Laser Cladding Of Boride-based Anti-oxidation Composite Coating On The Carbon Substrate

Carbon materials with excellent material properties are widely used in aerospace and civil fields.However,a fatal shortcoming of carbon materials is that they are easy to oxidize under aerobic conditions,which greatly limits the application space of carbon materials in high temperature and aerobic environment.In order to further expand the application space of carbon materials,most of the current research adopts coating technology to improve the oxidation resistance of carbon materials.Laser cladding technology has the advantages of small heat affected zone,low dilution rate,fast heating speed,convenient and flexible,low cost,little pollution,etc.Therefore,in this thesis,5MB₂（M=Ta,Zr,Nb,Hf,W）coating（5MB₂ coating）,Si-Ta Si₂-5MB₂coating,Si C-Si-Ta Si₂-5MB₂ coating and Si-Ta Si₂-Zr O₂/Si-Ta Si₂-5MB₂ double-layer coating were prepared on carbon matrix by laser cladding.The effects of coating composition on coating strength,phase composition,microstructure,ablative resistance and oxidation resistance were investigated.After laser cladding of 5MB₂ coating,diffraction peaks of（Zr,Nb）B₂ solid solution appear in some coatings.The formation of（Zr,Nb）B₂ solid solution can reduce the interfacial energy of diffusion between grains,which is beneficial to the densification of coating.Range analysis showed that the main factor affecting the scratch adhesion of coating was the content of Nb B₂ and the scratch adhesion of5MB₂（11）coating was 11.8±0.81 N.The addition of Si and Ta Si₂ significantly improved the cladding effect of the coating,and the formation of Si O₂-B₂O₃composite glass made the coating denser.The particles inside the S10T10 coating are distributed in gradient,which improves the oxygen resistance effect.The coating is closely bonded with the carbon matrix,and the scratch adhesion of the coating is13.4±1.12 N.The addition of Si C helps to regulate the thermal expansion coefficient of the coating and then adjusts the bonding strength between the coating and the carbon matrix.The oxidation layer on the surface of the C5B75 coating is less spalling,and the scratch adhesion is 18.6±1.22 N.After laser cladding of the double layer coating,the inner and outer coatings penetrate into each other closely,and the adhesion of the coating scratch is greatly improved.The scratch adhesion of S20T20Z60 coating is 36.7±4.83 N,the overall coating thickness can reach about50μm.The increase of coating thickness can effectively reduce the influence of heat transfer on carbon matrix materials.After five times laser ablation,the surface of 5MB₂ coating is more flat,but due to the influence of thermal shock during the ablation process,part of the B₂O₃ glass layer is lost and the oxide particles are exposed.In the repeated ablation process of5MB₂（11）coating,the particles inside the coating are more evenly distributed,the O content is always low,and the ablation resistance is better.The scratch adhesion of the coating increases to 18.8±1.13 N.The dendritic structure on the surface of Si-Ta Si₂-5MB₂ coating is mainly Ta oxide,which can transfer stress,avoid stress concentration and prevent cracking of coating.After the ablation of S10T10 coating,the internal particles are evenly distributed.Part of the composite glass flows to the bottom of the coating and contacts with the carbon matrix to act as a"binder",and the scratch adhesion of the coating increases to 15.8±1.38 N.Although there are micropores in some areas on the surface of C5B75 coating,the overall coating is relatively dense with fine oxide particles dispersed,and scratch adhesion of the coating increases to 19.0±1.75 N.The addition of a small amount of Si C can adjust the thermal expansion coefficient of the coating and increase the ablative ability of the coating,but the addition of excessive Si C will increase the defects of the coating,which has the opposite effect.The surface of the double-layer coating is more flat and compact,"mortise-and-tenon structure"and dendrite structure appear inside some coatings,and element diffusion occurs between the outer coating and the inner coating and between the inner coating and the carbon matrix,which makes the scratch adhesion of the coating still maintain a high level.The scratch adhesion of the S20T20Z60 double-layer coating is 40 N after 5 times laser surface ablation.After the coating was oxidized at 1500℃for 100 min,the Ta B₂ content of 5MB₂coating had the greatest influence on the coating oxidation weight gain.After oxidation,oxide particles with different morphologies are formed on the surface of the coating,and some areas have holes left by B₂O₃ gas escape.5MB₂（11）coating has the least surface defects,the least oxidation weight gain,and the oxidation activity factor A_x is 6.473.Compared with 5MB₂ coating,the oxidation weight gain of Si-Ta Si₂-5MB₂ coating decreases significantly after oxidation,which is due to the more excellent oxidation resistance of Si O₂-B₂O₃ composite glass.The oxidation activity factor A_x of S10T10 coating is 1.386.Zr Si O₄ and Hf Si O₄ were also found in the oxidized coating.Compared with Zr O₂ and Hf O₂,Zr Si O₄ and Hf Si O₄ had better"napping"effect,which could significantly improve the viscosity of the glass layer on the surface of the coating,and thus improve the oxidation resistance of the coating.Ta-O-Si-B dendritic glass was also found in the coating,which could inhibit the formation of microcracks and improve the strength of the coating.The surface morphology of the oxidized Si C-Si-Ta Si₂-5MB₂ coating changes with the addition of Si C content.When the content of Si C in the sample is high,the oxide of"pebble"structure with glass phase filler will appear in the coating.The antioxidant capacity of the coatings was higher than that of the coatings without Si C,in which C5B75showed the best antioxidant effect and A_x was 0.976.The overall oxidation weight gain of Si-Ta Si₂-Zr O₂ coating was small,and Ta Zr_2.75O₈ and Zr Si O₄ were generated in the coating during oxidation.The oxidation activity factor A_x of S20T20Z60 is 0.539.After oxidation,the oxide particles are uniformly dispersed in the coating,which enhances the toughness of the coating sample.The dense outer coating with Zr O₂ as the main phase can effectively block the diffusion of external heat and oxygen to the inside of the coating,and reduce the corrosion of the inner coating and carbon matrix material in the high temperature aerobic environment.The double-layer coating can effectively play the role of heat insulation and oxygen resistance to the carbon matrix material.