Evolution of the thermal conductivity of arc evaporated fcc-Ti1-x-yAlxTayN coatings with increasing Ta content

Hard coatings are commonly applied in severe cutting applications, where significant heat is generated. Thus, their thermal conductivity should be kept low to provide a heat barrier to the substrate and consequently to increase the service life time of the tools. Although, Ti _1-x-yAl _xTa _yN protective coatings have been applied successfully in the cutting industry, their thermal conductivity is barely investigated. The focus of this study is to determine the thermal conductivity of face-centered cubic (fcc)-Ti _1-x-yAl _xTa _yN coatings with a Ti/Al ratio of 1:1 and a Ta content increasing from 0 up to 23 at.%. The investigated coatings were deposited by cathodic arc evaporation to a coating thickness of 3.2 μm ± 0.4 μm. The microstructure and chemical composition were studied using X-ray diffraction and energy dispersive X-ray spectroscopy, respectively. Time-domain thermoreflectance measurements revealed a low thermal conductivity for fcc-Ti _1-xAl _xN with 5.7 W/(mK) and a further decrease with increasing Ta content to 2.4 W/(mK) for 23 at.% Ta. This trend can be explained by the small grain size caused by the Al addition leading to increased boundary scattering and the incorporation of Al and larger Ta atoms in the fcc-TiN lattice resulting additionally in alloy scattering, as the thermal conductivity decreases with increasing phonon scattering processes.