The Damage Mechanisms Of Carbon/Carbon Under Exposure Of Simulating Low Earth Orbit Environment And Its Modification

During the service as structural materials in Low Earth Orbit（LEO）environment,spacecraft will inevitably suffer radiating damage caused by a variety of high-energy particles.Such LEO damage particles mainly include Atomic Oxygen（AO）particles which are produced by ultraviolet light dissociating O₂ from the outer layer of the atmosphere and charged particles captured by the Van Allen belt,such as protons（P）and electrons（E）.When the structural material of spacecraft doing service at the speed of first cosmic velocity（7.9km/s）,it will collide with these particles at high speed,and then generate a series of physical and chemical interactions,and leading to the deterioration of material properties.It is the main space factor that threatens the service safety of spacecraft.Carbon fiber reinforced Carbon matrix composites（C/C）and a variety of modified composites have become the ideal choice of structural materials for space vehicles due to their excellent properties.Therefore,the study on the space radiation damage behavior of C/C and its modified composite materials is of great scientific and practical significance for the safety and stability of spacecraft in LEO environment.In order to understand the damage mechanisms of C/C in LEO space radiation environment,this paper adopted the experimental method of building an equivalent platform to simulate the real LEO space radiation environment on the ground to conduct long-term environmental simulation and assessment of C/C composite with its modified materials.The damage mechanism of single particle radiation and multiple particle superposition on specimen was studied in the light of various spacecraft operation modes.On this basis,the coating and matrix modification were studied to resist radiation,and then reexamined.The main research contents and results are as follows:（1）The single particle damage behavior of C/C composites in low earth orbit is studied.Results show that the AO particle erosion rate is at least one order of magnitude higher than that of P and E particles,is the main factor of space radiation environment damage in LEO and its damage mechanism shows a direct way of active oxide.Damaged structure presents the typical"honeycomb holes".the material consumption through“holes formation,holes growth,densifying of holes,rebuild holes"consumption mode,and then consume layer by layer on the surface of material.The damage caused by P particle is characterized by surface"hydrogenation"and mechanical impacting effect.The impact angle affects the damage effect and presents"bottom advantage"effect.The damage structure presents scratches,impact pits and other irregular defects.The damage of the C/C material caused by E particle irradiation is caused by the destruction of the Van der Waals force between the layers of the graphite structure in the C matrix,and the damage structure presents"pinhole"damage structure.At the initial stage of AO and P particle irradiation,mechanical energy transferred and the surface microstructure evolution will improve the mechanical properties of C/C materials,showing an effect similar to "stress releasing".（2）The superimposed damage effect of C/C composites in low earth orbit is studied.The results show that the superposition of AO and P particles can accelerate material damage.The damage is more severe at the P-AO（out of van Allen belt mode）than at the AO-P（into van Allen belt mode）.High-energy particles damage C/C composite by breaking through the surface,but have little effect on the internal structure of the materials.Superimposed radiation of AO and E particles will accelerate the damage of materials,among which the damage of E-AO radiation is more serious than that of AO-E radiation mode.AO superimposed E particles damage the surface of C/C through layer upon layer consumption,and their consumption behavior shows"top"preferred consumption.Under the condition of equal flux,the bending strength of P-AO decreases by about 3%more than AO-P.Surface roughness increases more than about 25.5%;The quality loss rate increased by over 16.7%.（3）AO particle radiation damage behavior of C/C composites modified by ZrC,TaC and SiC was studied.The results show that CVD SiC coating have the best resistance to AO particle radiation damage,followed by ICVI TaC coating,and PIP ZrC coating had a limited increase in the resistance to AO irradiation of C/C composites.For ZrC-C/C,Zr O₂ oxidized particles are the main products of chemical effect of AO particle.ZrC coating prepared by PIP method has a large surface porosity,and AO particles consume into material through the gap of ZrC phase,and causing damage to core structures such as fiber,interface and matrix inside.Zr O₂oxidized particles are weak in combination with the material surface,and are easy to be peeled off by the high kinetic energy of AO particles,resulting in large area of spalling.The main product of AO particles and TaC coating was Ta₂O₅.The radiation process was accompanied by obvious swelling of the surface,and the radiation burst with the increase of total flux.The damage process is oxidation effect firstly and then mechanical impingement effect.Oxidation effect presents"top advantage".The interaction between AO particles and TaC coating leads to surface expansion,and the closure of surface microcracks lead to the redistribution of surface stress,which is conducive to improving the mechanical properties of materials at the initial stage of irradiation.For CVD SiC coating,Si O₂ is the main oxidation product,and the generated Si O₂ grains grow up gradually with the increase of radiation time to form Si O₂ film,which improves the resistance to AO particle erosion.In the XPS analysis,the emergence of Zr loss,high-priced Ta and SiC_xO_y indicates that the final stable oxidation product in the irradiation process passes through the phase of unsteady intermediate-phase and high-valence compound.The transient high energy of AO particles will produce the unsteady transition products such as ZrC_xO_y,TaC_xO_y and SiC_xO_y.This process will also be accompanied by the precipitation of C phase.The crystallization degree of the precipitated phase increases with the increase of irradiation time,and finally forms a protective layer of DLC to improve the radiance resistance of the material.The order of precipitation degree of C phase was SiC>ZrC>TaC.（4）The mechanism of C phase precipitation of C/C composites modified by ZrC,TaC and SiC coatings and HfC matrix,as well as the kinetics,thermodynamics and crystal structure factors influencing the crystallization degree of C phase were studied.The results show that the AO particles in the Hf rich region of C/SiC-HfC composite were damaged more rapidly and severely,and the oxidation products were Hf O₂ particles accompanied by C phase precipitation.The C atoms are precipitated from the in site position of the carbide ceramic modified component lattice and grow in layers.The structure composition of precipitated C phase is related to bond energy,crystal structure similarity and entropy change of reaction system.Enough destroyed energy is the key factor for precipitation C phase reconstruction,crystal structure factor is a main conditions to influence the degree of crystallization precipitation phase,the bond energy and crystal structure factors of similar cases,the reaction process of entropy into the factors affecting the degree of crystallization precipitation phase.The order of crystallization degree is SiC>ZrC>HfC>TaC.AO particle denudation coating is carried out by"coating oxidation reaction;precipitation of C phase;C phase consumption;coating oxidation reaction" mode.