Fabrication and characterisation of novel ZnO/MWCNT duplex coating deposited on Mg alloy by PVD coupled with dip-coating techniques

Abstract

Zinc oxide (ZnO)/multi-walled carbon nanotube (MWCNT) duplex coating was synthesized on Mg-0.8Ca-3Zn alloy by physical vapour deposition (PVD) combined with dip coating. The ZnO inner layer was 1.1 μm thick and consisted of spherical nanoparticles. The outer layer of the MWCNTs was 10.2 μm thick, the diameter of the raw MWCNTs was approximately 42 nm, and their lengths were around a few microns. The ZnO/MWCNT duplex coating showed higher compressive strength than the single-layer coated ZnO and bare Mg alloy after immersion in simulated body fluid (SBF) solution. Polarisation and electrochemical impedance spectroscopy tests revealed that the ZnO/MWCNT duplex coating has lower corrosion current (icorr = 7.2 μA/cm2) and higher charge transfer resistance (Rt = 5250 Ω cm2) than the ZnO single-layer coated (Rt = 3420 Ω cm2; icorr = 42.3 μA/cm2) and bare Mg alloy (Rt = 1720 Ω cm2; icorr = 205.4 μA/cm2). In the immersion test, the ZnO/MWCNT duplex coating protected the substrate effectively in the SBF solution, while moderate to serious damage in this electrolyte was observed on the surface of the ZnO-coated and bare samples, respectively. The results indicated that the deposition of MWCNTs as the outer layer and ZnO as the inner layer on the surface of bare Mg alloy sample is a promising technique to enhance its corrosion resistance.