| Abstract: |
2D pores, including inter-splat pores and intra-splat cracks, play a dominant role on the thermal barrier effect and strain tolerance of plasma-sprayed thermal barrier coatings. Consequently, the healing of 2D pores during thermal exposure is mainly responsible for the sintering induced performance degradation. The differences in morphology, size and grain orientation between inter-splat pores and intra-splat cracks contribute to anisotropic healing kinetics of these 2D pores in commonly-used free-standing yttria-stabilized zirconia coatings (YSZ). This study examined the healing evolution of 2D pores in YSZ coatings attached to substrate. Results show that the anisotropic healing kinetics of 2D pores is distinctly enhanced on the coatings attached to substrate. At initial thermal exposure duration, the constraint on sintering-induced contraction of coatings and the positive mismatch of thermal expansion coefficients result in global tensile effect and compressed effect in in-plane di-rection and out-plane direction, respectively. Consequently, the inter-splat pores become narrowed, while the intra-splat cracks are widened. The stressed coatings lead to a much more severe healing of inter-splat pores than do intra-splat cracks. The properties evolution (like elastic modulus and ionic conductivity) also presents enhanced anisotropic evolution trends with respect to the free-standing coating. In other words, a faster and higher increment in out-plane direction occurs during thermal exposure. At an extended thermal exposure duration, the stiffened coating attributed by enhanced bonding ratio leads to some larger cracks in coating surface to release the constrained stress. Consequently, the healing of the micro intrasplat cracks near the large cracks was significantly enhanced. This study reveals that the 2D pore healing trend under stressed state is different from the free-standing coatings. By taking a much closer step to the actual service conditions, this enhanced anisotropic pore healing behavior in constrained coatings would shed light to the lifetime evaluation and structural tailoring towards the next generation advanced TBCs with both higher thermal-insulating effect and longer life time.
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