2.1.1. Substrate Pretreatment

PCrNi1MoA steels (10 × <sup>10</sup> × 1 mm<sup>3</sup> in size) were used as substrates. The substrates were mechanically ground using sandpaper from P240 up to P2000 and then polished by diamond suspension from 3 μm to 1 μm. The polished substrates were ultrasonically cleaned in acetone for 15 min and then dried with cold air. The chemical compositions of the alloy steel are shown in Table 1.


**Table 1.** Chemical composition of the alloy steel.

#### 2.1.2. Deposition Device

This experiment adopts DC magnetron sputtering (DCMS) technology and the equipment is single-target magnetron sputtering instrument (JCZK350A, Juzhi Vacuum Equipment Company, China), which mainly includes a deposition chamber, vacuum system, cooling system, gas supply system, heating system, control system, etc. Figure 1 shows a schematic diagram of the magnetron sputtering.

## 2.1.3. Deposition Conditions

The cleaned substrates were installed on a substrate holder and loaded into the chamber. A Ta target (purity: 99.95%) was used with dimensions of Φ50 mm × H4 mm. The substrate-to-target distance was 70 mm. A chamber pressure of below 6.0 × <sup>10</sup>−<sup>4</sup> Pa was reached before coating depositions. In deposition processes, Ar pressure and the deposition time were fixed at 0.5 Pa and 1 h, respectively. The substrate temperature (Tsub) was from 200 ◦C to 400 ◦C and the sputtering power (Pspu) was from 100 W to 175 W. The deposition parameters are listed in Table 2.

**Figure 1.** Schematic diagram of magnetron sputtering.

**Table 2.** Deposition parameters for Ta coatings.


#### *2.2. Characterization*

The crystalline phase of coatings was identified by X-ray diffraction (XRD, Empyrean, PANalytical, the Netherlands) with Cu-K<sup>α</sup> radiation (λ = 0.154 nm) over a 2θ range of 20◦–90◦. The surface and cross-section morphology were characterized using a field emission scanning electron microscope (FESEM, Quanta-250, FEI Company, Hillsboro, OR, USA) at an accelerating voltage of 20 kV. The three-dimensional profiler (ST400, Nanovea company, Irvine, CA, USA) equipped with confocal optical microprobe was used to collect the microscopic morphology, roughness, and flatness in a specific area of the sample surface. The collection range was 1 mm × 1 mm and the collection accuracy was 0.02 μm.

The hardness and elastic modulus of Ta coatings were measured by a nano-indentation tester (TI-980, Bruker, Billerica, MA, USA) at a loading force of 5 mN. Five measures were recorded for each sample to ensure reliable results and all indentation depths were less than 10% of the coating thickness to avoid the effect of the steel substrate.

The qualitative and quantitative adhesive properties of the samples were analyzed in 2 ways. Firstly, using Rockwell hardness tester (HRD-150, Bangyi Precision meter company, China), the loading force of the Rockwell hardness meter was set to 150 kgf, an indentation was pressed on the sample surface, and we observed the indentation under the optical microscope to evaluate qualitatively the adhesion. Secondly, we quantified the adhesion by the scratch tester (MST, Anton Paar, Austria) with a Rockwell C diamond indenter (200 μm of radius). The scratch length, loading rate, and maximum loading force were 5 mm, 25 N/min, and 30 N, respectively. The larger the measured critical load, the better the coating adhesive performance of the sample.
