Influence of substrate material on oxidation behavior and cyclic lifetime of EB-PVD TBC systems

doi:10.1016/S0257-8972(01)01481-5

Surface and Coatings Technology

Volumes 146–147, September–October 2001, Pages 117-123

https://doi.org/10.1016/S0257-8972(01)01481-5 Get rights and content

Abstract

EB-PVD NiCoCrAlY/P-YSZ TBCs on several polycrystalline, directionally solidified, and single crystalline (SX) substrate alloys were thermally cycled at 1100°C. TBC spallation does not correlate solely to TGO thickness, but depends also very much on the substrate alloy. The longest lifetimes are achieved on Hf-containing alloys while SX alloys suffer from early TBC spallation. The formation of the thermally grown oxide was investigated in detail by TEM. A mixed layer of alumina and zirconia exists in the as-coated condition. After initial slight thickening, the thickness of this mixed layer remains constant over a long period of time. During thermal exposure, a continuous layer of pure α-alumina forms and grows underneath the mixed zone by oxygen inward diffusion.

Introduction

Thermal barrier coatings (TBCs) used on advanced turbine blades can considerably increase engine efficiency and performance of the highly loaded blades. State-of-the-art TBCs consist of a PtAl diffusion or an MCrAlY overlay bond coat and a ceramic top coat of partially yttria stabilized zirconia (P-YSZ) deposited by electron beam physical vapor deposition (EB-PVD) or plasma spraying. The EB-PVD process offers the advantage of a superior strain and thermal shock tolerant behavior of the coatings due to their columnar microstructure. Zirconia, as a standard material for current TBC applications, has low thermal conductivity, a relatively high coefficient of thermal expansion and is chemically inert in combustion atmospheres. However, it is essentially transparent to oxygen at high temperatures. Consequently, a thermally grown oxide (TGO) layer forms between the bond coat and the P-YSZ top coat. The TGO plays an important role for TBCs performance; failure in EB-PVD TBCs is almost always initiated at or near the TGO, mostly between TGO and bond coat [1], [2], [3], [4].

Some investigations have already shown the great importance of the right balance between the constituents of a TBC system that includes substrate, bond coat, TGO and ceramic top coat [5], [6], [7]. Moreover, some authors report a dramatic decrease of TBC spalling life if the substrate material changes from conventional polycrystalline or directionally solidified (DS) to single crystalline (SX) alloy [7], [8], [9].

The aim of this study is to determine the influence of substrate material on cyclic TBC life for identical bond coat–top coat systems. For a better understanding of the spalling behavior, a detailed investigation of the TGO growth was performed by TEM for one specific substrate coating system.

Section snippets

Experimental

One hundred to 120-μm thick NiCoCrAlY (Ni–22Co–20Cr–12Al–0.1 to 0.2Y, in wt.%) coatings were deposited onto several cylindrical Ni-base superalloy substrates by EB-PVD at DLR. The composition of the substrates alloys used in this study is given in Table 1. All cylinders had a diameter of 6 mm except for PWA1483 substrates, which had a diameter of 10 mm and a bond coat thickness of 170 μm. IN 617 was used for analyses purposes as flat samples, but with the same coating system used on 6-mm

Results

The TBC lifetime in thermal-cyclic tests is shown for the different substrate materials in Fig. 1. Both SX alloys (PWA 1483 and CMSX-4) suffered from early TBC spallation whereas the two DS alloys (Rene 142 and MAR M002) showed no visible damage of the TBC after 2000 h, at which point testing was suspended. Samples of IN 100 showed a lifetime in between these two extremes. Note that the bars give the average of testing results from two samples with the exception of the base line IN 100 that

Discussion

The results have shown that the cyclic lifetime of TBCs not only depends on the bond coat but also very much on the substrate alloy. Similar to most other studies [6], [7], [8], the longest lifetimes were found for DS alloys while the SX alloys suffered from early TBC spallation. For the latter item, several points could be responsible for shortening the lifetime:

•
Less carbon in the SX alloys that ties up in DS or polycrystalline alloys detrimental elements diffusing from the substrate through

Conclusions

EB-PVD NiCoCrAlY/P-YSZ TBCs on several polycrystalline, directionally solidified, and single crystalline substrate alloys were thermally cycled at 1100°C. The following conclusions can be drawn:

1.
Hafnium-containing alloys (MAR M002 and Rene142) improve adhesion of the TGO on the bond coat, leading to the longest lifetimes. A rough interface between TGO and the bond coat with hafnia pegs was characteristic for these alloys.
2.
SX alloys suffer from early TBC spallation, although no difference in TGO

Acknowledgements

The authors gratefully acknowledge careful manufacture of the coatings by J. Brien, C. Kröder, H. Mangers and H. Schurmann as well as helpful comments on the manuscript by M. Peters at DLR. J. Münzer and U. Kaden performed some of the TGO thickness measurements in SEM. One of the authors (Y.Q.Y.) would like to thank the co-operation program between DLR and Chinese Aviation Establishment (CAE) for partially supporting this work. Special thanks to C. Levi for final corrections to the manuscript.

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Influence of substrate material on oxidation behavior and cyclic lifetime of EB-PVD TBC systems

Abstract

Introduction

Section snippets

Experimental

Results

Discussion

Conclusions

Acknowledgements

Surf. Coat. Technol.

Mater. Sci. Eng.

Mater. Sci. Eng

Acta Materialia

Surf. Coat. Technol.

Mater. Sci. Forum

Oxid. Metals

J. Mater. Sci.

J. Am. Ceram. Soc.