Development and characterization of metal diboride based composites toughened with ultra-fine SiC particulates

Two metal-diboride-based ceramics containing up to 15 vol%. ultra-fine alpha-SiC particulates were developed from commercially available powders. The primary matrix of the composites was ZrB2 or a mixture of ZrB2 and HfB2.With the assistance of 4.5 vol%. ZrN as a sintering aid, both the compositions achieved nearly full density after hot-pressing at 1,900 degree C. The microstructure was characterized by fine diboride grains (about 3 microns average size) and SiC particles dispersed uniformly. Limited amounts of secondary phases like MO2 and M(C,N), M = Zr or Zr/Hf, were found. The thermo-mechanical data of both the materials offered a promising combination of properties: about 16 GPa of microhardness, 5 MPam0.5 of fracture toughness and Young's moduli exceeding 470 GPa. The ZrB2-SiC composite showed values of strength in air of 635 ± 60 and 175 ± 15 MPa at 25 and 1,500 degree C, respectively. Likewise, the (ZrB2 + HfB2)-SiC composite exhibited values of strength in air of 590±25 and 190±20 MPa at 25 and 1,500 degree C, respectively. The composites also displayed good tolerance of conditions of repeated short exposures, 10 minutes each, at 1,700 degree C in stagnant air. In such oxidizing conditions, the resistance to oxidation was provided by the formation of a protective silica-based glass coating, the primary oxidation product of SiC. Such a coating encapsulated the specimen coherently, and provided protection to the faces exposed to the hot atmosphere.