Fabricationand Properties Of Nano-sized ZrB₂-based Ultrahigh Temperature Ceramics

ZrB₂-based ultra high temperature ceramics are thought to be one of promisingstructural materials used in thermal protection field due to excellent combination ofproperties such as high melting point, superb ablation and good scouring resistance, ect.However, ZrB₂-based ceramics with micrometer particles usually need very highsintering temperature to achieve full density owing to low sintering activity determinedby inherent crystal structure, which largely limit the kinds of reinforcement used inZrB₂-based ceramics due to the properties degradation of reinforcement at hightemperature. Consequently, it has significant meaning in engineering to sinterZrB₂-based ceramics at lower temperature which may be achieved by sintering ofnano-ZrB₂-based ceramics which has enhanced sintering activity. In this paper,nanosized ZrB₂-based ceramics were sintered at lower temperature in order to solve thehard sintering activity of ZrB₂ceramics.The densification and grain growth of pure ZrB₂were investigated systematically.It was found that nano-ZrB₂can be sintered at1300℃approximately500℃lower thanthat of micrometer ZrB₂. The relative density of sintered ZrB₂increased slightly form70%for1300℃to75%for1800℃. On the contrary, the grain size increasedsignificantly to micrometer when the sintering temperature elevated to1600℃. Theseresults can be attributed to the existence of densification and grain growth in the firststage of sintering of nanosized ZrB₂ceramics, the densification strained due to the lossof nano-particles caused by rapid grain growth. The relative density of ZrB₂ceramichardly increased even sintered by SPS technology because of temperature rather thantime is the dominant factor determining the grain size. In this respect, nano-ZrB₂can besintered at low temperature to full densification while maintaining nanosized grain.ZrB₂-SiC ceramics were sintered at low temperature by multi-step sintering, therelative density of80%and210nm grain size were achieved when sintered at1500℃,the flexural strength reached838.1MPa which attributed to the fine grain. However,only75%relative density,500nm grain size and420MPa flexural strength wereobtained after spark plasma sintering of ZrB₂at same temperature, which indicate thatthe grain size can be effectively controlled while increasing relative density in thesintering of nanosized ZrB₂-SiC ceramics by multi-step sintering. In addition, fine graincan be obtained when the green body was formed by wet processing and cold biostaticprocessing due to the grain growth can be effectively controlled by grain boundarywhich increased as the density of green body elevated. So the key to fabrication of nano-ZrB₂ceramics is how to obtain a compact green body with uniformmicrostructure.Carbon fiber reinforce nanosized ZrB₂-SiC ultra high temperature ceramics werefabricated at low sintering temperature and properties degradation of carbon fiberreduced largely, carbon extraction can be clearly observed in the fracture surface whichallow carbon fiber to strengthen and toughen ZrB₂-SiC ceramics. It provides a newapproach to solve intrinsic brittleness of ultra high temperature ceramics.