Ultrafast flame coating of carbon and chemical vapor deposition of graphene on NiTi alloy to enhance its corrosion resistance

To study their surface protection performance, both graphene and carbon layers have been deposited onto the surface of NiTi alloy via the common chemical vapor deposition (CVD) and a facile flame coating technique, respectively. The latter can be accomplished within tens of seconds with the combustion flame itself as the carbon source and alloy substrate as catalyst. The carbon layer not only inhibits effectively the Ni2+ releasing from NiTi alloy, but also displays lower surface roughness and higher corrosion resistance than that of the graphene coating. The effect of the flame coating time on the performance of resulted carbon layer has been studied comprehensively. The lowest corrosion current density (1.6 μA/cm2) of the flame coated alloy in simulated body fluid has been achieved by adjusting the flame time to be just 30s. This value is much less than that of graphene coating (2.4 μA/cm2) in the same test conditions. And it gets even less in 3.5 wt% NaCl solution (1.3 μA/cm2), implying the potential application of the flame coating method for more corrosive marine environments. Also, the Ni2+ releasing concentration of NiTi/C-30s is decreased to be around one fourth that of bare NiTi. Moreover, the excellent scratch resistance of the carbon layer and its strong adhesion to NiTi substrate have been also confirmed by both the cross-cut and scratching tests. Furthermore, both the elastic modulus and hardness of the alloy decreased marginally after the fast and mild-temperature flame coating process according to the nano-indentation test results. Compared with traditional surface engineering approaches, the flame coating method has many advantages, such as simple and fast process, high protective performance, environmental-friendly fabrication and protection. All of these benefit its promising application in various fields.