In the fabrication of a sputtered nitride film, if the process is not well controlled and some oxygen contaminations are detected in the coating. The hardness would decrease. The reason may result from the microstructure change (from compact structure to loose columnar structure). However, if the microstructure is similar, what other reason may cause the decrease of hardness when the oxygen content in the nitride film increased?

I'm thinking that since the nitrogen atoms are replaced by oxygen atoms, the bond strength would be different, and thus the hardness would decrease from the intrinsic hardness point of view. Can anyone kindly share some opinion, please?