Service Performance And Its Optimization Of C/SiC Composites Applied In High Specific Impulse Space Orbit Control Engine As Combustor

The technology of space orbit control engine with high specific impulse becomes the significant developing direction of modern space propulsion technology.And high performance materials of combustion chamber have been the key part in the developing of space orbit control engine with high specific impulse.Due to the performances of low density,high strength,oxidation resistance and anti-ablation,C/SiC composites have been one of the most promising materials as combustion chamber in high specific impulse engine.Nowadays,C/SiC composites combustion chamber developed by National University of Defense Technology has been successfully applied in small-lightweight and fast-respond attitude and orbit control engines of weapon system.Compared to small-lightweight and fast-respond attitude and orbit control engines,space orbit control engine with high specific impulse works in space environment for longer time and the working temperature is higher,the working time is longer.The combustor materials need more excellent performance.To realize the application of C/SiC composites combustor in high specific impulse engine,the service performance and its optimization of C /SiC composites applied in high specific impulse orbit control engine as combustion chamber have been investigated in this article.The service environment of C/SiC composites applied in high specific impulse engine as combustion chamber contains universe space environment?space environment for short?and combustion gas environment of the engine.According to relative test stardards,the effects of simulated space environments such as high vacuum,extra low temperature,thermal cycling,thermal cycling and random vibration,atomic oxygen and vacuum ultraviolet on the composition,microstructure and properties of C/SiC composites have been systematically researched.The results show that the composition,microstructure and properties of C/SiC composites have good stability in simulated space environments.The mass and mechanical properties of C /SiC composites have no obvious change in vacuum of 10-7 Pa for 24 h.And immerged in liquid nitrogen?-196??for 60 days,the mechanical properties of C/SiC composites have no obvious change.The mechanical properties of C /SiC composites have no significant change after up to 100 thermal cycles between-196? and 200?.Due to the release of the inner residual tensile stress in the thermal cycling environment between-100? and 180? associated with the random vibration environment with the acceleration of 40 g,the flexural strength of C/SiC composites increases about 20%.The mechanical properties of C/SiC composites have no obvious change in atomic oxygen environment for up to 15 h,in which the energy and flux of atomic oxygen reaches 5 e V and 9.6×1015?cm2·s?-1,respectively.The composition,microstructure and mechanical properties of C/SiC composites have no significant change in vacuum ultraviolet environment,in which the wavelength and radiation strength is about 100¹50 nm and 0.75W/m2,respectively.Owning outstanding heat-resistant performance is the precondition for the application of C/SiC composites as combustor.In this article,firstly,the heat-resistant performance of C/SiC composites at 1800? has been researched according to the thermal environment of the high impulse engine.And then the heat-resistant performance has been optimized based on the limitation mechanism by means of reducing the activity of carbon fiber and preparing protective coating on the fiber.The results show that the flexural strength of C/SiC composites heat treated at 1800? is less than 20% of that of the composites as received.And the limitation mechanism of the heat-resistant performance of C/SiC composites is that carbon fiber has been badly damaged in high temperature environment because of carbothermic reductions between carbon fiber and oxide impurity in matrix.The damage of carbon fiber leads to the fatal degradation in mechanical properties of C/SiC composites.Compared to T300 carbon fiber,T1000 carbon fiber has low activity,but the C/SiC composites reinforced by T1000 carbon fiber can not prevent the damage of carbon fiber from oxide impurity in matrix during heat treatment.As a result,the heat-resistant performance of C/SiC composites has not been improved.Because of the obvious difference in thermal expansion coefficient between carbon fiber and SiC coating,it is easy to form cracks in the coating under high temperature service environment.And these cracks can not protect carbon fiber from the erosion by oxide impurity in matrix.The heat-resistant performance of C/SiC composites can be significantly improved by preparing carbon coating on carbon fiber.The fiber carbon coating plays the role of isolation and victim.After treated at 1800? for 1h,the fiber carbon coating makes the flexural strength of C/SiC composites was 270 MPa,increasing from less than 20% to 70% of the strength of the composites as received.Combustion gas environment of the engine is a corrosive gas environment of high temperature,high pressure and high flow velocity.The ablation characteristic of C/SiC composites under combustion gas environment of high impulse engine has been investigated.N₂O₄ and methylhydrazine?MMH?are the oxidant and fuel of the propellant,respectively.The mole ratio of N₂O₄ and MMH is 1.65.The temperature in the center of combustion gas is about 3100 K and the temperature of combustion chamber wall is about 1800?.The pressure of combustion chamber is 1.0 MPa.The throat of C/SiC composite combustion chamber has been badly ablated after the engine working for 373 s.and the diameter of the throat expands about 4% of that of the throat as received.the ablation mechanism of C/SiC composites under combustion gas environment is oxidation/vaporization together with mechanical scouring.The oxidation by H₂O from combustion gas is the main oxidation during ablation.The SiC matrix has been oxidized to liquid SiO₂,gaseous SiO and Si?OH?₄.These oxidation products have been blown away by the combustion gas of high flow velocity.And then the oxidation of SiC matrix keeps on.Carbon fiber without the matrix has been oxidized by H₂O as well as scoured by the gas.To improve the ablation resistance of C/SiC composites under combustion gas environment of high impulse engine,ZrC coating has been deposited on the surface of C/SiC composites by chemical vapor deposition.The melt points of ZrC and its oxide ZrO₂ are 3420? and 2677?,respectively.The oxide ZrO₂ can adhere to the surface of the composites tightly.O n one hand,ZrO₂ can protect the substrate SiC from ablation,on the other hand,it can form ZrO₂ and SiO₂ glass layer with good oxidation and ablation resistance when the liquid SiO₂ fills the cracks in ZrO₂.The CVD system for ZrC has been designed.The Zr source can be transported quantificationally by screw.The available deposition area is the zone3 position in the constant temperature area.The composition and microstructure of CVD ZrC have been optimized.And the original performance of the optimized ZrC coating has been investigated.It can gain compact,uniform,pure and thick ZrC coating under 40 kPa at 1500?,as well as the C /Zr ratio in source gas is 1:1.The mechanical properties of ZrC optimized have been researched.The results show that the micro-Vickers hardness of the ZrC coating is 22.7 GPa,the nano-indentation hardness of the coating is 25.3 GPa,the plastic modulus of the coating is 327 GPa,and the fracture toughness of the coating is 0.88 MPa·m1/2.The bonding strengths of the coating with C/SiC composites,graphite and CVD SiC are 5.2 MPa,4.5 MPa and 1.5 MPa,respectively.The composition and microstructure of the ZrC coating under high temperature have been studied.It is found that the contents of element C and O decreases under high temperature because the impurity ZrO₂ and C in the coating can react.And as the temperature goes up,the growth of ZrC crystal trends to?111?crystalline plane,which has the lowest surface energy.Meanwhile,the micrograph of ZrC becomes samdwich with growth step gradually.Oxyacetylene torch is an approximate simulation of the combustion gas environment.The ablation of C/SiC composites with ZrC coating has been investigated by oxyacetylene torch.The temperature of the fire reaches about 3000 K,and the ablation time is 30 s.The results show that the ablation resistance of C/SiC composite has been obviously improved by ZrC coating.The mass ablation and linear ablation velocities of have been reduced from 52.5mg/s and 93.2?m/s of the composites as-received to 0.43mg/s and 1.7?m/s of the composites with ZrC coating.