Comparative study on thermal barrier coating lifetime of IN939 fabricated by casting and additive manufacturing

Although additively manufactured IN939 has been increasingly studied for gas-turbine hot-section applications, the suitability and thermal cycling durability of air plasma sprayed (APS) thermal barrier coatings on additively manufactured IN939 substrates remain insufficiently established. In this study, IN939 substrates produced by investment casting and powder bed fusion-laser beam (PBF-LB) were coated, via APS, with a NiCoCrAlYTa bond coat and a yttria-stabilized zirconia (YSZ) ceramic top coat. Thermal cycling was performed by flame-heating the coated surface to 1250 °C, followed by forced-air cooling, and continued until 30% spallation was reached. The coating architectures were largely comparable in terms of microstructure and phase constitution. Nevertheless, the coating on the additively manufactured substrate endured up to 197 cycles, whereas the coating on the cast substrate survived up to 106 cycles. Tensile testing in accordance with ASTM C633 showed a higher apparent adhesion strength on the additively manufactured substrate (9.04 ± 0.61 MPa) than on the cast substrate (7.16 ± 0.58 MPa). Dilatometry showed that the additively manufactured substrate exhibited a lower coefficient of thermal expansion over 90–1100 °C, thereby reducing the thermal expansion mismatch with the zirconia top coat in this system. Post-cycling microscopy and energy-dispersive spectroscopy confirmed the formation of an aluminum-rich oxide layer at the top-coat/bond-coat region where failure initiated. Under the applied surface preparation and APS conditions, the improved thermal-cycling lifetime is mainly linked to the lower thermal expansion and higher apparent adhesion strength of the additively manufactured substrate, while porosity and oxide effects are secondary.