Initial growth of TiN on different phases of high speed steel

High speed steel is a complex substrate material consisting of various phases including metal carbides such as MC and M₆C. The MC carbides, which mainly consists of VC_0.8, have the same crystal structure as TiN (NaCl B1) and a similar lattice parameter (4.16 Å) to that (4.24 Å) for TiN. Different nucleation and growth modes can thus be expected on the various phases during growth of TiN thin films. For example, on the MC carbides a local epitaxial growth can be expected. Deposition of TiN layers 40–60 nm thick by d.c. magnetron sputtering was carried out onto electrolytically thinned steel substrates for transmission electron microscopy examination. The substrate temperature was varied between 310 and 920 K. Air-exposed substrates were deposited with and without sputter etching. Examination of the as-deposited films shows fine equiaxed grains of TiN on the substrate that was not sputter etched. On the sputter-etched substrate, TiN grew epitaxially on the MC carbides and with fine equiaxed grains on the surrounding steel matrix. The size and number of TiN grains per unit area were the same for the non-sputter-etched substrates and for the steel matrix in the sputter-etched substrates. The carbide grains are randomly oriented and distributed in the steel matrix. Epitaxial growth of TiN is observed on the six most densely packed lattice planes of the MC carbides.