The Research Of Mechanic And Oxidation Properties Of ZrB₂-C_f Ceramic Matrix Composite

Because of its high melting temperature, relatively good oxidation resistance, high thermal conductivity, low thermal expansion and high hardness, ZrB₂ becomes the promising candidate material for the ultra-high temperature ceramics (UHTCs). However, the inherent brittleness of ZrB₂ is the key problem which limits its application for UHTCs, in this study Carbon fiber(C_f) is employed to improve its toughness. The phase and microstructure of the composites are examined by X-ray diffraction (XRD) and Scanning Electron Microscope (SEM). Hardness, flexural strength and fracture toughness of the composites are measured. The influence of the C_f on the properties of the composites is discussed. Thermal shock resistance and oxidation resistance of the composites are studied.The short C_f is dispersed in alcohol by ultrasonic with the dispersant agent C₁₂H₂₅SO₃Na. ZrB₂ powder, SiC powder and C_f are mixed by wet-milling for 6h to ensure the suitable length of C_f. The optimized hot-pressing sintering fabrication process of the composites is: heating up to 2000℃at the speed of 15℃/min, then insulating and maintaining pressure for 60 min. The relative density of the composites are all above 97%.XRD results show that a few ZrC is generated during hot-pressing sintering, due to the reaction between C_f and ZrB₂. The flexural strength and fracture toughness of composites with C_f content 5vol%, 10vol%, 15vol% and 20vol% are tested. The fracture toughness of composites is increasing with the increment of C_f content, but the flexural strength is unobvious, fracture toughness of the composite with 20 vol% is the highest 6.10MPa·m^1/2 ,but its flexural strength is little lower than that of material without C_f. 20vol%.The main toughening mechanics are studied.The flexural strength of the composite with 20vol% is studied at 300℃, 600℃and 900℃.The properties of composites with C_f are anisotropy.Thermal shock of the composite with 20vol% is outstanding due to its excellent toughness, low modulus of elasticity and low thermal expansion. During the process of rising temperature, the composite with 20vol% is oxidized to form ZrO₂ and SiO₂ coating on the surface of the composite. The protective coating inhibits the further oxidation of the composite. Therefore, the composite has excellent oxidization resistance.