Preparation And Toughness Mechanism Of Carbon Nanotube Aerogels/SiBCN Ceramics Composites

SiBCN ceramic is a novel ultra-high temperature prepared by crosslinking and pyrolyzing the polyborosilazane(PBSZ),which usually applies in aerospace,high temperature protection fields.Carbon nanotubes(CNTs)toughened SiBCN ceramic improved the cracks caused by shrinkage during pyrolysis.However,the agglomeration caused by van der Waals’ force(VDW force)among CNTs has negative effects on toughening.In this paper,the carbon nanotube aerogels with three-dimensional network structure is used as perform to prepare the CNTs/SiBCN composites with high well-dispersed CNTs.The distribution of CNTs,combination between CNTs and interface and toughened mechanism of CNTs in composite is studied.At the same time,the SiBCN coating CNTs composite is prepared using the PBSZ in toluene.The structure and the oxidation resistance of composites are discussed.The CNTs/SiBCN composites with 10 wt.% and 20 wt.% content of CNTs are prepared by vacuum impregnation followed by pyrolyzing.The content of CNTs is controlled by the density of CNTs in aerogel.The skeleton structure can support the composites to inhibit shrinkage during pyrolysis and the linear shrinkage of composites(19%)reduce 34% compared with pure SiBCN ceramic(29%).The three dimensional skeleton structure of aerogel avoids the agglomeration of CNTs,obtaining the high well-dispersed content CNTs combined with matrix closely.The CNTs aerogel has obviously effect on toughening similar with micrometer-level fiber showing that the nanometer-level CNTs can deflect or stop the cracks according to the observe the crack growth.It is benefit on the high-density dispersed CNTs which ensure to share the load.Different with micrometer-level fiber,there are amount of broken CNTs lengthened by tensile force appeared on the fracture surface.The length of broken CNTs increases by the increasing content of CNTs,indicating the CNTs in bundles can shear loads by slipping of CNTs.The study of cracks shows that the CNTs toughened SiBCN ceramics composites have four processes in crack propagation including ceramic breaking,CNTs bridging,CNT bundles slipping,interface slipping and CNTs breaking.The SiBCN@CNTs composites with the layer thickness of 10 nm and 20 nm are prepared by coating the PBSZ-toluene solution with different concentration onto CNTs via vacuum impregnation followed by pyrolyzing.The 10-nm-thickness layer composite retain the porous structure of aerogel.TG analysis shows the weight loss of SiBCN@CNTs composite is 9.1% in air,which reduced 87.3% compared to pure CNTs(71.6%).