| ZrC ceramics with some advantages of high melting point and hardness,low density and stable mechanical properties at high temperature,have become a hotspot in the field of ultra high temperature.However,the intrinsic brittleness of ceramics has severely limited their wide applications in load-bearing components.In order to improve their fracture toughness,many experts have added carbon fiber to achieve toughening of the ZrC ceramics.However,the uniform dispersion of carbon fiber with high volume in the matrix has not been solved.In this study,short carbon fibers(Csf)with high volume content of 40 vol.%and 50 vol.%,which were used as toughening phase,were dispersed by tape casting and then the light-weight ZrC-SiC ultra high temperature ceramics toughened with short carbon fibers were fabricated at 1900? and 30 MPa for 1 h by hot pressing.The most suitable process parameters of tape casting were studied.Meanwhile,the microstructure,mechanical properties,thermal shock resistance and oxidation resistance of the samples were analyzed in detail.Tape-casting substrates of the light-weight ZrC-SiC ultra high temperature ceramics toughened with short carbon fibers were produced by tape casting.Uniform dispersion of Csf in slurry was realized when the content of the binder and the stirring time were 40 wt.%(20 wt.%PVB + 20 wt.%PEG)and 9 h,respectively.It is necessary to try to shorten the repose time of slurry as much as possible.At this time,the viscosity of the slurry was 26 mPa·s and the tape-casting substrates had a suitable strength.When the green bodies were removed binder at 500? for 1h with a heating rate of 0.2 ?/min in N2 atmosphere,the tape-casting substrates were kept flat.The light-weight ZrC-SiC ultra high temperature ceramics toughened with short carbon fibers were successfully fabricated by hot pressing.The density of the ZrC ceramics was greatly reduced due to the addition of Csf with high volume content.The densities of the samples with Csf content of 40 vol.%(ZCsf-40)and 50 vol.%(ZCsf-50)were 3.80 g/cm3 and 3.11 g/cm3,respectively.Meanwhile,the relative density and bending strength of ZCSf-40 were 83.88%and 107±0 MPa,while those of the ZCsf-50 were 75.85%and 96±12 MPa.The relative density and bending strength of ZCsf-50 were lower than those of ZCsf-40 which was due to large amount of pores produced by the bridging of short carbon fibers in ZCsf-50.In addition,the fracture toughness of the ZCsf-40 was still as high as 3.6±0.3 MPa·m1/2 with such low bending strength,which was attributed to the consumption of fracture work by crack deflection,bridging and fiber debonding and pull-out.Thermal shock resistance tests showed that the bending strength of the ZCsf-40 and ZCsf-50 only decreased by 7%and 3%when the AT was 400?,indicating good thermal stability of the light-weight ZrC-SiC ultra high temperature ceramics toughened with short carbon fibers at low temperature.A large number of pores bridged the cracks and absorbed energy,thus leading to the stress relaxation in the crack tip.In addition,the cracks were bridged by Csf with high volume content,thus consuming energy.The synergy of pores and Csf made the light-weight ZrC-SiC ultra high temperature ceramics toughened with short carbon fibers have good thermal stability at low temperature.The light-weight ZrC-SiC ultra high temperature ceramics toughened with short carbon fibers had.a severe expansion after oxidation at 450? for 4h.The volume of the ZCsf-40 and ZCsf-50 increased by 29%and 68%,respectively.With the increase of oxidation time,the increaseing rate of oxidize mass slowed down.After oxidation at 450? for 4h,the weight gain of the ZCsf-40 and ZCsf-50 was 0.63 mg/mm2 and 0.71 mg/mm2,respectively.The flexural strength and fracture toughness of samples decreased substantially with the increase of oxidation time,which was attributed to the increasing defects such as large holes and cracks with large size. |
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