Effect of nitrogen on the structure and properties of Zr-Si-B-N coatings deposited by magnetron sputtering

Protective Zr-Si-B-N coatings were deposited by magnetron sputtering of the 66%ZrB2 + 26%ZrSi2 + 6%Si + 2%ZrO2 composite target at varied partial pressure of nitrogen. The coatings were studied in terms of structure, optical characteristics, mechanical and tribological properties, cyclic impact and oxidation resistance. All the coatings were characterized by a dense and defect-free structure, without any well-defined columnar elements. The base of the non-reactive Zr-Si-B coatings was the hexagonal ZrB2 phase with the crystallite size of about 20 nm. Nitrogen doping led to grain size reduction and coating amorphization. An increase in the nitrogen concentration leads to the formation of an amorphous phase а-Si(Zr)BN. The high content of nonmetallic bonds ensured the high transmittance (up to 90 %) in the visible and infrared spectral ranges for the coatings characterized by the maximum nitrogen content. Hardness of the coatings deposited in working gas Ar and Ar + 15%N2 was ~20 GPa. When the coatings were deposited in an atmosphere of pure nitrogen, this value was ≤15 GPa. Due to structure modification as well as increasing elastic recovery and elastic strain to failure, nitrogen doping of the coatings contributed to reduction of the friction coefficient and increased wear resistance under sliding friction conditions and cyclic impact loading. High-temperature oxidation resistance and thermal stability of the coatings decreased after nitrogen doping. The Zr-Si-B coating exhibited the highest oxidation resistance and withstood long-term exposure to 1200 °C and short-term exposure to 1400 °C. The high protective properties were ensured by formation of the silica-based surface protective layer with zirconia crystallites dispersed in it.