Search

The use of hard chromium electroplated coatings for industrial and decorative applications is widespread. However, there seem to be some defects like the existence of an extensive micro-cracks network that restrict their applications. They affect corrosion resistance due to formation of direct routes to the substrate through which corrosive agents could reach the main metal. Plasma nitriding process is proved an effective technique to overcome such problems. The objective of this study is to investigate the effect of plasma nitriding parameters namely, time and temperature, on the closure of the cracks. Samples of a hard chromium electroplated hot work tool steel were heat-treated via conventional plasma nitriding (CPN) and active screen plasma nitriding processes (ASPN) at 500 and 550 °C for 5 and 10 h, and at 550 and 600 °C for 10 and 15 h, respectively. The morphology and microstructure of the samples were characterized by the scanning electron microscopy (SEM) imaging. The coating phases were determined by the X-ray diffraction (XRD) method. The corrosion properties of samples were evaluated using anodic polarization tests in a 3.5 wt% NaCl solution at 25 °C. The results showed that both techniques filled up the cracks. However, the closure mechanisms were different. In CPN method, the cracks were disappeared due to the transformation of chromium to its nitrides, which occur with volume expansion. It is worth noting that this phenomenon occurs at lower temperatures and shorter times compared to ASPN method. In ASPN method, the iron nitrides detached from active screen were deposited inside the cracks to fill them up. Moreover, it was found that the corrosion resistances of the samples modified by the CPN process were superior to the ASPN samples. [ABSTRACT FROM AUTHOR]

In this research iron nitride coating was deposited on Al substrate using active screen plasma nitriding technique. Samples were plasma nitrided in an iron cage, at 550 °C, gas mixture of N 2 :H 2 % = 3:1 for 2.5, 5 and 7.5 h. The coatings were characterized using X-ray diffraction, scanning electron microscopy and atomic force microscopy. Corrosion resistance of samples was investigated using polarization technique. Mechanical properties including nano hardness, wear performance and adhesion strength were also studied. Results showed that the coatings were mainly consist of Fe 2-3 N particles whose size increased by nitriding at longer time periods. Moreover, it was found that at longer coating times such as 7.5 h, the nitrogen atoms diffused into the Al substrate and formed a thin layer of AlN under the Fe 2-3 N coating. Formation of the AlN coating increased the corrosion resistance of the coated sample, but it was still poor compared to uncoated Al. On the contrary, mechanical features of coated Al improved by increasing the nitriding time. Finally, the mechanical characteristics of the deposited coatings were compared to those of conventionally plasma nitrided Al. Despite the higher hardness of the AlN coating, Fe 2-3 N showed superior scratch hardness and wear resistance. [ABSTRACT FROM AUTHOR]

Highlights: [•] The surface hardness is increased about eight times. [•] Increasing nitriding time and temperature caused strengthening the Al diffused zone. [•] The inward diffusion of nitrogen and outward diffusion of iron changed aluminized zone microstructure. [•] Surface aluminum oxides are reduced by plasma nitriding in low pressure chamber with nitrogen and hydrogen in gas mixture. [ABSTRACT FROM AUTHOR]