Hardness, internal stress and thermal stability of TiB₂/TiC multilayer coatings synthesized by magnetron sputtering with and without substrate rotation

TiB₂/TiC multilayer coatings were synthesized in a dual-cathode, non-reactive, unbalanced magnetron sputtering system with stationary and rotating substrates. All coatings are polycrystalline with TiB₂(001) preferred orientation and have a multilayer structure. Coatings synthesized with stationary substrates have high compressive stress (4-7 GPa), and their hardness is slightly enhanced (∼25%) over the rule-of-mixture value. Coatings grown with substrate rotation have much lower compressive stress (<2 GPa) and high hardness (>60 GPa). Substrate rotation appears to improve mobility of surface species and allows more time for surface diffusion, thus resulting in denser films with lower void concentration. This may explain the substantial difference in hardness enhancement between these two cases. After being annealed in an inert environment (vacuum or argon) at 1273 K for 1 h, these multilayer coatings retain their layer structure. Moreover, most coatings have improved crystallinity and increased room-temperature hardness after such annealing.