本文采用纳米复合电沉积技术制备了一种由纳米晶Ni基体及Cr、Al纳米颗粒组成的新型Ni-Cr-Al纳米复合镀层。作为对比，同时制备了微米级Cr、Al颗粒弥散分布的Ni-Cr-Al复合镀层和Ni-Cr-Al（电弧熔炼法）。利用X射线衍射(XRD)、带能谱的扫描电子显微镜(SEM/EDAX)、原子力显微镜(AFM)、电子探针(EPMA)、透射电镜(TEM)、热重分析(TGA)等分析技术，研究了粒子尺寸对复合镀层氧化性能的影响，Ni-Cr-Al纳米复合镀层及成分相近的普通合金的高温氧化/热腐蚀性能以及初期氧化行为。另外，还研究了扩散合金化热处理对Ni-Al纳米复合镀层氧化性能及Al颗粒尺寸对由Ni-Al复合镀层转化的γ'/复合涂层氧化行为的影响。
900 oC空气中氧化及(0.9Na,0.1K)2SO4 熔盐热腐蚀实验表明：
(1) Ni-Cr-Al 纳米复合镀层（ENCs）依其氧化物组成可分为以下三种情况：(a) 当Cr和Al含量之和小于8 mass %时，生成NiO外氧化膜；当Cr和Al含量之和大于8 mass %，能生成一层连续的Cr2O3 和/或Al2O3膜 (取决于镀层中的Cr/Al比值)，并明显抑制NiO的生长；(b) 当Cr/Al1，形成Cr2O3外氧化膜(下面弥散分布的Al2O3或一层连续的Al2O3膜)；(c) 当Cr/Al 1，则形成Al2O3外氧化膜。
(2) 与成分相近复合微米级Cr、Al粒子的镀层（EMCs）相比较。EMC Ni-3Cr-16Al 的氧化膜具有双层结构，外层为很厚的NiO，内层为Cr、Al的混合氧化物。而ENC Ni-3Cr-11Al能够形成一连续的富Al氧化物，因而表现出优良的氧化抗力。
(3) 与同成分电弧熔炼合金相比较，Ni-Cr-Al纳米复合镀层表现出优良的高温氧化/热腐蚀抗力。Ni-6Cr-7Al合金表面的腐蚀产物主要是NiO，而纳米复合镀层表面却生成一层连续、致密的Al2O3膜。根据实验结果，认为这是由于纳米复合镀层独特的结构使其在氧化及熔盐腐蚀初期能快速形成连续的保护性氧化膜所致。其氧化过程描述如下：氧化时，Cr2O3和/或Al2O3在表面弥散分布的大量Cr、Al纳米颗粒以及部分纳米晶Ni基体晶界上快速形核。同时，纳米颗粒溶解产生的Cr、Al沿纳米晶晶界向氧化前沿快速扩散，促使Cr2O3和/或Al2O3核快速横向生长形成连续的氧化膜。
Ni-6Cr-7Al纳米复合膜在900 oC氧化5分钟后形成了一连续的Al2O3膜，而Ni-6Cr-7Al合金表面只生成NiO。这一初期氧化实验结果进一步证实了Ni-Cr-Al纳米复合膜具有快速形成保护性氧化膜的能力。
另外，通过600 oC、4小时真空扩散处理，Ni-Al纳米复合镀层可转变为一种新型超细晶γ'/涂层。1000 oC，20小时氧化实验表明：1) 在镀层中Al含量相近的情况下，复合Al纳米颗粒的γ'/ 转化涂层（7.1 mass % Al）的氧化性能优于复合微米颗粒转化的涂层（7.4 Al mass %）；2) 扩散后的合金涂层（13.4 mass % Al）具有比Ni-15 mass % Al纳米复合镀层更优良的抗氧化性。
其他摘要
Novel Ni-Cr-Al electrodeposited nanocomposites (ENCs) were fabricated by codeposition of Ni with appropriate Cr and Al nanoparticles from a nickel sulfate bath. The nanocomposites were composed of nanocrystalline Ni matrix and dispersed nanoparticles of Cr and Al. For comparison, arc-melted Ni-Cr-Al alloys and Ni-Cr-Al electrodeposited composites (ECs) with dispersion of micro-sized particles of Cr and Al were also prepared. The influence of particle size on the oxidation of the electrodeposits, the oxidation/hot corrosion as well as the initial oxidation of the ENCs Ni-Cr-Al were investigated using X-ray diffraction (XRD), scanning electron microscopy equipped with energy dispersive analysis of X-ray (SEM/EDAX), atomic force microscopy (AFM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and thermal-gravimetric analysis (TGA). In addition, the influence of heat-treatment on the oxidation of ENCs Ni-Al and the effect of particle size of Al on the oxidation behaviors of γ/γ' composites transformed from the annealed ENC and EC Ni-Al were also studied.
Based on the oxidation /hot corrosion in (0.9Na,0.1K)2SO4 molten salt at 900 oC in air, the primary results are summarized below:
(1) The ENCs Ni-Cr-Al, according to its oxide formation during oxidation at 900 oC, were divided into three groups. Group I - a NiO former (Cr+Al 8 mass %); Group II - a former of mixed oxides being composed of an outer Cr2O3 layer and an inner Al2O3 layer (Cr+Al 8 mass %, Cr/Al1); Group III - an Al2O3 former (Cr+Al 8 mass %, Cr/Al 1).
(2) ENC Ni-3Cr-11Al exhibited excellent oxidation resistance better than the EMC Ni-3Cr-16Al, due to the formation of a continuous Al-rich scale. In contrast, the EMC counterpart only formed a thick NiO scale.
(3) Compared with arc-melted Ni-6Cr-7Al alloy, ENC Ni-Cr-Al with a similar composition presented great oxidation /hot corrosion resistance. The main corrosion product was NiO on the alloy surface, while a continuous, compact Al2O3 film was formed on the ENC. The latter occurred is because that the unique microstructure of the nanocomposite favored the quick formation of the Al2O3 scale in the initial and transient oxidation stage. From the onset of oxidation, chromia and/or alumina easily nucleated on both chromium and/or aluminium nanoparticles and abundant Ni grain boundaries. Then, the chromia and/or alumina nuclei could be rapid linked together, as a result of their fast lateral growth through the diffusion of Cr and/or Al along the grain boundaries.
The rapid formation of a protective scale on ENC, was further evidence by comparing the initial oxidation of ENC Ni-6Cr-7Al and its alloy counterpart. It showed that the ENC grew a fine-grained Al2O3-rich scale during 5 minutes oxidation in air at 900 oC, whereas the alloy only formed a larger-grained NiO scale.
In addition, a γ/γ' composite with ultrafine-grained structure was developed by annealing an electrodeposited Ni-Al nanocomposite in vacuum at 600 °C for 4 h. The oxidation at 1000 °C for 20h showed that: 1) The oxidation resistance was greatly increased for the γ/γ' composite prepared from the ENC Ni-Al compared to that from the EC Ni-Al with a similar mass percentage of 7%; 2) the annealed Ni-13.4 mass % Al exhibited better oxidation resistance than the untreated ENC Ni-15 mass % Al.
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