*3.1. Crystalline Phase*

Figure 2 shows XRD patterns of Ta coatings deposited on steel substrates at Tsub from 200 ◦C to 400 ◦C as a function of power, respectively. The relevant texture coefficient (TC) and full width at half maximum (FWHM) are illustrated in the inset.

**Figure 2.** XRD patterns of Ta coatings at different Tsub and Pspu. (**a**) 200 ◦C, (**b**) 300 ◦C, and (**c**) 400 ◦C.

At Tsub = 200 ◦C (Figure 2a), both α-Ta (JCPDF 04-0788) and β-Ta (JCPDF 25-1280) are detected in the coatings at Pspu =100 W, 125 W, and 150 W. When Pspu is further increased to 175 W, all diffraction peaks can be indexed to α-Ta, indicating that single-phased α-Ta coatings can be obtained by adjusting Pspu. To more accurately characterize the change of the orientation degree, we introduce the texture coefficient *TC*(*hkl*), which can be evaluated using the following formula [17,18].

$$TC\_{(hkl)} = \frac{I\_{(hkl)} / I\_{\mathcal{O}(hkl)}}{1 / N \left[ \sum I\_{(hkl)} / I\_{\mathcal{O}(hkl)} \right]}$$

where, *I*(*hkl*) and *I*0(*hkl*) are the measured intensity of the (*hkl*) plane and standard intensity in the JCPDF card, respectively, and N is the number of reflections. The larger the *TC*(*hkl*), the higher the orientation degree of the (*hkl*) plane. The fluctuation of *TC*(110) and *TC*(211) of α-Ta as a function of Pspu is shown in the top-left corner of Figure 2a. It is discovered that the *TC*(110) value decreases while the *TC*(211) value grows with increasing Pspu from 100 W to 175 W. As is shown in Figure 2a, the different crystallization processes of coatings with different thicknesses will affect the crystallization orientation. In the early growth stage, the plane with low surface energy occupies a large coverage area. When the island growth ends, the plane with high surface energy and a high longitudinal growth rate becomes higher and subsequently expands into the surrounding area, contributing to the compression of the plane with low surface energy. In the middle and late growth stages, the proportion of the plane with high surface energy eventually exceeds the plane with low surface energy. Meanwhile, as can be seen in the top-right corner of Figure 2a, the FWHM of (110) and (211) decrease with increasing power, indicating better crystallinity at the higher power [19]. In addition, the FWHM is related to both crystallite size and the stress of the crystallites. These effects can be separated using the Williamson–Hall method [20]. With the increase in sputtering power, the ionization rate of Ar will lead to the increase in deposition rate and more Ta atoms will accumulate near the nucleation point, resulting in an increase in the grain size of the Ta coating. As can be seen from Figure 2a, the diffraction peaks of coatings are getting closer to the standard peak lines with increasing power, meaning reduced stress. Therefore, the increased crystallite size and reduced stress together promote the decrease in FWHM.

At Tsub = 300 ◦C (Figure 2b), the diffraction peaks are all α-Ta at any powers. However, different degrees of wrinkling and peeling from substrates macroscopically were observed at the Pspu of 100 W, 125 W, and 175 W. The highest *TC*(211) and the lowest *TC*(110) were obtained at 150 W. Furthermore, the FWHM at 150 W is lower than that of other powers. As is shown in Figure 2b, at the relatively low power, there is not enough energy to diffuse particles reaching the substrate due to the low kinetic energy, leading to poor adhesion and incomplete crystallization. With the increase in the sputtering power, the crystallization of the coating becomes better. However, when the power is too high, the particles deposited on the substrate cannot diffuse enough and are covered by the newly incident particles because of the fast deposition rate, which is not conducive to the crystallization of the coating. Indeed, high nucleation energy may lead to excessive stress, cracks, and other defects inside the coating.

At Tsub = 400 ◦C (Figure 2c), α-Ta was detected as the pure phase at Pspu = 100 W. When the Pspu was elevated from 125 W to 175 W, the main diffraction peaks were indexed from β-Ta to the mixed phase of α-Ta and β-Ta. The β-Ta phase further grew with crystal planes oriented along (002), which is the densest plane [21], as is shown in Figure 2c. Myers et al. [15] showed that the phase of Ta coating was correlated with its thickness. This conclusion was based on the increase in the substrate temperature with the deposition time, resulting in higher atomic mobility on the coating surface. Gregory Abadias et al. [22] showed that the deposition process parameters did not affect the stability of the crystal phase of α-Ta or β-Ta, indicating that the initial nucleation stage and the energy barrier of the stable α-Ta or β-Ta nucleation were decisive for the phase.

Based on the experimental results in Figure 2, Figure 3 summarizes the evolution of the phase composition of Ta coatings prepared under different Tsub and Pspu conditions. It can be seen that α-Ta is formed under specific temperature and power conditions. During the deposition process of coatings, the effect of temperature and power on the coating structure can be attributed to the surface activity of the substrate and the energy of the Ta particles reaching the substrate. Accordingly, we speculate that the α-Ta formation may be attributed to the deposition energy which needs to be maintained within a certain range. On the grounds of this finding, we have reviewed some of the literature results. Hua Ren et al. [23] reported that the coatings deposited at intermediate bias voltages had pure α-Ta while they showed either mixed α-Ta with β-Ta or pure β-Ta at higher or lower bias voltages. Kazuhide Ino et al. [24] found that the coatings exhibited β-Ta if the bombarding energy was greater than 25 eV. It was confirmed that an appropriate energy input to the deposition of Ta atoms is required to synthesize α-Ta and excessive high energy may be unnecessary or even deleterious to α-Ta formation.

**Figure 3.** Evolution of phase composition at different Tsub and Pspu, the α and β were indicated by and , respectively.

Therefore, Tsub and Pspu have a combined effect on growth and structural evolution of Ta coatings. They may interact with each other to keep a specific deposition energy. For the single-phased and homogeneous α-Ta coatings, the deposition conditions are 200 ◦C-175 W, 300 ◦C-150 W, and 400 ◦C-100 W.
