**1. Introduction**

Protective coatings with excellent properties, such as high strength, toughness, good wear, and ablation resistance, are essential materials to ensure long-term, stable, and safe service for key artifacts applied in extreme service environments. The widely used Cr plating has a high melting point and large hardness to mitigate ablation and wear at high temperatures. However, the Cr plating is brittle and often cracks, leading to limited protection of artifacts [1–3]. In this case, a series of new ablation-resistant materials, including alloys and ceramics [4,5], have been developed. Among them, Ta coating is considered to be an alternative material to Cr plating because of its higher melting point, more excellent anti-ablation ability, and better toughness [6,7].

It should be noted that Ta coatings display two crystalline phases with entirely distinct characteristics: the stable α-Ta (Im3m space group, a = 3.304 Å) with a body-centered cubic crystal lattice structure, which has a high melting temperature (2996 ◦C), moderate hardness (8–12 GPa), and good toughness, and the metastable β-Ta (P421m space group, a = 5.313 Å, c = 10.194 Å) with a tetragonal crystal lattice structure, which is harder (18–20 GPa) but brittle and thermally unstable over 700 ◦C [8–10]. Magnetron sputtering is commonly used to prepare Ta coatings. Unfortunately, β-Ta always nucleates priorly using this method, which is unsuitable for protective materials. Therefore, many studies have attempted to regulate the phase formation of Ta coatings by adjusting the sputtering

**Citation:** Liu, C.; Peng, J.; Xu, Z.; Shen, Q.; Wang, C. Combined Effect of Substrate Temperature and Sputtering Power on Phase Evolution and Mechanical Properties of Ta Hard Coatings. *Metals* **2023**, *13*, 583. https://doi.org/10.3390/ met13030583

Academic Editors: Andrea Di Schino and Claudio Testani

Received: 23 February 2023 Revised: 10 March 2023 Accepted: 11 March 2023 Published: 13 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

conditions, including substrate temperature [11], sputtering power [12], pressure [13], and post treatments [14], etc. Nevertheless, investigations on the phase formation in Ta coatings have yielded some conflicting results. For example, some studies [15] reported that a higher substrate temperature exceeding 365–375 ◦C was needed to promote α-Ta formation against β-Ta formation. On the contrary, recent experiments showed that pure α-Ta coatings could be fabricated even at a lower temperature of 200 ◦C [8]. The deposition energy is believed to be the most important factor determining the structural evolution of the coatings. An "energy window" was thus proposed, where α-Ta coatings could be formed between a specific energy range, while other energies higher or lower than this range would be beneficial to β-Ta coatings [16]. For magnetron sputtering, the substrate temperature [11] and sputtering power [12] are typically regarded as two important process variables concerned with the deposition energy, which can affect the surface activity of the substrate and the mobility of deposited Ta particles at the same time. The effects of substrate temperature [11] or sputtering power [12] on the preparation of Ta coatings have been investigated separately; however, their combined effect on the growth and phase evolution of Ta hard coatings has not yet been reported.

In this work, we conducted a comprehensive investigation on the structural evolution of Ta coatings using direct current magnetron sputtering by adjusting the substrate temperature (Tsub) and sputtering power (Pspu) simultaneously. The combined effect of Tsub and Pspu is discussed and the relationship between phase structure, surface morphology, mechanical properties, and the deposition parameters are established, to obtain single-phased α-Ta coatings with a fine structure and good mechanical properties.
