Characterization of nanostructured multiphase Ti–Al–B–N thin films with extremely small grain size

The microstructure and chemical composition of nanostructured multiphase Ti–Al–B–N films deposited by DC magnetron sputtering of a multiphase composite Ti–Al–B–N target (five phases) in a gaseous mixture of argon and nitrogen were studied by means of Auger electron spectroscopy, X-ray diffraction, conventional and high-resolution transmission electron microscopy and electron energy-loss spectroscopy. The (Ti+Al)/B ratio of films deposited at low nitrogen partial pressure replicates that of the target. At higher values of nitrogen partial pressure, the target is preferentially sputtered. The chemical composition of the films is not influenced significantly by either the substrate temperature or the bias voltage, whereas the phase composition is strongly affected by the PVD process parameters. Evidence for the formation of mixtures of nanocrystalline (Ti,Al)N, TiB2 and h-BN phases in an amorphous matrix is presented. Most of the Ti–Al–B–N films show a grain size in the range 0.6–4 nm. The grain size reduces further to 0.3–1 nm with decreasing N content (TiAl0.2B0.7N0.7 film). This is the smallest grain size determined from TEM investigations which has hitherto been observed, being only one–three unit cells in dimension. Evidence for the ordered structure of grain boundaries in the nanostructured TiAl0.3B0.5N1.9 film is obtained. The amorphous phase forms as individual grains rather than as a thin, intergranular amorphous layer of uniform thickness, even though only a limited number of grain boundaries were studied in detail. The applicability of the equilibrium phase diagram for the prediction of the phase composition in the Ti–Al–B–N films is also discussed.