| Zirconium(Zr)has been used as nuclear fuel cladding tubes and anti-corrosion materials under extreme conditions due to its low neutron absorption cross section and excellent corrosion resistance.At present,the cost of smelting and processing of Zr is very high,which severely limits its application scope.Zr coating can not only greatly reduce the consumption of Zr metal,but also exhibit excellent performance,so that the Zr coating is considered as one of the important means to expand the application scope.In theory,chemical vapor deposition(CVD)can prepare high-quality coatings on complex substrates;however currently the deposition temperature is high,which exceeds the heat treatment temperature of most structural materials.Therefore,designing a suitable vapor-phase precursor to reduce the deposition temperature is the key issue for preparing high-quality Zr coatings by CVD.Based on literature research and thermodynamic analysis,we found that ZrCl3 can not only be vaporized,but also release Zr atoms by its disproportionation reaction at a lower temperature,which provides feasibility for solving the above issue.Herein,to address the issue,the novel idea that disproportionation reaction of ZrCl3(g)was developed to fabricate Zr coatings.Firstly,the reaction behavior of Zr(s)-ZrCl4(s)system for synthesizing ZrCl3(s)precursor was systematically investigated and the the microstructure evolution of ZrCl3(s)was revealed.Secondly,the vaporization behavior of the as-synthesized ZrCl3(s)and the disproportionation reaction kinetic behavior of ZrCl3(g)for depositing Zr coating were systematically investigated.Finally,the interface reaction behavior of the deposited Zr coating on the steel substrate was systematically studied.The main innovative frndings achieved are as follows:(1)It was found that the disproportionation reaction of ZrCl(s)and ZrCl2(s)always takes precedence over theirselves vaporization so that they cannot be used as an effective precursor for CVD.However,ZrCl3(s)can be vaporized to ZrCl2(g),releasing fresh Zr atoms by a disproportionation reaction at a low temperature,which can be used as an effective precursor to deposit Zr coatings for low-temperature CVD.Herein,the kinetic conditions for the synthesis of ZrCl3(s)precursors were investigated based on the solid-phase reaction of Zr(s)and ZrCl4(s),and it was found that amorphous ZrCl2(s)precursors can be synthesized at 370-500℃.The yield rate of ZrCl3(s)increased with the increasing of the synthesis temperature and ratios of reactants N(nZrcl44/nzr),and it was found that the synthesis reaction is controlled by diffusion process.Finally,the vaporization behavior of ZrCl3(s)was clarified and the optimal vaporization temperature was 450℃.(2)The kinetic behavior of ZrCl3(g)disproportionation reaction to prepare Zr coatings was revealed.The lowest deposition temperature of Zr coating on Al2O3 substrate was about 600℃,and the optimal deposition temperature was about 970℃,which is about 45.5%lower than that of Zr coating prepared by traditional CVD(1100℃).At the same time,it was found that the deposition process was controlled by the surface chemical reactions,and the growth mode of the coating was island growth(Volmer-Weber mode).Based on these results,for the first time,dense Zr coatings were successfully prepared both on the internal and external surfaces of the tube substrates.(3)The deposition mechanism and interface reaction behaviors of Zr coating on steel substrate were revealed.The Fe,Cr,Ni elements in the steel substrate cannot react with ZrCl3(g),while the deposited Zr could diffuse into the matrix.When the temperature was lower than 700℃,a pure Zr coating was obtained on the steel substrate.However,above 700℃,the C atoms in the substrate were found to preferentially diffuse from the grain boundary to the substrate surface and react with ZrCl3(g)or the Zr atoms released by the disproportionation reaction of ZrCl3(g)to form ZrC,while the surface of the crystal grains of the substrate was still Zr coating,thereby forming a Zr-ZrC composite coating.Therefore,the growth mode of the Zr-ZrC composite coating on the steel substrate was a coupling growth mode,i.e.,ZrC induced by C atoms at grain boundary and Zr on the surface of the substrate crystal grains.Finally,a composite coating of soft phase Zr and hard phase ZrC was formed alternately instead of traditionally being stacked layer by layer.This special structure is expected to be used as a self-lubricating wear-resistant coating.At the same time,it was found that the Zr-ZrC composite coating can significantly increase hardness,which was roughly 2.8 times than that of the substrate. |
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