Advances in the characterisation of multilayered coatings using electron energy loss spectroscopy in the transmission electron microscope

The use of electron energy loss spectroscopy with parallel detection (PEELS) to study coatings is illustrated by results from TiN/Ti multilayers deposited by reactive arc evaporation. PEELS shows that the distribution of N close to the interface has a 10–90% width of ∼5 nm (determined by fitting an error function to the experimental data points) despite there being a well-defined sharp crystallographic interface. The crystallographic interface between the cubic TiN and α-Ti structures occurs at a N:Ti ratio of ∼0.2. The near edge fine structure (ELNES) on the N K-edge in TiN changes as the fraction of N decreases, this change being most rapid within 2 nm of the crystallographic interface with the Ti. No further change is seen after crossing into the Ti. In a similar manner, there is a change in the thresholds of the Ti L2,3-edges with decreasing N content in the TiN but not in the Ti. These changes reflect the changing electronic structure of the material. The relationship between mechanical and electronic properties is becoming clearer so that, in the future, this type of information will allow greater insight into coating properties. There are also features on the edge whose energies are determined by the lattice parameter. Thus these features offer a method of probing local strain in these highly strained materials. The TiN/Ti system shows a peak of ∼22 GPa in the hardness for a bi-layer spacing of ∼10 nm. At smaller values of the bi-layer spacing, the TiN layers are substantially sub-stoichiometric throughout their widths and this undoubtedly plays a role in the drop in the hardness for such small values of bi-layer thickness. On annealing, a monolithic layer of Ti2N is formed with a hardness of ∼38 GPa. This is similar to the value of ∼37 GPa found in a monolithic coating of TiN deposited under the same conditions. TiN/NbN multilayers deposited by arc evaporation show hardness values in the range of 40–50 GPa with relatively little variation with deposition conditions or bi-layer thickness. This is in contrast to single crystal and polycrystalline TiN/NbN multilayers prepared by magnetron sputtering.