**4. Conclusions**

Crystal plasticity-based simulations were used to investigate the suppressive effects of coatings on the operation of surface PSBs. The impact of both coating thickness and the applied strain amplitude (reflected by a variable density of PSBs) on the suppressive effects were evaluated. Four characteristics of the PSB–coating interactions—including (1) the suppressive effects of coatings on the operation of PSBs, (2) the normal stress developed in the coating layer along the loading direction, (3) the interfacial normal stress developed perpendicular to the interface, and (4) the interfacial shear stress developed parallel to the interface—were investigated.

The following conclusions can be drawn from this research:


In general, although thicker coatings may be beneficial to the fatigue performance of the coated parts, excessive coating thickness can lead to early crack initiation due to coating tensile fracture and coating–substrate delamination. The beneficial effect of the coatings on fatigue performance therefore is limited by the fracture toughness of the coating, and the adhesion strength of the coating–substrate interfaces. Therefore, the enhancement in the fatigue performance of a coating–substrate system hinges upon the careful selection of the correct coating–substrate material combination as well as the appropriate coating thickness.

**Author Contributions:** Conceptualization, S.S., J.W., and N.S.; methodology, S.S.; validation, M.S.D. and S.S.; formal analysis, M.S.D., S.S., J.W., and N.S.; investigation, M.S.D., S.S., J.W., and N.S.; resources, S.S., N.S., and J.W.; data curation, M.S.D. and S.S.; writing—original draft preparation, S.S.; writing—review and editing, M.S.D., S.S., J.W., and N.S.; visualization, M.S.D. and S.S.; supervision, S.S. and N.S.; project administration, S.S., J.W., and N.S.; funding acquisition, S.S., J.W., and N.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** M.S.D., S.S., and N.S. acknowledge the support by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0019378. J.W. acknowledges the support by the Department of Energy (DOE) Office of Nuclear Energy and Nuclear Energy University Program through Award No. DE-NEUP-18-15703.

**Acknowledgments:** The authors acknowledge the helpful discussions with Wenjin Meng, Michael M. Khonsari, and Shengmin Guo at Louisiana State University.

**Conflicts of Interest:** The authors declare no conflict of interest. Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency FA R&D Special TC NOVEMBER 2017- FF Page 4 of 12 thereof.
