**4. Conclusions**

This study investigated the correlation between the adhesion strength of cold-sprayed TiO2 on the SUS 304 stainless steel—annealed at temperatures ranging from room temperature to 1000 ◦C. The results of the study lead to the following conclusions:

• The annealing process plays an important role in the induced ductility of the austenitic stainless steel, SUS 304 especially when annealed at a high temperature such as 1000 ◦C. This will lead to a decrease in the hardness of the substrate and will make it softer. When the cold-sprayed TiO2 particle is impacted with a high velocity on the annealed 1000 ◦C SUS 304 surface, the plastic deformation of the substrate occurs and provides a large continuous contact zone between the particles and the substrate, resulting in bonding. Therefore, the adhesion strength of the TiO2 coating is high on the annealed 1000 ◦C SUS 304 substrate.

• The oxide layer of austenitic stainless steel, SUS 304 grows thicker as the annealed temperature of the substrate increases. The TEM/EDX result shows that the existence of the remaining interface of the amorphous layer is approximately 10 nm for the rebound region, R and 15 nm for the bonded region, B between the TiO2 particles and 1000 ◦C annealed SUS 304. Due to the restructuring of the interfacial layer upon a high-velocity particle impact, the adhesion between the brittle TiO2 particle and the stainless steel SUS 304 metal was attributed to a high-velocity collision and resulted in limited amorphization and atomic intermixing. Atomic intermixing of Ti/Fe/Cr at the interface may produce a strong adhesive bond between them due to chemical adhesion forces.

**Author Contributions:** Conceptualization and methodology, N.I.O., S.S., and M.K.; formal analysis, investigation, data curation, and writing—original draft preparation, N.I.O.; writing—review and editing, M.Y.; supervision, T.Y. and M.F.; funding acquisition, M.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research is supported by JSPS KAKENHI, gran<sup>t</sup> number JP17K06857 and was partially carried out at the Cooperative Research Facility Center at Toyohashi University of Technology.

**Acknowledgments:** We also acknowledge the Interface and Surface Fabrication lab, Majlis Amanah Rakyat, and the Technical University of Malacca for Noor Irinah's PhD scholarship.

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