Microstructure and thermal conductivity of Ti-Al-Si-N nanocomposite coatings deposited by modulated pulsed power magnetron sputtering

Hard Ti-Al-Si-N coatings are widely used in cutting tools, due to their excellent mechanical properties and superior thermal properties. In this study, Ti-Al-Si-N coatings are deposited by modulated pulsed power magnetron sputtering, with various substrate bias voltages from −35 V to −130 V. As the bias voltage goes up, the composition of coatings remained nearly unchanged, maintained as a constant of Ti_0.18Al_0.26Si_0.05N_0.51. However, the Ti-Al-Si-N coatings have a decrease in (200)-preferred orientation; dense columnar structure (Zone I) of Ti-Al-Si-N coatings gradually evolves into featureless and flat cross sections structure (Zone T). As increasing the substrate bias voltage, the hardness increases from 31.2 GPa to 37.5 GPa, the H/E* value increases from 0.079 to 0.090, while the compressive residual stress of coatings raises from -1.22 GPa to -2.15 GPa. The thermal conductivity of coatings is examined by transient thermoreflectance technique, which decreases from 5.4 W/m*K to 2.1 W/m*K with the bias voltage. The values of electric resistivity ρ for all coatings are very large, ranging from 147 kΩ⋅m to 173 kΩ⋅m. The electronic thermal conductivity has no contribution to the thermal conductivity of Ti-Al-Si-N coatings, which is mainly determined by the phonon thermal conductivity. As increasing the substrate bias voltage, the average grain size of Ti-Al-Si-N nanocomposite coatings decreases from 16 nm to 5 nm. The interfacial density per unit volume is therefore increased, and leading to more interface scattering of the phonons in the heat transport progress, which is the key parameter in determining thermal conductivity of Ti-Al-Si-N nanocomposite coatings.