CoNiCrAlTaHfY/Co Coating On C/C Composites And Its High Temperature Oxidation Properties

The C/C composites are carbon fiber reinforced carbon-based material system that replaced other high-speed aircraft and heavy-duty aircraft brake materials because of their high heat resistance,high frictional performance,stability and high mechanical and physical properties.However,C/C composites rapidly are oxidized at temperatures above 500?and the oxidation rate increases with oxidation temperature,which severely limits the wide application of this material.Matrix modification and anti-oxidation coating can effectively improve high temperature oxidation resistance of C/C composites.In this paper,CoNiCrAlTaHfY/Co composite coating was prepared based on the etched surface of C/C composite by H₂+Ar by using microwave plasma chemical vapor deposition,and Co transition layer by double glow plasma surface metallurgy technology?DGPSMT?.By optimizing the processing parameters of etching,Co and Co NiCrAl TaHfY alloying,the microstructure,composition,structure and bonding strength of the composite coating were analyzed,and the oxidation properties and mechanism of CoNiCrAlTaHfY/Co composite coating at 600?,800?and 1000?were investigated.The main contents and results are as follows:The C/C composites were etched for 120 min using microwave plasma chemical vapor deposition technique.The carbon substrate enveloping the carbon fiber was basically etched away and the top part was sharp and thin-walled.The C/C composites substrate did not damage while the etching only took place on the surface of the substrate.The enrichment of Cr carbide was formed in the interface between the CoNiCrAlTaHfY coating prepared directly on the surface of the unetched C/C composites,resulting in a mismatch between the coefficient of thermal expansion of the coating and the substrate,leading to through cracking and even spalling.The number and size of surface cracks of the CoNiCrAlTaHfY coating formed on the etched surface were significantly reduced,and the bond strength between the CoNiCrAlTaHfY coating and the substrate was improved.Co transition layer was prepared on the etched C/C composites by DGPSMT to further improve the bond strength.The thickness of the Co coating was about 18?m,which was grown in the columnar crystal,and the interface between the columnar crystals was obvious but tightly bonding.The CoNiCrAlTaHfY coating was prepared on the surface of Co coatingto form a CoNiCrAlTaHfY/Co multi-component composite coating.The thickness of the composite coating was about 30?m and was composed of small islands with small particles.The composite coating was compact and the components distributed uniformly.The coating phase was relatively complex,mainly including CrCoTa,Al₂Ta,Cr₂Ta,AlxCry,AlCo₂Ta,Co,Al_xNi_y and so on.The oxidation resistance of CoNiCrAl TaHfY/Co coating in air at 600?,800?and 1000?was obviously improved.The weight loss rate of CoNiCrAlTaHfY/Co coating after oxidating at 1000?for 240 min was only0.8%.During the oxidation process,dense anti-oxidation scales such as Al₂O₃and Cr₂O₃ were formed on the coating surface to prevent diffusing oxygen.The oxides of Ta,Hf,and Y can react with Al₂O₃ and Cr₂O₃ to improve the bonding strength and anti-oxidation properties of the coating and can also slow down the rate of failure of the coating.