Formation Mechanisms and Structural Properties of Aluminium-Chromium Based Oxynitride Thin Films Synthesized by Physical Vapor Deposition

Oxynitride thin films, a novel class of mixed anionic solid structures, have gained an intense interest during recent years related to the unique interplay of oxygen (O) and nitrogen (N) with metals in the material structure. This matter can result in specific physical and structural properties. Accordingly, by varying the anionic composition (O/(O+N)), it is possible to tune the properties of oxynitrides, and particularly to enhance mechanical strength and phase stability. Consequently, the understanding of the role of O and N in such structures is of great interest. Taking this into account, we studied the formation mechanisms and properties of AlCr-based oxynitride films synthesized by the reactive magnetron sputtering (650◦C) and cathodic arc evaporation (550◦C) with an emphasis for application in hard coat- ings. Our results revealed that the variation of the anionic composition (O/(O+N)) disclosed the existence of three distinct growth regimes including: (i) a nitride regime O/(O+N)≤ 0.6, (ii) a transition regime 0.6 < O/(O+N) < 0.97, and (iii) an oxide regime O/(O+N)≥ 0.97, which were correlated to the structural features, chemical environ- ment, physical and mechanical properties of the films within each regime. For the oxygen fraction up to 60%, transmission electron microscopy (TEM) and x-ray diffrac- tion (XRD) alongside indicate the development of films in the nitride-type structure with a continues anionic substitutions, i.e. solid solution of AlCrOx N1−x . With fur- ther increase of the oxygen content, the structure of the layers is that of a poorly crystallized phase, and a near-amorphous film was favored in accordance with the observed low nanohardness, i.e. transition regime. Finally, occurring at the highest oxygen content, oxide regime (O/(O+N)≥ 0.97) was assigned to the films with oxide- type structure; however, with some probable oxygen substitution by nitrogen. The composition/structure variation was consistent with the chemical environment of the involved atoms, evaluated by x-ray photoelectron spectroscopy (XPS), where the electronic properties of the films exhibited substantial changes from regime to regime. Moreover, the film properties observed within each regime were also strongly in- fluenced by the cationic composition and deposition conditions. Al-rich layers (Cr/Al ≤ 0.1) were found to crystallize in the hexagonal wurtzite (B4) structure in the nitride regime (O/(O+N) ≤ 0.55) and transformed to a fully amorphous network and cubic γ-(Al,Cr)2O3 in the transition and oxide regimes, respectively. Conversely, Cr-rich films (Cr/Al≃1) were crystallized in the cubic (B1) structure within the nitride regime and preserved their structure in the transition regime despite the large pro- portion of oxygen. Such coatings in the oxide regime consisted of a solid solution of α-(Al,Cr)2(O0.97,N0.03)3 with corundum lattice and favorable protective proper- ties. Furthermore, it was observed that the structure of the films deposited at lower temperatures (400◦C) remains in the transition regime, and the oxide structure is not developed. The presence of silicon (Si, 3-4 at.%) and yttrium (Y≤ 1 at.%) in the oxynitride films was found to change the thresholds and structural features within the growth regimes. Oxynitride layers exhibited an enhanced phase stability (i.e. cubic-B1) by the incorporation of both Si and Y. As a result, the nitride regime was extended in the range of O/(O+N)≤ 0.8. However, on the other hand, Si and Y prohibit the formation of corundum phase in the oxide regime suggesting retarded diffusional processes with the presence of such alloying elements. Considering the preparation conditions of the oxide coatings, we found a previously unobserved formation of cubic crystalline fcc-(Al,Cr)2O3 and its transition to the well-known corundum phase (α- (Al,Cr)2O3) at higher film thickness beyond 2 μm by employing an extra-high substrate bias during arc evaporation. The TEM, XRD, and XPS results alongside indicated that the cubic (B1) layer acquires the properties of a metastable monoxide (M1−x O), which is kinetically stabilized by the occurrence of vacancies. It was proposed that there is a thickness-dependent gradient in point defect density, and its modification leads to the destabilized fcc-(Al,Cr)2O3 and a cubic-to-corundum transformation occurs. Favorable phase stability and oxidation resistance together with the low wear rates and friction behavior observed for the oxynitride films underline their potential for hard coating of tools used under severe dry cutting conditions.