Effects of nitrogen content on microstructure and oxidation behaviors of Ti–B–N nanocomposite thin films

Abstract

Several different types of thin films, TiB0.65,TiB0.62N0.18,TiB0.61N1.04, and pure TiN, were deposited on Si(100) substrates at 500 °C by reactive unbalanced close-field dc-magnetron sputtering using two Ti and two TiB2 targets. The oxidation experiments of these films were carried out in air at fixed temperatures in a temperature regime of 600–1000 °C. As-deposited and oxidized films were characterized and analyzed using x-ray diffraction, plan-view and cross-sectional scanning electron microscopies, atomic force microscopy, and x-ray photoelectron spectroscopy (XPS). It was found that the microstructure and bonding configuration of Ti–B–N thin films were strongly dependent on nitrogen flow rate during deposition. Depending upon the amount of N addition, the films showed two- or three-phase nanocomposite structure. Nitrogen-free films were amorphous compound comprising of Ti and TiB2 (Ti–TiB2 compound). At 10 at. % N addition (TiB0.62N0.18), the films consisted of mainly TiN and TiB2 bondings with microstructures comprising of nanocrystalline (nc)-Ti(N) embedded in an amorphous (a)-TiB2 matrix. As the N concentration increased up to 38 at. % (TiB0.61N1.04), the films consisted of nc-TiN in a-(TiB2, BN) matrix. The oxidation experiment illustrated that the nanocomposite TiB0.61N1.04 thin films exhibited a much higher high-temperature oxidation resistance than TiN, TiB0.65, and TiB0.62N0.18 thin films. A two-stage oxidation process took place in these nanocomposite films in the whole temperature regime. A low oxidation rate accompanied with formation of small-grained Ti oxide occurred below 800 °C, while above 800 °C a rapid oxidation process accompanied with formation of large-grained Ti oxide with rough surface took place. It is believed that the two-stage oxidation process was related to oxidation resistance of nanocrystallites and thermal stability of amorphous matrix phase. By XPS, the oxides were determined to consist mainly of various types of Ti oxides in the oxidation temperatures of 600–1000 °C, such as TiO, TiNxOy, Ti2O3, and TiO2. It was also found that no elemental B was detected in oxide formed above 600 °C, which may be due to a low melting temperature of B2O3.