Aluminum rich oxynitride thin films were prepared using pulsed direct current (DC) magnetron sputtering from an Al95.5Cr2.5Si2 (at.%) target. Two series of films were deposited at 400 degrees C and 650 degrees C by changing the O-2/(O-2 + N-2) ratio in the reactive gas from 0% (pure nitrides) to 100% (pure oxides). The films were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and nanoindentation. The results showed the existence of three different regions of microstructure and properties with respect to the oxygen concentration. For the samples deposited at 650 degrees C in the nitrogen rich region (O-2/(O-2+N-2) <= 0.08), the formation of the h-AlN (002) and Al-N bond were confirmed by XRD and XPS measurements. The hardness of the films was around 30 GPa. In the intermediate region (0.08 <= O-2/(O-2 + N-2) <= 0.24), the presence of an amorphous structure and the shifting of the binding energies to lower values corresponding to non-stoichiometric compounds were observed and the hardness decreased to 12 GPa. The lowering of mechanical properties was attributed to the transition of the clean target to the reacted target under non-steady state deposition conditions. In the oxygen rich region (0.24 <= (O-2/(O-2 + N-2) <= 1), the existence of alpha-Al2O3-(113), alpha-Al2O3-(116) and Al-O bonds confirmed the domination of this phase in this region of deposition and the hardness increased again to 30-35 GPa. Films deposited at 400 degrees C showed the same behavior except in the oxygen rich region, where hardness remains low at about 12-14 GPa. (C) 2010 Elsevier B.V. All rights reserved.