*3.5. Adhesion*

From actual applications of Ta coatings, the adhesion and integrity of the coatingsubstrate interface under projected loads are key performance concerns and the basis for improving coating performance and extended applications [11].

The adhesive performance was evaluated according to the reference standard. The standard diagram is shown in Figure 9, where grades one to four are acceptable failure and five to six are unacceptable failure. Figure 10 shows the results of the testing of α-Ta coatings under different conditions. The coatings prepared at 200 ◦C-175 W and 300 ◦C-150 W showed cracks and delamination, and the coating prepared at 300 ◦C-150 W peeled off from the substrate more seriously. Therefore, the coating ratings at 200 ◦C-175 W and 300 ◦C-150 W were HF3 and HF4, respectively. The coating prepared at 400 ◦C-100 W showed good adhesion without obvious delamination around the indentation. Compared with Figure 9, the overall coating rating was HF1. The better adhesion of the coating under 400 ◦C-100 W may be related to the dense structure and higher hardness.

According to the above analysis with respect to the morphology, hardness, and indentation test, it is determined that 400 ◦C-100 W is the optimal condition for forming Ta coatings. However, the coating indentation morphology can only qualitatively represent the adhesion of the coating to a certain extent. In this work, a scratch tester was used to quantify the adhesive properties of the Ta coating at 400 ◦C-100 W. The sudden variation of the acoustic emission and friction force is the deciding factor of every scratch mechanism.

The scratch morphologies of the Ta coating at 400 ◦C-100 W are shown in Figure 11a. As the applied load increases, the scratch width of the Ta coating surface also gradually increases, and the scratch area is clearly visible. In the scratch test, Lc1 is the corresponding load when cracks begin to appear and Lc2 is the minimum load for continuous peeling between the coating and the substrate, indicating the failure of the adhesion of the coatingsubstrate system. Usually, Lc2 is used to determine the failure of the coating [35]. From the enlarged image of Lc1 in Figure 11b, the appearance of small cracks can be seen, and the enlarged image of Lc2 in Figure 11c demonstrates that the coating begins to peel off from the substrate at this time, exposing the color of the substrate. According to the micro scratch curves in Figure 11d, it can be seen that Lc1 = 7.38 N and Lc2 = 18.46 N. The larger the Lc1, the stronger the cracking resistance of the coating. The longer the distance between Lc1 and Lc2, the better the coating's resistance to crack propagation [36]. In previous work, Su et al. [32] found that mono-DC Ta coatings reached optimum adhesion compared with mono-PP and multi-PD coatings. The first fracture of mono-DC coating occurred when the load reached ~320 mN because the main diffraction peaks were β-Ta. Furthermore, Ay Ching Hee et al. [37], using DCMS technology, observed that the initiation of cracking in Ta coatings was at 2.5 N and the second critical load was at 18.6 N, with the coating separating from the substrate.

**Figure 9.** Reference standard for the evaluation of adhesion.

**Figure 10.** HRC indentation test of Ta coatings. (**a**) 200 ◦C-175 W, (**b**) 300 ◦C-150 W, and (**c**) 400 ◦C-100 W.

Compared with the studies in other literatures [32,37], the Ta coating prepared in this work has good adhesion as well as strong resistance to cracking.

**Figure 11.** Scratch results of Ta coating prepared at 400 ◦C-100 W (**a**) OM of the scratch, (**b**,**c**) SEM morphology of the selected areas in Figure 4a, and (**d**) the micro scratch curve.

#### **4. Conclusions**

In this study, Ta hard coatings were prepared on PCrNi1MoA steel substrates by direct current magnetron sputtering at various substrate temperatures (Tsub = 200–400 ◦C) and sputtering powers (Pspu = 100–175 W). The basic conclusions are as follows:

The growth and phase evolution were observed to be controlled by Tsub and Pspu simultaneously. Higher Pspu was required in order to obtain α-Ta coatings when the coatings are deposited at lower Tsub, and vice versa. Therefore, Tsub and Pspu interacted with each other and had a combined effect on coating growth. At a Tsub-Pspu at 200 ◦C-175 W, 300 ◦C-150 W, and 400 ◦C-100 W, single-phased and homogeneous α-Ta coatings were obtained.

The α-Ta coating deposited at Tsub-Pspu at 400 ◦C-100 W showed a relatively denser structure, finer grain (60–120 nm, and flatter surface, with the root mean square heights (Sq) of 0.0132 μm.

The α-Ta coating deposited at Tsub-Pspu at 400 ◦C-100 W exhibited a higher hardness (9 GPa), H/E (0.057), and H3/E2 (0.030 GPa), as well as an adhesion of 18.46 N.

**Author Contributions:** Conceptualization, C.L.; methodology, C.L.; software, C.L.; validation, C.L.; formal analysis, C.L.; investigation, C.L.; resources, J.P., Z.X. and Q.S.; data curation, C.L.; writing—original draft preparation, C.L.; writing—review and editing, J.P. and Z.X.; visualization, C.L.; supervision, C.W.; project administration, C.W. and Q.S.; funding acquisition, C.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Guangdong Major Project of Basic and Applied Basic Research (2021B0301030001), National Key R&D Program of China (2021YFB3802300), and Selfinnovation Research Funding Project of Hanjiang Laboratory (HJL202012A001, HJL202012A002, HJL202012A003). And the APC was funded by Guangdong Major Project of Basic and Applied Basic Research (2021B0301030001).

**Data Availability Statement:** The data that support the findings of this study are all own results of the authors.

**Conflicts of Interest:** The authors declare no conflict of interest.
