Processing and properties of ultra-refractory composites based on Zr- and Hf-borides: State of the art and perspectives

High performance Ultra-High-Temperature Composites, based on zirconium and hafnium borides, are characterized by relevant and unique thermo-physical and thermo-mechanical properties, suitable for applications in aerospace hot structures and in many industrial sectors where extreme conditions are present. In spite of the difficult sinterability of Zr- and Hf-diborides, recent results highlight that these ceramics can be produced with full density, fine microstructure and controlled mechanical and thermal properties, through different procedures: pressureless sintering and hot pressing with proper sintering aids, reactive synthesis/sintering procedures starting from precursors, field assisted technologies like spark plasma sintering (SPS). A proper selection of reinforcing phase (SiC, BC, TaSi, MoSi, etc) leads to improvements in mechanical properties and oxidation resistance of ZrB and HfB ceramic composites. Strength as high as ~800 MPa at room temperature and at 1,500°C in air can be obtained for HfB-based composites, after an accurate tailoring of compositions and processing parameters. SPS proved to be a very rapid fabrication process leading to refined microstructure and improved properties of ultra-refractory diborides-based composites. © 2010 Springer Science+Business Media B.V.