Influence of concentration of Ag on electron and crystallographic structure of reactively co-sputtered CFUBMS TiN/Ag nanocomposite coatings

Abstract

<title/>TiN/Ag coatings of 2·5-3 μm thickness were deposited on 304 austenitic stainless steel substrates by reactive closed field unbalanced magnetron sputtering (CFUBMS) in a Teer Coatings UDP350 rig. The titanium and silver targets were co-sputtered in pulsed DC mode in Ar-N2 atmospheres. In order to obtain different Ag contents in the coatings, the power applied to the Ag target was varied between 0 and 120 W, while the Ti target was sputtered at an average power of 1500 W. The concentration and behaviour of the silver atoms in the TiN matrix and their influence on the electron and crystallographic structure of the applied nanocomposite TiN/Ag coatings was studied by X-ray diffraction, density functional theory, scanning electron microscopy, energy dispersive X-ray spectroscopy and nanoindentation. The results show that the applied pure TiN and the composite coatings have polycrystalline structures built up by nanocrystals. A well pronounced {111} in-plane crystallographic texture was observed in all the coatings. Depending on their concentration the silver atoms are either dissolved into the TiN lattice forming substitutional solid solutions or in addition to that are also distributed as second phase nanoparticles along the boundaries of the nanocrystallites. In the pure TiN coating and in the coating with lower silver concentration compressive residual macrostresses formed, while in the coatings with higher silver content the stresses are tensile. The addition of silver decreases the valence electron density of TiN and thus weakens the metal bonding. This, together with the tensile residual stresses and softer Ag particles along the crystalline boundaries observed in the coatings with higher Ag concentration, is expected to cause the previously observed decrease in hardness with increasing silver content.