The Evaluation Of Microstructure Regulation And Mechanism Of The Double-layer Bond Coat In Thermal Barrier Coatings

The thermal expansion mismatch between layers in plasma-sprayed thermal barrier coating system?TBCs?dramatically affects the durability of TBCs.In order to reduce the detrimental effect,the TBCs with the double-layer bond coat consisted of porosity layer and oxidation layer was designed.The shrouded plasma spraying was employed to deposit the porosity layer that material is CoNiCrAlY on nickel-base superalloy Inconel 738 substrate,then the oxidation layer that material is CoNiCrAlY was sprayed on porosity layer via high-velocity oxygen fuel?HVOF?.Two type double-layer bond coats with different thickness ratio of porosity layer to oxidation layer?i.e.,type A:1:2 and type B:2:1?were prepared,respectively.The high temperature oxidation behaviors of sample A and sample B were addressed via Isothermal oxidation several times under 1000?,and the durability and failure mode of TBCs were also evaluated via thermal cycling test under 1120?.The results show that the porosity layer presents a typical lamellar structure and oxidation free,and the oxidation layer exhibits a denser microstructure with some uniformly distributed?-Al₂O₃.The thicknesses of porosity layer in these two bond coats are about 60?m and 120?m,and oxidation layer are about 120?m and 60?m.The coefficient of thermal expansion of oxidation layer is about 11.6×10^-6K^-1,which is rather lower than that of porosity layer(12.0×10^-6K^-1).The growth of thermally grown oxide?TGO?on two type bond coat obeys power function,the velocity of TGO is dramatic lower than that of traditional bond coat.In spite of depletion of a fraction of Al element during deposition,the mixed oxides were not found in surface of TGO,in 200 h oxidation under 1000?.The thermal cyclic lifetime of type A TBCs is about 60 cycles,which exceeds 20%of type B TBCs.The main factors are thermal expansion mismatch is eased and the growth velocity of TGO has decreased.The thicknesses of failure TGO are all about 4.0?m,the delamination cracks in two type TBCs all propagate in YSZ coating.