| Advanced environmental barrier coatings?EBCs?are being developed for next?generation gas turbine engines to protect engine hot?section SiC?based lightweight components in the harsh operating combustion environments and extend component lifetimes.As a research work,ceramic powders are synthesized by solid-liquid method followed by cold isostatic pressing and sintering.The EBC material prepared for the hot corrosion in this research was Yttrium disilicate?Y2Si2O7?.Different temperatures with different amount of additive LiYO2 for different time were tried for the synthesis of Y2Si2O7.When the temperature was at 1400oC to 1450oC for four hours,very little amount of Y4.67?SiO4?3O was observed.In this case Y2Si2O7 phase was obtained as more pure.So,it was concluded that the pure phase of Y2Si2O7 can be obtained at temperature of 1400oC to 1450oC for four hours.It is well noted that when the temperature was increased beyond1450oC,phase change was clearly observed and this is not the desirable case.For the measurement of thermal conductivity of Y2Si2O7 bulk experimental approach was adopted.It was confirmed that the thermal conductivity of Y2Si2O7 bulk is decreased when the temperature is increased.For the bulk prepared at 1400oC,low thermal conductivity was measured with the value of 3 W/m·K at above 600K.The calculated minimum thermal conductivity was 2.2 W/m·K at above 1400 K.Also,it was seen that the values of coefficient of thermal expansion does not have linear relation with the temperature.The minimum thermal expansion coefficient was seen for bulk prepared at1500oC.TEC obtained for 1500oC Y2Si2O7 bulk was 4.5×10-6 K-1 for temperature 1200 K.And,TEC obtained for temperature 1600 K was 3.9×10-6 K-1.The Y2Si2O7 bulk was fabricated at 1400oC,1500oC and 1550oC for two hours.In this research the hot corrosion tests were done at these temperatures and time.When the exposure time is further increased from 2 hours to 32 hours,in stepwise manner,there was no effect of reactions seen in the microstructure.This explanation is derived from the SEM images which show almost no changes in the microstructure.Change in microstructure along with observation of cracks and pores could be seen when this hot corrosion is performed for wide range of temperature and time.For the case of temperature up to1300oC for 32 hours,Y2Si2O7 EBC ceramic is good hot corrosion resistant.For the preparation of CMAS the raw powder of CaO,MgO,Al2O3 and SiO2 was taken.This powder was prepared in lab by mixing 35 mol.%CaO,10 mol.%MgO,7 mol.%Al2O3 and 48 mol.%SiO2 with isopropyl alcohol and was then ball milled for proper mixing.The Y2Si2O7 bulk was fabricated at 1400oC,1500oC and 1500oC for two hours.In case of 2 hours of CMAS test,CMAS powder started to melt.With the increase in reaction time,due to CMAS corrosion effect,pores started to form and they start to expand over the time period.This fact is also supported and further verified by microstructure which also gives the nature and composition of elements for each case.From this it could be understood that the presence of additional elements like Mg,Ca and Al suggested the reaction occurred with the CMAS powder and their variation in amount over the time period proves the fluctuating and worsening outcome on the microstructure of the bulk.In sum,the thermal expansion coefficient determined from the test in this research is very close to that of silicon nitride and silicon carbide and thermal conductivity obtained for Y2Si2O7 is also very low.This combination of low thermal expansion coefficient and low thermal conductivity of Y2Si2O7 proves that it a very suitable candidate for high temperature structural components and at environment barrier coatings.Y2Si2O7 are easily corroded due to the chemical reaction with calcium-magnesiu,-alumino-silicate?CMAS?.With the intake of air in the operational gas turbine,these CMAS also enter through the as-rich atmosphere.These composites start to melt above 1200oC.In this research the test were done to observe the CMAS corrosion above 1300oC and have seen that the CMAS has already melted at this stage.Moreover,the phenomenon of adverse effect of CMAS on the bulk surface aggravated with the increase of time from melting to pores formation and cracks.Two processes of CMAS corrosion are identified as dissolving of Y2Si2O7 in CMAS molten salt and the other process includes CMAS molten salt seeping into the substrate Y2Si2O7 through pores and grain boundary.Thus,conclusion can be drawn that Y2Si2O7 easily reacts with CMAS and reduces the melting point of EBCs.As a future research work it is suggested to work on methods to limit the effects of CMAS corrosion and one of the methods could be doping of certain oxides. |
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