Transition Metal Oxynitrides: Enhancing Performance by Adding Oxygen – paper presented at ICMCTF 2011.
“The different way nitrogen and oxygen bond with transition metals often results in completely different crystallographic and electronic properties between nitrides and oxides. This translates in divergent functionalities for various technological applications. Interestingly, by varying the O/(N+O) atomic ratio in several transition metal oxynitride coatings, it is possible to tune their electronic, optical and crystallographic properties and also enhance their hardness, thermal stability and oxidation resistance. Therefore, the understanding of the role of oxygen and nitrogen in such compounds is of great interest for a wide range of applications such as microelectronics, heterogeneous catalysis, magnetism and protective coatings. As a model example, it will be shown how the crystal structure and electronic properties of Cr-O-N and Cr-Si-O-N thin films change by varying the oxygen content, and how this variation may result in an enhancement of their mechanical properties, phase stability and oxidation resistance. Although Cr-Si and Cr oxynitride coatings exhibit similar crystallographic, electronic structure and mechanical properties, the Cr-SiO-N films are superior in terms of phase, nanocrystalline stability and oxidation resistance. The cubic/rhombohedral transition occurs in the O/(N+O) range between 70 and 80%. The oxygen is substitutionally incorporated into the nitride cubic structure resulting in a decrease of the lattice parameter and the development of a strong (002) crystallographic preferred orientation as the oxygen concentration increases. According to SEM, all the coatings have dense and smooth microstructure. The maximum hardness is obtained for a oxygen concentration of 97% (28 Gpa) and 44 % (27 Gpa) for the Cr-O-N and Cr-Si-O-N coatings, respectively. Both Cr-based oxynitride coatings with the cubic structure exhibit an excellent phase stability and oxidation resistance in vacuum and in air, which is higher than 1000 °C for the Cr-Si-O-N coatings with a O/(N+O) ratio of 44%.”