Synthesis and characterization of (Ti1-xAlx)B2+Δ thin films from combinatorial magnetron sputtering

(Ti1-xAlx)B2+Δ films with a lateral composition gradient of x = [0.30–0.66] and Δ = [0.07–1.22] were deposited on an Al2O3 wafer by dual magnetron sputtering at 400 °C from sintered TiB2 and AlB2 targets. Composition analysis indicates that higher Ti:Al ratios favor overstoichiometry in B and a reduced incorporation of O. Transmission electron microscopy reveals distinctly different microstructures of Ti- and Al-rich compositions, with formation of characteristic conical growth features for the latter along with a lower degree of crystallinity and significantly less tissue phase from B segregation at the grain boundaries. For Al-rich films, phase separation into Ti- and Al-rich diboride nanometer-size domains is observed and interpreted as surface-initiated spinodal decomposition. The hardness of the films ranges from 14 to 28 GPa, where the higher values were obtained for the Ti-rich regions of the metal boride.