# Effect of Ti-Based Thin Solid Films on Tribological and Mechanical Properties of AL7075-T7351

Authors: Dohyeon Kim; Hyunji Kim; Sunghoon Im; Hyomin Jeong; Jungpil Noh; Sunchul Huh
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

Aluminum alloy itself does not provide sufficient wear resistance for structural parts. Therefore, there is need to protect against the wear issue in practical applications. The objective of this study is to investigate different parameters wear characteristics of direct current (dc) magnetron sputtered titanium films on Al7075-T7351 using a wear test. Each parameter has three levels which include the direct current (DC) power, deposition time, and substrate surface roughness. The thickness of the coatings was measured using a focused ion beam scanning electron microscopy (FIB-SEM) from approximately 0.5μm to 4.4 μm. The titanium thin film coatings were then evaluated using a PD102 wear tester under conditions of 60 rpm, 2N, 30 minutes. As a result, the coefficient of friction of the coating was reduced, and the wear resistance of the coating was improved as the applied power and deposition time increased. The hardness of titanium-coated specimens is increased significantly up to 272 HV, while the hardness of uncoated specimens was 160 HV.


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## Body

## 1. Introduction

Generally, aerospace, automobile parts, and energy-saving strategies have promoted research studies in the field of lightweight materials, especially on alloys based on aluminum. Aluminum has low hardness and does not provide sufficient mechanical strength for structural parts. Therefore, it is necessary to improve surface properties required in practical applications, especially when aluminum is in contact with other parts to protect against wear issues [1–4]. The resistance of the materials can be increased by improving surface conditions such as hardness and roughness [5, 6]. The creation of a titanium coating is one of the most effective methods to improve the wear resistance of a material surface undergoing contact motion. Surface coating technology using titanium thin films with high strength, corrosion resistance, and excellent mechanical properties is widely used to protect materials from external environments [7–13]. Many studies on developing wear resistance thin films for machining applications exist; for example, according to Nguyen et al. [14], the advantage of the titanium nitride coatings on aluminum alloy, which are well known for cutting tools, could be greatly enhanced with improved surface finishing in appearance, corrosion resistance, and wear resistance. Singh et al. showed that Ti-MoS2 films tribological performance is influenced by humidity, temperature, and deposition process and attributed titanium content with the increase in load-bearing capacity, higher wear resistance, and lower humidity [15]. Many of the coatings that can be applied by wet plating, sputtering, and thermal evaporation are used for some physical properties, but the coatings that have importance in tribological systems are comparatively few. Disadvantages of thermal evaporation and wet plating are deposits that may have poor adhesion. Magnetron sputter coating is a basic vacuum coating process, and a target (or cathode) plate is bombarded by energetic ions generated in a glow discharge plasma, situated in front of the target, which is used in this study because of its flexible coating technique that can be used to coat almost any material [16–18]. Magnetron sputtering is deposited by a titanium thin film and typically has crystallographic orientations of crystals (100), (001), and (101) parallel to the surface [19, 20]. The properties of nanoscale thin films are different from those of bulk materials, and the properties of these thin films start from the growth of nuclei of adsorption atoms that reach the substrate. The properties of thin films show different growth patterns depending on the substrate, deposition equipment, and process parameters. Thin films have properties distinguished from those of bulk materials, and they show delicate changes in their properties within a few nanoscales. Changes in the microstructure and surface condition determine this change in properties according to the thin film deposition equipment’s process parameters, which affects the mechanical properties of the thin film material [21–23]. The number of various and independent deposition parameters reliable for optimizing deposited layers is limited. Achieving thin films of desired properties for specific applications is limited due to the lack of suitable methods for sputtering process control parameters. It is difficult for the sputtering to control the film properties such as film thickness, grain size, and step coverage. Many studies on magnetron sputtered Ti thin films have been reported. For example, Martin et al. researched the influence of bias power on some properties of Ti coatings [24], Chawla et al. reported that the average surface roughness of the Ti films has increased with an increase in substrate temperature [25]. However, these studies do not deal with a systematic investigation of the wear characteristic of Ti films in the sputtering process with respect to deposition parameters. The objective of this study is to investigate different parameters wear characteristics of direct current (dc) magnetron sputtered titanium films on Al7075-T7351 using a wear test. Each parameter has three levels which include the direct current (DC) power, deposition time, and substrate surface roughness. The surface roughness was calculated through applying a surface roughness to the base metal, and a Ti thin film was deposited to perform the wear test [26]. The objectives in such cases are to extend the wear resistance performance of the sputtering process beyond which either process can achieve on its own and allow the use of suitable base coating materials in high-performance aerospace applications.


## 2. Experimental Work

### 2.1. Experimental Material

In this study, experimental work was carried out on Al7075-T7351 alloyed specimens with dimensions of 10 mm in length and 32 mm in diameter in accordance with standard ISO 7148 [27]. To investigate wear resistance according to the surface roughness of the aluminum alloys and thin film DC power, working pressure, and deposition time, a total of 8 specimens were prepared. Table 1 shows the physical properties of the substrate. Polishing was performed using #400 abrasive paper and alumina hardener (0.3 um) powder. Table 2 shows the chemical composition of the Al7075-T7351. The roughness values according to the polishing type and data on average values were calculated using a surface roughness tester (model: AR-132C, manufacturer: AMITTARI, China). Table 3 shows that the average roughness value was 0.45 μm and 0.25 μm, respectively.Table 1
Mechanical properties of Al7075-T7351.
Mechanical propertiesTensile strength (MPa)Yield strength (MPa)Elongation (%)Modulus of elasticity (GPa)Hardness, Vickers (Hv)AL7075- T7351462∼505386∼435872155Table 2
Chemical compositions of Al7075-T7351.
AlZnCuMgZrFeSiMnCrTiEtc.Al7075Bal.5.7∼6.72.0∼2.61.9∼2.60.08∼0.150.150.120.10.040.060.15Table 3
Experimental parameters of surface roughness tests.
ParametersConditionsSurface roughness (μm)0.45, 0.25


### 2.2. DC Magnetron Sputtering Process

Ti thin films were fabricated on Al alloy by DC magnetron sputtering system with pure Ti (99.9%) target for the development of titanium coating. Each specimen was washed for 15 minutes with an ultrasonic cleaner (model: SD-300H, manufacturer: SUNGDONG, Korea) using ethyl alcohol. Ultrasonically cleaned specimens were positioned in the coating chamber and loaded with pure titanium target. The distance between the Ti target and Al7075 substrates was 80 mm. Figure1 shows a schematic diagram of the DC magnetron sputtering process. The sputtering device (model: SDC1022A, manufacturer: PSPLASMA, Korea) was composed of a vacuum system, sputtering target, D.C power supply device, substrate material support, and gas injection device. First, Ti was deposited under the following conditions: substrate temperature room temperature, Ar gas flow rate of 20 sccm, initial pressure of 5.0 × 10−6 Torr, working pressure of 2.0 × 10−3 Torr, and 3.0 × 10−3 Torr. The deposition time was 30 min and 90 min, and DC power of the Ti target was 100 W and 300 W. Before starting the actual experiment, the presputtering was performed for 15 minutes to remove the surface oxide of the Ti target. Experimental work was carried out on DC power and deposition time as variables to find optimum conditions for wear resistance of the titanium thin film. Table 4 shows the sputtering conditions for titanium thin film deposition process.Figure 1
Schematic of the DC magnetron sputtering.Table 4
Experimental parameters of DC magnetron sputtering.
Deposition conditionDC magnetron sputteringTarget materialTitaniumBase pressure (Torr)5.0×10−6Working pressure (Torr)2.0×10−3Pressure gasArgonPower (W)100, 300Deposition time (min)30, 90TemperatureRT


### 2.3. Coating Surface Characterization

The coating method was implemented for approximately 30 to 90 min to achieve coating thickness in the range of 0.5–4.5μm. The cross section view and surface changes in the morphology of the Ti films were characterized using a focused ion beam scanning electron microscope (model: AMBER G, manufacturer: TESCAN). In addition, the component analysis was carried out using EDS (energy dispersive spectroscopy). A platinum (Pt) coating was fabricated locally to the specimen surface to prevent damage to the titanium thin film layer’s surface during FIB processing. After FIB processing, the titanium thin film’s cross section was observed by tilting it at an angle of 55 degrees.


### 2.4. Wear Test

The friction and wear tests were conducted using a wear tester (model: PD102, manufacturer: R&B, Korea). The wear tests were conducted at a laboratory condition temperature of 20°C and under dry friction. During the wear test, a zirconia (ZrO2) ball with a diameter of 12.7 mm and a hardness of 1400Hv was used as a counter body. Table 5 shows that the tribological conditions were performed by ball-on disc type and rotation speed of 60 RPM and a normal load of 2 N for 30 minutes. The wear test sliding track diameter was 11.5 mm. The wear test parameter was set to a low-speed rotation to prevent the generation of frictional heat and to set the linear speed to 100 mm/sec or less to prevent the lifting phenomenon. The amount of wear was based on the specimen’s average weight before and after the experiment measured ten times. The wear track was observed using a scanning electron microscope (model: JSM6010LA, manufacturer: JEOL, Japan).Table 5
Experimental parameters of wear tests.
Test parametersTestMaterialsZrO2Diameter (mm)12.7Sliding speed (RPM)60Time (sec)1800Load (N)2TemperatureRT


### 2.5. Hardness Test

The hardness tests were conducted using a nanoindentation tester (model: AIS 3000, manufacturer: Frontics, Korea). The nanoindentation tests were conducted at depth load resolutions of 10µm, which were performed across the point at 2 mm intervals. The maximum applied load and testing speed were 50 kgf and 0.3 mm/min. 10 indentation measurements were made under a controlled load of 0.5 kgf for each measurement. The hardness values we conducted by using a quadrilateral conical indentation tip.


## 2.1. Experimental Material

In this study, experimental work was carried out on Al7075-T7351 alloyed specimens with dimensions of 10 mm in length and 32 mm in diameter in accordance with standard ISO 7148 [27]. To investigate wear resistance according to the surface roughness of the aluminum alloys and thin film DC power, working pressure, and deposition time, a total of 8 specimens were prepared. Table 1 shows the physical properties of the substrate. Polishing was performed using #400 abrasive paper and alumina hardener (0.3 um) powder. Table 2 shows the chemical composition of the Al7075-T7351. The roughness values according to the polishing type and data on average values were calculated using a surface roughness tester (model: AR-132C, manufacturer: AMITTARI, China). Table 3 shows that the average roughness value was 0.45 μm and 0.25 μm, respectively.Table 1
Mechanical properties of Al7075-T7351.
Mechanical propertiesTensile strength (MPa)Yield strength (MPa)Elongation (%)Modulus of elasticity (GPa)Hardness, Vickers (Hv)AL7075- T7351462∼505386∼435872155Table 2
Chemical compositions of Al7075-T7351.
AlZnCuMgZrFeSiMnCrTiEtc.Al7075Bal.5.7∼6.72.0∼2.61.9∼2.60.08∼0.150.150.120.10.040.060.15Table 3
Experimental parameters of surface roughness tests.
ParametersConditionsSurface roughness (μm)0.45, 0.25


## 2.2. DC Magnetron Sputtering Process

Ti thin films were fabricated on Al alloy by DC magnetron sputtering system with pure Ti (99.9%) target for the development of titanium coating. Each specimen was washed for 15 minutes with an ultrasonic cleaner (model: SD-300H, manufacturer: SUNGDONG, Korea) using ethyl alcohol. Ultrasonically cleaned specimens were positioned in the coating chamber and loaded with pure titanium target. The distance between the Ti target and Al7075 substrates was 80 mm. Figure1 shows a schematic diagram of the DC magnetron sputtering process. The sputtering device (model: SDC1022A, manufacturer: PSPLASMA, Korea) was composed of a vacuum system, sputtering target, D.C power supply device, substrate material support, and gas injection device. First, Ti was deposited under the following conditions: substrate temperature room temperature, Ar gas flow rate of 20 sccm, initial pressure of 5.0 × 10−6 Torr, working pressure of 2.0 × 10−3 Torr, and 3.0 × 10−3 Torr. The deposition time was 30 min and 90 min, and DC power of the Ti target was 100 W and 300 W. Before starting the actual experiment, the presputtering was performed for 15 minutes to remove the surface oxide of the Ti target. Experimental work was carried out on DC power and deposition time as variables to find optimum conditions for wear resistance of the titanium thin film. Table 4 shows the sputtering conditions for titanium thin film deposition process.Figure 1
Schematic of the DC magnetron sputtering.Table 4
Experimental parameters of DC magnetron sputtering.
Deposition conditionDC magnetron sputteringTarget materialTitaniumBase pressure (Torr)5.0×10−6Working pressure (Torr)2.0×10−3Pressure gasArgonPower (W)100, 300Deposition time (min)30, 90TemperatureRT


## 2.3. Coating Surface Characterization

The coating method was implemented for approximately 30 to 90 min to achieve coating thickness in the range of 0.5–4.5μm. The cross section view and surface changes in the morphology of the Ti films were characterized using a focused ion beam scanning electron microscope (model: AMBER G, manufacturer: TESCAN). In addition, the component analysis was carried out using EDS (energy dispersive spectroscopy). A platinum (Pt) coating was fabricated locally to the specimen surface to prevent damage to the titanium thin film layer’s surface during FIB processing. After FIB processing, the titanium thin film’s cross section was observed by tilting it at an angle of 55 degrees.


## 2.4. Wear Test

The friction and wear tests were conducted using a wear tester (model: PD102, manufacturer: R&B, Korea). The wear tests were conducted at a laboratory condition temperature of 20°C and under dry friction. During the wear test, a zirconia (ZrO2) ball with a diameter of 12.7 mm and a hardness of 1400Hv was used as a counter body. Table 5 shows that the tribological conditions were performed by ball-on disc type and rotation speed of 60 RPM and a normal load of 2 N for 30 minutes. The wear test sliding track diameter was 11.5 mm. The wear test parameter was set to a low-speed rotation to prevent the generation of frictional heat and to set the linear speed to 100 mm/sec or less to prevent the lifting phenomenon. The amount of wear was based on the specimen’s average weight before and after the experiment measured ten times. The wear track was observed using a scanning electron microscope (model: JSM6010LA, manufacturer: JEOL, Japan).Table 5
Experimental parameters of wear tests.
Test parametersTestMaterialsZrO2Diameter (mm)12.7Sliding speed (RPM)60Time (sec)1800Load (N)2TemperatureRT


## 2.5. Hardness Test

The hardness tests were conducted using a nanoindentation tester (model: AIS 3000, manufacturer: Frontics, Korea). The nanoindentation tests were conducted at depth load resolutions of 10µm, which were performed across the point at 2 mm intervals. The maximum applied load and testing speed were 50 kgf and 0.3 mm/min. 10 indentation measurements were made under a controlled load of 0.5 kgf for each measurement. The hardness values we conducted by using a quadrilateral conical indentation tip.


## 3. Results and Discussion

### 3.1. Characterization of Titanium Thin Films

To evaluate the surface features of Ti thin films, FIB-SEM images were collected at various DC magnetron sputtering processes and different fields of view. Figure2 shows an image of the shape of the thickness measured by FIB-SEM. Figure 2 shows four sets of surface morphologies of Ti films deposited on Al7075 alloy with different sputtering DC power ranging from 100 W to 300 W and deposition time ranging from 30 min to 90 min. The lower part of the interface is an aluminum alloy layer, and titanium thin film was located between the Pt coating layer and the aluminum alloy layer [28]. The thickness of the titanium thin film in Figures 2(a), 2(b), 2(c), and 2(d) was approximately 0.5 μm, 1.6 μm, 1.5 μm, and 4.4 μm, respectively. Figure 3 shows a cross section of the titanium thin film after FIB processing through EDS (energy dispersive spectrometer mapping). Figure 3(a) shows a cross section image before composition analysis. The components were separately graphed into a titanium layer through EDS mapping analysis as shown in Figure 3(b), an aluminum layer as shown in Figure 3(c), and a platinum layer as shown in Figure 3(d). The platinum layer component was got from a platinum coating component that was used to prevent surface damage for internal processing with an ion beam. The component analysis results confirmed that the titanium thin film was uniformly formed on the aluminum alloy’s surface.Figure 2
SEM image of cross-sectional view: (a) 30 min 100 W; (b) 90 min 100 W; (c) 30 min 300 W; (d) 90 min 300 W.
(a)(b)(c)(d)Figure 3
EDS mapping image of the interface: (a) cross-sectional view of sample; (b) Ti layer; (c) Al layer; (d) Pt layer.
(a)(b)(c)(d)


### 3.2. Surface Morphology

Figures4(a)∼4(d) show the surface morphology of the coated titanium thin film on aluminum alloys [(a) deposition time of 30 minutes and applied power of 100 W; (b) deposition time of 90 minutes and applied power of 100 W; (c) deposition time of 30 minutes and applied power of 300 W; (d) deposition time of 90 minutes and applied power of 300 W]. In Figure 4, the fine-grained structure of thin film samples are more explicitly revealed in images (c) and (d). The larger grains in the structure of (c) and (d) films compared with (a) and (b) films could be easily perceived. This phenomenon affects showing rough surface morphology with an increase in the plasma applied power. According to In-Kwon Oh [29], the acceleration of charged electrons or argon ions occurs, and the energy released after collision with a target increased too when plasma applied electric power increases, resulting in the rise of high temperature due to atomic momentum when high-energy titanium collides with the substrate. As a result, it increases the mobility of titanium atoms on the surface, which in turn roughens the surface. As seen in Figures 4(b) and 4(c), the thin film has a similar thickness. However, small and dense shapes were observed in Titanium thin film surface morphology. The developed coatings are characterized by fine particles exhibiting crystallinity, and their size increases when the applied DC power is increased from 100 W to 300 W. The titanium thin film particles’ size was the largest with a deposition time of 90 minutes and applied power of 300 W as shown in Figure 4(d). According to Jin et al. [30], crystallization was not observed at a low sputtering power of 100 W for depositing a titanium thin film, and the microstructure property with 100 W applied power was amorphous. On the other hand, the specimen with 300 W applied power showed spherical hexagonal particles with a uniform distribution. This change in the surface’s shape was attributed to the fact that the particles’ size grew as the applied DC power increased.Figure 4
SEM image of surface morphology (×20000): (a) 30 min 100 W; (b) 90 min 100 W; (c) 30 min 300 W; (d) 90 min 300 W.
(a)(b)(c)(d)


### 3.3. Wear Behavior

Figure5 shows a graph of the friction coefficients according to the deposition time and DC power during the sputtering process. Figure 5(a) is a graph comparing friction coefficients of specimens A0, A1, A2, A3, and A4 polished with #400 abrasive paper. Graphs were compared based on the substrate material. The friction coefficients of A1 and A2 deposited with 100 W were higher than those of the substrate material, while the friction coefficients of A3 and A4 deposited with 300 W were lower than those of the substrate material. The shape of the particles covering the thin film’s surface changed from amorphous particles to crystalline particles as the applied DC power increased during the deposition of the titanium thin film. Wear resistance for crystallized particles was improved because the crystallized particles increased in size and strength. Therefore, the A4 specimen, which had the thickest crystalline particles, showed the best friction coefficient. Figure 5(b) shows the comparison between friction coefficients of specimens B0, B1, B2, B3, and B4 polished with alumina. Figure 5(a) shows that the friction coefficients of B1 and B2 deposited with 100 W showed a similar pattern to those of the substrate material, and friction coefficients of B3 and B4 deposited with 300 W showed a relatively linear curve. However, unlike the A4 specimen, the friction coefficient of the B4 specimen was high from the beginning of the experiment because a thick titanium thin film was deposited on the surface with low surface roughness, resulting in the weakening of the bonding strength with the substrate material.Figure 5
Friction coefficient curves by deposition conditions: (a) A0∼A4; (b) B0∼B4.
(a)(b)Figure6 shows the result of SEM analysis of the wear track formed on the titanium thin film’s surface after the wear test. Wear analysis was performed on only two samples (A4 and B1) using a ball-on-disc wear machine. Figure 6(a) shows the wear track of the A4 specimen with the best wear resistance and a width of 0.79 mm. In the A4 specimen, the titanium thin film shows diminished and shallow wear tracks and small-scale delamination of titanium thin film particles from the aluminum substrate matrix [31]. Such properties display the pattern of grinding wear. Figure 6(b) shows that the track width of the B1 specimen was the worst result at 1.43 mm. The worn-out surface was relatively coarse and had many irregular pits due to the removal and sliding of the titanium thin films particles. Majority of debris was formed in irregular pits due to the thin film whose bonding strength was weakened and peeled off by the wear test, thus indicating an abrasive type of wear mechanism [32]. This is an adhesive wear pattern in which loose debris adheres to the wear track’s edge along the turning radius. The repeated sliding action of the abrasive wear on the B1 sample caused grooves, surface cracks, and ploughed. The displaced materials can be observed in the SEM image shown in Figure 6(b) [33]. Table 6 shows the amount of hardness, the width of the wear track, and the average friction coefficient for each deposition condition of the titanium thin film. The wear amounts of A0 and B0 were 2.67 mg and 2.17 mg, respectively, for two types of substrate materials with different surface roughness, showing a difference of approximately 0.50 mg. It was confirmed that the width of the wear track for B0 was relatively narrow. This indicates that the lower the surface roughness value of the substrate material on which the thin film was not deposited, the better the wear resistance was improved after the wear test. Specimens A1, A3, and A4 processed with #400 abrasive paper showed that the amount of wear was 0.67 ∼ 1.7 mg lower than that of the substrate material. On the other hand, the amount of wear of specimens A2 was similar to that of the substrate material. The amount of wear for the B specimens was lower than that of the substrate material in most cases. However, the amount of wear for the B1 specimen was similar to that of the substrate material. This can be because the comparatively thin film reduces the adhesive force with the substrate and easily peels off [26]. As shown in Figure 6(a), the amount of wear for the A4 specimen can be observed as a minimum according to shallow grooves and narrow wear track width. The amount of wear of the titanium thin film can be adjusted for better stability by increasing applied power and deposition time. It was confirmed that the amount of wear on all the specimens coated with titanium thin film formed was lower than that of the substrate material. These results confirmed that the A4 specimen deposited with a thin film with 300 W applied DC power for 90 minutes after polishing with #400 abrasive paper had the best wear resistance.Figure 6
SEM image of the wear track surface morphology (×30): (a) A4; (b) B1.
(a)(b)Table 6
Results of hardness and wear behavior.
ParametersHardness (Hv)Wear track (mm)Friction coefficientWear loss (mg)SpecimensABCA0000.451601.370.3432.67A130 min100 W0.452721.180.3452.00A290 min100 W0.452511.230.2632.50A330 min300 W0.452481.120.2841.70A490 min300 W0.451730.790.2940.93B0000.251661.340.3382.17B130 min100 W0.251961.430.3972.13B290 min100 W0.252700.930.2331.10B330 min300 W0.252370.850.2041.37B490 min300 W0.251481.060.3611.20


### 3.4. Hardness of the Titanium Thin Films

Nanoindentation measurements indicate that the hardness values of the films are 173 Hv to 272 Hv. The results of the nanoindentation test on titanium thin film confirm the shape change of the non-stress-free film according to the press-in load curve as shown in. Figure7 shows the concave depth of the same load-displacement curve 10 um. This indicates that the loading and unloading curves are all nonlinear. As a result of the measurement, the hardness tends to decrease as DC power increases from 100 W to 300 W as shown in Figure 8. It can be observed that the hardness of the coating is affected by the change in DC power. If the thickness of the film is too small, the size effect will increase and plastic deformation under the nanoindentation will occur because the area is expanded and the hardness of the substrate material affects the measured hardness value [34]. Figure 8 shows traces of nanoindentation press-in by SEM for hardness measurement. Traces of pressing into the shape of diamond press-in can be seen, compressive stress was applied to the film, and peeling of the film layer can be seen. Figure 9 shows that the hardness of Ti deposited parameters tends to be higher than that of the parent material. The hardness of A4 and B4 is low, but it is assumed that the adhesion between the parent material and the thin film is low and the hardness is not improved. Hardness changes due to surface roughness conditions of the parent material have no effect.Figure 7
Nanoindentation force curves by penetration depth.Figure 8
SEM image of the nanoindentation region.Figure 9
Surface hardness curves by deposition conditions.


## 3.1. Characterization of Titanium Thin Films

To evaluate the surface features of Ti thin films, FIB-SEM images were collected at various DC magnetron sputtering processes and different fields of view. Figure2 shows an image of the shape of the thickness measured by FIB-SEM. Figure 2 shows four sets of surface morphologies of Ti films deposited on Al7075 alloy with different sputtering DC power ranging from 100 W to 300 W and deposition time ranging from 30 min to 90 min. The lower part of the interface is an aluminum alloy layer, and titanium thin film was located between the Pt coating layer and the aluminum alloy layer [28]. The thickness of the titanium thin film in Figures 2(a), 2(b), 2(c), and 2(d) was approximately 0.5 μm, 1.6 μm, 1.5 μm, and 4.4 μm, respectively. Figure 3 shows a cross section of the titanium thin film after FIB processing through EDS (energy dispersive spectrometer mapping). Figure 3(a) shows a cross section image before composition analysis. The components were separately graphed into a titanium layer through EDS mapping analysis as shown in Figure 3(b), an aluminum layer as shown in Figure 3(c), and a platinum layer as shown in Figure 3(d). The platinum layer component was got from a platinum coating component that was used to prevent surface damage for internal processing with an ion beam. The component analysis results confirmed that the titanium thin film was uniformly formed on the aluminum alloy’s surface.Figure 2
SEM image of cross-sectional view: (a) 30 min 100 W; (b) 90 min 100 W; (c) 30 min 300 W; (d) 90 min 300 W.
(a)(b)(c)(d)Figure 3
EDS mapping image of the interface: (a) cross-sectional view of sample; (b) Ti layer; (c) Al layer; (d) Pt layer.
(a)(b)(c)(d)


## 3.2. Surface Morphology

Figures4(a)∼4(d) show the surface morphology of the coated titanium thin film on aluminum alloys [(a) deposition time of 30 minutes and applied power of 100 W; (b) deposition time of 90 minutes and applied power of 100 W; (c) deposition time of 30 minutes and applied power of 300 W; (d) deposition time of 90 minutes and applied power of 300 W]. In Figure 4, the fine-grained structure of thin film samples are more explicitly revealed in images (c) and (d). The larger grains in the structure of (c) and (d) films compared with (a) and (b) films could be easily perceived. This phenomenon affects showing rough surface morphology with an increase in the plasma applied power. According to In-Kwon Oh [29], the acceleration of charged electrons or argon ions occurs, and the energy released after collision with a target increased too when plasma applied electric power increases, resulting in the rise of high temperature due to atomic momentum when high-energy titanium collides with the substrate. As a result, it increases the mobility of titanium atoms on the surface, which in turn roughens the surface. As seen in Figures 4(b) and 4(c), the thin film has a similar thickness. However, small and dense shapes were observed in Titanium thin film surface morphology. The developed coatings are characterized by fine particles exhibiting crystallinity, and their size increases when the applied DC power is increased from 100 W to 300 W. The titanium thin film particles’ size was the largest with a deposition time of 90 minutes and applied power of 300 W as shown in Figure 4(d). According to Jin et al. [30], crystallization was not observed at a low sputtering power of 100 W for depositing a titanium thin film, and the microstructure property with 100 W applied power was amorphous. On the other hand, the specimen with 300 W applied power showed spherical hexagonal particles with a uniform distribution. This change in the surface’s shape was attributed to the fact that the particles’ size grew as the applied DC power increased.Figure 4
SEM image of surface morphology (×20000): (a) 30 min 100 W; (b) 90 min 100 W; (c) 30 min 300 W; (d) 90 min 300 W.
(a)(b)(c)(d)


## 3.3. Wear Behavior

Figure5 shows a graph of the friction coefficients according to the deposition time and DC power during the sputtering process. Figure 5(a) is a graph comparing friction coefficients of specimens A0, A1, A2, A3, and A4 polished with #400 abrasive paper. Graphs were compared based on the substrate material. The friction coefficients of A1 and A2 deposited with 100 W were higher than those of the substrate material, while the friction coefficients of A3 and A4 deposited with 300 W were lower than those of the substrate material. The shape of the particles covering the thin film’s surface changed from amorphous particles to crystalline particles as the applied DC power increased during the deposition of the titanium thin film. Wear resistance for crystallized particles was improved because the crystallized particles increased in size and strength. Therefore, the A4 specimen, which had the thickest crystalline particles, showed the best friction coefficient. Figure 5(b) shows the comparison between friction coefficients of specimens B0, B1, B2, B3, and B4 polished with alumina. Figure 5(a) shows that the friction coefficients of B1 and B2 deposited with 100 W showed a similar pattern to those of the substrate material, and friction coefficients of B3 and B4 deposited with 300 W showed a relatively linear curve. However, unlike the A4 specimen, the friction coefficient of the B4 specimen was high from the beginning of the experiment because a thick titanium thin film was deposited on the surface with low surface roughness, resulting in the weakening of the bonding strength with the substrate material.Figure 5
Friction coefficient curves by deposition conditions: (a) A0∼A4; (b) B0∼B4.
(a)(b)Figure6 shows the result of SEM analysis of the wear track formed on the titanium thin film’s surface after the wear test. Wear analysis was performed on only two samples (A4 and B1) using a ball-on-disc wear machine. Figure 6(a) shows the wear track of the A4 specimen with the best wear resistance and a width of 0.79 mm. In the A4 specimen, the titanium thin film shows diminished and shallow wear tracks and small-scale delamination of titanium thin film particles from the aluminum substrate matrix [31]. Such properties display the pattern of grinding wear. Figure 6(b) shows that the track width of the B1 specimen was the worst result at 1.43 mm. The worn-out surface was relatively coarse and had many irregular pits due to the removal and sliding of the titanium thin films particles. Majority of debris was formed in irregular pits due to the thin film whose bonding strength was weakened and peeled off by the wear test, thus indicating an abrasive type of wear mechanism [32]. This is an adhesive wear pattern in which loose debris adheres to the wear track’s edge along the turning radius. The repeated sliding action of the abrasive wear on the B1 sample caused grooves, surface cracks, and ploughed. The displaced materials can be observed in the SEM image shown in Figure 6(b) [33]. Table 6 shows the amount of hardness, the width of the wear track, and the average friction coefficient for each deposition condition of the titanium thin film. The wear amounts of A0 and B0 were 2.67 mg and 2.17 mg, respectively, for two types of substrate materials with different surface roughness, showing a difference of approximately 0.50 mg. It was confirmed that the width of the wear track for B0 was relatively narrow. This indicates that the lower the surface roughness value of the substrate material on which the thin film was not deposited, the better the wear resistance was improved after the wear test. Specimens A1, A3, and A4 processed with #400 abrasive paper showed that the amount of wear was 0.67 ∼ 1.7 mg lower than that of the substrate material. On the other hand, the amount of wear of specimens A2 was similar to that of the substrate material. The amount of wear for the B specimens was lower than that of the substrate material in most cases. However, the amount of wear for the B1 specimen was similar to that of the substrate material. This can be because the comparatively thin film reduces the adhesive force with the substrate and easily peels off [26]. As shown in Figure 6(a), the amount of wear for the A4 specimen can be observed as a minimum according to shallow grooves and narrow wear track width. The amount of wear of the titanium thin film can be adjusted for better stability by increasing applied power and deposition time. It was confirmed that the amount of wear on all the specimens coated with titanium thin film formed was lower than that of the substrate material. These results confirmed that the A4 specimen deposited with a thin film with 300 W applied DC power for 90 minutes after polishing with #400 abrasive paper had the best wear resistance.Figure 6
SEM image of the wear track surface morphology (×30): (a) A4; (b) B1.
(a)(b)Table 6
Results of hardness and wear behavior.
ParametersHardness (Hv)Wear track (mm)Friction coefficientWear loss (mg)SpecimensABCA0000.451601.370.3432.67A130 min100 W0.452721.180.3452.00A290 min100 W0.452511.230.2632.50A330 min300 W0.452481.120.2841.70A490 min300 W0.451730.790.2940.93B0000.251661.340.3382.17B130 min100 W0.251961.430.3972.13B290 min100 W0.252700.930.2331.10B330 min300 W0.252370.850.2041.37B490 min300 W0.251481.060.3611.20


## 3.4. Hardness of the Titanium Thin Films

Nanoindentation measurements indicate that the hardness values of the films are 173 Hv to 272 Hv. The results of the nanoindentation test on titanium thin film confirm the shape change of the non-stress-free film according to the press-in load curve as shown in. Figure7 shows the concave depth of the same load-displacement curve 10 um. This indicates that the loading and unloading curves are all nonlinear. As a result of the measurement, the hardness tends to decrease as DC power increases from 100 W to 300 W as shown in Figure 8. It can be observed that the hardness of the coating is affected by the change in DC power. If the thickness of the film is too small, the size effect will increase and plastic deformation under the nanoindentation will occur because the area is expanded and the hardness of the substrate material affects the measured hardness value [34]. Figure 8 shows traces of nanoindentation press-in by SEM for hardness measurement. Traces of pressing into the shape of diamond press-in can be seen, compressive stress was applied to the film, and peeling of the film layer can be seen. Figure 9 shows that the hardness of Ti deposited parameters tends to be higher than that of the parent material. The hardness of A4 and B4 is low, but it is assumed that the adhesion between the parent material and the thin film is low and the hardness is not improved. Hardness changes due to surface roughness conditions of the parent material have no effect.Figure 7
Nanoindentation force curves by penetration depth.Figure 8
SEM image of the nanoindentation region.Figure 9
Surface hardness curves by deposition conditions.


## 4. Conclusions

In this study, an Al7075-T7351 aluminum material was polished under two roughness conditions: #400 abrasive and alumina, and then sputtering was performed using different applied electric power and deposition time. A wear test was performed on the prepared specimen under such conditions. The surface shape and thickness of the titanium thin film were then compared and analyzed through SEM.(1)
As a result of the titanium thin film thickness measurement, the titanium thin film was measured from approximately 0.5μm to 4.4 μm. The sample polished with #400 abrasive paper and the sample polished with alumina has a roughness of approximately 0.45 μm and 0.25 μm, respectively.(2)
As a result of titanium thin film surface morphology, the rough surface fine-grained structure is observed when direct current power of 300 W is applied. This phenomenon has an effect on the rough surface fine-grained structure showing a rough surface morphology as plasma applied power increases.(3)
As a result of the friction coefficient analysis, the increasing deposition time of the test causes a decrease in friction coefficient value. Observing the wear track indicated that the wear track’s width of specimens deposited with 100 W power was relatively broader than that of specimens deposited with 300 W power, indicating that applied direct current power has a significant effect on the improvement of wear resistance. This indicates that the A4 specimen in which a thin film was deposited with 300 W power for 90 minutes after polishing with #400 abrasive paper had the best wear resistance.(4)
In the titanium coating on Al7075-T7351 alloy by sputtering, to obtain the highest surface hardness for the specific test range 272 HV, the use of 100 W power is recommended. The applied direct current power of 100 W showed a higher hardness of the film than that of 300 W, and a higher hardness was observed when the titanium was deposited compared to the substrate material.



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*Source: 5791263-2022-04-25.xml*