Stress analysis and microstructures of PVD monolayer TiN and multilayer TiN/(Ti, Al)N coatings

Two PVD titanium nitride based coatings; monolayer TiN and multilayer resulting from the stacking of TiN and (Ti,Al)N layers were evaluated with respect to their stress state and microstructure. The TiN was deposited by triode evaporation ion plating, whereas the TiNy(Ti,Al)N was deposited using a reactive hybrid deposition process consisting of a combination of electron beam evaporation of Ti and DC magnetron sputtering of a Ti-Al alloy. The structural and mechanical state characterisations of the as-deposited coatings on steel substrates were performed using X-ray diffraction methods. The Bragg-Brentano geometry was used to study the texture and the sin c method was applied to obtain the stress-free lattice parameter, the Poisson's ratio 2 and the residual stresses. The monolayer exhibited a preferred orientation with (1 1 1) parallel to the surface. However, the TiN and (Ti,Al)N layers from the multilayer revealed a slightly (3 1 1) preferred orientation. All coatings were in a state of compressive stress ranging from 10.1 to 2.7 GPa, depending logically on the substrate material, layer thickness and deposition processes. The microstructure and composition of the coatings were investigated using a combination of scanning electron microscopy, plan-view and cross-sectional transmission electron microscopy, energy-dispersive spectroscopy and electron energyloss spectroscopy. The TiN exhibited a fibrous microstructure where only a few columns extended through the whole coating thickness. The TiNy(Ti,Al)N multilayer revealed a more pronounced columnar microstructure with the columns extending throughout the film thickness. Micrometer-sized macroparticles were present in the multilayer at various distances from the substrate, but never at the substrate surface. The results showed that they were incorporated in the growing film in the solid state and consisted of a core structure with equiaxed grains having the a-Ti phase and an outer layer of TiN. Evidence was found of nitrogen diffusion, presumably from both the working gas into the solidifying Ti droplet during migration to the film and through the TiN outer layer.