Microstructure And Performance Of Si-Al-Y Co-deposition Coating On The Surface Of TiAl Alloy

This thesis has put forward the failure of TiAl alloy that resulted from oxidation,wear, hot corrosion and erosion during applications. The Si-Al-Y co-depositioncoatings on a TiAl alloy were prepared by pack cementation processes. Scanningelectron microscope (SEM), energy dispersive X-ray spectrometer (EDS) and X-raydiffractmeter (XRD) were employed to investigate the phase constituents,compositional distribution and microstructure of the coatings in this paper. Theoxidation resistance, hot corrosion, erosion and tribological behaviors of both TiAlalloy and the Si-Al-Y co-deposition coatings were investigated, aiming at elucidatingthe anti-oxidation mechanism and the mechanism of friction, hot corrosion and wear.The main contents and conclusions of this work are listed as follows:The microstructure and composition of Si-Al-Y co-deposition coatings wereaffected by process parameters including the type of catalysts, content of catalyst,content of rare earth and Al, testing temperature and soaking time. The deposition ofAl and Si occurred in a sequential manner during the pack cementation process ofSi-Al-Y co-deposition coating. The Si-Al-Y co-deposition coating prepared by theoptimized parameters (A pack mixture with a composition10Si-10Al-1Y2O3-8AlCl36H2O-71Al2O3(wt.%), co-deposition temperature:1050℃, co-depositiontime:4h) have a multiple layer structure: a10μm thick outer layer composed of TiSi2,a7μm thick middle layer composed of (Ti,X)5Si4and (Ti,X)5Si3(X represents Nb andCr elements), an15μm thick inner layer composed of TiAl2, γ-TiAl phases and a5μm thickAl-rich inter-diffusion zone.The oxidation kinetic of the Si-Al-Y co-deposition coating at1000℃followed aparabolic law, and the parabolic rate constant of the coating was lower than that of theTiAl alloy by about2orders of magnitude. The excellent oxidation resistance of thecoating was attributed to the formation of a dense scale mainly consisted of TiO2,SiO2and Al2O3, which inhibits the O element diffusing into the coating effectively.However, the internal diffusion of Si, external diffusion of Ti and both internal andexternal diffusion of Al lead to the crack and failure of Si-Al-Y co-deposition coating.The wear mechanisms of TiAl alloy and Si-Al-Y co-deposition coating wereobvious various when under the different experimental conditions, which could bereflected in the curves of friction coefficient, mass loss in sliding and wear morphology. Taken as a whole, the Si-Al-Y coating had much better wear resistancethan that of TiAl alloy at either normal or high temperature.The hot corrosion behaviors of TiAl alloy and Si-Al-Y co-deposition coating in75%Na2SO4+25%K2SO4mixed melts at900℃were investigated. The results showedthat the inter-phases were selective corrosed, which caused the corrosion pits initiatedfrom lamellar interfaces and the preferential corrosion of α2phases, was observed atthe initial stage of hot corrosion of TiAl alloy, followed by catastrophic hot corrosion.The Si-Al-Y co-deposition showed better hot corrosion resistance at the initial stagesof hot corrosion. However, with the time prolongation, the Si-Al-Y co-depositioncoating failed for the formation of internal sulfidation, which lead to the occurrence ofcracks. The hot corrosion behaviors of TiAl alloy and Si-Al-Y co-deposition coatingin75%Na2SO4+25%NaCl mixed melts at850℃were also investigated. The resultsshowed that the hot corrosion of both TiAl alloy and Si-Al-Y co-deposition coating inmolten sulfate including NaCl is much more serious and rapid than that in puremolten sulfate.The erosive resistance of the TiAl alloy and Si-Al-Y co-deposition coating weredifferent under distinct impact angles, and the Si-Al-Y co-deposition coating has gooderosive resistance than TiAl alloy at lower impact angles such as15°and30°, whichis cutting injury. However, the erosive resistance of Si-Al-Y co-deposition coatingdecreased significantly with increasing the impact angles up to60°and90°, which isfatigue damage, and hardness is one of influencing facters of the erosion rate ofmaterial.