| Based on the plasma nitrocarburizing, a new kind of plasma duplex-treatment method - plasma nitrocarburizing plus post-oxidation has been brought forward. The sample material was middle carbon steel AISI 5140. After treatment, the micro-structure and the surface morphology of the complex layers were investigated. The influence factors in the duplex-treatment process on the complex layers were investigated and the formation of the complex layers was discussed. At the same time, the corrosion resistance of the complex layers was investigated.By a series of experiments in choosing parameters, the influence factors on the compound layer, such as gas compositions, duplex-treatment temperature and time, were investigated in the plasma nitrocarburizing treatment process and the post-oxidation treatment process. Based on a number of process experiments, the duplex treatment process parameters were optimized.The surface morphology, micro-structure and microhardness of the complex layers were observed and analyzed by optical microscope, XRD and micro-hardness tester. In the case of oxidation after nitrocarburizing, an oxide layer was formed on the surface of the compound layer. As a result, the complex layers were made up of oxidation layer, compound layer and diffusion layer from outside to inside. The oxidation layer was made up of Fe3O+4 and its thickness was about 13μm. The compound layer was made up of Fe23N and its thickness was about 1020μm. The basic fundamentals of the oxide layer formation on the nitrocarburized surface are discussed.The corrosion test results of the complex layer were shown that the oxidation of the nitrocarburized layer led to a significant improvement of the corrosion resistance. The corrosion resistance was much higher than plasma nitrocarburizing treatment, and blackening. Because electrode potential of the oxidation layer was high and the structure of the oxidation layer was compact, it is possible to restrain corrosion.
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