Research On The Microstructure And Properties Of Cr-alloyed MoSi₂and Mo₃Si Nanocomposite Coatings

Owing to its unique combination of a high melting temperature, good corrosion and oxidationresistance, molybdenum disilicide is becoming one of the most potential candidatematerials for structural applications at ultra-high temperature. The “Pest Oxidation” greatlyreduces its reliability and capability in practical applications. Much effort has been devotedto address these issues through solid solution alloying or forming nanocomposite by theincorporation of a suitable second phase. To date, there are relatively few studies for Mo3Si, So thelatter part of this article is intended to Mo3Si and performance changes after adding Cr elementsstudied.Four kinds of nanocrystalline (Mo1-xCrx)Si2(x-=0,0.1,0.2,0.3) coatings are prepared onto a TC4substrates through a double cathode glow discharge apparatus. XRD, SEM and TEM results showedthat the coating is mainly concluded two layers. The microstructures are dense and homogeneouswithout any visible defects. Scratch experiments showed that the Cr content is higher, the bindingforce between coating and substrate is also stronger. Wear experiments showed that the wearresistance of nanocrystalline (Mo1-xCrx)Si2films were improved by the Cr alloying.The oxidation experiments of nanocrystalline (Mo1-xCrx)Si2coatings have been studied at500℃.Results show that the “Pest Oxidation” occurred in the MoSi2coating at500℃. However,followingthe Cr content increase, it can effectively prevent the entry of oxygen to improve the oxidationresistance of the coatings. Electrochemical measurements were used to analyse, after nanocrystalline(Mo1-xCrx)Si2films were oxided at500℃with different time.The electrochemical results showed:The Cr content is higher, the porosity is smaller, the well dense oxide film which takes bettercorrosion resistance.Four kinds of nanocrystalline (Mo1-xCrx)3Si(x-=0,0.09,0.20,0.29) coatings are prepared onto aTC4substrates by the same method. By XRD, SEM and other means of testing show that the coatingis dense and no obvious holes, projections and other structural defects exist. The coating is alsoconcluded two layers. The nanoindentation tests showed that H and E of the nanocrystalline(Mo1-xCrx)3Si films decreased with increasing Cr content. We want to determine the toughness of thecoating by Vickers indentation method. The results show that the Cr content increases, the hardnessvalue of nanocrystalline Mo3Si coatings decreased slightly, but the toughness of the coating has beenimproved. Finally, to gain a well understanding of the effect of Cr alloying on the electrochemicalbehavior of the (Mo1-xCrx)3Si coatings, electronic structure and Mulliken populations have been calculated by first-principles theoretical calculations.