Microstructural, Mechanical and Anti-microbial Characterization of Electrodeposited and DC Magnetron Sputter Deposited Copper Coatings on Aluminium

Abstract

Copper is a well-known antimicrobial metal which could be used in the form of coating on the aluminium touch surfaces, especially, those in hospitals. Experiments were conducted by copper coating on the double zincated aluminium samples by two processes, viz : i) Electrodeposition and ii) DC magnetron sputter deposition. Double zincation of the aluminium had been carried out, mainly, to improve the adhesion between the coating and the substrate. The electrodeposition was carried out by using two types of non-cyanide alkaline electrolyte: Bath-I (with ammonium nitrate) and Bath-II (without ammonium nitrate). In the case of electrodeposition, the coating characteristics were changed by varying the current density while in the case of DC magnetron sputter deposition, they were changed by varying the sputtering power. SEM characterization shows the nodular morphology of the deposits in the electrodeposited as well as the DC magnetron sputter deposited coating. TEM study of the coating shows that each nodule is made up of nano-sized crystallites. Moreover, the crystallite size varied with the coating parameters. XRD study of the coating revealed the texture and the microstrain of the coating. Further, the mechanical characteristics like hardness, scratch resistance and adhesion strength of the coatings has been determined. The scratch hardness and microhardness were significantly increased due to the nano-crystallinity, nano-twins and preferred texture of the coating. Adequate adhesion strength of the coating was observed in the cross-hatch cut test as well as in the pull-off adhesion test. Antimicrobial activity of the coating was evaluated against gram negative bacteria (Escherichia coli) and gram positive bacteria (methicillin-resistant Staphylococcus aureus (MRSA)). These pathogens are most harmful and ever present on the hospital touch surfaces. The colony forming unit (CFU) count of the bacteria on the copper coated samples was found to be less than that on the bulk copper. TEM and SEM analysis showed that Escherichia coli cells exposed to copper coating were found to be wrinkled and damaged because cell wall was broken and intracellular fluid was leaked. Whereas, in the case of MRSA, small pits weredeveloped on the copper treated cells. But the morphology of the cell was maintained due to the thick cell wall in the MRSA.