Segregation and microstructural evolution at interfaces of atmospheric plasma sprayed thermal barrier coatings during thermal cycling

Segregation and microstructural evolution at interfaces of thermal barrier coatings (TBCs) during thermal cycling were systematically investigated using scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The as-sprayed TBCs were thermally cycled between 1100 °C and ambient temperature up to 3000 cycles. Results show that phosphorus, sulfur, and yttrium co-segregate at the top coat (TC)/bond coat (BC) interface after 3000 cycles. The above co-segregation at the BC/substrate interface was delayed by 1000 cycles. Chemical components of thermally grown oxide (TGO) layers at both the TC/BC and BC/substrate interfaces change from O and Al to O, Al, Cr, Co and Ni as the thermal cycling proceeded. The corresponding oxides in the TGO layers evolved from α-Al₂O₃ to α-Al₂O₃, Cr₂O₃, NiO and spinel phases. Controlled by the element diffusion rate and selective internal oxidation, evolution of the thickness of the TGO layers followed a natural logarithmic law.