illustrate some of the recent results on a range of UHTC-based ceramics with a prospective view. The materials investigated include damage tolerant ultra-high temperature ceramic matrix composites (UHTCMCs) or extremely strong ceramics, to also touch the class of compositionally complex (CC) UHTCs better recognized as “high-entropy”.
First, UHTCs 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 have obtained ceramics with strengths above 1 GPa at 1800°C, when normally their strengths at these temperatures are around 200 MPa, composites with fracture toughness of 10 MPa m^1/2, a huge improvement over conventional bulk UHTCs (typically 3.5 MPa m^1/2), and UHTCMCs that are extremely resistant to oxidation and ablation, thus being suitable for reentry and propulsion applications.
Finally, synthesis and sintering of different CC diborides was object of our attention: hardness and thermal expansion were analyzed.