The work aims to improve the mechanical and wear properties of Cr2O3-based coatings prepared by plasma spraying through the introduction of niobium oxide (Nb2O5) and molybdenum (Mo) in an attempt to apply them to the hard sealing surfaces of slag-locking valve balls and to enhance their service life. Slag locking valves play a crucial role in coal chemical production, mainly for the precise separation of slag and slag water. However, hard seal surface wear caused by slag has become an important issue in recent years. In this study, two types of coatings were prepared by high-energy plasma jet (HEPJ) technology with a matrix consisting of Cr2O3-5%SiO2-3%TiO2. Molybdenum (4%) and niobium oxide (4%) were added to the coatings, respectively. The spraying parameters were set as follows: voltage 115 V, current 370 A, powder feeding rate 35 g/min, spraying distance 120 mm, horizontal gun travelling speed 800 mm/s, and gun perpendicular to the substrate. The resulting coating thickness was 300 μm, including a 100 μm transition layer and a 200 μm deposition layer. In order to simulate the actual working conditions, wear tests were carried out with a multifunctional wear tester (MS M9000) at a load of 15 N, with a wear length of 5 mm, a frequency of 2 Hz and a time of 60 minutes, using 316L stainless steel balls for reciprocating friction. Electrochemical corrosion tests were carried out in a mixture of 5% HCl and 5% H2SO4 with CHI600F electrochemical workstation. The experimental results showed that the addition of 4% Mo and 4% Nb2O5 decreased the porosity from (2.653±0.03)% to (1.308±0.03)% and (2.038±0.03)%, respectively, which indicated an improvement in the densification of the coatings. The mechanical properties of the coatings were all improved, and friction tests showed that the friction properties between the contact pairs were significantly improved. Specifically, the average friction coefficient decreased from 0.576 2 to 0.525 6 with the addition of Mo, and further to 0.431 1 with the addition of Nb2O5, while the depth of the wear track decreased from 13.4 μm to 4.7 μm and 3.7 μm, respectively, and the wear rate decreased from 1.58×10-5 mm3/(N·m) to 2.77×10-5 and 2.18×10-5 mm3/(N·m) respectively. It was also found that the main wear mechanisms of the coatings were fatigue wear and abrasive wear. In addition, the self-corrosion current density of the coatings decreased from 2.275×10-5 A/cm² to 1.103×10-6 A/cm² (with Mo) and 5.937×10-6 A/cm² (with Nb2O5), while the corrosion potentials increased, and the Nb2O5-added coating had the largest EIS half-arc and the highest value of |Z| in the low-frequency region, indicating that the corrosion resistance of the coating was significantly improved in the chloride ion environment. In conclusion, the addition of 4% Nb2O5 shows the best effect in improving the comprehensive performance of the coating, which not only improves the wear and corrosion resistance of the coating, but also provides a more reliable material support for slag locking valves in harsh working environments. These findings suggest that modified coatings have the potential to be an ideal solution for improving the durability and extending the service life of critical industrial components.
Key words
slag valve /
supersonic plasma spraying /
chromium oxide coating /
friction wear resistance /
corrosion resistance
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Funding
Special Funding for Special Pump and Valve Materials Key Laboratory in Wenzhou, Zhejiang Province, China (2024ZDSYS)
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