Study On Multi-scale Interface Modification And Performance Of C/C Composites

Better service performances of C/C composites are required by the R&D and manufacturing of the next generation aerospace crafts.Proper optimizations of interfaces are the key to improve the mechanical performance of C/C composites.The interfaces inside C/C composites can be briefly classified into three type according to the scale of the interfaces:interface inside the pyrocarbon matrix,fiber-matrix interface and the interface between neighboring plies.Most of the existing work focus on the reinforcement of interfaces at a single spatial scale,which leads to unsatisfactory imporovements of the performance.To solve this problem,we propose the strategy of multi-scale interface modification in the present research.We firstly study the failure behavior of C/C composites under various loading states.Subsequently,we attempt to optimize the interfaces from micro-scale to meso-scale by means of matrix microstructure tailoring and the dopping of nanomaterials.Finally,the fatigue behaviors of CNT-and SiCNW-C/C composites are analyzed and a possible mechanism responsible for the fatigue strengthening behavior of C/C composites is proposed.The main contents and conclusions of this work are shown as followed:The failure behaviors of C/C composites with different preformed structures are discussed.The results indicate that under tensile stress,the load transfer within the unidirectional C/C composites shows typical localized-load-shearing(LLS)features.Failures of sub-fiber-bundles can be observed at the fracture of unidirectional C/C composites,which is caused by the synergy effect of LLS and poor cohesive strength of the pyrocarbon.When the orientation angle between the main stress and fiber axial direction excesses a threshold value,the failure of C/C is dominated by matrix cracking.The influence of matrix and interphases with different texture on the interfacial failure of C/C composites are investigated.Low textured(LT)pyrocarbon shows better inherent cohesive,which is not sensitive to the variation of loading rates.High textured(HT)pyrocarbon shows high anisotropy compared with LT pyrocarbon,and the bonding between graphite crystallites are poor.Thus,the stress transfer within HT pyrocarbon shows significant rate-dependent feature.A proper residual stress mismatch is induced between neighboring laminates by an alternatingly stacking of low textured and high textured laminates.This residual stress efficiently decreases the stress intensity at the crack tips,which facilitates the deflection and bifurcation of the delamination cracks.By introducing the interlaminar stress mismatch,the flexural strength of the composites increases by 66% compared with the original specimens and the Weibull modulus of the composites also increase from 4.9 to 8.3,indicating a significant increase of reliability.The in-situ growth of SiC nanowires on carbon fiber surface can simultaneously improve the fiber-matrix interfaces and the interlaminar interfaces,which significantly enhances the anti-shear capability of the C/C composites.However,the in-situ growth method results in severe damage to the fiber and cause degradation of the flexural strength.Applying a pyrocarbon coating on the fiber surface before the in-situ growth process can optimize the interfaces with improved shear and flexural strengths.The synergy effect of pyrocarbon texture and nano reinforcement has been analyzed.The LT matrix shows poor synergy with CNTs because of the overly bonded interfaces and brittle fracture itself.Meanwhile,the HT matrix exhibits better synergy with CNTs,and the introducing of CNTs can significantly enhance the interfaces within the composites and inherent cohesive strength of the matrix,which increases the average flexural strength of the composites by ?90%.The effect of nano reinforcements type on the microstructure of the matrix and multifunctional performances of C/C composites has been discussed.CNTs can induce the formation of isotropic carbon during the CVI process and enhance the fiber-matrix interface efficiently.SiC nanowires can induce the formation of SiCWN-pyrocarbon coaxial structures with larger in-plane and stacking-up crystallite size.The introducing of CNTs not only directly enhances the fiber-matrix interfaces and increases the z-directional thermal comductance of the laminated composite,but also indirectly induces the formation of protuberances within the matrix and decreases stress concentrations within the matrix.Under high-velocity oxyacetylene ablation,local thermal damage is significantly relieved and matrix peeling off by high-velocity gas is inhibited.As a result,mass ablation of C/C composites decreases by 64% with an optimized CNT content.Meanwhile,the introducing of SiC nanowires can only enhance the matrix but show little influence on the fiber-matrix interfaces.The coaxial structure induced by the SiC nanowires increases the electron loops in the matrix and greatly improves the inherent connectivity of the pyrocarbon.The average electromagnetic shielding efficiency in the X band(8.2–12.4GHz)increases from 35.6 dB to 60.5 dB with an optimized nanowire content.The fatigue behaviors of plain-weaved CNT-C/C and SiCNW-C/C composites are investigated.Our results indicate that matrix microcracks are the main damage form under cyclic loadings.The subcritical cracks formed at the 0° matrix benefits the deflection of the main crack,decreasing the stress concentration at the tip of main crack.On the other hand,damage cumulated inside the 90° ply is the main reason for the decrease of residual strength,which significantly decreases the bear-capability of the 90° ply under flexural stress.The propagation of subcritical cracks within the carbon matrix is analyzed.The results indicate that the growth of these subcritical cracks is self-arrested,where the growth of the cracks slows down or even stops with increased cyclic number.Further analysis reveals that these subcritical cracks are closely related to the anisotropy of the matrix and result from the debonding of turbostratic stacked pyrocarbon grains,which efficiently induces crack tip bifurcations and decreases the local stress intensity.