**2. Materials and methods**

#### **2.1 Process**

*Material Flow Analysis*

bonding metallurgically.

particles and the substrate [2].

a comparatively reasonable cost, they are utilized as functional material [2]. The crystalline framework of TiO2 has a major impact upon its photocatalytic performance where TiO2 in anatase stage supplies greater photocatalytic activity than in its rutile stage. At temperatures exceeding 900°C, which is above the melting point of TiO2 (1908°C), the anatase stage irreversibly changes into the rutile stage. At temperatures above the melting point of TiO2, the deposition of molten or semimolten droplets form thermal impossible to avert the phase transformation of TiO2 in thermal spray procedures [2]. Therefore, cold spraying is the best solution

In terms of the mechanism for bonding, numerous works have been conducted in the past 10 years to enhance our understanding of this phenomenon [3–14]. At present, the manifestation at the interface of adiabatic shear instability, (ASI) represents the most accepted bonding-mechanism theory, resulting from the high rate of strain as well as the intense local deforming that occurs through the process of depositing the particles. In the vicinity of adiabatic shear instability occurrence, adiabatic heating-induced thermal softening dominates the hardening through work, and therefore the metals' behavior is similar to viscous material and leads to extrusion from the interface, with the formation of an outward metal jet at the rim [4–5]. The presence of the metal jet with viscosity assists in the cleaning of the native-oxide film that is cracked and originates from the surfaces of particle or substrate, facilitating contact between the metals and therefore the occurrence of metallic bonding [6]. Two mechanisms widely perceived to dominate in terms of metallic bonding in cold spray are interlocking through mechanical means and

It is interesting that cold spraying can also be used to deposited ceramic materials, although initially it appears impossible as cold spraying require plastic deformation to work. Previous studies have reported the possibility of cold spraying thick pure titanium dioxide, TiO2 within range 300 μm [15] but the bonding mechanism of cold sprayed TiO2 is not fully understood. Several experiment result regarding the bonding mechanism of cold spraying TiO2 particles onto metal and ceramic substrates have been published. Toibah et.al reported as-synthesized TiO2 powders that calcined at 200°C and 300°C showed the successful deposition of TiO2 coating on the ceramic tile substrate by the cold spray method. The coating deposition occurred due to tamping effect through slipping of nanoparticles due to high impact during the particle collision. Mechanism responsible for the TiO2 deposition is chemical bonding between TiO2 particles and substrate or among particles during cold sprayed process may lead to coating formation [16]. Kliemann et al. used 3-50 μm TiO2 agglomerates formed from 5 to 15 nm primary particles for the continuous coating on pure titanium, stainless steel, copper and aluminum alloy substrate. They identified ductile substrate that allow shear instability to happen as primary bonding mechanism between the particles and the substrate [17]. Gardon et al. claimed that the mechanism responsible for the TiO2 deposition by the cold spraying process on the stainless-steel substrate is the chemical bonding between the particles and the substrate. They showed that the previous layer of titanium sub-oxide prepares the substrate with the appropriate surface roughness needed for the TiO2 particle deposition. Moreover, the substrate composition is also important for the deposition because it can provide chemical affinities during the particle interaction after impact. The substrate hardness may also ease the interaction between the

Based on the above mention meaningful findings, it shown a lot of factor influenced toward bonding mechanism of cold sprayed TiO2 coating and one of it is substrate surface. Previous studies reported the possibility of cold spraying thick

because it sprays below the material melting temperature.

**40**

In all coating experiments, cold-spraying equipment with a De-Laval 24TC nozzle (CGT KINETIKS 4000; Cold Gas Technology, Ampfing, Germany) was used. Nitrogen was used as the process gas with a 500°C operating temperature, and a 3 MPa pressure. The spray distance was 20 mm, with a process traverse speed of 10 mm/s. The coatings were deposited on grit-blasted annealed pure aluminum (Al 1050). The substrates were annealed with an electric furnace to preheat the grit-blasted substrate to four different temperatures respectively (i.e., 100°C, 200°C, 300°C and 400°C) before spraying. In all cases, the temperature of the annealed substrate during spraying was room temperature.

#### **2.2 Materials**

As a feedstock, we applied agglomerated TiO2 powder (TAYCA Corporation, WP0097) containing a pure anatase crystalline structure with an average particle size of about 7.55 μm. The material chemical composition of substrates used is presented in **Table 1**.

#### **2.3 Characterization**

#### *2.3.1 Tensile-strength testing*

In accordance with JIS H 8402, specimens measuring Ø25 mm × 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). 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 25mmx10mm sample into a hardenable resin. The hardened sample embedded in hardened resin was ground with silica papers to a #3000 grit size and finally polished with 1 μm and 0.3 μm alumina suspension.


**Table 1.**

*Material chemical composition (wt.%).*


**Table 2.**

*XPS parameters for substrate oxide layer analysis.*

## *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). 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 5 tests taken at approximately the same points for each substrate.
