HIGH-TEMPERATURE OXIDATION RESISTANCE OF NI-AL ALLOY-FILMS PREPARED BY HEATING ELECTRODEPOSITED COMPOSITE

Nickel-Al particle composites of 5-23 mol%Al were electrodeposited and heated at 1073 K to get Ni-Al alloy films dispersed with A12O3 particles. Then, the Ni-Al alloy films were isothermally oxidized in air at 1173 approximately 1373 K, or subjected to a cyclic oxidation test between room temperature and 1273 K. This study was performed to elucidate the Al content which will provide a good oxidation resistance to the alloy film. The structure of the alloy films before and after the oxidation was traced by X-ray diffraction, scanning electron microscopy and X-ray microanalysis. It was found that a less mass gain and a good adhesion of the surface oxide phases are obtained when the Al contents of the Ni-Al alloy films are equal to or greater than 16 mol%. Although the surface oxide of the Ni-16 mol% Al alloy film was NiO and rather thick, it did not spall off the substrate during the cyclic oxidation for 259 ks. In this case, we found a network structure of Al2O3 in the substrate, and which was linked with the bottom of the surface NiO layer. When the Al content of the alloy film was 23 mol%, we found an Al2O3 scale of about 1 mum-thickness, which did not spall off the substrate even after the cyclic oxidation for 259 ks.