| Cf/SiC composites are advanced structural materials in the aerospace field because oftheir excellent high-temperature performance, but in high temperature oxidation andablation will lead to the degradation of performance of Cf/SiC composites, and limit thewide application. Preparing a coating on C/SiC composites is an effective method to solvethis problem. In this paper, Several ablative coatings applied to different temperature havebeen designed and desposited by plasma spraying. The ablation resistance of coatings istested by oxyacetylene flame, and The ablative mechanism is analyzed, The main researchcontents and results are as follows:Al2O3has been chosen as the bonding layer between the Cf/SiC composites and ZrO2coating due to its good high temperature stability and low coefficient of thermal expansion.The three structural coatings of Al2O3/ZrO2, Al2O3/ZrO2+SiO2and Al2O3/ZrO2/SiO2havebeen designed and prepared by plasma spraying (APS). The Al2O3is suitable to plasmaspraying and there is a good physical and chemical compatiblity between the Al2O3andCf/SiC composites, which provide excellent bonding strength for the coating. After ablationat1600℃using the oxyacetylene flame, the Al2O3/ZrO2/SiO2coating shows a goodablation resistance performance. It is mainly because that the SiO2could melt and fill thesurface defects and internal cracks of ZrO2coating at high temperature, which caneffectively inhibit the mechanical denudation of ZrO2coating and oxidation of Cf/SiCsubstrate. The failure mechanism of Al2O3/ZrO2/SiO2coating is mainly physical andchemical corrosion of SiO2, While the failure mechanism of the other structure coatings arethe mechanical denudation and oxidation of Cf/SiC substrate as result of the through cracksof ZrO2coating.The oxidation characteristic of materials at2000℃has been calculated using thethermodynamic method. TiC was selected as the coating material under the condition of2000℃. TiC coating, prepared by plasma spraying, wascomparatively dense. However,TiC has a high melting point, which resulted that TiC powder melt insufficiently in thespraying process and some small particles were formed when TiC powder impact thesubstrate with a high speed. At the same time, the coating contains a small amount of TiO2because the ambient air could be drawn into plasma spraying arc during the APS process.TiC coating exhibited a good ablation resistance at2000℃. This is mainly because, underthe action of the high temperature flame flow, TiC coating was oxidized into TiO2. TiO2melted and flow viscously due to being washed by the flame flowc and surface tension. TiO2experienced the microstructure evolution process of point-like fusion, linearconnection and net connection, and a fully dense TiO2layer was finally formed, whereuponreducing the mechanical denudation as well as mass ablation rate. The ablation resistanceof TiC coating could be further improved by adden theZrO2second phase. Melting TiO2could adhere around the high melting point ZrO2solid particles, and a synergistic effectwas formed, which promote the quick formation of densification structure, and improve theablation resistance of the coating. The ZrO2second phase also provides a good tougheningeffect for coating, so the composite coating showed a good thermal shock resistance in thecooling process after ablation and no crack generated.ZrC was selected as the ablation resistance coating materials for the3000℃ultra hightemperature. The coating of W/ZrC was designed, and then prepared by APS. ZrC powderapplied to APS could be prepared by spray drying. After1250℃calcination treatment, ZrCpowder had a good cohesive strength and mobility. There was a better coating quality usingthe powder with60-80μm, and the coating quality could be further improved after apre-oxidation treatment of ZrC powder. ZrO2thin layer in the ZrC powder surface wasformed after pre-oxidation treatment, which promoted the combination between theparticles. After ablation at3000℃for300s, W/ZrC coating exhibited a good ablationresistance and different regional characteristics. In the central ablation region, there was thehighest temperature, and the ZrO2and WO3formed by oxidation could provide asynergistic effect, which melt and fill the defects and cracks inner coating. Finally, a denselayer was formed, which could effectively reduce the mechanical denudation opportunity.The thermal chemical ablation played a principal role in the central ablation region. Nodense layer was formed in the low temperature region during ablation, and the ablationwere mainly thermochemical corrosion and mechanical denudation. |
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