**4. Conclusions**

We have investigated the effect of continuous vs. periodically interrupted plasma exposure during cathodic arc evaporation on the elastic modulus as well as the residual stress state of metastable cubic TiAlN coatings. The elastic modulus of TiAlN grown at floating potential with continuous plasma exposure is 7%–11% larger than for coatings grown with periodically interrupted plasma exposure due to substrate rotation. Excellent agreemen<sup>t</sup> between the experimental and predicted stress-dependent elastic modulus data was obtained. Growth experiments with periodically interrupted plasma exposure due to shutters allowed to infer that the exposure time of the substrate to the plasma is not decisive for the residual stress state. The macroparticle surface coverage exhibits a strong angular dependence, since both density and size of incorporated macroparticles are significantly lower due to

continuous plasma exposure. Employing scanning transmission electron microscopy in combination with energy dispersive X-ray spectroscopy, it was revealed that underdense boundary regions are formed between the matrix and TiN-rich macroparticles and the estimated porosity is in the order of 1%. Hence, based on effective medium theory an elastic modulus reduction of 5%–9% has to be expected. It appears reasonable to assume that these underdense boundary regions enable stress relaxation causing the experimentally determined reduction in elastic modulus as the population of macroparticles is increased.

**Author Contributions:** Conceptualization, M.H. and J.M.S.; Methodology, M.H., D.M., B.S., D.P. and J.R.; Investigation, M.H., L.P., D.M., D.M.H., S.E., V.S., B.S., D.P., B.V., B.W., J.R.; Writing, all the authors; Project Administration, A.O.E., M.A., H.R. and J.M.S.

**Funding:** This research was partly funded by German Research Foundation (DFG, SFB-TR 87/3) "Pulsed high power plasmas for the synthesis of nanostructured functional layers", Jülich Aachen Research Alliance-High Performance Computing (JARA-HPC, project JARA0151), Swedish Research Council for Research Infrastructures (VR-RFI, contract 821-2012-5144) and Swedish Foundation for Strategic Research (SSF, contract RIF14-0053). Parts of this research were carried out at beamline P02.1 of the light source PETRA III at Deutsches Elektronen-Synchrotron (DESY), a member of the Helmholtz Association (HGF).

**Conflicts of Interest:** The authors declare no conflict of interest.
