Study on Possibility and Characteristics of ADI Treated by Low-Temperature Coating Processes

• Main Authors: Jung-Kai Lu, 呂榕凱
• Other Authors: Cheng-Hsun Hsu
• Format: Others
• Language: en_US
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/84779218817650642352

博士 === 大同大學 === 材料工程學系(所) === 93 === In this study, both ENi (Electroless nickel) and CAD (Cathodic arc deposition) surface technologies were used to coat ENi, DLC, TiN and TiAlN films on the surface of ADI substrate for evaluating the possibility about surface modification of ADI. Furthermore, various properties of ADI such as mechanical and chemical properties before and after surface treatment were further analyzed and discussed. The results showed that the microstructure of ductile iron was transformed from ferrite/pearlitic constituents into interwoven ferrite and retained austenite phases in the matrix of ADI after the austempering treatment. Due to the above unique microstructure obtained, thus ADI’s mechanical properties such as hardness, tensile strength and fatigue limit were increased and corrosion performance in 3.5 wt. % NaCl or in 10 vol. % HCl aqueous solutions was also better than that of ductile iron. According to Raman spectrum and XRD analysis, it was found that surface treatment of ADI by ENi and CAD technologies at low processing temperature could be successfully to coat DLC, TiN and TiAlN coatings on the surface of ADI substrate; moreover, the microstructure of ADI was not changed due to both the coating temperatures (ENi: 90℃, CAD: 270~300℃) controlled not beyond ADI’s processing temperature (360℃). The surface roughness of ENi coated specimens was almost the same as uncoated specimen. However, the surface of CAD coated specimens was roughened resulting from CAD processing characteristics, especially for the treated specimen at higher target current (75A). Both DLC-ADI and DLC/ENi-ADI coated specimens had a reduction in the fatigue limit and it was ascribed that the initial sites of fatigue fracture took place at the defects (such as inclusion, gas hole) or graphite of subsurface. The tensile strength of coated specimens with ENi, DLC and DLC/ENi coatings had an increase about 10-14%. In particular, ENi-ADI had the best behavior in tensile properties due to its well adhesion. The wear performance of ENi coated specimen was almost analogous the uncoated specimen because of the same hardness value. DLC coated specimens had the best wearability because it had the hardest surface among the coated specimens. In chemical property, the corrosion currents (Icorr.) of all coated specimens were lower than that of uncoated specimen after polarization test in 3.5 wt.% NaCl aqueous solution. It implied that ADI utilizing surface coating treatment could have an improvement in corrosion resistance. In particular, DLC/ENi-ADI duplex-coated specimen had the best corrosion resistance in all coated specimens. From SEM observation, it was found that pitting corrosion of coated specimens was occurred in the sites of graphite. After immersion test in 10 vol. % HCl aqueous solution, similarly, the result obviously showed that the corrosion resistance of all coated specimens was better than that of uncoated specimen. Finally, comparison the relationship among DLC, TiN and TiAlN coatings, it was found that DLC had the best surface hardness and wearability but the poor adhesion. In the performance of corrosion resistance, TiN behaved between DLC and TiAlN. Also, it was noted that the surface hardness of TiAlN was less than that of DLC but it had the best corrosion resistance among the coated specimens.