*2.3. Characterization*

## 2.3.1. Tensile-Strength Testing

In accordance with JIS H 8402, specimens measuring Ø25 × 10 mm were used to assess the coatings' adhesion strength, given as the fracture load value measured by a universal testing machine (Autograph AGS-J Series 10 kN, Shimadzu, Japan). We measured the adhesion strength over an average of five specimens for each of the spraying conditions.

## 2.3.2. Coatings Evaluation

A scanning electron microscope (SEM: JSM-6390, JEOL, Tokyo, Japan) was used to observe the TiO2 coating's cross-sectional microstructures on annealed substrates. The observation sample of the TiO2 coating was prepared by embedding a 25 mm × 10 mm sample into a hardenable resin. The hardened sample embedded in the hardened resin was ground with silica papers to a #3000 grit size and finally polished with 1 and 0.3 μm alumina suspension.

#### 2.3.3. Micro-Vickers Hardness

To investigate the relationship between the annealed substrate surface hardness and the adhesion strength of the TiO2 coating on the annealed substrate, the substrate hardness was measured using an HMV-G micro-Vickers hardness tester (Shimadzu, Japan). The measurement showed a hardness of HV 0.1; the test load on the cross section was 98.07 mN. The final micro-hardness value was the average of five tests taken at approximately the same points for each substrate.

#### 2.3.4. Substrate Oxide Evaluations

X-ray photoelectron spectroscopy (XPS) is a versatile surface analysis technique used for compositional and chemical state analyses. In this study, XPS analysis (ULVAC-PHI, PHI Quantera SXM-CI, Kanagawa, Japan) using a monochromatic Al Kα source (15 mA, 10 kV) was performed. Wide (0–1000 eV) and narrow scans of Fe 2*p*, Cr 2*p*, and O 1*s* for different annealed substrates were collected. The measured binding energies were then corrected with C 1*s* at 285.0 eV. When pre-sputtering to clean the surface was performed, the sample surface was reduced and the measurements were affected, so XPS analysis was performed without pre-sputtering. Table 2 shows the XPS analysis conditions for substrates oxide analysis.


**Table 2.** XPS parameter for substrate oxide layer analysis.

## 2.3.5. Wipe Test

A CGT Kinetiks 4000 cold-spray system (Cold Gas Technology, Ampfing, Germany) with a custom-made suction nozzle was used to perform the wipe test and coating using TiO2 powder onto the annealed 1000 ◦C stainless steel substrates. The wipe test was conducted to study the deformation behavior of a single particle on this substrate. Prior to deposition, the substrate was ground and polished until a mirror finish surface was obtained. The temperature of the process gas and the pressure used were 500 ◦C and 3 MPa, respectively. Nitrogen has been used as a process gas. The distance between the exit of the nozzle and the substrate was fixed at 20 mm. The traverse speed of the process was 2000 mm/s. Prior to spraying, the substrates were rinsed with acetone. The FEI Helios Dual

Beam 650 field emission SEM (FESEM, FEI, Oregon, USA) and focused ion beam (FIB, FEI, Oregon, USA) microscope was used to investigate the single particle TiO2 deposition on mirror polished annealed substrate.
