Superstoichiometric (Al,Cr)N: Nitrogen's whereabouts and role in structure-property relationships

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)Nx 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)N1.08, with nitrogen near the threshold, and (Al,Cr)N1.11, which exceeds it. While (Al,Cr)N1.08 features densely packed, elongated fibrous grains with a strong (111) growth orientation, (Al,Cr)N1.11 develops a fine-grained microstructure with a (220) growth orientation. These structural changes significantly affect mechanical properties: (Al,Cr)N1.08 is 9 % harder (34.4 GPa vs. 31.6 GPa) with superior abrasive resistance, while (Al,Cr)N1.11 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)Nx coatings but also highlight the broader potential of superstoichiometric nitrides and ceramics for advanced applications.