Phase evolution and high-temperature wear behavior of non-equiatomic metastable CoCrNiTiMox HEA coatings fabricated by high-velocity oxy-fuel technique

Abstract

The current research aims to enhance the tribological performance of maraging steels at high temperatures by surface modification techniques. CoCrNiTiMo<inf>x</inf> (x; molar fraction, x = 0.5, 1.5) high-entropy alloy (HEA) coatings with dense lamellar microstructures were deposited onto maraging steels using high-velocity oxy-fuel spray (HVOF). In order to achieve a uniform distribution of constituent elements for thermal spray deposition, mechanical alloying was employed to synthesize the HEA feedstock. The phases and microstructure of the synthesized HEA powder, as-sprayed coatings, and worn surfaces were examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The as-sprayed HEA coatings exhibited metastability, with a BCC phase solid solution, NiTiO<inf>3</inf> spinel, and an intermetallic MoNi phase for CoCrNiTiMo<inf>0.5</inf> and Co<inf>2</inf>Mo<inf>3</inf> phase for CoCrNiTiMo<inf>1.5</inf>. The average microhardness of CoCrNiTiMo<inf>0.5</inf> and CoCrNiTiMo<inf>1.5</inf> HEA coatings were 841 ± 62 HV<inf>0.3</inf> and 952 ± 23 HV<inf>0.3</inf>, respectively. The specific wear rate and friction coefficients of CoCrNiTiMo<inf>x</inf> HEA coatings exhibited a decreasing trend with an increase in temperature, owing to the formation of tribofilms on the worn surface. X-ray diffraction studies revealed the formation of NiMoO<inf>4</inf> spinel for CoCrNiTiMo<inf>0.5</inf> and MoO<inf>2</inf>, Co<inf>3</inf>O<inf>4</inf> phases for CoCrNiTiMo<inf>1.5</inf> HEA at a wear temperature of 600 °C. The investigation of worn surfaces showed a transformation in wear mechanisms from abrasive wear at room temperature to oxidative wear with mild fatigue at elevated temperatures. © 2023 Elsevier Ltd