*3.2. Surface Morphology*

Figure 4 shows the surface morphology and grain distribution histogram of the Ta coatings deposited at Tsub-Pspu of 200 ◦C-175 W (Figure 4a,d), 300 ◦C-150 W (Figure 4b,e), and 400 ◦C-100 W (Figure 4c,f), respectively. The size distribution histograms were obtained by measuring more than 100 grains from the FESEM images.

As can be seen from the photographs, the surface morphology of single-phased α-Ta coatings deposited in these different conditions are all characterized by triangular pyramid-shaped particles with clear grain gaps, consistent with results published by M. Grosser et al. [25]. According to the Thornton structural zone model, the deposition pattern of Ta coating is the Z1 structure. This indicates that the diffusion distance of the particles is very small, and the initial nucleus tends to capture the orientation of the atoms on the coating surface at T/Tm < 0.1. While the substrate temperature increases from 200 ◦C–400 ◦C (0.1 < T/Tm < 0.3), the diffusion kinetic energy of surface adsorbed atoms increases, leading to increasing mobility of adsorbed atoms. The initially adsorbed atoms rapidly self-diffuse towards the equilibrium position, eventually trapped by larger grains to form the pyramid-shaped structure. The grain sizes of Ta coatings prepared at

200 ◦C-175 W and 300 ◦C-150 W range from 120 nm to 240 nm, while the grains of the sample prepared at 400 ◦C-100 W are smaller, within 60–120 nm, the surface of which is also denser than that in the other two conditions.

**Figure 4.** FESEM images of surface morphology and grain size of the Ta coatings. (**a**,**d**) 200 ◦C-175 W, (**b**,**e**) 300 ◦C-150 W, and (**c**,**f**) 400 ◦C-100 W.

Figure 5 shows the 3D profiles of Ta coatings in different deposition conditions. Compared with the microstructural images obtained through SEM observation, the 3D profile contains a larger observation interval which is more suitable for observing the overall morphology distribution of millimeter-level samples. The surface height data of the Ta coating under different conditions were obtained according to the 3D profiles, as is listed in Table 3. Root mean square heights (Sq) of Ta coatings at 200 ◦C-175 W, 300 ◦C-150 W, and 400 ◦C-100 W were 0.0402 μm, 0.0184 μm, and 0.0132 μm, respectively. Arithmetic mean heights (Sa) of Ta coatings at 200 ◦C-175 W, 300 ◦C-150 W, and 400 ◦C-100 W were 0.0319 μm, 0.0145 μm, and 0.0103 μm, respectively. The results indicate that the surface of the Ta coating at 400 ◦C-100 W is flatter, while the roughness of the Ta coating at 200 ◦C-175 W is higher. According to the histogram results of the grain distribution in Figure 4, the coating grain size distributions at 400 ◦C-100 W and 300 ◦C-150 W have a concentrated grain size distribution, while the coatings at 200 ◦C-175 W are quite distributed between 130 nm and 170 nm, resulting in higher roughness.

**Figure 5.** The 3D profile of the Ta coatings. (**a**) 200 ◦C-175 W, (**b**) 300 ◦C-150 W, and (**c**) 400 ◦C-100 W.


**Table 3.** Height parameters of the Ta coatings in different conditions.

## *3.3. Cross-Sectional Morphology*

Figure 6 shows the cross-sectional morphology of the Ta coatings deposited at Tsub-Pspu of 200 ◦C-175 W, 300 ◦C-150 W, and 400 ◦C-100 W.

**Figure 6.** FESEM images of cross-sections of Ta coatings. (**a**) 200 ◦C-175 W, (**b**) 300 ◦C-150 W, and (**c**) 400 ◦C-100 W.

The coatings exhibit a typical columnar crystal growth pattern mainly affected by the shadowing effect. The anisotropy of columnar crystal growth leads to a large competitive advantage for those crystals growing perpendicular to the interface, and columnar crystals in other directions are submerged as the coating thickens. In addition, the thicknesses of Ta coatings are approximately 3.78 μm, 3.34 μm, and 1.87 μm, respectively. When the sputtering power increases, the sputtered Ta atoms from the target increase, resulting in a high sputtering rate. However, according to the theory proposed by Kim et al. [26], a lower deposition rate enables atoms to move through the crystals across larger regions, and atoms are unlikely to be covered by subsequent adatom flux before being dampened in a particular location, potentially improving the mechanical properties of materials.
