| Carbon-carbon?C/C?composites is wildly applied in aerospace,military,medical and civilian areas because of their excellent properties,such as high strength,thermal shock and ablation resistances,high thermal conductivity,and low specific gravity.With the rapid development of aerospace technology and high-tech weapons,as the thermal structural materials,C/C composites' service conditions will become more severe,and the oxidation behavior in extreme environment is also becoming more and more prominent problem for the performance of weapon property.Oxidation resistance coatings are considered as an effective choice to protect C/C composites from oxidation at high temperature.In this paper,Zr-alloyed and Cr-alloyed coatings of as well as the complex coatings of Ni+Zr,Ni+Cr and Ni+Zr-Cr were prepared on the surface of C/C composites by double glow plasma surface metallurgy technology.Effects of etching and Ni-intermediate layer on the microstructure of coatings were studied.The phase structure,microstructure and composition of the Ni+Zr,Ni+Cr,Ni+Zr-Cr complex coatings after different time alloying were investigated as well.The oxidation resistance and mechanism of complexcoatings were analyzed.The main experimental results show as follow:?1?The Zr coating of 6.5 ?m in thickness and The Cr coating of 5.5 ?m in thickness were prepared on the surface of C/C composites by double glow plasma surface metallurgy technology,respectively.The Zr coating is composed of the outer Zr-rich layer and Zr-C diffusion layer and mainly contain ZrC,Zr3C2,and ZrC0.7.The Cr coating is formed by the outer Cr-rich layer/Cr-C diffusion layer/substrate,which mainly contains Cr23C6,Cr7C3,and Cr3C2.Some cracks were found between coatings and substrates as well as in both coating surfaces,or even surface spalling for Zr coating.?2?The C/C composites were etched for different times prior to plasma alloying.Honeycomb-like structures was formed between the carbon fibers after etched 60 min.Carbon spheres appeared on the surface of C/C composites with etching time,causing damage to the C/C composites.?3?The compact coatings with 6 ?m thickness were formed on the surface of C/C composites by Zr-alloying and Cr-alloying after etching.The phases of Zr and Cr coatings are Zr?ZrC?Zr3C2?ZrC0.7 and Cr?Cr23C6?Cr7C3?Cr3C2repectively.Honeycomb-like holes accelerate the diffusing of elements and the formation of the pining combination between coatings and substrate,resulting in stronger bonding..?4?The Ni coating,27 ?m in thickness,was obtained on the surface of C/C composites after etching and subsequent Ni-alloying.The carbon fibers were packed by crack-free Ni coating,a transition layer with outer Ni-rich layer and Ni-C diffusion layer.?5?Complex coatings can be gained on the Ni-transition layer by alloying of Zr,Cr and Zr-Cr,respectively.The thickness of Ni+Zr,Ni+Cr,Ni+Zr-Cr complex coatings is separately about 58 ?m,56 ?m,44 ?m after 4 h alloying.The phase is of NiZr2,ZrC,Zr for Ni+Zr coatings;Cr,Cr23C6,{Cr,Ni},Ni,C for Ni+Cr coatings;and Cr?ZrC?Zr for Ni+Zr-Cr coatings.The strength of thediffraction peak intensity of phases in three coatings changes with alloying time.?6?The result of the oxidation in the air at 900 °C indicates that the weight loss of untreated C/C composites is 132.23 mg/cm2 after exposed for 3 h,and after Ni+Zr,Ni+Cr,Ni+Zr-Cr complex treatments,the oxidation resistances of treated C/C composites were imporved compared with untreated C/C composites.Especially the weight loss of Ni+Cr coatings is 0.35 mg/cm2 after oxidation for 4 h,indicating a significant oxidation resistance due to a continuous and compact Cr2O3 oxide layer formed on Cr-coating surface.The weight loss of the Ni+Zr specimen is 5.29 mg/cm2 after exposed for 30 min,and then the oxidation rate increased due to spalling of the ZrO2 layer.After 30 min oxidation,the weight loss of the Ni+Zr-Cr complex treated specimen is 2.54mg/cm2,and the oxide layer consists of ZrO2 and Cr2O3. |
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