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采用Si-Cr包埋共渗法在Ti-Nb-Si基高温合金表面制备了Cr改性的硅化物涂层,共渗温度为1250和1300℃,时间为10h.利用SEM,EDS和XRD等检测手段分析了涂层的结构、元素分布及相组成等,并对涂层的形成机理进行了讨论.结果表明:Si-Cr共渗温度为1250℃时,降低渗剂中的催化剂NaF含量会降低Si和Cr的反应扩散速度并且改变了涂层的结构和相组成.催化剂NaF含量为8 wt%,涂层外层由(Nb,Ti) Si_2及少量(Ti,X)_5Si_3(X代表Nb,Cr和Hf等元素)组成,中间层由(Ti,X),Si_4组成,过渡层由(Nb,Ti)_5Si_3组成;降低NaF含量至5 wt%,Si-Cr共渗温度仍为1250C时,涂层外层由(Ti,X)_5Si_3组成,且有较多孔洞出现,中间层为(Ti,X)_5Si_4,而过渡层很薄.与渗Si涂层相比,Si-Cr共渗涂层中的裂纹明显减少,但在涂层外层存在较多孔洞且涂层厚度明显减小.提高包埋共渗温度至1300℃时,Cr的反应扩散速度得到提高,且在涂层外层出现了(Nb_(1.95)Cr_(1.05))C_(r2)Si_3三元相.

Si and Cr were co-deposited on a Ti-Nb-Si based high temperature alloy by pack cementation processes at 1250 and 1300 ℃ for 10 h, respectively.XRD,SEM and EDS were used to identify the structure, phases and composition distribution of the coatings.The results show that the Si-Cr co-deposition coatings prepared with different pack mixtures at 1250 ℃ exhibit different atructure and phase constituents.Both the reaction and diffusion speeds of Si and Cr are reduced with the decrease of the content of activator NaF in the pack mixtures.The coating fabricated at 1250 ℃ with activator NaF of 8 wt% in the pack mixture is composed of (Nb,Ti) Si2 outer layer with a small quantity of (Ti,X)_5Si_3 (X denotes Nb ,Cr and Hf) phase,(Ti,X)_5Si_4 intermediate layer and (Nb,Ti)_5Si_3 transitional layer.In the coating prepared with 5 wt% NaF in the pack mixture at 1250 ℃,single (Ti,X)_5Si_3 phase is found in its outer layer and many micropores are observed,and the intermediate layer is composed of (Ti,X)_5Si_4 and the transition layer is very thin.Compared with the Si deposition coating,the amount of cracks decrease evidently,but more micro-pores are detected in the outer layer for the Si-Cr codeposition coatings,and thickness of the Si-Cr co-deposition coatings decreases obviously.(Nb_(1.95)Cr_(1.05)) Cr_2Si_3 ternary phase and (Ti,X)_5Si_3 phase form in outer layer of the coating when the co-deposition temperature is raised up to 1300 ℃ due to the increase of reaction and diffusion speed of Cr atoms.