• • At 1050 °C and 80 vol.% water vapor, the spinel oxide fraction in the TGO of APS-sprayed bond coats increased markedly compared to 0 vol.% and 45 vol.% conditions, confirming that water vapor accelerates outward diffusion of metal ions and spinel growth; this directly threatens coating durability in hydrogen-blended turbines where steam partial pressures are high.
• • HVOF-sprayed bond coats with dense lamellar interfaces reduced spinel oxide formation relative to APS-sprayed coats, because the porous, weakly bonded APS lamellae provide fast diffusion pathways for corrosive species; industrial adoption of HVOF for bond coat deposition can therefore extend TBC service intervals under high water vapor exposure.
• • The NiCoCrAlTaY bond coat generated the lowest spinel oxide content after 100 h at 1050 °C across all water vapor levels, as Ta promotes rapid formation of a stable Al2O3 scale and suppresses diffusion of other metal cations; this composition offers a viable route for turbines operating with high humidity or hydrogen combustion, though performance above 1050 °C remains unverified.
• • All TBC systems formed a dual-layer oxide scale with continuous, dense Al2O3 beneath uneven, porous spinel oxide; the spinel's brittleness and accelerated growth rate readily initiate cracks within the TGO, leading to premature failure, so controlling spinel content is a direct lever for extending coating life in commercial gas turbines.