| Molybdenum?Mo?and its alloys are widely used in aerospace,glass,electronics,metallurgy,solar cells and other industries due to their excellent high-temperature comprehensive properties.However,the volatilization of MoO3 generated by the reaction of Mo with oxygen leads to the structural failure of molybdenum.The high temperature performance of MoSi2 coating is stable,and the surface of the coating can form a stable and dense SiO2 in the oxidation,preventing the reaction between oxygen and the Mo substrate.However,the thermal expansion coefficient of MoSi2 and Mo substrate is very different,and Si atoms in MoSi2 are easily diffused at high temperatures,resulting in failure of MoSi2coating.It is found that the addition of boron?B?during the preparation of MoSi2 coating can form a boride structure that prevents the high-temperature diffusion of Si in MoSi2.However,the preparation process of B modified MoSi2 coating is mostly co-deposition method,in which the coating has low density,the structure gradient is not obvious,and the high-temperature oxidation resistance needs to be further improved.Therefore,in this paper,a rare earth yttrium?Y?modified Si-B coating was prepared on the surface of molybdenum by a two-step pack cementation process.The process of boronizing the molybdenum surface and siliconizing on the surface of the Mo-B coating was optimized.The morphology,phase composition and composition distribution of the coating were analyzed using XRD,EDS,and SEM,respectively.The oxidation resistance of Si-B-Y coating prepared by different processes and the Si-B-Y oxidation and failure mechanisms prepared by the two-step pack cementation process were studied.The conclusions are as follows:?1?The outer layer of the Mo-B coating prepared by the pack cementation on the surface of substrate is Mo2B5,the sub-outer layer is MoB,and the Mo2B is close to the substrate.The outer layer of the Mo-Si-B coating prepared by the pack cementation on the surface of Mo-B coating is MoSi2,the inner layer is MoB,and the side close to the substrate is Mo2B.The density of the outer MoSi2 layer is high and the structure gradient of the coating layers is obvious.The addition of 2wt.%Y to the entrapment agent can promote the formation of Mo2B5 on the surface of Mo-B coating,increase the thickness of the Mo-B coating and refine the grain size of MoSi2.?2?The Si-B-Y coating prepared by the two-step pack cementation has better comprehensive oxidation resistance than the Si-B-Y co-deposition coating.The parabolic oxidation rate constant(Kw=1.20×10-4 mg2/?cm4·h?)of Si-B-Y coating prepared by two-step pack cementation at 1150? for 105 h is lower than that of Si-B-Y co-deposition coating(Kw=5.60×10-4 mg2/?cm4·h?).When the oxidation time was prolonged to 275 h,the Si-B-Y coating structure prepared by two-step pack cementation still had no failure with its Kw value of 3.96×10-4 mg2/?cm4·h?.After oxidizing at 1600? for 3 h,Si-B-Y co-deposition coating showed weight loss with a weight loss of 0.45%,while the two-step pack cementation Si-B-Y coating showed a weight gain with a weight gain of 0.076%.?3?The main reason for the high temperature oxidation failure of the two-step pack cementation Si-B-Y coating is the diffusion of Si,which is characterized by the thickening of the Mo5Si3 layer.Compared with MoSi2 coating,the growth rate of Mo5Si3 is low at 5.35×10-4?m2/s when Si-B-Y coating is prepared by two-step pack cementation at 1150?.With the oxidation temperature increased to 1600?,the Mo5Si3 growth rate in the coating is still significantly lower than the MoSi2 coating,indicating that Si-B-Y coating prepared by two-step pack cementation effectively slows the diffusion of Si and prolongs the life of the oxidation resistance coating. |
PDF Full Text Request