Calcium-magnesium-alumina-silicate (CMAS) delamination mechanisms in EB-PVD thermal barrier coatings

Thermal barrier coatings (TBCs) comprise thermally insulating materials having sufficient thickness and durability that they can sustain an appreciable temperature difference between the load bearing alloy and the coating surface. TBC exhibit multiple failure modes. Among them, in-service erosion caused by the deposition of significant amount of calcium-magnesium-alumina-silicate (CMAS) at high temperature was found to be one of the most prevalent failure modes. The large thermal expansion mismatch between the CMAS and TBC and the extra strain energy stored in the CMAS layer can lead to delamination cracks between the TBC and bond coat (BC). In this study, the energy release rate and mode mixity of a propagating delamination crack are calculated by using the finite element analysis. The columnar microstructure of EB-PVD TBC is factored into the approach. The effects of CMAS layer thickness, mechanical and thermal properties are examined, and the steady-state energy release rates are compared with a theoretical model. Two failure mechanisms associated with CMAS deposition are analyzed: cracking within individual columns and spallation of a large TBC layer. It is believed that both mechanisms have contributed to the CMAS delamination failure. Failure criterions are derived which provide useful insights on how to improve the resistance of CMAS delamination. (c) 2005 Elsevier B.V. All rights reserved.