Gradient stiffening induced interfacial cracking and strain tolerant design in thermal barrier coatings

Strain tolerant structures endow thermal barrier coatings (TBCs) with exceptional capability to bear various strains generated during service. However, the strain tolerance of TBCs inevitably degrades when stiffening occurs at high temperatures. Herein, we tailored a strain tolerant structure to resist degradation based on an understanding of the unique stiffening behavior and consequent failure mechanism of TBCs. The degree of gradient stiffening across the thickness of TBCs is caused by temperature-dependent sintering kinetics of ceramic coatings. As a result, vertical and in-plane cracks are formed in a scale-progressive way. Simulation results reveal that the differential degree of stiffening is a main cause of interfacial cracking. Subsequently, we proposed a strain tolerant structure that is tailored by lowering the stiffening rate of regions exposed to higher temperatures. Due to the weakened differential stiffening effect, the driving force that extends the interfacial cracks was significantly lowered. Thus, this strain tolerant TBC is expected to be able to resist degradation caused by sintering. These results will guide advanced design of TBCs for future applications.