With the continuous advancement of materials science, the demand for high-performance alloys with superior corrosion resistance and wear resistance is steadily increasing. This is particularly true in industries such as aerospace, energy, and electronics, where the performance requirements for coating materials are constantly rising. Traditional powder-production methods, including powder metallurgy and mechanical milling, are increasingly unable to meet the need for precise control and functionalization of complex materials. Against this backdrop, ultrasonic atomization—a green and highly efficient new powder-production technology—has gradually emerged as a focal point in the research and development of advanced materials.

This paper provides an in-depth examination of the principles, characteristics, and advantages of ultrasonic atomization powdering technology, as well as its applications in novel materials such as high-entropy alloys (HEAs), with a particular focus on innovative breakthroughs in the preparation of high-entropy amorphous alloy powders and coatings.

Ultrasonic Atomization Powder-Production Technology

Ultrasonic atomization powdering technology leverages the high-frequency energy of ultrasonic vibrations to convert liquid metals or alloy materials into fine droplets or mist-like particles, which are then subjected to controlled cooling or solidification to produce powders with uniform particle size and controllable morphology.

1. Ultrasonic atomization process:

• Ultrasonic excitation of liquids: Ultrasonic transducers (such as piezoelectric ceramics) generate high-frequency vibrations, which transmit energy to the liquid surface.
• Formation of metastable droplets: Vibration generates micron-sized droplets, resulting in a fine, mist-like particle cloud.
• Cooling and solidification: The droplets rapidly cool upon leaving the atomization source, forming a solid powder.

The key to this process lies in the high-frequency ultrasonic vibrations—typically ranging from 20 kHz to 200 kHz—and the focused delivery of wave energy, which ensures the formation of uniform, fine droplets.

2. Features and Technical Specifications:

• Controllable particle size: By adjusting parameters such as ultrasonic frequency, amplitude, and liquid surface tension, the particle size can be tuned over a range from several tens of nanometers to several micrometers.
• Uniform powder formation: The droplet size is uniform, significantly improving the particle size distribution of the powder.
• High efficiency: rapid preparation, high yield, and suitable for mass production.
• Environmental protection and energy conservation: no chemical additives are required, reducing pollution and lowering energy consumption.

Advantages of Ultrasonic Atomization for Powder Production

1. Uniform particle size and adjustable dimensions

Ultrasonic milling can produce powders with highly uniform particle sizes, thereby ensuring high-performance and consistent material properties in subsequent processing and coating applications.

2. High purity and pollution-free

No chemical additives are required, which reduces impurity contamination and ensures high powder purity, thereby enhancing alloy performance.

3. Micron- and Nanoscale Particle Control

It is particularly suitable for the preparation of ultrafine powders, meeting the stringent requirements for microstructural control in high-end materials.

4. Precision Preparation of Complex Alloys

Multi-element high-entropy alloys exhibit a complex multi-component composition, and ultrasonic atomization can achieve efficient and uniform mixing of multiple elements, thereby ensuring their homogeneous distribution.

5. High adaptability and wide range of applications

In addition to metallic materials, it can also be used for powder preparation in various systems, such as ceramics and composite materials.

High-entropy alloy

1. Introduction to High-Entropy Alloys

Traditional alloys are typically composed of only a few elements, with the elemental composition heavily biased toward a single dominant element. In contrast, high-entropy alloys break with this convention by incorporating five or more elements in equimolar or near-equimolar proportions, thereby forming a multi-component alloy system. The high concentrations of these constituent elements give rise to increased mixing entropy, which stabilizes the alloy’s unique microstructure.

2. Structural Characteristics of High-Entropy Amorphous Alloys

High-entropy amorphous alloys possess an amorphous (disordered) structure, which distinguishes them from crystalline alloys. This amorphous structure imparts exceptional corrosion resistance, wear resistance, and high strength. Moreover, the structure exhibits excellent resistance to microcrack propagation.

Preparation Process and Structural Characteristics

1. Selection of Raw Materials and Preparation Methods

This study employed an alloy composition of (ZrTiHfNi)85Nb15 and prepared high-entropy amorphous alloy ribbons via rapid quenching. The specific process includes:

• Alloy smelting: Thoroughly mix and melt the elements according to their specified proportions.
• Rapid cooling: Employ rapid cooling techniques such as arc thinning or spray forming to ensure the formation of an amorphous structure.
• Powder preparation: Spherical powders with particle sizes ranging from 20 to 80 micrometers are obtained using ultrasonic metal atomization powdering technology.
• Plasma Spraying (APS): This technique is used to convert powders into coatings with a thickness of approximately 200 micrometers.

2. Structural Analysis

• Microstructure: Observed by scanning electron microscopy (SEM), the coating exhibits a composite structure consisting of an amorphous phase and a small fraction of nanocrystalline phases.
• Phase composition: X-ray diffraction (XRD) analysis confirms that the coating is predominantly amorphous, with minor FCC and BCC crystalline phases and trace amounts of oxides.
• Elemental distribution: Energy-dispersive spectroscopy reveals that the elements are uniformly distributed throughout the coating, and the particles exhibit excellent sphericity and high skeletal integrity.

Performance Study

01 Mechanical Properties

• Hardness: The average hardness of the coating reaches 10.5 GPa, significantly exceeding that of conventional alloys and standard coatings, thereby demonstrating outstanding wear resistance.
• Elastic modulus: Nanoindentation tests indicate that the coating exhibits a high elastic modulus, effectively mitigating crack propagation.

02 Wear Resistance

• Room-Temperature Wear Resistance: Tested at room temperature, the coating exhibits significant resistance to fatigue wear, with the primary failure modes being fatigue wear and mild oxidation.
• High-temperature wear resistance: Under high-temperature conditions of 400°C, microstructural deformation increases markedly and oxidation reactions are intensified, leading to increased softening and adhesive wear. At this point, the cracking and spalling of the oxide film become the primary failure mechanisms.

03 Corrosion Resistance

• Corrosion testing: In a 3.5 wt.% NaCl solution, electrochemical measurements reveal that the coating’s corrosion current density (I_pass) is significantly lower than that of the substrate and stainless steel, indicating excellent corrosion resistance.
• Mechanistic analysis: Thermodynamic calculations indicate that Nb exhibits low reactivity during the formation of the oxide film, making it difficult to establish a protective layer in the initial stage; however, through subsequent secondary oxidation, its corrosion resistance is continuously enhanced.

Illustrated Guide

Figure 1. Morphology of (ZrTiHfNi)85Nb15 powder (a–b); particle size distribution curve (c); SEM/EDS images showing the cross-section and elemental composition of (ZrTiHfNi)85Nb15 powder (d).

Figure 2. XRD patterns of (ZrTiHfNi)85Nb15 powder, coating, and ribbon (a), and DSC curves of (ZrTiHfNi)85Nb15 powder and ribbon (b).

Figure 3. Macroscopic and cross-sectional morphologies of the coating (a, c), surface morphology of the coating (b), and schematic diagram of porosity statistics (d).

Figure 4. EDS image of the coating cross-section.

Figure 5. Load–displacement nanoindentation curves for the (ZrTiHfNi)85Nb15 ribbon, powder, and coating.

Figure 6. Three-dimensional topography of the coating wear tracks at room temperature (a) and 400°C (b); coefficient of friction of the coating at room temperature and 400°C (c); profile curves of the wear tracks (d).

Figure 7. Wear rate of the coating measured at room temperature and 400°C.

Figure 8. Wear surface morphologies after wear testing at room temperature (a, b) and 400°C (c, d); the insets on the right show typical EDS analyses of debris.

Figure 9. Illustrative models of the tribological processes at room temperature (a) and 400°C (b).

Figure 10. Polarization curves of (ZrTiHfNi)85Nb15 strip, coating, 316L stainless steel, and Q235 steel in a 3.5 wt.% NaCl solution.

Figure 11. (a) Potentiostatic polarization curves; (b) Equivalent circuit; (c) Nyquist plot and (d) Bode plot of the electrochemical impedance spectrum in a 3.5 wt.% NaCl solution.

Figure 12. XPS spectra of the coating passivation film (a–f) and the stripes (h–l) in 3.5 wt.% NaCl.

Figure 13. Histogram of the accumulation of metal cation content in the passivation film.

Figure 14. Schematic diagram of Nb5+ substitution for Ti4+ and the continuous cation diffusion process based on the point-defect model.

High-entropy amorphous alloys, as a novel class of multicomponent materials characterized by high strength and excellent corrosion resistance, demonstrate tremendous application potential. Advanced ultrasonic atomization powder-production technology enables the fabrication of powders with uniform microstructure and superior properties, which can then be used to prepare high-quality coatings. This provides effective technical support for extending the service life of industrial equipment and reducing maintenance costs. In the future, as materials-design methodologies continue to evolve and ultrasonic atomization processes are further optimized, high-entropy amorphous alloys are expected to find widespread application and adoption across an even broader range of fields.

High-entropy alloys represent the future direction of materials science, owing to their unique microstructure and multifunctional properties, which confer significant advantages in corrosion resistance and wear resistance. With the continuous advancement of ultrasonic atomization powder-production technology, this emerging class of materials is poised to deliver further breakthroughs and transformative changes in industrial manufacturing, ushering in a new era of material applications.