Nanocrystallite superhard Titanium Nitride film in Multi-Arc ion plating

Titanium nitride (TiN) films synthesized by multi-arc ion plating (AIP) normally have a columnar microstructure, and are likely to induce surface defects due to the formation of macroparticles and neutral particles in the vicinity of cathode arc sources. Hence, the achievable microhardness of the normal AIP TiN films only ranges between 20˜30 GPa. A systematic study for fabricating an adherent nano-superhard titanium nitride (TiN) film on M2 high speed steel substrate by a vacuum cathode multi-arc ion-plating (AIP) system was initiated. To understand the relationship of the film processing-structure-property, their microhardness, film-to-substrate adhesion, frictional property, and microstructure of the film were investigated using Vickers hardometer, scratch tester, ball-on-disc tester, X-ray diffractometer, and transmission electron microscope. Results show that: (i) the achievable film microhardness ranges between 35 GPa and 45 GPa; (ii) the critical load (Lc) of the superhard TiN film is at 64 N approximately; (iii) the friction coefficient, under a high-load and a high rotating-speed, of the film is ranging from 0.5 to 0.8; and (iv) the nm scale mean main grain-sizes of the film are approximately 12.7 nm for TiN₁₁₁, 19.7 nm for TiN₂₀₀ and 9.6 nm for TiN₂₂₀. The maximum achievable microhardness 45 GPa is more than twice of the 22 GPa for standard TiN film. Such hardness enhancement is anticipated as mainly due to: (a) the formation of nanoscaled crystalline grains; (b) the preferential orientation and growth of grains in the close-packed plane (111); and (c) the induced residual stress within the film by ion bombardment.