**3. Results**

#### *3.1. Strength of Adhesion*

The adhesion strength of the cold-sprayed TiO2 coating on annealed SUS 304 stainless steel are shown in Figure 2. The TiO2 coating on the annealed hard substrates showed an increased trend of adhesion strength from room temperature to 1000 ◦C, with values from 0.51 to 2.55 MPa.

Figure 3 shows the fracture coating of TiO2 on annealed SUS 304 from room temperature to 1000 ◦C after adhesion-strength testing. The interface fracture occurred between the coating and substrate for annealed SUS 304 substrates in all conditions, as shown in Figure 3a–e.

**Figure2.**Adhesionstrengthofthe TiO2coatingonannealedSUS304.

**Figure 3.** Fracture surface substrate and TiO2 coating after tensile strength testing on SUS 304. (**a**) Room temperature; (**b**) annealed at 300 ◦C; (**c**) annealed at 500 ◦C; (**d**) annealed at 700 ◦C; and (**e**) annealed at 1000 ◦C.

#### *3.2. SEM Cross-Section Microstructure of TiO2 Coatings on Annealed SUS 304 Substrates*

Figure 4a–e shows the TiO2 coating cross-sectional area on SUS 304 for various substrate annealing temperatures. The figures show, in all conditions, a dense coating with a thickness of 300 μm, indicating that a critical velocity was reached for this hard material. This suggests that the TiO2 coating adhered well to the annealed SUS 304 substrate from room temperature to 1000 ◦C annealing.

**Figure 4.** Cross-section microstructure of TiO2 coatings on SUS 304. (**a**) Room temperature; (**b**) annealed at 300 ◦C; (**c**) annealed at 500 ◦C; (**d**) annealed at 700 ◦C; and (**e**) annealed at 1000 ◦C.

We can categorize the cold-spraying procedure into two stages: (1) Adhesion and (2) cohesion bonding. Adhesion or the formation of the interface between the substrate and particle is the first stage. The annealed substrates can clearly implement this stage, which forms the first coating layer, particularly for the hard material, SUS 304.

#### *3.3. Substrate Vickers Microhardness*

Figure 5 shows the annealing substrate hardness of SUS 304 from room temperature to 1000 ◦C. The stainless steel, SUS 304 showed a decreasing trend from 345.90 Hv for room temperature to 173.00 Hv for 1000 ◦C annealing.

Based on the iron-carbon phase diagram, when the austenite stainless steel, SUS 304 is annealed at 1000 ◦C, which is above the eutectoid temperature of 727 ◦C and slow cooled in the furnace using air medium, the phase transformation involved is austenite to pearlite (ferrite + cementite). This microstructure transformation is associated with a reduction of substrate hardness for 1000 ◦C annealed SUS 304 and it becomes softer [13].

The reduction of the hardness of SUS 304 may be one of the factors that contributed to the trend of the increase in the TiO2 coating adhesion strength with increasing SUS 304 substrate annealing temperature. As the substrate becomes softer at higher annealed temperatures such as 1000 ◦C and when cold sprayed TiO2 impacts the substrate surface with a high impact velocity, it is associated with substrate deformation that may contribute to the bonding. The mechanical anchoring factor is discussed later in Sections 3.5 and 3.6.

**Figure 5.** Annealed substrate microhardness of SUS 304 from room temperature to 1000 ◦C annealing.

#### *3.4. Depth Profile of the Oxide Layer*

The result of the depth analysis of room temperature substrate and annealed 700 ◦C by X-ray photoelectron spectroscopy for the SUS 304 substrate is shown accordingly in Figure 6a,b. The atomic composition of oxygen in the deepest part of the oxide layer increases significantly as the annealing substrate temperature increases from RT to 700 ◦C. This shows that the oxide layer of stainless steel grows thicker as the annealing temperature of the substrate increases. We also expect the oxide layer to be thicker on the annealed 1000 ◦C SUS 304 substrate.

**Figure 6.** Depth profile analysis of SUS 304 stainless steel. (**a**) Room temperature, (**b**) annealed at 700 ◦C.

#### *3.5. FIB Splat TiO2 Particle on 1000* ◦*C Annealed Substrates*

Figure 7 shows the FIB result of the TiO2 particles on the 1000 ◦C annealed SUS 304. This wipe test was conducted to further understand the bonding mechanism of the TiO2 particle on the annealed SUS 304 substrate. Only the 1000 ◦C annealed SUS 304 was selected because it had a high adhesion strength. The results obtained revealed that the TiO2 particle was found unchanged after the collision and the substrate surface of the 1000 ◦C annealed SUS 304 experienced a deformation due to impacting during the cold-spraying process, as shown by Figure 7. A previous study undertaken by Trompetter et al. demonstrated that for a solid particle impacting on a substrate, the substrate hardness played a significant role in the as-produced solid particles [14]. This condition can be understood by the fact that SUS 304 experienced microstructure transformation during the annealing process at 1000 ◦C, therefore the substrates hardness was reduced and it became softer.

**Figure 7.** FIB cross-section of a single particle of TiO2 on 1000 ◦C annealed SUS 304. J indicates the jetted-out region; B is the bonded region; R is the rebound region.

Referring to Figure 7, since the shear instability starts at a position away from the bottom center of the TiO2 particle, the bottom region of the deposited particle can be divided into three regions along the particle–substrate boundary: (i) The particle jetted out region (J) generated by the severe shear plastic strain induced by adiabatic shear instability (ASI); (ii) the well-bonded region (B) where the particle and the substrate are intimately bonded; and (iii) the rebound region (R) where the shear instability did not occur and the accumulated elastic energy from the impact of a sprayed particle detached the particle from the substrate. At the boundary of B and R, ASI is accompanied by severe shear stress, and an abnormal increase in temperature can easily expel the particles, and consequently the oxide covering the surface of particle or substrate can be broken and removed [3,15–20].

The adhesion strength of the TiO2 coating on annealed SUS 304 showed an increased trend as the annealed substrate temperature is increased. This indicates that substrate deformation or mechanical anchoring is one of the factors that influence the adhesion bonding of the annealed SUS 304 with TiO2 at the annealing temperature of 1000 ◦C. This result is supported by other reports of cold-sprayed TiO2 onto hard substrate such as titanium and stainless steel. Schmidt et al. used 0.1–10 μm of TiO2 particles that were cold sprayed onto the flat polished surface of a titanium substrate. They identified that the plastic deformation of the substrate leads to a large continuous contact zone between the particles and the substrate and thus to a durable bonding [8]. Kliemann et al. used 3–50 μm TiO2 agglomerates formed from 5 to 15 nm of primary particles for the continuous coating of steel substrate. They identified ductile substrate that allows shear instability to happen as the primary bonding mechanism between the particles and the substrate [9].

#### *3.6. TEM Analysis on Interface Oxide Layer between TiO2 Particle on 1000* ◦*C Annealed Substrates*

The TEM result is shown as the STEM image interface of the rebound region between single-particle TiO2 and 1000 ◦C annealed SUS 304, as shown by Figure 8. It confirms the existence of the remaining interface layer after the cold-sprayed TiO2 impacted, with a thickness of approximately 10 nm at the rebound region interface, R and 15 nm at the bonded region, B between single-particle TiO2, and 1000 ◦C annealed SUS 304, as revealed in Figures 9 and 10. Kim et al. used kinetic spraying of single titanium particles on mirrored steel substrates. They showed that some portion of a thin amorphous oxide remained between the particle–substrate interface, and even a severe plastic deformation was associated with the impacts of the particles onto the substrate. The remaining oxide provided a bond between a particle or particle–substrate [21]. Our data also reported that the same, even interface thickness in the bonded region B is thicker than in the rebound region, R.

**Figure 8.** STEM of the TiO2/1000 ◦C annealed SUS 304 interface at the rebound region, R.

**Figure 9.** High-magnification images of the TiO2/1000 ◦C annealed SUS 304 interface at the rebound region, R.

**Figure 10.** High-magnification images of the TiO2/1000 ◦C annealed SUS 304 interface at the bonded region, B.

Further analysis of the EDS (JEOL, Tokyo, Japan) on the rebound region interface layer R showed the elemental composition, as shown in Table 3 and Figure 11. The results showed that the oxide layer that occurs at the interface rebound area, R of TiO2 particles and 1000 ◦C annealed SUS 304 consist of 96.33 at% oxygen, 2.35 at% titanium, 0.95 at% iron, and 0.38 at% chromium. In addition, the EDS analysis of the interface layer in the bonded region B reveals the elemental composition, as shown in Table 4 and Figure 12. The results show that the elemental composition was oxygen at 91.88 at%, titanium at 4.22 at%, iron at 2.77 at%, chromium at 0.83 at%, and nitrogen at 0.30 at%. This EDS analysis showed that nitrogen is present in the bonded region B, which is a gas carrier that was used during the cold-spraying process. In addition, at% of titanium, iron, and chromium in the bonded region B was also slightly higher than the rebound region R.

**Table 3.** Chemical composition analysis by EDS for the interface at the rebound region, R of TiO2/1000 ◦C annealed SUS 304.


**Figure 11.** EDX elemental mappings of the TiO2/1000 ◦C annealed SUS 304 interface at the rebound region, R: (**a**) oxygen; (**b**) titanium; (**c**) chromium; (**d**) ferum.

**Table 4.** Chemical composition analysis by EDS for the interface at the bonded region, B of TiO2/1000 ◦C annealed SUS 304.


**Figure 12.** EDX elemental mappings at the TiO2/1000 ◦C annealed SUS 304 interface at the bonded region, B: (**a**) oxygen; (**b**) titanium; (**c**) nitrogen; (**d**) chromium; (**e**) ferum.

Ko et al. cold-sprayed soft Al particles on the hard, but deformable substrate, Fe. They showed that the atomic intermixing of Al/Fe occurring at an amorphous 10 nm oxide-layer interface could produce a strong adhesive bond between Al and Fe due to some of the chemical adhesion forces [2]. They were also cold-sprayed Cu particles on the AIN substrate and Al particles on the ZrO2. TEM images show the formation of a 10 nm-thick amorphous layer at the Cu/AIN interface and approximately 5 nm amorphous layer at the Al/ZrO2 interface. Due to the restructuring of the interfacial layer upon a high-velocity particle impact, the adhesion between the malleable cold-sprayed metals (Cu and Al) on the brittle ceramic (AIN and ZrO2) substrate was attributed to the high-velocity collision, instead of mechanical interlocking—resulting in limited amorphization and atomic intermixing. The degree of this type of restructuring depends on particle velocity, hardness, and mechanical deformability that is different between the particle and substrate [22]. Our experiments are consistent with the result (Ko. et al. 2016 [22]) that TEM images showed the formation of an amorphous layer approximately 10-nm from the rebound area of the interface, R and 15 nm at the bonded area, B. The EDX results confirm the elemental composition of the amorphous layer consisting of atomic mixing of Ti/Fe/Cr. Therefore, atomic intermixing also contributed to the bonding mechanism between the TiO2/SUS 304 substrate due to some chemical adhesion forces. These findings provide considerable progress related to the bonding mechanism of cold-sprayed TiO2 onto annealed SUS 304 at higher temperatures in terms of explaining the increased adhesion strength of the TiO2 coating as the annealing temperature of the substrate also increased. Substrate deformation and atomic intermixing at the amorphous layer at interface TiO2/SUS 304 are the two factors involved here.
