Modification of NiCoCrAlY with Pt: Part II. Application in TBC with pure metastable tetragonal (t′) phase YSZ and thermal cycling behavior

doi:10.1016/j.jmst.2018.09.040

Abstract

Abstract:

A thermal barrier coating system comprising Pt-modified NiCoCrAlY bond coating and nanostructured 4?mol.% yttria stabilized zirconia (4YSZ, hereafter) top coat was fabricated on a second generation Ni-base superalloy. Thermal cycling behavior of NiCoCrAlY-4YSZ thermal barrier coatings (TBCs) with and without Pt modification was evaluated in ambient air at 1100?°C up to 1000 cycles, aiming to investigate the effect of Pt on formation of thermally grown oxide (TGO) and oxidation resistance. Results indicated that a dual layered TGO, which consisted of top (Ni,Co)(Cr,Al)₂O₄ spinel and underlying α-Al₂O₃, was formed at the NiCoCrAlY/4YSZ interface with thickness of 8.4?μm, accompanying with visible cracks at the interface. In contrast, a single-layer and adherent α-Al₂O₃ scale with thickness of 5.6?μm was formed at the interface of Pt-modified NiCoCrAlY and 4YSZ top coating. The modification of Pt on NiCoCrAlY favored the exclusive formation of α-Al₂O₃ and the reduction of TGO growth rate, and thus could effectively improve overall oxidation performance and extend service life of TBCs. Oxidation and degradation mechanisms of the TBCs with/without Pt-modification were discussed.

Key words: Pt-modification, NiCoCrAlY, Thermal barrier coating (TBC), Thermal cycling, Thermally grown oxide (TGO)

Chuntang Yu, He Liu, Chengyang Jiang, Zebin Bao, Shenglong Zhu, Fuhui Wang. Modification of NiCoCrAlY with Pt: Part II. Application in TBC with pure metastable tetragonal (t′) phase YSZ and thermal cycling behavior[J]. J. Mater. Sci. Technol., 2019, 35(3): 350-359.

Figures/Tables 17

Table 1 Condition and parameters for spraying 4YSZ via APS.

Current (A)	Voltage (V)	Flow rate (SCFH)		Spraying distance (cm)	Powder feeding rate (g/min)	Nozzle scanning speed (mm/s)
Current (A)	Voltage (V)	Ar	H₂	Spraying distance (cm)	Powder feeding rate (g/min)	Nozzle scanning speed (mm/s)
600	70	120	20	10	35	300

Fig. 1. Preparation steps for the TBCs on Pt-free (a) and Pt-modified (b) NiCoCrAlY bond coats.

Fig. 2. 3D topographies of as-deposited NiCoCrAlY coating specimens without (a) and with (b) Pt-modification.

Fig. 3. Surface morphology of the TBCs coating after vacuum annealing treatment.

Fig. 4. Cross-sectional morphologies of APS 4YSZ on normal (a) and Pt-modified (b) NiCoCrAlY bond coat after vacuum-annealing treatment.

Fig. 5. Element distributions of NiCoCrAlY (a) and Pt-modified NiCoCrAlY (b) bond coats (by EPMA).

Fig. 6. Phase constituents of 4YSZ in the states of powder, as-deposited coating and after thermal exposure at 1100?°C for 1000 cycles.

Fig. 7. Mass change curves of the TBCs with Pt-free and Pt-modified NiCoCrAlY bond coats during the thermal cycling test at 1100?°C.

Fig. 8. Macroscopic morphologies of the 4YSZ TBCs with Pt-free (a) and Pt-modified (b) NiCoCrAlY bond coats after 1000 cycles of thermal exposure at 1100?°C.

Fig. 9. Cross-section morphologies of 4YSZ TBCs with Pt-free (a and c) and Pt-modified (b and d) NiCoCrAlY bond coats after 1000 cycles of thermal exposure at 1100?°C.

Fig. 10. EDS results of different zones as shown in Fig. 9.

Table 2 Compositions of different zones shown in Fig. 9 (by EDS).

Element (at.%)	NiCoCrAlY		NiCoCrAlY?+?Pt
Element (at.%)	Zone I	Zone II	Zone III
O	64.31	65.23	65.55
Al	15.25	33.02	33.44
Cr	10.09	—	—
Co	4.69	—	—
Ni	5.66	—	—
Y	—	0.65	1.01

Fig. 11. Elemental mapping of 4YSZ TBCs with normal NiCoCrAlY bond coat after thermal exposure for 1000 cycles at 1100?°C.

Fig. 12. Elemental mapping of 4YSZ TBCs with Pt-modified NiCoCrAlY bond coat after thermal exposure for 1000 cycles at 1100?°C.

Table 3 Standard Gibbs free energy change of forming various oxides at 1100?°C.

Oxide	ΔGfθ (kJ/mol)
Al₂O₃	-1239.1
CoO	-135.7
NiO	-122.8
Cr₂O₃	-769.6

Fig. 13. Schematic diagrams illustrating the oxidation mechanisms and failure processes of the TBCs with Pt-free (a) and Pt-modified NiCoCrAlY (b) bond coats.

Fig. 14. XRD patterns showing phases of the NiCoCrAlY bond coats with and without Pt-modification after vacuum-annealing at 1000?°C for 6?h (< 6?×?10-3 Pa).

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