| In the era of rapid development in the aerospace industry,more stringent performance requirements are imposed on ultra-high temperature composite materials,and carbon-carbon composite materials play a unique advantage in the high temperature field.Based on C/C composite ceramics,three kinds of carbides,TiC,ZrC,HfC/ZrC,HfC/ZrC/TiC,were studied.The sintering properties,from sintering densification,Vickers hardness and fracture toughness,microstructure fracture mode,crack propagation and mechanical properties change,discuss the relationship between the microstructure,microstructure and mechanical properties of the material.The main conclusions are as follows:(1)Effect of different kinds of sintering aids on densification in TiC/ZrC ceramics.The results of sintering density and relative density show that the sintered density of TiC/ZrC/Si C samples is 5.4637 g/cm3 and the relative density is 92.6%.The silicon carbide is evenly distributed between TiC and ZrC,which makes the interface and interface very good.The wettability,better promotes sintering,and also because of the high sintering speed,leaving more pores.In the TiC/ZrC/Ti&C samples,due to the SPS sintering process requirements,the sintering powder has higher conductivity requirements,and the titanium powder and the carbon powder have better conductivity,and related literature reports are favorable for activation sintering,and The presence of titanium and carbon is more conducive to the solid solution of titanium carbide and zirconium carbide than other additives.The bulk density after sintering was 5.4962 g/cm3,and the displacement of the pressure bar before and after the complete sintering was 3.47 mm,and the relative density of sintering was 96.2%.In the TiC/ZrC/MWCNTs samples,the sintering densification is better,the sintered density is 6.2191 g/cm3,and the relative density is 95.113%;the length/radial ratio of MWCNTs is high,the Young's modulus is high,and the toughness is extremely high.In combination with the TiC/ZrC ceramic matrix,the fiber interface strength is moderate,improving the mechanical strength and toughness of the ceramic.(2)The composite sintering process has a significant improvement on the mechanical properties of TiC/ZrC.15wt% TiC/ZrC is compared with Vickers hardness and fracture toughness by direct SPS sintering and composite sintering(cold isostatic pressing + SPS).The direct SPS Vickers hardness is 14.0±0.5 GPa and the fracture toughness KIC is 3.5±0.3 MPa·m1/2.The improved process is composite sintering,and the Vickers hardness value is 22.0±1.8 GPa,and the fracture toughness KIC is 5.1±0.3MPa·m1/2.Comparing the data obtained from two different sintering processes,the Vickers hardness of the latter is increased by 57% and the fracture toughness is improved by 45.7%,which proves that the innovative sintering process has a significant improvement on the sinterability(strong toughness)of ceramics..The composite sintering process has a significant improvement on the oxidation resistance of TiC/ZrC ceramics.After the simulated oxidation of the ordinary sintered ceramics,the surface layer is detached from a large area,and the unevenness is uneven.There is a large amount of scattered powder.However,after the simulated composite oxidation of the same ceramic sample,the surface morphology does not change much,and the surface is flat and neat.With powder,the densification performance is better.(3)Comparison of sintering properties of composite sintered HfC/ZrC series ceramics.HfC/ZrC series of burningThe relative density,Vickers hardness and fracture toughness of the knot are all bell-shaped.The pure phase ZrC has the lowest value,the relative density is 87.93%,the Vickers hardness is 6.79 GPa,and the fracture toughness is 3.87MPa· m1/2.With the increase of HfC content,Binary composites have the best performance of 5% HfC/ZrC and 7wt% HfC/ZrC,with a relative density of 98.63%,a Vickers hardness of 16.38 GPa and a fracture toughness of 4.74MPa· m1/2.With the further increase of HfC content,the sintering performance has declined.It is verified whether the strength and hardness of the ceramic sample can be significantly improved and the fracture toughness is better when the HfC/ZrC carbon-carbon composite sintering is controlled to a certain amount of solute content.5wt% HfC/ZrC and 7wt% HfC/ZrC have good toughness.(4)The fracture modes of pure phase ZrC and HfC/ZrC composite phase are the same,the typical brittle port characteristics dominated by transgranular fracture,and the smooth intergranular fracture of small crystals under back dispersion is mostly sintered without denseness leaving a certain gap.Formed by a weak surface.The difference in microstructure forms a stress concentration at the interface between the region A and the region B.When it is impacted by an external force,it becomes the origin of the microcrack,and it is selected to develop toward the A surface or the B surface,depending on the difference in sintering performance and strength.Abnormal grains of different sizes,the strength is even worse,the cracks tend to expand toward the area where the resistance is small,and the micro-cracks gradually gather to form a larger size.When the abnormal grains are gone,or die,or ? deflection occurs,the facets are formed.Tear ribs or large cracks.(5)The composite ceramics are basically brittle fracture characteristics of transgranular fracture.The mechanical properties such as Vickers hardness and fracture toughness are strongly related to the number of transgranularity and the behavior of tearing edges on the crystal plane.The better the sintering performance of the composite ceramic,the more the number of transgranular,the more and more the tearing edge on the step plane,the more the fracture surface energy of the composite ceramic increases,and the better the fracture toughness.(6)HfC/ZrC through three kinds of sintering process performance feedback,CIP / SPS composite sintering process is the most ideal,the toughness is better than the other two,which proves that HfC / ZrC composite ceramics to obtain excellent performance,large enough The duration of pressure and insulation is an important consideration while ensuring the size of the ceramic grain.The process of low temperature insulation after SPS is expected to achieve better sintering effect through improved process.(7)The variation of Vickers hardness and fracture toughness of HfC/ZrC/TiC series is analyzed.The value of fracture toughness changes little,and the sintering process plays a major role in the properties of raw materials.The Vickers hardness is affected by the microstructure,and the change is irregular.Although the size is small,the dispersed Zr O0.9 first increases the hardness of the coarse impurities,but the C4 Vickers hardness does not have any impurities and has good sintering properties..(8)Two fracture modes were analyzed,and the fracture transgranular fracture was characterized by typical brittle fracture.The beginning of the crack disappeared,explaining the reason why the micro-fractures were uneven,and the micro-cracks disappeared at the highest point of the step and the lowest point of the step.(9)In the HfC/ZrC/TiC system,the stable content of Ti is constant.When the TiC content increases(>10%Wt),the Ti element can not be better with the matrix due to the presence of HfC in the ZrC matrix.Sintering,dispersion and ZrC and HfC affect the expansion and mechanical properties of microcracks. |
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