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Item Elevated temperature tribological performance of non-equiatomic CoCrNiTiWx high entropy alloy coatings developed by mechanical alloying and high-velocity oxy-fuel spray(Elsevier B.V., 2024) Addepalli, S.N.; Joladarashi, S.; Ramesh, M.R.High entropy alloys (HEA) have applications in multiple fields owing to their exceptional mechanical and physical properties. In the current study, mechanical alloyed CoCrNiTiWx (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 CoCrNiTiW0.5 and CoCrNiTiW1.5 HEA coatings at elevated temperatures was studied extensively using a Pin-on-Disc tribometer. The CoCrNiTiW0.5 and CoCrNiTiW1.5 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 CoCrNiTiW0.5 and CoCrNiTiW1.5 HEA coatings were found to be 1.69 ± 0.32 % and 1.51 ± 0.37 % respectively.Consequently, the CoCrNiTiW0.5 and CoCrNiTiW1.5 HEA coatings displayed average microhardness values of 863 ± 52 HV0.3 and 1025 ± 39 HV0.3, respectively. Further, the wear rates of coatings exhibited a significant reduction at elevated temperatures, owing to the formation of TiO2, NiCr2O4 oxide tribofilms for CoCrNiTiW0.5, and CoCr2O4, NiWO4, WO3 oxides for CoCrNiTiW1.5. The specific wear rate of CoCrNiTiW0.5 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 CoCrNiTiW1.5 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 CoCrNiTiW0.5 HEA dropped from 0.504 ± 0.015 to 0.397 ± 0.005, while CoCrNiTiW1.5 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 CoCrNiTiWx coatings were dominated by adhesive wear. However, at elevated temperatures, a shift towards oxidative wear was observed. © 2023 Elsevier B.V.Item Microstructure, Mechanical, and Dry Sliding Wear Performance of Equimolar CoCrNiTiMo and CoCrNiTiW High-Entropy Alloy Coatings(Springer, 2025) Addepalli, S.N.; Joladarashi, S.; Ramesh, M.R.In the present investigation, mechanical alloyed CoCrNiTiMo and CoCrNiTiW equimolar HEA powders were employed as feedstock in the development of dense coatings using high-velocity oxy-fuel technique. The dry sliding wear behavior of uncoated substrate and high-entropy alloy (HEA) coatings were extensively investigated at different temperatures and loads using a pin-on-disk tribometer. The microstructures and phases of the mechanical alloyed powders, deposited coatings, and worn surfaces were thoroughly studied. The mechanical alloyed CoCrNiTiMo and CoCrNiTiW HEAs demonstrated the evolution of two BCC solid solutions. However, the deposited coatings reported the formation of additional phases, including Co3Ti intermetallic and NiTiO3 spinel. The microstructural analysis of CoCrNiTiMo and CoCrNiTiW coatings unveiled a compact lamellar structure characterized by robust mechanical interlocking to the substrate. The CoCrNiTiMo and CoCrNiTiW HEA coatings displayed porosities of 1.12 ± 0.05% and 1.39 ± 0.03%, respectively. Additionally, the microhardness assessments revealed superior values for CoCrNiTiMo and CoCrNiTiW HEA coatings, measuring at 927 ± 45 HV0.3 and 951 ± 38 HV0.3, correspondingly. The wear rate of CoCrNiTiMo HEA coating dropped by 70.5%, from 17.34 ± 2.8 × 10?6 mm3/N-m to 5.1 ± 1.6 × 10?6 mm3/N-m with an increment in the wear testing temperature from ambient to 600 °C. Concurrently, the CoCrNiTiW coating experienced a 76.3% drop in the wear rates from 15.8 ± 3.7 × 10?6 mm3/N-m to 3.73 ± 2.1 × 10?6 mm3/N-m. The significant fall in the wear rates at higher temperatures was accredited to the development of oxide tribofilms. CoCrNiTiMo exhibited discernible oxide phases, including CoMoO4, TiO2, and NiO. In contrast, its counterpart, CoCrNiTiW, generated WO3, CoWO4, and TiO2 oxides at a temperature of 600 °C. The adhesive wear at RT transitioned to predominant oxidative wear with slight fatigue and abrasive wear at high temperatures. © ASM International 2025.