In situ stress analysis of multilayer environmental barrier coatings

Abstract

The biaxial stress and thermal expansion of multilayer doped-aluminosilicate environmental barrier coatings were measured in situ during cooling using microfocused high-energy X-rays in transmission. Coating stresses during cooling from 1000 ° C were measured for as-sprayed and thermally cycled samples. In the as-sprayed state, tensile stresses as high as 75 MPa were measured in the doped-aluminosilicate topcoat at 375 °C, after which a drop in the sti;ess occurred accompanied by through-thickness cracking of the two outermost layers. After thermally cycling the samples, the stress in the topcoat was reduced to approximately 50 MPa, and there was no drop in stress upon cooling. This stress reduction was attributed to a crystallographic phase transformation of the topcoat and the accompanying change in thermal expansion coefficient. The addition of a doped aluminosilicate to the mullite layer did not lower the stress in the topcoat, but may offer increased durability due to an increased compressive stress.

Yb2Si2O7 Environmental barrier coatings with reduced bond coat oxidation rates via chemical modifications for long life

“…Most of the current EBCs are variants of the second‐generation EBCs. Various aspects of current EBCs, including processing, volatility in water vapor, silica activity in Y 2 O 3 ‐SiO 2 and Yb 2 O 3 ‐SiO 2 systems, interactions with CMAS, stress analysis, fracture mechanism, impact damage, and oxidation, have been reported.…”

Structure, composition, and defect control during plasma spray deposition of ytterbium silicate coatings

“…The air plasma spray (APS) process is an economical and often reliable method for the deposition of such coatings, and has attracted attention for the deposition of EBCs on SiC components [24,25,28,29,[34][35][36][37][38]. This is a logical extension to its already widespread use for the deposition of the thermal barrier coating (TBC) systems used to protect superalloy components in current gas turbine engines [39][40][41][42][43][44].…”

Thermal spray coatings in environmental barrier coatings

“…In order to use mullite effectively as an intermediate layer, silicon must be deposited onto the silicon-based ceramic to promote the adhesion of mullite (Lee, 2000). The addition of SAS did not lower the stress in the other layers; however, it did increase the compressive stress, which may increase the resistance to surface cracking, thus improving the durability of the BSAS topcoat (Harder et al, 2009b). More et al (2002) mixed BSAS with mullite to form a mullite + BSAS composite layer.…”

“…It is possible to achieve low porosity in plasma-sprayed coatings, if the parameters are optimized (Moldovan et al, 2004). However, the large difference in temperature between the particles and the substrate causes the particle to solidify quickly, which can form amorphous phases (Lee, 2000), generate undesirable phases (Harder et al, 2009b;Moldovan et al, 2004;Weyant et al, 2005), or generate residual stresses due to resolidification (Weyant et al, 2005). Heating the substrate to an appropriate temperature during plasma spraying will reduce the quenching rate of the deposited particles, which can eliminate the formation of amorphous phases, such as that observed in mullite coatings (Lee et al, 1995).…”

“…Issues such as phase transformations (Harder et al, 2009b;Moldovan et al, 2004) and stresses from resolidification (Moldovan et al, 2004) are prominent in plasmasprayed coatings; however, these issues can be reduced by heat treatment. Weyant et al (2005) had shown that it is possible to heal macrocracking via heat treatment at 1200°C, but the length of heat treatment must be controlled because microcracks can form due to grain growth.…”