Mechanical and Tribological Properties and High-Speed Drilling Performance of NbTiN Coatings Prepared by High-Power Impulse Magnetron Sputtering with Varying Nitrogen and Acetylene Flux Rates

Abstract

NbTiNx and NbTiN25-CHx coatings (where x indicates the nitrogen or acetylene flux rate) were deposited on SKH51 substrates by high-power impulse magnetron sputtering. The study comprised three stages. In the first stage, NbTiNx coatings with nitrogen flux rates varying from 0 to 50 sccm were deposited to determine the nitrogen flux rate which yielded the best mechanical and tribological properties. In the second stage, NbTiN25-CHx coatings with acetylene flux rates varying in the range of 5-25 sccm were prepared using the optimal nitrogen flux rate determined in the first stage in an attempt to further improve the tribological performance. In the final stage, the NbTiN25-CHx coating with the best tribological properties was deposited on micro-drills and used to perform a series of high-speed through-hole drilling tests under dry conditions using printed circuit board specimens. Among the various NbTiNx coatings, the NbTiN25 coating showed an excellent adhesive strength (Lc > 100 N) and superior tribological properties. The NbTiN25 coating was thus selected for further experimentation using various acetylene additions. The NbTiN25-CH25 coating was found to possess a sufficient carbon content (34.9 at.%) to produce a transition from a metal nitride crystalline (NbTiN25) structure to a diamond-like carbon structure with a high sp2 concentration. The coating exhibited a high hardness/elastic modulus ratio of 0.063 and an adhesive strength of Lc > 100 N. Consequently, a carbon-rich solid lubricant layer was formed at the contact interface under ball-on-disk sliding, which resulted in an excellent tribological performance. The NbTiN25-CH25 coating was thus chosen for deposition on micro-drills. The coating increased the lifetime of the micro-drill by around three times compared to that of an uncoated drill. Even after drilling 6000 holes, the holes showed an excellent quality with a low nail head ratio and surface roughness and a diameter error of less than 1.5%.