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

Oxygen implantation (5×1017 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 O2 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× deeper (e.g. 10% MgO at 500 nm) than the O-ion range (∼50 nm). Al remains practically metallic. The nanohardness and Youngʹs modulus are enhanced 2–4× 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.