Micro-structure And Anti-ablation Resistance Of ZrC-based Composite Coatings

Zirconium carbide（ZrC）is a promising candidate to act as thermal protection system（TPS）to provide protection for the hypersonic vehicle.However,due to its poor anti-oxidation and volume change of oxide layer,ablation resistance of ZrC is not satisfactory in the extreme environment.In this work,MoSi₂,SiC and TiC incorporated with ZrC composite coating were prepared to improve the ablation resistance of ZrC coating.It is hoped that the oxidation products（Si O₂ and TiO₂）could fill the gaps,act as oxygen diffusion barrier and bond Zr O₂ particles.The composite powder were prepared by spray drying.Plasma flame and laser were used to simulate the extreme environment with low and high oxygen partial pressure,respectively.The ablation resistance was investigated in different environment by adjusting the heat flux.The main results were shown as follows:1.ZrC powder and ZrC-MoSi₂,ZrC-SiC and ZrC-TiC composite powders were prepared by spray drying.The composite powders showed the uniform particle size and sharp size distribution.The powders was suitable for plasma spraying.2.Zr C coating and ZrC-MoSi₂,ZrC-SiC and ZrC-TiC composite coatings were fabricated by vacuum plasma spray.No obvious cracks was detected at the interface between coating and SiC transition layer,which means that there was a good bonding strength at the interface.Plastic deformation of MoSi₂ was beneficial to increasing the density of the composite coating.The decomposition phenomenon of SiC during the spraying process was unfavorable to improve the deposition efficiency and density of the composite coating.Due to the high melting point of TiC,lots of semi-melted particles were observed in the ZrC-TiC composite coating.3.Plasma flame was chosen to assess the ablation resistance of ZrC-MoSi₂composite coatings.Under the ablation environment with the heat flux of 1.94MW/m²,MoSi₂ additive can effectively improve the ablation resistance of Zr C coating,Si derived from the decomposition of MoSi₂ could enrich the SiO₂ source of the oxide layer,which is conducive to maintaining the integrity and thickness of SiO₂ layer.Under the ablation environment with the heat flux of 3.01MW/m²,the rapid volatilization of SiO₂ caused that the oxygen partial pressure beneath SiO₂ layer increased and formed a large number of gaseous MoO₃,which leaded to the occurrence of bubble burst.4.Plasma flame was used to assess the ablation resistance of ZrC-SiC composite coatings.Under the ablation environment with the heat flux of 3.01MW/m²,SiC could effectively improve the ablation resistance of ZrC coating.During the ablation process,gaseous oxidized products（CO or CO₂）could be timely released,which successfully avoided the occurrence of the bubble burst.However,Si C-depleted zone may cause the oxide layer to peel off.Under the same ablation environment,ZrC-MoSi₂/ZrC-SiC with double-layerd structure showed excellent ablation resistance.The ZrC-SiC external layer acted as oxygen diffusion barrier to protect the ZrC-MoSi₂inner layer and the ZrC-MoSi₂ inner layer offered extra Si source for the ZrC-SiC external layer,which greatly reduced the destructive effect of SiC-depleted zone and MoO₃ gas.5.Plasma flame was used to assess the ablation resistance of ZrC-TiC composite coatings.Under the ablation environment with heat fluxs of 3.01 and 4.02MW/m²,TiC could effectively improve the ablation resistance of ZrC coating,（Zr,Ti）O₂ could bond ZrO₂ particles and fill the gaps.6.Laser was chosen to assess the ablation resistance of ZrC,ZrC-MoSi₂,ZrC-SiC and Zr C-TiC coatings.The oxide layer with porous structure of ZrC coating could not limit the oxygen diffusion;Destructive MoO₃ from Zr C-MoSi₂ coating destroyed the integrity of the SiO₂ layer,the structure of oxide layer was similar to ZrC oxide layer;ZrO₂-SiO₂ layer formed on the surface of ZrC-SiC coating,the ZrO₂-Si O₂ layer could effectively reduce the oxygen diffusion;The oxide layer structure of ZrC-TiC coating was not dense,which means that the ablation resistance of ZrC-SiC coating was better than that of ZrC-TiC coating.However,under the high heat flux,liquid phase derived from ZrC-TiC was more than that under low heat flux,the liquid phase could effectively cover the substrate because of higher temperature.Therefore,TiC additive is suitable for improving the ablation resistance of ZrC coating under high heat flux.