Superstoichiometric (Al,Cr)N: Nitrogen's whereabouts and role in structure-property relationships
Fedor F. Klimashin, M. Učík, Martin Matas, David Holec, M. Beutner, Matthias Hackert‐Oschätzchen, Angelos Xomalis, Jakob Schwiedrzik, J. Klusoň, Mojmír Jílek, Andreas Lümkemann, Johann Michler, Thomas Edward James Edwards
Abstract
The role of light elements in ceramic materials cannot be overestimated. By tuning their concentration, a whole multitude of mechanisms can be activated, directly influencing the material's functional properties. However, accurate determination of light element concentrations remains a challenge and is often, especially in superstoichiometric compounds, overlooked. Here, we provide atomic-scale insight into nitrogen incorporation in superstoichiometric (Al,Cr)N x coatings ( x = 1.01–1.11) with similar Al/(Al+Cr) ratios (0.59–0.63) by combining elemental concentrations, stress-free lattice parameters, and ab initio calculations. Below a threshold of x ≈ 1.06–1.08, excess nitrogen predominantly occupies interstitial lattice sites, while beyond this point, it can also incorporate into vacant metal sites (anti-sites). We then conducted a detailed investigation on two superstoichiometric coatings: (Al,Cr)N 1.08 , with nitrogen near the threshold, and (Al,Cr)N 1.11 , which exceeds it. While (Al,Cr)N 1.08 features densely packed, elongated fibrous grains with a strong (111) growth orientation, (Al,Cr)N 1.11 develops a fine-grained microstructure with a (220) growth orientation. These structural changes significantly affect mechanical properties: (Al,Cr)N 1.08 is 9% harder (34.4 GPa vs. 31.6 GPa) with superior abrasive resistance, while (Al,Cr)N 1.11 has 9% higher fracture toughness (4.15 MPa√m vs. 3.80 MPa√m) and enhanced microcracking and crack-branching behaviour. Our findings not only demonstrate the tunability of mechanical properties in superstoichiometric (Al,Cr)N x coatings but also highlight the broader potential of superstoichiometric nitrides and ceramics for advanced applications.