Crack Initiation Criteria in EBC under Thermal Stress

Abstract

For design of multi-layered environmental barrier coatings (EBCs), it is essential to assure mechanical reliability against interface crack initiation and propagation induced by thermal stress owing to a misfit of the coefficients of thermal expansion between the coating layers and SiC/SiC substrate. We conducted finite element method (FEM) analyses to evaluate energy release rate (ERR) for interface cracks and performed experiment to obtain interface fracture toughness to assess mechanical reliability of an EBC with a function of thermal barrier (T/EBC; SiC/SiAlON/mullite/Yb-silicate gradient composition layer/Yb 2 SiO 5 with porous segment structure) on an SiC/SiC substrate under thermal stress due to cooling in fabrication process. Our FEM analysis revealed that a thinner SiAlON layer and a thicker mullite layer are most suitable to reduce ERRs for crack initiation at the SiC/SiAlON, SiAlON/mullite and mullite/Yb 2 Si 2 O 7 interfaces. Interface fracture tests of the T/EBC with layer thicknesses within the proposed range exhibited fracture at the SiC/SiAlON and SiAlON/mullite interfaces. We also estimated the approximate fracture toughness for the SiC/SiAlON and SiAlON/mullite interfaces and lower limit of fracture toughness for the mullite/Yb 2 Si 2 O 7 interface. Comparison between ERR and fracture toughness indicates that the fabricated T/EBC possesses sufficient mechanical reliability against interface crack initiation and propagation.Coatings 2019, 9, 697 2 of 27 with the following properties as the water vapor shielding layer; significant wall-thinning resistance in the combustion gas environment, and coefficients of thermal expansion (CTEs) being close to that of the SiC/SiC. Klemm conducted burner-rig tests for various heat-resistant materials under the condition simulating the actual combustion gas environment to quantitatively evaluate relationship between CTEs and wall-thinning rate [8]. The result indicated that ytterbium silicate (Yb 2 SiO 5 ; YbMS and Yb 2 Si 2 O 7 ; YbDS) has prominent wall-thinning resistance. He reported that the resistance of YbMS is particularly outstanding and that the CTE of YbMS is slightly larger than that of the SiC/SiC substrate, which is almost equal to that of YbDS. Yb-silicate is unlikely to fracture by phase transition during thermal cycle because phase transition causing the large volume change does not appear in the material throughout a wide temperature range from room temperature to high temperature. On the basis of these features, Yb-silicate is considered to be an attractive material as the water vapor shielding layer. By the way, YbMS is expected to be applied as the thermal barrier coating (TBC) for SiC/SiC substrate because the thermal conductivity of YbMS [9] is smaller than that of Y 2 O 3 -ZrO 2 (YSZ), which is used as TBC for Nickel-based super alloys.Under the combustion gas environment, EBCs are exposed to a steep gradient of oxygen chemical potential (dµ O ). It is known that, in an oxide layer under the environment of high temperature and dµ O , th...

Homogenized model of environmental barrier coatings for evaluation of energy release rate

“…We assume that stress is zero everywhere in the EBC model at 1400°C, that is, before the cooling process. We consider a crack with a length of 5, 10, 20, and 50 µm from the left edge of mullite/SiAlON interface where significant stress concentration occurs 13 . The interface crack was modeled by disconnecting the upper and lower surfaces of the interface crack.…”

“…Conventional EBCs have a multilayer structure consisting of a vapor‐barrier layer, an oxygen‐barrier layer, and a bond layer piled up on a SiC/SiC substrate, as depicted in Figure 1. In recent years, EBCs having a columnar layer, which is an aggregate of a number of fine columns, have been intensively developed with the aim to achieve better resilience against thermal shock due to start and shutdown of the engine 6,11‐13 . The layers in an EBC are formed at a high temperature followed by cooling down to the room temperature, resulting in severe thermal stress in the multilayer structure of EBCs due to the difference in the coefficients of thermal expansion (CTE) of the layers.…”

“…In designing of EBCs structure for sufficient mechanical reliability, the stress state and ERR in EBC must be evaluated by FEM calculations for EBC models with varying structure conditions. For example, to optimize thicknesses of the coating layers by conducting FEM simulations for thermal/EBC models, a substantial number of calculations with varying layer thicknesses and crack lengths are needed 13 . Moreover, if the FEM model deals with a number of columns in the columnar layer separately and individually to calculate ERR, the calculation can be daunting and substantially inefficient owing to the following reasons: (a) a very fine mesh is required to represent a lot of narrow and long columns; (b) deformation associated with crack propagation may cause contact between neighboring columns, resulting in exhaustingly time‐consuming and error‐prone calculations.…”

A modified simple interface fracture test for ceramic environmental barrier coating on ceramic matrix composite

“…These constitutive materials have different Young's moduli and coefficients of thermal expansion (CTE); thermal stresses are induced on the order of several hundreds of MPa after processing and during heat cycles in use. 6), 7) Thus, thermomechanical durability is required for EBCs as well as chemical stability in the environment, even in quite severe heat cycle conditions ranging from ambient temperature to service temperatures of 13001400°C.…”

An Improved Cohesive Zone Model for Interface Mixed-Mode Fractures of Railway Slab Tracks

“…In CZM, material failure is characterized by a traction-separation law, which relates the traction across the crack to the corresponding separation [13]. The approach ensures that CZM maintains the continuity conditions mathematically and removes the singularity present in Linear Elastic Fracture Mechanics (LEFM) [14][15][16]. With the development of CZM over the past six decades since it was proposed by Barenblatt [17] and Dugdale [18], a large variety of traction-separation laws have been established.…”