| 其他摘要 | Ni-Cr composites were fabricated by codeposition of Ni with appropriate Cr particles from a nickel sulfate bath. The Cr particles used had an average size of 14 m、2.4m、96nm and 62 nm, respectively. The influences of particle size and Chromium content on the oxidation of the composites, as well as the initial oxidation of the composites were investigated using X-ray diffraction (XRD), scanning electron microscopy equipped with energy dispersive analysis of X-ray (SEM/EDAX), transmission electron microscopy (TEM), atomic force microscopy (AFM) and thermal-gravimetric analysis (TGA). In addition, the influence of annealing treatment and CeO2 addition on the oxidation behavior of electrodeposited microcomposite Ni-Cr were also studied.
Based on the oxidation at 900oC in air, the primary results are summarized below:
(1) The oxidation behavior of electrodeposited Ni-Cr composite was increased when the content of Cr was increased and existed a critical content of Cr for the formation of Cr2O3 scale on the composites. When the Cr particle content was below the critical content, the oxidation resistance was even worse than that of the electrodeposited pure Ni coating because Cr played a doping effect in accelerating the growth of NiO in this case. When the Cr content was over the critical value, the oxidation resistance was greatly improved.
(2) The oxidation behavior of electrodeposited Ni-Cr composite strongly depended on the Cr particle size. For electrodeposited Ni-Cr nanocomposite, although it has a Cr content much lower than those of the electrodeposited Ni-Cr microcomposite, had an improved oxidation resistance. The reason was that with the reduction of particle size, the transient oxidation period for the formation of a continuous chromia layer was shortened due to an increase of the nucleation sites for chromia and simultaneously a promotion of the oxide lateral growth through the increased grain boundary diffusion of Cr to the oxidation front due to a abundant grain boundary within the nanocomposites. The result demonstrates: the finer the codeposited Cr particles, the lower critical content of Cr for the formation of Cr2O3 scale and the better the oxidation resistance of the composites.
(3) The initial oxidation of the nanocomposite at 900oC in air was investigated. For comparison, the initial oxidation of an arc-melted Ni-20Cr alloy was also studied. The result showed that a Cr-rich oxide scale formed on the nanocomposite, whereas a Ni-rich oxide scale formed on the alloy. The reason lies in three aspects: firstly, the numerous dispersed Cr nanoparticles as well as abundant grain boundaries act as the easy nucleation and growth of Cr2O3; secondly, the abundant grain boundaries in the nanocrystalline Ni matrix, as “short-circuit” diffusion path of Cr to oxidation front, promote quick formation of Cr2O3; third, the growth of NiO was restrained after a continuous Cr2O3 layer was formed which induced a surface smoothening of the sample.
For microcomposites, the critical value for the formation of a continuous Cr2O3 layer was higher than that of namocomposite. This was related to the dispersion of Cr particles in EMCs. On these bases, it is assumed that the oxidation resistance of EMCs may be increased by increasing the distribution uniformity of Cr through a homogenization treatment. So, the effect of annealing treatment on the oxidation of EMC Ni-Cr was studied.Additionally, taking into account the beneficial effect of reactive element oxides (RE oxides) on oxidation resistance, small amount of CeO2 was added into the EMC and the effect of CeO2 addition on the composites with and without annealing treatment was investigated. The oxidation results showed that, at a comparable Cr content, the oxidation resistance was greatly increased after annealing treatment. This was mainly because of the distribution uniformity of Cr through a homogenization treatment. After annealing, Cr was more homogeneously dispersed. Chromia nuclei could form on Cr particles remained on the composite surface. In addition, the Cr2O3 may nucleate on the surface of Ni with solid solubility of Cr, particularly at its grain boundaries (GBs). This may reduce the spacing between the initially formed chromia nuclei, which, in turn, would reduce the time required for the interconnection of the chromia nuclei to form a compact and continuous layer by their lateral growth. The addition of CeO2 particles had little effect on the oxidation resistance of microcomposite with the Cr content lower than the critical content. However, the CeO2-dispersed annealed microcomposite showed better oxidation resistance than the annealed microcomposite. The beneficial effects may result from reducing the growth rate of the chromia scale by turning the scaling mechanism from dominant outward chromium diffusion into dominant inward oxygen diffusion. |
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