Tribological Performance of High-Entropy Coatings (HECs): A Review

Abstract

Surface coatings that operate effectively at elevated temperatures provide compatibility with critical service conditions as well as improved tribological performance of the components. High-entropy coatings (HECs), including metallic, ceramics, and composites, have gained attention all over the world and developed rapidly over the past 18 years, due to their excellent mechanical and tribological properties. High-entropy alloys (HEAs) are defined as alloys containing five or more principal elements in equal or close to equal atomic percentage. Owing to the high configurational entropy compared to conventional alloys, HEAs are usually composed of a simple solid solution phase, such as the BCC and FCC phases, instead of complex, brittle intermetallic phases. Several researchers have investigated the mechanical, oxidation, corrosion and wear properties of high-entropy oxides, carbides, borides, and silicates using various coating and testing techniques. More recently, the friction and wear characteristics of high-entropy coatings (HECs) have gained interest within various industrial sectors, mainly due to their favourable mechanical and tribological properties at high temperatures. In this review article, the authors identified the research studies and developments in high-entropy coatings (HECs) fabricated on various substrate materials using different synthesis methods. In addition, the current understanding of the HECs characteristics is critically reviewed, including the fabrication routes of targets/feedstock, synthesis methods utilized in various research studies, microstructural and tribological behaviour from room temperature to high temperatures.

High-Entropy Alloys: Advantages and Applications in Challenging Environments

“…The system, which holds up the rotating shaft, comprises a top and a bump foil. The top foil and the journal, which are in touch with the shaft when it is stationary, are separated by a thin layer or gap when moving at higher speeds because of hydrodynamic pressure [69]. As a result, the bearing strikes the race at a slow rate during startup acceleration at standard temperature and shutdown deceleration at high temperature.…”

Strategic Selection of Refractory High-Entropy Alloy Coatings for Hot-Forging Dies by Applying Decision Science

“…This elemental composition seems to promote high configurational entropy and the formation of BCC phases. The correlation between the top five RHEAs and the BCC crystal structure hints at high hardness, translating into low wear rates [26]. These findings strongly support the idea that the top-ranked RHEAs excel in wear resistance, making them highly suitable for use as coating materials for hot-forging dies, which is evident in the WMoTaNb RHEA, which has a BCC crystal structure and exhibits a relatively high wear resistance (or low wear rate) at ambient and elevated temperatures [60].…”

“…Die coatings can be categorized into three main groups [18,19,21]: metallic coatings (comprising transition metals, refractory metals, and lightweight metals), ceramic coatings (encompassing oxides, borides, carbides, and nitrides), and composite coatings (involving ceramic-reinforced and hybrid coatings). Upon careful evaluation, it becomes evident that refractory HEAs (RHEAs) exhibit exceptional suitability as coatings for conventional hot forge die materials, offering a remarkable combination of properties ideally suited for their intended application as coatings on hot forge dies, employing various coating techniques [19,[21][22][23][24][25][26][27].…”

Wear Characterization of Cold-Sprayed HEA Coatings by Means of Active–Passive Thermography and Tribometer

“…Previous research has suggested that materials possessing favorable mechanical qualities, such as high hardness, elastic modulus, and fracture toughness, can lead to improved tribological performance in various applications [7,8]. High-entropy alloys (HEAs), a recent advancement in alloy development, have garnered considerable interest due to their potential properties [9][10][11][12][13].…”