Preparation,microstructure And Properties Of Hierarchical SiC_f/SiC Composites Reinforced With BNNTs Grown In Situ

Conventional silicon carbide fiber reinforced SiC matrix（SiC_f/SiC）composites have been regarded as the most promising candidates for hot section components in next-generation and high-performance aero-engines due to their excellent properties,such as low density,good thermal stability,outstanding oxidation resistance,high specific strength and modulus.The incorporation of fibers and a proper design of fibers/matrix interface can arouse energy dissipation mechanisms such as interfacial debonding,crack deflection and so on,which endows SiC_f/SiC composites a non-catastrophic failure mode.However,inter-fiber,inter-fiber-bundle and inter-laminar matrix cannot be strengthened and toughened by the above-mentioned mechanisms and thus still shows a brittle behavior.The threshold stress or strain for damage initiation is rather low in such matrix and cracks propagate easily in matrix without being hindered,which limits mechanical properties of the macroscopic composites eventually.Such matrix at the micron scale is weak point in composites and its mechanical properties remain to be further improved.Boron nitride nanotubes（BNNTs）exhibit excellent mechanical properties,thermal and chemical stability and possess good oxidation resistance up to 900℃,which endow BNNTs a great potential application as nanoscale reinforcements.Introducing BNNTs into matrix has been deemed as an effective way to solve above-mentioned problem.BNNTs are expected to strengthen and toughen the matrix at nanoscale and at last optimize mechanical properties of the macroscopic composites.A simplified ball milling method,consisting of ball milling of boron powder and thermal annealing in high pressure nitrogen gas,was developed to in situ grow BNNTs on fiber surface and BNNTs/fiber hierarchical reinforcements were fabricated.Effects of main process parameters on the morphologies of BNNTs were investigated.Results show that growth density and length of BNNTs can be tailored by controlling the content of catalyst,the concentration of precursor slurry,physical and chemical states of raw material,growth temperature and pressure.Further study on the microstructure,morphology and chemical composition of BNNTs was conducted.Results display that BNNTs exhibit a multi-walled and bamboo-like structure and possess two distinct morphologies of tube walls:flat walls and bubble-chain walls,respectively.Catalysts are encapsulated inside and at the tips of BNNTs with bubble-chain walls while for BNNTs with flat walls catalysts are located at the bottoms of nanotubes.Based on the analysis described above and investigation on phase,particle size distribution and surface chemical state of precursor slurry,the growth process and possible formation mechanisms of BNNTs were proposed.Results indicate that in situ growth of BNNTs with bubble-chain walls and flat walls can be well interpreted by stress-induced sequential growth mode in VLS mechanism.The distinction of formation mechanism for these two types of BNNTs is that the former are formed following the tip growth mode while the base growth mode for the latter.This distinction is ascribed to the different strength of the interaction between catalyst and the fiber surface.After the build of BNNTs/fiber hierarchical reinforcements,SiC_f/BNNTs-SiC hierarchical composites were fabricated via chemical vapor infiltration（CVI）and polymer impregnation and pyrolysis（PIP）methods.Effects of BNNTs on the microstructure and mechanical properties of composites were studied.Results demonstrate that energy dissipation mechanisms at nanoscale are triggered by BNNTs such as pullout,debonding,crack deflection and crack bridging,which certifies the strengthening and toughening effects of BNNTs.Meanwhile,BNNTs put a restriction on densification process of composites,leading to an improper infiltration of matrix into fiber bundle.This negative effect may offset the real contribution of BNNTs and hence mechanical properties of composites have not been raised.It is found that densification to fiber bundle by PIP in advance before subsequent matrix infiltration is expected to solve above issue.BNNTs/matrix interfacial bonding was tailored via depositing a BN interphase on BNNT surface by chemical vapor deposition（CVD）method and effect of BNNTs/matrix interface tailoring was investigated elaborately.Results show that BNNTs/matrix and fiber/matrix interfacial bonding can be optimized by the deposition of BN interphase.It can activate strengthening and toughening effects of BNNTs and fibers.As a result,fracture toughness of composites is improved evidently.When the thickness of BN interphase is 10-20,30-45 and 45-70 nm,the fracture toughness is improved from 11±0.4 MPa.m^1/2 to 13.0±1.2,14.0±1.1 and 12.2±0.3 MPa.m^1/2,respectively.The flexural strength is also raised from 296.8±17.6 MPa to 342.5±11.9MPa when the thickness of BN interphase is 10-20 nm.Additionally,in-depth study on the crack deflection mode demonstrates that debonding cracks at the BNNTs/matrix interface zone are deflected within the interphase.This mode can produce branched multiple cracks and is beneficial for BNNTs to give full scope to strengthening and toughening effects in matrix.Damage initiation and evolution of SiC_f/BNNTs-SiC hierarchical composites were also scrutinized.Results display that BNNTs without and with an interphase deposited can improve damage development thresholds and delay early damage progression of composites by interdicting and deflecting the cracks,respectively.The former is considered as“hard”mode and the delay effect is not so successful while the latter is“soft”mode and the delay effect is more pronounced.Both BNNTs can induce premature failure of composites.However,in the latter,fiber/matrix interfacial bonding is weaken thanks to the interphase and the bonding strength is slightly higher than that in virgin composite.Consequently,final failure is just a bit earlier than that of virgin composites.When employed in hot section components of aero-engines,good oxidation resistance is the key for SiC_f/SiC composites to serve long-timely and reliably.In this thesis,oxidation behavior and mechanism of oxidation resistance of SiC_f/BNNTs-SiC hierarchical composites were examined systematically.Results indicate that hierarchical composites have a better oxidation resistance than virgin composites,which is attributed to the protection for PyC interphase from glassy phase B₂O₃ and the remained strengthening and toughening effects from surviving BNNTs.When oxidized at 900℃without pre-fabricated cracks,glassy phase B₂O₃ with low viscosity,produced by the oxidation of the residual boron powder after the growth of BNNTs,can seal the diffusion channel of oxygen and protect PyC interphase.Furthermore,BNNTs can survive and maintain the strengthening and toughening effects.When oxidized at1000℃without pre-fabricated cracks,volatilization of glassy phase is serious and the diffusion channel of oxygen cannot be sealed,thus exacerbating the oxidation of PyC interphase.However,BNNTs still can arouse the strengthening and toughening mechanisms.When oxidized at 900℃with pre-fabricated cracks,glassy phase originates from both the residual boron powder and some BNNTs because pre-fabricated cracks go through some BNNTs.PyC interphase still can be protected by glassy phase.Meanwhile,due to the loss of some oxidized nanotubes,strengthening and toughening effects of BNNTs are weaken.In contrast,after depositing a BN interphase on nanotube surface,few cracks are pre-fabricated in composites and glassy phase is only from the residual boron powder.In the meantime,thanks to the protection from BN interphase,the remained strengthening and toughening effects of surviving BNNTs are more effective.