Microstructure of Al95.5Cr2.5Si2(N1-xOx) thin films covering from nitrides to oxides

Al95.5Cr2.5Si2(N1-xOx) thin films were prepared using pulsed DC-magnetron sputtering at 650 C by changing the oxygen to nitrogen ratio (O2/(O2 + N2)) in the reactive gas. The films were characterised by XRD, XPS, SEM, HRTEM and nano-indentation. By testing the structural and mechanical properties of the films, the presence of three distinct modes were confirmed in the synthesis stages with respect to the oxygen content, x. In the nitride mode with x up to 0.6, where the wurtzite lattice still survives, films with a well-developed columnar structure were obtained. At low oxygen concentrations, the formation of a mixture of either round grains or triangular facets was assigned to (002) and non-(002) orientations, respectively. When the oxygen content increased (x about 55%), the (002) was the dominant reflection and an amorphous layer was formed, surrounding the nitride grains, as confirmed by HRTEM. The hardness of the films was about 30 GPa, which increased to 33 GPa with the addition of oxygen before decreasing slowly at higher oxygen concentrations. In the transition mode between nitrides and oxides, x = 0.6-0.95, the structure collapsed into a completely amorphous network and the observed hardness in this region decreased to 12 GPa. The third mode was formed by coatings with x > 0.95, where the formation of nanocrystalline gamma-(Al,Cr)2O3 was observed by the appearance of (400) and (440) reflections and the hardness increased further to 30-35 GPa.