Elevated Temperature Sliding Wear Behavior Of Cocrnitimox And Cocrnitiwx High Entropy Alloys Processed Using Mechanical Alloying and High-Velocity Oxy-Fuel Spray

Abstract

Maraging steels, widely used in the aircraft landing gear components were subjected to wear due to the harsh working conditions. Surface modification of these components by the deposition of advanced coating materials prolong their life. High entropy alloys (HEA) are a contemporary class of materials with multiple primary elements having applications in different fields, owing to their exceptional mechanical and physical properties. Therefore the curent research is aimed at enhancing the wear performance of maraging steels, by the deposition of HEA coatings. CoCrNiTiMox and CoCrNiTiWx (x: molar ratio; x= 0.5, 1, 1.5) HEAs were processed by mechanical alloying of pure metal powders for further application as feedstock in the High velocity oxy-fuel (HVOF) technique. The phase and microstructural transformation of the ball milled powders is investigated in detail by optimizing the milling time and speeds. The milling process is extended for 50 h and milled powder samples were collected at regular intervals of 10, 20, 30, 40 and 50 h to characterize the samples for their suitability to deposit using thermal spray techniques. The milled powders were characterized with respect to the phases, particle morphology, chemical homogeneity, particle size and crystallite sizes. Based on the characterization studies, the powders milled at a speed of 200 rpm for 10 h were selected as feedstock for HVOF deposition. After the deposition of coatings, the microstructural and mechanical characterization of coatings were performed. The phases and microstructure of the deposited HEA coatings were determined by X-ray diffraction (XRD) and scanning electron microscope (SEM). The microhardness of the coating was determined by using a vickers indenter on the coatings cross-section, with a load of 300 g and a dwell time of 15 s. The deposited coatings fracture toughness was determined by using the Evans and Wilshaw’s approach. The tribological behaviour of CoCrNiTiMox and CoCrNiTiWx HEA coatings at elevated temperatures was studied extensively using a Pin-on-Disc tribometer. The deposited coatings exhibited a lamellar structure and good mechanical bonding with the substrate. The porosities of CoCrNiTiMox and CoCrNiTiWx HEA coatings, as calculated using ImageJ software, were found to be in the range of 1-2%. i The mechanical performance of the CoCrNiTiMox and CoCrNiTiWx HEA coatings revealed superior values, when compared to other HEA coatings. The microhardness of CoCrNiTiMo0.5, CoCrNiTiMo, and CoCrNiTiMo1.5 HEA coatings were found to be 841±62 HV0.3, 927 ± 45 HV0.3 and 952±23 HV0.3, respectively. On the other hand, the microhardness of CoCrNiTiW0.5, CoCrNiTiW, and CoCrNiTiW1.5 HEA coatings were found to be 863±52 HV0.3, 951 ± 38 HV0.3 and 1025±39 HV0.3, respectively. The fracture toughness of CoCrNiTiMo0.5, CoCrNiTiMo, and CoCrNiTiMo1.5 HEA coatings were found to be 2.89 ± 0.31 (Mpa m1/2), 3.26 ± 0.25 (Mpa m1/2) and 3.79 ± 0.35 (Mpa m1/2) respectively. Likewise, the fracture toughness of CoCrNiTiW0.5, CoCrNiTiW, and CoCrNiTiW1.5 HEA coatings, were found to be 3.22 ± 0.26 (Mpa m1/2), 3.54 ± 0.32 (Mpa m1/2) and 3.87 ± 0.3 (Mpa m1/2) respectively. Further, it can be witnessed that the as-sprayed HEA coatings exhibited a steady increment in the mechanical properties with an increment in the molar fraction of Molybdenum and Tungsten. The specific wear rate of CoCrNiTiMo HEA coating dropped by 70.5%, declining from 17.34 ± 2.8 x10-6 mm3/N-m to 5.1 ± 1.6 x10-6 mm3/N-m, while CoCrNiTiW dropped by 76.3%, decreasing from 15.8 ± 3.7 x10-6 mm3/N-m to 3.73 ± 2.1 x10-6 mm3/N-m, with an increase in the temperature from RT to 600 °C. The wear rates of coatings exhibited a significant reduction at elevated temperatures, owing to the formation of TiO2, CoMoO4, NiO tribofilms for CoCrNiTiMo, and TiO2, CoWO4, WO3 oxides for CoCrNiTiW. Further, the CoCrNiTiMo1.5 HEA coatings offered better wear resistance, as compared to CoCrNiTiMo0.5 HEA coatings, at any temperature and loading condition, due to the increment in the molar fraction of Molybdenum. Additionally, the CoCrNiTiW1.5 HEA coatings exhibited superior wear performance, when compared to all the six compositions in the current research. The investigation of worn surfaces showed a transformation in wear mechanisms from adhesive and abrasive wear at room temperature to oxidative wear at elevated temperatures.