The influence of rotation during sputtering on the stoichiometry of TiAlN/CrNx multilayer coating

Multilayer coatings TiAlN/CrNx were prepared by magnetron sputtering with planetary rotation of samples alongside TiAl and Cr targets. Coatings were investigated by transmission electron microscope (TEM). Thickness of individual layers was 50–80 nm. Chemical composition across TiAlN and CrNx layers was measured by electron energy-loss spectroscopy (EELS) point-by-point analysis. It was found that stoichiometry in CrNx varied strongly while the composition in TiAlN was approximately constant. TEM images revealed ~ 25 nm thick lamellae inside some of the CrNx layers. It was found that lamellae correspond to hexagonal Cr2N phase. Variations of stoichiometry in CrNx and existence of Cr2N phase can be explained by differences in the deposition rate which occur due to rotation of sample. During the deposition process the nitrogen partial pressure was constant, therefore the only variable that could influence stoichiometry was deposition rate. In certain positions of sample the deposition rate was high enough for growth of Cr2N phase. Changes of stoichiometry did not occur in TiAlN indicating that nitrogen partial pressure was high enough for growth of stoichiometric TiAlN layers but not high enough for growth of stoichiometric CrN. A previously developed model, in which we calculated the deposition rate in dependence of trajectory of sample, can explain observed differences in stoichiometry and growth of Cr2N phase.