Research On Mechanical And Antioxidant Properties Of CNT Buckypaper/SiC Self-sealing Laminated Composite

Carbon nanotubes（CNTs）have been considered the most ideal strengthening and toughening phase for ceramic matrix composites due to their excellent mechanical and physical properties such as low density,high strength,high modulus,and high thermal conductivity.However,CNTs are prone to agglomeration and have poor bonding with the SiC matrix,which hinders the preparation and development of high-performance CNT-reinforced SiC ceramic matrix composites,making the performance of composites fail to achieve the desired expected results,and sometimes even reduce the performance of composites.Moreover,the traditional straight plate laminated composite has weak interlayer bonding force,and the propagation of cracks between the layers can easily lead to spallation.Therefore,solving the dispersibility of CNTs in the SiC matrix and the bonding with the SiC matrix and improving the ability to resist interlaminar crack propagation have become the key technologies for the preparation of CNTs reinforced SiC matrix laminated composites.In this thesis,an ideal interface bond between CNTs and SiC matrix was obtained by introducing a B₄C-modified resin-derived carbon intermediate interface layer;CNTs were made into CNT buckypaper preforms,which not only avoids the shortcomings of uneven distribution of CNTs in the ceramic matrix,but also can prepare ceramic matrix composites with high CNTs volume fraction.Taking the CNT buckypaper macro body as the preform,Si powder and two kinds of B₄C micropowders with different particle diameters were introduced into the CNT buckypaper preform,and the boron-modified phenolic resin solution was impregnated as a binder.High temperature sintering generated SiC in situ in the pores of the CNT buckypaper,thereby preparing a CNT buckypaper preform reinforced SiC ceramic matrix（CNT buckypaper/SiC）laminated composites.In this thesis,by introducing two kinds of B₄ C modified resin carbon intermediate interface layers with different particle diameters,the ideal interface bonding between CNTs and SiC matrix was obtained;high volume content was achieved by making CNTs into CNT buckypaper preforms combined with Si in-situ generation method.The uniform dispersion of CNTs in the SiC matrix.Moreover,this thesis designs two novel structures of CNT buckypaper/SiC ceramic matrix laminated composites,one of which was a self-sealing graded laminated structure,the SiC content increases layer by layer from the center to the surroundings;the other was a fiber monolithic laminated structure,and each fiber in this structure was also a self-sealing laminated structure.This self-sealing structure has outstanding advantages in preventing crack propagation,maintaining the integrity of the structure,and resisting environmental interference and corrosion.In this thesis,the mechanical properties and oxidation resistance of these two different structures of SiC matrix composites were compared in detail and systmatically,and the unique microstructure of the composites and their effects on performance were analyzed.The main research contents and results of this thesis are as follows:1.The preparation and microstructure of CNT buckypaper/SiC ceramic matrix graded composites were studied.When the density of the self-sealing graded laminated composite was 1.63 g/cm3,the open porosity was 4.19%;when the density of the fiber monolithic layered composite was 1.63 g/cm3,the open porosity was 5.89%.The pores were mostly concentrated at the interface between layers.The volume content of CNTs in the two composite materials exceeded 35%,and the CNTs were uniformly dispersed in the SiC matrix.The self-sealing laminated composites of the two structures were composed of a strong structural layer and a weak interlayer interface.The thickness of the structural layer was 100-200 μm,and the thickness of the interface layer was 10-100 μm.The structural layer was composed of B₄C dispersion and toughened CNT buckypaper/SiC composites;the interface layer was composed of expanded graphite and SiC whisker（SiC whisker,SiCw）dispersed and toughened resin carbon.The SiC content of the self-sealing laminated graded composite gradually increases from the center to the periphery,forming a closed-loop structure that gradually increases in a circle.The fiber monolithic laminated composite was composed of a plurality of fibers with a self-sealing layered structure,and each fiber formed a self-sealing structure,and the fibers were bonded by a weak interface.2.The mechanical properties of CNT buckypaper/SiC ceramic based gradient composites were studied.The results show that the flexural strength of the self-sealing graded laminated composite and the fiber monolithic laminated composite were 97 MPa and 117 MPa,respectively.The compressive strength were 329 MPa and 377 MPa,respectively.The flexural load-displacement curves and compressive loaddisplacement curves of the two self-sealing laminated composites were zigzag curves,and both had high strain,showing a typical failure mode of nonbrittle fracture,indicating that CNTs plays a very good role in strengthening and toughening the SiC matrix.Scanning electron microscopy（SEM）analysis of the cross section shows that the micro-toughening mechanism in the layer of the CNT buckypaper/SiC selfsealing laminated composite included the debonding,bridging and pulling out of CNTs and B₄ C particles;interlayer crack deflection,bridging and pulling out of SiCw whiskers,layer bending and pulling out,interlayer crack deflection and other strengthening and toughening mechanisms.The combined effect of these toughening mechanisms improves the mechanical properties of the composites.3.The oxidation resistance of the two different structures of CNT buckypaper/SiC ceramic matrix self-sealing laminated composites was studied.The two-structure CNT buckypaper/SiC ceramic matrix self-sealing laminated composites were oxidized at 1400 ℃ for 2 h,and there was almost no mass change,no change in bulk density and open porosity,and no obvious microstructure change.SEM and X-ray diffraction analysis（XRD）show that only a small amount of SiC on the surfaces of the two composites was oxidized to Si O2,thereby protecting the internal composition and structure and making it intact.No signs of internal oxidation were observed.It shows that the self-sealing structure has excellent resistance to environmental interference and erosion.