Improved thermal stability, mechanical and tribological properties of reactively sputtered Si doped TiAlC nanostructured hard coatings

• TiAlSiC hard coatings were deposited by reactive pulsed DC magnetron sputtering.
• XRD, XPS and Raman data reveal the presence of a-Si, a-AlC and a-C in the coating.
• High hardness and low friction coefficient were achieved for low Si contents.
• Maximum toughness value of 2.14 MPa m1/2 was obtained for 8 at.% Si content.
• Increase in the Si content improves the thermal stability of the TiAlSiC coating.

The influence of different Si contents on TiAlSiC nanocomposite coatings deposited on stainless steel and silicon (100) substrates using reactive unbalanced direct current magnetron sputtering has been studied. A comprehensive study of microstructure, mechanical, tribological and thermal stability of Si doped TiAlC has been carried out. X-ray diffraction (XRD) analysis reveals that the TiAlSiC coating exhibits fcc NaCl type TiC phase. The presence of a-AlC and a-Si phases was observed by correlating the X-ray photoelectron spectroscopy and XRD data. Moreover, micro-Raman spectroscopy studies indicate the existence of graphitic rich layer of a-C in the coating which limits the columnar growth on account of a-Si. Friction coefficient, adhesion, toughness and mechanical properties of the TiAlSiC coatings were characterized by nanoscratch tester and nanoindentation measurements. A high hardness ~ 28 GPa and a friction co-efficient of ~ 0.49 was achieved for TiAlSiC coatings prepared at a Si contents of 8 at.%. The maximum toughness of 2.14 MPa m1/2 was obtained for the optimized coating due to grain boundary strengthening. Although, increase in the Si content deteriorates the mechanical and tribological performance, but it improves the thermal stability of the coatings. A remarkable thermal stability up to 1000 °C in air for 4 h has been observed for higher Si contents (> 12 at.%).