Comparison of the Oxidation Behavior of Nanocrystalline and Coarse-Grain Copper

The Oxidation of an electro-deposited nanocrystalline Cu (nc Cu) and a conventional coarse-grain Cu (cg Cu) was investigated at 30–800 ° C under 1 atm of oxygen. Both Cu samples formed external scales of copper oxide (Cu2O+CuO). At the lower temperature (30–300 °C) the very slow oxidation rates of both the nc and cg Cu might be attributed to the formation of a protective Cu2O surface layer. However at the higher temperature (300–700 °C), oxidation rates of the nc Cu were obviously faster than those of the cg Cu, which was attributed to faster diffusion of various species along grain boundaries both in the metal and in the scale. In particular, the scale grew faster on the nc Cu by means of not only rapid external oxidation as a result of outward diffusion of Cu-ions but also a significant contribution from inward diffusion of oxygen along the grain boundaries in the scales. Compared with the cg Cu, dissolved O2 in the nc Cu may have a certain effect on the faster oxidation of the nc Cu. Above 700°C, the difference seemed to disappear as a result of the ineffectiveness of grain-boundary diffusion.