Study On Microstructure And Preparation Of Mosi2 Coating For A Nickel-base Alloy

Molybdenum disilicide(MoSi₂) has long been regarded as an attractive coating material for refractory metal, graphite, and C/C composites used in an oxidative atmosphere at high temperature because of its ability to from an adherent and continuous SiO2 film on the surface. Nickel-alloy has been widely applied in various fields for its excellent mechanical properties at high temperature. However, it exhibits poor high temperature oxidation resistance when the temperature is above 1000℃. Protection coating such as aluminide coating, thermal barrier coating(TBC) and microcrystallized coating except MoSi₂ coating has been prepared on nickel-alloys. In this paper, the effect of main parameters, such as spraying power, spraying distance, Ar gas flow rate and powder feed rate, on the phase composition and microstructure of MoSi₂ coating, which prepared by plasma spraying of MoSi₂ powders by self-propagating high-temperature synthesized (SHS) and agglomerated, on Nickel-base alloy was investigated. Subsequently the bonding mechanism of coating also was analyzed. The results are shown as follows:1.The effect of spraying parameters on coating is obvious, when MoSi₂ powders synthesized by SHS as the raw material. While oxidation took place during spraying due to over small size of the powder, rich-Mo phase in coating is hardly reduced. Relatively ideal MoSi₂ coating is prepared with 30kw of spraying power, 100mm of spraying distance, 50L/min of Ar gas flow rate and 18g/min of powder feed rate. However, there are still more rich-Mo phase in the coating.2.The problem of unfavorable phase composition in the MoSi₂ coating, when MoSi₂ powders synthesized by SHS as the raw material, can be solved by taking MoSi₂ agglomerated powders as the raw material with the size of 30～54μm. The coating in microstructure is compact and had little pore. With this material, the influence of spraying power and Ar gas flow rate on the composition of coating phase is less dramatic, while the influence of spraying distance and powder feed rate is distinct. The best spraying parameters show as the following: 35kw of spraying power, 100mm of spraying distance, 45L/min of Ar gas flow rate and 21g/min of powder feed rate.3.The bonding strength of the MoSi₂ coating is 20.3MPa. The interface of NiCoCrAlY layer and MoSi₂ layer shows a mechanical bonding form, while there are some thick flat pores in partial area. The interface of NiCoCrAlY layer and substrate shows metallurgical bonding from. The interface of NiCoCrAlY layer and MoSi₂ layer is the weak part of this coating, which may be due to the mismatch of thermal expansion coefficient.