Crack Initiation Criteria in EBC under Thermal Stress

Kawai Kawai,Akatsu Akatsu,Kakisawa Kakisawa,Umeno Umeno,Yamaguchi Yamaguchi,Kubo Kubo,Yokoi Yokoi,Kitaoka Kitaoka

doi:10.3390/coatings9110697

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/Yb2SiO5 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/Yb2Si2O7 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/Yb2Si2O7 interface. Comparison between ERR and fracture toughness indicates that the fabricated T/EBC possesses sufficient mechanical reliability against interface crack initiation and propagation.

Highlights

Silicon carbide (SiC) fiber reinforced SiC matrix (SiC/SiC) composites are one-third lighter and have approximate 100–200 K higher heat resistance than current heat-resistant super alloys (Nickel-based super alloys) [1,2,3]
K, and the development of stress high temperature in the deposition process, and we evaluate the thermal stress state in T/environmental barrier coatings (EBCs) after distribution wastemperature calculated at a cooling of 20 K/min creep wasis considered within cooling to room from the highrate temperature
The results shown reveal that the T/EBC with the proposed layer thicknesses can be fabricated without delamination along interfaces by cooling in the fabrication process

Summary

Introduction

Silicon carbide (SiC) fiber reinforced SiC matrix (SiC/SiC) composites are one-third lighter and have approximate 100–200 K higher heat resistance than current heat-resistant super alloys (Nickel-based super alloys) [1,2,3]. Application of SiC/SiC to advanced hot-section components in airplane engines is expected for improving fuel consumption and curbing emission of carbon dioxide [2,3]. SiC/SiC composites react with oxygen to form silica, which hinders degradation of the composites at high temperature over 1373–1473 K [4,5]. In the condition of high temperature and humidity (i.e., combustion gas environment), the silica reacts with water vapor to form silicic acid (Si(OH4 )) gas [4,5], leading to wall thinning of the composites. Application of the SiC/SiC composites to the hot-section components inevitably requires environmental barrier coatings (EBCs). For using EBCs at about 1673 K, which is the durable temperature of the heat-resistant SiC fiber, it is of immense importance to select a material

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