Self-crystallization Behavior Of Environmental Deposits And Its Reaction Mechanism With Ti₂AlC For Thermal Barrier Coating Applications

Thermal barrier coatings(TBCs)are one of key techniques for blades of turbine engines.Recently,the failure of TBCs induced by the corrosion of environmental deposits with the main composition of CaO-MgO-Al₂O₃-SiO₂(CMAS)has attracted considerable attention.In this paper,the self-crystallization behavior of CMAS was first investigated and then a MAX phase of Ti₂AlC was proposed as the protective layer against CMAS attack for TBCs.The reaction behavior between Ti₂AlC and CMAS under high temperature and the characteristics of reaction crystallization products were studied.Also,the mechanism of crystallized CMAS promoted by Ti₂AlC and the mechanism of Ti₂AlC arresting CMAS penetration were clarified.The above researches provide theoretical basis for CMAS protection studies for TBCs.CMAS glass with a composition of 33CaO-9MgO-13AlO_1.5-45SiO₂(mol%)was prepared by melting-quenching method.DSC result showed that CMAS self-crystallizations occurred in the temperature range of?950?-1230?.The self-crystallization behavior of CMAS and factors affecting crystallization in the process of heating(the peak temperatures were 1050?,1150?and 1250?,and the heating rates were 10?/min and 3?/min),cooling(air quenching,furnace cooling with door opening,furnace cooling and 10?/min cooling)and constant temperatures(holding at1150?for 0.5 h and 1.5 h)were studied systematically.The results showed that there was a positive correlation between the degree of self crystallization of CMAS and its residence time in the temperature range referring to crystallization.The self-crystallized products of this CMAS were melilite,diopside,wollastonite and anorthite,and the phase evolutions occurred with the change of peak temperature and holding time.The interactions between Ti₂AlC,pre-oxidized Ti₂AlC and CMAS under high temperature were studied.The results showed that a crystalline layer mainly composed of Al₂O₃,melilite and anorthite was formed between Ti₂AlC and CMAS after heat treatment at 1250?for 0.5 h.After pre-oxidation at 1200?for 10 h,a microstructure of double oxide layer with the outer layer of Al₂O₃ and the inner layer of Ti O₂ was formed on the surface of Ti₂AlC pellet.The results of interactions between pre-oxidized Ti₂AlC and CMAS revealed that only a layer of anorthite was formed after heat treatments at 1250?for 1 min,2 h,8 h and 16 h,and its number and morphology did not change significantly with reaction time.CMAS melts with different Ti content were designed by various methods(dissolution&flowing method,design of pre-oxidation condition and adopting mixed powder of CMAS-Ti O₂)and the effects of Ti and its content on CMAS crystallization were examined.The results showed that Ti in the form of precipitated phase or solute could promote CMAS crystallization.However,when Ti existed in the form of solute,there was a critical content(1.53 mol%)to trigger the crystallization of CMAS,and the promotion effect of Ti on molten CMAS increased with the increase of Ti content until precipitation occurred when Ti content exceeded the solubility.Under CMAS corrosion,both Ti₂AlC and its pre-oxidized counterpart could effectively prevent the infiltration of molten CMAS by forming a dense crystallization layer right above the surface of the samples.However,the corrosion resistance of Ti₂AlC after pre-oxidation was better,because the pre-existing dense Al₂O₃ layer could not only accelerate the formation of anorthite,but also completely block the infiltration path of molten CMAS.