Thermal Stability of WB2 and W-B-N Films Deposited by Magnetron Sputtering

The work is mainly to study the thermal stability including the phase stability, microstructure and tribo-mechanical properties of the AlB₂-type WB₂ and W-B-N (5.6 at.% N) films annealed in vacuum at various temperatures, which are deposited on Si and GY8 substrates by magnetron sputtering. For the WB₂ and W-B-N films deposited on Si wafers, as the annealing temperature increases from 700 to 1000 °C, a-WB (700 °C) and Mo₂B₅-type WB₂ (1000 °C) are successively observed in the AlB₂-type WB₂ films, which show many cracks at the temperature ≥?800 °C resulting in the performance failure; by contrast, only slight α-WB is observed at 1000 °C in the W-B-N films due to the stabilization effect of a-BN phase, and the hardness increases to 34.1 GPa first due to the improved crystallinity and then decreases to 31.5 GPa ascribed to the formation of α-WB. For the WB₂ and the W-B-N films deposited on WC-Co substrates, both the WB₂ and W-B-N films react with the YG8 (WC-Co) substrates leading to the formation of CoWB, CoW₂B₂ and CoW₃B₃ with the annealing temperature increasing to 900 °C; a large number of linear cracks occur on the surface of these two films annealed at ≥?800 °C leading to the film failure; after vacuum annealing at 700 °C, the friction performance of the W-B-N films is higher than that of the deposited W-B-N films, while the wear resistance of the WB₂ films shows a slight decrease compared with that of the deposited WB₂ films.