Preparation Of MoSi₂-based High Temperature Oxidation Resistance Coating From Spent Molybdenum Disilicide Heating Elements

The affection of resource consumption on ecological environment is one of the problems must be faced in the course of social development,and the recycling of secondary resources is an important part to solve it.While widely employed by industrial production and experimental research,spent molybdenum disilicide,as commonly used heating elements,will also be heavy consumption.This results in a large number of spent molybdenum disilicide heating elements.The main component of spent molybdenum disilicide heating elements is molybdenum disilicide（MoSi₂）,the molybdenum content of MoSi₂ up to 63wt%.Molybdenum not only plays an important role in manufacturing and petrochemical industry,but also is an irreplaceable part of non-fossil-fuel energy generation in future.Therefore,the utilization of waste molybdenum disilicide rods have great significance.MoSi₂ has good high temperature oxidation resistance,which is especially suitable for providing oxidation protection for high temperature structural materials as oxidation protection coating.In this paper,recycling waste MoSi₂ rods to fabricate high temperature oxidation resistance coating is proposed,which can totally depose of waste MoSi₂ rods,improve the oxidation resistance of high temperature structural materials,and achieve the green recovery of molybdenum secondary resources.In this paper,the preparation and high temperature oxidation resistance of silicide coating were studied.Spent molybdenum disilicide heating elements,main raw materials for coating preparation,was used in coating fabrication on the molybdenum alloys and graphite.Four coating preparation processes include atmosphere plasma spray（APS）,liquid phase sintering,slurry method and spark plasma sintering（SPS）were adopted,and the oxidation test at 1400℃ were also carried out.Then,the influence of simple substance on microstructure and high temperature oxidation resistance of MoSi₂-based coating were also investigated.The MoSi₂-based coating with boron and carbon added was prepared by spark plasma sintering,and the oxidation test at 1500℃ were also carried out.The main results are as follows:（1）The four coating preparation processes were compared.The coating deposited by APS has some holes about 20μm,which is caused by the air involved in the spraying process.The thickness of SPS fabricated coating is 500μm,due to the radial pressure during sintering process,the coating was dense and without any cracks or holes.The coating after liquid phase sintering and densification is 100μm thick without any defects,which shows the densification process can reduced surface defects.There are pores in the coating sintered by slurry method,which is an unavoidable defect in slurry process.（2）In oxidation test at 1400℃,the coating sintered by slurry method has mass gain 5.88mg/cm² after 20h oxidation,the coating after liquid phase sintering and densification has mass loss 0.23mg/cm²,and SPS fabricated coating has mass gain9.93mg/cm² after 90h oxidation.These coatings all show good oxidation resistance.The reason is that the Si of MoSi₂ will be oxidized to SiO₂ which can prevent the diffusion of oxygen during oxidation period.（3）The addition of carbon has generated Mo_4.8Si₃C_0.6 besides transition the oxides in coating into network distribution.However,the net distribution of oxides will lead to internal oxidation of substrates.The addition of boron only generates some borides of molybdenum.During the oxidation period,the additional of boron not only improves the fluidity of the oxide film,but also forms improve the high temperature stability of the oxide film by the formation of ribbons of MoB layer in the oxide film.After 20h oxidation,the MoSi₂ coating with 10 vol%carbon addition was mass loss 1.78mg/cm²and the mass gain of MoSi₂ coating with 10 vol%boron addition was 1.41mg/cm².（4）Oxidation test shows that the MoSi₂-based coating prepared from spent molybdenum disilicide heating elements has the potential to serve in high-temperature anti-oxidation environments for a long time,it is a new technology of recovering spent molybdenum disilicide heating elements.