# Pt Effect on the Bond-Coating/Superalloy Interdiffusion: Case of a Pt-Modified NiCoCrAlYTa Coating

Aurélie Vande Put, Enrica Epifano, Fabrice Crabos, Daniel Monceau

► To cite this version:

Aurélie Vande Put, Enrica Epifano, Fabrice Crabos, Daniel Monceau. Pt Effect on the Bond-Coating/Superalloy Interdiffusion: Case of a Pt-Modified NiCoCrAlYTa Coating. High Temperature Corrosion of Materials, 2024, 101 (6), pp.1527-1542. 10.1007/s11085-024-10314-2. hal-04792789v2

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# Pt Effect on the Bond-Coating/Superalloy Interdiffusion: Case of a Pt-Modified NiCoCrAlYTa Coating

Aurélie Vande Put<fnref n="1" /> · Enrica Epifano<fnref n="1" /> · Fabrice Crabos<fnref n="2" /> · Daniel Monceau<fnref n="1" />

Received: 13 September 2024 / Revised: 13 September 2024 / Accepted: 17 September 2024 /
Published online: 23 September 2024
© The Author(s) 2024

## Abstract

Heat treatments and oxidation tests were performed on systems composed of a single-crystal AM3 superalloy with a NiCoCrAlYTa+Pt coating to investigate the effect of Pt on Ta and Ti diffusion at high temperature. Experimental results were compared to an AM3 superalloy directly coated with Pt. For all the studied systems, the effect of Pt on elemental activities was evaluated through thermodynamic calculations. Pt diffusion was found to be faster in the NiCoCrAlYTa coating than in AM3. High Ta contents were measured in the Pt-rich γ' phase below the surface and significant Al and Cr transport toward the surface was observed, in agreement with thermodynamic calculations which predicted an important decrease in their activities in the presence of Pt. An outward diffusion of Ti was also noticed, whereas calculations did not show a decrease in Ti activity due to Pt. Other discrepancies between experiments and thermodynamic calculations were noted and are discussed in this work.

**Keywords** Bond-coating · Superalloy · Platinum · Oxidation · Interdiffusion

<figure><img src="image_2.png" /></figure>

<footnote>Aurélie Vande Put
aurelie.vandeput@toulouse-inp.fr</footnote>

<footnote>Enrica Epifano
enrica.epifano@toulouse-inp.fr</footnote>

<footnote>Fabrice Crabos
fabrice.crabos@safrangroup.com</footnote>

<footnote>Daniel Monceau
daniel.monceau@toulouse-inp.fr</footnote>

<footnote n="1">¹ CIRIMAT, Toulouse INP, Université Toulouse 3 Paul Sabatier, CNRS, Université de Toulouse, 4 allée Emile Monso - BP44362, 31030 Toulouse cedex 4, France</footnote>

<footnote n="2">² Safran Helicopter Engines, Avenue Joseph Szydlowski, 64510 Bordes, France</footnote>
## Introduction

A thermal barrier coating (TBC) system is a multilayered system composed of a superalloy, an Al-rich metallic coating (the bond-coating, BC) and an insulating top coat (the thermal barrier coating). The low thermal conductivity of the ceramic used as a thermal barrier decreases the temperature of the internally cooled underlying substrate. Meanwhile, the high Al content of the BC enables the formation of a protective oxide layer (α-alumina), during the reaction with oxygen. The formation and growth of this protective oxide layer are key factors in the resistance to degradation of a TBC system and depend on many parameters such as composition, microstructure and fabrication process of the BC. Regarding the composition, adding Pt to the BC significantly enhances oxidation resistance of TBC systems [1]. Furthermore, Pt decreases Al activity, leading to an Al uphill diffusion from the superalloy toward the surface [2], which enables the development of Pt-rich $\gamma + \gamma'$ BC [3], martensite in MCrAlY [4] or α-NiPtAl phase in NiCrAl model alloy [5]. The composition of the superalloy also strongly affects the oxidation resistance of TBC systems [6, 7]. Ti, an element present in most superalloys and often detected in the oxide scale, partitions into $\gamma'$ rather than $\gamma$, similar to Al and Ta. Therefore, when a Ti- and Ta-rich superalloy is coated with Pt in order to obtain a Pt-rich $\gamma + \gamma'$ BC, the $\gamma'$ phase becomes enriched in Ti and Ta compared to $\gamma$ [8]. Even with an overlay MCrAlY-type BC which is Ti-free after deposition, a noticeable content of Ti was detected in the oxide scale after short oxidation times [7, 9]. A significant content of Ti was also measured in the oxide scale of a Pt-modified NiCoCrAlYTa BC after exposures at 1100 °C [10, 11]. This was attributed to the absence of TaC carbides (known to trap Ti) in the microstructure of the Pt-modified NiCoCrAlYTa, compared to the unmodified coating [10]. These experimental results suggest a possible effect of Pt on Ta and Ti activities, as proposed by [4, 12] and [12, 13], respectively. In order to investigate the effect of Pt on the activity and diffusion of Ta and Ti, heat treatments under vacuum and oxidation tests were carried out on Pt-enriched systems. The materials were then characterized, and the experimental results were compared to thermodynamic calculations.

## Materials, Experiments and Computational Methods

### Materials and Experiments

Three coatings were applied to the AM3 superalloy: a Pt layer only, a Pt-modified NiCoCrAlYTa BC and a Pt-modified NiCoCrAlYTa BC with a TBC (made of yttria-stabilized zirconia, YSZ) on top. The AM3 composition was Ni-12.85Al-8.89Cr-5.88Co-1.28Ta-2.41Ti-1.2Mo-1.57W in at.%, and the AM3 samples were disks with a diameter of 19.6 mm and a thickness of 2 mm. The “AM3-Pt” system was obtained by depositing a 7 µm ± 1 µm thick Pt layer by
sputtering, on the grit blasted surface of an AM3 sample. A heat treatment of 1 h at 1100 °C was performed to allow Pt interdiffusion with the superalloy. The “AM3-NiCoCrAlYTa+Pt” system was fabricated following three steps. First, the AMDRY 997 powder (Ni-20.9Co-20.9Cr-16.9Al-1.2Ta-0.4Y in at.%) was deposited on grit blasted samples of the AM3 superalloy by plasma spraying under vacuum (VPS) to make a 70–80 µm thick NiCoCrAlYTa coating. Then, the NiCoCrAlYTa surface was partially polished using P1200 SiC paper to decrease the roughness resulting from the VPS process and a 7 µm ± 1 µm thick Pt layer was deposited by sputtering. Two samples were subjected to a heat treatment under vacuum for 6 h at 1080 °C and then, only one was coated by the thermal barrier by EB-PVD (the “AM3-NiCoCrAlYTa+Pt-TBC” system). To grasp the first minutes of Pt diffusion, a 10 min heat treatment under vacuum was performed at 1100 °C on the “AM3-NiCoCrAlYTa+Pt” sample right after Pt deposition. One part of this sample was cut and the remaining part was subjected to a new heat treatment at 1100 °C under vacuum of 20 min. The temperature of these short heat treatments was set to 1100 °C to compare with the heat treatment performed on the “AM3-Pt” system and with previous works on Pt diffusion on model alloys. These short heat treatments were performed in a tubular furnace. The sample was first placed in an alumina boat, which was connected to the hot chamber of the furnace preheated to 1100 °C. The entire setup was under secondary vacuum. A rod allowed rapid movement of the sample between the enclosure and the hot isothermal zone. Once the sample holder was positioned in the hot zone, the temperature reached 1090 °C after 3.6 min, and 1100 °C after 5.1 min. Whatever the heat treatment, the pressure was inferior to 10⁻⁴ mbar within the furnace. The cooling was done inside the furnace in the case of the heat treatments of 1 h at 1100 °C and 6 h at 1080 °C, and in the cold chamber for the 10 min and 10+20 min heat treatments.

Additionally, an oxidation test of 150 h at 1100 °C was performed in a carbolite muffle furnace on the “AM3-NiCoCrAlYTa+Pt” system and a thermal cycling test was carried out on the TBC system using a vertical cyclic oxidation rig, both under laboratory air. Each cycle was composed of a 1 h dwell at 1100 °C (including a fast heating) and a cooling of 15 min to ambient temperature. More details on the sample fabrication process and on the cyclic oxidation tests can be found in [4, 11, 14].

<figure><img src="image_3.png" /><figcaption>**Fig. 1** Diagram of the expected diffusion fluxes for the 3 studied systems, based solely on composition gradients and alumina growth (in the case of the cyclic oxidation test)</figcaption></figure>
Figure 1 schematically presents the three studied systems and the diffu-
sion fluxes expected during the heat treatments (for the “AM3-Pt” and “AM3-
NiCoCrAlYTa+Pt” systems) or the cyclic oxidation (for the TBC system), based
solely on the concentration differences between the materials and the growth of an
alumina layer (when oxidation occurs).

Characterization of the coatings after heat treatments and after oxidation was car-
ried out using secondary electron microscopy (SEM) and energy-dispersive X-ray
spectroscopy (EDS). SEM observations of polished cross sections were performed
using the backscattered imaging mode (BSE) with a LEO 435VP microscope and
a PGT IMIX-PC system for EDS for the “AM3-NiCoCrAlYTa+Pt” system heat
treated for 6 h at 1080 °C and the “AM3-NiCoCrAlYTa+Pt-TBC” system after ther-
mal cycling or using a FEI Quanta450 microscope with a Bruker Quantax detector
for EDS after the short heat treatments. For these latter, the EDS analysis was done
along a 40 µm line, from the coating surface toward the bulk, and the quantification
was made based on the data collected for each analyzed spot. In the case of the long
heat treatment and after thermal cycling, the analyzed areas were divided in rectan-
gles 3.5 µm wide and 89 µm long, parallel to the coating surface. A quantification
was then made based on the data collected for each rectangle. In each case, quantifi-
cation was based on real standards.

An EDS analysis was also made on a thin foil of the “AM3-NiCoCrAlYTa+Pt” system after 6 h at 1080 °C, during TEM observations using a Jeol JEM 2010 microscope, operating at 200 kV and equipped with a “Tracor” EDS spectrometer (details on the preparation of the foil can be found in [4]). The EDS spectra were treated semi-quantitatively using the “Cliff and Lorimer K factors.”

Phase proportions were estimated by image analysis using ImageJ software (con-
sidering the volume proportion was equal to the surface proportion).

**Computational Methods**

Experimental results were compared to thermodynamic calculations, made using the Thermo-Calc software and the TCNI9 database. In particular, activity calculations as a function of the Pt concentrations were performed for both NiCoCrAlYTa and AM3 alloys. For these computations, the number of moles of each element in the alloy was fixed to the nominal composition of the material while the number of moles of Pt was increased. This corresponds to a progressive dilution of all the alloying elements, to reproduce the evolution of the NiCoCrAlYTa or AM3 composition with Pt diffusion. Only Y was not included in the NiCoCrAlYTa composition, to avoid the Ni₅Y phase in the calculated equilibria (this is why the material will be called NiCoCrAlTa in Figs. 5 and 6).

Pt diffusion coefficients were determined using TCNI9 and MOBNI5 databases. For this calculation, the number of moles of each element was fixed to the nominal composition of the alloy (as in the previous case) and an additional content of 11.11 mol of Pt was added to the system. This corresponds to a Pt concentration of 10 at.% with a homogeneous dilution of all the other elements.

<figure><img src="image_4.png" /></figure>
# Experimental Results

## Short Heat Treatments

The cross section of the “AM3-NiCoCrAlYTa+Pt” system after heat treatment(s) of 10 and 10+20 min at 1100 °C and of the “AM3-Pt” system after the heat treatment of 1 h at 1100 °C are shown in Fig. 2. The difference in terms of surface preparation before Pt deposition (grit blasting or partial polishing) led to a rougher surface for the “AM3-Pt” system compared to the “AM3-NiCoCrAlYTa+Pt” system. After different times at high temperature and despite similar deposition process and thickness layer of Pt, the diffusion of Pt resulted in pore formation at the Pt diffusion front of the “AM3-Pt” system while only a few voids were visible in the “AM3-NiCoCrAlYTa+Pt” system. In this system, small voids were at a depth that could correspond to the former Pt/NiCoCrAlYTa interface and bigger ones were visible within the inner part of the coating. These last could be attributed to the VPS process of the NiCoCrAlYTa coating (very large pores within the NiCoCrAlYTa were probably due to metallographic preparation). Also, one striking observation is the disappearance of TaC carbides (in white on the SEM images) in the inner part of the NiCoCrAlYTa+Pt coating with Pt diffusion (Fig. 2a, b).

To complete these observations, EDS analyses were then carried out on the first 40 µm from the surface of these three systems (only Y was not considered), Fig. 3. First, it can be noticed that Pt diffusion within the NiCoCrAlYTa coating was faster than in the AM3 superalloy. While Pt diffusion reached up a depth of about 22.5–25 µm in AM3 after 1 h at 1100 °C, Pt was measured up to 35–40 µm in depth for the “AM3-NiCoCrAlYTa+Pt” system after only 10+20 min at 1100 °C (Fig. 3b, c). Also, as already observed in previous works [4], the Pt diffusion led to an Al

<figure><img src="image_5.png" /><figcaption>**Fig. 2** SEM/BSE images of the cross section of the “AM3-NiCoCrAlYa+Pt” system after a heat treatment under vacuum of a 10 min and b 10+20 min at 1100 °C and of c the “AM3-Pt” system after a heat treatment under vacuum of 1 h at 1100 °C</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 3** SEM/BSE images and corresponding EDS concentration profiles obtained on the cross section of samples heat treated in vacuum, at 1100 °C: AM3-NiCoCrAlYTa+Pt after a 10 min and b 10+20 min and c AM3-Pt after 1 h (Al and Cr contents in at.% in NiCoCrAlYTa and AM3: 16.9/20.9 and 12.9/8.9, respectively)</figcaption></figure>

uphill diffusion from the bulk of the NiCoCrAlYTa coating toward the surface (with Al contents up to 17–18.5 at.%), below which an area depleted in Al compared to the unmodified NiCoCrAlYTa was observed after 10+20 min at 1100 °C. (Al contents as low as 8.4–11.2 at.% were measured between 15.1 and 36.7 µm in depth.) Regarding the Al content of the “AM3-Pt” system, it does not exceed 13.2 at.% over the Pt diffusion depth (of around 20 µm). The EDS profiles also revealed an “alter nation” of Al- and Cr-rich layers. While the outermost surface contains more Al than Cr, a second area below the first one is richer in Cr than Al, before the Al content once again exceeds that of Cr. The depth and position of the Cr-rich “layer” depend on the alloy composition in which Pt diffused and on the diffusion time.

Regarding the Ti content after diffusion, it was close to zero in the Pt-rich depth of the “AM3-NiCoCrAlYTa+Pt” system. This was not surprising consider ing that the NiCoCrAlYTa did not contain Ti, was 70–80 µm thick and that the heat treatment was very short (Fig. 3a, b). On the other hand, in the “AM3-Pt” system, uphill diffusion of Ti was observed. Indeed, after 1 h at 1100 °C, its con tent reached up nearly 4 at.% in the Pt-rich depth (Fig. 3c), while AM3 is com posed of only 2.4 at.% of Ti. Moreover, it can be noticed that the Ti concentration profile appears to be the replica of the Al profile, with lower values. In this same system, the variations in the Ta concentration profile are less pronounced than those of Ti, but the Ta content is still slightly higher in one part of the Pt-rich

<figure><img src="image_7.png" /></figure>
area compared to the superalloy composition (the Ta content in AM3 is half that of Ti). Despite a similar Ta content between the AM3 alloy and the NiCoCrAlYTa coating, and in spite of a shorter heat treatment (10+20 min at 1100 °C), the Ta content rises from less than 2 at.% in the first 8 µm from the surface to levels greater than 4 at.% in the “AM3-NiCoCrAlYTa+Pt” system (Fig. 3b).

## Long Heat Treatment

To complete these data and study any Ti diffusion due to Pt within the “AM3-NiCoCrAlYTa+Pt” system, a similar work of characterization was performed on this system after a heat treatment of 6 h at 1080 °C. The obtained concentration profiles are plotted in Fig. 4a. The values measured at the extreme surface are not discussed as the surface was not perfectly planar, distorting the results. However, it is obvious that Pt diffusion during the heat treatment of 6 h at 1080 °C led to a large Al uphill diffusion. The powder used to make the coating contained 16.9 at.% of Al. After Pt diffusion, the upper part of the coating contained more than 30 at.% of Al while an Al content of around 10 at.% was measured in the coating inner part. This led to the formation of a martensite and $\gamma'$-rich microstructure close to the surface, while the coating core was composed of both $\gamma'$ and $\gamma$ phases. Also, a peak in Ta content at around 19 µm in depth was noticed, which corresponds to an area mainly composed of $\gamma'$, and no Ta carbides were observed through the entire coating depth.

<figure><img src="image_8.png" /><figcaption>**Fig. 4** SEM/BSE images and corresponding EDS concentration profiles through the bond-coating a after heat treatment under vacuum of 6 h at 1080 °C of the “AM3-NiCoCrAlYTa+Pt” system and b after 92 cycles at 1100 °C under laboratory air of the “AM3-NiCoCrAlYTa+Pt-TBC” system. On the SEM images, “M” stands for “martensite” and “IDZ” for “interdiffusion zone”</figcaption></figure>
## Cyclic Oxidation

Finally, the TBC system was oxidized for 92 cycles at 1100 °C in air. The cross section of the BC after thermal cycling is shown in Fig. 4b. The thermal barrier spalled, so it is no longer visible. The dark contrast at the BC surface is the oxide scale, which was formed during the thermal cycling, through Al consumption. This led to a decrease in Al content in the alloy and a consequent disappearance of the martensite (Fig. 4b). However, the Al concentration was sufficient to maintain a large $\gamma'$ proportion in the upper part of the coating and it decreased slightly in the inner part to reach 9.3 at.%. The high Ta content of the BC helped to stabilize the $\gamma'$ phase. Up to 2.8 at.% of Ta was measured in this phase (its content in the NiCoCrAlYTa powder was 1.2 at.%). Also, a noticeable Ti enrichment was observed in the $\gamma'$ phase. While Ti contents lower than 0.4 at.% were detected in the coating after 6 h at 1080 °C, concentrations up to 1.7–1.9 at.% were measured between 8 to 15 µm below the surface after 92 cycles at 1100 °C. Finally, the Pt profile flattened during cyclic oxidation, due to its diffusion toward the superalloy. Cr and Ni concentrations increased in the upper part of the coating compared to the state after heat treatment. This last observation is a consequence of Pt diffusion and Al consumption due to oxidation.

## Determination of Phase Composition

To complete these data, some additional SEM/EDS and TEM/EDS analyses were performed on the Pt-modified NiCoCrAlYTa BC after a heat treatment of 6 h at 1080 °C followed by 150 h of oxidation at 1100 °C (case 1) or right after the 6 h heat treatment (case 2), respectively. The compositions of neighboring $\gamma/\gamma'$ regions were measured, and the data are reported in Table 1. The obtained values indicate that elements such as Al, Pt, Ta and Ti preferentially partition in $\gamma'$ rather than $\gamma$, while the opposite is true for Co and Cr.

Thermodynamic calculations were made to compare the experimental results with the phase compositions predicted at equilibrium. To perform the calculation, the overall composition of the biphasic region was taken as the weighted sum of the concentrations of each element measured by EDS in both the $\gamma$ and $\gamma'$ phases. The sum was weighted by the volume fraction of the phases. These volume fractions (about 50/50%) were measured by image analysis using the SEM and TEM micrographs (based on one micrograph only in each case) and considering they were equal to the surface fractions. The computational results are also reported in Table 1 for $\gamma$ and $\gamma'$. A small proportion of Ni₅Y (<1 mol.%) was also predicted but its composition is not reported in Table 1. The obtained phase fractions at equilibrium were far from the experimental ones: for case 1, they were 65.6 vol.% and 33.5 vol.% for $\gamma$ and $\gamma'$, respectively; for case 2, they were 86.2 vol.% and 12.9 vol.% for $\gamma$ and $\gamma'$, respectively. Concerning the partition of elements between the $\gamma$ and $\gamma'$, a very good agreement between experimental and computation data is obtained for Al and—at least for case 1—for Ta and Ti. Conversely, the calculated Pt content was

<figure><img src="image_9.png" /></figure>
<table><caption>**Table 1** EDS composition of γ' phases and phase proportion measured and overall compositions considered for thermodynamic calculations for the Pt-modified NiCoCrAlYTa coating of the “AM3-NiCoCrAlYTa+Pt” system after (case 1) the heat treatment of 6 h at 1080 °C followed by an oxidation of 150 h at 1100 °C (SEM-EDS analysis) and (case 2) the heat treatment of 6 h at 1080 °C (TEM-EDS analysis), and comparison with calculated γ and γ' phase compositions</caption><thead><tr><th rowspan="2">Case</th><th rowspan="2">Type of data</th><th rowspan="2">Phase proportion (vol.%)</th><th colspan="8">Composition (at.%)</th></tr><tr><th>Ni</th><th>Al</th><th>Pt</th><th>Ta</th><th>Ti</th><th>Co</th><th>Cr</th><th>W</th><th>Mo</th><th>Y</th></tr></thead><tbody><tr><td rowspan="5">1</td><td>Exp. in γ</td><td>50</td><td>44.3</td><td>9.1</td><td>4.1</td><td>0.8</td><td>0.7</td><td>19.1</td><td>20.5</td><td>0.7</td><td>0.6</td><td>0</td></tr><tr><td>Exp. in γ'</td><td>50</td><td>52.4</td><td>16.4</td><td>7.1</td><td>3.8</td><td>2.5</td><td>10.8</td><td>5.6</td><td>0.9</td><td>0.3</td><td>0.28</td></tr><tr><td>Overall considered composition</td><td></td><td>48.4</td><td>12.8</td><td>5.6</td><td>2.3</td><td>1.6</td><td>15.0</td><td>13.1</td><td>0.8</td><td>0.5</td><td>0.14</td></tr><tr><td>Calc. in γ</td><td>65.6</td><td>40.8</td><td>10.2</td><td>7.3</td><td>1.5</td><td>0.7</td><td>19.4</td><td>18.5</td><td>1.1</td><td>0.6</td><td>0</td></tr><tr><td>Calc. in γ'</td><td>33.5</td><td>68.0</td><td>17.6</td><td>2.5</td><td>3.9</td><td>3.4</td><td>6.9</td><td>2.8</td><td>0.3</td><td>0.1</td><td>0</td></tr><tr><td rowspan="5">2</td><td>Exp. in γ</td><td>50</td><td>42.3</td><td>4.0</td><td>1.3</td><td>0</td><td>3.0</td><td>23.9</td><td>24.0</td><td>0.6</td><td>0.9</td><td>0.10</td></tr><tr><td>Exp. in γ'</td><td>50</td><td>60.3</td><td>10.1</td><td>2.3</td><td>4.1</td><td>4.5</td><td>11.4</td><td>5.8</td><td>0.6</td><td>0.9</td><td>0.20</td></tr><tr><td>Overall considered composition</td><td></td><td>51.3</td><td>7.1</td><td>1.8</td><td>2.1</td><td>3.8</td><td>17.7</td><td>14.9</td><td>0.6</td><td>0.9</td><td>0.15</td></tr><tr><td>Calc. in γ</td><td>86.2</td><td>48.7</td><td>6.1</td><td>2.0</td><td>1.9</td><td>3.0</td><td>19.6</td><td>16.9</td><td>0.6</td><td>1.0</td><td>0</td></tr><tr><td>Calc. in γ'</td><td>12.9</td><td>66.5</td><td>12.1</td><td>0.4</td><td>2.9</td><td>9.2</td><td>6.5</td><td>2.1</td><td>0.1</td><td>0.1</td><td>0</td></tr></tbody></table>
significantly lower in γ' compared to γ, in contradiction with the experimental data. Other differences between experimental and calculated compositions can be noted for Cr and W.

## Thermodynamic Computations of Activities

Thermodynamic computations were performed in order to verify if the Calphad modeling in the TCNI9 database was consistent with the effect induced by Pt on the activities of the alloying elements. Figure 5 shows the elemental activities and the predicted phases at 1080 °C (the heat treatment temperature of the Pt-modified NiCoCrAlYTa coating), as a function of the Pt concentration for three systems. The first system is a “modified” NiCoCrAlTa alloy with 1 at.% of Ni replaced by 1 at.% of Ti (Fig. 5a, d). This system has been considered because it is representative of the BC experimentally investigated in this work. The “Ti modification” has been introduced in order to get insights on the behavior of this element within this system. Indeed, even if Ti is not initially present in the BC, it diffuses from the AM3 alloy, as shown in Fig. 4. For this first system, all the phases included in the databases were considered in the calculation and the computed equilibrium predicted β, γ' and γ phases. Due to concerns about the accuracy of the calculations regarding the β phase (see below), the same calculations were performed for this system, but including only the γ and γ' phases (Fig. 5b, e). The third system considered is an AM3 alloy (γ+γ' phases) to which Pt was progressively added (Fig. 5c, f).

According to the calculations on the “modified” NiCoCrAlTa alloy (Fig. 5a, d), the Cr and Ta activities are continuously decreasing with the Pt addition while

<figure><img src="image_10.png" /><figcaption>Fig. 5 a–c Calculated activities and d–f phase molar fractions versus Pt concentration at 1080 °C, for a “NiCoCrAlTa+1 at.% Ti” alloy considering a, d all phases or b, e only γ and γ' phases and c, f for the addition of Pt to the AM3 composition</figcaption></figure>

<figure><img src="image_11.png" /></figure>
no significant effect is observed for Ni, Co and—quite surprisingly—Ti activities. Moreover, when $\beta$, $\gamma'$ and $\gamma$ phases are considered, the Al activity is almost constant for Pt concentration lower than 15 at.%, while it continuously decreases above this value. It can be noticed that this concentration value seems to coincide with the disappearance of the $\beta$ phase. This led to doubt the reliability of the modeling of this phase. For this reason, the second calculation was performed including only the $\gamma$ and $\gamma'$ phases. In this second case (Fig. 5b, e), the Al activity continuously decreases with the Pt addition, for the entire concentration range. Concerning the other elements, their activity evolutions are almost identical to the first calculation. For the “AM3-Pt” system, a steep decrease in the activity of several elements can be observed by increasing Pt concentration: Cr, Al, Ta but also Mo and, in a less remarkable way, W (not shown here for seek of simplicity). Ni, Co and Ti activities are only slightly affected by Pt addition. For the latter, a slight increase is observed at first, followed by a decrease for Pt concentration higher than 28 at.%.

It can be noticed that, whatever the Pt content in the NiCoCrAlTa+1 at.%Ti, the activity of Ni is always higher in the superalloy than in the coating. On the contrary, the activity of 1 at.% Ti is always higher in the coating than in the superalloy. For Al, Ta and Cr, it depends on the amount of Pt. For low amounts of Pt in the coating, the activities of the three elements are higher in the coating than in the superalloy without Pt. With sufficient Pt in the coating, it is the opposite, the activities become lower than in the superalloy without Pt.

Finally, it can be noticed that, according to the calculation, the $\gamma$ fraction continuously increases with the Pt content (Fig. 5).

To complete these calculations, the computed phase compositions for the diffusion of Pt in the modified NiCoCrAlTa+Ti and AM3 systems are shown in Fig. 6, considering only the $\gamma+\gamma'$ phases for both cases. According to the calculations, Pt should be more concentrated in the $\gamma$ phase rather than $\gamma'$, with the only exception of the AM3 system for overall Pt concentrations above 25 at.%. This is the opposite of what was observed in the previously presented EDS analysis.

## Discussion

After 1 h at 1100 °C, pores were observed at the Pt diffusion front of the “AM3-Pt” system (Fig. 2c). Large buckles had already been observed within a Pt-modified NiAl coating belonging to a thermally cycled TBC system (on AM3 and CMSX-4 superalloys) and for which the Pt layer was deposited by sputtering [14]. In this study, the pores were considered being due to the deposition of a dense Pt layer coupled with an out-of-pack aluminizing process. In the present study, the Pt layer was deposited using the same sputtering parameters and no aluminizing was carried out. The presence of pores at the Pt diffusion front with a Ni-13Al alloy and Ni-12Al-10Cr (in at.%) had already been observed after short exposures at 1100 °C by Audigié et al. [15] and [5], respectively. In both studies, void formation was shown to be due to the Kirkendall effect. The Kirkendall effect could also explain the pore formation in the present study, because of their morphology and their location at the diffusion front of Pt [5, 15].
<figure><img src="image_12.png" /><figcaption>**Fig. 6** a, b γ' phase composition and c, d γ phase composition calculated by Thermo-Calc at 1080 °C for a, c the addition of Pt to the “NiCoCrAlTa+1 at.% Ti” alloy and for b, d the addition of Pt to the AM3 superalloy</figcaption></figure>

Experimentally, a large Al outward diffusion was noticed after 1 h of Pt diffusion at 1100 °C in the AM3 superalloy, Fig. 3c. Also, an Al maximum content of 13.2 at.% was measured in the Pt-enriched surface, value that is higher than the Al content in the superalloy. This significant Al diffusion toward the surface is in agreement with previous studies on Pt-rich γ+γ' coatings, either produced from a 7 µm or thicker Pt electroplated layer with a longer heat treatment at higher temperature [13, 16], or from the diffusion of a 5 µm Pt electroplated layer in the AM1 superalloy for 1 h at 1100 °C [12]. Also, after Pt diffusion in the NiCoCrAlYTa coating for 6 h at 1080 °C, the upper part of the coating contained more than 30 at.% of Al (Fig. 4a). This led to the formation of a martensite and γ'-rich microstructure close to the surface, while the coating core was composed of both γ' and γ phases, as described in [4]. This differs from the microstructure of the unmodified NiCoCrAlYTa coating, mainly composed of the γ and β phases with a small proportion of TaC carbides. As explained in [4] based on the works of Gleeson et al. [2], the great decrease in Al activity due to Pt addition resulted in an important Al uphill diffusion. The high Al content (>30 at.%) in the Pt-enriched region (in the outer part of the coating) resulted in the formation of martensite. This Pt effect on Al activity is also responsible for the large Al outward diffusion observed in the case of AM3 superalloy and is in agreement with the performed thermodynamic calculations highlighting a continuous decrease in Al activity with Pt addition when a γ+γ' microstructure was

<figure><img src="image_13.png" /></figure>
considered (Fig. 5b, c for Ti-enriched NiCoCrAlTa and AM3 alloys, respectively). Nevertheless, when calculations were made considering all the predicted phases at equilibrium (β, γ' and γ) (Fig. 5a, d), an almost constant value of Al activity for Pt concentrations lower than 15 at.% was predicted. This was unexpected since an Al outward diffusion was observed from the very beginning of the heat treatments (Fig. 3). Looking at the predicted phases at equilibrium (Fig. 5d), it can be noticed that the beginning of the decrease in Al activity corresponds to the disappearance of the β-NiAl phase and the entering into the γ/γ' two-phase domain. Only in this biphasic domain, as explained, a continuous decrease in Al activity with Pt addition was predicted, Fig. 5b, e. This discrepancy highlights possible issues in the database regarding the β phase with Pt.

Whatever the system, the EDS profiles also highlighted an “alternation” of Al- and Cr-rich layers after short heat treatments (10 min, 10+20 min or 1 h at 1100 °C) (Fig. 3). Audigié et al. studied the first minutes of diffusion of a 7 µm electroplated Pt layer within a Ni-12Al-10Cr alloy at 1100 °C under Ar [5]. While the Al content was larger than the Cr content at the outermost surface of the material after a duration of 1 or 60 min, the Cr content was higher than the Al content after 15 min of diffusion [5]. To explain this “alternation” of Al- and Cr-rich layers in the Pt-rich area after short heat treatments, the transport of these two elements in the NiCoCrAlYTa system has to be considered. First, in a similar way than for Al, Cr outward diffusion is consistent with the decrease in Cr activity with Pt addition (Fig. 5b, c). As known from diffusion theory [17], this transport is controlled by a thermodynamic factor, related to the activity gradient, and by a kinetic factor, i.e., the mobility. The activity gradients due to Pt and mobilities of both elements have to be considered. Based on these thermodynamic calculations, the Al activity ratio is slightly lower than the one of Cr when the Pt molar fraction increases from $10^{-2}$ to 0.33 (for a $γ+γ'$ NiCoCrAlTa with Ti). According to calculations performed with the MOBNI5 database, Al and Cr mobilities are pretty close (in the γ phase, $5.4 \times 10^{-18}$ m$^2 \cdot s^{-1} \cdot J^{-1} \cdot mol$ for Al and Cr, respectively). Moreover, in agreement with these results, according to Audigié et al. work on a Pt/Ni-12Al-10Cr (in at.%) diffusion couple, the Al and Cr diffusion coefficients are pretty close [5]. Then, the Cr outward flux could be equal or slightly higher than the Al one. However, the Pt-rich external region contains a higher quantity of Al than Cr. One possible explanation could be the low solubility of Cr in the Al-rich phases. Indeed, the atom probe tomographic study of the Pt-modified NiCoCrAlYTa coating revealed a low Cr content in the L1₀ martensite phase (<10 at.%) and α-Cr precipitates within the martensite lattes [18]. It is then proposed that the fast and important Al transport toward the surface led to the formation of an Al-rich phase (α-NiPtAl, β-(Ni,Pt)Al or γ'-Ni₃Al depending on the diffusion time), preventing Cr to reach the extreme surface. Cr, diffusing outward in the coating, accumulated below the Al-rich area and favored the formation of a γ layer. As Al segregates preferentially in γ' rather than in γ, Al diffusing outward accumulated below the Cr-rich region leading to the formation of a γ'-rich layer.

The disappearance of TaC carbides (in white on the BSE-SEM images) in the inner part of the NiCoCrAlYTa+Pt coating with Pt diffusion was also noticed (Fig. 2a, b). Large Ta and Ti uphill diffusion was also noticed during the short and
long heat treatments (Figs. 3 and 4a), and high Ta and Ti contents were measured
in the γ' phase after 92 cycles of oxidation at 1100 °C in the external part of the BC
in the “AM3-NiCoCrAlYTa+Pt-TBC” system (Fig. 4b). These experimental results
are in agreement with previous works. In 2010, Vande Put et al. proposed that Pt
could decrease Ta activity, to explain the absence of TaC when Pt had diffused
within a NiCoCrAlYTa coating [4]. Later, Bennett et al. noticed a higher Ti content
at the surface of a Pt-rich γ+γ' coating compared to the NiAl or NiPtAl coatings
on CMSX-4 superalloy after 100 h of oxidation at 1100 °C [19]. In 2016, Haynes
et al. measured Ti content up to 4.3 and 10.1 at.% after 1000 h at 1050 °C or 400 h
at 1100 °C, respectively, in a Pt-rich γ+γ' coating on 1483 superalloy (composed of
4.9 at.% of Ti) [13]. They suggested that Pt could decrease Ti activity as it does for
Al activity, resulting in an increased Ti uphill diffusion. Also, higher Cr, Ti and Ta
concentrations than in the AM1 superalloy were measured in Pt-rich γ+γ' coatings
thermally oxidized at 1100 °C for more than 12,000 1 h-cycles by Audigié et al.
[12]. These authors noticed a greater Ti and Ta contents in the coatings containing
higher Pt concentrations, and they suggested that the detrimental effect of Ti and Ta
on the oxidation resistance of the coated superalloy could be amplified by a higher
Pt content. All these experimental results and proposed explanations on Pt effect are
consistent with the decrease in Ta activity with Pt addition (Fig. 5b, c). However,
the large Ti contents measured close to the alloy surface in the Pt-rich area (Fig. 4b)
cannot be explained by a Pt effect on Ti activity, according to thermodynamic calcu-
lations (Fig. 5b, c). Indeed, Ti activity exhibits a slight increase for low Pt contents
and a slight decrease for higher Pt contents. Also, it is interesting to notice that based
on thermodynamic calculations, Ti activity is always higher in AM3 than in a NiCo-
CrAlTa containing 1 at.% of Ti, whatever the Pt content (Fig. 5b, c). Then, above 1
at.% of Ti, no further diffusion of this element from AM3 toward the Pt-rich NiCo-
CrAlYTa alloy would be expected, which is contrary to what was experimentally
measured. Despite its low content in the AM3, Ti outward diffusion was noticeable
after a short Pt diffusion within the superalloy (and greater than the diffusion of Ta
whose content in AM3 is twice lower than the one of Ti), Fig. 3c. It is possible that a
thermodynamic interaction with another element in the system could be responsible
for the uphill diffusion of Ti. Another explanation could be that the TCNI9 database
has some weaknesses for the system with Pt and Ti.

Regarding interdiffusion, a faster Pt diffusion in NiCoCrAlYTa than in
AM3 was highlighted by the EDS in the SEM concentration profiles (Fig. 3).
Such a difference in Pt diffusion could be due to the presence of grain boundaries
in the NiCoCrAlYTa coating while AM3 is a single-crystal superalloy. However,
this could also be due to differences in terms of alloy composition and phases.
To answer this question, Pt diffusivity was calculated considering Pt-modified
NiCoCrAlTa and AM3 alloys. In these materials, Pt was added to the alloy com-
position so that its final molar fraction was equal to 0.10 and the Pt diffusivity
was calculated, at 1100 °C, for each phase predicted to be in equilibrium. For
the Pt-modified NiCoCrAlTa alloy, Pt diffusivity is equal to 2.3 × 10⁻¹³ m²/s in
the β phase and 9.1 × 10⁻¹⁵ m²/s in γ. In the case of the Pt-modified AM3, the
Pt diffusivity is 3.8 × 10⁻¹⁵ m²/s and 1.2 × 10⁻¹⁵ m²/s in γ and γ', respectively.
Although caution is advised when relying on calculations, these results highlight
a significantly faster Pt diffusion in β, which could slightly increase the overall Pt diffusion in the coating, depending on the volume fraction and morphology of β precipitates.

A discrepancy between experiments and calculations was noted regarding the partition of some elements (Pt, Cr, W) between γ and γ' phases enriched in Pt (Table 1 and Fig. 6). Some additional computations (not shown here) were performed in order to better understand the origin of the problem on Pt partition between γ and γ'. When working on the simpler ternary system Ni-Pt-Al, the calculations are in reasonable agreement with the experimental evidences, showing a preferential partitioning of platinum in γ'. Discrepancies start to be observed for quaternary systems, in particular the Ni-Cr-Pt-Al system. Considering the high Cr content in the NiCoCrAlYTa coating, it is necessary to exercise caution regarding the results of the thermodynamic calculations performed with the TCNI9 database in this particular system.

**Conclusion**

Heat treatments and oxidation tests were performed on systems composed of a NiCoCrAlYTa+Pt coating, and materials were characterized after exposure to investigate any effect of Pt on Al, Cr, Ta and Ti diffusion. Experimental results were compared to the AM3 superalloy coated by Pt. Thermodynamic calculations with Calphad-based approach were performed to be confronted to the characterization work. Pt diffusion was found to be faster in the NiCoCrAlYTa coating than in AM3 superalloy. After heat treatment, high Ta contents were measured in the Pt-rich γ' phase below the surface, while significant Al and Cr transport toward the surface was observed within both the AM3 superalloy and the NiCoCrAlYTa coating. These findings align well with thermodynamic calculations, which predicted a significant decrease in their activities due to Pt addition. An uphill diffusion of Ti was also noticed, but thermodynamic calculations did not show a decrease in Ti activity induced by Pt. It is possible that the titanium behavior could be due to a thermodynamic interaction with another element of the system. However, some doubts have arisen regarding the reliability of the adopted thermodynamic description due to some observed discrepancies with experimental results. For instance, the predicted partitioning of certain elements—notably Pt—between the γ and γ' phases was not correct.

**Acknowledgements** Authors thank the LERMPS laboratory (Sévenans, France) for the deposition of the NiCoCrAlYTa coating and Cranfield University for the deposition of the Pt layer and carrying out of the heat treatments of 1 h at 1100 °C and of 6 h at 1080 °C.

**Author Contribution** AVP wrote the main manuscript text and performed the experiments with the help of EE. AVP made the characterization and prepared figures and tables. EE made the thermodynamic calculations. AVP, EE and DM analyzed the results. All authors reviewed the manuscript.

**Funding** Open access funding provided by Institut National Polytechnique de Toulouse.
**Data Availability** No datasets were generated or analyzed during the current study.

**Declarations**

**Conflict of 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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