Ultra-High-Temperature-Ceramics: potentialities and barriers to the application in hot structures

High performance Ultra-High-Temperature Composites (based on zirconium-, hafnium-, tantalum- borides and carbides) are characterized by relevant and unique thermo-physical and thermo-mechanical properties, suitable for applications in thermo-protection systems for aerospace applications. In spite of the difficult sinterability of borides and carbides of Zr, Hf, Ta, recent results highlighted that UHTC ceramics can be successfully produced with full density, fine and uniform microstructure and controlled mechanical and thermal properties, through different procedures: pressureless sintering and hot pressing combined with the use of proper sintering aids, reactive synthesis/sintering procedures starting from precursors, field assisted technologies like spark plasma sintering (SPS). The selection of reinforcing phase (SiC, B4C, TaSi2, MoSi2, etc) is suitable to improve mechanical properties and oxidation resistance of ceramics like ZrB2, HfB2, HfC, TaC, ZrC. Strength as high as ~ 800MPa at room temperature and up to 600 MPa at 1500?C in air were obtained, consequently to tailored compositions and processing control. SPS proved to be a very rapid fabrication process leading to refined microstructure and high properties of UHTC composites. However, barriers still exist toward the application in hot structures for aerospace and other industrial applications: i) UHTC suffer from low toughness: attempts to improve this property are presented; ii) it is necessary to develop techniques for the production of complex shapes. In this respect, examples are shown about the fabrication of components either by electrical discharge machining from simple dense shapes, or through forming techniques like slip casting and tape casting combined to gas pressure sintering. Several examples are presented for the production of dense or porous bodie