Feasibility of plasma nitriding for effective surface treatment of pure aluminum

Cast aluminum with 99.99% purity was successfully plasma nitrided using nitrogen and hydrogen mixed gas. Pre-sputtering was carried out prior to plasma nitriding in order to eliminate surface oxide film. Sputtering and nitriding durations were varied from 3.6 to 18 ks and 72 ks to 252 ks, respectively. The samples were nitrided at 823 and 873 K to observe the effect of nitriding temperature. The nitrided samples were analyzed by GIXD, XPS, and TEM. Through GIXD and XPS results, formation of AlN was distinctly detected as a nitrided surface. Cross-sectional microstructure of nitrided samples showed that AlN was formed with the thickness up to 3-5 μm. AlN formation is controlled by the diffusion process. The thickness of AlN layer was determined by the nitriding time and temperature. Partial degradation of AlN in the vicinity of the free surface occurred due to its reaction with moisture in air. Partial detachment of AlN layer occurred due to the residual thermal stress, which was caused by the difference in thermal expansion coefficient between AlN and substrate of Al.