Microstructure of Al-containing magnetron sputtered TiB2 thin films

Abstract The synthesis of ternary metal diboride thin films as solid solution with potentially enhanced properties is recently attracting large interest. This study reports on Al-containing TiB 2 thin films grown in single-phase solid solution (Ti,Al)B 2 structure at substrate temperatures below 200 °C on cemented carbide substrates. The thin films were synthesized by non-reactive magnetron co-sputtering of a TiB 2 and an Al target. The Al content of the (Ti,Al)B 2 thin films was systematically varied between 10 and 16% metal fraction via tuning the power supplied to the Al target, while all other process parameters were kept constant. Thereby also the boron:metal ratio reduces, approaching the 2:1 stoichiometry at an Al metal-fraction of 16% (Ti 0.84 Al 0.16 B 2.00 ). High resolution transmission electron microscopy show that growth and microstructure is clearly impacted by the Al content and changes from (00 l ) oriented growth of TiB 2.33 coatings towards various crystalline orientations of the solid solution (Ti,Al)B 2 lattice. For low Al concentration, the films exhibit moderate boron excess at grain boundaries, whereas at higher Al concentration, the (Ti,Al)B 2 thin films exhibit a microstructure with only few defects, and nearly no excess boron at boundaries (being in-line with the change in boron:metal ratio). In agreement with this observation, the Vickers microhardness of the thin films decreases slightly with increasing Al content. However, all films exhibit hardness values above 2908 HV 0.05 (30.8 GPa).