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.

Titanium alloys are widely used in demanding applications due to their exceptional strength-to-weight ratio, high-temperature resilience, and excellent corrosion resistance. Understanding their tribological behavior is critical, as the performance and durability of several mechanical systems, particularly in gas turbine engines, are often constrained by friction and wear in complex contacting and mobile assemblies. This study investigates the tribological behavior of two widely used titanium alloys, Ti–6–4 and Ti–6–2–4–2, focusing on their interfacial phenomena under varied operational conditions. Tribological testing was conducted using a reciprocating tribometer at different temperatures and loading conditions. Ex situ analyses revealed that wear mechanisms were heavily influenced by the properties of the oxide layer formed during sliding. Under higher loads, the oxide layer on the alloy surface fractured, resulting in the generation of flake-like debris, which contributed to third-body abrasion. Additionally, the study examined the transfer film formation on the alumina counterface under various conditions, correlating friction, and wear behavior with interfacial processes, particularly the oxide formation on the worn surfaces. This study enhances the understanding of the tribological behavior of titanium alloys, paving the way for improved performance in demanding applications through advanced surface modification techniques.