Isothermal oxidation and TGO growth behavior of NiCoCrAlY-YSZ thermal barrier coatings on a Ni-based superalloy

https://doi.org/10.1016/j.jallcom.2020.156093Get rights and content

Highlights

  • TBCs reduce the thicknesses of oxide scale and internal oxidation zone of substrate.

  • Oxides of TGO layer at 1000 °C are mainly α-Al2O3, (Ni,Co)O, (Co,Ni)Cr2O4 and (Ni,Co)Al2O4.

  • The growth stresses of α-Al2O3, NiCr2O4, NiO and CoO have been evaluated.

  • The growth kinetics of TGO layer and the interface oxidation mechanism of YSZ TBC are analyzed.

Abstract

The isothermal oxidation of 8 wt% Y2O3 stabilized ZrO2 (YSZ) thermal barrier coating (TBC) and growth behavior of thermal growth oxide (TGO) have been investigated. The average thicknesses of oxide scale and internal oxidation zone of the substrate with coating are significantly smaller compared with the bare substrate. The TGO layer evolves from a black single-layer to a double-layer structure. The lower black layer is composed of α-Al2O3, and the upper gray layer consists of mixed oxides of (Ni,Co)O, (Co,Ni)Cr2O4 and (Ni,Co)Al2O4 at 1000 °C by TEM analysis. The formation of the solid solution oxides is probably related to the intense diffusion at high temperature. The growth stresses of α-Al2O3, NiCr2O4, NiO and CoO are calculated to be 30.88, 62.99, 34.52 and 35.85 GPa, respectively, indicating that mixed oxides, especially the spinel phase, are very detrimental to the stability of the TBC. The growth kinetics of TGO layer and the oxidation mechanism of YSZ TBC are illustrated.

Introduction

As an advanced high temperature structural material, Ni-based superalloys are usually used as substrate materials for hot section components of gas turbine engines due to their excellent mechanical properties and high reliability at elevated temperature [1,2]. With the increasing of turbine inlet temperature, the material surface faces increasingly severe high temperature oxidative damage. Although alloying elements such as Al, Si and Cr can improve the oxidation resistance of alloys, mechanical properties of alloys are always deteriorated [[3], [4], [5]]. In contrast to alloying, deposition coating is a more effective and economical method, because it can improve the oxidation resistance without deteriorating mechanical properties of alloys [6,7]. Comparing with the first and second generation aluminide coatings as well as the third generation overlay coatings, the fourth generation thermal barrier coatings (TBCs) show superior performance in terms of temperature improvement and service life [1]. The use of TBCs can effectively decrease the operating temperature of superalloys surface and prevent the diffusion of oxygen to the alloys surface, thus increasing the durability of hot section components [[8], [9], [10]].

A common TBCs system with two different layers consisting of ceramic top coating and metallic bond one above a superalloy substrate. The metallic bond coating with 75–150 μm in thickness is generally made of MCrAlY (M = Co, Ni, NiCo, CoNi) or aluminides of Ni and Pt. The bond coating not only improves the physical compatibility between the substrate and the ceramic top coating, but acts as a protective layer against high temperature oxidation, hot corrosion and erosion [[11], [12], [13]]. Weng et al. [14] have observed that NiCoCrAlY bond coating with higher Al content preserves better oxidation resistance than CoNiCrAlY bond coating. Chen et al. [15] have reported that the addition of Ta element in NiCoCrAlY coating inhibits the formation of Al2O3 scale on the coating surface, which is detrimental to the oxidation resistance of the coating. The ceramic top coating with 100–400 μm in thickness provides the thermal insulation at high temperature. YSZ is commonly the preferred material for ceramic top coating due to the low thermal conductivity, high thermal expansion coefficient and good thermal shock resistance [11,16]. There are many preparation techniques used in the production of the TBC system, such as high velocity oxygen fuel (HVOF), air/vacuum plasma spraying (A/VPS), electron beam physical vapor deposition (EB-PVD), laser cladding (LC) and cold gas dynamic spray (CGDS), etc [7,[17], [18], [19], [20]]. In addition, CGDS process is usually employed to prepare bond coating due to its lower spraying temperature. Air plasma spraying (APS) and EB-PVD are both widely used methods. Compared with EB-PVD, APS method has the advantages of high efficiency, low cost, relatively simple equipment and a wide range of spray materials. TBCs with lamellar structure prepared by APS are widely used on combustion chambers of gas turbine and turbine guide vanes [11,21].

It is well known that the formation of TGO is one of the main reasons for the failure of TBCs. The growth stress caused by the thickening of the TGO layer and the thermal stress caused by the mismatch of the thermal expansion coefficient lead to the failure of TBCs [22,23]. The YSZ/TGO interface is generally rough, and undulation peaks are the concentration point of the tensile stress, so the formation and propagation of crack tip usually occur near the peaks of the TGO interface [10,24]. The formation of mixed oxides in TGO, such as porous spinel phase, can easily lead to crack nucleation, which is related to the rapid volume expansion and poor toughness of mixed oxides [25]. In addition, Dong et al. [8] have found that YSZ TBC prepared by APS have a critical TGO thickness of about 6.0 μm, over which the thermal cyclic lifetime of the TBC reduces more rapidly, and the failure mode changes from cracking within YSZ coating to through YSZ/TGO interface. The cracks caused by formation and growth of TGO destroys the integrity of TBCs structure and can provide more channels for diffusion of oxygen into the bond coating, which significantly accelerate the degradation of TBCs [11,26,27]. Thus, it is of utmost importance to illustrate the formation and growth behavior of TGO for improving the lifetime of TBCs.

In order to obtain a TBC with superior oxidation resistance, corrosion resistance and service life, enormous studies on TBCs have been conducted in recent years. The main ways to improve the oxidation resistance of TBCs include: (i) Reducing thermal stress by improving the mismatch of thermal expansion coefficient [19]; (ii) Decreasing oxygen permeation [28,29]; (iii) Reducing thermal conductivity [18]; (iv) Increasing thermal stability of ceramic materials [22]. At present, the development of TBCs mainly have functionally graded coatings (FGC), nanostructured TBCs, double-ceramic-layer (DCL) TBCs, new ceramic material (rare earth zirconate etc.) and laser surface treatment, etc [18,19,22,28,29]. Although the above-mentioned methods can improve the oxidation resistance of TBCs to a certain extent, there are also some limitations. The preparation process of FGC is complex and has low repeatability. DCL TBCs have higher thermal stress due to the mismatch of thermal expansion coefficient. The toughness and thermal expansion coefficient of the rare earth zirconate material are relatively low. Laser re-melting may lead to stress cracking. Thus, when the service temperature is lower than the phase transformation temperature (1443 K) of YSZ, the typical YSZ is still the preferred material considering the stability and service life [16]. Nevertheless, the research on the interface oxidation behavior of YSZ TBC as well as formation and growth of TGO layer is still lacking in the literature and indispensable to analysis the failure and oxidation mechanism of coatings.

In this work, the typical YSZ TBC is prepared by APS method to protect the substrate from high temperature oxidation. YSZ TBC is exposed to oxidation tests at 900 and 1000 °C for different durations in order to investigate the oxidation behavior. The microstructure and basic properties of as-sprayed YSZ TBC are characterized. The thicknesses of oxide scale and inner oxidation zone of alloy with and without coating are compared and analyzed. Meanwhile, the formation and growth behavior of TGO during oxidation are studied, and the phase constitutes of the TGO layer is also analyzed to investigate the failure reason and oxidation mechanism of YSZ TBC at high temperature.

Section snippets

Substrate and coating preparation

The rolled-state Ni–20Cr–18W superalloy was used as substrate material, and chemical compositions of Ni–20Cr–18W alloy and APS feedstocks were listed in Table 1. Rectangular specimens with the dimensions of 20 mm × 10 mm × 2 mm were cut by wire electrical discharge machine. The surface of samples were ground with sandpaper to remove oxides and ultrasonically cleaned with alcohol for degreasing. Before spraying, one surface of samples was sandblasted with SiC particles under 0.45 Mpa pressure to

Microstructure of as-sprayed TBC

Surface and cross-sectional morphologies of as-sprayed YSZ TBC are shown in Fig. 2. Surface of YSZ coating exhibits a common lamellar structure with a certain number of open pores and micro-cracks in Fig. 2(a), which is a characteristic feature of the coating fabricated by plasma spraying [11]. During plasma spraying, the molten or semi-molten droplets are spread out on the surface of the formed coating and solidified instantaneously to form lamellar structure. In Fig. 2(b), the YSZ TBC

Conclusion

  • 1)

    The average thicknesses of oxide scale and internal oxidation zone of substrate with coating are significantly smaller than that of bare substrate. YSZ TBC effectively improves the high temperature oxidation resistance of the superalloy substrate.

  • 2)

    The TGO layer evolves from a black single-layer to a double-layer structure composed of lower black layer and upper gray layer during oxidation at 900 and 1000 °C. The lower black layer is composed of α-Al2O3, and the upper gray layer consists of mixed

CRediT authorship contribution statement

Jiaqi Shi: Methodology, Data curation, Validation, Writing - original draft. Tiebang Zhang: Conceptualization, Supervision. Bing Sun: Software, Formal analysis. Bing Wang: Investigation. Xuhu Zhang: Formal analysis. Lin Song: Visualization, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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