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 obtained films were characterized by various techniques including XRD, XPS, SEM, HRTEM and nano-indentation. By probing the structural and mechanical properties of the films, the existences of three distinct modes were confirmed in the synthesis stages with respect to the oxygen content, x. In the nitride mode with x ≤ 0.6, where the wurtzite lattice still survives, films with 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. Whereas upon increasing the oxygen content (x ≈ 55%), the (002) is 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 first increased to 33 GPa with the addition of oxygen before decreasing slowly at higher oxygen concentrations. In the transition mode between nitrides and oxides, 0.6 < x ≤ 0.95, the structure collapsed into 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 γ-(Al,Cr)2O3 was observed by the appearance of (400) and (440) reflections and the hardness increased again to 30–35 GPa.

Research highlights
► We prepare a large series of Al95.5Cr2.5Si2(N1-xOx)thin films.
► We study changes in microstructure with the oxygen content of the films.
► We find the existences of three distinct modes of films the synthesis stages.
► Nitride mode, transition mode and oxide mode can be assigned to x ≤ 0.6, 0.6 < x ≤ 0.95 and x > 0.95, respectively.
► The structure of films in the transition region collapses to an amorphous network.