Influence of original powders on the microstructure and properties of thermal barrier coatings deposited by supersonic atmospheric plasma spraying, part II: Properties

Abstract

In this paper, the thermal shock resistance, oxidation resistance and thermal insulation performance of “conventional” microsized and nanostructured coatings (named as MC and NC respectively) deposited by supersonic atmospheric plasma spraying (SAPS) were investigated. The results showed that due to the improvement of intersplat cohesion and bonding strength between the top coat and bond coat, the MC presented a higher thermal shock resistance, which was distinct from the previous reports about the property difference between the conventional and nanostructured coatings. In addition, the result from isothermal oxidation test indicated that the oxidation kinetics of both coatings followed a parabolic law. The weight gain of MC was slightly lower than that of NC before 600 h at 1100 °C. However, due to the formation of vertical cracks which penetrated the whole top coat, the weight gain for MC after 1000 h was slightly higher than that of NC, indicating that the NC had a higher oxidation resistance after long time exposure. Meantime, because lots of fine cracks were formed between the unmelted particles and lamellar structures, the temperature drop (ΔT) of NC increased with the increase of testing times. The nanostructured SAPS-coating showed better oxidation resistance and thermal insulation properties, but the inter-splat cohesion and top coat/ bond coat interface adhesion are still needed to be improved in order to increase its thermal cycling life.

Introduction

Thermal barrier coatings (TBCs) have been widely applied to protect the hot-section components of modern gas turbine engine from the extremely aggressive environments [1]. The TBC typically consists of a ceramic Y₂O₃ partially stabilised ZrO₂ (YSZ) top coat and a metallic bond coat, usually MCrAlY, where M is usually nickel, cobalt, or combination of these two. The bond coat enhances the adhesion and provides a good thermal expansion match between the top coat and the substrate. In addition, during high-temperature service, oxidation of the bond coat can lead to the formation of thermally grown oxides (TGOs) at the original top coat/bond coat interface.

Among the thermo-mechanical properties of TBCs related to the high temperature application, the thermal shock resistance, oxidation resistance and thermal insulation capacity are of central importance. TBCs have a tendency to spall or debond under thermal cycling conditions at high temperature. It was found that the failure of TBCs during the thermal cycles was associated with the spallation of the ceramic coat due to the buildup of thermal stresses, which generated by the temperature gradients in service, ceramic sintering, phase transformation, corrosive and erosive attack and residual stresses arising from the deposition process [2], [3]. In addition, during high temperature exposure or thermal cycling, cracks initiated and propagated within or close to the TGO layer as the result of oxidation of bond coat at high-temperature exposure. These large delamination cracks can finally cause the spallation of the top ceramic coat and failure of TBC system. Due to the porous and microcracked structure of plasma sprayed coatings, some studies considered that the top ceramic coat was transparent to oxygen penetration [4], [5]. However, some studies have suggested that the presence of the top coat can reduce the oxidation rate of bond coat to some extent [6], [7]. Therefore, the effect of composition and microstructure of top coat on the oxidation behavior of bond coat is needed to be further investigated.

The thermal insulation property of TBCs is usually evaluated by thermal conductivity, which had a close relationship with the microstructure of coating. The plasma sprayed TBCs exhibited typical layered and porous microstructures. The interlamellar boundaries or cracks running parallel to the ceramic/metal interface can effectively reduce the high-temperature thermal conductivity of coating. However, the above defects are considered as a source of weakness, which can result in the failure of TBC system since they leave the coating relatively less strain-tolerant [8]. Therefore, how to tailor the microstructure, which can provide a balance between low thermal conductivity and high thermal cycling life, is still a hot research topic in the field of TBCs.

In part I [9] of this two-part series, the microstructural difference between the microsized and nanosized coatings deposited by supersonic atmospheric plasma spraying (SAPS) was studied. In this part, the property difference between the above two types of coatings, including thermal shock resistance, oxidation resistance and thermal insulation property will be discussed in detail.