Self-healing Gradient MoSi₂-based Anti-oxidation Coatings Prepared By SAPS

In a high-temperature oxygenated environment,coating is an effective route to protect carbon/carbon?C/C?composites from oxidation.The excellent performance of Mo Si₂ at elevated temperature make it promising candidates for coating.However,at low temperature Mo Si₂ is prone to brittle cracking and could only protect the substrate in a narrow temperature range.Besides,its application is seriously limited due to the poor designability of the preparation technology.In this dissertation,with the aim of widening the oxygen protection temperature range of Mo Si₂ coating,supersonic plasma spray technology?SAPS?was employed to prepare the coatings,and the effects of preparation process,healing phases and gradient structure design on microstructure and property of Mo Si₂ coating were systematically investigated.SEM,XRD,EDS,WDS,tensile strength test and finite element analysis were employed to characterize the morphology,phase composition and distribution,mechanical property and residual stress of the coatings.The self-healing performance,oxidation and thermal cycle resistance and mechanism of the coatings were studied.Detailed research contents and main conclusions attained were listed below:The effects of particle diameter and post treatment process on the density and interfacial bonding strength of Zr B₂-Mo Si₂ coatings were investigated.Results showed that the smaller of the particle diameter,the higher degree of molten particle unfold and oxidation,and then the density and bonding strength became higher.Heat treatment in argon atmosphere could reduce the viscosity of Si O₂ and B₂O₃,and accelerate element mutual diffusion in the coatings,which were benefit for density and bonding strength of the coatings.The effect of Zr B₂ on the oxidation resistance of Mo Si₂-based coatings was investigated at the temperatures ranged from 900 to 1500°C.Results showed that 20?30%Zr B₂ could significantly improve the oxidation resistance,which was mainly attributed to Zr Si O₄ pinned Si O₂ glass layer to restrain oxygen permission and to form multi-phase mosaic microstructure with original coating to inhibit crack growth.The failure of the coatings at 900°C was due to interfacial thermal mismatch and brittle nature of ceramic coatings,which induced cracks difficult to be healed by insufficient B₂O₃.At 1500°C,the failure of the coatings was due to the destruction of protective oxide layer by vigorous evaporation of B₂O₃ and phase transition of Zr O₂ with higher CTE accompanied by volume change.Introducing nano-Si B₆ dramatically improved the oxidation resistance of Mo Si₂-based coatings at low and medium temperature regions.15%Si B₆-Mo Si₂ coating could effectively protect the substrate from oxidation for 84h and 120h at 900°C and 1200°C with mass gain of only 0.24%and 0.93%.The outstanding property of the coating was ascribed to nano-Si B₆,which reduced lamellar interfaces and splat cracking of as-sprayed coating.At the same time,B₂O₃·Si O₂,oxidation product of Si B₆,could flow to heal the cracks.The crack healing behavior of pre-cracked Si B₆-Mo Si₂ ceramics was studied at 900°C and 1200°C.Results showed that enhancing the oxidation temperature and the content of Si B₆ could speed up the crack healing rate.A theoretical calculation model of Si B₆-Mo Si₂ ceramics during oxidation was built to estimate crack healing time and temperature preliminarily.Introducing Zr Si₂ alloy could improve self-healing property and oxidation resistance of Mo Si₂ coating at high temperature.Zr Si₂ alloy possessed the abilities to relieve interfacial stress concentration,induce surface crack deflection,reduce the width of large size cracks and increase the length of micro-cracks,which were beneficial to heal cracks immediately.Following oxidation at 1500°C for 10h,the cracks prefabricated in the surface of 20?60%Zr Si₂-Mo Si₂ coatings were completely healed.The crack healing rate was positively related with the content of Zr Si₂.Among these coatings,40%Zr Si₂-Mo Si₂ presented the best oxidation resistance,which gained mass of 1.89%after oxidized at 1500°C for 42h.Its distinct protective performance was attributed to four aspects:?1?The reduction of crack width speeded up the healing rate;?2?The increase of Si source raised the production of glass phase;?3?The volume expansion of Zr Si₂ was 1.5 times higher than Mo Si₂ during oxidation,which was helpful for crack reduction;?4?Moderate amount of Zr O₂ formation could react with Si O₂ to produce the more stable phase-Zr Si O₄,which pinned glass layer to prevent cracking and avoid damaging protective layer by plentiful Zr O₂ accompanied with volume change during phase transition.Gradient Si-Zr-Mo and self-healing B-Si-Zr-Mo coatings were designed and prepared on Si C coated C/C composites.The residual stress in the coated composites was analyzed by Abaqus FEA and XRD.Results showed that interfacial stress concentration was remarkable relieved through gradient structural design,and the position of maximum stress transferred from the interface to the surface of the coatings.These two kinds of multilayer coatings could withstand 30 and 40 thermal cycles between 900°C and room temperature.On one hand,the superior thermal cycle resistance of self-healing coating was due to thermal stress relief by gradient structure design.On the other hand,the coating was oxidized at low temperature to form liquid B₂O₃·Si O₂ phase and Zr Si O₄ particles,which healed cracks and further restrained crack propagation to prevent coating failure.These two kinds of multilayer coatings exhibited outstanding oxidation resistance in wide temperature range,especially the self-healing B-Si-Zr-Mo coating,whose protection time increased from 10h,30h,22h to over 107h,125h,130h at 900,1200 and 1500°C with mass gains of only 0.41%,1.7%,1.1%,respectively.The superior protective performance was influenced by three factors:?1?The inhibition of crack growth through gradient structure design;?2?At low temperature,Si B₆ was oxidized to produce liquid B₂O₃,which facilitated Zr Si O₄ particles and Si O₂ to heal cracks and form protective layer;?3?At high temperature,the inward diffusion of oxygen and production of B₂O₃ volatile phase,which were fatal to the protective layer,were avoided by forming a dense Zr Si O₄ pinned Si O₂ protective layer through oxidation of Zr Si₂.