【作者】 张敏；

【导师】 凌国平；

【作者基本信息】 浙江大学 ， 材料加工工程， 2015， 博士

【摘要】 α-Al2O3具有热力学稳定、机械强度高、耐磨、耐高温、抗氧化腐蚀等优良性能,因而被广泛用作金属防腐蚀膜、刀具保护涂层等。在热核聚变计划(ITER)中,α-Al2O3膜因具有优异的阻氚能力,被优先选择为阻氚涂层。然而,传统的方法中,α-Al2O3的制备温度高达1000℃,这不但需要消耗大量的能量,更重要的是会降低基体材料的力学性能。因此低温制备α-Al2O3膜成为一个亟需解决的课题。此外,如何使α-Al2O3膜具有自修复性能,也是一个重要的工程问题。本文用离子液体电沉积法制备Al/Cr复合镀层及薄膜,采用XRD、SEM、 EDS、DTA、XPS、FIB、TEM等手段,研究了Al/Cr复合薄膜的互扩散及其Al-Cr合金的相组成、Al-Cr合金涂层与不锈钢基体的互扩散、不锈钢上Al-Cr涂层的低温热氧化,讨论了α-Al2O3膜的形成机理,得出以下结论：对成分为16.0 at.%Cr (Al11Cr2)的Al/Cr复合薄膜的研究结果表明：在低于Al熔点温度下5 min热处理,540℃时Al层和Cr层之间已形成固溶层,640℃时Al反应完全,但Cr有残留；在高于Al熔点的690℃温度下5 min热处理,Al层、Cr层已完全反应,形成由Al7Cr、Al4Cr、Al11Cr4和Al9Cr4组成的多相合金。690℃下不同时间热处理,Al、Cr扩散的初始形成相为Al7Cr,随着热处理时间的延长,靠近Cr层的界面成分由Al7Cr变为Al4Cro Cr层反应完全后,原始Cr层外侧的界面成分依次由高Cr含量的合金层转变为相对低Cr的合金相：Al9Cr4→Al11Cr4→Al4C,而原始Al层外侧的界面成分一直为Al7Cr;最终复合薄膜转变为由Al7Cr和Al4Cr相组成的合金。而该成分的薄膜经820℃、2 h热处理,得到以Al11Cr2相为主的合金。证明Al/Cr复合薄膜低温热处理不会形成Al11Cr2。此外,成分为25.4 at.% Cr (Al11Cr4)的Al/Cr复合薄膜经热处理形成Al11Cr4单相,证明了Al-Cr相图中Al11Cr4相区的存在,该相可分解为Al4Cr和Al8Cr5,分解的起始温度为829℃。对1Cr17铁素体不锈钢基体上成分为31 at.%Cr(Al9Cr4)的Al/Cr复合镀层的研究结果表明：通过540℃～650℃热处理5min～4h,最外层的合金层依次为Al、Al4Cr、Al11Cr4相,最内层的合金依次为Al7Cr、Al4Cr、Al9Cr4和Al8Cr5,得到具有多相合金的Al-Cr涂层；但在690℃～740℃热处理4h, Al-Cr涂层与基体发生互扩散。研究热处理时间对互扩散的影响,一旦和1Crl7基体相邻的Al-Cr合金层转变为Al9Cr4时,Al-Cr涂层与基体发生互扩散,形成Al-Cr-Fe涂层；并且,随热处理时间延长,涂层中的Al会不断向基体扩散,使涂层的组成向高Cr含量转变。即使温度低至600℃,热处理25 h后,Al9Cr4也会与基体发生互扩散。对1Cr17基体上成分为13.0 at.% Cr (Al7Cr)和19.0 at.% Cr (Al4Cr)的Al/Cr复合镀层,在600 ℃低温下短时间热处理,当与基体相邻的Al-Cr合金层转变为Al11Cr4时,Al-Cr涂层与基体发生互扩散。此外,当基体为316L奥氏体不锈钢时,即使涂层成分为Al8Cr5, Al-Cr涂层也会与基体发生互扩散。在1Cr17铁素体不锈钢基体上制备成分为40.0 at.% Cr (Al8Cr5)的复合镀层,经740 ℃热处理16 h,成功制备出Al8Cr5涂层；表面机械打磨的Al8Cr5涂层在720 ℃氩气气氛下氧化100 h得到γ-Al2O3膜；去除表面孔洞的抛光态的Al8Cr5涂层在氩气气氛中氧化100 h得到α-Al2O3膜,厚度为110 nm,Al8Cr5涂层与基体之间不发生互扩散；抛光态的Al8Cr5涂层在真空和空气气氛中氧化100 h得到α-Al2O3膜,厚度为分别为112 nm和207nm。Al8Cr5涂层表面前处理对α-Al2O3膜形成的影响,与抛光表面阻碍非晶氧化铝的长大,使其难以达到临界相变厚度,从而不能转变为γ-Al2O3有关。更多还原

【Abstract】 α-Al2O3 film has been widely used as high temperature oxidation resistant coating of metals and as protective coating of cutting tools, due to its high thermodynamic stability, high hardness and high corrosion resistance,etc. Especially in International Thermonuclear Experimental Reactor (ITER), α-Al2O3 film is identified to be the promising tritium permeation barriercoating on structural materials. However, α-Al2O3 was conventionally prepared above 1000 ℃,leading to degradation of the mechanical properties of substrate, and thus greatly limits its applications. Therefore, it is urgently demanded to prepare α-Al2O3 at low temperature.In the present paper, Al/Cr composite film and coating were prepared by electrodeposition method in ionic liquid, the interdiffusion and the Al-Cr alloy composition in Al/Cr composite film, the interdiffusion of Al-Cr alloy coating and stainless steel substrate, and the low temperature oxidation of Al-Cr coating on stainless steel substrate were studied by X-ray diffraction(XRD), scaning electro microscope(SEM), energy-dispersive spectrum(EDS), differential temperature analysis(DTA), X-ray photoelectron spectroscopy(XPS), focused ion beam (FIB), transmission electron microscope(TEM), etc. Also, the formation mechanism of α-Al2O3 film was discussed. The conclusions are as fllows:The Al/Cr composite films with composition of 16.0 at%Cr(Al11Cr2) were firstly heat treated in 5min at low temperature below the Al melting point. Analysis shows that the solid solution was formed between Al coating and Cr coating at 540℃, and no Al coating and little Cr coating appeared at 640℃. When heated at 690 ℃ above the Al melting point, Al coating and Cr coating were consumed over with multi alloy phase of Al7Cr, Al4Cr, Al11Cr4 and Al9Cr4 appearing in the coating within 5 min. And the result of different heat treat time at 690 ℃ suggests that the first Al-Cr phase formed in the coating was found to be Al7Cr and the phase at the interface near to Cr coating was changed to Al4Cr from Al7Cr with heat time increased. Till the end of Cr coating consumption, the phase of the alloy coating at the interface near to Cr coating was changed to low Cr content phase from high Cr coating phase:Al9Cr→Al11Cr4→ Al4Cr→Al7Cr, while the phase at interface near to Al coating remained to be Al7Cr. The final composite film was composed of Al7Cr and Al4Cr alloy. Meanwhile, main Al11Cr2 phase coating was obtained at 820℃ for 2h, which suggests that Al11Cr2 phase would not form by low temperature heat treated Al/Cr composite film. Otherwise, the single Al11Cr2 phase coating can be obtained by 690℃ 2h heat treated the composite film with 25.4 at.%Cr(Al11Cr4), demonstrating the existence of Al11Cr4 phase in Al-Cr phase diagram. The decomposition temperature of Al11Cr4 was found to be 829℃.The date of the Al/Cr composite coating with 31 at.%Cr(Al9Cr4) electrodeposited on SUS 430 substrate shows that the alloy coatings at outermost layer were Al, Al4Cr and Al11Cr4 while the Al7Cr, Al4Cr, Al9Cr4 and Al8Cr5 phase were exiseted at innermost layer in order for the sample heat treated at 540 ℃～650 ℃ within 5min-4h, the interdiffusion of Al-Cr coating and substrate happened at 690 ℃-740 ℃ for 4h. When the Al-Cr alloy layers near to SUS 430 substrate turned into Al9Cr4, the Al element would diffuse to substrate and make the composition of coating to be high Cr content phase. Even the heat treat temperature lowed to 600 ℃, the Al9Cr4 would still diffuse to substrate within 25h. Meanwhile, the Al-Cr-Fe coating can also be obtained by diffusion between Al11Cr4 alloy coating and substrate through the composite coating with 3.0 at.% Cr(Al7Cr) and 19.0 at.%Cr(Al4Cr) heat treated at 600℃ for short time. Aside, when substrate changed to 316L austenite stainless steel, the interdiffusion would also happen to Al8Cr5 coating and substrate.Al8Cr5 coating was sucessfully prepared by heat treated the composite coating with 40.0 at.%Cr(Al8Cr5) at 740 ℃ for 16h on SUS 430 substrate. γ-Al2O3film was obtained by thermal oxidation of Al8Cr5 coating with mechanical grinded surface for 100h at 720℃ under argon atmospere, while α-Al2O3 film ca.110nm was obtained by thermal oxidation of Al8Cr5 coating with mirror polished surface under same condition. The interdiffusion did not happen to Al8Cr5 phase coating and substrate, and α-Al2O3 film with thickness of 112nm and 207nm can also be prepared by oxidation of the mirror polished Al8Cr5 coating for 100h in vacuum and in air atmosphere respectively. The effect of pre-treat of Al8Cr5 coating on the formation of α-Al2O3 film may be related to the prohibit function of polished surface to amorphous Al2O3 growth, leading to the critical phase transformation thickness of amorphous Al2O3 can not meet the demand, and thus the formation of γ-Al2O3 can not finish.更多还原