Study On Preparation And Properties Of Silicides Coating On The Molybdenum Surface By In-situ Synthesizing

Molybdenum has the extremely high melting point (2622℃) as well as the good resistance to wear, the corrosion resistance and the electricity heat conductivity, which is often used as high temperature heating element. But the molybdenum trioxide formed on the surface of molybdeum in the high temperature atmospheric environment is easy to be volatilizned. Therefore the molybdenum only used in vacuum or the protective atmosphere as high temperature heating element. Molybdenum disilicide can be used as anti-oxidation coating of refractory metals molybdenum and alloys for high temperature, which is due to the higher melting point, thermal shock resistance, good thermal conductivity and excellent high temperature oxidation resistance. However, the difference of thermal expansion coefficient between MoSi₂(8.1×10^-6K^-1) and Mo(5.8×10^-6K^-1 ) is much larger, the mismatch of thermal expansion between MoSi₂ coating and molybdenum substrate results in the cracks easily occur during the cyclic oxidation process. The nitrogen element can change the microstructure of Mo-Si coatings and improve the thermal performance matching. Silicide dopped with boron element has better oxidation resistance property due to the protective film of borosilicate glass. Therefore, ammonia nitriding, molten salt technique for siliciding and boronizing are used to prepare Mo-Si-N-B system silicide coating. The main parameters such as salt content, temperature and holding time on the microstructure, phase composition and oxidation properties of the Mo-Si-N-B system coating are discussed, and the anti-oxidation mechanism is studied. The main results are as follows:1.A Mo-Si coating on Mo substrate was prepared by molten salt technique.The microstructures of the coating were MoSi₂, Mo₅Si₃ and matrix from the outside to the inside. Activator plays a great impact on the thickness of coating. When Na2SiF6 together with NaF in the molten salt, their activation is more evident.The thickness of coating increases with the increase of temperature and holding time, and effect of temperature on the thickness of coating is more sensitive than that of holding time.The Mo-Si coating prepared at 1000℃for 10h has good high-temperature oxidation resistance, the cyclic oxidation at 1200℃can continue to more than 100 hours.2.A Mo-Si-N coating on Mo substrate was prepared by molten salt technique pretreated by ammonia nitridation. The phase of MoSi₂ and Si₃N₄ were formed on the surface of coating. Mo-Si-N coating on Mo substrate has more excellent oxidation resistance than Mo-Si coating. The coating has good high-temperature oxidation resistance prepared by ammonia nitriding at 1000℃and at flow rate 200ml/min for 2h and by molten salt at 1000℃for 2h, high temperature cyclic oxidation life of the Mo-Si-N coating on Mo substrate is continued to 78h at 1450℃and is continued to 48h at 1600℃.3.A Mo-Si-N-B coating on Mo substrate was prepared by molten salt technique. The phases of MoSi₂, Mo₂B₅ and Si₃N₄ are formed on the surface of coating. Mo-Si-N-B coating has excellent high temperature cyclic oxidation resistance and the cyclic oxidation life can hold 100h at 1450℃.4.The formation of a compact SiO₂ layer on the surface of the Mo-Si-N-B system coating improved high temperature cyclic oxidation resistance. The nitrogen element can increase the density of coating, and the boron element can improve the liquidity of borosilicate glass phase at high temperature. With the diffusion reaction accelerating, the thickness of Mo₅Si₃ increased, and the oxidation resistance performance decreased, the continuous SiO₂ film was destroyed by the volatile MoO3, which resultes in the coating failure after the coating oxidized for a certain time.