Fracture mechanisms of ytterbium monosilicate environmental barrier coatings during cyclic thermal exposure

A recently optimized air plasma spray process has been used to deposit a model tri-layer Yb2SiO(5)/Al6Si2O13/Si environmental barrier coating (EBC) system on alpha-SiC substrates using low power deposition parameters to reduce silicon losses, improve interface adherence and decrease defect concentrations. During cooling, tensile stresses developed in the ytterbium monosilicate layer since its coefficient of thermal expansion exceeded that of the substrate. These stresses drove vertical mud cracks that underwent crack branching either within the Al6Si2O13 (mullite) layer or at one of its interfaces. Upon subsequent thermal cycling between temperatures of 1316 degrees C and 110 degrees C in a 90% H2O + 10% O-2 environment, the branched mud cracks propagated into the Si bond coat and grew laterally along the mid-plane of this layer. The faces of the branched cracks were accessible to the steam environment resulting in the formation of a cristobalite surface layer, which mud cracked due to repeated beta <-> alpha cristobalite phase transformations during thermal cycling. After extended cycling, these cracks linked to cause partial spallation of the coating. The crack branching phenomenon was analyzed using finite element analysis, and the crack trajectory was assessed in terms of the crack driving force controlling kinking from the tip of the mud cracks. A comparison between the present optimized deposition process (performed at low deposition power) with a previous study of a non-optimized process (performed at high power) highlights the importance of reducing the crack driving force and controlling microstructural defects. Finite element simulations provided an effective means to quantify the susceptibility of coating design to failure by the various cracking modalities. (c) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.