Development of Thin Aluminide Coatings on Alloy 800 Substrate

Magnetron sputtering using aluminium as a target material was employed on Fe–Ni–Cr containing austenitic Alloy 800 substrates. Electron microscopic examination with elemental analysis on magnetron-sputtered substrates indicated formation of ~1.0 µm thick oxygen-rich Al–Fe–Ni–Cr containing layer just below the uppermost aluminium-rich deposited layer. Magnetron-sputtered + thermally–treated (1273 K for 18 h) substrates revealed formation of thin (~5 µm) aluminide layers at the upper surface comprising primarily Fe–Al rich layer with adjoining layers of Fe–Ni-Cr–Al type followed by Ni–Al rich layer. Thin multilayered alloy substrate exhibited microhardness value of ~405 (±15) Vickers hardness number (VHN) while for alloy substrate, that was recorded as 310 (±10) VHN. The scratch test conducted on thin multilayered alloy substrate in the load range of 1–10 N indicated friction coefficient and penetration depth of ~0.1 and 11 µm, respectively. A good adherence was recorded for thin layers as no peeling off was noticed, which was confirmed by acoustic emission signals. The exposure of magnetron-sputtered + thermally treated alloy substrates to 70% K2SO4 + 30% KCl salts at 973 K in air revealed retention of surface oxide predominantly composed of Al–Fe–Cr–Ni type oxide layer indicating its good resistance to aggressive environment.