Strain-Induced Cracking Behavior of Coating/Substrate Systems and Strain Tolerant Design for Thick Coatings

Abstract

The life span for a coating attached to its substrate is basic support for their desired protective function. Therefore, it is necessary to find out the causes responsible for the failure of coatings during service. This paper developed a finite element model to investigate the cracking behavior of plasma-sprayed ceramic coatings induced by the mismatch strain of thermal expansion between coating and substrate. Crack propagation affected by coating thicknesses was realized by the virtual crack closure technique (VCCT). The residual stresses (σ22 and σ12) and the strain energy release rate (SERR) induced at the tip of pre-crack in ceramic coatings are calculated. Results show that the σ22 and σ12 at the tip of the pre-crack increases continuously with the thickening ceramic coatings. The SERRs at the tip of the pre-crack in top-coat (TC) were increased with the thickness of ceramic coatings, resulting in the propagation of cracks. The crack length increases with the thickening of ceramic coatings. The crack propagation and coalescence lead to coating spallation, which is one of the main failure modes for plasma sprayed ceramic coatings during service. Given that, strain tolerant design was developed by inserting vertical pores in coatings. It was found that the SERRs were decreased with the increase in the number of vertical pores, as well as their depth. Moreover, the coatings with vertical pores appear to be crack-resistant, in particular for the thicker coatings. This suggests that the strain tolerant design is helpful to extend the life span of thick coatings, which makes a fundamental contribution to the design and preparation of advanced protective coatings in future applications.

Tailoring Periodic Vertical Cracks in Thermal Barrier Coatings Enabling High Strain Tolerance

“…To achieve some significant data, the distribution of stress in the ceramic TC layer was primarily investigated at the pre-crack tip due to thermal mismatch strain between coatings and substrate, which caused the degradation of coating. Figure 3 Our previous work [53] suggested that the maximum stresses at the pre-crack tip decreased as the vertical crack numbers in ceramic top-coat increased, and also decreased with an increase in their depth. This study further indicated that, when the SVC was controlled up to a suitable value, the SVC liberated the concentration of stress of thick TBCs.…”

“…The residual stresses (σ22 & σ12) are at minimum (σ22 = 585 MPa and σ12 = 436 MPa) at the precrack tip in the ceramic top-coat when the SVC is at its minimum, which is 0.5 mm. Our previous work [53] suggested that the maximum stresses at the pre-crack tip decreased as the vertical crack numbers in ceramic top-coat increased, and also decreased with an increase in their depth. This study further indicated that, when the SVC was controlled up to a suitable value, the SVC liberated the concentration of stress of thick TBCs.…”

“…Therefo have various applications in engineering-for example, they are essentia sorption [55,62]. However, in our previous study [53], thick TBCs often Figure 9 represents the variation of crack length in the ceramic top-coat with the SVC. The SVC has influenced the growth of the crack.…”

“…Zhou and Kokini et al [51,52] analytically examined the interfacial fracture mechanism of the TBC system with multiple vertical cracks. In our previous paper [53], the strain tolerant design was elaborated by embedding vertical cracks in coatings. The results show that the crack-driving force was reduced as the number and depth of vertical cracks increased.…”

“…This result suggests that coatings with vertical cracks increase strain tolerance, which has a significant influence on the preparation and design of advanced protective coatings in future applications. However, many previous investigations have disregarded the interaction of vertical cracks and propagation of cracks, while a few studies investigate the influence of crack space on the coating delamination and the durability of the TBCs [51][52][53]55]. This is the original motivation of the present study.…”