Plasma nitriding and in situ characterisation of aluminium

Substrates of aluminium alloy 2011 were plasma nitrided using an inductively coupled plasma source and characterised in situ using X-ray photoelectron spectroscopy (XPS). The maximum entropy method (MEM), used for sharpening the spectra, has enabled the identification of metallic Al and AlN at binding energies of 72.7 and 74.4 eV, respectively. Other peaks at 73.4, 75.0, 75.5 and 76.2 eV are ascribed to Al oxide species. Nitriding carried out for 3 h at 300 and 400 °C gave rise to a higher relative surface AlN concentrations compared to treatments carried out at lower temperatures or for shorter times. Oxidation of the substrate surface rather than nitriding occurred if the substrate surfaces were treated for shorter times at high temperatures. The substrates were also characterised ex situ by dynamic time-of-flight secondary ion mass spectroscopy (ToF-SIMS) and scanning electron microscopy. AlN layers were found to extend up to 15 nm into the metal. A large number of CuAl2 precipitants are formed on the grain faces as well as on the grain boundaries after treatment at 400 °C. Faster cooling rates after the nitriding process and/or lower nitriding temperatures are required to prevent segregation of the precipitants at the grain boundaries. After exposure to air, further oxidation occurs and the thickness of the surface oxide layer increases.