Study On Oxidation Behavior And Properties Of Si Implantation-coated Molybdenum Alloy In Wide Temperature Range

Molybdenum electrodes are often subjected to service conditions in the temperature range from 400?to 1600?.The design of molybdenum alloy surface protective coatings and the study of molybdenum alloy in wide temperature range oxidation performance are of great significance.In this paper,MoSi₂-based coatings were prepared on pure molybdenum,W-doped molybdenum-tungsten alloy and rare earth La₂O₃-doped molybdenum-lanthanum alloy substrates by encapsulation method.The oxidation properties of MoSi₂ coatings with W and La doping in wide temperature range were systematically studied.The microstructure,composition and oxidation resistance of the coating were tested using a scanning electron microscope,an X-ray diffractometer,and a high temperature muffle furnace.The results show that:?1?MoSi₂ is the main phase and Mo₅Si₃ is the transition phase after Si infiltration treatment of Mo substrate.The weight loss of pure Mo reaches-540.20 mg/cm²after 60minutes of oxidation at 1600?.The oxidation life of pure Mo matrix increases to 25 h after Si infiltration treatment,and the weight loss is only-0.40 mg/cm².MoSi₂ coating still has excellent protection ability after long-term oxidation at 1200?and 800?,and the high temperature oxidation resistance is significantly improved.However,during the constant temperature oxidation at 500?,the dense oxide film on the surface of MoSi₂ coatings was destroyed after 400 hours,and the"Pest"effect was produced.?2?The W element is doped into the MoSi₂ coating to form a?Mo,W?Si₂ solid solution,and the microstructure of the coating is becoming more densified.The time of oxidation failure of?Mo,W?Si₂ coatings was significantly increased under the condition of static oxidation at 1600?.The addition of W element hinders the diffusion reaction between Si and matrix,and reduces the consumption rate of Si in the coating.Thereby,the high temperature oxidation resistance of the?Mo,W?Si₂ coating is remarkably enhanced.Under the static oxidation condition of 500?under low temperature,compared with the MoSi₂ coating,the?Mo,W?Si₂ coating produced a significant"Pest"phenomenon at the initial stage of oxidation,and the coating was severely powdered and failed.The addition of W reduces the diffusion rate of Si element in the coating,which results in thedense oxide layer unable to form in time on the coating surface at low temperature,and further aggravates the rapid oxidation of the coating.?3?The addition of La element acts to refine the grain,the inside of the coating is uniform and dense,and defects such as cracks and holes are significantly reduced.Under the static oxidation conditions at 1600?and 1200?,the La element doping has a certain negative effect on the high temperature oxidation resistance of the MoSi₂ coating.The addition of La reduces the thermal expansion coefficient of MoSi₂ coatings,which leads to the increase of thermal expansion imbalance between coatings and substrates,thus causing the oxidation interface between coatings and substrates to fail earlier.The addition of La accelerates the diffusion of Si element in the coating at medium and low temperature under static oxidation condition,and can form a dense oxide layer on the surface faster,which effectively hinders the permeation of oxygen.At the same time,the grain size in the coating is refined,the grain boundary is increased,and the oxygen diffusion potential can be decomposed by more diffusion paths,which makes the oxygen diffusion more likely to terminate at the grain boundary,thus significantly enhancing the oxidation resistance at medium and low temperature in the coating.