Modification of mechanical properties of nitrogen-sputtered boron nitride films by ion bombardment

Boron nitride is a fascinating coating material, both in the electronics industry and for tribological applications. For the various applications the crystalline structure is important, and there is a need for studies of its basic nature and of its surface modification by ion beams. A large variety of high temperature processes for production of boron nitride exists, whereas there are only a few reports on low temperature processes, such as reactive r.f. sputtering. In these cases, only boron nitride targets have been used and usually in argon-nitrogen sputtering mixtures. The authors of the present paper have, however, shown that boron nitride can be deposited by a reactive nitrogen-sputtering process from a boron metal target without argon at all. The acoustic scratch test technique was used as a kind of mechanical test for nitrogen-sputtered BN deposited on cemented carbide. The effect of a negative substrate bias and sputter gas mixtures of nitrogen and krypton was studied. Although nitrogen-krypton sputter gas mixtures had only a marginal effect on sputter rates, they had a significant effect on the mechanical film properties. A post-ion bombardment of nitrogen-sputtered BN coatings with nitrogen ions in the hundreds of kiloelectronvolts range was also effective for modification of the mechanical properties. The possibility of ion implanting a lithium catalyst for a crystalline transformation from hexagonal to cubic crystalline structure was discussed, and lithium ion implantation did show a modification of surface friction properties. This novel process of nitrogen sputtering of boron may improve understanding of the fundamental aspects of phase control in the deposition of boron nitride films.