Abstract
Thermal barrier coatings (TBCs) were made on cylindrical substrates with radii of 5, 10 and 15 mm. In addition to the cylindrical substrates, the TBC were also made on a plate substrate. First, NiCoCrAlY was pressureless plasma sprayed as a bond coating, and then zirconia with 8 mol% yittria was air plasma sprayed as the top coating. The TBC specimens were exposed in an atmosphere of air at 1373 K for 0, 200 and 500 h. The radial strain was measured with the strain scanning method using hard synchrotron X-rays. The hoop and axial stresses of the cylindrical specimens were measured using laboratory X-rays with successive removal of the surface by polishing. Using these measured stresses and strains, the distributions of the 3-dimensional components of the residual stress in the top coating were determined. The spalling stress became a maximum at about 50 mm from the interface. This position corresponded to the location of initiation of the spalling cracks. The value of the measured maximum spalling stress increased with the decrease in the radius of curvature. The difference of the measured distribution of the residual stress from the calculated distribution of the thermal stress was explained on the basis of the characteristics of the top coatings, such as the stress relaxation due to sliding between lamellae, a binding effect by the interface roughness and the spalling stress induced by the interface roughness.