Microstructures and mechanical properties of ZrB₂–SiC–Ni ceramic composites prepared by spark plasma sintering

ZrB₂ based ultra-high temperature ceramic is one of the most promising thermal protection materials that can be used in hypersonic vehicles. Four different ZrB₂-25 mol.% SiC–Ni ceramic composites are prepared by spark plasma sintering at 1700 ^οC, in which the content of Ni are 2, 4, 6 and 8 mol.%, respectively. To make a comparison, ZrB₂–SiC ceramic is also sintered with the same method. The densifications and microstructures are carefully checked. Nanoindentation tests are performed to determine the hardness and elastic modulus of sintered specimens. The fracture toughness of ceramics is evaluated by a series of single-edge notched-beam tests. Results show that densities of the ZrB₂–SiC–Ni ceramic composites obviously increase with the increase of the content of Ni. The hardness and elastic modulus of the specimens slightly decline, while the fracture toughness significantly increase with the increase of the Ni content. The fracture toughness of ZrB₂-25 mol.% SiC-8 mol.% Ni reaches 8.3 MPa m^1/2, which is about 1.7 times that of the ZrB₂–SiC composites. The results in this paper show that the introduction of Ni can greatly improve the fracture toughness of ZrB₂–SiC ceramics.