Study On Properties Of Rare Earth-based Thermal Barrier Coatings With Pre-implanted Vertical Microcracks

The service lifetime of thermal barrier coatings(TBCs)mainly depends on the preparation process and microstructures.TBCs prepared by atmospheric plasma spraying(APS)have a layered structure and lower thermal conductivity,but shorter thermal cycling lifetime.TBCs prepared by electron beam-physical vapor deposition(EB-PVD)have a feather-like columnar crystal microstructure,which have a higher strain tolerance and longer thermal cycling lifetime,but their higher thermal conductivity reduces the thermal insulation.At the same time,glassy deposit(33Ca O-9Mg O-13Al O_1.5-45Si O₂,CMAS)easily penetrates into the interior of TBCs along the columnar crystal gap.The long-term use temperature of the traditional 7-8 wt.%Y₂O₃stabilized Zr O₂(YSZ)TBC is lower than 1200°C,which obviously cannot meet the development needs of gas turbines that develop higher gas temperatures.The research work of this thesis mainly focuses on the new rare earth-based TBCs(LaMgAl₁₁O₁₉/LMA,La₂Ce₂O₇/LC?La₂(Zr_0.7Ce_0.3)₂O₇/LZ7C3)materials.Based on the control of pores structure,the in-situ dry-ice blasting was used during the plasma spraying,the vertical microcracks were implanted in the rare earth-based TBCs'ceramic layer by controlling the“quenching stress”of the droplet spreading.The thermal cycling lifetime and CMAS corrosion behaviour of TBCs were further studied to establish the intrinsic relationship between the pre-implanted vertical microcracks and the thermal cycling lifetime and CMAS corrosion behaviourFirstly,LaMgAl₁₁O₁₉,La₂Ce_2.5O₈ and La₂(Zr_0.6Ce_0.4)₂O₇ ceramic powders were synthesized by high-temperature solid-state reaction.And the as-synthesized ceramic powders were spray-dried to obtain the free-flowing globular powders with a particle size of 32-125?m.Secondly,LMA,LC and LZ7C3 TBCs were prepared by APS coupled with dry-ice blasting,and the thermal cycling lifetime and CMAS corrosion behaviour were evaluated.TBCs'properties were characterized by XRD,SEM,EDS,TEM,DSC,microcomputer control electronic universal testing machine and roughness instrument.Meanwhile,based on the cross-sectional SEM images,the total porosity and microcracks content of as-sprayed TBCs were statistically analyzed by Image J software.The results are as follows:The total porosity of LMA coatings prepared by APS coupled with dry-ice blasting is significantly reduced and the proportion of vertical microcracks is significantly increased,thereby prolonging the thermal cycling lifetime.Furthermore,LMA coatings with pre-implanted vertical microcracks have better CMAS corrosion resistance.Usually,as-sprayed LMA coatings contain a large amount of amorphous phase,and the amorphous phase disappeared after heat treatment,but the significantly increased and widened vertical microcracks provide a channel for the infiltration of CMAS melt,resulting in a more severe CMAS corrosion of the heat-treated LMA coatings.Due to the rapid cooling of dry-ice particles,the oxygen vacancies in LC coatings increase,the shearing motion of the M-O-M'bond is stronger than the horizontal motion,resulting in intensified hot shrinkage.Consequently,LC coatings are prone to premature peeling and failure during the thermal and cooling cycling.There is no significant change in the CMAS corrosion behaviour of LC coatings pre-implanted vertical microcracks.The proportion of microcracks in LZ7C3 coatings prepared by coupled process was not obvious,but the content of macropores is significantly reduced,which helps to improve the tensile bonding strength and thermal cycling lifetime.In addition,the CMAS corrosion behaviour of LZ7C3 coatings pre-implanted vertical microcracks has not changed significantly.Dry-ice blasting process has certain material applicability.The effect of pre-implanted vertical microcracks on materials with low fracture toughness and low temperature shrinkage properties(such as LC and LZ7C3)is not obvious.The mechanical impact of dry-ice blasting can remove weakly unmelted/broken/spottered particles during plasma spraying,thereby reducing surface roughness and micro-defects such as internal macropores.