Surface morphology and texture of TiAlN/CrN multilayer coatings

Abstract

Multilayer thin films of TiAlN/CrN were deposited at varying bilayer periods (λ=2, 4, 8 and 16nm), along with a co-deposited (TiAlCr)N film and constituent TiAlN and CrN films, in order to examine the effects of periodicity on grain structure, texture and surface morphology. Characterization was performed using electron microscopy, atomic force microscopy and X-ray diffraction analysis. All films exhibited a rock-salt B1-type structure. Transmission and scanning electron microscopy cross-sections showed a columnar morphology with lateral coarsening of the columns during film growth that is more pronounced in the multilayer films with smaller bilayer periods. The surface morphology was found to be dependent on period with sharp pyramidal surface features at λ=2nm, an increase in secondary faceting with increasing periodicity, and nearly rounded column tops at λ=16nm. Consistent with these morphological observations, surface roughness measurements showed an increasing roughness with decreasing bilayer period. X-ray diffraction pole figures revealed that the multilayer films had a crystallographic texture that could be described as preferential [200] poles tilted approximately 15° away from the substrate normal and clustered near the deposition direction. The distribution of the [111] poles was nearly random for the films with larger periodicities (8 and 16nm), but a small [111] texture component was noted for the 2 and 4nm period films. A mechanism for the effect of the multilayer period on the surface morphology is proposed wherein the higher interfacial energy density in the smaller period multilayers promotes a higher degree of (200) faceting at the film surface, reducing grain orientation effects on adatom mobility.