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

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 beta- to alpha-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 degrees C for 100 h. The phase transformation was mapped at 200-260 degrees 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 alpha ->beta 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.