Bond Strength and Stress Measurements in Thermal Barrier Coatings

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.