Elevated temperature tribological performance of non-equiatomic CoCrNiTiWx high entropy alloy coatings developed by mechanical alloying and high-velocity oxy-fuel spray

Abstract

High entropy alloys (HEA) have applications in multiple fields owing to their exceptional mechanical and physical properties. In the current study, mechanical alloyed CoCrNiTiW<inf>x</inf> (x; a molar fraction, x = 0.5 and 1.5) HEA feedstock powders were deposited on maraging steel substrate using high-velocity oxy-fuel spray (HVOF). The phase evolution and the microstructure of the milled powders and as-sprayed coatings were analysed by X-ray diffraction (XRD) and scanning electron microscope (SEM). The tribological behaviour of CoCrNiTiW<inf>0.5</inf> and CoCrNiTiW<inf>1.5</inf> HEA coatings at elevated temperatures was studied extensively using a Pin-on-Disc tribometer. The CoCrNiTiW<inf>0.5</inf> and CoCrNiTiW<inf>1.5</inf> HEA coatings retained the BCC solid solution phases formed during the milling stage. However, additional oxide and intermetallic phases were formed owing to the in-flight oxidation and high temperatures experienced during the HVOF deposition. The deposited coatings exhibited a lamellar structure and good mechanical bonding with the substrate. The porosities of CoCrNiTiW<inf>0.5</inf> and CoCrNiTiW<inf>1.5</inf> HEA coatings were found to be 1.69 ± 0.32 % and 1.51 ± 0.37 % respectively.Consequently, the CoCrNiTiW<inf>0.5</inf> and CoCrNiTiW<inf>1.5</inf> HEA coatings displayed average microhardness values of 863 ± 52 HV<inf>0.3</inf> and 1025 ± 39 HV<inf>0.3,</inf> respectively. Further, the wear rates of coatings exhibited a significant reduction at elevated temperatures, owing to the formation of TiO<inf>2</inf>, NiCr<inf>2</inf>O<inf>4</inf> oxide tribofilms for CoCrNiTiW<inf>0.5</inf>, and CoCr<inf>2</inf>O<inf>4</inf>, NiWO<inf>4</inf>, WO<inf>3</inf> oxides for CoCrNiTiW<inf>1.5</inf>. The specific wear rate of CoCrNiTiW<inf>0.5</inf> HEA coating dropped by 73.6 % from 22.7 ± 2.6 × 10−6 mm3/N-m to 5.99 ± 1.9 × 10−6 mm3/N-m, while CoCrNiTiW<inf>1.5</inf> dropped by 78.8 % from 11.86 ± 3.5 × 10−6 mm3/N-m to 2.51 ± 1.5 × 10−6 mm3/N-m, with a rise in the temperature from RT to 600 °C. Likewise, The frictional coefficients of CoCrNiTiW<inf>0.5</inf> HEA dropped from 0.504 ± 0.015 to 0.397 ± 0.005, while CoCrNiTiW<inf>1.5</inf> HEA dropped from 0.578 ± 0.025 to 0.471 ± 0.004, with a rise in temperature from RT to 600 °C. At room temperature, the wear mechanisms of the as-sprayed CoCrNiTiW<inf>x</inf> coatings were dominated by adhesive wear. However, at elevated temperatures, a shift towards oxidative wear was observed. © 2023 Elsevier B.V.