Study of Integrity of NiO Oxide Film by Acoustic Emission Method

Isothermal and cyclic oxidation behaviors of pure and yttrium-implanted nickel were studied at 1000°C in air. Scanning electronic microscopy (SEM) was used to examine the micro-morphology and structure of oxide scales formed on nickel substrate. It was found that Y-implantation greatly improved the anti-oxidation ability of nickel both in isothermal and cyclic oxidizing experiments. Acoustic emission (AE) technique was used in situ to monitor the cracking and spalling of oxide films in isothermal oxidizing stage and air-cooling stage, and to examine the number and size distribution of oxide/substrate interfacial defects. Laser Raman microscopy was also used to study the stress status of oxide scales formed on nickel with and without yttrium. The main reason for the improvement in anti-oxidation of nickel was that Y-implantation greatly reduced the growing speed and grain size of NiO. This fine-grained NiO oxide film might have better high temperature plasticity and could relieve parts of compressive stress by means of creeping, and maintained ridge character and relatively lower internal stress level. In the meantime, Y-implantation reduced the size and number of interfacial defects, hence remarkably enhanced the adhesion of protective NiO oxide scale formed on nickel substrate.