A Novel (AlCrNbTaTi)N Multilayer Hard High-Entropy Alloy Nitride Coating with Variable Aluminum Content Deposited by Cathodic Arc Ion Plating

Abstract

Traditional binary coatings like TiN and CrN display limited thermal stability and wear resistance under extreme conditions. High-entropy alloy nitride (HEAN) coatings offer a promising solution due to their customizable composition and unique properties, including high hardness, corrosion resistance, and thermal stability. This study focused on (AlCrNbTaTi)N HEAN coatings to address a critical need for materials capable of enduring extreme mechanical and tribological demands by examining the impact of aluminum content on their structural and mechanical properties, providing insights for optimizing coatings in harsh conditions through a self-assembled nanolayer structure with enhanced resilience and performance. The coatings were deposited via a cathodic arc by employing an AlCrNbTaTi alloy target composed of aluminum (20, 50, 60, 70%) and equal molar ratios of Cr, Nb, Ta, and Ti. The coatings were characterized through grazing incidence X-ray diffraction, SEM, HR-TEM, a nano-indentation test, and a friction and wear test. The results indicated that with increasing Al content, the structure of (AlCrNbTaTi)N coatings shifted from FCC to an amorphous state, leading to a reduction in the hardness and elastic modulus, accompanied by an increase in the wear rate and friction coefficient. The (AlCrNbTaTi)N coating, with an equal atomic ratio of metallic elements, showed potential as a hard tool coating. It demonstrated outstanding mechanical and tribological properties, with a 34.5 GPa hardness, 369 GPa modulus, 0.35 friction coefficient, and 8.2 × 10⁻¹⁹ m²·N⁻¹ wear rate. The findings highlight the potential of (AlCrNbTaTi)N coatings to extend tool life and improve operational efficiency, helping advance materials engineering for industrial applications.

1. Introduction

Surface coatings have been significant in protecting traditional tools over the past few years. Binary coatings, involving CrN [1] and TiN [2], are often employed to safeguard tools because of their outstanding chemical stability and enhanced wear resistance characteristics. For instance, the considerable heat produced in the cutting process could remarkably influence tool efficiency, leading to an enhanced need for tool coatings with improved heat resistance. Aluminum (Al) can significantly improve binary coatings’ thermal characteristics and protect them from additional oxidation by forming a thick oxide layer on their surface. Thus, coatings including AlCrN [3,4] and AlTiN [5], with improved hardness, outstanding resistance to elevated-temperature oxidation, and thermal stability [6,7,8], are well suited for high-speed, high-temperature applications. Furthermore, previous research [9] has demonstrated that as long as AlTiN and AlCrN coatings retain their face-centered cubic (FCC) structure, their hardness increases with rising Al content. Furthermore, it has been reported that the Al content in AlTiN and AlCrN may reach up to 67 to 70% [10].

Based on the above-described facts, Yeh et al. introduced the concept of high-entropy alloys (HEAs) [11], protective nitride coatings based on HEAs have been widely investigated due to their outstanding oxidation resistance, thermal stability, and impressive hardness. This evolution in coating technology aligns with the need for materials that can withstand increasingly demanding applications. Although HEANs deposited with equimolar metals, involving (AlCrMoSiTi)N [12,13], (TiAlCrSiV)N [14], (AlCrTaTiZr)N [15], (AlCrNbSiTiV)N [16,17], and (AlCrTaTiZr)–Six–N [18] demonstrated high configurational entropy, it has been suggested that their performance can be further optimized by adjusting the elemental ratios in the target composition. The development of non-equimolar coatings, such as (AlCrNbSiTi)N, offers improved oxidation resistance due to increased Al and Cr concentrations [19], highlighting a pathway for tailoring coatings to meet specific application demands.

In light of these evolving industry requirements, this study focused on synthesizing (AlCrNbTaTi)N high-entropy alloy nitride (HEAN) coatings via cathodic arc ion plating, aiming to create a hard, protective coating with enhanced mechanical strength and tribological performance for high-temperature applications. To examine the effects of aluminum content on the structure and mechanical characteristics of these coatings, a series of coatings with varying aluminum ratios were systematically characterized through grazing incidence X-ray diffraction, scanning electron microscopy, nano-indentation testing, and wear testing. This study hypothesized that increasing aluminum content would influence phase stability and hardness, and the findings revealed a transition from FCC to an amorphous structure, correlating with a reduction in the hardness and elastic modulus and an increase in the wear rate and friction. This analysis provided a comprehensive view of how aluminum content affects coating performance, offering insights for optimizing HEAN coatings for extreme conditions. To improve the adhesion, structural stability, and overall performance of the (AlCrNbTaTi)N coatings, a CrN sublayer was incorporated as an interlayer. This sublayer plays a pivotal role in mitigating stress, providing a robust foundation and contributing to the wear resistance of the composite coating system.

2. Materials and Methods

2.1. Preparation of (AlCrNbTaTi)N Coatings

The (AlCrNbTaTi)N coatings were fabricated on silicon (100) substrates using cathodic arc ion plating with a Hauzer Flexicoat^® 850 system, which includes a vacuum chamber to facilitate the deposition process. Chromium targets (99.95% purity) and an AlCrNbTaTi alloy were used as cathodes. The AlCrNbTaTi alloy target was composed of Al at varying atomic percentages (20, 50, 60, 70 at. %), along with equal molar ratios of Cr, Nb, Ta, and Ti. The target-to-substrate distance was 175 mm. The coating process involved heating, plasma etching, CrN interlayer deposition, (AlCrN-bTaTi)N layer deposition, and cooling. The substrates were directly heated in the vacuum chamber via a controlled heating system, maintaining a substrate temperature of 450 °C during the deposition. This temperature was optimized to enhance deposition rate and improve coating adhesion and quality. The background pressure in the chamber was maintained at 5.0 × 10⁻³ Pa. During plasma etching, the substrate surface was treated with Ar-H2 plasma for 30 min. Notably, the deposition process included a CrN sublayer, which was designed to enhance adhesion between the substrate and the main coating layer. This sublayer also served to distribute residual stresses and provided a stable base for the subsequent (AlCrNbTaTi)N deposition, thereby improving the coating’s overall durability and wear resistance. The deposition parameters for the CrN and (AlCrNbTaTi)N layers are detailed in

Table 1. Process parameters for the deposition of CrN interlayer and (AlCrNbTaTi)N layer.

Process Parameters	Unit	CrN	(AlCrNbTaTi)N
Pressure	[Pa]	0.7	2.0
Arc Current	[A]	80	100
Bias Voltage	[V]	−200	−100
Time	[s]	1200	3000

.

2.2. Characterization

The thickness and cross-sectional morphology of the coatings were evaluated through a scanning electron microscope (SEM, JSM 7500F, JEOL, Tokyo, Japan). The phase structure was characterized using a grazing incidence X-ray diffractometer (GIXRD, Empyrean, PANalytical, Almelo, The Netherlands). Micro-hardness measurements were conducted with a nanoindenter (Hysitron TI950, Bruker, Billerica, MA, USA) in Nano Dynamic Mechanical Analysis mode, ensuring accuracy by testing each sample at six different locations. The wear resistance was assessed while employing a ball–disk reciprocating friction instrument (CSEM, Neuchâtel, Switzerland) under conditions with a temperature of (12 ± 1) °C and 55% relative humidity. The grinding component consisted of a GCr15 ball with a diameter (6 mm), applying 0.5 N load, a 3.77 cm/s sliding speed, and a 1000 RPM frequency over a reciprocating stroke of 40 m. Wear marks were examined for depth and morphology through a step meter (Surface Profilomer XP-2, Ambios, Santa Cruz, CA, USA) and an optical microscope (MX6R, Sunny Optical Technology, Yuyao, China). Spherical aberration-corrected transmission electron microscope (ACTEM, Titan Themis 80-300, FEI, Hillsboro, OR, USA) was employed to analyze the microstructure. Line scan analyses of samples were conducted with energy-dispersive X-ray (EDX, 4-quadrant) spectroscopy at 300 kV, involving a 0.2 nm point-to-point resolution and a 0.06 nm maximum resolution in high-angle annular dark field (HAADF) high-resolution scanning transmission electron microscopy (STEM). Selected area diffraction (SAD) was employed to analyze the local crystal framework. Cross-sectional TEM images were produced using a focused ion beam (FIB) system (HELIOS Nano Lab 600i, FEI, Hillsboro, OR, USA).

3. Results and Discussion

SEM analysis was employed to provide an initial assessment of the morphology and structure of the (AlCrNbTaTi)N coating. The cross-sectional SEM images of the (AlCrNbTaTi)N coatings at varying Al contents (20%, 50%, 60%, and 70%) are presented in Figure 1. According to Figure 1, the coating comprised a two-layer structure: a CrN interlayer measuring approximately 0.3–0.5 μm in thickness and a sublayer of (AlCrNbTaTi)N with a thickness of about 1.3–1.6 μm. The CrN interlayer had a columnar grain structure, while the (AlCrNbTaTi)N sublayer appeared very dense and lacked a distinct columnar formation. The SEM analysis revealed a CrN interlayer with a columnar grain structure, which not only facilitated strong adhesion to the substrate, but also acted as a stress-relief layer. This structural feature supports the dense (AlCrNbTaTi)N layer above, ensuring enhanced mechanical stability and reduced delamination risks.

Moreover, GIXRD was employed to analyze the phase structures of the (AlCrNbTaTi)N coatings containing Al at concentrations of 20, 50, 60, and 70%, and the observed findings are presented in Figure 2. The XRD pattern of the (AlCrNbTaTi)N with Al content of 20% showed the (111) and (200) peaks of cubic MeN. Each peak aligned with its theoretically predicted 2θ angle, as specified in

Table 2. Lattice constants and theoretically individual 2θ angle of (111) and (200) reflection for each MeN nitride in (AlCrNbTaTi)N coatings.

Nitride (JCPDS Number)	AlN (46-1200)	CrN (11-0065)	NbN (38-1155)	TaN (49-1283)	TiN (38-1420)
Lattice constant (Å)	4.04	4.14	4.40	4.34	4.24
2θ of (111) (deg)	38.530	37.538	35.365	35.830	36.662
2θ of (200) (deg)	44.771	43.737	41.068	41.605	42.596

, reflecting variations in the lattice constants. The diffraction peaks corresponding to the (111) and (200) crystal planes of the equiatomic (AlCrNbTaTi)N coating appeared at 36.7° and 42.7°, respectively. These peaks closely approximated the average diffraction peak positions for the (111) and (200) planes of the five binary compounds of AlN, CrN, NbN, TiN, and TaN measured at 36.8° and 42.8°. With increasing Al content from 20 to 50%, the intensity of the (111) peak decreases, and the intensity of the (111) peak decreases, while its position shifts to 36.8°. When the aluminum content was increased to 60%, the (111) peak intensity continued to decline, with its position shifting further to 37.2°. In contrast, the (200) peak was observed at 43.1° and 44.3°. Among the five binary compounds of AlN, CrN, NbN, TiN, and TaN, AlN demonstrated the largest lattice constant. The observed upward shift in these peaks was attributed to the increasing Al content in the coating. However, the decrease in the intensity of the diffraction peak indicated the poor crystallinity of the coating. This amorphization with higher Al content can be attributed to the destabilization of the FCC structure due to the significant size mismatch between Al atoms and other constituents. Similar trends have been observed in AlTiN and AlCrN systems, where high Al concentrations induce lattice distortions and amorphization. This behavior is critical for tailoring coatings for applications requiring reduced brittleness while maintaining sufficient hardness.

Figure 3 shows the relationship between the Young’s modulus and microhardness of the (AlCrNbTaTi)N coating relative to the Al content. The friction coefficient for the coatings with different Al contents is presented in Figure 4. A consistent trend was observed, where the friction coefficient decreased with increasing Al content, suggesting a reduction in friction and improved wear resistance. The coating with 20% Al content exhibited the highest friction coefficient, which progressively decreased as the Al content increased to 50%, 60%, and 70%. This trend correlates well with the intrinsic hardness of Al-containing nitrides and their ability to form protective oxide layers during sliding. However, the decline in hardness and modulus beyond 50% Al indicates a trade-off between structural integrity and amorphization. The dispersion of the friction coefficient varied significantly across the different Al concentrations. Coatings with higher Al content (50%, 60%, and 70%) exhibited lower variability in friction coefficients, indicating more stable tribological performance. In contrast, the 20% Al coatings, which retained a more crystalline structure, displayed greater variability in friction values, which is likely due to their microstructural features and higher susceptibility to wear. This observation suggests that the transition to an amorphous structure with higher Al content leads to more uniform and stable tribological behavior, while the crystalline coatings show more fluctuation in their friction performance. Compared to conventional binary systems, such as TiAlN, which achieve a hardness of approximately 30 GPa, the (AlCrNbTaTi)N coating’s equimolar composition surpasses these benchmarks, showcasing the benefits of high-entropy alloy configurations. The low wear rate and friction coefficient are attributable to the synergistic effects of its nanomultilayer structure and optimized H/E ratio. This design mitigates crack propagation and wear, as corroborated by studies on multilayered TiAlSiN and AlCrN coatings. The observed tribological performance is therefore indicative of the potential of (AlCrNbTaTi)N coatings to outperform traditional systems under similar testing conditions. The H/E and H³/E² of the coating were 0.09 and 0.30 GPa, respectively, indicating that the coating possessed remarkable toughness due to its favorable mechanical properties [10]. The equichemical ratio (AlCrNbTaTi)N coating demonstrated an exceptionally low wear rate of 8.2 × 10⁻¹⁹ m²·N⁻¹, surpassing that of various advanced wear-resistant materials, such as TiAlN [11], TiAlSiN [12] and other coatings. The observed improved wear resistance can be attributed to the coating’s high hardness, impressive toughness, and low friction coefficient. While previous research on binary coatings, such as TiN and AlTiN, demonstrated excellent high-temperature stability and wear resistance, these coatings mainly have limitations in terms of phase stability and oxidation resistance under extreme conditions. Recent studies, including those by Yeh et al. [11] on HEAs, have introduced more complex coatings, such as (AlCrNbSiTi)N, providing enhanced oxidation resistance and thermal stability. However, these studies primarily concentrated on equimolar compositions, in which limited exploration into how varying aluminum content can influence coating performance. This study extended this knowledge by investigating the effects of different Al concentrations in the (AlCrNbTaTi)N system, with the hypothesis that adjusting the Al content could further enhance mechanical and tribological properties.

The coating containing 20% aluminum emerges as a promising candidate for hard tool applications. In this paper, the structure of the coating was further analyzed by TEM (Figure 5). The coating comprised a columnar crystal structure of CrN and a nanostructured multilayer of (AlCrNbTaTi)N. The CrN layer appeared at 340 nm in thickness, while the (AlCrNbTaTi)N layer demonstrated a thickness of about 1380 nm. This multilayer comprised roughly 50 uniform layers, each with a thickness of 28 nm, resulting in a distinctive pattern of alternating light and dark stripes. Figure 6 shows the brightfield and darkfield phases of the enlarged local layout. Cross-disciplinary elemental (CDE) mapping revealed that the dark grains visible in the brightfield corresponded to those in the darkfield. Significantly high concentrations of Cr and Ti were found, while Al, Nb, and Ta were uniformly distributed throughout the coating. The electron diffraction patterns revealed that the crystal showed an FCC structure with a lattice constant of 2.02 Å, and the grains are presented in Figure 7. The SAD patterns revealed that the coating’s hybrid structure was characterized by an FCC configuration, demonstrating significant planes at (200), (111), and (220). The coating comprised around 50 distinct layers, reflecting the rotational cycle of the workpiece. As the specimen rotates, it periodically transitions between high-density plasma regions and lower-density areas on the target surface. This process enabled the deposition of material directly in front of the target while simultaneously etching away the coating in regions farther from it. Therefore, the coating demonstrated compositional fluctuations, attributed to the varying yields of the different metallic elements involved. This study not only confirmed the findings from previous studies on HEA coatings, such as those by Yeh et al. [11] and studies on TiAlN and AlCrN [17], but it also provided new insights into the structural evolution of coatings with varying aluminum content. Zhao et al. [20] revealed that as temperature increases, a transition from hcp to fcc occurs inconcentrated solid solution alloys with negative stacking fault energies, driven byintrinsic vibrational entropy. Inoue et al. [21] highlighted the advantages of metastable Al-based alloys, including a tensile strength of 1500 Pa in amorphous alloys, elevated temperature strength of 364 MPa at 573 K in nanoquasicrystalline alloys, a strength of1000 MPa at room temperature and 520 MPa at 473 K in nanocrystalline alloys, a strength of 596 MPa coupled with 16% elongation in nanocomposite alloys, and a strength of 900 MPa with 5% elongation in supersaturated fcc-Al solid solutions Manivasagam and Suwas [22] provided a detailed discussion on the development of new Mg alloys and their corrosion properties. They also reviewed recent advancements in coating techniques for Mg alloys aimed at controlling their degradation rates. The observed transition from crystalline to amorphous phases with increasing Al concentration, leading to improved wear resistance and hardness, possesses a distinct advantage over previous systems such as TiAlN and AlCrN, which primarily maintain crystalline structures under similar conditions. Furthermore, the improved wear resistance of the (AlCrNbTaTi)N coating with a 20% Al content surpasses that of several existing high-performance coatings, including TiAlN and TiAlSiN, thereby presenting a more optimized solution for industrial applications. Chim et al. [23] conducted an in-depth analysis of the oxidation behavior of AlTiN and AlCrN coatings, revealing that AlCrN demonstrated a markedly reduced oxidation rate compared to its AlTiN counterpart. This enhanced oxidation resistance of AlCrN relative to AlTiN was corroborated by analogous findings in the works of Singh et al. [24] and Feng et al. [25]. From the cumulative body of prior research, it is evident that during the high-temperature oxidation of AlTiSiN coatings, the rapid outward diffusion of Ti ions to the surface catalyzed the accelerated formation of TiO2. This process hindered the establishment of a continuous, protective Al₂O₃ layer, thereby compromising the coating’s oxidative stability [26,27]. Conversely, the intricate architecture of the ML (AlCrBN/TiAlNbSiN) multilayer coating, characterized by its sublayer interfaces, served as robust diffusion barriers. These interfaces effectively inhibited the inward permeation of oxygen and the outward migration of metallic ions, such as Ti, thereby conferring substantial improvements in the oxidation resistance of the multilayer coatings [28,29,30].

4. Conclusions

The CrN sublayer played a critical role in improving the adhesion between the substrate and the (AlCrNbTaTi)N coating. Its columnar grain structure contributed to stress mitigation and structural stability, which are essential for the durability and wear resistance of the overall coating system. This highlights the value of incorporating functional sublayers in high-performance coatings. (AlCrNbTaTi)N high-entropy alloy nitride coatings were successfully deposited using cathodic arc deposition. The elements Cr, Nb, Ta, and Ti were employed in an equimolar ratio of 1:1:1:1, while Al content varied between 20, 50, 60, and 70%. In comparison to existing studies on Al-containing binary and equimolar HEA coatings, this research provided a deeper understanding of how varying aluminum content could influence the structural and mechanical properties of high-entropy nitride coatings. The (AlCrNbTaTi)N coating, particularly with 20% Al, demonstrated superior wear resistance and hardness, positioning it as an advanced alternative to coatings, such as TiAlN and TiAlSiN, for high-temperature, high-wear applications. This research therefore provided the basis for further optimization of HEAN coatings tailored to meet the evolving demands of cutting-edge industrial applications. Based on the results observed in this study, the following conclusion can be obtained:

• With increasing aluminum content, the coating structure transitioned from a nanomultilayer configuration at 20%–50% Al content to a fully amorphous state at 70%. The XRD results indicated peak shifts and reductions in crystallinity, consistent with the amorphization trend.
• The equichemical (AlCrNbTaTi)N coating demonstrated a nanomultilayer structure, with a hardness of 34.5 GPa and a modulus of 369 GPa. These mechanical properties, combined with a wear rate of 8.2 × 10⁻¹⁹ m²·N⁻¹, highlight the advantages of the multilayer arrangement in dissipating stress and resisting deformation under high mechanical loads.
• The superior tribological performance of the equimolar (AlCrNbTaTi)N coating, with its exceptionally low wear rate, makes it an outstanding candidate for hard tool applications. This unique combination of mechanical properties, high hardness, excellent wear resistance, and structural integrity demonstrates the potential for significantly extending tool life in industrial applications requiring high-performance materials.

While this study highlighted the significant potential of (AlCrNbTaTi)N coatings, future research should explore their long-term thermal stability, oxidation resistance under extreme conditions, and scalability for industrial applications.