Study On Novel Environmental Barrier Coatings By First Principles Calculation And Experimental Methods

Consindering that the water vapor resistance of rare earth silicates is inconsistent under different test conditions,we used the first-principles to calculate the Si-O bond strength of rare earth monosilicates,in order to characterize the strength of water vapor resistance by comparing the strength of Si-O bond.The results show that the order is that Sc2SiO5>Dy2SiO5>Y2SiO5>Ho2SiO5>Er2SiO5>Yb2SiO5>Tm2SiO5>Lu₂SiO₅ and except for scandium silicate,the water vapor resistance of the other rare earth silicates is very close,which explains the inconsistency of the water vapor resistance of different experimental results.Rare earth ytterbium silicate is widely used in environmental barrier coatings because of its low thermal expansion coefficient,excellent water vapor resistance,superior thermal stability and good compatibility with substrate and bond layer.At present,the common ytterbium silicate is divided into single and double ytterbium silicate.In order to compare the service life of the two types of ytterbium silicate,ytterbium monosilicate and ytterbium disilicate are coated on the matrix of SiC_f/SiC ceramic matrix composites,and the service life is tested by thermal shock test.At the same time,in order to explore the durability of different ytterbium monosilicate and ytterbium disilicate composite coatings,three different proportions of ytterbium silicate composite coatings were also prepared.The results show that Yb₂Si₂O₇·Yb₂SiO₅ and Yb2Si2O7 have nearly the number of thermal shock cycles,about 360 times.In addition,the thermal expansion coefficients of Yb₂Si₂O₇·Yb₂SiO₅ and Yb₂Si₂O₇ are,respectively,5.2×10^-6?^-11 and 5.5×10^-6?^-1,and the fracture toughness of them exceeds 2.6MPa·m^0.5.The lifetimes and fracture toughness of ytterbium silicate coating increase while thermal expansion efficience of them decreases with an increase of ytterbium silicate composition.Rare earth phosphate has been explored for the application of environmental barrier coatings due to its good water vapor resistance.In this paper,the elastic modulus,thermal expansion coefficients,thermal conductivity,water vapor resistance and compatibility with matrix and bond layer of rare earth phosphate are studied.The results show REPO₄ have a Young's moduli of 130?225 GPa,thermal conductivity of 2?7W?m^-1?K^-1 at 300°C?1500°C,average TECs of 5?6.5×10^-6?^-1 and recession rate of 1.5?4×10^-4 mg/cm²/h in high temperature water vapor environment.Compared with rare earth silicates which are currently used widely,phosphates have higher elastic modulus and better water vapor resistance.They are potential candidates for environmental barrier coatings.The point that environmental barrier coatings is applied a layer of thermal barrier coating to enhance their water vapor resistance has existed for a long time.In this paper,lanthanum zirconate as thermal barrier coatings with low thermal conductivity can be applied on the surface of rare earth silicates.However,the fracture toughness of lanthanum zirconate is relatively low.Compressive stress toughening and fine grain toughening will commonly reduce the toughening effect at high temperature because of the reduction of compressive stress at high temperature and the easy sintering of fine grains.Therefore,ferroelastic toughening,which is still useful at high temperature,is used to toughen lanthanum zirconate matrix.The selected ferroelastic material is lanthanum aluminate and the results show that the fracture toughness of lanthanum zirconate matrix is improved by ferroelastic materials.Especially for x LaAlO₃/?1-x?La2Zr2O7 composite ceramics with x=0.3 and 0.5,the fracture toughness is close to 3MPa·m^0.5.