# Oxidation of Ti-6Al-4V titanium alloy in air plasma

Adrien Brault, Samuel Jouen, David Gibouin, Pascal Boubert

► To cite this version:

Adrien Brault, Samuel Jouen, David Gibouin, Pascal Boubert. Oxidation of Ti-6Al-4V titanium alloy in air plasma. CEAS Space Journal, 2025, AeroThermoDynamics for Space Debris Mitigation Solutions, 10.1007/s12567-025-00611-0. hal-05081958

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# Oxidation of Ti–6Al–4V titanium alloy in air plasma

Adrien Brault<fnref n="1" /> · Samuel Jouen<fnref n="2" /> · David Gibouin<fnref n="2" /> · Pascal Boubert<fnref n="1" />

Received: 21 December 2024 / Revised: 20 February 2025 / Accepted: 31 March 2025
© The Author(s) 2025, corrected publication 2025

## Abstract

The intensification of space activities has led to an increase in orbital debris, making it crucial to study its behavior during atmospheric re-entry. This study focuses on the oxidation of Ti–6Al–4V alloy, a material used in the manufacture of pressurized fuel tanks for the aerospace industry. This alloy was exposed to an inductively coupled air plasma in the SOUPLIN set-up at the CORIA laboratory. Two operating points were studied: (1400 Pa, 4.6 MJ kg⁻<fnref n="1" />) and (4500 Pa, 7.3 MJ kg⁻<fnref n="1" />), for oxidation times of up to 120 s. Surface analyses (XRD, Raman spectroscopy, and SEM-EDS) reveal a predominance of TiO₂ rutile throughout the oxide layer, as well as the localized presence of Al₂ O₃ and Al₂ TiO₅ at temperatures above 1150 °C.

**Keywords** Inductively coupled air plasma · Ti–6Al–4V · Atmospheric re-entry · Oxidation

## 1 Introduction

Ti–6Al–4V is the world’s most widely produced titanium alloy, accounting for over 50% of global titanium alloy production [1]. Composed of 6% aluminum and 4% vanadium, Ti–6Al–4V is characterized by its low density, high mechanical strength, and high resistance to oxidation. These properties make it a material used in a wide variety of sectors, including aerospace, chemicals, medical, and military. In aerospace, it is particularly used in the manufacture of pressurized fuel tanks.

The intensification of space activities, marked by the multiplication of satellites in Earth orbit, has considerably increased the risk of collisions and, consequently, the amount of space debris. During atmospheric re-entry, these debris interact with an air plasma in thermodynamic non-equilibrium, where chemical reactions, notably oxidation, alter their mass and modify their surface composition. Understanding these phenomena is essential to accurately model the behavior of debris during atmospheric re-entry. Thus, the study of the oxidation phase, particularly for materials such as Ti–6Al–4V, is crucial.

Many studies have already been carried out on the oxidation of Ti–6Al–4V under standard atmosphere in furnaces over a wide range of temperatures (400–1550 °C) and oxidation times (0.5 h to 140 days). Champin et al. [2] observed up to 700 °C and 160 h a parabolic evolution of the mass variation, indicating the formation of a protective oxide layer, while at 800°C, a transition to a linear regime was reported, indicating a loss of protection. This transition was also studied by Guleryuz et al. [3], who noted its earlier appearance, as early as 700 °C after 36 h of oxidation. Their analysis of the oxidized layer revealed a surface oxide composed mainly of TiO₂ in rutile form, with traces of anatase, two allotropic forms of TiO₂. Kumar et al. [4] detected the additional presence of TiO on the oxide surface for a temperature of 800 °C and reported oxide layer thicknesses of 17 and 35 µm at 650 °C for oxidation times of 8 and 16 h, respectively. El Bakali et al. [5] observed that TiO₂ is formed from 600 °C, while Al₂ O₃ appears from 900 °C. For an

Samuel Jouen, David Gibouin, and Pascal Boubert have contributed equally to this work.

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

Adrien Brault
braulta@coria.fr

Samuel Jouen
samuel.jouen@univ-rouen.fr

David Gibouin
david.gibouin@univ-rouen.fr

Pascal Boubert
pascal.boubert@coria.fr

<footnote n="1">1 CORIA, UMR 6614, Université de Rouen Normandie, Avenue de l'Université, 76801 Saint-Etienne du Rouvray, Normandie, France</footnote>

<footnote n="2">2 GPM, UMR 6634, Université de Rouen Normandie, Avenue de l'Université, 76801 Saint-Etienne du Rouvray, Normandie, France</footnote>
oxidation time of 1 h, oxide thicknesses of 0.78 µm at 600 °C and 50 µm at 1000 °C were measured, showing rapid thickening of the oxide layer as temperature increases. Garbacz et al. showed that at 900 °C, the oxide is mainly $TiO_2$ with small proportions of $Al_2O_3$ on the surface and $V_2O_5$ distributed in the oxide layer [6]. For higher oxidation temperatures up to 1550 °C, Rabaji et al. [7] identified on the oxide surface the formation of $Al_2TiO_5$, in addition to $TiO_2$ and $TiVO_4$, as well as significant spalling from 1230 °C. After 1 h of oxidation, they measured oxide layer thicknesses of 30.9 and 41.8 µm at temperatures of 1120 and 1230 °C, respectively.

However, oxidation in a standard atmosphere does not reproduce the conditions of atmospheric re-entry. Thermodynamic non-equilibrium, complex plasma composition, and pressure levels imply that results obtained in a standard atmosphere cannot be directly transposed to a plasma environment. Although a few studies [8, 9] have explored oxidation in plasma environments, these have been conducted under conditions different from those encountered during atmospheric re-entry. Additionally, as part of studies [10, 11], Ti-6Al-4V samples were oxidized in plasma wind tunnels, to test their resistance to oxidation without investigating the oxidation mechanisms.

Buckner [12] studied the oxidation of a Ti-6Al-4V X-link component from the NASA's Space Shuttle Columbia, which was exposed to an air plasma produced by an arc-jet set-up. The oxidized layer contained species, such as $Ti_3O$, $Ti_6O$, $Ti_2O_3$, $Ti_4O_7$, $Ti_2O$, and $Ti_3N_{1.29}$. The least degraded samples were those containing nitrides, while the most extreme cases of oxidation resulted in less oxygen-rich oxides.

The following studies, carried out at the PROMES laboratory using the MESOX (Moyen d'Essai Solaire d'OXydation) set-up, are the closest to real atmospheric re-entry conditions. The MESOX set-up generates a microwave air plasma in thermodynamic non-equilibrium with pressures ranging from 100 to 5000 Pa [13] and an oxygen dissociation rate of about 80% [14].

Prévereaud et al. [15] studied the oxidation of Ti-6Al-4V in an air plasma at 500 Pa, over a temperature range of 1330-1730 °C, with maximum oxidation times up to 7 min. For an oxidation time of 5 min, the oxide layer delaminates at 1330 °C, cracks appear at 1630 °C and porosity develops at 1730 °C. At 1630 °C, for oxidation times below 122 s, the oxide layer remains adherent, after 244 s, it starts to crack and after 320 s, it detaches, showing visible spalling. For the same temperature of 1630 °C, oxide layer thicknesses of 24 and 43 µm were measured after 66 and 124 s of oxidation, respectively.

Balat-Pichelin et al. [16] conducted their studies for temperatures ranging from 1030 to 1630 °C and oxidation times between 50 and 470 s. The results show that the oxidation layer is composed mainly of $TiO_2$ rutile with traces of TiO, $Ti_2O_3$ and $TiO_2$ anatase only for short oxidation times not exceeding 85 s. Oxidation kinetics follow a linear regime, indicating a non-protective, non-adherent oxide layer that rapidly delaminates. These observations highlight distinct oxidation mechanisms in plasma, influenced by the specific conditions of atmospheric re-entry.

In this context, the present study aims to provide a better understanding of the oxidation mechanisms of Ti-6Al-4V in a plasma environment, under specific conditions distinct from those explored by the MESOX set-up. The experiments were carried out in an inductively coupled air plasma in thermodynamic non-equilibrium, an environment with a lower oxygen dissociation rate and higher specific enthalpy. The main objective is to evaluate oxidation kinetics and identify the chemical species formed on the surface, as well as their distribution in the oxide layer, to understand the oxidation mechanism in this plasma environment.

# 2 Material and experimental set-up

## 2.1 Material

The Ti-6Al-4V used in this study was supplied by CGM (Comptoir Général des Métaux). According to the supplier, the material was annealed at 750 °C and air-cooled for 60 min. Its chemical composition, given by Baoji First Titanium Industry (Group) Co, Ltd., is shown in Table 1. The circular samples have a thickness of 3 mm and diameters of 26.5 mm for the plasma-interacting side and 30.0 mm for the opposite side. The surface interacting with the plasma was polished with SiC abrasive papers to grade #2400. The samples were successively cleaned in an ultrasonic bath containing acetone, then in propanol, before being dried. The mass of the samples was measured before and after oxidation using an electronic balance to an accuracy of ±0.5×10⁻³ g.

The X-ray diffraction (XRD) analyses were carried out using an Empyrean diffractometer (PAnalytical) using Co-Kα₁ radiation (λ<sub>Co</sub> = 0.178886 nm) at a voltage of 40 kV and a current of 40 mA. Diffractograms were recorded over a 2θ range of 30-110° with a step of 0.01° and phase identification was carried out using DIFFRAC.EVA (Bruker AXS). The sample is rotated during analysis, to eliminate any textural effects and obtain a representative

<table><caption>**Table 1** Chemical composition in wt% of the Ti-6Al-4V alloy used in this study</caption><thead><tr><th>Ti</th><th>Al</th><th>V</th><th>Fe</th><th>O</th><th>C</th><th>N</th><th>H</th><th>Y</th><th>Residuals</th></tr></thead><tbody><tr><td>Balance</td><td>6.43</td><td>4.10</td><td>0.17</td><td>0.165</td><td>0.02</td><td>0.014</td><td>0.001</td><td>&lt; 0.001</td><td>&lt; 0.040</td></tr></tbody></table>
<figure><img src="image_3.png" /><figcaption>**Fig. 1** Scheme of the SOUPLIN set-up</figcaption></figure>

response from the whole sample. Raman spectroscopy was performed with a Horiba Jobin-Yvon LabRam Aramis spectrometer, equipped with two lasers of wavelengths 632.8 nm and 532 nm. The laser beam was focused on the sample using a 100× objective, giving a spot size smaller than 1 µm. Finally, Scanning Electron Microscopy coupled to Energy-Dispersive X-ray Spectroscopy (SEM-EDS) was carried out using a JEOL JSM-7900F microscope with a voltage of 15 kV and a current of 2 nA.

## 2.2 Experimental set-up

The SOUPLIN (SOUfflerie à PLasma INductif) set-up at the CORIA laboratory, shown schematically in Fig. 1, is a wind tunnel equipped with an inductively coupled plasma torch. The pumping system provides a vacuum pressure of

around 100 Pa and an air plasma is generated in a pressure range from 1300 to 20,000 Pa. The torch is powered by a TEKNA System 105 radio-frequency source operating at a frequency of 1.8 MHz, enabling a maximum power of 60 kW to be injected into the plasma. The electromagnetic field produced by the coil surrounds a quartz tube and inductively heats the injected air at a rate of 2.4 g s⁻¹, generating an air plasma with a specific enthalpy from 5 to 15 MJ kg⁻¹. The plasma is then expanded in a wind tunnel, where the plasma jet interacts with the sample. The sample is positioned on a sample holder with a SiC cap and a K-type thermocouple, held in contact with the back of the sample by a spring, is used to measure its temperature. Finally, an aluminum foil is placed in the wind tunnel beneath the sample holder to collect any oxide particles delaminated during interaction with the plasma or during cooling of the sample. For more details on the

<figure><img src="image_4.png" /><figcaption>**Fig. 2** Temporal evolution of the temperature at the back of the samples</figcaption></figure>
**Fig. 3** Temporal evolution of the variation in surface mass of oxidized samples, the PROMES laboratory results come from the study by Balat-Pichelin et al. [16]. Atomic oxygen density values were obtained by Two-photon Absorption Laser-Induced Fluorescence (TALIF) for the SOUPLIN set-up used in this study [18] and by actinometry for the MESOX set-up [14]

<figure><img src="image_5.png" /></figure>

SOUPLIN set-up, further information can be found in the previous studies [17, 18].

# 3 Experimental results

Air pressure and specific enthalpy are two key parameters for defining plasma conditions during atmospheric re-entry of a space object. The aim of this study is not to reproduce specific points in the trajectory of a space object, but rather to investigate the oxidation of the material under significantly different conditions. Two operating points were

therefore selected: (1400 Pa, 4.6 MJ kg⁻¹) and (4500 Pa, 7.3 MJ kg⁻¹). For the first point, samples were exposed to the plasma for durations of 30, 60, 90, and 120 s. For the second point, exposure times were 30, 60, and 90 s. Exposure times were limited to 120 s for the first point and 90 s for the second, as longer exposure times cause considerable damage to the sample holder.

## 3.1 Temporal evolution of temperature

During oxidation tests, the sample remains in interaction with the plasma for the duration of the oxidation, and

<figure><img src="image_6.png" /><figcaption>**Fig. 4** Pictures of unoxidized and oxidized Ti-6Al-4V samples</figcaption></figure>

<figure><img src="image_7.png" /></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 5 Diffractograms with Miller indices (hkl) of oxidized Ti-6Al-4V samples at 1400 Pa and 4.6 MJ kg⁻¹</figcaption></figure>

plasma operation is stopped once the target time has been reached. The temporal evolution of sample temperature for different oxidation conditions is shown in Fig. 2. The results show that for identical plasma operating conditions, sample temperature rise is reproducible. For oxidations performed at 4500 Pa and 7.3 MJ kg⁻¹, it takes 50 s of plasma interaction for the sample to reach a stable maximum temperature, measured at 1150 °C. After 30 s of plasma interaction, the maximum temperature of the sample does not exceed 900 °C. For oxidations carried out at 1400 Pa and 4.6 MJ kg⁻¹, the temperature rise is slower and does not stabilize, even after 120 s. These results show that sample oxidation does not take place under isothermal conditions, in contrast to studies carried out in furnaces or in the PROMES laboratory, where the temperature is stationary. These observations are essential for a better understanding of oxidation mechanisms in an inductively coupled plasma environment.

Taking into account the thermal conductivity [19] and emissivity [20] of Ti-6Al-4V as a function of temperature, as well as the plasma flux density estimated from the specific enthalpy, the temperature difference between the front and back faces of the sample is estimated to be of the order of a few tens of degrees Celsius at the maximum temperature of the back face. It suggests that measuring the temperature of the back face provides a reasonable estimate of the temperature of the front face.

## 3.2 Weight gain measurements

Under isothermal conditions, oxidation kinetics can be described by the following simplified equation:

$$ \left( \frac{\Delta m}{S} \right)^n = kt, \qquad (1) $$
where $\Delta m$ is the mass gain, $S$ is the surface area of the sample exposed to the plasma, $k$ is the oxidation rate constant, $t$ is the oxidation time, and $n$ is an exponent.

The variation in sample mass as a function of plasma exposure time is shown in Fig. 3. The mass measured after oxidation takes into account the oxide formed on the sample as well as the oxide that may have detached from the surface during the test or during the handling of the sample. For the same oxidation time, the variation in mass for conditions (4500 Pa, 7.3 MJ kg⁻¹) is significantly greater than for (1400 Pa, 4.6 MJ kg⁻¹).

Although the tests were not carried out under isothermal conditions, the experimental points were fitted with Eq. 1 to determine $k$ and $n$ values specific to these oxidation conditions, and therefore not comparable with the other studies. The calculated values, plotted in Fig. 3, indicate a higher $k$ rate constant for the condition (4500 Pa, 7.3 MJ kg⁻¹). Values of $n$ close to 0.5 indicate that the severity of oxidation increases with the time when the material is exposed to the plasma under the two conditions tested. As the rate of oxidation reactions increases with temperature, these results are in accordance with the maximum temperatures reached on the material according to plasma condition and exposure time (Fig. 2).

The mass changes measured in this study are close to those obtained by Balat-Pichelin et al. [16] for similar oxidation times using the MESOX set-up at the PROMES laboratory. However, in the PROMES study, oxidation conditions are different, since a higher, stationary temperature is reached rapidly, atomic oxygen density is higher, and specific enthalpy is lower.

## 3.3 Visual observations

Images of oxidized Ti–6Al–4V samples, shown in Fig. 4, reveal significant delamination of the oxide layer from 60 s for both oxidation conditions tested. However, when the substrate/oxide interface has cracked during the cooling, the oxide remains on the surface until it spalled during sample handling. The Pilling–Bedworth ratio, $R_{BP}=V_{m, oxide}/V_{m, substrate}$ considering $TiO_2$ rutile oxide and TA6V, is 1.7, indicating that the oxide layer is in compression. Oxide spalling is therefore the result of this compressive stress, which is greatly accentuated during the cooling of the material due to the difference in the coefficient of thermal expansion between the Ti–6Al–4V alloy and the oxides formed on the surface.

For similar cooling rates which is the case here, as shown in Fig. 2, the critical temperature drop required for spalling is inversely proportional to oxide thickness [21]. As the oxide layer thickens with increasing oxidation time, this critical temperature drop decreases, making spalling more probable during cooling for longer oxidation time.

## 3.4 XRD phase identification

Diffractograms of Ti–6Al–4V samples before and after oxidation under the conditions (1400 Pa, 4.6 MJ kg⁻¹) are shown in Fig. 5. The diffractograms of the unoxidized sample show the diffraction peaks of the solid solution $\alpha$Ti (PDF 89-2762) of HCP structure, which is the stable phase of titanium at room temperature. The weak peak at 46.1° corresponds to neither $\alpha$Ti nor $\beta$Ti. It was attribute to an

<figure><img src="image_9.png" /><figcaption>**Fig. 6** Diffractograms of the substrate/oxide and plasma/oxide interfaces of an oxide scale detached from the surface of a Ti–6Al–4V sample oxidized 90 s at 4500 Pa and 7.3 MJ kg⁻¹</figcaption></figure>
Al₃Ti peak (PDF 65-7847), even if it was not expected with the composition of Ti–6Al–4V.

After 30 s of oxidation, the oxide layer is not thick enough to be detected by XRD and only the substrate peaks are visible. Additional diffraction peaks appear on the diffractograms after 60 s of oxidation. The position of these peaks indicates the formation of rutile TiO₂ (PDF 65-0191) on the surface of the material. The relative intensity of the TiO₂ diffraction peaks varies from one exposure time to another, which can be explained by the more or less pronounced spalling of the sample surface.

Two characteristic TiN nitride weak diffraction peaks (PDF 65-5774) appear at 43.1° and 50.2°. TiN formation has not been observed in studies of Ti–6Al–4V oxidation in air, even if another nitride with a different stoichiometry has already been identified by Buckner [12]. Its presence here could be explained by the highly dissociated nature of the plasma environment, which favors reactions involving atomic nitrogen.

For the second plasma condition (4500 Pa, 7.3 MJ kg⁻¹), TiO₂ in rutile form as the main phase and the peaks attributed to TiN are also observed. However, TiO₂ diffraction peaks can be detected as early as 30 s, and TiN as early as 60 s, confirming more intense oxidation in this plasma condition.

DRX analyses were also carried out on both sides of an oxide scale detached from the surface: plasma/oxide and substrate/oxide interfaces. Figure 6 shows the diffractograms obtained on a Ti–6Al–4V sample oxidized for 90 s at 4500 Pa and 7.3 MJ kg⁻¹. On the external face of the scale at the plasma/oxide interface, TiO₂ rutile diffraction peaks are mainly distinguishable, as well as weaker diffraction peaks of the ternary oxide Al₂TiO₅ (PDF 41-0258). At the substrate/oxide interface, only TiO₂ rutile is detected.

Al₂TiO₅ is not observable on the surface of the sample, as the DRX analyses were performed on the whole sample, because a high proportion of the oxide layer was detached from the surface.

## 3.5 SEM-EDS and Raman analyses of the plasma/oxide and substrate/oxide interfaces

As the surface of the samples was highly scaled after oxidation, surface analyses by SEM-EDS and Raman spectroscopy focused on two zones: one where the oxide was still adhering to the substrate, and another where the oxide had detached from the surface. This makes it possible to compare the morphologies and chemical composition of the oxides formed at the plasma/oxide and substrate/oxide interfaces.

Figures 7a and 7b show SEM images of the surfaces of Ti–6Al–4V samples oxidized at 1400 Pa and 4.6 MJ kg⁻¹ for, respectively, 90 and 120 s. At the plasma/oxide interface,

<figure><img src="image_10.png" /><figcaption>Fig. 7 SEM image of the surface of the Ti–6Al–4V sample oxidized a 90 s and b 120 s at 1400 Pa and 4.6 MJ kg⁻¹</figcaption></figure>

<table><caption>Table 2 EDS analysis in at% of oxide at the plasma/oxide and substrate/oxide interfaces of the Ti–6Al–4V samples oxidized 90 and 120 s at 1400 Pa and 4.6 MJ kg⁻¹</caption><thead><tr><th></th><th></th><th>O</th><th>Ti</th><th>Al</th><th>V</th></tr></thead><tbody><tr><td rowspan="2">90 s</td><td>Plasma/oxide interface</td><td>63.5</td><td>20.4</td><td>15.3</td><td>0.8</td></tr><tr><td>Substrate/oxide interface</td><td>69.4</td><td>24.5</td><td>5.2</td><td>0.9</td></tr><tr><td rowspan="2">120 s</td><td>Plasma/oxide interface</td><td>64.0</td><td>21.7</td><td>13.4</td><td>0.9</td></tr><tr><td>Substrate/oxide interface</td><td>71.2</td><td>21.8</td><td>6.3</td><td>0.7</td></tr></tbody></table>

oxides grain size increases from 2–7 for 90 s to 5–20 µm for 120 s, while at the substrate/oxide interface, it increases from 0.5 to 2 to 1–5 µm. In addition, at the plasma/oxide interface, the grains appear more irregular and present greater morphological variability with longer oxidation time. EDS analysis of both samples (Table 2) shows that the chemical
<figure><img src="image_11.png" /><figcaption>**Fig. 8** Raman spectrum of plasma/oxide and substrate/oxide interfaces of Ti-6Al-4V sample oxidized 90 s at 1400 Pa and 4.6 MJ kg⁻¹</figcaption></figure>

compositions of the plasma/oxide and substrate/oxide interfaces remain unchanged between 90 and 120 s of oxidation. Aluminum concentration is higher at the plasma/oxide interface, while vanadium is present in very small quantities in

<figure><img src="image_12.png" /><figcaption>**Fig. 9** SEM cross-section image of an oxide scaled from the Ti-6Al-4V sample oxidized 60 s at 4500 Pa and 7.3 MJ kg⁻¹, plasma/oxide interface (left) and substrate/oxide interface (right)</figcaption></figure>

the oxide layer at both interfaces, indicating that it does not contribute significantly to oxide formation. Furthermore, no visible traces of vanadium containing oxide were detected by XRD and Raman analyses likely because the quantities formed were too small to be detected. Vanadium could also dissolve in titanium oxide without significant shifting the Raman peaks of TiO₂ [22, 23]. The most stable vanadium oxide, V₂O₅, has a melting point of 678 °C [24]. Given the high temperatures reached, it is possible that a part of the vanadium oxide vaporized along oxidation.

Raman spectroscopy analyses were carried out on these two surfaces. Representative spectra are shown in Fig. 8. The spectrum recorded at the plasma/oxide interface shows intense peaks whose positions (140, 234, 444 and 609 cm⁻¹) are characteristics of the TiO₂ rutile [25] already highlighted by DRX. Two other peaks with Raman shifts of 1370 and 1400 cm⁻¹ indicate the presence of alumina α-Al₂O₃ [26] in the oxide layer, which is consistent with the higher proportion of Al detected by EDS at this interface. The Raman peak located around 800 cm⁻¹ was previously observed by Dymshits et al. [27] in aluminotitanate and attributed to Ti-O vibrations.

α-Al₂O₃ is detected by Raman in the outer surface of oxide layer for all oxidized samples, except for the one oxidized at 90 s, 4500 Pa, and 7.3 MJ kg⁻¹ in which Al₂TiO₅ is formed. The formation of Al₂TiO₅ requires a temperature higher than 1140°C [28], which is exceeded when the oxidation is performed 90 s with the highest specific enthalpy. For this condition, the presence of aluminotitanate on the outer surface of the oxide layer is consistent with the detection of Al₂TiO₅ by XRD.

<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /><figcaption>**Table 3** Oxide scale thickness of samples oxidized at 4500 Pa and 7.3 MJ kg⁻¹</figcaption></figure>

<table><thead><tr><th></th><th>30 s</th><th>60 s</th><th>90 s</th></tr></thead><tbody><tr><td>Oxide scale thickness</td><td>7 µm</td><td>26 µm</td><td>57 µm</td></tr></tbody></table>

At the substrate/oxide interface, the intensity of the Raman peaks is very low, indicating that the oxide thickness is small and that the breaking of the oxide layer during cooling therefore takes place close to the substrate/oxide interface. Only the TiO₂ rutile peaks are visible at this interface.

The TiN phase detected by XRD after 90 s of plasma exposure under the conditions (1400 Pa, 4.6 MJ kg⁻¹) is not highlighted on the Raman spectra due to its low proportion according to the intensity of the diffraction peaks and due to the characteristics of TiN Raman peaks which are broad, weak, and overlap with those of rutile TiO₂ [29].

## 3.6 SEM-EDS analysis of oxide scale cross-section

Since XRD and Raman analyses probe just a few micrometers beneath the analysis of the entire oxide layer requires further analysis of the section. The fragility of the oxide layer at the substrate/oxide interface made it impossible to preserve the oxide layer on the substrate surface using the conventional coating techniques. On the spalled parts of the material, the oxide layer still adhering to the substrate is very thin, in the order of 1 µm. Analyses of oxide layers are therefore carried out directly on oxide scales detached from the substrate.

In Fig. 9, an SEM cross-section image of the oxide scale of the Ti-6Al-4V sample oxidized for 60 s at 4500 Pa and 7.3 MJ kg⁻¹ is presented. This observation is complemented by an EDS analysis (Fig. 10). The SEM image reveals columnar grains with porosity, suggesting a fragile structure likely to favor delamination. EDS analysis reveals significant fluctuations in atomic quantifications. Two distinct areas are clearly identifiable:

<figure><img src="image_15.png" /><figcaption>**Fig. 11** Scheme of the oxide layer structure for Ti-6Al-4V samples oxidized at less than 1150°C (left) and at more than 1150°C (right)</figcaption></figure>
* Area 1 (0 to 16 µm): Rich in O, Ti, and Al, this area can be correlated with the phases identified above as being mainly TiO₂ with a smaller proportion of Al₂O₃.

* Area 2 (16–26 µm): Rich in O and Ti, this area corresponds to TiO₂ in rutile form.

The oxide scale thicknesses of samples oxidized at 4500 Pa and 7.3 MJ kg⁻¹, measured by SEM analysis, are presented in Table 3. Although the oxidation conditions are different, for similar oxidation times, the measured oxide scale thicknesses are close to those of Prévereaud et al. [15]. Nitrogen was detected, but since its quantification is subject to significant errors in EDS analysis, it was not considered in these analyses.

## 4 Conclusion

The oxidation of Ti–6Al–4V alloy in an air plasma generated by the SOUPLIN set-up at the CORIA laboratory was studied for two distinct operating points: (1400 Pa, 4.6 MJ kg⁻¹) and (4500 Pa, 7.3 MJ kg⁻¹). The variations in mass per unit area measured are comparable in order of magnitude to the results obtained by Balat-Pichelin et al. [16] in the MESOX set-up. However, this comparison must take into account that there are significant differences between the two set-ups, including plasma type, atomic oxygen density, sample temperature, and specific enthalpy.

Based on surface and cross-sectional analyses of the samples, the oxide layer structure shown in Fig. 11 was determined. Rutile TiO₂ is identified as the main oxide in the whole thickness. Al₂O₃ is observed in smaller proportions in the outer layer oxide, at the plasma/oxide interface for all samples oxidized at temperatures below 1150°C. At temperatures around 1150°C, the Al₂TiO₅ phase resulting from the reaction between TiO₂ and Al₂O₃ appears at the plasma/oxide interface. Finally, TiN nitride detected by XRD may be present throughout the oxide layer in very low proportions.

**Author contributions** A.B. wrote the main text, took part in all the experiments and processed all the results. S.J. participated in the choice of oxidation conditions, as well as in the analysis and interpretation of all oxidation results. D.G. participated in all SEM-EDS experiments at the GPM laboratory. P.B. was involved in the choice of oxidation conditions, as well as in all the oxidation experiments carried out in the SOUPLIN set-up at the CORIA laboratory. All authors reviewed the manuscript.

**Funding** Open access funding provided by Région Normandie and Métropole Rouen Normandie.

**Data availability** No datasets were generated or analyzed during the current study.

## Declarations

**Conflict of interest** The authors declare no competing interests.

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