Experimental investigation and first-principles calculations of Nb and W alloying effects on the microstructure and properties of MoSi2 coatings fabricated via arc cladding

Abstract

MoSi2 is one of the most promising refractory metal silicide materials, but its further use as a structural material is limited by its drawbacks such as poor room-temperature toughness and low high-temperature strength. The work performed a comprehensive investigation combining first-principles calculations and arc cladding experiments to explore the effects of Nb and W doping on the mechanical properties and electronic structure of MoSi2 coatings. The first-principles calculations revealed that Nb addition improved the B/G value and Poisson’s ratio of MoSi2, indicating enhanced ductility. W addition yields the opposite effect and led to a higher elastic modulus and improved hardness. Experimental results demonstrated that the arc-cladding MoSi2 coating mainly consisted of MoSi2 and Mo5Si3 phases with a dendritic microstructure. Upon doping with Nb and W, additional t-(Mo,Nb)Si2 and t-(Mo,W)Si2 phases were formed, which resulted in a denser and finer microstructure. Nb addition contributed to the solid-solution toughening of the coating, while W addition enhanced hardness but reduced toughness. Remarkably, the synergistic alloying of Nb and W significantly increased the hardness and fracture toughness of the coating by 30.7 and 70.7%, respectively, compared to pure MoSi2. The strengthening mechanism of the coating was attributed to solid-solution softening and fine-grain strengthening, while the crack extension mechanism involved the crack deflection and bridging. Furthermore, the coatings doped with 2% Nb and 4% W exhibited the lowest wear weight loss and superior wear resistance. The dominant wear mechanisms were oxidation wear and abrasive wear.