**1. Introduction**

Cold spraying is a process in which the powder particles are used to form a coating by means of ballistic impingement upon a suitable substrate. Powders range in particle sizes from 5 to 100 μm and are accelerated by injection into a high-velocity stream of gas. The particles are then accelerated by the main nozzle gas flow and impacted on the substrate. Upon impact, the solid particles deform and create a bond with the substrate [1]. When solid particles are sprayed toward a substrate, there are various phenomena that are generally observed on the substrate surface in relation to the process parameters such as substrate hardness, ductility, velocity size, incident angle, etc. However, if the velocity of particles is su fficiently fast, they can be embedded in the surface through a deposition process [2]. Therefore, substrate properties such as hardness, temperature, and degree of oxidation play a significant role in the bonding between particles and substrate [3].

It is well known that cold spraying of ceramic materials can be di fficult because cold spraying requires plastic deformation of the feedstock particles for adhesion to the substrate. The challenge lies in the di fficulty of plastically deforming hard and brittle ceramic materials, such as TiO2. Previous studies have reported the possibility of cold spraying thick pure TiO2 [4] but the bonding mechanism of cold sprayed TiO2 is not fully understood.

Several experimental results have been published on the bonding mechanism of cold-spraying Ti or TiO2 particles onto metal and ceramic substrates. Winnicki et al. used a low-pressure cold-spray system for cold-sprayed amorphous, anatase, and rutile TiO2 powders with a particle size of 10–70 μm and similar shape, which were prepared by the sol-gel process. The 100 μm TiO2 coatings were prepared on aluminum and the mechanism responsible for powder deposition was the mechanical interlocking of submicron powders with a local presence of agglomerates. They also indicate that the key parameter for the process seems to be the working gas temperature [5]. The same author also cold-sprayed amorphous and anatase TiO2 powder on the ABS substrate with varying gas temperatures of 200 and 300 ◦C. The bond strength of the coating was tested using a tensile strength test machine and the highest value was 2 MPa for TiO2 amorphous+anatase at 200 and 300 ◦C gas temperature [6]. Hajipour et al. cold sprayed two types of TiO2 powder; nanocrystal particles with a particle diameter of about 100 ± 15.3 nm and agglomerating ultra-fine particle with a diameter of about 80 ± 11 μm on aluminum substrate. The thickness of the coating is about 490 nm for nanocrystal particles and 15–20 μm for the agglomerating ultra-fine particle. They indicated that for a brittle material such as TiO2, the first layer is achieved by plastic deformation of the ductile metallic substrate when the particles are embedded into the substrate without any additional binding agen<sup>t</sup> or calcination procedure. The coating/substrate interface is relatively rough when the particle hits the substrate at a high speed. As a result, the titanium dioxide particles are embedded in the Al substrate. Roughness causes mechanical entanglement that might also play an important role in the buildup stage. They also conducted a coating bond strength testing by ultrasonic cleanout of 185 W for 1 min. The adhesion was assessed according to the spalling state of the coating. They identified that there was no spalling of the coating after 1 min [7]. Schmidt et al. used 0.1–10 μm of TiO2 particles that were cold sprayed onto the flat polished surface of the 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 durable bonding. They also tested the bond strength of the coating by ultrasonic cleaning with a maximum intensity of 40.8 <sup>W</sup>/cm2. No local changes in the number, positions, and volumes of the particles could be observed after the cleaning cycle, indicating that the bonding of all the TiO2 particles to the substrate resists cleaning up to a maximum intensity of 40.8 <sup>W</sup>/cm<sup>2</sup> in the ultrasonic bath [8]. Kliemann et al. used 3–50 μm TiO2 agglomerates formed from 5 to 15 nm of primary particles for continuous coating of steel, Cu, Ti, and AlMg3 substrates. They identified ductile substrates that allow shear instabilities to happen as the primary bonding mechanism between the particles and the substrate [9]. Gutzmann et al. obtained the impact morphology of single TiO2 particles and studied the deposition of di fferent particles on substrates with di fferent temperatures. They showed that there were concentric rings on the impacted substrates such as the shear instability zone. The deposition of a single TiO2 particle could be achieved only when the substrate temperature was above a certain value. The softer the substrate, the higher the deposition e fficiency. It has been proposed that the preheating of the substrates could make them soft and facilitate the substrate shear instability, thus helping the deposit of the coating [10]. Gardon et al. reported that the mechanism responsible for the deposition of TiO2 on the stainless steel substrate in the cold-spraying process is the chemical bonding between the particles and the substrate. They have shown that the previous layer of titanium sub-oxide prepares the substrate with the appropriate surface roughness needed for the deposition of the TiO2 particle. In addition, the composition of the substrate is also important for deposition as it can provide chemical a ffinities during the particle interaction after impact. Substrate hardness may also ease the interaction between the particles and the substrate [11]. Salim et al. prepared 400 and 150 μm TiO2 coatings on metal and tiles, respectively. They reported that the adhesion strength changed little with the changing spraying parameters. It was discovered that the hardness and the oxidizability of the substrate a ffected the adhesion strength of the TiO2 coatings. The adhesion strength of the TiO2 coatings could be improved by altering the surface chemistry of the substrate. It was proposed that the chemical or physical bonding mechanism can the main bonding mechanism for ceramic coatings [12], and that preheating could increase the oxidizability of the substrate, thus deteriorating the adhesion strength of the coating. The result seems to contradict Gutzmann's experiment, which showed that the preheating of the substrate improved the deposition of the TiO2 particle.

Clearly, there is still considerable uncertainty concerning the preheating of the substrate and the influence of any remaining amorphous oxide layer present at the interface of particle/substrate in relation to the bonding mechanism of cold sprayed TiO2. Therefore, to further understand the bonding mechanism of cold-spraying TiO2 onto metal substrates, in this study, we investigated the correlation between the remaining oxide amorphous phases after cold spraying, and their impacts on the particle/substrate interface toward the adhesion strength of agglomerated nano-TiO2 coatings on annealed metal substrates. Stainless steel (SUS 304) was chosen as the substrates to investigate the bonding mechanism involved and subjected to heat treatment of annealing. An annealing is a heat treatment that changes the physical and chemical properties of a material in order to improve its ductility and reduce its hardness, making it more workable [13]. By increasing the ductility of stainless steel after annealing at higher temperature, we expect better bonding between the TiO2 coating and the SUS 304 annealed stainless steel.

#### **2. Materials and Methods**
