Microstructural Evolution and Mechanical Properties of Plasma Arc Cladded FeCoNiAlTi High-Entropy Alloy Coatings

Literature Overview

Published in Materials Protection (Vol. 58, No. 4, 2025, pp. 56-69), this study investigates the influence of cladding current on the microstructure and mechanical properties of FeCoNiAlTi high-entropy alloy (HEA) coatings deposited by plasma arc cladding. The research team from Liaoning University of Science and Technology, supported by the National Key R&D Program of China (2021YFB3702003), systematically varied the cladding current from 140 A to 220 A in 20 A increments and evaluated the resulting coating properties through XRD, OM, SEM, Vickers hardness testing, profilometry, tribological testing, and tensile testing.

High-Entropy Alloy Background

The FeCoNiAlTi system is a representative equiatomic high-entropy alloy that combines the attractive properties of multiple principal elements. In the context of surface engineering, HEA coatings offer the potential for enhanced combination of strength, toughness, and corrosion resistance that is difficult to achieve with conventional alloy systems. The plasma arc cladding process provides a practical manufacturing route for HEA coatings on industrial components, though the process parameters must be carefully controlled to manage the complex solidification behavior of multi-principal element alloys.

Effect of Cladding Current on Microstructure

Porosity and Elemental Segregation

Increasing the cladding current from 140 A to 220 A significantly improved both porosity and elemental segregation in the coatings. Higher currents increase the thermal input, which extends the liquid pool lifetime and allows better gas escape and more homogeneous element mixing. However, excessively high currents may lead to excessive dilution from the substrate, which could compromise the HEA character of the coating.

Phase Composition

The phase composition remained relatively stable across the current range, consisting primarily of a typical FCC solid solution phase and Co3Ti precipitate phase. This stability is important for engineering applications because it means that the current can be adjusted to optimize porosity and segregation without fundamentally changing the phase architecture. The FCC matrix provides excellent ductility and toughness, while the Co3Ti precipitates contribute to precipitation strengthening.

Optimal Current Condition

The 180 A condition (referred to as T3 coating) produced the best overall performance:

Property	T3 Coating (180 A)	Substrate	Improvement Factor
Surface hardness	322.77 HV0.2	~160 HV0.2	~2×
Maximum wear depth	2.144 μm	—	—
Average friction coefficient	0.362	—	—
Wear rate	3.83×10⁻⁶ mm³/(N·m)	—	Minimum
Tensile strength	948 MPa	—	—
Elongation	26.61%	—	—

Mechanical Property Analysis

Hardness and Strengthening Mechanisms

The 322.77 HV0.2 hardness of the T3 coating represents approximately a doubling of the substrate hardness. This enhancement is attributed to multiple strengthening mechanisms operating simultaneously: solid solution strengthening from the equiatomic Fe-Co-Ni-Al-Ti matrix, precipitation strengthening from Co3Ti particles, and grain refinement effects from the plasma arc solidification conditions. The relatively low porosity at 180 A also contributes to the effective hardness by ensuring full density of the coating.

Tribological Performance

The T3 coating exhibited the best wear resistance with a maximum wear depth of only 2.144 μm and an average friction coefficient of 0.362. The low wear rate of 3.83×10⁻⁶ mm³/(N·m) indicates excellent resistance to adhesive and abrasive wear mechanisms. The combination of high hardness (322.77 HV) and good ductility (26.61% elongation) is particularly advantageous for wear applications because it provides both resistance to material removal and tolerance to impact and cyclic loading without catastrophic failure.

Tensile Performance

The T3 coating demonstrated a tensile strength of 948 MPa with an elongation of 26.61%, representing an excellent strength-toughness combination. This is remarkable for a plasma arc cladded coating, which often suffers from cracking due to high residual stresses and thermal mismatch with the substrate. The good elongation suggests that the coating can accommodate significant plastic deformation before failure, which is critical for applications involving thermal cycling or mechanical shock.

Process Parameter Window Analysis

The study reveals a clear process parameter window:

Below 180 A: Insufficient thermal input leads to higher porosity and worse elemental segregation, degrading coating properties.
At 180 A: Optimal balance of thermal input for porosity elimination, segregation reduction, and phase stability.
Above 180 A: While porosity continues to decrease, the benefits diminish and there is a risk of excessive dilution from the substrate, which would alter the HEA composition and potentially degrade the coating's unique properties.

Engineering Practice Considerations

For engineers considering HEA coatings for industrial applications, this study provides several important insights:

Plasma arc cladding is a viable manufacturing route for HEA coatings: The process produces dense, well-bonded coatings with controllable microstructure and excellent mechanical properties.
The 180 A current provides a robust process window: The relatively broad optimal range (around 180 A) suggests that the process is not overly sensitive to minor current variations, which is important for industrial scalability.
The FCC + Co3Ti phase architecture is stable across the current range: This means that the coating's fundamental properties are predictable and reproducible, which is essential for quality assurance in production environments.
The strength-toughness combination (948 MPa / 26.61%) is competitive with bulk HEA alloys: This suggests that plasma arc cladding can produce coatings with properties approaching those of wrought HEA materials, which is a significant achievement for a surface engineering process.

Key Reflections

The study raises an important question about the long-term stability of the Co3Ti precipitates under thermal cycling conditions. In service applications involving temperature fluctuations, precipitate coarsening (Ostwald ripening) could reduce the precipitation strengthening effect over time. Additionally, the interfacial residual stress between the HEA coating and the substrate should be characterized, as thermal mismatch during cooling can generate significant tensile or compressive stresses that affect coating adhesion and fatigue life.

The tensile testing of the coating is particularly noteworthy because it is uncommon in cladding studies, which typically focus on hardness and wear properties. The demonstration of 26.61% elongation in a plasma arc cladded HEA coating suggests that the coating has excellent crack tolerance, which is a critical requirement for components subjected to cyclic loading or thermal fatigue.

Summary

This research demonstrates that plasma arc cladding of FeCoNiAlTi high-entropy alloy produces coatings with exceptional mechanical properties, including approximately double the substrate hardness, excellent wear resistance, and a remarkable strength-toughness combination of 948 MPa with 26.61% elongation. The optimal cladding current of 180 A provides a practical process window that balances porosity elimination, elemental homogeneity, and phase stability. The FCC solid solution with Co3Ti precipitate phase architecture is stable across the tested current range, providing predictable and reproducible coating properties. For industrial applications requiring combined wear resistance, corrosion resistance, and mechanical toughness, this HEA coating system represents a promising advancement in surface engineering technology.