The production of stresses in thermal barrier coating systems by high temperature oxidation

Two kinds of Thermal Barrier Coating (TBC) were examined in this study. A duplex TBC consists of a thermally-insulating ceramic top coat on an oxidation-resistant metallic bond coat. A graded density TBC also has an intermediate layer where bond coat and ceramic are mixed. Experiments on thin-coated substrates have shown that some TBC systems, when heated in air (1,000 C and 1,100 C), produce stresses which can bend their substrates; uncoated control substrates remained unbent. For all the systems examined, X-ray diffraction measurements of the phase of the outer partially stabilized zirconia layer showed that no significant phase differences were produced by the heat exposure; furthermore X-ray diffraction could detect no strain in the top layers. Cross-sectional backscattered electron microscopy showed that the curvature was due to the lateral expansion of the bond coat material as it oxidized. This was confirmed by the large compressive strains found in the oxide by X-ray diffraction techniques. The curvature could occur for duplex and graded density coatings but the higher metallic surface area in the graded density systems was thought to have caused more extensive oxidation. This oxidation-induced bending might make some sorts of TBCs unsuitable for coating certain thin gas turbine components such as combustion-can liners