Fabrication of ZrC/ZrB<sub>2</sub>/C Composites Having High Mechanical Properties at Elevated Temperatures

Abstract

Ultra-high temperature ceramics (UHTCs) are most recently getting much attention for structural parts of hypersonic missiles with their cruising speed of more than Mach 5. Most of the UHTCs are poor sinterability carbides, nitrides, and borides. Therefore, they have been studied and developed for a long time. However, there are still many problems to solve. In this paper, based on the solid-state reaction presented as an equation of (x + y)·ZrC + 2·y·B → x·ZrC + y·ZrB2 + y·C, three-phase ZrC/ZrB2/C composites have been fabricated from ZrC and amorphous B powders using pulsed electric-current pressure sintering at 1373 to 2173 K for 6.0 × 102 s under 50 MPa in a vacuum. ZrC/ZrB2/C = 30/70/C~70/30/C vol% composites with the relative densities Dr of 96.6 to 98.7% were obtained at 2073 K. The 60/40/C vol% composite revealed high bending strength σb (554 MPa), Vickers hardness Hv (19.2 GPa) and moderate fracture toughness KIC (5.25 MPa·m1/2) at room temperature. Furthermore, all composites showed elastic deformation up to 1873 K and revealed σb more than 600 MPa at this temperature, in addition, some composites showed higher σb than 900 MPa at the same temperature. These high mechanical behaviors are discussed with those of the simple binary ZrC/ZrB2 composites which were fabricated under the same conditions except for their starting materials. The best mechanical properties of binary composites were σb (474 MPa), Hv (18.5 GPa), and KIC (4.45 MPa·m1/2) at room temperature, and σb of 400 - 700 MPa at 1873 K. Overall, three-phase composites, nevertheless including soft carbon, have higher mechanical properties than the binary composites.