Thermal cyclic lifetime and oxidation behavior of air plasma sprayed CoNiCrAlY bond coats for thermal barrier coatings

Furnace thermal cycling lifetime and microstructural degradation are reported for thermal barrier coatings (TBCs) with air plasma sprayed (APS) ZrO2-8 wt.%Y2O3 (YSZ) and APS CoNiCrAlY bond coats. TBCs examined in this study consisted of 600 μm-thick APS YSZ, 175 μm-thick APS CoNiCrAlYSi and 5 mm-thick Haynes 230 superalloy substrate. Furnace thermal cycling was carried out using 1-, 10- and 50-hour thermal cycles that consisted of 10-minute heat-up to 1121 °C, dwell at 1121 °C and followed by 10-minute forced-air-quench. To further document microstructural degradation of APS CoNiCrAlY coatings, free-standing APS CoNiCrAlY coatings were produced and isothermally oxidized at 1124C for up to 300 h. Despite the significant internal oxidation of APS CoNiCrAlY bond coats, APS TBCs exhibited excellent thermal cycling lifetime. Microstructural evolution of TBCs with an emphasis on the development of thermally grown oxide including internal oxidation was examined by scanning electron microscopy, transmission electron microscopy (TEM) and scanning TEM. Site-specific TEM specimen was prepared by using Focus Ion Beam In-situ Lift-out technique. Extensive internal oxidation was observed including formation of Al2O3, (Al,Cr)2O3 and (Ni,Co)(Al,Cr)2O4 identified by electron diffraction. Quantitative microscopy and a simple rule of mixture were employed to understand the excellent performance of these TBCs with APS CoNiCrAlY bond coats based on reduced thermal expansion mismatch.