# NiO whiskers growth driven by the presence of Pt nanoparticles during Ni-base superalloy oxidation

Renaud Podor, Joseph Lautru, Titouan Duprat, Stéphane Mathieu, Michel Vilasi, Jonathan Cormier, Guillaume Burlot, Louis Hunault, Fernando Pedraza

► To cite this version:

Renaud Podor, Joseph Lautru, Titouan Duprat, Stéphane Mathieu, Michel Vilasi, et al., NiO whiskers growth driven by the presence of Pt nanoparticles during Ni-base superalloy oxidation. Materials Characterization, 2025, 230, pp.115754. 10.1016/j.matchar.2025.115754. hal-05422627

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Submitted on 18 Dec 2025

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Distributed under a Creative Commons Attribution 4.0 International License
NiO whiskers growth driven by the presence of Pt nanoparticles during Ni-base
superalloy oxidation

Renaud PODOR¹*, Joseph LAUTRU¹, Titouan DUPRAT¹, Stéphane MATHIEU², Michel VILASI², Jonathan

CORMIER³, Guillaume BURLOT³, Louis HUNAULT⁴, Fernando PEDRAZA⁴

¹ ICSM, Univ Montpellier, CNRS, CEA, ENSCM, Marcoule, France

² Université de Lorraine, CNRS, IJL, Nancy, France

³ Institut Pprime, UPR CNRS 3346, Physics and Mechanics of Materials Department, ISAE-ENSMA, 1
Avenue Clément Ader, BP 40109, 86961 Futuroscope, Chasseneuil, France

⁴ La Rochelle Université, LaSIE, La Rochelle, France

*Corresponding author (Tel : (33) 04 66 33 92 02) / email : [renaud.podor@cea.fr](mailto:renaud.podor@cea.fr) / Adress : ICSM -
UMR 5257, Site de Marcoule – Bâtiment 426 - BP 17171 - F-30207 Bagnols-sur-Cèze cedex, FRANCE)
Graphical abstract

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

<figure><img src="image_3.png" /></figure>

High temperature Environmental Scanning Electron Microscopy is used to observe continuously the oxidation of a single crystal Ni-base superalloy at 900°C. The formation of a new type of NiO whiskers covered by a Pt nanoparticle is reported and a formation mechanism is proposed.
**Abstract**

The formation and growth of NiO whiskers on a platinum-containing nickel-base superalloy was monitored continuously up to 113 hours, at 920°C, under 50 Pa of air, using High-Temperature Environmental Scanning Electron Microscopy. The images recorded revealed that two types of whiskers form and grow at two different stages of the oxidation process. The first type of whisker forms as soon as the isothermal plateau at 920°C is reached. The growth rate is greater than 500 nm/min. After a few hours of isothermal oxidation, these whiskers have grown and are integrated into the oxide layer covering the surface of the sample. The second type of NiO whiskers forms after 20 hours of isothermal stabilization. They grow at a rate of around 1 to 10 nm/min for several tens of hours. The formation and growth of these whiskers is directly linked to the presence of platinum nanoparticles on the surface of the alloy. The mechanisms of whiskers formation and growth are discussed.

**Keywords:** SEM, in situ, superalloy, whisker, oxidation, Ni-base

**Highlights:**

- Whiskers formation is observed during Ni-base superalloy oxidation

- High-Temperature Environmental Scanning Electron Microscopy is used to monitor NiO whiskers formation

- Two types of whiskers form and grow at two different stages of the oxidation process

- Growth mechanisms of the whiskers are proposed
# 1. Introduction

Nickel-based superalloys are materials designed to maintain their mechanical strength, corrosion resistance and oxidation resistance at high temperatures. They are widely used for applications in the hot sections of gas turbines and jet engines. When exposed to oxidizing conditions at high temperatures, different oxide layers form on their surface. These layers are generally covering and protective, and are limiting the degradation of superalloys [1]. However, various undesirable surface reactions may occur [2]. Among them, the formation of whiskers is a potentially damaging phenomenon, given that their growth does not lead to the formation of a continuous, protective layer, which can limit the performance and durability of superalloys in operating conditions [3].

Whiskers are long, fine nano- or micrometric crystals. The vast majority of whiskers reported in the literature are metallic, and very often Sn whiskers. They have been the subject of numerous studies because they are considered to be a significant problem for the electronics industry [4]. Numerous studies on their growth mechanisms have been reported [4]. In recent years, these one-dimensional (1D) materials have been viewed in a new light because they potentially have unique chemical and physical properties and may find potential applications in the field of nanomaterials [5]. Studies on the fabrication of pure Sn whiskers (potentially Ga or In) from MAX phases are also being conducted [5,6]. This technique can also be used to fabricate whiskers of SnBi or SnPb intermetallic compounds, or solid solutions, paving the way for control over the composition of manufactured objects [7]. At the same time, the formation and manufacture of oxide whiskers, particularly ZnO, is also being reported and designed [8,9]. Whiskers can also form when oxidation products grow preferentially in certain directions, leading to the development of needle-like structures that extend outwards from the surface [10]. This process is observed when the alloys are exposed to temperatures above 700°C, where oxidation is both thermodynamically favorable and kinetically
rapid. Their formation and growth is related to the stresses generated within the oxide layers formed
at these temperatures [11,12].

In addition, the growth of Cr₂O₃ and Al₂O₃ whiskers on top of Ni-base superalloys (or similar
compounds) has been reported [3,13,14] to be generally associated with the presence of water in gas
which reacts with the metallic substrate. The formation of NiO whiskers during the oxidation of
nickel-containing alloys has only been reported to a very limited extent [15]. The growth of
unidirectional γ-Al₂O₃ with a NiO nanoparticle at the tip of the nanowire is reported in the work of
Zhu et al. [16]. The formation of NiO whiskers has been observed when the substrate is Ni. They can
grow at 480 °C after 24 hours when Ni is in contact with a steam containing atmosphere [17]. They
have also been reported to form after heat treatment at 1500-1600 °C for 10 to 60 minutes in air or
sometimes in an atmosphere containing argon and oxygen [12]. They can also be obtained at 800°C
when the atmosphere contains water vapour instead of steam [18]. In this case, the authors report
that their growth may be linked to the presence of a Ni nanoparticle or an impurity at the tip of the
whisker growth zone. In recent studies, the fabrication of NiO nanowires by thermal oxidation-driven
nanoparticles [19] or Ni films [20] has been reported. This leads to the formation of objects ranging
from 100 to 1500 nm in length. These authors underline the possible applications of NiO nanoobjects
as electrochromic devices, electrode materials in battery systems, electrochemical supercapacitors,
gas sensors.

In the present study, high-temperature environmental scanning electron microscopy is used
to directly observe and describe the formation of whiskers during the oxidation of a Ni based
superalloy at 920°C for durations up to 113 hours. This work also aims to demonstrate the value of
the HT-ESEM technique for studying local phenomena that are limited in time and not necessarily
visible when the sample is observed post-mortem (i.e. after cooling at the end of the heat
treatment). This technique has been used in the past to observe oxidation processes on alloys
[21,22,23,24]. Recent developments in microscopes and associated software allow for more precise
(short image capture times) and longer durations of these processes [25,26,27].

## 2. Materials and methods

An environmental scanning electron microscope (ESEM) of the Quattro S type
(ThermoFisher) was used. It was combined with an HT1400 furnace (ThermoFisher) that could heat
samples up to 1400°C. This furnace is equipped with an additional thermocouple [28] onto which the
sample is deposited directly. It allows the temperature of the sample to be measured with an
accuracy of a few degrees.

A Karmen high-temperature backscattered electron detector (Crytur society) was used to
form the images in this experiment [29].

After positioning the sample in the furnace, a partial vacuum of 50 Pa of air is maintained in
the chamber. The temperature is increased at a rate of 30°C/min to 920 (±5)°C. This temperature was
chosen because it is within the typical temperature range for nickel-based superalloys. Once this
temperature has been reached, an isothermal dwell step is maintained for the expected duration.
The duration of the experiments varies from 64 to 113 hours, which corresponds roughly to the
duration of the 4-day tests (100 h) traditionally carried out during thermogravimetric oxidation tests.
The variations in duration are linked to the experimental constraints imposed by the operating
conditions of the microscope. The results obtained from an experiment carried out on a frosted
sample for 69.5 hours are mainly used in this work. The results of the other experiments are used to
validate the reproducibility of the phenomena observed and to clarify their limits.

The microscope is then controlled by a software specially developed in the laboratory. This
software automatically focuses the microscope (focus and astigmatism) each time an image at the
highest magnification is recorded. It allows a region of interest to be recorded and refocused
regularly to limit image offset. Finally, it allows images to be recorded regularly at different magnifications. The magnifications selected here are 16 000, 8 000, 4 000, 2 000, 1 000, 500 and 250X to obtain information on the behavior of the sample at different scales during heat treatment. The images are recorded at 12 kV to maintain a separating power (equivalent to the lateral resolution on the image) of around 20 nm, in line with the size of the details to be observed during the oxidation process, and to limit the electron beam effect on the sample surface. After 2 hours of isothermal dwell step at 920°C, each image recorded corresponds to the average of 32 images of 1 second. This improves the signal-to-noise ratio, reduces any distortion of the images caused by oven regulation and limits the number of images recorded over the whole experiment.

The newly developed Ni-based single-crystal γ/γ' TROPEA superalloy containing 0.6 at% Pt material was employed in this work [30]. The specimens were used in fully heat-treated conditions, including a solution heat-treatment of 10 h at 1300 °C/Air Quench (AQ), followed by 1 h at 1200 °C/AQ and then 16 h at 870 °C/AQ. The samples have specific coupon dimensions (1 mm x 0.5 mm x 0.5 mm) and the surfaces are polished with a SiC P1200 grit papers (ground sample). For one experiment, the sample are polished with a 1 µm diamond paste and then with 100 nm colloidal silica in order to remove the main part of the defects present on the surface of the sample (polished sample).

After the experiments, local chemical analyses and elemental distribution mapping by EDS are carried out at room temperature both on the raw surface of the sample and on a cross-section of the sample obtained after resin embedding and polishing with 1 µm diamond paste. The EDS detector used is an XFlash 6-100 (Bruker). Analyses are carried out at 8 kV, with a photon flux of 100 kcps. These conditions ensure a lateral resolution of around 100 nm on the elemental distribution maps.

The 3D reconstructions of the sample surface are based on images recorded in secondary electrons mode, once the sample has cooled, using Mex software (Alicona).
SEM images of whiskers are a 2-dimensional projection of a 3-dimensional object. The terms used to designate the dimensions of the whiskers are defined as follows and shown in Figure 1. The base of the whisker is the area of contact between the whisker and the crystal that supports it. The center of the whisker is the central part and the tip of the whisker is the part in contact with the platinum nanoparticle (when present). The dimensions in the width direction are called "size". The length defines the distance between the base and the tip of the whisker.

<figure><img src="image_4.png" /><figcaption>Figure 1. a) List of terms used to designate the different dimensions measured from SEM images. b) SEM image of a whisker.</figcaption></figure>

### 3. Results

The results used were obtained during an experiment carried out at 920°C for 69.5 hours. The series of images obtained are shown in Supplementary Information SI1a to SI1g. Two other image series, respectively recorded at 920°C on a ground sample for 113 hours and on a polished sample for 64.5 hours are reported in Supplementary Information SI2 and SI3. It is important to watch these films, because it is by viewing them that the explanations and descriptions given in the rest of this work take on their full meaning.

The results indicate that there is a different behavior depending on the initial surface state of the sample. For a ground sample, the recorded images indicate that oxidation of the sample occurs from around 480-500°C. The oxides form according to nodules of a few tens of nanometers in size,
which grow rapidly by coalescence (see the very beginning of movie reported in SI1g). Whiskers form at two different times during the experiment. The earliest whisker formation is observed as soon as the temperature of the sample reaches the isothermal plateau of 920°C. A second series of whiskers forms after the sample has been held at this temperature for several tens of hours. Each of the two growth periods leads to the formation of whiskers with different properties, which will be dealt with separately in the rest of this work. The whiskers develop over the entire surface of the sample observed, at the minimum magnification at which they remain visible (surface area covered of 42 x 64 µm²). At the end of the experiment, the number of whiskers was counted manually on different images (each corresponding to a given area of the material's surface). This number was then normalized to 1 mm² so that comparisons could be made. The density of whiskers still observed was around 75 000 whiskers/mm². Similar results have been obtained during two distinct experiments (see SI1f and SI2).

When the sample surface is polished, and the experiment is carried out in the same way, the formation of whiskers is also observed (SI3). When the sample surface is polished, the density of first-stage whiskers is high (120 000 whiskers/mm²). In contrast, the whiskers do not develop during the second stage. In this case, only few Pt nanoparticles were observed on the surface of the oxides.

Experiments carried out in the ESEM chamber on polished and ground samples indicate that the first stage of NiO whisker formation is always observed. The second stage of NiO whisker formation appears to be conditioned by the presence of Pt nanoparticles. In both cases, the composition of the whiskers formed is NiO. In the following work, particular attention will be paid to the results obtained on ground samples, which are more representative of the conditions under which Ni-base superalloys are used. Particular attention will be paid to the formation conditions, kinetics and growth mechanisms of whiskers.
These whiskers form as soon as the 920°C isothermal plateau is reached and within the
following 10 to 20 minutes. Their anchoring point seems to be located between the first oxide
crystals present on the surface of the sample. Their rapid growth rate is greater than 500 nm/min.
The growth direction of these whiskers is mainly linear and their length is ranging between a few
hundred nanometers to a few micrometers. An example of the formation, growth and magnification
of one of these whiskers is shown in Figure 2a.

After a very rapid growth (see the first 2 images of Figure 2a), the whiskers get bigger
(following images of Figure 2a). Sizes at the base (where whisker formation begins), center and tip
were measured on one of the whiskers observed (Figure 2b). These parameters increase with time
and after a few hours, the whiskers have grown and become grains integrated into the surface oxide
layer. So, if the experiment had been stopped at that point, and without having access to the full
series of images, it would not have been possible to distinguish a 'classically' formed oxide grain from
a whisker whose morphology had evolved with the time it was kept at temperature.

The tip of these whiskers has no platinum nanoparticles. The EDS analyses performed after
cooling of the sample to room temperature indicate that they are composed exclusively of NiO (see
Supplementary Information SI4a).

<figure><img src="image_5.png" /><figcaption>Figure 2. a) Images recorded in HT-ESEM at 920°C showing the formation, growth and coarsening of a whisker formed above the TROPEA2 alloy, after thermal equilibration at 920°C, under 50 Pa of air - Scale bar = 250 nm. b) Variation in the length of this whisker, as well as the sizes measured at the base, center and tip</figcaption></figure>
- Whiskers - Second stage / slow growth

After 20 hours of isothermal dwell at 920°C, other whiskers form during a second stage. They differ from the whiskers formed in the first stage in several respects.

The formation of these whiskers starts at the location of a platinum nanoparticle on the surface of the oxides. This nanoparticle then remains on the tip of the whisker (Figure 3a). The initial diameter of the whisker is approximately the same as the diameter of the nanoparticle (typically 30 nm). The inserts shown in the first images in Figure 3a show a magnified view of this nanoparticle. These images show that the diameter of the nanoparticle decreases with time as the temperature is maintained. From the 9th image onwards, the Pt nanoparticle is no longer present on the whisker head. From this image onwards, the whisker no longer grows. It is clear that whisker growth is halted as soon as the nanoparticle disappears. The presence of this nanoparticle is directly related to the formation and growth of whiskers.

The growth rate, measured on 8 separate whiskers, is of the order of 1 to 10 nm/min. This speed can vary over time for a given whisker (Figure 3b). These variations point to changes in the growth of the whisker, which could be related to a change in the direction of growth. The growth can indeed slow down or stop and then restart without any particular event being observable, at least on the scale at which the observations are made. It should be noted here that the continuous? growth of a given whisker was observed for 48 hours, with a resolution of around 20 nm on the recorded images.

The observations made on several whiskers (Figure 3a et S1g) indicate that their diameter increases over time. Measurements of the size of the base, middle and tip of a particular whisker (Figure 3a – Arrows in white in the series of images) whose direction of growth always remains the same (Figure 3b), show that the size of the whisker tip (contact zone with the Pt nanoparticle) remains almost the same, i.e. equivalent to the diameter of the nanoparticle attached to NiO. The
size of the base of this whisker increases with the duration of the experiment and the size of the
center of the wire also increases, but less rapidly than that measured at the base (Figure 3c).

However, some whiskers behave differently, such as the one marked with black arrows in Figure 3a
(image 4 and subsequent). In this case, it is the tip of the whisker that grows the most rapidly. From
the ninth image shown (15:05:42), this whisker has the morphology of a NiO grain integrated into the
surface oxides, and no longer that of a needle.

<figure><img src="image_6.png" /><figcaption>Figure 3. a) Series of images showing the growth of a NiO whisker captured by a Pt nanoparticle (top series) - Scale bar = 500 nm - The insert (95 nm X 95 nm) shows the nanoparticle present at the tip of the whisker. The first image of the series is recorded after 22 hours of isothermal holding at 920°C. b) Total length and growth rate associated with this whisker (bottom series). c) Variation in the length of this whisker, as well as the sizes measured at the base, center and tip of this whisker.</figcaption></figure>
The morphology of these whiskers evolves relatively slowly, and their position can still be
easily identified after the experiment, once the sample has been cooled to room temperature. An
image recorded at 920°C showing the surface of the sample before cooling (Figure 4a, b) is compared
with the same zone observed after cooling in secondary electrons mode (SE) (Figure 4a, b). Even if
the general morphology of the oxides remains the same, it is clear that the whisker morphology
changed upon cooling. After cooling, the sizes of the whiskers observed on the 2 images appear to be
20 to 50% larger than the sizes measured at temperature. Conversely, the lengths of the whiskers
observed seem to be 20 to 50% smaller after cooling than at temperature. At the same time, the
distance between the “conventional” oxides is not modified upon cooling. This clearly indicates that
only the whiskers evolve upon cooling with a change in both shape (not cylindrical anymore) and
dimensions. Similarly, the Pt nanoparticle that was present on the whisker head during its formation
and growth is no longer visible after cooling, which is consistent with the observations made at high
temperature.

The 3D reconstructions of the surface of the sample in the zone observed at high
temperature indicate that the oxide layers developed on the surface of this sample do not show
strong heterogeneities, the difference between the upper and lower part of this layer being a few
hundred nanometers. The whisker, on which the measurements shown in Figure 3b and Figure 3c
were made, has no particular topography (Figure 4d) compared to other oxides. It appears to be
laying on top of these oxides. Another whisker has a particular morphology and stands out from the
plane of the sample. The end of this whisker is positioned approximately 1.2 µm above the surface of
the sample (Figure 4d). It presents an angle of 45° with the surface formed by the oxides developed
on this sample. The measurement of this inclination of the whisker in relation to the substrate
indicates that the measurement of whisker growth rates may be underestimated. It may be 50%
higher than previously reported values.
Semi-quantitative analyses of the chemical composition of the compounds formed on top of
the layer and in the layer (cross-section) were carried out by EDS. The data are shown in Figure 4c
and Figure 4e (as well as in Supplementary Information SI4b). These data indicate that the whiskers
(marked by arrows on the BSE images associated with the elemental distribution maps) are mainly
composed of NiO. To our knowledge, this is the first time that NiO whiskers formed during high-
temperature oxidation of a Ni-based superalloy have been reported in the literature. Specific
analyses performed at low-voltage on a whisker with a nanoparticle on its head show a local Pt
enrichment at the position of the nanoparticle located at the head of the whisker. Thus, considering
the minimum analyzed volume using energy dispersive spectrometry, this result indicates that these
nanoparticles are Pt-nanoparticles. Furthermore, such nanoparticles have been observed and
characterized directly by TEM on the NiO grains formed at the sample surface. They were previously
identified [31] as Pt-nanoparticles.

Last, the same type of whiskers were also observed at the surface of a sample heat treated
under 1 atm ambient air in a muffle furnace at 920°C for 96 hours, indicating the HT-ESEM
experiments reproduce accurately the oxidation behavior of Ni-base superalloys under atmospheric
conditions.
<figure><img src="image_7.png" /><figcaption>Figure 4. Observation before (a) and after cooling of the same area of the sample heat-treated at 920°C for 69.5h. Details of the whiskers shown in figure 2(b). Elemental distribution maps of the area observed on the surface (c). 3D images reconstructed after sample cooling at RT (d). Elemental distribution maps of the sample surface seen in cross-section. (e) A whisker seen in cross-section is observed on the surface of the sample (white arrow).</figcaption></figure>
4. Discussion

The results obtained allow a direct and continuous observation of NiO whiskers development
at the surface of a Ni base single-crystal y/y' superalloy at 920°C over a period of 69.5 hours, at a
maximum magnification of 16 000X. The observations of the sample surface at different
magnifications indicates that there was no beam effect and that the observed processes are not
activated by the electron beam (as it can sometimes be the case in experiments carried out in a SEM
or TEM [32]).

The formation and growth of two different types of whiskers was observed. The origin of
each type of whisker is different, but is linked to the presence of heterogeneities at the surface of the
sample. Similarly, their growth mechanisms are different.

In the case of whiskers formed during the first phase (as soon as the isothermal plateau
temperature is reached), the heterogeneity from which these whiskers form is an interface between
two oxide crystals present on the surface of the sample. The whisker probably forms according to a
mechanism close to that proposed by Luo et al. [15]. The growth of the whisker is driven by the
substrate through a diffusion short cut of Ni along the interfaces of the crystals located at the base of
the whisker. Gulbransen [33] proposed a model to explain the growth of oxide whiskers which
assumes that the whiskers are hollow or contain a screw dislocation in the center. Rapid diffusion of
the metallic element from the substrate to the tip would occur through these defects. Tallman and
Gulbransen [34] have shown, in a study of iron oxide whiskers, that these whiskers are single crystals
that exhibit axial twist, which is proof for the existence of a screw dislocation at the center of the
whisker. According to the same authors, whisker growth occurs by diffusion of iron atoms or ions
through the dislocation and then by reaction with oxygen at the tip of the whisker. Oxygen is present
in the atmosphere and its supply does not limit the growth of the NiO needle. As the 2 fluxes are fed
rapidly and continuously, the unidirectional growth of the whisker is rapid. Tallman and Gulbransen
also stated that the whisker remains thin because axial diffusion of metal species is more favorable
than radial diffusion [34]. The stop in rapid whisker growth could be linked to a local morphological
change at the base of the whisker due to the growth of oxide grains (as can be seen directly on the
SEM images). Structural modifications to the screw dislocation, which would stop the flow of nickel
towards the tip of the whisker, could also explain the stop in the rapid growth of the whisker along
the axis direction. Then, the supply of nickel through atomic addition at the base contributes to the
lateral growth of the whisker, which is rapidly transformed into a grain of NiO and integrated into the
surface oxide layer.

In the case of whiskers formed during the second phase (after 20 hours of isothermal
holding), the surface heterogeneity at the origin of their development is the presence of platinum
nanoparticles on the sample surface. These nanoparticles, formed during the oxidation of the sample
by the association of Pt atoms not participating in the oxide layer [31], initially move to the oxide
surface. The catalytic effect of the Pt nanoparticle on the formation and subsequent growth of
whiskers is clearly established here. Indeed, the whiskers that form are all covered by a Pt
nanoparticle (Figure 3a). As soon as this nanoparticle is no longer present on the whisker, whisker
growth is stopped in the length direction. Then, the whisker continues to grow (not in length, but in
width) and leads to the formation of a large crystal by atomic addition at the base. A different growth
mechanism using a Vapor Liquid Solid method have been reported in the literature for metallic
materials to explain the growth of nanowires whose tip is covered with a nanoparticle
[10,35,36,37,38] or oxides [16,39]. In their work, Zhu et al. show that this oxide nanowire growth
process does not involve the formation of a droplet of catalytic liquid at the end of the nanowire
[16]. Instead, the solid NiO nanoparticle formed from the oxidation of NiAl acts as a catalyst in the
dissociative adsorption of O₂ gas in the surrounding atmosphere. This results in an inward diffusion
flux of atomic oxygen from the surface of the NiO nanoparticle towards the NiO/γ-Al₂O₃ interface to
react with the incoming flux of Al³⁺ cations provided by upward diffusion through the γ-Al₂O₃
nanowire crystal lattice, leading to elongation of the nanowire.
A similar mechanism, where growth takes place at the level of the Pt nanoparticle, can be proposed. At some stage, a whisker begins to form at the position of a Pt nanoparticle and starts to grow. At this point, there is likely a lattice coherence between the Pt and NiO structures. Oxygen dissociates at the Pt nanoparticle and atomic oxygen or oxygen ions diffuses from the Pt nanoparticle to the NiO oxide below. The rising flow of Ni²⁺ leads to the localized and accelerated formation of the first atomic planes of the whisker, which begins to grow along the length direction. Secondly, matter transfer initially leads to an increase in whisker length and a significant increase in whisker base diameter. This could indicate that Ni²⁺ flux is limited at the base of the whisker, while oxygen transport through the NiO needle is rapid. Whisker growth would then be limited by the Ni²⁺ supply at the Pt nanoparticle level. This hypothesis is corroborated by the fact that the needle diameter does not increase at the Pt nanoparticle level (at the tip), and remains roughly equal to the diameter of this nanoparticle as long as the whisker grows. Furthermore, the diameter of the base of these whiskers increases more rapidly than the diameter at the center and tip. This can be explained by the fact that Ni²⁺ (and electrons) coming from the base of the whiskers diffuses outwardly in the planes perpendicular to the whisker axis. On the surface of the whisker, it can react with oxygen and lead to its lateral growth (see inset in Figure 5b). This mechanism is consistent with the literature [40], which states that the outward flow of nickel is far higher than that of interstitial oxygen. Oxygen migration into the crystal is minor, such that oxygen reacts at the surface of the crystal, forming there the new building blocks.

The diffusion paths are clearly different from those proposed to explain the growth of whiskers observed at the beginning of oxidation. In this case, there is probably no diffusion short circuit (such as screw dislocation) and the nickel transfers are limited by the solid-state diffusion of Ni²⁺ in NiO (the self-diffusion coefficient of Ni²⁺ in NiO is of the order of 10⁻¹¹-10⁻¹² cm²/s at 920°C and at an oxygen partial pressure P>O₂ = 1 atm [41]). This is probably the reason why the growth rate of this second type of whiskers remains slow.
The slow decrease in platinum nanoparticle diameter could be explained either by the integration of a few Pt atoms into the nanowire as it forms, or by the thermally activated migration of Pt atoms onto the nanowire surface [42]. These mechanisms lead to the disappearance of the Pt nanoparticle after a given time at high temperature.

A summary diagram showing the whisker growth mechanisms proposed is reported in Figure 5.

<figure><img src="image_8.png" /><figcaption>Figure 5. Schematic illustrations of whisker growth mechanisms during the first (a) and second (b) stages identified (adapted from Tallman and Gulbransen [34]). The insert in (b) illustrates the mechanism of lateral whisker growth through the addition of NiO layers.</figcaption></figure>

Specific events related to whisker growth can modify the $Ni^{2+}$ flux to the whisker. For example, if the whisker comes into contact with a NiO crystal already formed on the surface of the
superalloy, areas other than the foot of the whisker may grow faster. Morphological changes in the nickel source crystal can also lead to accelerated or stopped whisker growth, as the diffusion rate of $Ni^{2+}$ within this crystal can be modified. Here, the development of the chromia layer or the formation of a Ta-rich oxide layer beneath the $NiO$ crystals formed at the start of alloy oxidation can modify the $Ni^{2+}$ delivery mechanisms to the whisker's "source", i.e. the $NiO$ crystal [Erreur ! Signet non défini.]. This is probably one of the reasons why whisker growth rates decrease with time.

During their formation, the growth of some whiskers is slowed down or even stopped, and then restarted ($Si_{5a}$ and $Si_{5b}$). These changes could also be linked to platinum nanoparticle movements (such as those observed on the surface of $NiO$ grains formed above the superalloy) and to potential changes in the orientation of these nanoparticles relative to the orientation of the $NiO$ whiskers. Epitaxial relationships between the <100> orientations of Pt and $NiO$ are reported in the literature [43]. Whisker growth rate could be directly linked to this epitaxial relationship between platinum nanoparticles and $NiO$ whiskers. Any change in orientation of the platinum nanoparticle would alter this epitaxial relationship and halt whisker growth. Whisker growth can only resume once this relationship has been re-established.

The apparent morphological changes in the whiskers observed during cooling (expansion of their diameter by 20 to 50%) and contraction of their length (by around 20%) are probably linked to the change in the angle of observation of these objects, which are not perfectly cylindrical. The images recorded before and after cooling, as well as those recorded at different tilt angles (in order to produce a 3D reconstruction of the surface), show that they tend to adopt a more or less flattened cylindrical morphology, the circumference of which varies with the distance from their base.

All these observations show that whiskers formed at a given moment can remain present at the surface of the sample for a long time (up to 2 days). However, whether whiskers are formed very early in the oxidation process, or later during a second stage, they do not remain present in the form of needles and are integrated into the continuous layer of oxides after a growth process. Thus, their
presence above the oxides seems to be only transient. Only in situ experiments such as those carried
out in this study can highlight and describe these transient phenomena. It is important to note that
these whiskers appear to be able to form at any point during the oxidation cycle and potentially
interact (during service) with nearby parts. This study also highlighted very different mechanisms and
kinetics involved in the formation of these whiskers. The formation of these objects has already been
reported on various alloys. The data obtained in this work sheds new light that will likely lead to a
better understanding of whisker formation on other alloys or metals [3].

**5. Conclusions**

In this work, for the first time, the HT-ESEM technique, combined with automated ESEM control [27], enabled images to be recorded at different magnifications, continuously, over several tens of hours. This technique proved particularly effective in highlighting and understanding transient phenomena at the submicroscopic scale directly at high temperature. The growth of two types of NiO whiskers associated with different stages of the oxidation process was observed during the oxidation of a nickel-based superalloy at 920 °C. The associated formation and growth mechanisms were proposed for each of the two stages. These results constitute a body of knowledge that enriches our understanding of the possibilities for the formation of 1D objects, and even 1D composite objects (in this case, NiO nanowires with a Pt nanoparticle on the tip), while proposing original formation mechanisms, through high temperature oxidation of Ni-based superalloys.

The HT-ESEM technique will be used to conduct an in-depth and ongoing exploration of the
formation of oxide layers on nickel-based superalloys. This technique will make it possible to
evaluate the various events that occur on the surface of these alloys during oxidation and will
provide information that can be compared with the reactivity of these alloys in different
atmospheres, air, water vapour, other gas mixtures, or even in the presence of corrosive compounds
(Na₂SO₄).
## Fundings

The authors acknowledge the French National Agency for Research for funding the ANR project ALUPLAT (ANR-21-CE08-0013-01).

## Acknowledgements

Safran Aircraft Engines is gratefully acknowledged for providing TROPEA alloy single-crystalline bars. Dr. Jérémy Rame, formerly at Safran Aircraft Engines and now at NAAREA is gratefully acknowledged for fruitful discussions regarding TROPEA alloy development.
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