Raman spectroscopic characterization of SiO₂ phase transformation and Si substrate stress relevant to EBC performance

Abstract

To accurately model the long‐term durability of environmental barrier coatings (EBCs), a more complete understanding of the phase composition and transformations of the thermally grown oxide SiO2 (TGO) is desired. For the TGO formed during thermal cycling in steam, cristobalite formation and the subsequent β‐ to α‐cristobalite transformation has been identified as a potentially life‐limiting mechanism. In this study, Raman micro‐spectroscopy was used to quantify the cristobalite transformation on a polycrystalline Si coupon that was exposed to steam at 1350°C for 100 h. The phase transformation was mapped at 200–260°C on the TGO surface at different ramp rates using a heating stage and a micro‐positioning stage. The stress in the Si substrate was also determined using Raman spectroscopy by measuring the stress induced peak shift. The α→β phase transformation produced a 300–500 MPa tensile stress in the Si substrate, which compared well to the stress predicted from the volumetric expansion of the cristobalite. Quantifying the phase transformation and residual stress are critical tools in developing the next generation of high performance EBCs.

Holistic comparison of environmental barrier coating material candidates through design of a figure of merit

“…59 6% volume reduction, further damaging the EBC-bond coat interface. 35 EBC candidates should have low oxygen and water vapor diffusivity, where an industry limit has been placed at a maximum oxygen diffusion coefficient of 1 × 10 −11 cm 2 /s, 36 assuming the oxidant can also quickly reach the bond coat through microstructural defects, such as porosity or cracks, resulting from either the APS process, the thermal stress release upon engine cycling, or from the porosity created with the EBC-steam reaction process.…”

Capturing and visualizing the phase transition mediated thermal stress of thermal barrier coating materials via a cross-scale integrated computational approach

“…Thermal barrier coatings (TBCs) are widely used in gas turbine engines to obtain elevated working temperatures and improve engine efficiency [1−5]. TBCs consist of multilayered structures, including a top ceramic coat (TC), bond coat (BC), and superalloy substrate (SUB) [6,7]. The failure of TBCs is generally attributed to the following factors such as the rapid growth of thermally grown oxide (TGO) [8], phase transformation within ceramic layers [9−11], and thermal expansion misfit between multilayered structures [12,13], leading to the accumulation of the thermal stress (σ) in coatings, especially at the interface region.…”