**5. Conclusions**

The importance of the combined evaluation of the scratch tests on thin films was presented. In this combined approach, the standardly used visual observation of the residual groove and the depth change record were correlated with the simultaneous acoustic emission (AE) record. The illustrative examples were presented for the SiC and SiCN thin films deposited on Si substrates.

It was shown that proper analysis of the AE signal and its correlation with standard techniques can significantly increase the informative value of the scratch test. As a result, more reliable conclusions about the material response to external loading can be drawn. The present combined scratch/acoustic emission test evaluation has provided an excellent insight into the physical behaviour of SiC and SiCN films on silicon substrates during the single asperity scratching contact simulated by the diamond indenter.

The AE method provides unique information that enables the elucidation of the dynamics of the material failure with the potential to differentiate between the film and substrate cracking even at nano/micro scale. The use of AE in scratch testing is especially beneficial in the case of nontransparent thin films, where the AE signal can reveal subsurface cracking. Ignoring and/or neglecting the internal cracking may dramatically affect the scratch test results/findings. When they are used as input parameters into finite element modeling then the simulation findings can be distorted. Besides, it was shown that AE always detects critical loads sooner than visual observation.

**Author Contributions:** R.C. and J.T. conceived and designed the experiments, J.T. focused on scratch test experiments—their design, performance, and evaluation. R.C. and M.D. focused on crosschecking of acoustic emission data and advance Hit and Hit's spectra analysis. J.M. performed the FIB milling and SEM. J.T. and R.C. wrote the paper.

**Acknowledgments:** The authors gratefully acknowledge the support by the Operational Programme Research, Development and Education—European Regional Development Fund, project no CZ.02.1.01/0.0/0.0/ 16\_019/0000754 of the Ministry of Education, Youth and Sports of the Czech Republic and the project TH03020245 of the Technology Agency of the Czech Republic, J.T. also acknowledges the Internal Grant of Palacky University (IGA\_PrF\_2018\_009). J.M. acknowledges the support of the project LM2015087 of the Czech Ministry of Education, Youth and Sports.

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