Effect Of Thermal Barrier Coating Porosity On CMAS Corrosion Behavior

For ceramic-based thermal barrier coatings with excellent thermal insulation properties in aero-engines,Penetration and corrosion of thermal barrier coatings by molten silicates(CMAS)is becoming an increasingly important issue affecting thermal barrier coating life.Therefore,this thesis takes the whole process of CMAS erosion atmospheric plasma spraying Zr O2-8%Y2O3(YSZ)as the research object,the change of coating pore structure in the whole process of CMAS erosion was traced and characterized,and investigate the failure mechanism.Also,the effect of thermal barrier coating pore structure on the erosion behavior of CMAS was further illustrated by preparing thermal barrier coatings with different structures.The main conclusions and research contents are as follows:(1)The CMAS erosion behavior of thermal barrier coatings is divided into two processes:a fast penetration process and a slow degradation process.The molten CMAS penetrated the YSZ coating with a thickness of about 200 μm within 30 s,which reduces the porosity of the coating from 12.8% to 4%.When the pore diameter in the YSZ coating is larger than 3 μm,the capillary force is too small and the large pores are not easily filled.Incorporating the effective porosity(ωd≤3 μm)into the calculation of CMAS penetration rate can reduce the deviation between the calculated result and the measured result to within 5%.Within 30 minutes after CMAS permeation,YSZ did not undergo significant phase change,and the permeation behavior was much faster than the corrosion degradation behavior.The content of microcracks with pore diameters less than 3 μm in the coating determines the penetration rate,while the tortuous degree of the microcracks has little effect on the penetration rate.(2)The YSZ coating has obvious sintering behavior after being penetrated by CMAS.After penetration,sintered at constant temperature for 30 minutes,The microhardness of the YSZ coating is increased by about 120%,and the rigidity of the coating is significantly increased.Within 4 hours before CMAS corrosion,with the increase of corrosion time,the severely corroded area gradually increased,but after 4h,the severely corroded area of the coating was basically stable.CMAS corrosion leads to the transformation of t-Zr O2 grains from colμmnar grains to spherical grains.(3)The degradation of the YSZ coating resulted in a large number of microporous aggregation areas and lath-like crack corrosion zones in the coating structure.Under long-term corrosion,the pores with spherical pore diameters between 3.0 μm and 6.0 μm are partially degraded,resulting in a large nμmber of micro-pore aggregation areas in the coating.The large pore structure with a diameter of more than 6 μm has good structural stability without cracking.The failure and cracking modes of coatings can be roughly divided into three behaviors: pore cracking and recombination behavior,interlayer crack propagation behavior and intergranular cracking behavior.(4)Network cracks of YSZ coating can significantly promote the transformation of t-Zr O2→m-Zr O2,and the network crack will have a wide range of crack propagation in the early stage of CMAS erosion,corrosion areas are more diffuse.Spherical pores have a certain ability to resist coating cracking caused by coating degradation,however,the phenomenon of cracking and recombination of pores is more obvious,a large area of corrosion pits appeared,and the damage to the coating was more serious.Therefore,while introducing a spherical pore structure to resist the degradation of the coating,it should be considered to retain a certain proportion of the network pore structure,so as to reduce the influence of the cracking and recombination behavior of pores.