Characteristics of duplex surface coatings on austempered ductile iron substrates

Austempered ductile iron (ADI) is an austemper-treated ductile iron containing acicular ferrite and high-carbon austenite constituents in its microstructure. It is well known that the traditional surface treatment at high temperature is not available to treat ADI because of the austempering temperature in the range of Ms ∼ 450 °C. This study utilized electroless nickel (EN) and cathodic arc deposition (CAD) technologies with lower processing temperature to treat ADI and then evaluated the availability of applying the EN and CAD-DLC duplex coatings on ADI. Characteristics of the resultant coatings such as roughness, hardness, and adhesion were analyzed and microstructures of ADI before and after surface treatment were observed. Also, polarization curve test was carried out for further understanding the effects of the coatings on the corrosion resistance of ADI. The results showed that microstructures of ADI did not deteriorate after EN and CAD surface treatments. Moreover, both the EN and CAD-DLC coatings were identified to be amorphous type and they could be well deposited on the ADI substrate. The surface of CAD-DLC-coated specimen was rougher than that of EN-coated specimen due to the globular particles deposited by CAD process. In the case of hardness, it showed that the duplex coated DLC/EN-ADI had the highest hardness (1312 HV0.05), followed by DLC-ADI (1088 HV0.05), EN-ADI (409 HV0.05) and then uncoated ADI (396 HV0.05). In the performance of corrosion resistance, all coated specimens were better than that of the uncoated one in 3.5 wt.% NaCl aqueous solution, and the sequence was DLC/EN-ADI > EN-ADI > DLC-ADI > ADI. Also, corrosion resistance of uncoated ADI was better than that of uncoated as-cast iron due to ADI's unique microstructure. It implied that the austempering treatment could improve the corrosion resistance of ductile iron in NaCl solution.