Fabrication And Properties Of ZrB₂Ultra High Temperature Ceramic Composites With High Thermal Conductivity Graphite Fibers

ZrB2based ultra-high temperature ceramic materials (UHTCs) are widely usedin the aerospace field due to their refractory, high temperature resistance, erosion,anti-erosion and other fine features, became applicable to the extreme environmentof the new high-temperature structural materials. In this study, because the lowertoughness, low thermal conductivity and low thermal shock resistance of theshortcomings of ZrB2-SiC composites, added high-conductivity graphite fibers toZrB2-SiC composites. The ZrB2-SiC-Gf(ZSGf) ultra-high temperature ceramiccomposites was prepared by hot-pressed and achieved near full density, used1950oC and1850oC two kinds of sintering temperature,30MPa/inert gas protection.The organizational structure and properties ZSGfcomposites were studied by X-raydiffraction (XRD), scanning electron microscopy (SEM), and energy dispersivespectroscopy (EDS).The effect of sintering temperature on the density of ZSGfcomposites has beenstudied. Under the same conditions component, the density of material sintered at1950oC was higher than the material sintered at1850oC. The relative density ofZSGfcomposite materials decreased with the increasing of high conductivitygraphite fibers content; at the same high levels of conductivity graphite fiberscontent, high temperature improved the bending strength of the materials. Theflexural strength and fracture toughness of ZSGfcomposites increased with highconductivity graphite fibers content; when the high conductivity graphite fiberscontent was more than20vol%, the bending strength decreased obviously while thefracture toughness slightly increased.Thermophysical properties of ZSGfcomposites were tested, including thespecific heat capacity, thermal diffusivity and thermal conductivity. Heat capacityobtained by DSC, which selected sapphire as test standard; thermal diffusivity ofZSGfcomposites sintered at two temperature increased with increasing of highconductivity of graphite fibers content. Under the same component conditions,parallel to the pressing surface direction, the thermal diffusivity of the ZSGfcomposites sintered at1950oC was higher than the materials sintered at1850oC.While perpendicular to the pressing direction, the thermal diffusivity of ZSGfcomposites sintered at1850oC was higher than the materials sintered at1950oC;ZSGfcomposites sintered at two sintering temperature perpendicular to the pressingsurface of the thermal diffusivity was higher than the direction parallel to thedirection of the pressing surface; ZSGfcomposites sintered at two sinteringtemperature perpendicular to the pressing surface of the thermal conductivity washigher than the direction parallel to the direction of the pressing surface. The ablation behavior of ZSGfcomposites has been studied by usingoxyacetylene ablation experiments. After the ablation of270s, the surfacetemperature up to2300oC, the samples were still good, no cracks took place,indicating good thermal shock resistance and oxidation resistance of the material s.An appropriate increase in graphite fiber helped to improve thermal shockresistance of materials; excessive high-conductivity graphite fibers decreased themechanical properties of the materials, causing the thermal shock performance ofthe materials declined.