Influence of pores on mechanical properties of plasma sprayed coatings: Case study of YSZ thermal barrier coatings

This paper presents a thorough study on plasma sprayed 8 wt% Y2O3-stabilized ZrO2 (8YSZ) thermal barrier coatings intended to show the effect of the internal microstructure especially the pores on mechanical properties of plasma sprayed coatings. The internal microstructure of 8YSZ thermal barrier coatings was analyzed comprehensively by image processing technology combined with fractal theory. The mechanical properties of 8YSZ coatings were tested by the instrumented indentation testing. And the stress distribution in three models with different microstructure of coatings was analyzed in depth by the finite element method. It was found that plasma sprayed 8YSZ thermal barrier coatings have a typical layered structure and contain many microscopic defects such as microcracks and pores. The pore size distribution of plasma sprayed 8YSZ coatings obey the logarithmic normal distribution, and the pore diameter inside the coating is mainly concentrated between 1 and 10 pm. With the increase of high temperature thermal exposure time, the average diameter of pores gradually decreased from 9.30 mu m to 5.96 mu m, and the porosity of the ceramic top coat is reduced from 9.38% to 5.79%. Fractal characterization results showed that after a certain period of thermal exposure, the ceramic top coat become densified, and the internal pore structure is regularized with the fractal dimension decreased from the initial value of D-1 = 1.26 to the D-6 = 1.12 after 200 h. The indentation testing results showed that after 200 h of high temperature thermal exposure, the hardness of 8YSZ coatings increased from 2.52 GPa to 3.12 GPa, and the elastic modulus increased from 32.3 GPa to 43.6 GPa. The results of finite element analysis showed that the complex pore structure inside the coating will lead to stress concentration, which were largely affects the mechanical properties of 8YSZ coatings.