Bond Strength and Stress Measurements in Thermal Barrier Coatings

Abstract

The failure mode for thermal barrier coatings in gas turbine engines is spallation at or near the ceramic to metal interface. We propose that the two most important factors leading to this failure are the change in the bond strength and bond stress with cycling. Five methods of measuring stress near the ceramic bond coat interface and four methods of bond strength measurement were investigated. Laser fluorescence and enhanced laboratory x-ray methods have the most potential for stress measurement, while the promising bond strength measurement methods are direct pull testing, chevron notch fracture toughness, and laser induced ultrasonic spallation. The stress and strength as a function of thermal cycles was determined for one set of yttria stabilized zirconia coated single crystal samples. Both a decreasing residual compression stress in the oxide layer that joins the ceramic to the metal and a decreasing bond strength were measured as a function of thermal cycles.

Influence of High-Temperature Exposure on Interfacial Fracture Toughness of Thermal Barrier Coating

“…Many attempts have been made to measure TBC interfacial fracture toughness. Jordan et al (3) have conducted a tensile fracture test using a sandwich-type chevron-notched TBC specimen and obtained TBC interfacial fracture toughness G c . Yamazaki et al (4) have applied a Charallambides-type four-point bending interfacial fracture test method to measure TBC interfacial fracture toughness.…”