| 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. |
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