Microstructure and lifetime of EB-PVD TBCs with Hf-doped bond coat and Gd-zirconate ceramic top coat on CMSX-4 substrates

    Microstructure and lifetime of EB-PVD TBCs with Hf-doped bond coat and Gd-zirconate ceramic top coat on CMSX-4 substrates Ahmed Umar Munawar, Uwe Schulz, Muhammad Shahid PII: DOI: Reference:

S0257-8972(16)30366-8 doi: 10.1016/j.surfcoat.2016.05.005 SCT 21158

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

29 January 2016 1 May 2016 2 May 2016

Please cite this article as: Ahmed Umar Munawar, Uwe Schulz, Muhammad Shahid, Microstructure and lifetime of EB-PVD TBCs with Hf-doped bond coat and Gd-zirconate ceramic top coat on CMSX-4 substrates, Surface & Coatings Technology (2016), doi: 10.1016/j.surfcoat.2016.05.005

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ACCEPTED MANUSCRIPT Microstructure and lifetime of EB-PVD TBCs with Hf-doped bond coat and Gd-zirconate ceramic top coat on CMSX-4 substrates Ahmed Umar Munawara, b, *, Uwe Schulza, Muhammad Shahidb a

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Corresponding author

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*

German Aerospace Center (DLR), Linder Hoehe, Cologne 51147, Germany National University of Science and Technology, H-12, Islamabad 44000, Pakistan

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b

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Abstract

Gadolinium zirconate (GZO) with its lower thermal conductivity and higher thermal stability

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compared to the industrial standard 7YSZ is a new promising material for thermal barrier coating (TBC) applications. In this study, top coats of GZO and 7YSZ were deposited on NiCoCrAlY

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and Hf-doped NiCoCrAlY bond coats with CMSX-4 as substrate material. The bond coats as well

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as the ceramic top coats were manufactured by electron-beam physical vapor deposition (EB-

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PVD). The lifetimes of these new TBC systems were investigated by thermal cycling at 1100°C. In comparison to the standard 7YSZ TBC, GZO deposited on the NiCoCrAlY bond coat exhibited a significantly enhanced lifetime. Doping NiCoCrAlY with 0.6wt. % Hf resulted in around 10times increase in the lifetime for the 7YSZ top coat. However, no significant difference in lifetimes was observed when 7YSZ is replaced by GZO on NiCoCrAlY-Hf bond coats. During thermal cycling, a chemical reaction between GZO and the thermally grown oxide (TGO) formed on the NiCoCrAlY bond coat was observed; however, such a chemical reaction did not occur when GZO was deposited on NiCoCrAlY-Hf bond coats. A faster TGO growth has been observed for the Hf- based systems, resulting in TGO thicknesses as large as 20µm. The role of Hf-doping in the bond coat, the individual TGO microstructure, and the diffusion of refractory elements from the substrate into the bond coats are discussed along with the lifetime measurements of the different TBC systems.

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Introduction

Thermal Barrier Coatings (TBCs) are multi-layer and multi-material systems which are applied

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on internally cooled turbine engine components to improve their high temperature capability and

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lifetimes. A TBC system consists of a ceramic top coat and a metallic bond coat deposited on top

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of a super alloy substrate. During the deposition and subsequent high temperature exposure of TBCs during service, an oxide layer starts to develop at the bond coat- ceramic top coat interface.

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This oxide layer is called thermally grown oxide (TGO) and its composition and microstructure

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largely depends on the bond coat material. Currently, two type of bond coats are mostly used which are PtAl type diffusion coatings and MCrAlY type overlay coatings where (M = Ni, Co or

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a combination of both). For the ceramic top coat, 7wt.%yttria stabilized zirconia (7YSZ) has been

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the state-of-the-art material for the last 2 decades. There is immense literature available covering

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different aspects of TBC systems [1-7]. The Ceramic top coat material must possess a very low thermal conductivity and a very high thermal stability so that the efficiency of gas turbines can be improved by achieving the highest possible turbine inlet temperature (TIT). However, the upper temperature limit for the use of 7YSZ has been found to be around 1200°C as 7YSZ on prolonged exposure at higher temperature decomposes into high yttria and low yttria phases. The low yttria phase on cooling transforms into monoclinic phase accompanying a large volume increase associated with this phase change [7-10]. This phase change eventually proves to be catastrophic and results in TBC spallation. As a result, new ceramic top coat compositions are sought which can allow a higher TIT. Gadolinium zirconate (GZO) is one such material which is well known for its low thermal conductivity and high thermal stability [11-14]. Recent studies have also indicated a higher resistance of GZO against CMAS [15-18] and volcanic ash attack [19-21]. However, a lower

ACCEPTED MANUSCRIPT lifetime of GZO based TBCs deposited by air plasma spraying has also been reported due to its lower fracture toughness and due to the incompatibility between GZO and alumina TGO [22-24]. To overcome these problems, double-layer structures have been studied by some groups where a

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thin layer of 7YSZ is deposited between GZO and the bond coat [23-24]. In case of plasma-

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sprayed systems, such double-layer GZO TBCs have shown longer lifetimes than the single-layer

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ones [24]. However, in case of EB-PVD systems, the application of a 7YSZ base layer is not necessary on IN100 substrate covered by a NiCoCrAlY bond coat, and the single-layer GZO

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TBCs have shown longer lifetimes than the double-layer TBCs [26].

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Pre-mature spallation of TBCs during service has been a serious concern as it exposes the underlying metal to the dangerous hot gases. To improve the lifetime of TBCs, several

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approaches have been used which include not only the use of different compositions for the

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substrate, bond coat and/ or the ceramic top coat material but also different pre-treatments and

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use of different parameters for the bond coat and the ceramic top coat deposition [23-31]. Therefore, the compatibility between the different constituents in a TBC system needs to be considered as well while selecting a suitable TBC system. A lot of development has been done to improve the strength and high temperature stability of superalloys. Several refractory elements such as W, Ta, Re and Ru are added in amounts up to 20wt. % and more in single-crystal (SX) super alloys to mainly improve their high temperature creep-strength by the solid solution strengthening mechanism [33-35]. However, in some cases these refractory elements have been found to diffuse towards the TGO forming oxides which result in a lower adhesion of the ceramic top coat, thereby lowering the lifetime of TBC systems [27, 36]. As an example, when NiCoCrAlY was deposited on CSMX-4 as the bond coat, a much lower TBC lifetime was found in comparison to the same TBC system on IN100, Rene142 or MAR M002 for both 7YSZ and

ACCEPTED MANUSCRIPT GZO top layers. Therefore, a lot of research has been focused to improve the adhesion between ceramic top coat, TGO, and bond coat.

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Addition of Hf and/ or other rare-earth (RE) elements such as Y and Zr to bond coats has been such an effort. Although still under debate which mechanism is dominant for the improved TGO

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adherence and prolonged TBC life by RE doping of bond coats, current consent is their ability to

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getter detrimental sulfur from the substrate alloy, to reduce void formation in or underneath the

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TGO, to enhance TGO adhesion, and to suppress outward cation transport which promotes inward scale growth [37, 38]. An optimum content of the RE element(s) seems to exist for each

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individual substrate -bond coat- ceramic top coat system that leads to a maximum beneficial RE effect. Below the optimum content, the positive effects are not fully exploited and above

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additional negative effects such as internal oxidation and enhanced scale growth rates start to

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dominate. This optimum RE content for TBC lifetime improvement seems to be rather small for

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NiAl model substrate alloys or Ni(Pt)Al bond coats (around 0.05 to 0.08at% for Zr& Y) [39-41] and larger for doped coatings on superalloy substrates (around 0.5at% Zr in a systematic study with variation in the Zr content) [38]. No studies are known that have systematically evaluated the influence of theHf-content in bond coats on TBC lifetime. Note that the conversion from at% to wt% (the latter is used in the current study) in a typical NiCoCrAlY bond coat leads to roughly two-fold higher numbers for Zr-doping and four-fold for Hf-doping. RE-doping in MCrAlYs typically results in the formation of Hf-oxide stringers that can reach deeply into the bond coat. They are believed to improve the TGO-bond coat adhesion by anchoring the TGO, most likely since crack propagation along such a wavy interface requires more driving force than along a flat interface. An extra crack initiation at those oxide pegs is not likely [42], which finally results in an improved TBC lifetime if an optimum RE-content is used [30, 38, 43, 63]. In this study, CMSX-4 superalloy substrates have been deposited with NiCoCrAlY or

ACCEPTED MANUSCRIPT NiCoCrAlY-Hf bond coats with either 7YSZ or GZO as the ceramic top coat material. Furnace cyclic testing (FCT) has been performed to determine the lifetime of these TBC systems and

Experimental procedure

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2.

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investigations have been carried out to discuss the dominating failure mechanisms.

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The samples consisted of cylindrical rods of CMSX-4 superalloy substrates with a diameter of around 6mm and a length of around 60mm. A NiCoCrAlY bond coat has been coated by a 60kW

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EB-PVD coater while a Hf-doped NiCoCrAlY, referred to as NiCoCrAlY-Hf bond coat, has been

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deposited by a 150kW EB-PVD coater in one run using two ingots for achieving the Hf- doping. One ingot was the same NiCoCrAlY ingot type (designated PWA 1370) used for single source

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evaporation, and the other one was a pure Hf ingot. The main difference between the two bond

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coats is their Hf content while all the other elements are quite similar. All bond coats had a thickness between 80-100µm. The composition of these two bond coats that was measured by X-

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ray Fluorescence Spectroscopy along with the composition of the superalloy substrate and the ceramic top coats are presented in table 1. For both bond coats, peening and vacuum annealing at 1080°C for 4h was applied before the deposition of the ceramic top coat layers. The deposition of ceramic top coats has been done in the 150kW EB-PVD coater by using commercial ingots of 62.5mm diameter. All the samples have been pre-heated in the same manner in a separate chamber before ceramic top coat deposition. During deposition the samples have been rotated on a horizontal axis at 12rpm. All ceramic top coat depositions have been carried out at a substrate temperature of around 1000°C by using single source evaporation. Furnace cyclic testing (FCT) of the TBC systems has been done by keeping the samples in a preheated furnace at 1100°C for 50minutes and then cooling them to nearly room temperature by forced air cooling for 10minutes. During the FCT testing, no internal cooling has been used

ACCEPTED MANUSCRIPT which means that there is no temperature gradient across the thickness of the TBC systems once the samples are fully heated through. Failure of the TBC systems has been defined as the spallation of TBC of an area with one of the dimensions greater than 5mm. For each version, two

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samples have been tested. In order to understand the evolution of TGO microstructure, the

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NiCoCrAlY-Hf+7YSZ systems have been additionally thermally cycled for 100, 200, 500 and

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1000 cycles and cross-sections have been investigated for TGO microstructure.

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To perform cross-sectional investigations, the samples were mechanically cut and prepared by standard metallographic techniques. The microstructural analysis has been carried out in an

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analytical SEM (LEO Gemini 982) equipped with an EDS (energy dispersive x-ray spectroscopy)

Results Lifetime results

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3.1

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system.

The standard CMSX-4+NiCoCrAlY+7YSZ TBC system shows an average lifetime of 311 cycles. This average lifetime value has been determined after testing more than 10 samples in previous studies. However, there is a remarkable improvement in the lifetime when the NiCoCrAlY bond coat is doped with 0.6wt. % Hf with the same ceramic top coat and substrate materials. The 2 samples tested for NiCoCrAlY-Hf bond coat showed lifetimes of 2929 and 3039 cycles which are almost 10times more than the lifetimes obtained for the standard system. When the ceramic top coat 7YSZ is replaced by GZO on the NiCoCrAlY bond coat, the 2 samples endured 943 and 1187 cycles which is again a significant improvement when compared with the lifetime of the standard system tested in this study. When CMSX-4 is deposited with both the new compositions i.e. NiCoCrAlY-Hf and GZO, the 2 samples showed lifetimes 3246 and 2486 which is much higher than the standard TBC system,

ACCEPTED MANUSCRIPT however, the average lifetime is very similar to the one obtained for the NiCoCrAlY-Hf+7YSZ system. The lifetime of the TBC systems tested in this study are given in Fig. 1.

As-coated TBCs

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3.2

The column morphology of the as-coated 7YSZ and GZO ceramic top coats has been explained

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in detail in our previous publications [26-27, 45]. The as-coated GZO columns are curved in the

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direction of rotation and have more columns branching at the column tips. In addition, the

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columns have a larger diameter at the column tips than in the middle region of the columns. Such morphology has not been observed for 7YSZ columns where the columns are straight and the

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diameter is almost uniform over the columns length. This was independent of the substrate

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material and was found on CMSX-4 in the present study as well.

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The TGO in the as-coated condition for the NiCoCrAlY based systems has been found to be smoother than the Hf-doped systems and consists mainly of alumina with white zirconia particles

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in it. The TGO thickness in the as-coated condition is in the range of 0.5-1µm. Yttrium is the 1st element to oxidize in the system and forms yttria islands at the TBC-TGO interface. These yttria islands are frequently present at the TBC-TGO interface for the NiCoCrAlY- based systems. This TGO microstructure was similar for both top coats and also similar to the one on other substrate alloys such as IN100 and has been investigated in detail elsewhere, including mixed zone formation [25, 27, 29, 46-47]. In case of as-coated NiCoCrAlY- Hf systems, some Hf oxide can be seen within the alumina TGO at the TBC-TGO interface which shows that Hf is the 1st element to oxidize in the NiCoCrAlY-Hf system (brightest particles in Fig. 2). In addition, zirconia particles are present within the TGO that occur in the classical mixed zone configuration. The TGO is slightly more undulated compared to pure NiCoCrAlY with some starting oxide protrusions at the TGO-bond

ACCEPTED MANUSCRIPT coat interface. The TGO thickness in the as-coated condition is slightly more than the NiCoCrAlY-based systems and has been found to be around 0.7-2µm. At locations, where the ascoated TGO has a slightly higher thickness, Hf oxide can be seen at the bottom (Fig. 2) which

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shows that Hf results in a faster oxidation of the bond coat. Hf oxide that is not connected to the

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TGO and is quite often surrounded by a shell of alumina was detected within the bond coat,

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confirming a strong oxygen affinity of Hf.

Evolution of TGO microstructure

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3.3

With thermal cycling, the TGO growth takes place for all TBC systems predominantly by an

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inward diffusion of oxygen. The as-coated TGO on NiCoCrAlY bond coats transforms into a double layer structure consisting of an upper mixed zone where bright zirconia particles are

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finely dispersed in alumina, and a lower alumina zone. In the latter zone yttria-pegs are present

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which have a limited depth within the bond coat and have been discussed in several previous

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studies [26, 29, 46, 47]. The TGO growth for both 7YSZ and GZO on NiCoCrAlY bond coats has been found to be very similar to each other and to the IN100 based systems tested in our previous study [26, 45].

Thermal cycling of the Hf- doped systems results in the formation of oxide-stringers which run very deep into the bond coat in a very short time. The stringers contain in most locations Hfoxide (bright particles in Fig. 3a) which are surrounded by alumina. After only 100 cycles, the oxide-stringers have a higher depth than the dense and continuous TGO thickness at several locations. Hf seems to be the main driving force for the diffusion of oxygen as Hf present deeper in the bond coat is also oxidized in a very short time, along with the aluminum around it (Fig. 3a). Due to this preferred growth of the TGO towards Hf, the bond coat is “entrapped” in the TGO at several places as shown in Fig. 3a. It can be seen in Fig. 3b that the continuous thermal cycling results in growth of the TGO in thickness, however, the hafnia stringers do not grow at

ACCEPTED MANUSCRIPT the same rate and consequently after 3200 cycles the stringers have a roughly 50% depth compared with the main TGO thickness. In addition, NiCoCrAl-spinels form within the TGO at several locations for both ceramic top coats (see Fig. 3b). It is important to mention here that the

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gap within the TGO in both Fig. 3a and 3b has developed during the mounting step of the

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metallographic sample preparation and does not show the failure location of the TBC systems.

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As thermal cycling is continued, underneath the TGO a continuous zone with different contrast

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and composition starts to form. It has a thickness of around 1µm after 500 cycles but after around 3000cycles it is as thick as 4µm. Careful EDS measurements gives some insight into the chemical

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composition of the phases and revealed this zone contains very little Hf and is rich in refractory elements such as Ta, W and Re while depleted in Al (first 3 points in Fig. 4b).

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A chemical reaction between GZO and the TGO has been observed for the standard NiCoCrAlY based systems, similarly as the one found on IN100 substrates with the same TBC systems. Two

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distinct oxide phases had been identified there at the interface that contain Gd, Al and Y in different amounts [26]. This chemical reaction takes place on CMSX-4 more slowly with thermal cycling and after 1000cycles the new resulting phase can be found only in patches at the TBCTGO interface (see light grey phase at the interface in Fig. 5a). However, no such chemical reaction and formation of a new phase has been observed in the NiCoCrAlY-Hf+GZO system at the TBC-TGO interface, as shown in Fig. 5b. Even after quite long thermal cycling at around 3000cycles, the new phase does not form. In some areas, a lighter grey phase was present at the interface that contains mainly Al, but considerable amounts of Ni, Co, and some minor amounts of Cr and Gd. It is assumed that this is also a spinel phase that has dissolved some Gd from the GZO. More spinel phases can be found within the TGO in the middle region. Limited amounts of Al have been detected within the GZO at several locations close to the TGO interface (up to only a few microns away from it) after thermal cycling in the

ACCEPTED MANUSCRIPT range of 2500cycles. This finding is in accordance with the Al-diffusion towards the GZO in our previous publication [26].

Diffusion of elements

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3.4

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reaction between the TBC and the TGO has been observed.

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For the standard 7YSZ top coat on both NiCoCrAlY and NiCoCrAlY-Hf systems, no chemical

The refractory elements from the CMSX-4 substrate diffuse through both the bond coat towards

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the TGO during high temperature exposure. Fig. 6a shows the location where EDS measurements

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have been carried out. The length of the line scan measurement is around 5µm while the arrow head shows the direction of the line scan measurement. Fig. 6b shows the total refractory (Ta, W

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and Re) content in wt. % after 100 cycles and 1187 cycles in the NiCoCrAlY+GZO system. It is

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evident from Fig. 6b that the refractory elements diffused faster towards the TGO in the

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beginning and after only 100cycles the total refractory content close to the TGO is around 3-4%. However, with continuous thermal cycling the diffusion becomes slightly slower and after around 1200cycles the total refractory content is in the range of 5.5-7.5wt%. Out of the three refractory elements diffusing towards the TGO, W has been found to be the fastest which means that at most of the locations the amount of W is higher than the amount of any of Ta or Re (Fig. 6c). The scatter of the data is a consequence of the different phases of beta, gamma and gamma prime that these bond coats consist of, and the random arrangement of the phases with regard to the measurements points.

3.5

TGO growth & macroscopic failure pattern

Fig. 7 shows the TGO thickness as a function of cycles for all the versions investigated in this study. For comparison, TGO thickness data from previous investigations of NiCoCrAlY+7YSZ on both IN100 and CMSX-4 have been included. In case of the IN100 substrate, the average

ACCEPTED MANUSCRIPT trend line has been plotted as a dotted line which is based on TGO thickness values from a large number of samples. TGO thickness has been measured at min. 3 locations for each sample and only the dense, continuous part of the TGO was considered for the thickness measurement which

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means that the irregularities like hafnia stringers or yttria pegs have not been included in the TGO

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thickness data.

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In case of NiCoCrAlY based TBC systems, a slower TGO growth has been observed when

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compared with the NiCoCrAlY-Hf based systems. Furthermore, the GZO based systems showed a slightly slower TGO growth than the 7YSZ based ones, but overall the TGO thicknesses are

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quite similar for all versions and fall close to the base line values on the IN100 substrate. For the NiCoCrAlY-Hf systems, the TGO thicknesses are considerably larger and it can be clearly seen

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that the TGO growth for the GZO based system is faster than for the 7YSZ based ones. For the

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GZO based system, the TGO thickness for the 2 samples has been measured after they failed at

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2486 and 3246 cycles, however for the 7YSZ based system TGO thickness has been measured after 100, 500, 1000 and after spallation at 2929 and 3039 cycles. For the as-coated TGOs, NiCoCrAlY-Hf based TBC systems showed similar TGO thicknesses for both 7YSZ and GZO ceramic top coats.

The macroscopic failure patterns of the NiCoCrAlY based systems with both 7YSZ or GZOis characterized by large scale spallation of the TBC (Fig. 8a). In case of NiCoCrAlY-Hf based systems, 7YSZ on NiCoCrAlY-Hf reveals a buckling type of failure (Fig. 8b) while GZO on NiCoCrAlY-Hf shows a macroscopic failure pattern which seems to be predominantly buckling although the spallation takes place through piece by piece spallation as in case of mud-cracking (Fig. 8c). The failure location for all the NiCoCrAlY-based systems has been found to be predominantly at the TGO- bond coat interface. In case of NiCoCrAlY-Hf+7YSZ, the failure location was also at

ACCEPTED MANUSCRIPT the TGO- bond coat interface, however, most of the Hf- stringers still adhered to the bond coat even after TBC spallation. For the GZO on NiCoCrAlY-Hf bond coat, the failure location was mostly at the TBC- TGO interface with some areas of additional delamination between the TGO

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Discussion

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4.

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and the bond coat, as shown in Fig. 8d.

The present results confirm that the composition of all the constituents of a TBC system has a

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profound effect on the TBC lifetime. A lower lifetime of CMSX-4 based TBCs with NiCoCrAlY bond coat and 7YSZ ceramic top coat in comparison to the same TBC system on polycrystalline

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or directionally solidified alloys has been mentioned previously [25, 36]. A similar shorter lifetime has been observed in the present study with NiCoCrAlY+GZO on CMSX-4 (1065

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cycles) when compared to the longer lifetime of 2896 cycles of the same system on IN100 [26]. Surprisingly, the reduction in the lifetime is nearly the same for 7YSZ (38%) and GZO (37%)

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when CMSX-4 and IN100 substrates are compared, although the number of samples is far too low to have good statistics in the data. Several attempts have been made to find out the exact reason for this lower lifetime on SX alloys [25, 27, 36]. One reason might be their lower coefficient of thermal expansion (CTE) when compared with several polycrystalline alloys [25, 54]. This causes lower stresses in the ceramic top coat but at the same time higher stresses that accumulate in the system due to the larger difference in the CTE between the substrate and the bond coat which can result in a lower lifetime of single-crystal super alloys based TBC systems. Some other studies have argued that a higher refractory content and a low level of Cr in the single crystal super alloys have a negative impact on the TBC lifetime [36, 55, 59-60]. The reduction of Cr content is necessary to accommodate the addition of Re in order to maintain the microstructural stability [59]. Such variations in the alloy chemistry, including a lower level of grain boundary strengthening RE-elements in SX alloys in comparison to DS- or polycrystalline

ACCEPTED MANUSCRIPT alloys, have a positive impact on the alloy strength, but a negative impact on the performance of the coating systems [60]. The single- crystal super alloys have a lower carbon content that, in directionally solidified and polycrystalline alloys, ties up the detrimental elements diffusing from

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the substrate towards the TGO [25].

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In one of the studies, where the effect of each refractory element present in CMSX-4 on the TBC

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lifetime has been studied separately and in detail, Ta in the substrate has been found to cause the

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maximum reduction in the TBC lifetime [36]. In another study, where different versions of Rene N5 alloys have been tested, segregation of Ta in the TGO-bond coat interface has been reported

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[48]. In the present study W has a higher concentration than Ta and Re at most of the locations in the bond coat because of its faster diffusion, however, Ta is still present in large amounts (Fig.

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6c). The diffusion of refractory elements from the substrate towards the TGO has been reported

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previously as well [27]. The inter-diffusion of W, Ta and Re in γ-Ni has been studied in the

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temperature range of 900-1300°C and it has been reported that Ta has the highest diffusivity in γNi while Re has the lowest [48-49]. Elements like Ta, Co, Cr and W get enriched in the grain boundaries of the bond coat, leading to the formation of less protective and less adherent phases at these regions [49]. Although not fully clear which of the proposed mechanisms is responsible for the reduction in lifetime for SX alloys, the present study clearly shows that the effect is also present with GZO as the top coat. The positive effects of Hf-doping have been already reported for both the cases when Hf is present in the substrate [25] or in the bond coat [30]. In a previous study, where around 1wt. % Hf has been added to the EB-PVD NiCoCrAlY bond coat along with a minor quantity of Si, a 15times increase in the lifetime has been observed for the CMSX-4 based TBC systems with 7YSZ top coat [30]. However, no Si was observed in the microstructure although it was present in the ingot along with the Hf. The present study, where 0.6wt. % Hf has caused around 10 times

ACCEPTED MANUSCRIPT improvement in TBC lifetime, confirms that Hf even without Si has a positive impact on the lifetime of CMSX-4 based TBC systems. The lifetime improvement as a function of the Hfcontent in the NiCoCrAlY bond coat on CMSX-4 substrate for the previous [30] and the present

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study is shown in Fig. 9. There is a clear trend of a prolonged lifetime with increasing Hf-content

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in NiCoCrAlY. A low lifetime of 403 cycles has been achieved for a EB-PVD NiCoCrAlY bond

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coat doped with only 0.1wt% Hf that was manufactured and tested in parallel to this study (see additional data point in Fig. 9). This clearly shows that doping of MCrAlY with minor

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concentrations of RE elements does not improve the TBC life. In case of NiAl substrates or

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coatings, smaller amounts of even 0.1wt% RE caused a drastic improvement in the TBC lifetime [39-41]. For the Zr-doping in an NiAlCr bond coat, the optimum value for the reactive element

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has been found to be 0.5at. % which is equal to around 1wt. % [38]. A roughly 3.5times

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improvement of the lifetime was reported there. The same 0.5at. % for Hf- doping (if this is

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hypothetically considered as the optimum value) would give around 2wt. % Hf in the NiCoCrAlY bond coats, however, this value of Hf- doping has not been tested so far for the CMSX-4 based systems. It remains to further research what amount of Hf causes the maximum improvement in TBC lifetime. In case of Zr+Y co-doping in amounts of 0.6wt. %, a fourfold improvement has been observed in the NiCoCrAlY-based EB-PVD TBC lifetime under cyclic conditions [63] while a lower lifetime has been observed when Zr is added in a CoNiCrAlY bond coat [64]. Other studies did not find an improvement in TBC lifetime by 0.3at. % Hf-doping in NiAlCr(Pt, Pd) bond coats [66] which clearly shows that engineering the optimum RE content in a bond coat is crucial for TBC lifetime. Hf shows a stronger affinity towards oxygen & diffuses quickly towards the TGO. The driving force for this diffusion can be attributed to the oxygen potential gradient across the metal-oxidegas system [53]. However, with continuous thermal cycling, Hf in the bond coat is used up in the

ACCEPTED MANUSCRIPT growing TGO and consequently, the continuous diffusion of refractory elements creates a region at the TGO- bond coat interface which is rich in refractory elements (Fig. 2c). This region contains lower amounts of Hf and Al since both elements have been consumed in the growing

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TGO.

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In case of NiCoCrAlY bond coats where Hf is not present, Yttrium is the 1st element to oxidize

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and can be found at the TGO-TBC interface in the form of yttria islands [26, 28, 46]. While Y

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diffuses during vacuum annealing along grain boundaries to the surface of the bond coat and oxidizes nearly exclusively there, Hf attracts oxygen also in parts of the bond coat that are not

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directly exposed to the atmosphere (Fig. 2). Because of this stronger affinity of Hf towards oxygen and the preferential diffusion of Hf along grain boundaries, it gets enriched over there.

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Hafnia stringers are formed very quickly as the TBCs are exposed to high temperature. These

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stringers run very deep into the bond coat and even after only 100 cycles, Hf-stringers with a

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depth of 20µm have been observed (Fig. 3a). Similar oxide pegging and stringer formation due to the addition of reactive elements in the bond coat has been reported in the literature [30, 38, 6163]. These hafnia stringers are believed to act as crack stoppers because the interface cracks have to change their propagation direction repeatedly if they intimately follow the TGO-bond coat interface which possesses normally the lowest interface toughness in those TBC systems. In addition to the anchoring effect the oxide stringers may getter detrimental sulfur and change the local CTE which can also explain the long TBC life. As a result of this “abnormal” growth of the TGO, the metallic bond coat gets entrapped within the TGO between the stringers at several locations and these bond coat regions later oxidize during prolonged high temperature exposure (Fig. 3b). Since these regions close to the TGO are depleted in aluminum (additionally also depleted in Hf) a spinel forms, which is in general believed to be detrimental since it is a nonprotective oxide. [55]. However, in the present study, the formation of spinels has not been found

ACCEPTED MANUSCRIPT detrimental for the lifetime of NiCoCrAlY-Hf based TBC systems which could be due to the formation of the hafnia stringers. Even spinel formation at the interface close to the TBC did not deteriorate lifetime. Spinel formation was similar to earlier findings of our group on the 1wt. %

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Hf (+Si) doped NiCoCrAlY bond coat [30, 62], and in accordance with literature [61, 64].

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Hf can improve the adhesion between TGO and bond coat which has been explained by

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shortening of metal- oxide distance due to strong bond formation between Hf and the neighboring

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oxygen atoms in Al2O3. The open d-shell structure of Hf allows it to form a mixture of ionic, covalent and donor-accepter character, which increases the bond density by 25% and as a result

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makes the bonds particularly strong [56]. The investigation of the mechanism responsible for the RE-effect is beyond the scope of this paper, but the oxide stringer formation and anchoring has

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been found for both top coats and seems to be a plausible cause for the improvement in TBC

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lifetime. The additional version doped with only around 0.1wt% Hf, that was mentioned above,

argument.

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did neither show stringers nor spinel phases and had a low lifetime, which further supports this

When GZO is deposited directly on top of alumina, a GdAlO3 perovskite phase forms as a result of a chemical reaction between alumina and GZO (Fig. 5a), which can lead to porosity [22] and can cause a reduction in TBC lifetime [22]. For the EB-PVD systems on polycrystalline IN100 substrates, the chemical reaction between GZO and the TGO resulted in the formation of two distinct oxide phases that contain Gd, Al and Y in different amounts [26]. However, this GZOalumina interaction has not been found detrimental for the TBC lifetime as longer lifetimes have been observed when GZO was deposited as a single layer on IN100 based TBCs [26]. The same has been observed for the GZO-NiCoCrAlY system in the present study (Fig. 3a). However, due to the lower lifetime on the CMSX-4 substrate, the new phase resulting from the GZO-TGO

ACCEPTED MANUSCRIPT interaction has not been found to be as frequently present at the TBC-TGO interface as in the case of IN100 based systems where the lifetime has been significantly longer [26].

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The improvement in lifetime by using GZO in comparison to 7YSZ is quite similar for the two substrate materials investigated (3.5 on IN100 for the standard NiCoCrAlY bond coat [26] and

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3.4 for CMSX-4 in the current study). It can be concluded that the EB-PVD columnar structure is

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more strain tolerant and hence the larger CTE mismatch between GZO (that possesses a lower

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CTE than 7 YSZ) and the underlying bond coat and the substrate does not cause a major problem. It is perhaps the lower sintering rate of GZO and the associated lower rate of increase in stiffness

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that may have contributed to the considerable longer lifetime of GZO on NiCoCrAlY. In addition, chemical aspects and TGO formation seem to play a more important role for failure than the

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thermo-mechanical effects in case of EB-PVD TBC systems [57]. A good compilation of FCT

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data for comparison is found in a recent paper [67].

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In the TBC systems, where GZO is deposited on NiCoCrAlY-Hf bond coats, no significant chemical reaction between GZO and TGO has been observed even after thermal cycling for a very long time (Fig. 5b). This is a surprising result since there are on a first view only small changes within the TGO on NiCoCrAlY-Hf compared to the Hf-free counterpart. However, the chemical reaction between GZO and TGO depends a lot on the composition of oxides adjacent to GZO [22, 26]. In the present study, the TGO on NiCoCrAlY- Hf bond coat consists mostly of alumina along with some hafnia and spinels (as detailed above) which can be a reason of retarded GZO-TGO interaction. TGO growth is one of the most important factors determining the lifetime of TBCs. It results in stress accumulation which proves to be detrimental for their lifetime. Therefore, a slow TGO growth is appreciated in the TBC systems. On the NiCoCrAlY bond coat, GZO based TBC systems show a slightly slower TGO growth than the 7YSZ based systems (Fig. 7). This finding

ACCEPTED MANUSCRIPT is in accordance with our previous study on IN100 [26]. A possible explanation for this is the formation of the new phase at the TBC-TGO interface which becomes more continuous as a result of long thermal cycling. It may change the diffusion conditions for oxygen ions through the

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TGO [26]. Further research is under way to explore why GZO on NiCoCrAlY slightly reduces

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the TGO growth rate. In this study, Hf in the bond coat has been found to increase the TGO

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growth rate as it is found in most other investigations when overlay coatings are doped with reactive elements in larger amounts [30, 61-63]. This can be attributed to the irregular TGO

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growth as the bond coat gets entrapped within the TGO at several places and transforms into

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spinels due to subsequent oxidation. As a result, the Hf addition in the bond coat not only forms hafnia stringers in the TGO but also increase the bulk TGO growth rate. For the NiCoCrAlY-Hf-

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based TBC systems, the TGO growth is faster for the GZO based system than for the 7YSZ based

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ones. This finding is totally opposite to the one observed for NiCoCrAlY- based TBC systems. As

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the column morphology, phase changes and inter-columnar sintering of GZO is the same on both bond coats, the only difference is the formation of the new phase on NiCoCrAlY as a result of the chemical reaction between GZO and the TGO at the TBC-TGO interface which might affect the oxygen diffusion. Consequently, in the NiCoCrAlY-Hf+GZO system where this phase formation was not observed, a higher TGO thickness has been observed at comparable times (see Fig. 7). The location of failure in TBCs points to the weakest point in the system, see Fig. 8. In case of the standard 7YSZ-NiCoCrAlY system with both bond coats, the failure location has been found to be mainly at the TGO- bond coat interface. However, for the NiCoCrAlY-Hf+7YSZ system, it was within the TGO close to the interface to the bond coat despite the hafnia stringers staying intact within the bond coat even after the spallation. Cracks run basically along the bond coatTGO interface of the “normal” TGO and bridge the oxide stringers. These stringers run in the beginning very deep into the bond coat and have a depth of around 150% of the TGO thickness.

ACCEPTED MANUSCRIPT With continuous thermal cycling, they do not grow much deeper and this proportion decreases and after around 3000cycles the depth of Hf- stringers is around 50% of the TGO thickness. As a result, the anchoring effect of the hafnia stringers slowly diminishes and accumulation of

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refractory elements at the TGO-bond coat interface reduces the adhesion considerably causing

Conclusions

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5.

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TBC spallation.

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A variety of EB-PVD TBC systems with NiCoCrAlY or NiCoCrAlY-Hf bond coats and 7YSZ or gadolinium zirconate (GZO) ceramic top coats has been deposited on CMSX-4 substrates.

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Furnace cyclic testing has been performed at 1100°C to determine lifetimes and to investigate the

Hf-doping in NiCoCrAlY increases the TGO growth rate when compared with the

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1-

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microstructural changes in bond coat and TGO. The findings can be summarized as follows:

standard NiCoCrAlY bond coats. This is mainly due to the high oxygen affinity of Hf which

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results in the formation of hafnia stringers. This preferential TGO growth also causes the metallic bond coat to get entrapped within the TGO which results in spinel formation in the TGO upon prolonged testing. 2-

Hf-doping in NiCoCrAlY bond coats improves the lifetime of CMSX-4 based TBC

systems. In case of 7YSZ top coats, the lifetime improvement is in the order of 10 times while for GZO it is nearly 3 times. Formation of the hafnia stringers that anchor the TGO is believed to contribute to this prolonged lifetime. Neither spinel formation within the TGO nor enhanced TGO growth rates were detrimental. 3-

On the standard NiCoCrAlY bond coat, replacement of 7YSZ by GZO causes an around 3

times improvement in TBC lifetime. Such a lifetime improvement has not been observed on the

ACCEPTED MANUSCRIPT NiCoCrAlY-Hf bond coat where the lifetimes of both top coats are quite similar and astonishing long. GZO on the NiCoCrAlY bond coat undergoes a chemical reaction with the TGO, whereas

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4-

no chemical reaction has been observed between GZO and TGO on the NiCoCrAlY-Hf bond

Diffusion of refractory elements such as Ta, W, and Re from the CMSX-4 substrates

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5-

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coat.

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towards the TGO has been observed independent of the bond coat material. The concentration of

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W in the bond coat close to the TGO was higher than that of Ta or Re.

Acknowledgements

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The authors acknowledge the technical assistance during the experiments at DLR by J. Brien, D. Peters, and A. Handwerk. The authors would like to thank Prof. Cerri from the University

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of Roma Tre and Hendrik Lau from DLR for their valuable discussions and comments on the manuscript.

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ACCEPTED MANUSCRIPT Table I: Composition of different materials used in the study. Material

Composition in wt. %

Substrate

CMSX-4

9 Co, 6.5 Cr, 5.6 Al, 0.6 Mo, 6 W, 6.5 Ta, 3 Re, balance Ni

Bond Coat

NiCoCrAlY

Ni balance, 22 Co, 19 Cr, 12 Al, 0.15 Y

Bond Coat 2

NiCoCrAlY-Hf

Ni balance, 22 Co, 20 Cr, 12 Al, 0.20 Y- 0.6 Hf

Ceramic Topcoat

7YSZ

7 Y2O3 in ZrO2

Ceramic Topcoat 2

GZO

60 Gd2O3 + 40 ZrO2

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1: Cyclic lifetime of individual samples at 1100°C of various TBC systems. The left bar for

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the standard NiCoCrAlY + 7YSZ system gives an average value taken from [25] Fig. 2: TGO microstructure of the NiCoCrAlY-Hf + 7YSZ system in the as-coated condition.

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Fig. 3: TGO microstructure of a) NiCoCrAlY-Hf- 7YSZ TBC after 100cycles, b) NiCoCrAlY-

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Hf-GZO TBC after 2486 cycles and c) Hf-depleted region in NiCoCrAlY-Hf- 7YSZ TBC after

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3039cycles.

Fig. 4: a) EDX line scan of Hf- depleted region after 3039 cycles and b) Hf and the refractory

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content at the TGO- bond coat interface.

Fig. 5: Interface between GZO and TGO on a) NiCoCrAlY after 1187cycles, b) and c) on NiCo-

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Fig. 6: Diffusion of elements from the CMSX-4 substrate into NiCoCrAlY+GZO a) Location and

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the length of the EDS measurement, b) total refractory content (wt. % of Ta + W + Re) after 100 and 1187 cycles and c) wt. % of Ta, W and Re in the bond coat after 1187 cycles. Fig. 7: TGO growth vs. number of cycles at 1100°C for various NiCoCrAlY based TBC systems on CMSX-4. Data for CMSX-4 with NiCoCrAlY+7 YSZ and the same TBC system on IN100 which is added for comparison were taken from [25]. Fig. 8: a) Macroscopic failure pattern of 7YSZ- NiCoCrAlY-Hf, b) GZO- NiCoCrAlY, c) GZONiCoCrAlY-Hf and d) failure location of GZO- NiCoCrAlY-Hf. Fig. 9: Cyclic lifetime of 7YSZ EB-PVD TBCs vs. Hf-content in NiCoCrAlY bond coats on CMSX-4 substrate.

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Fig. 1: Cyclic lifetime of individual samples at 1100°C of various TBC systems. The left bar for the standard NiCoCrAlY + 7YSZ system gives an average value taken from [25]

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Fig. 2: TGO microstructure of the NiCoCrAlY-Hf + 7YSZ system in the as-coated condition.

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ACCEPTED MANUSCRIPT Fig. 3: TGO microstructure of a) NiCoCrAlY-Hf- 7YSZ TBC after 100cycles, b) NiCoCrAlY-HfGZO TBC after 2486 cycles and c) Hf-depleted region in NiCoCrAlY-Hf- 7YSZ TBC after

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Fig. 4: a) EDX line scan of Hf- depleted region after 3039 cycles and b) Hf and the refractory content at the TGO- bond coat interface.

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Fig. 5: Interface between GZO and TGO on a) NiCoCrAlY after 1187cycles, b) and c) on NiCoCrAlY-Hf after 2486cycles. The arrow in a) points to the reaction phase that formed at the TGO to TBC interface.

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Fig. 6: Diffusion of elements from the CMSX-4 substrate into NiCoCrAlY+GZO a) Location and the length of the EDS measurement, b) total refractory content (wt. % of Ta + W + Re) after 100 and 1187 cycles and c) wt. % of Ta, W and Re in the bond coat after 1187 cycles.

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Fig. 7: TGO growth vs. number of cycles at 1100°C for various NiCoCrAlY based TBC systems on CMSX-4. Data for CMSX-4 with NiCoCrAlY+7 YSZ and the same TBC system on IN100 which is added for comparison were taken from [25].

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ACCEPTED MANUSCRIPT Fig. 8: a) Macroscopic failure pattern of 7YSZ- NiCoCrAlY-Hf, b) GZO- NiCoCrAlY, c) GZO-

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NiCoCrAlY-Hf and d) failure location of GZO- NiCoCrAlY-Hf.

Fig. 9: Cyclic lifetime of 7YSZ EB-PVD TBCs vs. Hf-content in NiCoCrAlY bond coats on CMSX-4.

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Highlights Hf-doping in NiCoCrAlY bond coat improves the lifetime of TBC systems.

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GZO on NiCoCrAlY shows longer lifetime than the 7YSZ based systems.

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On NiCoCrAlY, GZO undergoes a chemical reaction with the TGO.

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Diffusion of refractory elements takes place from the substrate towards the TGO.

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On NiCoCrAlY-Hf, no chemical reaction between GZO and the TGO is observed.

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