High Temperature Oxidation Resistance of Y-doped Mo-Si-B Coating by Spark Plasma Sintering
Nb-Si based ultra-high-temperature structural materials exhibit high melting points and excellent mechanical properties at high temperatures,which are research focus areas of structural materials for next-generation aero engines.However,the high-temperature oxidation resistance of Nb-Si based alloys is poor because non-protected Nb-containing oxides are spontaneously produced without the formation of a protective oxide scale during oxidation.Because of the formation of a continuous and dense SiO2 oxide scale,MoSi2-based coatings are expected to be protective coatings for Nb-Si-based alloys.However,it suffers from pest oxidation at temperatures between 400 ℃ and 600 ℃.To avoid pest oxidation of MoSi2,B was introduced to form a continuous and dense borosilicate(B2O3-SiO2).Borosilicate has higher fluidity than SiO2,which can accelerate the formation of the oxide scale and improve the intermediate-temperature oxidation resistance of the MoSi2 coating.Studies have shown that rare-earth elements,including Y,can improve the oxidation resistance of materials.However,the effect of Y on the oxidation resistance of Mo-Si-B coatings remains unclear.In this paper,Nb-Si based alloys were used as substrates,and spark plasma sintering(SPS)was employed to prepare 33Mo-62Si-5B(at.％)and Y-doped Mo-Si-B coatings on Nb-Si-based alloys.The phase constitution and microstructure of the coatings were characterized by X-ray diffraction and scanning electron microscopy,respectively.The presence of Y in the coating was confirmed by transmission electron microscopy and selective electron diffraction patterns.Subsequently,the high-temperature oxidation behavior and effect of Y on the high-temperature oxidation resistance of the coatings were studied during high-temperature oxidation experiments at 1 250 ℃ for 100 h.The results showed that the Mo-Si-B and Y-doped Mo-Si-B coatings had low porosities of 0.46％and 0.56％,respectively.Both coatings consisted of MoSi2,MoB,Mo5Si3 and SiO2.Mo5Si3 and SiO2 were derived from the oxidation of MoSi2 during SPS.In addition,Y was segregated in the interior of SiO2 in the form of Y2Si2O7 in the Y-doped Mo-Si-B coatings.During the initial stage of the oxidation experiment,the Nb-Si-based alloy coated with the Mo-Si-B coating had weight loss owing to the volatilization of MoO3.The oxidation weight increase of the alloy coated with Mo-Si-B coating after 100 h was 0.421 mg cm-2,which was higher than that of the alloy samples coated with Y-doped Mo-Si-B coating(0.351 mg·cm-2).Moreover,the oxidation rate constant(kp)of the Mo-Si-B coating was 2.0×10-3 mg2·cm-4·h-1,which was also higher than that of the Y-doped Mo-Si-B coating.The oxidation kinetics of the coatings indicate that the Y-doped coating has better high-temperature oxidation resistance.After oxidation,an oxide scale was formed on the surface of the Mo-Si-B coating,consisting of crystal SiO2 and borosilicate glass.Additionally,Y2Si2O7 emerged in the oxide scale of the Y-doped Mo-Si-B coating.Owing to its low Gibbs free energy,Y2O3 was preferentially formed on the surface of the Y-doped Mo-Si-B coating.Y2O3 particles provided nucleation points,accelerating the formation of crystalline SiO2 and borosilicate glass.The subsequent formation of Y2Si2O7 prevented the inward diffusion of O and improved the oxidation resistance of the coatings.This study proposes a new method for oxidation protection of Nb-Si-based alloys and promotes the application of Nb-Si-based alloys in aero-engine high-pressure turbine blades.
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