Effect of nitrogen partial pressure on the structure, physical and mechanical properties of CrB2 and Cr-B-N films

The investigation of structure and properties of Cr-B and Cr-B-N films deposited by direct current magnetron sputtering of CrB₂ target in argon and argon-nitrogen environments, respectively is presented. The nitrogen partial pressure was kept at 10, 15, and 25% of the total pressure. The microstructure, phase and chemical composition of Cr-B-(N) films were studied by means of X-ray diffraction, transmission- and scanning-electron microscopy, X-ray photoelectron spectroscopy, electron probe microanalysis and secondary neutral mass-spectrometry. The films were characterized in terms of their electrical resistivity, optical reflectivity and transmittance. Measurements of hardness and elastic modulus were performed by depth sensing nanoindentation. The results obtained show that the films deposited in pure Ar had a hexagonal AlB₂-type structure with crystallites, 15-17 nm in lateral size, elongated in the direction of film growth. The Cr-B-N films consisted of nanocrystalline nc-CrB₂ and amorphous a-BN phases. As the nitrogen content in films was raised, the volume fraction of the nc-CrB₂ phase decreased and a-BN phase increased. When nitrogen was added to the gas discharge during deposition, the nc-CrB₂ crystallite size decreased down to 3-5 nm. Without nitrogen, the films exhibited a columnar morphology. Nitrogen introduction suppressed the columnar growth of films because formation of a-BN intergranular layers. The films deposited under optimal parameters showed hardness in the range of 36-43 GPa and Young's modulus below 300 GPa. For all films, electrical resistivity values between approximately 200 and 700 µΩ cm were recorded.