Preparation And Oxidation And Ablation Resistances Of ZrSi₂-Based Coated Carbon/Carbon Composites

Carbon/carbon(C/C)composites have outstanding properites at high temperature,and become the important materials in aerospace field for their high mechanics performance in innert inviroments at high temperature.However,easily oxidized at high temperature is the main technical bottleneck to limit their applications.Oxidation resistance coating is one of the effective methods to solve the aboved problem.ZrSi₂-based ceramic is the effective coating material due to quickly produce dense oxygen resistance layer at about 500?-900?.In this paper,for improving the oxidation and ablation resistance of the C/C composites,the SiC/ZrSi₂-based anti-oxidation coatings were deposited on the surface of C/C composites by SAPS and pack cemention.The surface and cross-section morphologies,phase composition,the distribution of elements and the bonding strength were analyzed by SEM,EDS,XRD,Raman,DSC/TG and Scratch tests.The spraying preparation process parameters were optimized.The oxidation and ablation resistances of the coatings are also studied by isothermal oxidation,thermal shock and Oxy-acetylene ablation tests.The relationship between preparation technology,microstructure and oxidation and ablation of the coatings was built.The oxidation and ablation mechanisms of the coatings were also studied.The major contents and results are as follows:The effects of spraying power,main gas(Ar)flow and powder feed change on the microstructure,phase composition and interfacial bonding strength of the coatings were studied.The results show that with the aboved parameters increase,the microstructure of the coatings exhibit the transformation from gradual dense to loose.When the values of spraying power,Ar flux rate and powder feed rate are 40 k W,75 L/min and 20 g/min,the deposition rate is up to the maximum,due to the complete melting and flattening of the particles.Moreover,the coating prepared by the aboved parameters has excellent microstructure without obvious cracks and pores,and its maximum bonding strength is 11�1.23 N.The SiC/ZrSi₂-Y₂O₃double-layer composite coatings with different contents of Y₂O₃were deposited on the surface of C/C composites by SAPS and pack cementation.The effects of adding different contents of Y₂O₃ on the microstructure,phase composition,oxidation resistance of coating samples were studied.The results show that with the content of Y₂O₃increase,the density of the ZrSi₂-Y₂O₃ coatings first increases and then decreases,and the oxidation resistance of the coatings exhibits the trend from increase to decrease.The SiC/ZrSi₂-10 wt.%Y₂O₃coating exhibits excellent microstructure and oxidation resistance,and the loss rate is only 0.13%after oxidation 301h in air at 1500?.After experiencing 19times thermal shock cycles from 1500?to room temperature,the weight loss of the SiC/ZrSi₂-10 wt.%Y₂O₃coating was 0.65%.The improvement of oxidation resistance is attributed to two reasons.First,Y₂O₃ inhibits the phase transformation of the oxidion product Zr O₂.Second,the thermal stabilty of the coating increases due to the Zr O₂ and dense glassy Si O₂ layer mosaic structure formed.The SiC/CrSi₂-ZrSi₂-10 wt.%Y₂O₃composite coatings with different contents of CrSi₂were prepared.The effects of adding different contents of CrSi₂ on the microstructure,phase composition,oxidation resistance of the coatings were studied.The results show that with the increase of CrSi₂ content,surface roughness decreases first and then increases,pore rate gradually decreases,bonding strength first increases and then decreases.When the CrSi₂content is 30 wt.%,surface roughness and pore rate of the coating is up to the minimum values of 8.63?m and 1.1�0.2%,bonding strength reaches the maximum value of 15.1�1.3 N,beacause the flowing CrSi₂ at high temperature fills the pores and microscracks existed in the coating and coating interface to increase the dense of the coating.The coating has the mass loss of 1.2%after oxidation 288h in air at 1500?and could experience 15 times thermal shock cycles from 1500?to room temperature.The reason of oxidation resistance improvement of the coating is that the dense Cr-O-Si composite glass barrier layer is formed on the coating surface.The ablation resistance of the SiC/30 wt.%CrSi₂-ZrSi₂-10 wt.%Y₂O₃ composite coating were studied.The results show that after the ablation of 80 s,the coating exhibits good ablation resistance and its linear ablation rate and mass ablation rate are-1.0�0.03?m�s^-1 and-0.16�0.014 mg�s^-1,respectively.With the increase of ablation time,ablation central region of the coating surface was oxidized to form Cr-O-Si liquid film,which could form a passive layer to prevent oxygen from entering the coating.Subsequently,with the temperature of coating surface raised,the oxidition product Zr O₂ blending in the liquid film to form pinning struture could increase the viscosity of the liquid film and resist to the scour of high temperature airflow,which improved the ablation reistance of the coated composites.In order to further improve the ablation resistance of the SiC coated C/C composites,the TaSi₂ ablation resistance coating was prepared on the surface of SiC coated C/C composites by SAPS.The results show that with the spraying power increase,the microstructure of the coating appears to be loose to dense.When the spraying power is 50 k W,particles deposition rate is up to the maximum value of 39.8%,the coating bonding strength has the maximum value of 15.3�2.3N due to dense stack of completely melted particles.After the ablation of 80s in the oxyacetylene flame,the linear ablation rate and mass ablation rate of the coating are0.9�0.06?m�s^-1 and-0.4�0.02 mg�s^-1,respectively.The reason of the ablation resistance improvement is that the Si O₂-Ta₂O₅ composite protective film formed on the surface of the SiC/TaSi₂ coating could effectively protect C/C composites.