Microstructure evolution in TiN films reactively sputter deposited on multiphase substrates

Abstract

Cross-sectional transmission electron microscopy (X-TEM) has been used to investigate the microstructure of TiN films deposited by reactive magnetron sputtering onto ASP 30 high speed steel (HSS) substrates. ASP 30 is a multiphase material consisting of a martensitic matrix with MC and M6C transition–metal particles embedded in it. The TiN films were ∼4 μm thick and were grown at substrate temperatures Ts between 200 and 550 °C. The X-TEM results showed that the film microstructure was very nonhomogeneous. A dense columnar morphology was observed above both the martensitic and M6C substrate phases. The average column diameter 〈d〉 ranged from 15 nm (Ts=200 °C) to 50 nm (Ts=550 °C) near the film/substrate interface over the martensitic phase and was in a factor of 2 less in the film regions adjacent to the M6C phase. 〈d〉 increased in both regions to 100 nm (Ts=200 °C) to 150 nm (Ts=550 °C) near the surface of the films. TiN grew epitaxially on MC particles but contained a high dislocation density which (except at 550 °C where some of the epitaxial grains extended throughout the entire film thickness) eventually coalesced to form low-angle grain boundaries and, finally, high-angle boundaries. Thus the nucleation and initial growth kinetics were strongly influenced by the local substrate structure while the growth temperature had relatively little effect other than to increase adatom surface diffusivities, and hence the average column size above the martensitic and M6C phases, and to decrease the extended defect concentration in the epitaxial regions. Grain boundary migration and secondary recrystallization were not observed even at the highest growth temperatures investigated.