Effect of Vacuum Hot Pressing on Plasma-Sprayed Molybdenum Coatings on Rail Vehicle Axle Steel
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Effect of Vacuum Hot Pressing on Plasma-Sprayed Molybdenum Coatings on Rail Vehicle Axle Steel Chunming Liang1 • Min Yu1 • Hao Ma1 • Hui Chen1
Submitted: 5 December 2018 / in revised form: 26 March 2019 / Published online: 2 May 2019 Ó ASM International 2019
Abstract Railway axles suffer severe friction damage during service. In this work, a pure molybdenum coating was prepared on EA4T axle steel by air plasma spraying and hot pressing to improve the steel lifetime. The phase composition, microstructure, hardness, and tribological properties of the coatings were characterized. The results showed that the Mo coating had typical lamellar structure with low porosity and oxidation. Hot pressing compacted the coating and improved the microstructure uniformity. The hardness of the Mo coating and its bonding strength with the substrate were increased after hot pressing under 60 MPa at temperatures ranging between 400 and 500 °C, due to reduced porosity and enhanced mechanical bonding between the coating and substrate. In addition, both the friction coefficient and wear loss of the Mo coating were lower than those of the EA4T steel, illustrating its lubricating effect. Observation of worn surfaces showed that the delamination wear of the Mo coating could be relieved after proper hot pressing due to the strengthened bonding between lamellae. Keywords air plasma spraying EA4T steel microstructure molybdenum coating rail vehicle tribological properties vacuum hot pressing
& Min Yu [email protected] 1
Research Center of Sichuan Advanced Welding and Surface Engineering, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610063, People’s Republic of China
Introduction Axles are some of the most critical parts of railway vehicles. The hollow axles used for high-speed trains usually experience higher speed and stronger vibrations than solid ones, thus such axles held by an interference fit with the wheel seat often suffer severe friction damage and scratching after some time in service (Ref 1). Surface treatment by cladding the surface with a hard coating is usually applied for friction and wear control (Ref 2-4). Thermally sprayed molybdenum (Mo) coating is considered to be one of the best materials for achieving high resistance to scuffing and wear (Ref 5). Application of plasma-sprayed Mo coatings has been reported for automotive parts such as synchronizer rings, cylinder bores, and piston rings (Ref 6, 7). Liu et al. (Ref 8) performed sliding tests on flame-sprayed Mo coatings under boundary lubrication at loads ranging from 300 to 2000 N. It was reported that the wear resistance of the Mo coating was better by a factor of 2-18 compared with the uncoated hardened steel. Laribi et al. (Ref 9) reported that a flame-sprayed Mo coating on 35CrMo4 (Euronorm) steel presented not only good sliding wear resistance but also good antiimpact fatigue property. However, the occurrence of oxidation in the Mo coating during flame spraying can greatly shorten its service lif
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