Reactively sputtered TiN/SiO2 multilayer coatings with designed anisotropic thermal conductivity – From theoretical conceptualization to experimental validation

Wear resistant coatings are frequently used for cutting applications due to their high hardness, chemical stability and low thermal conductivity. The low thermal conductivity is generally believed to redirect the heat generated during cutting into the chip, while the heat transfer into the tool is kept low. However, a low isotropic thermal conductivity of the applied coatings consequently causes high lateral thermal gradients close to the heat affected zone. These gradients in turn might cause local degradation of mechanical properties, increased local wear and pronounced local thermal stress. A promising concept for thermal management during cutting is the use of coatings with designed anisotropic thermal conductivity. Thereby, the heat flow into the tool can be still kept low, while heat dissipation within the coating can be maximized. Following this concept, the focus of this work is the design, deposition and characterization of coatings with tailored anisotropic thermal conductivity. TiN/SiO ₂ multilayer coatings with periodicities between ~100 and ~400 nm and different TiN/SiO ₂ thickness ratios resulting in theoretical anisotropy factors ranging from ~1.7 to ~4.0 were designed. Subsequently, corresponding coatings were deposited by unbalanced reactive magnetron sputtering to a total thickness of ~12 μm using Ti and Si targets. The multilayers were prepared for cross-sectional measurements of their in-plane and surface measurements of their cross-plane thermal conductivity by time-domain thermoreflectance to successfully verify the predicted anisotropies.