Preparation And Strengthening Mechanism Of 2.5D Woven SiC_f/SiC Composites

SiC_f/SiC composites have broad application prospects in the hot-end parts of aeroengines.With the continuous improvement of the Mach number of aerospace vehicles and the thrust-to-weight ratio of aeroengines,higher requirements for strength and toughness of SiC_f/SiC composites are put forward.At present,domestic researches on SiC_f/SiC composites are mostly focused on the first and second generation SiC fibers,while the strengthening and toughening mechanism of the third generation SiC fibers and their composites with significantly improved oxidation resistance is rarely reported.Therefore,the research on the preparation and mechanical properties of the third generation SiC fiber reinforced SiC matrix composites has important application value.The interfacial layer and matrix are the key microstructure units of SiC_f/SiC composites,which have important influence on their strength and toughness.In this paper,SiC_f/SiC composites with four kinds of interfacial phases and two kinds of matrix systems were designed and prepared for the third generation of Domestic SiC fiber with 2.5D woven structure as the skeleton structure.Acoustic emission（AE）and X-ray tomography（Micro-CT）nondestructive testing technology combined with normal temperature fracture toughness were used.The mechanical behavior and progressive damage process of 2.5D machine SiC_f/SiC composites were analyzed,and the failure mechanism was effectively revealed.Specific research contents and conclusions include:（1）The 2.5D woven layer-by-layer angular interlocking preform with the third generation of SiC fiber was used as the skeleton structure.Four interlayer systems of Py C,Py C/SiC,BN,BN/SiC and two matrix systems of PCS and VHPCS were designed and selected.2.5D woven SiC_f/SiC composites were prepared by chemical vapor deposition（CVD）,chemical vapor osmosis（CVI）and precursor infiltration pyrolysis（PIP）.On this basis,the microstructure of2.5D woven composites with different matrix and interface layers was characterized.The results show that the deposition of interfacial layer and the type of precursor will affect the density and porosity of the final composites.Among them,PCS as matrix precursor will form more pores in the matrix during the pyrolysis process,which is not conducive to improving the density of the composite.Compared with the sample with single interfacial layer,the sample with composite interfacial layer has higher density and lower porosity.（2）Focusing on PCS as matrix precursor,combined with AE testing technology,the normal temperature fracture toughness of 2.5D woven SiC_f/SiC composites containing Py C,Py C/SiC,BN and BN/SiC interfaces was studied.Load-deflection curves and AE signal curves were obtained to analyze the damage modes.In order to reveal the damage mechanism of 2.5D woven SiC_f/SiC composites with different interfacial layers.The results show that the sample with composite interfacial layer exhibits higher fracture toughness,and the introduction of SiC layer significantly improves the energy dissipation mechanism of the material,while the BN interfacial layer is damaged and exhibits strong interfacial bonding,and the mechanical properties of the material decrease.In addition,acoustic emission technology can completely describe the real-time damage process of SiC_f/SiC composites.The sample with composite interfacial layer has more interfacial damage due to the blocking effect of SiC layer on the crack before the failure of a large number of fibers,which also produces higher energy AE events.（3）In order to study the influence of different precursor systems on the properties of SiC_f/SiC composites,PCS and VHPCS were selected to prepare SiC matrix.Combined with AE and Micro-CT nondestructive testing technology,the progressive fracture damage behavior of 2.5D woven SiC_f/SiC composites was carried out.In addition,the influence of component materials on the mechanical properties and microscopic morphology is compared and analyzed,revealing the progressive damage failure mechanism of the materials.With the help of Micro-CT damage visualization and damage quantitative analysis,the different damage development of B/S-SiC_f/SiC_PCS and B/S-SiC_f/SiC_VHPCS can be observed.The distribution of pore defects has an important influence on the progressive damage behavior and final damage mechanism of SiC_f/SiC composites.The sample B/S-SiC_f/SiC_PCS contains obvious pore distribution characteristics,which can effectively adjust the stress concentration when the material is loaded,so that the area of crack propagation is larger,which ensures the SiC_f/SiC composite has good strength and toughness,and also generates more acoustic emission damage signals.Although the densification degree of B/S-SiC_f/SiC_VHPCS has been improved,there are many micro-cracks in the matrix.During the loading process,the stress at the crack tip cannot be released in time,resulting in the mechanical properties of the composite material has not been greatly improved.