Oxidation-Induced Failure of Thermal Barrier Coatings.
Abstract
On prolonged high-temperature exposure in air, thermal barrier coatings (TBCs) on bond-coated superalloys fail by spalling. We have focused on identifying the underlying mechanisms of failure and sought to establish whether it is a direct consequence of the failure of the underlying thermally grown oxide. We have discovered a new TBC failure mode, one in which failure is associated with moisture-enhanced sub-critical crack-growth along the bond-coat/thermally grown oxide interface. By making concurrent piezospectroscopy measurements, the interfacial fracture energy was determined to be ^ 10 J/m(exp 2) - a considerably smaller value than that of sapphire/metal interfaces prepared in the laboratory but consistent with measurements of the effects of segregation on metal/ceramic interfaces. New insights into the mechanism underlying failure of the thermally grown oxide have come from direct optical microscopy. These indicate that failure is associated with surface roughness of the bond-coat and specifically that the thermally-grown oxide separates from the bond-coat on cooling at the concave ("crests") surface features. These locally separated regions grow with oxidation time and are seen to link-up. These events are believed to be the precursor events that grow to provide the critical-sized flaws from which buckling and spalling of thermal barrier coatings occur.
Document Details
- Document Type
- Technical Report
- Publication Date
- Dec 01, 1998
- Accession Number
- ADA360036
Entities
People
- David R Clarke
Organizations
- University of California, Santa Barbara