Research On Damage Behavior Of SiC Nanowire Reinforced Ceramic Matrix Composites SiC_f/SiC In Stress

The high requirements of the new generation of high-performance aeroengines have caused the engine's supercharging ratio,combustion chamber temperature,and turbine temperature to continue to increase.However,the current use temperature of superalloys is close to the limit,and it is difficult to meet the design requirements of the new generation of advanced aeroengines.Continuous fiber reinforced ceramic matrix composites(Si C/Si C composites)have the advantages of high temperature resistance,low density,high specific strength,oxidation resistance,ablation resistance,and high reliability,which can effectively reduce the weight of the structure by 50-70%and improve the working temperature to above 200?higher,and the amount of cooling gas can be greatly reduced,and the working efficiency and thrust-to-weight ratio of the engine can be improved.It is an ideal material for the heated end parts of the next generation aero engine.However,when the Si C/Si C composite is in service for a long time,its safe service stress is limited by the matrix cracking stress.At this time,microcracks and penetrating cracks appear in the matrix,and the external oxidation phase will extend along the microcracks to the interface phase and fiber.The interface phase and fiber are oxidized,and the life expectancy drops sharply.The current main way to improve the oxidation resistance of composite materials under high stress conditions is the self-healing mechanism of the multi-layer matrix structure.However,it is limited by the chemical vapor deposition process.The material has a high porosity(10%),and each insufficiency of the mechanical properties of the layer matrix unit itself and the brittle nature of each layer of the matrix(brittle ceramic phase).When the thermal stress and the stress during service are repeatedly loaded and unloaded,the matrix will still form large-scale penetrating cracks,and the cracks can grow directly as far as the interface phase and fibers are concerned,the self-healing phase is difficult to fill and heal newly formed large cracks quickly and effectively,and the matrix has poor fatigue resistance,which affects the service life.The service life of composite materials under high fatigue stress is an order of magnitude lower than that under low stress conditions.Therefore,to improve the service life of composite materials in a high-stress oxidizing atmosphere,it is also necessary to start with the strengthening and toughening of the structural components inside the matrix to improve the anti-cracking ability and fatigue resistance of the micro-domain matrix.The purpose of this research is to modify the matrix of Si C/Si C ceramic matrix composites by introducing Si C nanowires,strengthen some weak areas of the matrix away from the fiber bundles,and focus on regulating the damage behavior of the material before the transverse penetration crack is formed.Affect the size and number of cracks as the main control method.Acoustic emission technology is introduced to realize damage characterization of the cracks inside the material in the form of energy-carrying and counted damage events.The three-point bending test,Vickers indentation and other techniques are used to verify the reinforcing effect of nanowires on the composite matrix.The results show that after the introduction of Si C nanowires,the bending mechanical properties of the material are significantly enhanced,the bending proportional limit is increased by 61.4%,and the bending strength is increased by42.1%.In addition,the development of early damage is delayed,and the growth of microcracks is inhibited when encountering Si C nanowires or deflected on the interphase while the surface of the nanowires is modified by weak bonding interface.The first damage event corresponding stress(?_min)is lifted by 33.7%;In the middle stage of damage development,nanowires can improve the size of transverse cracks through ductile mechanical behaviors like fiber bridging,slippage,pull-out,etc.,and the brittleness of the composite matrix is significantly improved.The damage tolerance when the external stress is less than the proportional limit is obviously lifted,and the acoustic emission high-energy stress(?_onset)is increased by 47.4%.In addition,through monotonic stretching and dynamic external load stretching,it has been found that Si C nanowires can positively modify the overall damage development of the material,improve the fatigue resistance of the material,and significantly reduce the damage caused by the unit increasing external stress.