Influence Of Microstructure Of Air Plasma Sprayed Thermal Barrier Coatings On Stress State

In order to get higher thrust-weight ratio and combustion efficiency, the working temperature of gas turbine has been increased again and again. With the application of thermal barrier coatings (TBCs),the superalloy can be used up to its melting point.In terms of air plasma sprayed TBCs, the YSZ coating is easy to crack under thermal cycling due to its lamellar structure, which also decrease the service lifetime seriously. Correspondingly, based on the ideal microstructure model of plasma sprayed ceramic coatings,the effect of non-bonded interface was studied on the stress distribution of coatings through using Finite Element Method (FEM).Additionally, the influence of spraying parameters was investigated on the length of non-bonded interfaces of the as-sprayed coatings, the crack density of coatings after thermal cycling. Thus, a new type of multilayer TBC with different microstructures was designed and deposited based on the above research.The microstructures include segmentation cracks, conventional horizontal cracks and pores and dense columnar crystals.It was found that the effect of non-bonded lamellar interfaces is the most obvious one on the stress state of coatings, compared with the distance between them and the lamellar thickness.Experimental results indicate the length of non-bonded lamellar interfaces in the as-deposited coatings shows an increasing tendency with the spraying distance. The total crack length of coatings increases with the number of thermal cycles and the spraying distance after thermal shock tests.With the increment of feeding rate, the density of segmentation cracks and length of branch cracks increased firstly and then decreased in the certain adjusting range when the coatings were deposited by hot spraying process.The density of vertical cracks showed an increment tendency with the growth of substrate preheating temperature. However, the length of branch cracks increased firstly, followed by a decreasing trend.Under different hot spraying process,the average length of single branch cracks could be up to 90μm.It was shown that the lifetime of the top YSZ coatings can be improved effectively through restraining the total length and number of branch cracks in coating and maintaining the density of segmentation cracks at a certain level.Compared with the conventionally plasma sprayed coatings, the infiltration of CMAS glass was not different obviously for the new-type of multilayer TBC.However, through the verifications of thermal cycling test and oxidation experiment, the multilayers TBC has better performance than the conventional specimen.