Investigation On The Properties Of Mo₅Si₃/MoSi₂Gradient Nano/Nanocomposite Coating

Molybdenum disilicide （MoSi₂） is one of the most potential candidates for high-temperaturestructural applications, primarily due to its high melting temperature, relatively low density, goodoxidation resistance and metal-like thermal and electrical conductivities. Unfortunately, its lowtoughness at ambient temperature remains a major obstacle to a wide range of practical applications.In general, this drawback of MoSi₂can be effectively improved by several strategies, including theincorporation of a suitable reinforcements, solid solution alloying and grain refinement.Monolithic nanocrystalline MoSi₂coating with a mean grain size of5nm has been prepared ontopure Ti substrates successfully by double glow discharge. Then, three kinds of MoSi₂/Mo₅Si₃gradientnano/nanocomposite coatings and three kinds of SiNx/Mo₅Si₃/MoSi₂amorphous/nanocompositecoatings with different Al content （0at%,5at%,10at%） were deposited onto the substrates bychanging the stochiometric ratios of the targets and deposition processes. Whereas the extent oftoughening achievable with single strategy is relatively limited, this study will try to find new possibleway to bypass the shortcoming of low fracture toughness of monolithic MoSi₂by overlap of multifoldtoughening strategies.The microstructure and chemical composition of the as-deposited coatings were studied using XRD（D8ADVANCE with Cu Kα radiation）, SEM （Quanta200, FEI Company）, FIB （Nova Nanolab200,FEI Company）, TEM （Tecnai G220） and energy-dispersive X-ray spectroscopy （EDX）. All coatingsare quite well adhesive to the substrate and composed of deposited layer and diffusion layer. Theresults revealed that the MoSi₂/Mo₅Si₃gradient nano/nanocomposite coatings possess a bimodalmicrostructure in plan-view, which are composed of C40-MoSi₂with a mean grain size of5nm andD8m-Mo₅Si₃with a mean grain size of50nm, and exhibit a typical columnar microstructure incross-section, which is separated by the wavy-like Mo₅Si₃interlayers due to the up-hill diffusion ofMo element. The SiNx/Mo₅Si₃/MoSi₂amorphous/nanocomposite coatings consist of C40-MoSi₂witha mean grain size of5nm and D8m-Mo₅Si₃with a mean grain size of10nm, as well as large mounts ofgrain boundry located amorphous SiNx. The results from mechanical properties tests show that withthe increasing amount of Al addition the hardness of MoSi₂/Mo₅Si₃gradient nano/nanocompositecoatings slightly decreased, whereas the toughness firstly increases and then decreases. However, boththe hardness and toughness are enhanced with the increasing of Al addition for SiNx/Mo₅Si₃/MoSi₂amorphous/nanocomposite coatings. The results of wear tests indicate that all the MoSi₂/Mo₅Si₃ gradient nano/nanocomposite coatings have superior wear resistance than monolithic nanocrystallineMoSi₂coating. The wear resistance of the MoSi₂/Mo₅Si₃gradient nano/nanocomposite coatings andthe SiNx/Mo₅Si₃/MoSi₂amorphous/nanocomposite coatings are improved with the increase of contentof Al by changing the dominant wear mechanism. During electrochemical test at room-temperature in3.5wt%NaCl solution, the existence of Mo₅Si₃is detrimental to the corrosion resistance ofnanocrystalline MoSi₂, whereas the addition of Al evidently improves the corrosion resistance of theas-deposited coatings.