Microstructure And Properties Of Ytterbium Silicates Coating Deposited By Atmospheric Plasma Spray

Ytterbium silicates,including ytterbium monosilicate(Yb2SiO5)and ytterbium disilicate(Yb2Si2O7),are promising materials for multilayed environmental barrier coating(EBC)on fiber reinforced SiC matrix composites(SiC/SiC,C/SiC and SiC)because of their superior high-temperature stability,relatively low coefficient of thermal expansion(CTE),long durability in water vapor.This work focused on Yb2SiO5 and Yb2Si2O7,including the synthesis of powder,coating fabrication,high-temperature stability of coatings and design of coating systems.The coatings were fabricated by atmospheric plasma spray(APS)technique and systematical characterization has been taken in microstructure and thermomechanical properties.Microstructure evolution and its influence on thermomechanical properties of plasma-sprayed Yb2SiO5 and Yb2Si2O7 coating during thermal aging have been investigated.Failure mechanism of different coating systems were discussed according to the thermal shock test and thermal cycling in water vapor.The objective of this study is to develop new rare-earth silicates EBC systems with excellent thermal shock resistance,crack propagation resistance,good durability in water vapor and long service life.The main results obtained are described as follows:1.Pure Yb2SiO5 and Yb2Si2O7 powders were fabricated by the solid-state reaction.The Yb2SiO5 and Yb2Si2O7 coatings were fabricated by APS technique,and variation of the deposition parameters had significant impact on microstructure,such as phases and porosity of the coating.Decomposition phenomenon and porosity in the ytterbium silicate coating can be reduced by using reasonable spray parameters.2.Both Yb2SiOs and Yb2Si2O7 coating exhibited dense structure with defects like pores and microcracks.Amorphous phase crystallized in the as-sprayed(b2SiO5 coating during the heating process,which resulted in the sudden change of volume and increase of thermal conductivity.The average CTE and minimum thermal conductivity of the as-sprayed Yb2SiO5 coating were about 7.24×10-6 K-1 and 0.67 W/(m·K),respectively.Yb2SiO5 existed in the amorphous phase which resulted from the dissociation of Yb2Si2O7 during deposition process.A linearly thermal expansion can be observed in the as-sprayed Yb2Si2O7 coating during heating process,and the average CTE and minimum thermal conductivity were about 4.25×10-6 K-1 and 0.68 W/(m K),respectively.3.Significant microstructure evolution was observed in both Yb2SiO5 and Yb2Si2O7 coating during thermal aging.Defects were gradually reduced and significant grain growth took place under thermal aging.The grain size of the Yb2Si2O7 coating was larger than that of the Yb2SiO5 coating.The calculated activation energies of grain growth for the Yb2SiO5 and Yb2Si2O7 coatings were 366 kJ mol-1 and 219 kJ mol-1,respectively.Extensive plastic deformation such as twinning and dislocation were confirmed in the thermal-aged Yb2Si2O7 coating,indicating excellent damage tolerance,while the plastic deformation was not obvious in the thermal-aged Yb2SiOs coating.4Microstructure evolution had significant influence on the thermomechanical properties of the Yb2SiO5 and Yb2Si2O7 coating during thermal aging.The Vickers hardness(Hv)and elastic modulus(E)values of the Yb2SiO5 and Yb2Si2O7 coatings increased with temperature increasing,while the Hv and E values of the Yb2Si2O7 coating were lower than that of the Yb2SiO5 coating.With the thermal-aged temperature increasing,the thermal conductivities of both coatings increased substantially,and the Yb2SiO5 coating had lower thermal conductivity than the Yb2Si2O7 coating.The Yb2Si1O7 coating exhibited lower CTE than the Yb2SiO5 coating,and the CTE of the Yb2SiO5 coating increased obviously during thermal aging.5.Thermal shock resistance of the Yb2Si2O7/Si system was much better than that of the Yb2SiO5/Si system.The tri-layer Yb2SiO5/Yb2Si2O7/Si coating possessed excellent thermal shock resistance and excellent crack propagation resistance.Combining plastic deformation,low CTE and elastic modulus,Yb2Si2O7 could be a promising interlayer material for tri-layer rare-earth silicates EBC system.6.The water vapor corrosion behavior for Yb2SiO5 and Yb2Si2O7 coatings were systemically studied in 1400 ? water vapor environment containing Al2O3.Results showed that the Yb2SiO5 coating had good resistance to high-temperature water vapor corrosion,while it was corroded by Al2O3.The silica component in the Yb2Si2O7 was removed by water vapor and polycrystalline Yb2SiO5 phase generated in the Yb2Si2O7 coating,however,the Yb2Si2O7 is not corroded by Al2O3.The Yb2SiO5 coating has better anti-corrosive substance penetration than the Yb2Si2O7 coating.7.Results of thermal cycling in water vapor showed that the Si bond layer was oxidized and TGO layer formed in both Yb2SiO5/Si and Yb2Si207/Si system,and the breakage of TGO layer contributed to the failure of both coating systems.The Yb2SiO5/Yb2Si2O7/Si system had good resistance to water vapor corrosion,and it is a promising coating system for rare-earth silicates EBCs.