Nanoindentation and nanostructural characterization of ZrB₂–SiC composite doped with graphite nano-flakes

Nano-sized graphite was used as a dopant for fabrication of ZrB₂–SiC ceramic via spark plasma sintering at 1800 °C for 8 min under 35 MPa. As-sintered composite was characterized by XRD, SEM, EDS, STEM, TEM and nanoindentation in order to study the micro/nanostructure and mechanical properties of the sample. A near fully-dense ternary composite was obtained after densification process. In-situ formation of ZrC was attributed to the chemical reaction of graphite nano-flakes with ZrO₂ nano-layers covered the surface of starting ZrB₂ powders. Reactive role of graphite as an effective sintering aid, via removal of oxide impurities, was illustrated by TEM, as some ultrafine porosities were remained in the sintered bulk in graphite-free areas. The hardness and elastic modulus of the composite, obtained by the nanoindentation method, showed an excellent harmony with the reported data in the literature. The average hardness of 15.2, 18.3 and 10.7 GPa were achieved for ZrB₂, SiC and ZrB₂/SiC interface, respectively. Average Young's moduli of matrix and reinforcement phases were measured as 328 and 306 GPa, respectively, which showed favorable adaption in mechanical properties of composite components. The nano-indentational characteristics of composite, especially pop-ins in the load-displacement curves, were also discussed.