Plasma nitriding of cold-pressed and BMA-forged Al compacts

Aluminum powder compacts were successfully plasma-nitrided under nitrogen-hydrogen mixture gas for various holding temperatures from 723 K to 823 K for 36 ks (10 h) to 144ks (40 h). Two kinds of specimens were prepared for pure aluminum and Al-1 mass%Ti alloy; the cold-pressed aluminum powder compact, and the refined aluminum powder compact via Bulk Mechanical Alloying (BMA). Before nitriding, the samples were sputtered to in situ eliminate surface oxide film by nitrogen plasma for varying the holding time from 0 s to 15 ks (5 h). Glancing angle X-ray Diffraction (GIXD) at 1° incident angle and X-ray Photoelectron Spectroscope (XPS) were used for qualitative analysis of the nitrided surface. Aluminum nitride (A1N) was formed with the thickness up to 15μm. The refined microstructure by BMA enhanced the formation of A1N by increasing the fraction of high diffusion paths through the grain boundary. Degradation of AJN film adjacent to surface can be caused by its reaction with moisture. Detachment of A1N layer was partially observed in several samples. Detachment of nitrided layer might be introduced by the residual thermal stress in A1N layer during cooling down from the nitriding temperature.