| In recent years,with the development of propulsion system,the sharp leading edge and nose tip of hypersonic vehicles are subjected extreme environments of high temperature,oxidation and ion erosion,which represent a challenge to the selection of structure material to be used in such harsh working conditions.ZrB2 ceramic,which is termed ultra-high temperature ceramics(UHTCs),has high melting point,low density,excellent oxidation and ablation resistance and is among the most promising structure material for long-term service under hypersonic conditions.Based on this,ZrB2-SiC ceramics have attracted much attention because it has better oxidation resistance and mechanical properties than monomer ZrB2 ceramics.Fine SiC particles,especially nano-sized SiC particles,can not only improve the sintering behavior but also the mechanical properties of ZrB2-SiC ceramics.However,it is difficult to disperse the nano-sized SiC particles to other matrix homogeneously,as the nano-sized particles easily agglomerate even milling for a long time.Furthermore,the susceptibility to brittle fracture of ZrB2-SiC ceramics resulting their small damage tolerance and poor crack growth resistance,which limit the engineering application of ZrB2-SiC ceramics.Therefore,the research of fabrication of ZrB2-SiC based composites with uniformly dispersed nano-SiC and the toughening technology for such composites is extremely important to improve the service performance of ZrB2-SiC based composites.In the present work,firstly,the ZrB2-SiC composite powder mixtures with uniformly dispersed nano-SiC were synthesized by solid-state chemical reaction.And the sintering property of the synthesized powder was investigated via hot pressing.On this basis,in-situ reaction hot pressing was developed to fabricate the ZrB2-SiC-ZrSi2 ceramics.Then,chopped carbon fibers(Csf)and silicon carbide nanowires(SiCnw)were used as reinforcements to improve the mechanical properties of ZrB2-SiC-ZrSi2 ceramic matrix composites.Furthermore,the synergistic effect of micro-scale Csf and nano-scale SiCnw was explored.Finally,the effects of in-situ interlocking microstructure,Csf,SiCnw and Csf-SiCnw hybrid micro-nano reinforcements on the fracture properties of ZrB2-SiC-ZrSi2 ceramic matrix composites were analyzed.The main research contents and results are summarized as follows:(1)The ZrB2-SiC composite powder mixtures with uniformly dispersed nano-SiC were synthesized by using ZrSi2,B4C and C as raw materials.And the sintering property of the synthesized powder was investigated via hot pressing.The result indicated that the synthesized ZrB2 particles present a platelet morphology and numerous nano-SiC particles adhered to the surface of ZrB2 particles.The grain size of ZrB2 and SiC increased with the synthesis temperature,and the ZrB2 grain grew faster than SiC grain.When the synthesis temperature was 1200 ?,the particle size of SiC and ZrB2 were 40 run and 300 nm,respectively.The nano-SiC can significantly promote the densification of ZrB2-SiC ceramics.With 15vol%ZrSi2 as sintering aid,the ZrB2-SiC-ZrSi2 ceramic hot pressed at 1550 ? could reach to a high relative density of 97.7%.(2)Combining the synthesis and densification process into one step,in-situ reactive hot pressing method was developed to fabricate ZrB2-SiC-ZrSi2 ceramics.The influence of microstructure on the mechanical properties of the ceramics was investigated.The results indicated that ZrB2 grains with platelet morphology arrayed perpendicular to the pressing direction and formed interlocking microstructure.Meanwhile,the orientation factor of ZrB2 grain increased significantly with the introduction of ZrSi2 as the sintering aid.This in-situ interlocking microstructure improves the strength and toughness of the ZrB2-SiC-ZrSi2 ceramics.The sample with 20vol ZrSi2 added present the highest mechanical properties,but it remained brittle fracture.(3)To reduce the brittleness of the ZrB2-SiC-ZrSi2 ceramics,chopped carbon fibers(Csf)were used as the reinforcement to prepare Csr/ZrB2-SiC-ZrSi2 composites.It was found that the introduction of Csf changed the failure mode of the composites from typical brittle fracture to quasi-ductile fracture.Compared with pure matrix ceramics,when the fiber content was 20vol%,the work of fracture of Csf/ZrB2-SiC-ZrSi2 composites increased by 76.7%and had lager critical crack.(4)The SiC nanowires(SiCnw)synthesized by a catalyst free method were used as the reinforcement to prepare SiCnw/ZrB2-SiC-ZrSi2 and SiCnw-Csf/ZrB2-SiC-ZrSi2 composites.It was found that SiCnw exhibits significant toughening and reinforcing effect in composites,which could to some extent solve the problem of strength reduction for the composites caused by the introduction of Csf.The Csf-SiCnw hybrid micro-nano reinforcements acted to increase the ability of composites to prevent crack propagation from nano to micro then macro scale,resulting the more pronounced quasi-ductile fracture behavior.Compared with 20vol%Csf/ZrB2-SiC-ZrSi2 composites,the flexural strength,fracture toughness and work of fracture of 20vol%Csf-5vol%SiCnw/ZrB2-SiC-ZrSi2 composites increased by 19.8%,13.5%and 26%,respectively.(5)For improvement the fracture properties of ZrB2-SiC-ZrSi2 ceramic matrix composites,the comparative analysis of the reinforcements indicated the following results:1)the in-situ interlocking microstructure exhibits excellent self-reinforcing effect,and the SiCnw can further improve the mechanical properties of the composites.However,they cannot change the brittle fracture feature of the composites;2)Although Csf cause the strength reduction of the composites,they can significantly improve the work of fracture of the composites and result the quasi-ductile fracture behavior.3)The Csf-SiCnw hybrid micro-nano reinforcements can not only improve the strength weakness caused by introduction of Csf but also improve the fracture toughness and fracture work of the composites.And the composite present more pronounced quasi-ductile fracture behavior and avoid the catastrophic damage. |
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