Tribological Properties of CoCrFeNiW_x High-Entropy Alloys at Room Temperature and 900 ℃

In aerospace and aviation industry, there is an urgent need for the materials with good mechanical properties, wear resistance performance and self-lubricating properties at high temperatures. High entropy alloys (HEAs), also known as multi-principal element alloys are a kind of novel metal materials that contain five or more elements in near equiatomic proportions with four core effects: high entropy, sluggish diffusion, severe lattice distortion, and cocktail effects. Because of their unconventional structures, the HEAs exhibit excellent strength, hardness, fracture toughness, wear resistance and high temperature stability, all these advantages make HEAs have promising for high temperature applications in the field of aerospace and equipment manufacturing.

In this study, in order to obtain a HEA with excellent mechanical properties and tribological properties, a series of CoCrFeNiW_x (x=0.25, 0.5, 0.75 and 1.0) HEAs were prepared by vacuum arc melting. The effects of W content on the crystal structure, microstructure, mechanical properties and tribological properties of CoCrFeNiW_x HEAs at room temperature as well as 900 ℃ were studied. The experimental results suggested that single-phase face-centered cubic (FCC) solid solution was formed in the alloy with low W content, and the intermetallic compound μ-phase was promoted when the W content was higher than x=0.25. With the increase of W content, the microstructure of the CoCrFeNiW_x HEAs evolved from single-phase cellular dendritic FCC structure (x=0.25) to dendritic FCC plus interdendritic lamellar (FCC+μ) eutectic structure (x=0.5, 0.75), and then to coarse dendritic μ phase on FCC matrix (x=1.0).

Due to the effect of solution strengthen of W atoms and the precipitation strengthen of the in-situ formed intermetallic compound μ- phase, the mechanical properties such as yield strength and compressive strength of the HEAs are significantly increased while the plasticity is decreased. The Vickers hardness of the CoCrFeNiW_x HEAs were improved significantly with the increasing W content both at room temperature and 900 ℃, especially the CoCrFeNiW_0.75 and CoCrFeNiW HEAs still maintained higher hardness at 900 ℃, which indicated that the addition of W was helpful to improve the high-temperature softening resistance of the CoCrFeNiW_x HEAs.

Ball-on-disk sliding friction and wear tests of CoCrFeNiW_x HEAs coupled with Si₃N₄ ball were carried out under a contact load of 10 N and a sliding speed 0.3 m/s at room temperature and 900 ℃, the results showed the addition of W element significantly improved the room temperature wear resistance of CoCrFeNiW_x HEAs, but made little contribution to the friction coefficient. At 900 ℃, the friction coefficients and wear rates of the CoCrFeNiW_x HEAs was sharply reduced with the increase of W content, which was mainly attributed to the fact that the multi-component composite oxide glaze layer mainly composed of Fe₂O₃, CoO, Co₃O₄, Cr₂O₃, NiO and WO₃ formed on the worn surface had a good friction reduction and anti-wear effect at high temperature, especially the WO₃ played an important role in the reduction of friction coefficient. The detailed characterization of the worn surface revealed that abrasire wear and fatigue wear were the dominant mechanisms of CoCrFeNiW_x HEAs at room temperature but oxidative wear at 900 ℃.

Among these W contained HEAs studied in this paper, the CoCrFeNiW_0.75 HEA had a yield strength of 863 MPa, a compressive strength of 1 250 MPa, and a hardness of 446 HV at 900 ℃, both good tribological and wear properties at room temperature and 900 ℃, which had a good prospect of engineering application.