Growth Behavior And Environmental Damage Mechanism Of BN/SiC Interfacial Coating Of SiC_f/SiC Composite

Ceramic matrix composites（CMC）have excellent properties including light weight,good temperature resistance,ablation resistance,and oxidation resistance.It is a kind of high-temperature structural material which is potential candidate for superalloy and intermetallic compound.It can be applied to the aeroengine with 12～15 and higher thrust-weight ratio.The introduction of continuous fiber into the ceramic materials can overcome its inherent brittleness and improve the fracture toughness of CMC,which makes the application of ceramic materials in aeroengine possible.The interfacial coatings between continuous fiber and ceramic matrix,especially BN and SiC coatings,is the key factor for the application of ceramic matrix composites in aeroengine.The introduction of interfacial coatings can not only overcome the brittle fracture of ceramic materials,but also play an important role in improving the oxidation resistance and corrosion resistance of CMC in high-temperature gas environment.It is the guarantee of ceramic matrix composite material used in the engine environment for a long time.Up to now,the growth mechanism of BN and SiC interfacial coatings and the damage mechanism of SiC_f/SiC with interfacial coatings under various environmental conditions are not clear.Therefore,it is necessary to systematically and deeply study the nucleation and growth behavior of different interfacial coatings and the damage evolution characteristics of CMC with interface layers in different environments.In this paper,it is proposed to prepare single BN,BN/SiC,and multilayered（BN/SiC）₂ interfacial coatings by chemical vapor infiltration technique and prepare Mini-SiC_f/SiC and 2D-SiC_f/SiC composites with different interfacial coatings by precursor infiltration pyrolysis method.The influences of different processing parameters on the growth behavior of BN and SiC interfacial coatings were studied.The microstructure control technology,performance evaluation such as oxidation resistance and the tensile strength of fiber tows with interfacial coatings were also systemically studied.The damage mechanism of SiC_f/SiC with different interfacial coatings under oxidation environment and mechanical load was obtained.The main research contents and conclusions are as follows:（1）The influence of different CVI processing parameters on the growth behavior of BN interfacial coatings was studied.The results showed that decreasing the deposition temperature,increasing the total pressure of the system,and increasing the partial pressure of H₂ are beneficial to increase the yield of BN interfacial coatings in the temperature range of 750～1150℃from BCl₃-NH₃-H₂ system.At 850～1050℃,with the increase of deposition temperature,the interfacial coating changed from amorphous structure to polycrystalline and hexagonal structure.When BCl₃/NH₃ flow ratio is too low,strumae tended to grow on the surface of BN coating.The yield of BN coating increased first and then decreased with the increase of H₂partial pressure in reactant system.The deposition process of BN coating was controlled by surface reaction.After being reduced by H₂,BCl₃ formed BH_xCl_3-x droplets on the fiber surface,and then NH₃ dissolved in the droplet to form BN.BN grains on the surface of the interfacial coating grew in island growth mode at 843℃,while the growth mode of BN coating changed from island growth mode to layer-by-layer mode with the increase of deposition temperature.（2）The growth mechanism of SiC interfacial coating prepared from MTS-H₂ system was explored.The results showed that increasing the deposition temperature and the initial content of H₂ in the system are beneficial to increase the yield of SiC interfacial coating in the temperature range of 860～1160℃.The surface of SiC coating deposited at 860℃and 960℃was smooth,while the density of the interfacial coating was low duo to the rapid growth of SiC at 1060℃and 1160℃.SiC interfacial coating deposited at 860℃was polycrystalline,while the coating deposited at 960～1160℃had high crystallinity and the phase structure of it was cubic.When the temperature was lower than or higher than 1060℃,the deposition process of SiC coating was controlled by surface reaction and diffusion,respectively.SiC interfacial coating deposited at 960℃was composed ofβ-SiC and（111）plane is the preferred growth plane of SiC grains.（3）The microstructure control technology of BN interfacial coating and the performance evaluation technology of BN/SiC interfacial coating were developed.The results showed that the amorphous BN in the coating deposited at 860℃could transformed into hexagonal BN through the heat-treatment at 1050℃,1150℃,and 1250℃.The crystallinity of BN increased with the increase of heat treatment temperature.The ordering transformation of BN coating could also be achieved by controlling the oxygen content in BN interfacial coating.By controlling the vacuum degree of CVI deposition process,the surface morphology of BN interfacial coating could be effectively controlled.After the long time exposure in air at room-temperature and in air at 800℃and 1200℃for 20 h,the single BN interfacial coating was seriously oxidized.As to BN/SiC interfacial coating,SiC layer played a protective function to BN coating,effectively preventing the corrosion of BN coating by humid air and high-temperature oxidation atmosphere.In addition,according to Weibull distribution,the dispersion of tensile strength of SiC fiber bundles increased after high temperature heat-treatment and deposition of single BN and BN/SiC interfacial coatings.（4）The damage mechanism of Mini-SiC_f/SiC with different interfacial coatings in medium and high temperature oxidation environment was proposed.The results showed that the weight gain curves of single BN,BN/SiC and multilayered（BN/SiC）₂ coatings changed linearly in dry air at 800℃.The damage of the composite came from the oxidation of BN interfacial coating,and the oxidation kinetics of BN was always controlled by chemical reaction.The three kinds of Mini-SiC_f/SiC containing BN interfacial coating exhibited linear weight loss after a short weight gain in water vapor/oxygen environment at 800℃.The presence of water vapor in the environment made the oxidation of BN to B₂O₃ compete with the formation of volatile H_xB_yO_z,and the latter was dominant after 1 h.The three kinds of Mini-SiC_f/SiC experienced three stages of rapid weight loss,slow weight loss and weight gain in dry air at 800℃.SiC at the end of the composite was oxidized to form glassy Si O₂,which could seal the internal pores and cracks of the composite.Parts of glassy Si O₂ would be consumed to form gaseous Si（OH）₄,which counteracted the oxidation weight gain of SiC in water vapor/oxygen environment at 1200℃.At this temperature,the SiC layer in Mini-SiC_f/SiC with BN/SiC and multilayered（BN/SiC）₂coatings blocked the diffusion of oxidation atmosphere along the direction perpendicular to the fiber axis.Then,the oxidation of the composite mainly came from the oxygen diffusion parallel to the fiber axis.（5）The damage mechanism of SiC_f/SiC with different interfacial coatings under mechanical load was explored.The results showed that the introduction of single BN and BN/SiC interfacial coating could increase the fracture strength of un-coated Mini-SiC_f/SiC from416.4 MPa to 478.6 MPa and 638.1 MPa,respectively.The fracture strength of Mini-SiC_f/SiC with multilayer（BN/SiC）₂ interfacial coating was similar to that of composite with BN/SiC coating.The introduction of BN/SiC and multilayer（BN/SiC）₂ interfacial coating could significantly improve the tensile strength and bending strength of un-coated 2D-SiC_f/SiC.According to the AE results,the fracture process of Mini-SiC_f/SiC without interfacial coating under tensile load included three stages of microcrack initiation,matrix crack initiation and propagation,and fiber fracture.After the introduction of single BN,BN/SiC,and multilayered（BN/SiC）₂ coating,matrix cracks deflected and propagated along the interface between the components of composite and interface debonding occurred before fiber fracture.The corresponding stress range of this stage increased in the three kinds of composites,which improves the fracture properties of the composites.