Abstract
Microscopical nonuniformity of mechanical properties caused by phase transformation is one of the main reasons for the failure of the materials in engineering applications. Accurate measurement of the mechanical properties of each phase is of virtual importance, in which the traditional approach like Vickers hardness cannot accomplish, due to the large testing range. In this study, nanoindentation is firstly used to analyze the mechanical properties of each phase and demonstrate the phase transformation in thermal barrier coatings during high-temperature aging. The distribution of T-prime metastable tetragonal phase, cubic and tetragonal phase is determined by mapping mode of nanoindentation and confirmed with X-ray diffraction and scanning electron microscope observation. The results show that during 1300 °C aging, the phase transition of metastable Yttria-Stabilized Zirconia induces the quick decrease of T′ phase content and an increase of T and C phases accordingly. It is found that there are some fluctuations in the mechanical properties of individual phase during annealing. The hardness and Young’s modulus of T′ increase at first 9 h, due to the precipitation of Y3+ lean T phase and then decrease to a constant value accompanied by the precipitation of Y3+ rich C phase. The relevant property of C phases also increases a little firstly and then decreases to a constant, due to the homogenization of Y3+ content, while the hardness and Young’s modulus of T phase remain unchanged. After aging of 24h the hardness of T′, C and T phases are 20.5 GPa, 21.3 GPa and 19.1 GPa, respectively. The Young’s modulus of T′, C and T phases are 274 GPa, 275 GPa and 265 GPa, respectively. Present work reveals the availability of nanoindentation method to demonstrate the phase transformation and measure mechanical properties of composites. It also provides an efficient application for single phase identification of ceramics.
Highlights
In order to prolong gas turbine lifetime and increase operation temperature, thermal barrier coatings (TBCs) are required to protect the hot parts of the surface from high temperature, high pressure and corrosive gases [1,2]. 8 wt.% Yttria-stabilized zirconia (8YSZ) has been effectively applied as TBCs [3,4], exhibiting low thermal conductivity [2,5,6], superior mechanical properties and high thermal expansion coefficient [7]
It usually occurs in the non-equilibrium fabrication processes, like air plasma spray (APS) or electron beam physical vapor deposition (EB-PVD) [8]
When the operating temperature is higher than 1250 ◦ C, the metastable T0 phase in TBCs would decompose into thermodynamically stable tetragonal (T) phase and cubic (C) phase accompanied by Y ion diffusion [10,11,12]
Summary
In order to prolong gas turbine lifetime and increase operation temperature, thermal barrier coatings (TBCs) are required to protect the hot parts of the surface from high temperature, high pressure and corrosive gases [1,2]. 8 wt.% Yttria-stabilized zirconia (8YSZ) has been effectively applied as TBCs [3,4], exhibiting low thermal conductivity [2,5,6], superior mechanical properties and high thermal expansion coefficient [7]. According to the phase diagram of ZrO2 -Y2 O3 [9], the T0 phase containing 8 wt.% Yttria does not exist as an equilibrium phase in a solid state. It usually occurs in the non-equilibrium fabrication processes, like APS or EB-PVD [8]. When the operating temperature is higher than 1250 ◦ C, the metastable T0 phase in TBCs would decompose into thermodynamically stable tetragonal (T) phase and cubic (C) phase accompanied by Y ion diffusion [10,11,12]. The transformation from T0 phase to T and C phases would result in the degradation of mechanical properties [11]
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