A flexural test for the in situ measurement of the delamination toughness of the interface between a thermally grown oxide and a bond coat in the presence of a thermal barrier coating (TBC) has been implemented. To accomplish the testing, a section of the substrate was removed by microelectro-discharge machining and a precrack introduced through the TBC by center point loading. This was followed by application of an asymmetric single-point load to extend a delamination along the interface. A displacement and strain mapping method was used to locate the delamination and to ascertain its extension. To relate the energy release rate and mode mixity to the crack extension and the loads, a finite-element method was implemented. The ensuing fracture resistance was found to vary along the interface with values in the range of 25-95 J/m(2).