Transmission electron microscopy of nanocomposite CrB–N thin films

This paper reports on the preparation and characterization of CrBN nanocomposite coatings for low friction, low wear and high thermostability applications. Sputtered CrBN thin films were prepared in order to obtain a composite structure consisting of hard CrB2 and CrN crystallites as well as hexagonal BN lubricant phase by unbalanced magnetron sputtering (UBM) of a CrB2 target in an Ar/N2 gas discharge. Coatings, with a total thickness of 4.5–5.5 μm, were deposited at 450 °C on silicon single-crystal substrates. A nanocomposite structure was obtained by increasing the nitrogen content of the sputtering gas. The coating microstructure was investigated on selected samples by high-resolution transmission electron microscopy. The films were generally found to consist of crystallites of a 1–4 nm size embedded in amorphous matrix. This crystalline phase was identified by electron diffraction as hexagonal CrB2 for low nitrogen content and cubic CrN for high nitrogen content. In the medium composition range, the structure was amorphous, still keeping the two-phase morphology. The use of high-resolution imaging mode helped to reveal the composition of the amorphous phase which seems predominantly to consist of boron nitride.