Deep segregation and hardening in Al-Mg-Zn-Cu-Cr alloy treated by plasma source oxygen ion implantation

Oxygen implantation (5 X 10(17) O cm(-2), 30 keV) of the Al-7075 alloy (Al-Mg-Zn-Cu-Cr) was performed using an electron cyclotron resonance plasma source. The chemical implantation effects were studied by X-ray photoelectron spectroscopy and secondary ion mass spectrometry depth profiling, and the mechanical effects by atomic force microscopy. Strong Mg segregation (up to -100% MgO) was found. For purely thermal near-surface (-15 nm) segregation, in O-2 gas, our measured activation energy of 68 kJ mol(-1) is consistent with other results. Following O-ion irradiation, however, MgO is surprisingly observed 10 x deeper (e.g. 10% MgO at 500 nm) than the O-ion range ( 50 nm). Al remains practically metallic. The nanohardness and Young's modulus are enhanced 2-4 x to 1.5 GPa and - 300 GPa, respectively. An increase in rms roughness from 4.5 to 47 nm, likely due to MgO precipitates, does not modify the nanoscale friction coefficient (0.026), dominated by adhesive forces. (C) 2001 Elsevier Science B.V. All rights reserved.