**3.2 SEM cross-section microstructure of TiO2 coatings on annealed Al 1050 substrates**

**Figure 2a**–**e** shows the TiO2 coating cross-sectional area on Al 1050 for various substrate annealing temperatures. Under all condition figures, the figures show a dense coating with a thickness in the range of 200 to 300 μm, indicating that a critical velocity was reached for this soft material. This suggests that the TiO2 coating adhered well to the annealed Al 1050 substrate from room temperature to 400°C annealing.

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 soft material, Al 1050.

### **3.3 Substrate Vickers microhardness**

*Material Flow Analysis*

*2.3.3 Micro-Vickers hardness*

*XPS parameters for substrate oxide layer analysis.*

**Table 2.**

*2.3.4 Substrate oxide evaluation*

*2.3.5 Wipe test*

**3. Results and discussion**

**3.1 Strength of adhesion**

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

**Measured regions Al 2p, O1s,**

Measured X-Ray output [W] 10 Probe diameter[μm] 50 Time per step [ms] 30 Pass energy 140 Cycle 30

X-ray photoelectron spectroscopy (XPS) is a versatile surface analysis technique

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 400°C pure aluminum substrates. The wipe test was conducted to study the deformation behavior of a single particle on this substrate. Prior to deposition, substrate was ground and polished until a mirror finish surface was obtained. The process gas temperature and pressure used were 500°C and 3 MPa, respectively. Nitrogen was used as the 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. An FEI Helios Dual Beam 650 field emission SEM (FESEM) and focused ion beam (FIB) microscope was used to inves-

tigate the single particle TiO2 deposition onto mirror polish annealed substrate.

The adhesion strength of the cold-sprayed TiO2 coating on annealed Al 1050 pure aluminum are shown in **Figure 1**. The TiO2 coating on the annealed soft

used for compositional and chemical state analyses. In this study, XPS analysis (ULVAC-PHI, PHI Quantera SXM-CI) using a monochromatic Al Kα source (15 mA, 10 kV) was performed. Wide (0–1000 eV) and narrow scans of Al 2P and O1s 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

tests taken at approximately the same points for each substrate.

XPS analysis conditions for substrates oxide analysis.

**42**

**Figure 3** shows the annealing substrate hardness of Al 1050 from room temperature to 400°C. The pure aluminum, Al 1050 showed a decreasing trend from 45.36Hv for room temperature to 27.70Hv for 400°C annealing.

**Figure 1.** *Adhesion strength of the TiO2 coating on annealed Al 1050.*

#### **Figure 2.**

*Cross-section microstructure of TiO2 coatings on Al 1050. (a) Room temperature; (b) annealed 100°C; (c) annealed 200°C; (d) annealed 300°C and (e) annealed 400°C.*

**Figure 3.** *Annealed substrate microhardness of Al 1050 from room temperature to 400°C- annealed.*

This trend appears because pure materials experienced recrystallization process at 400°C and slow cooling in the electric furnace; allow the grains to growth larger and reduce the substrate hardness. Based on decrease trend shown by adhesion strength TiO2 coating on annealed pure aluminum, substrate deformation (mechanical anchoring) due to TiO2 particle impacting during cold spraying is not the main factors that influence adhesion bonding between TiO2 and annealed pure aluminum.

#### **3.4 Depth profile of the oxide layer on Al 1050 substrate**

The results of the depth analysis of room temperature substrate and annealed 400°C by X-ray photoelectron spectroscopy for Al 1050 substrates is shown accordingly in **Figure 4**. The composition as a function of depth can be analyzed by in-situ argon ion beam sputtering, found on most surface analytical equipment.

Al 1050 as shown by **Figure 4(a)** and **(b)**, shown that the atomic composition of Oxygen in the deepest part of the oxide layer increases as the annealing substrate temperature increases from RT to 400°C. This indicate that the oxide layer of pure aluminum grows thicker as the annealing substrate temperature is increased. According to W. Ya Li et.al stated that the increasing of oxide film thickness, it will need more kinetic energy to break up and extrude the oxide film, thus a higher particle velocity is needed for bonding. In other words, the effective bonding area is decreased under the same particle impact conditions [18]. It could explain the decreased trend of adhesion strength TiO2 coating on 400°C annealed pure aluminum because the particle velocity was constant in all condition in this experiment.

#### **3.5 Oxide layer composition**

X-Ray photoelectron spectroscopy (XPS) is a versatile surface analysis technique that can be used for compositional and chemical state analysis. In this experiment, total of 30 cycles is involved and to plot binding energy graph versus intensity to study the chemical state changes, outermost surface referring to the 1st cycle data, mid-layer referring to the 15th cycle data and deepest part referring to the 30th cycle data for each substrate conditions.

The peak position of the aluminum oxide layer, Al2O3 on outermost surface at 74.8 eV and aluminum metal at approximately 72.8 eV was prominently present in the mid-layer and deepest part for the room-temperature substrate Al 1050 shown by **Figure 5(a)**. The peak position of the O1s shown by **Figure 5(b)** also support the present of aluminum oxide layer, Al2O3 on outermost surface at 531.3 eV and

**45**

**Figure 4.**

*Influence of Annealed Aluminum Properties on Adhesion Bonding of Cold Sprayed Titanium…*

slightly presented of aluminum in hydroxide on mid-layer and deepest part given by 532.4 eV, supported that substrate mostly Al metal state for mid layer and deepest part at room temperature substrate. Meanwhile, at the annealing temperature of 400°C for Al 1050 substrate, **Figure 6(a)**, the peak position of aluminum oxide layer, which was at approximately 74.8 eV, was detected at the outermost surface, mid-layer, and deepest part of the substrates. O1s supported that by the present of aluminum in hydroxide/oxyhydroxide state at 532.4 eV [19]. This indicate Al 1050 substrate experienced chemical composition changes from Al metal state in room temperature substrate to aluminum in hydroxide condition in 400°C annealed.

**Figure 7** shows the top view of TiO2 particles on 400°C-annealed Al 1050. This wipe test was conducted to further understand the bonding mechanism of TiO2 particle on annealed Al 1050 substrate. Only 400°C-annealed Al 1050 was selected because it had the highest oxide thickness. The results obtained revealed that the TiO2 particle were found unchanged after the collision and the substrate surface of the 400°C-annealed Al 1050 experienced a deformation due to impacting during the cold-spraying process, as shown by **Figure 7**. Soft substrates such as aluminum, the TiO2 particles were impacting the surface with minimal deformation, and the particles rebounded after the impact, leaving craters on the surface of the substrate. This shown by arrow in the **Figure 7**. K.-R. Ernst et al. [20] also mentioned that for soft substrate, impact energy that was generated during the spraying process was also used for deformation of the substrate. Since 400°C

**3.6 FIB splat TiO2 particle on 400°C annealed Al 1050**

*Depth profile analysis of Al 1050 (a) room temperature (b) 400°C annealed.*

*DOI: http://dx.doi.org/10.5772/intechopen.94097*

*Influence of Annealed Aluminum Properties on Adhesion Bonding of Cold Sprayed Titanium… DOI: http://dx.doi.org/10.5772/intechopen.94097*

**Figure 4.** *Depth profile analysis of Al 1050 (a) room temperature (b) 400°C annealed.*

slightly presented of aluminum in hydroxide on mid-layer and deepest part given by 532.4 eV, supported that substrate mostly Al metal state for mid layer and deepest part at room temperature substrate. Meanwhile, at the annealing temperature of 400°C for Al 1050 substrate, **Figure 6(a)**, the peak position of aluminum oxide layer, which was at approximately 74.8 eV, was detected at the outermost surface, mid-layer, and deepest part of the substrates. O1s supported that by the present of aluminum in hydroxide/oxyhydroxide state at 532.4 eV [19]. This indicate Al 1050 substrate experienced chemical composition changes from Al metal state in room temperature substrate to aluminum in hydroxide condition in 400°C annealed.

### **3.6 FIB splat TiO2 particle on 400°C annealed Al 1050**

**Figure 7** shows the top view of TiO2 particles on 400°C-annealed Al 1050. This wipe test was conducted to further understand the bonding mechanism of TiO2 particle on annealed Al 1050 substrate. Only 400°C-annealed Al 1050 was selected because it had the highest oxide thickness. The results obtained revealed that the TiO2 particle were found unchanged after the collision and the substrate surface of the 400°C-annealed Al 1050 experienced a deformation due to impacting during the cold-spraying process, as shown by **Figure 7**. Soft substrates such as aluminum, the TiO2 particles were impacting the surface with minimal deformation, and the particles rebounded after the impact, leaving craters on the surface of the substrate. This shown by arrow in the **Figure 7**. K.-R. Ernst et al. [20] also mentioned that for soft substrate, impact energy that was generated during the spraying process was also used for deformation of the substrate. Since 400°C

*Material Flow Analysis*

aluminum.

**Figure 3.**

**3.5 Oxide layer composition**

cycle data for each substrate conditions.

This trend appears because pure materials experienced recrystallization process at 400°C and slow cooling in the electric furnace; allow the grains to growth larger and reduce the substrate hardness. Based on decrease trend shown by adhesion strength TiO2 coating on annealed pure aluminum, substrate deformation (mechanical anchoring) due to TiO2 particle impacting during cold spraying is not the main factors that influence adhesion bonding between TiO2 and annealed pure

*Annealed substrate microhardness of Al 1050 from room temperature to 400°C- annealed.*

The results of the depth analysis of room temperature substrate and annealed

accordingly in **Figure 4**. The composition as a function of depth can be analyzed by in-situ argon ion beam sputtering, found on most surface analytical equipment. Al 1050 as shown by **Figure 4(a)** and **(b)**, shown that the atomic composition of Oxygen in the deepest part of the oxide layer increases as the annealing substrate temperature increases from RT to 400°C. This indicate that the oxide layer of pure aluminum grows thicker as the annealing substrate temperature is increased. According to W. Ya Li et.al stated that the increasing of oxide film thickness, it will need more kinetic energy to break up and extrude the oxide film, thus a higher particle velocity is needed for bonding. In other words, the effective bonding area is decreased under the same particle impact conditions [18]. It could explain the decreased trend of adhesion strength TiO2 coating on 400°C annealed pure aluminum because the particle velocity was constant in all condition in this experiment.

X-Ray photoelectron spectroscopy (XPS) is a versatile surface analysis technique that can be used for compositional and chemical state analysis. In this experiment, total of 30 cycles is involved and to plot binding energy graph versus intensity to study the chemical state changes, outermost surface referring to the 1st cycle data, mid-layer referring to the 15th cycle data and deepest part referring to the 30th

The peak position of the aluminum oxide layer, Al2O3 on outermost surface at 74.8 eV and aluminum metal at approximately 72.8 eV was prominently present in the mid-layer and deepest part for the room-temperature substrate Al 1050 shown by **Figure 5(a)**. The peak position of the O1s shown by **Figure 5(b)** also support the present of aluminum oxide layer, Al2O3 on outermost surface at 531.3 eV and

400°C by X-ray photoelectron spectroscopy for Al 1050 substrates is shown

**3.4 Depth profile of the oxide layer on Al 1050 substrate**

**44**

**Figure 5.** *XPS spectra of Al 2P for (a) room temperature and O1s for (b) room temperature.*

annealed aluminum has lower hardness due to recrystallization, the depth of the craters was deeper and experienced heavy damage, as seen in the SEM image in **Figure 7**.

**Figure 8** shown a cross-section of a single particle of TiO2 on 400°C annealed 1050. Referring to **Figure 8**, since the shear instability starts at a position away from the bottom center of a TiO2 particle, the bottom region of such a 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 increases in temperature can easily expel the particles, and consequently the oxide covering the surface of particle or substrate can be broken and removed [3, 4, 11, 21–24].

The adhesion strength of the TiO2 coating on annealed Al 1050, showed a decreased trend as the annealed substrate temperature is increased from room temperature to 400°C annealed. This indicate that substrate deformation or mechanical anchoring is not one of factors that influence the adhesion bonding for annealed Al 1050 with TiO2 at an annealing temperature of 400°C. Referring to arrow indicate in **Figure 8**, there were a gap observe between deposited TiO2 particle and 400°C annealed 1050 substrate at rebound, R and bonded, B region. This result indicate that even severe plastic deformation is occurred due to TiO2 impacting during cold spraying, thin amorphous oxide will remain between particle-substrate [25].

**47**

**Figure 7.**

*Top view of TiO2 particles on 400C annealed Al 1050.*

**Figure 6.**

*Influence of Annealed Aluminum Properties on Adhesion Bonding of Cold Sprayed Titanium…*

Moreover, due to thicker oxide on 400°C annealed Al 1050 or the fresh surfaces of metallic such as aluminum may be able to be re-oxidized due to their high reactivity with oxygen in air during deformation [25], it prevent bonding to form between deposited TiO2 particle and deform substrate surface of 400°C annealed Al 1050. This may explain that bonding mechanism involved between TiO2 coating and

*XPS spectra of Al 2P for (a) 400°C annealed and O1s for (b) 400°C annealed.*

*DOI: http://dx.doi.org/10.5772/intechopen.94097*

*Influence of Annealed Aluminum Properties on Adhesion Bonding of Cold Sprayed Titanium… DOI: http://dx.doi.org/10.5772/intechopen.94097*

#### **Figure 6.**

*Material Flow Analysis*

annealed aluminum has lower hardness due to recrystallization, the depth of the craters was deeper and experienced heavy damage, as seen in the SEM image in

*XPS spectra of Al 2P for (a) room temperature and O1s for (b) room temperature.*

**Figure 8** shown a cross-section of a single particle of TiO2 on 400°C annealed 1050. Referring to **Figure 8**, since the shear instability starts at a position away from the bottom center of a TiO2 particle, the bottom region of such a 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 increases in temperature can easily expel the particles, and consequently the oxide covering the

surface of particle or substrate can be broken and removed [3, 4, 11, 21–24]. The adhesion strength of the TiO2 coating on annealed Al 1050, showed a decreased trend as the annealed substrate temperature is increased from room temperature to 400°C annealed. This indicate that substrate deformation or mechanical anchoring is not one of factors that influence the adhesion bonding for annealed Al 1050 with TiO2 at an annealing temperature of 400°C. Referring to arrow indicate in **Figure 8**, there were a gap observe between deposited TiO2 particle and 400°C annealed 1050 substrate at rebound, R and bonded, B region. This result indicate that even severe plastic deformation is occurred due to TiO2 impacting during cold spraying, thin amorphous oxide will remain between particle-substrate [25].

**46**

**Figure 7**.

**Figure 5.**

*XPS spectra of Al 2P for (a) 400°C annealed and O1s for (b) 400°C annealed.*

#### **Figure 7.**

*Top view of TiO2 particles on 400C annealed Al 1050.*

Moreover, due to thicker oxide on 400°C annealed Al 1050 or the fresh surfaces of metallic such as aluminum may be able to be re-oxidized due to their high reactivity with oxygen in air during deformation [25], it prevent bonding to form between deposited TiO2 particle and deform substrate surface of 400°C annealed Al 1050. This may explain that bonding mechanism involved between TiO2 coating and

#### **Figure 8.**

*FIB cross-section of a single particle of TiO2 on 400°C-annealed Al 1050. J indicates the jetted-out region; B is the bonded region; R is the rebound region.*

400°C annealed 1050 is newly formed substrate surface that free from oxide with TiO2 particle which is similar with the cold spraying of metallic materials that known as metallurgical bonding.
