| 其他摘要 | Copper or Cu-based alloys have excellent electrical and thermal conductivity. However, the poor oxidation performance of these materials limits their widespread use at elevated temperature. Adding Cr is expected to improve the oxidation resistance of Cu-based alloys since chromia scale is dense, slow-growing and thermodynamically stable. Unfortunately, Cr is almost insoluble in Cu, and its diffusion rate in Cu is extremely low. This leads to that a protective chromia scale is difficult to grow on the alloys even though they contain a rather high content of Cr. However, alloy grain refinement is an alternative to promote selective oxidation of Cr to form a protective layer. In this work, novel Cu-Ni-Cr nanocomposite coatings were prepared by nano-composite electrodeposition, and their microstructures and oxidation properties were investigated.
Cu-Ni-Cr nanocomposite coatings were electrodeposited from a sodium citrate bath with the addition of Cr nanoparticles. The composition of the coatings can be adjusted by changing the contents of CuSO4∙5H2O and Cr nanopartices in the bath. The Cu-Ni-Cr nanocomposite coatings were composed of nanocrystalline Cu-Ni alloy matrix with Cr nanoparticles.
The oxidation behaviors of the Cu-Ni-Cr nanocomposite coatings at 800oC in air were examined. The main results are presented as follows:
(1) Effect of the content of Cr on the oxidation behavior of Cu-Ni-Cr nanocomposite coatings. The electrodeposited Cu-50Ni alloy coating oxidized very fast and formed very thick oxide scales, consisting of CuO, Cu2O and NiO. The Cu-Ni-Cr nanocomposite coatings (weight percentage ratio of Cu/Ni 1) containing 15 wt. % Cr exhibited a rather low oxidation rate as compared with the Cu-50Ni alloy coating, because of the formation of a continuous Cr2O3 scale. The reduction of oxidation rate of the nanocomposite coatings was more significantly when the content of codeposited Cr further increased.
(2) Oxidation comparison of the Cu-Ni-Cr nanocomposite coatings and the Cu-Ni-Cr alloys prepared by arc melting (AM). The Cu-30Ni-20Cr alloy prepared by arc melting was composed of Cu-rich phase, Ni-rich phase and a small amount of Cr-rich phase. Cu-30Ni-20Cr alloy formed a triple-layer scale, which included a thick CuO layer on the outside, followed by a mixture layer of Cu and Ni oxides and NiCr2O4, then a thin Cr2O3 scale layer around the phase. The AM Ni-30Cu-20Cr alloy consisted of the phase and a little phase. During the oxidation, Cr2O3 scale quickly formed on the phase, and then laterally grew along the interface between CuO and phase. Finally, a continuous Cr2O3 layer formed, which significantly decreased the oxidation rate of the Ni-30Cu-20Cr alloy. The results demonstrate that the phase, which has high content of Ni and Cr, increased the diffusion of Cr from the bulk alloy to the oxidation front and consequently the formation of a protective Cr2O3 layer. For the Cu-30Ni-20Cr nanocomposite coating exhibited a low oxidation rate similar to the Ni-30Cu-20Cr alloy, because the exclusive formation of external Cr2O3 scale. The latter is due to the unique microstructure of the nanocomposite coating, from onset of oxidation, chromia easily nucleated on both chromium nanoparticles and abundant Cu-Ni grain boundaries. Then, the chromia nuclei could be fast linked together as a result of their fast lateral growth through the diffusion of Cr along the grain boundaries. The Ni-30Cu-20Cr nanocomposite coating has a slightly reduced oxidation rate compared to the Cu-30Ni-20Cr nanocomposite coating, because the continuous Cr2O3 scale on the former could form more quickly.
(3) Oxidation comparison of the Cu-30Ni-20Cr nanocomposite and the Cu-30Ni-20Cr nanocrystalline alloys prepared by magnetron sputtering (MS) or surface mechanical attrition treatment (SMAT). Continuous Cr2O3 scale could also form on the MS/SMAT Cu-30Ni-20Cr. However, the inward growth of Cr2O3 along the boundaries of the columnar grains (normal to the gas/scale interface) in the MS Cu-30Ni-20Cr was seen. For the SMAT Cu-30Ni-20Cr, a thick CuO scale also formed on the phase. By contrast, external Cr2O3 scale could exclusively form on the Cu-30Ni-20Cr nanocomposite coating. These results indicate that the nanocomposite coating exhibited a superior oxidation resistance.
In addition, the hardness and Young’s modulus of electrodeposeited Cu-Ni based coatings and the alloys were respectively examined using Hv hardness test and nanoindentation method. The electrodeposited Cu-50Ni alloy coating exhibited a much higher Hv hardness than the Cu-50Ni alloy. Codeposition of the Cr nanoparticles further increased the hardness of Cu-Ni alloy coating. Nanoindentation tests showed that the hardness of Cu-30Ni-20Cr nanocomposite coating was greatly enhanced as compared with that of the AM Cu-30Ni-20Cr alloy, but the value was much similar to that of the MS Cu-30Ni-20Cr nanocrystalline coating. The Cu-Ni-Cr nanocomposite coating had a high hardness due to that it was strengthened by the fined-grained structure and the dispersion of Cr nanoparticles. The Cu-30Ni-20Cr nanocomposite coating exhibited the same Young’s modulus as the AM Cu-30Ni-20Cr. However, the MS Cu-30Ni-20Cr had a low Young’s modulus mainly resulted from the structural heterogeneity in the density and grain size. |
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