A study of the microstructure evolution of hard Zr-B-C-N films by high-resolution transmission electron microscopy

Pulsed reactive magnetron sputtering was used to deposit Zr-B-C-N films as a function of the N2/Ar ratio in the plasma. The microstructure evolution of the films was studied by high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy and nanoindentation. Zr-B-C-N films with a chemical composition (in at.% without 1-2 at.% H and < 1 at.% O and Ar) of Zr61B27C6N3, Zr41B30C8N20, Zr26B26C5N42 and Zr24B19C6N49 were produced using a nitrogen fraction of 0%, 5%, 10% and 15%. The Zr61B27C6N3 film consists of fcc B-rich Zr(B,C,N) nano-columnar structures concealed by an amorphous matrix. The fcc B-rich Zr(B,C,N) columns exhibit a preferred orientation with their [1 1 1] parallel to the film growth direction. The Zr41B30C8N20 film possesses the highest hardness and consists of nano-needle structures (∼40 nm long and ∼10 nm wide) separated by amorphous boundaries. The nano-needles have a face-centered cubic (fcc) structure and are composed of ZrN and/or Zr(B,C,N) nano-domain structures (∼2 nm) that are semi-coherently joined via Zr-N monolayer interfaces. The Zr26B26C5N42 film is composed mainly of refined crystalline ZrN nano-needle structures (∼2 nm) embedded in an amorphous structure, and the Zr24B19C6N49 film has an amorphous-like structure. In the Zr41B30C8N20 film, the formation of ZrN and/or Zr(B,C,N) nano-domain structures, semi-coherently joined via Zr-N monolayer interfaces within the nano-needle structures, play a critical role in achieving high hardness and modulus. The annihilation of such a structure and the introduction of additional amorphous structure into the films by changing the N/Zr ratio via varying the N2 fraction in the gas mixture result in a significant decrease in the mechanical properties.