In situ synthesis of α-alumina layer on thermal barrier coating for protection against CMAS (CaO–MgO–Al2O3–SiO2) corrosion

• An α-Al2O3 layer was in situ synthesized at the top 7YSZ TBC.
• Anorthite CaAl2Si2O8 and MgAl2O4 spinels were observed after CMAS corrosion.
• The TBC with α-Al2O3 overlay has better CMAS resistance than the as-sprayed TBC.

In general, it is accepted that thermal barrier coating (TBC) with good CMAS (CaO–MgO–Al2O3–SiO2) corrosion resistance has long lifetime. A columnar Al film was firstly deposited in this work at the top of 7 wt.% yttria-stabilized zirconia (7YSZ) TBC by magnetron sputtering. Then a vacuum treatment of Al-deposited TBC was carried out at 700 °C for 1 h and 900 °C for 5 h. An α-Al2O3 layer was in situ synthesized at the top of 7YSZ TBC through the reaction of Al and ZrO2 after vacuum heat treatment. The CMAS corrosion resistance of as-sprayed TBC and treated TBC was compared after isothermal test of 1200 °C for 24 h. The results indicated that the Al-deposited TBC after vacuum heat treatment has better CMAS corrosion resistance than the as-sprayed TBC. Two mechanisms are considered to contribute to the protective effect of in situ synthesized α-Al2O3 layer: the first is α-Al2O3 partially filling existed open pores and cracks in the top 7YSZ coating to hinder the penetration of molten CMAS; and the second is α-Al2O3 reacted with CMAS forming CaAl2Si2O8 and MgAl2O4 that play the role in inhibiting further infiltration of CMAS.