Ultra-refractory borides for extreme environments

Materials based on borides of transition metals have been broadly investigated as ultra-high temperature ceramics (UHTCs), thanks to their unique combination of physical-chemical and structural properties and potential employ in aerospace field. Research on UHTCs has generally involved two major fields: the improvement of mechanical properties and oxidation resistance. High performances are achieved upon careful tailoring of the microstructure, obtained through an aware control of the sintering additives, secondary phases and densification process. Zirconium diboride (ZrB2) may have strengths of 500-600 MPa up to 1500°C when doped with suitable secondary phases and densified using the proper sintering technique. However, it generally collapses down to ~200 MPa at higher temperatures. In order for ZrB2 to be employed in the ultra-high temperature regime, exploration of their performance at higher temperatures is necessary. Here, we present an exceptional strength behavior at temperatures up to 2100°C and show the effects of microstructure tailoring in terms of secondary phases, grain morphology and grain size. Strengths exceeding 1 GPa have been achieved at 1800°C and analysis showed that this behavior was due to sub-grain refinement which occurred as a consequence of intra-granular plasticity under load. In addition, some aspects of the oxidation behavior of a series of ZrB2 composites are reviewed in the 1500 - 1800°C temperature range. Microstructural features disclosed by SEM and TEM coupled to thermodynamics enabled to shed light on the basic phenomena occurring at the various temperatures.