| With the rapid development of industrial science and technology in China,the development and demand for mineral resources are increasing,and a large number of waste resources are generated.The accumulation of industrial solid waste not only causes environmental pollution,but also leads to serious resources waste.The realization of"zero waste"through comprehensive utilization of waste resources has become the key to the sustainable development of China’s mining resources.As a kind of resistance heating element with high-temperature resistance and oxidation resistance,the maximum working temperature of Mo Si2 heating elements can reach 1800℃.As Mo Si2 heating elements are used in high-temperature oxygen-containing environment,a continuous and dense Si O2 oxide scale can be formed on its surface,which will protect the inner layer of the Mo Si2 heating elements from oxidation.With the development of industrial technology,the use of Mo Si2 heating elements has been increasing,and a large number of spent Mo Si2 heating elements were abandoned in factories and laboratories after long-term use,which results in the resources waste and environmental pollution.In consideration of the excellent anti-oxidation properties of Mo Si2 heating elements,it is of great significance to develop a new kind of high-temperature anti-oxidation coatings on the surface of refractory metals by the recycling of spent Mo Si2heating elements.In this work,spent Mo Si2 heating elements were used as main coating raw materials,and refractory metals such as Niobium and Molybdenum were used as substrates.Mo Si2-based composite coatings were prepared on the surface of refractory metals by Spark Plasma Sintering and slurry.The phase composition and microstructural evolution of the coatings before and after high-temperature oxidation were characterized by XRD,SEM,EPMA,TEM.The thermal expansion coefficient of the refractory metal oxides and transition layers of refractory metal borides in Mo Si2-based coating system was analyzed,and related effects on the microstructure,morphologies and high-temperature oxidation resistance of Mo Si2-based composite coatings were investigated.A finite element model of residual thermal stress of for the the coatings after high-temperature sintering was established,and the growth behavior of the composite oxide scales of the coatings in high-temperature environments was explored,and relevant high-temperature antioxidant protection mechanism was studied.The preparation substrate and the mechanism of high-temperature oxidation resistance of Mo Si2 coating on niobium were studied by spark splasma sintering.The results showed that the spent Mo Si2 powders were made up of Mo Si2,Mo5Si3 and small amounts of bentonite（8.61 wt.%）,and Mo Si2 phase was covered by bentonite.After SPS,MS coating is dense and crack-free with low porosity（1.48%）.Although pores were formed in MC coating to promote oxygen diffusion at high temperature,（Mo,Nb）5Si3 diffusion layers were formed after sintering in the coatings,which strengthened the bonding between the substrate and coating.After oxidation at 1500°C for 40 h,Si O2 oxide scales were formed on the surfaces of MS and MC coatings,but voids were found on the oxide scale surface of MC coating while it was dense and crack-free for MS coating.The mass gain of MS coating was only 2.78 mg/cm2,indicating a few Mo Si2 phases were consumed during high-temperature oxidation,thus showing better high-temperature oxidation resistance than MC coating.The effect of refractory metal oxides on the microstructure,morphologies and high-temperature oxidation resistance of Mo Si2-based composite coatings on niobium substrates were investigated using spark plasma sintering.The results showed that cracks were observed in Mo Si2 coating after sintering,and the addition of Zr O2,Hf O2and Ta2O5 into Mo Si2 can reduce the thermal expansion coefficient（CTE）mismatch between the substrate and coating,and the three composite coatings are dense and complete with crack-free morphologies,and diffusion layer was formed between the substrate and coating.After oxidation of 20 h at 1500℃,cracks still existed in the surface of the oxide scale of Mo Si2 coating while the oxide scales of Mo Si2-Zr O2,Mo Si2-Hf O2 and Mo Si2-Ta2O5 composite coatings showed low oxygen permeability,high viscosity and high stability after oxidation,presenting excellent high-temperature oxidation resistance.The influence of refractory metal silicides and Si C on the microstructural evolution and high-temperature oxidation resistance of Mo Si2 composite coatings on niobium substrate were studied by spark plasma sintering.The results showed that obvious cracks appeared in the Mo Si2 coating after SPS sintering,and the addition of Si C alleviated the mismatch of CTE between the coating and substrate,and complete and dense Mo Si2-Zr Si2-Si C composite coating was formed after sintering.During the oxidation process at 1500℃,Si O2 oxide scale with cracks was formed on the surface of Mo Si2 coating,which resulted in a substantial and continuous increase in the mass gain of the coating.Zr-Si-O composite oxide scale with high viscosity and low oxygen permeability was formed on Mo Si2-Zr Si2-Si C composite coating after oxidation,and the crack-free oxide scale inhibited the diffusion of oxygen,thus the composite coating showed low mass gain.Mo Si2 coating was oxidized at 1700℃for 120 min,and the circular Mo5Si3 phase appeared with large numbers of pores on the surface while the Mo Si2-Zr Si2-Si C composite coating had complete and dense oxide scale.During the oxidation process from room temperature to 1700℃,it still maintained complete and defect-free layered structure,thus showing excellent oxygen inhibition properties.The preparation and high-temperature oxidation resistance mechanism of Mo Si2-based coating on molybdenum substrate were explored by spark plasma sintering and slurry,respectively.The results showed that cracks appeared in Mo Si2 coating after SPS while Mo Si2-bentonite composite coating that prepared by the reduced Mo Si2 heating elements fabrication process obtained complete and dense morphologies,and defect-free Mo5Si3 diffusion layer was also observed.After oxidation at 1500°C for 300 min,the bentonite-containing composite coating obtained relatively lower mass gain and thinner oxide scale during high-temperature oxidation than Mo Si2 coating,indicating that complete Si O2 oxide scale can be formed on the composite coatings after only small amount of oxidation.In addition,Mo Si2 coatings prepared by slurry exhibited lower porosity and crack-free morphologies after high-temperature sintering.After oxidation at 1500℃for 20 h,the mass gain of the coating was only 6.28 mg/cm2,therefore showing excellent high temperature oxidation resistance.The effect of refractory metal borides on the microstructure,morphologies and high-temperature oxidation resistance of Mo Si2-based gradient coatings on molybdenum substrate were investigated using a two-step method of slurry and spark plasma sintering.The results showed that Mo Si2 coating possessed cracks after sintering while Mo Si2/Mo B gradient coating has only tiny cracks after sintering,and Mo B and Mo2B diffusion layers were formed under the coating,showing good metallurgical bonding.Dense and continuous oxide scale was generated on the gradient coating during the oxidation process at 1500℃,and lower mass gain than the Mo Si2coating was also obtained.In addition,Mo Si2-Zr B2/Mo Si2 gradient coating also showed crack-free morphologies after high-temperature sintering,and Mo5Si3 and Mo B diffusion layers were formed in the interface of coating/substrate layers.After oxidation of 100 h at 1500°C,Mo Si2-Zr B2/Mo Si2 gradient coating exhibited low oxidation rate constant and mass change due to its continuous and crack-free Si O2 oxide scale,thus showing excellent high-temperature anti-oxidation properties. |
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