# Exploring the impact of carbon fiber surface chemistry on the structure of their alumina-silica protective coatings: A comparative study of high resistance, medium and high modulus fibers

Elise Des Ligneris, Diane Samélor, Abderrahime Sekkat, Claudie Josse, Jean-Bernard Ledeuil, Alessandro Pugliara, Constantin Vahlas, Brigitte Caussat

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Elise Des Ligneris, Diane Samélor, Abderrahime Sekkat, Claudie Josse, Jean-Bernard Ledeuil, et al. Exploring the impact of carbon fiber surface chemistry on the structure of their alumina-silica protective coatings: A comparative study of high resistance, medium and high modulus fibers. Carbon, 2025, 238, pp.120235. 10.1016/j.carbon.2025.120235. hal-05022681

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# Exploring the impact of carbon fiber surface chemistry on the structure of their alumina-silica protective coatings: A comparative study of high resistance, medium and high modulus fibers

Elise des Ligneris<sup>a,∗</sup> <sup>●</sup>, Diane Samélor<sup>a</sup>, Abderrahime Sekkat<sup>b,●</sup>, Claudie Josse<sup>c</sup>, Jean-Bernard Ledeuil<sup>d</sup>, Alessandro Pugliara<sup>a,c</sup>, Constantin Vahlas<sup>a</sup>, Brigitte Caussat<sup>b,●</sup>

## ARTICLE INFO

### Keywords:

- Carbon fibers
- High resistance
- High modulus
- Alumina-silica thin films
- Environmental barrier coatings
- ALD

## ABSTRACT

Due to the ease of supply, it has become usual for investigations of carbon fiber coatings to be carried out on high resistance carbon fibers, for intended applications at high temperatures in air. Yet, medium modulus and high modulus carbon fibers, for which carbon fiber coatings serves further purpose or are initially designed for, present significant variation in surface roughness and graphite plane stacking from their HR counterparts. As such, this study focuses on the transposition of the deposition of an environmental barrier coating onto HR, MM and HM carbon fibers, and further compares the coated fibers following a thermal cycle. To do so, a 20 nm bilayer thin film of amorphous alumina/alumina-silica was deposited using both atomic layer deposition (ALD) and catalytic ALD, and further annealed in inert atmosphere at 1100 °C. It was demonstrated that the HR fibers required a prior annealing to outgas the fiber core, while residual elements in the surface region of HR and HM fibers impeded the integrity of the film. Furthermore, the density of grooves at the carbon fiber surface was shown to hold substantial impact on the kinetics of crystallisation of the thin films, being determinant to the fiber surface O/C atom ratio.

## 1. Introduction

Considerable research has been devoted to the tackling of thermal oxidation of carbon materials to extend their lifespan in extreme temperatures, paving the way for novel high value-added applications [1–6]. Exploiting chemical inertness, creep resistance, microwave absorption capability, high modulus and compression strength, carbon fibers (CFs) and carbon-carbon composites C/C pieces were previously reported to hold promising potential for applications in high temperature composites for atmosphere re-entry in space vehicles, in barrier layers against fuel contaminants in aircraft engines, as well as in radar for submarine engines [1–6]. As such, investigations of deposition of refractory oxygen diffusion barrier layers (r-OBLs) on carbon materials (also referred to as themal barrier coatings (TBCs), environmental

barrier coatings (EBCs) and hot corrosion barriers (HBCs)), as a protection from carbon degradation above 500 °C in air, flourished [1,2,7]. In recent years, the protection of individual carbon fibers via thin film deposition received particular attention, yearning to enable oxidation protection while preserving the fiber mechanical properties [8–12]. Nonetheless, the ease of supply of high resistance (HR) carbon fibers, prompted their use as a surrogate of medium and high modulus (MM/HM) carbon fibers on investigations of r-OBLs deposition and performance [8–13]. However, HR carbon fibers, obtained after significantly lighter graphitization treatment than MM and HM fibers, vary in fiber chemistry and surface roughness [14–18]. Therefore, the extent of the righteousness of the transposition of thin film investigation results performed on HR carbon fibers onto MM and HM carbon fibers remains undefined.

<footnote><sup>a</sup> Centre Interuniversitaire de Recherche et d'Ingénierie des Matériaux CIRIMAT, Toulouse INP, Université de Toulouse 3 Paul Sabatier, CNRS UMR 5085, 4 allée Emile Monso, 31030, Toulouse, France</footnote>

<footnote><sup>b</sup> Laboratoire de Génie Chimique LGC, Toulouse INP, Université de Toulouse 3 Paul Sabatier, CNRS UMR 5503, 4 allée Emile Monso, 31030, Toulouse, France</footnote>

<footnote><sup>c</sup> Centre de Microcaractérisation Raimond Castaing, Toulouse INP, Université de Toulouse 3 Paul Sabatier, INSA Toulouse, CNRS UAR 3623, 3 rue Caroline Aigle, 31400, Toulouse, France</footnote>

<footnote><sup>d</sup> Institut des Sciences Analytiques et de Physicochimie pour l'Environnement et les Matériaux IPREM, Université de Pau et des pays de l'Adour, CNRS UMR 5254, 2 avenue du président Angot, 64053, Pau, France</footnote>

<footnote>* Corresponding author.
E-mail address: elise.desligneris@toulouse-inp.fr (E. des Ligneris).</footnote>
The most often reported thin film deposition techniques for OBLs on 3D high aspect-ratio carbon-based surfaces are chemical gas-phase deposition methods, and among them atomic layer deposition (ALD) [5,19–21]. The ALD of a metal oxide typically relies on the sequential exposure of the substrate surface to a metal precursor in a first half-cycle followed by an oxidant precursor in a second half-cycle [22]. The anchorage of the first precursor to specific surface sites and the co-reaction of all of its remaining ligands with the second precursor ensures the self-termination of the reaction cycle, conferring atomic-scale control of the deposited structure and a conformal aspect of deposition [22]. Abundantly reported in the field of gas diffusion barrier layers, alumina-silica films combine intrinsic properties of low intra-diffusion, heat resistance, and improved toughness [20,23–26]. Furthermore, the use of multilayered or nanolaminated film structures incurs an increase in the tortuosity of the diffusion path [27]. In this perspective, alumina-silica thin films on carbon-based surfaces have received a focus of attention [23,27–30]. Amorphously deposited as a bilayer, the alumina layer at the fiber-film interface prevents from a carbothermal reaction of the carbon fiber with the upper silica layer, which would entail the formation of pore channels upon annealing [9, 31,32]. Furthermore, strong interfacial diffusion within the alumina-silica film results in a monolayer film after annealing, albeit of heterogeneous microstructure, such as a crystalline distribution within an amorphous matrix [33,34]. Thus, thin film surface topology directly determined by the underneath carbon fiber in the case of conformal ALD coatings could hold a substantial impact on in-film crystallisation kinetics. Considering the oxidation-sensitive nature of carbon surfaces, protocols of low-temperature ALD for amorphous alumina deposition were previously reported [23,35]. In the case of silica thin films, protocols of low temperature catalytic ALD involving the regular inter-spersion of catalytic aluminum were recently purposely developed to circumvent the oxidative nature of conventional silica ALD deposition routes [23,28].

With the chemical deposition of a below 0.1 µm thick film on a carbon fiber exhibiting surface roughness in the submicron range, the scope of this article is to define the impactfulness a grade of carbon fiber holds onto the thermostructural behaviour of its above-deposited thin film. As such, this article encompasses the structural investigation of carbon fibers coated with an alumina-silica thin film, as a function of the fiber initial physico-chemistry and coated fiber annealing route. In detail, this article first presents the intrinsic structural variations in carbon fibers as a function of their temperature of graphitization, and their impact onto the growth of the alumina-silica thin film amorphously deposited above by ALD and ALD-derived techniques. Then, the influence of thermal pretreatment of carbon fibers prior-to-deposition as a mean of limitation of thin film blistering events during post-deposition annealing is reviewed. Finally, thin film crystallisation kinetics observed on HR, MM and HM fibers are compared in light of film retraction amplitude and fiber surface anchorage of alumina crystals, respectively.

# 2. Materials and methods

## 2.1. Materials

High resistance carbon fibers were purchased from Sicomin, in a plain woven fabric of Toray HR T300 fibers, fiber count 1K per roving, and fiber diameter of 7 µm. Medium modulus carbon fibers of Hexcel HexTow IM7 and high modulus carbon fibers of Toray M40J were obtained privately, with a fiber diameter of 5 µm and a fiber count of 6 K per roving. Trimethylaluminum (TMA, Al₂(CH₃)₆, 98 % purity) and tris (tert-pentoxy)silanol (TPS, (CH₃CH₂C(CH₃)₂O)₃SiOH, 99.999 %-Si) were purchased from Strem Chemicals.

## 2.2. Carbon fibers desizing and thin film amorphous deposition

MM fibers were purchased without sizing agent. HR and HM carbon fibers were desized following a thermal procedure inside a GERO F-VS 70–200/13 tube furnace in nitrogen atmosphere at 700 °C with a heating ramp of 10 °C.min⁻¹ and gas flow of 0.3 L min⁻¹. Further details can be found elsewhere [35]. The desizing procedure was complemented with a sizing debris cleaning procedure consisting in a 24 h bath in DI water without stirring. The fibers were then dried in oven prior to thin film deposition. Carbon fibers were coated with amorphous alumina-silica thin films inside a thermal mini-ALD® reactor from Modular Flow, with a chamber volume limited below 5 cm³, at a set base pressure of 1.2 Torr. The amorphous alumina deposition was carried out at 120 °C for 50 cycles following the sequence TMA 10 ms/N₂ purge 30 s/H₂O 10 ms/N₂ purge 40 s [35]. The amorphous alumina-silica deposition was carried out at 250 °C for 13 cycles following the sequence TMA 10 ms/N₂ purge 30 s/TPS 100 s/N₂ purge 120 s [28].

## 2.3. Annealing of coated carbon fibers

Coated carbon fibers were annealed using a Gero F-VS 70–200/13 tube furnace equipped with a Leybold vacuum PT50 Turbotronik NT10 vacuum pump for atmosphere preconditioning. Annealing procedures were carried out at a heating rate of 10 °C.min⁻¹ in 0.3 slm N₂ flow, followed by natural cooling. Prior-to-deposition annealing procedures were undertaken following the same protocol.

## 2.4. Material characterization

### 2.4.1. Theoretical added linear mass of the coating to the carbon fiber roving

The volume of the deposited thin film can be approximatively determined mathematically, under the assumption that a carbon fiber can be assimilated to a cylinder, following Equations 1, 2 [9]:

$$V_{\text{film}} = NL\pi((r + t_{\text{film}})^2 - r^2) \quad (1)$$

$$V_{\text{film}} = \frac{m_{\text{film}}}{\rho_{\text{film}}} \quad (2)$$

Where $V_{\text{film}}$ represents the volume of the deposited thin film, N the fiber count per roving, L the roving length in cm, r the fiber radius in cm, $t_{\text{film}}$ the thin film thickness in cm, $m_{\text{film}}$ the thin film mass in g and $\rho_{\text{film}}$ the thin film volumetric mass in g.cm⁻³, respectively.

The thin film mass $m_{\text{film}}$ can thus be expressed as in Equation (3):

$$m_{\text{film}} = \rho_{\text{film}} N L \pi ((r + t_{\text{film}})^2 - r^2) \quad (3)$$

In this theoretical study, bulk material densities from common thermal oxidation barriers were gathered in Table 1.

<table><caption>**Table 1**

Bulk densities of pyrocarbon and common thermal oxidation barrier materials.</caption><thead><tr><th>Material</th><th>Bulk density (g.cm<sup>-3</sup>)</th><th>Reference</th></tr></thead><tbody><tr><td>Pyrocarbon (PyC)</td><td>&lt;2.2</td><td>[36,37]</td></tr><tr><td>SiC</td><td>3.2</td><td>[38]</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>4.0</td><td>[39]</td></tr><tr><td>SiO<sub>2</sub></td><td>2.7</td><td>[40]</td></tr><tr><td>HfN</td><td>13.7</td><td>[41]</td></tr><tr><td>HfO<sub>2</sub></td><td>9.7</td><td>[42]</td></tr><tr><td>ZrO<sub>2</sub></td><td>5.7</td><td>[43]</td></tr><tr><td>MoSi<sub>2</sub></td><td>6.3</td><td>[44]</td></tr><tr><td>TaSi<sub>2</sub></td><td>9.3</td><td>[44]</td></tr><tr><td>Ta<sub>2</sub>O<sub>5</sub></td><td>8.2</td><td>[45]</td></tr><tr><td>BN</td><td>2.1</td><td>[46]</td></tr><tr><td>B<sub>2</sub>O<sub>3</sub></td><td>2.5</td><td>[43]</td></tr></tbody></table>
### 2.4.2. Surface morphology of carbon fibers

The surface morphology of carbon fibers was characterized by scanning electron microscopy using a FIB FEG SEM ThermoFisher HE- LIOS 600i (TLD detector, operating at a voltage of 5 kV, 4 mm working distance). Cross-section images of the coated fibers perpendicular to the fiber axis were realised using the FIB (local C and Pt coating, tilt 52°, ionic column 30 kV, 4.0 mm WD, current intensity 0.79 nA. Thickness measurement of the thin films were realised using a tilt correction). Energy Dispersive Spectroscopy EDS was performed at a voltage of 5 kV, current intensity of 2.8 nA and magnification of x12k.

### 2.4.3. Distribution in graphite states within carbon fibers

The structural chemistry of carbon fibers was characterized by Raman spectroscopy using a Horiba Scientific Labram HR Evolution equipped with an optical microscope ($\lambda_{ex}$ = 532 nm, ND filter 10 %, Objective x100 Long Working Distance, 60s acquisition time, 3 accum- mulations, grating of 1800 g mm$^{-1}$). Samples for Raman spectroscopy were prepared by extraction of individual fibers from the woven fabric, and sticking on a glass slide, with the fibers in perpendicular alignment to the laser beam.

### 2.4.4. Surface composition of carbon fibers

X-ray photoelectron spectroscopy (XPS) measurements were per- formed on a Thermo Scientific K-alpha spectrometer equipped with a 120 nm mean radius Hemi-spherical Analyzer (HSA) and a micro- focused monochromated radiation (Al Ka,1486.6 eV, micro-spot continuously variable from 30 to 400 µm diameter) operating under UHV conditions (residual pressure of 2.10$^{-7}$ Pa). The HSA to surface angle was 90° (TOA angle) so that the photoelectrons were collected within the maximum depth of 1–6 nm according to their Kinetic Energy (range about 1–1400 eV) and the inelastic mean free path of electrons in solid. The X-ray power source was operating at 72 W (12 kV, 6 mA) for a typical 400 µm beam diameter. The spectrometer was fitted with a dry argon glove box directly connected to the fast entry load lock to prevent sample exposure to air and moisture (concentration of H$_2$O and O$_2$ < 1 ppm). Samples were prepared first by careful extraction of a roving from the woven fabric, then by sandwiching the roving extremities between two layers of copper tape on a silicon wafer for optimal electrical connection to the sample holder. All spectra were recorded in the con- stant Pass Energy (PE) mode CAE (Constant Analyser Energy), either for wide high sensitivity survey spectra (PE = 200 eV) or high energy res- olution analyses core peak spectrum (PE = 20 eV) for chemical quan- titative resolved analyses. Charge effects were compensated by the use of a dual beam charge neutralization system (low energy electrons and Ar$^+$ ions flood), which has the unique ability to provide consistent charge compensation. All neutralizer parameters remained constant during the analysis and allowed for the calibration of the C$_{1s}$ peak binding energy for adventitious carbon at 284.8 eV. Each spectrum was fitted using the Casa XPS software (v2.3.25), with a least squares algo- rithm and a Shirley non-linear baseline. The experimental curves fitting was defined by a combination of Gaussian and Lorentzian distributions, in a 70:30 ratio. Quantification was performed using ThermoScientific relative sensitivity factors based on the Scofield cross-sections.

### 2.4.5. Microstructure of alumina-silica thin films

The microstructure of the alumina-silica thin films was investigated by transmission electron microscopy using a TEM JEOL JEM 2100 F, operating at 200 kV with an emission current of 118 µA and current density of 0.8 pA cm$^{-2}$, with the thin film samples casted on a carbon lacey 300 nm mesh nickel grid. To separate the thin film from the carbon fiber, the coated fibers were dispersed in ethanol and placed in an ultrasonication bath, operating at 200 W for 1h.

## 3. Results and discussion

### 3.1. Surface morphology and chemistry of desized HR, MM and HM carbon fibers

Following wet spinning, carbon fibers are synthetised through a process of stabilization, carbonisation up until 1500 °C for HR and further graphitization above 2000 °C to generate HM carbon fibers [47, 48]. Carbon fibers are best described by a turbostratic structure of gra- phene layer stacks aligned along the fiber axis and showing after car- bonisation a significant variation in stacking height and lateral extension of the graphene sheets [49]. Graphitization further enables to decrease the degree of disorder in the carbon fibers to generate a porous graphitic structure with three dimensional order from a sheath of gra- phene layers stacks to a mixed matrix of oriented graphene nanoclusters forming networks between the fiber sheath and core [47,48]. Concern- ing the process-structure relationships of carbon fibers, tensile modulus is reported to linearly increase with graphitization temperature, while a maximum tensile strength is obtained following carbonisation but prior to graphitization [48]. In detail, a lower interlayer spacing between the graphene sheets and higher order of preferred orientation improved the tensile strength with increasing carbonisation treatment. However, a threshold in temperature is defined at 1500 °C from which among concurring events the decrease in the N/C atom ratio and decrease in transverse bonds between the stacked graphene layers entail a decreasing tensile strength [48].

The structural changes undergone by carbonisation and graphitiza- tion induce a degree of variation in surface topography of carbon fibers, related directly to the post-treatment temperature. The striated surface morphology and chemistry of desized and surface-cleansed HR, MM and HM carbon fibers used in this study is presented in Fig. 1, through comparative SEM images of the fiber local surface roughness as depicted in the insert and corresponding XPS spectra. The XPS O1s core peak spectrum, quantification survey as well as the comparative XPS spectra and quantification survey of sized fibers are available in Supporting Information (Figs. S1–S2, Table S1). The comparative Raman scattering shown in Supporting Information (Fig. S3) was found consistent with previous reports of low temperature below 1500 °C carbonized fibers for HR and MM fibers and below 2000 °C for HM fibers [50].

The comparative morphology of the fiber surface shown in Figure 1 (a–c) showcases the amplitude of fiber surface smoothing between HR and MM/HM fibers. The HR fiber surface grooves were found of in average 200 ± 60 nm in width, with significant inter-groove spacings capped below 250 nm. In comparison, the graphitized MM and HM fi- bers showed well-aligned grooves with negligible inter-groove spacings and of 100 ± 30 nm and 70 ± 20 nm average width, respectively. The O/C atom ratio at the surface of the desized and surface-cleansed fibers presented in Fig. 1 (d) confirmed a steady increase as the fiber post- treatment temperature increased, from 10 % on HR to 22 % on HM. It should be noted that the fibers underwent a DI water bath to cleanse their surface from sizing debris prior measurement. Concerning residual elements at the fibers surface, the XPS survey revealed the presence of boron and nitrogen in HR fibers in a B:N ratio of 2:1. In the case of MM fibers, no boron was detected but a significant proportion in nitrogen was found. Concerning HM fibers, boron and sulfur were detected albeit with a concentration below 1 at%. The C$_{1s}$ spectra shown in Fig. 1 (e) revealed typical graphitic structures with $\pi$- $\pi^*$ scattering and a decreasing sp$^2$:sp$^3$ ratio with increasing post-treatment temperature [51, 52]. A narrower C$_{1s}$ main peak was observed as the fiber module increased, consistent with an alignment of the graphitic planes [50,53]. Similarly, the distribution in carbon states was found to narrow as the fiber state of graphitization increased [54]. Concerning thin film depo- sition, the significant decrease in the inter-groove spacing between HR fibers and MM/HM fibers can have an impact on the threshold in film thickness required for a cohesive film [28]. While the decrease in N content between HR and HM fibers incurred by volatilisation during
<figure><img src="image_3.png" /><figcaption>**Fig. 1. Morphology and surface chemistry of desized HR, MM, and HM carbon fibers.** SEM images of the surface of HR (a), MM (b), HM (c) carbon fibers, with pictogram of the zone of view in inset. Comparative XPS survey (d) presenting the O/C atom ratio and the concentration of residual elements of B, N, and S. Comparative XPS C1s region scan (e), with I, II, III, IV representing the contribution of ethers, carbonyls, carboxyls, and carbonates, respectively.</figcaption></figure>

graphitization is consistent with the literature, the higher N/C atom ratio of MM fibers can be explained by their different manufacture compared to HR and HM fibers [54]. The increase in O/C atom ratio is overall consistent with the observed increasing density of fiber surface grooves with increasing post-treatment temperature. Indeed, the grooves exhibited at the fiber surface are rich in electronic defects with edges of graphitic planes, hence the increase in surface hydroxyls groups

upon exposure to air [28,35]. Thus, despite the apparent surface smoothing, the surface of higher modulus carbon fibers is bound to enable the nucleation of a thin film via a chemical-based deposition process as that of high resistance fibers.

As carbon fibers are mainly used bundled into carbon fiber rovings, Fig. 2 illustrates an impact thin film deposition holds onto the added linear mass to the roving, as a function of thin film thickness, density,

<figure><img src="image_4.png" /><figcaption>**Fig. 2. A perspective on the extent of linear mass addition to the roving from thin film deposition.** Relationship between film characteristics of thickness and density and the resulting weight addition on the carbon fiber roving, expressed in linear mass and applied to the two rovings characteristics used in this study: namely 1k fiber count of 7 µm diameter (a), and 6k fiber count of 5 µm diameter (b). The density of pyrocarbon was set at 1.6 g cm⁻³.</figcaption></figure>
and characteristics of the rovings used in this study. The added linear mass is further benchmarked against common composition of thermal oxidation barriers, as well as to pyrocarbon, a usual fiber-matrix interphase in composites. Among the approximations made to realise this Figure, carbon fiber volume was assimilated to that of a cylinder, and material densities were based on bulk material data.

One aspect potentially impactful in the thin film design lies in the fiber count per roving and the linear mass added to the roving as a function of film thickness for a given material density. Whilst rovings of 1k and 6k fiber count were used in this study, respectively shown in Fig. 2 (a) and in Fig. 2 (b), other rovings with a fiber count over 10k are commonly available [55]. That is, the required film thickness to ensure a dense coverage over the carbon fiber surface ridges and in the inter-groove spacings could be put in perspective with the added linear density to the roving. In this view, the roving fiber count could hold a determinant impact on the permissible density and hence composition of the thin film. In previous works, an alumina minimal film thickness of 20 nm was reported to ensure dense self-standing alumina microtubes upon thermal oxidation of the HR fiber underneath [35]. However, the brittleness of crystalline alumina thin films on flexible carbon fibers upon annealing in inert atmosphere prompted the combined use of a vitreous phase, such as alumina-silica [28,35]. Compared to a pyrocarbon composite interphase in a thickness range below 100 nm and of a density below 2.2 g cm⁻³, set at 1.6 g cm⁻³ in Fig. 2, 20 nm film thickness of alumina-silica composition represents a smaller linear mass addition [36,37]. Furthermore, alumina-silica possesses a bulk density

in average inferior to other common thermal oxidation barriers compositions, taking further into account the necessity of a sharp fiber-film interface provided by alumina to prevent from carbothermal reaction [28,31,32]. The extent of pore channel formation within the silica catalytic ALD layer when deposited directly on a HR carbon fiber can be found in Supporting Information (Fig. S3).

## 3.2. Thin film amorphous deposition on HR, MM and HM carbon fibers

Recent advances in research made the conformal deposition of dense amorphous alumina-silica thin films easily achievable, even on oxidation-sensitive, surface ruguous, flexible 3D substrates such as carbon fibers [12,28,35,56]. The comparative morphology of amorphously alumina-silica coated HR, MM and HM fibers is presented in Fig. 3, across a set of SEM images taken along a longitudinal and cross-section axis of the carbon fiber. The amorphous alumina-silica thin film was deposited via a bilayer of 50 cycles alumina ALD and of 13 cycles alumina-silica catalytic ALD processes performed at 120 °C and 250 °C, respectively. Comparative fiber Raman scattering underneath the coating as well as XPS thin film surface composition acquired on coated HR, MM and HM fibers can be found in Supporting Information (Figs. S5–S6, Table S2).

Further than the uniform aspect of deposition retained on all fibers, ALD smoothing by the junction of proximate fiber grooves can be observed on at the surface of HR fibers, shown in Figure 3(a and b) [28, 35]. This observation can also be transposed less significantly to MM

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Variation in carbon fiber morphology induced by ALD coatings. SEM images of surface and cross-section of carbon fibers coated by an amorphous alumina silica thin film, obtained by 50 cycles alumina ALD and 13 cycles silica catalytic ALD, for HR (a, b), MM (c, d) and HM (e, f) fibers, respectively. Orange rectangles guide the eye on deposition defects.</figcaption></figure>
fibers, presented in Figure 3(c and d). Thin film deposition on MM fibers resulted as well in local events of round-shaped nanoparticle deposition and surface groove widening by less than 10 nm, respectively. Nano-particle deposition was also detected at the surface of HM fibers, as presented in Figure 3(e and f). However, surface groove widening and fiber surface smoothing could not be here observed. Among the cross-section images, the uniform aspect of deposition was found improved on smoother fiber surfaces. Average film thickness was found similar on all fibers, estimated at 16 ± 2 nm. However, film thickness variation in the inter-groove pits was found subtlely wider in the case of HR fibers, estimated at 25 ± 4 nm compared to 23 ± 2 nm and 20 ± 2 nm for MM and HM fibers, respectively. Difference in fiber diameter of 7 µm for HR and 5 µm for MM/HM fibers, was not observed to hold significance in the kinetics of deposition. Furthermore, no significant impact of the grade of carbon fiber on the above thin film composition could be monitored from the XPS spectra (Fig. S6).

The limited discrepancies observed on the character of uniform deposition among the fiber grades can within reason be ascribed to the initial state of fiber surface roughness. Furthermore, the SEM analysis confirmed good fiber-film adhesion, regardless of the carbon fiber type, as previously reported in the case of HR fibers [8,9,56]. Nevertheless, the increased O/C ratio was found to prompt local events of silicon oxycarbide particle deposition at fewer number of deposition cycles, as previously observed with unoptimized catalytic ALD protocols [28]. Other than the exhibited O/C ratio, the state of graphitization of the carbon fiber held a minor impact on the irregular growth of the thin film with accelerated kinetics on selected groove edges, incurring a widening capped below 50 % of the deposited film thickness. As such, discrepancies arise when transposing a thin film deposition protocol from HR to MM and HM fibers, albeit of limited significance.

In complement, thin film deposition on HR carbon fibers implies some peculiarities intrinsic to their non-graphitized nature, that are showcased in Fig. 4.

A close-up view of the inter-groove spacing of desized and uncoated HR fibers shown in Fig. 4 (a) revealed aligned bridges in-between two surface grooves. On HR fibers, those bridges can then act as a seed for

particle growth during thin film deposition, as shown in Figure 4(b and c) after 90 nm thick controlled deposition of alumina ALD. Strings of nanoparticle outgrowths aligned parallel to the fiber axis are local events that were observed throughout this study exclusively and regularly on HR fibers. Another specificity of HR fibers concerns the free volume contained within the fiber, in-between the core and sheath, as presented in Fig. 4 (d). Undiscerned on graphitized MM and HM fibers, microsized pores were observed in varying proportions in cross-section images of HR fibers throughout this study. As a major asset of carbon fibers is their applicability in harsh environments and in particular at high temperature, this study was pursued with a focus on the comparative microstructure of the coated fibers obtained after annealing. In the case of HR fibers, the volatilisation of the fiber-contained free volume as well as of residual elements upon annealing is bound to affect the above-deposited film in a different manner compared to their MM and HM counterparts.

### 3.3. Impact of prior-to-deposition annealing of HR and HM fibers on thin film blisters

The morphology of desized HR carbon fibers coated with alumina-silica thin film and annealed post-deposition in inert atmosphere at 1100 °C for 3 h is shown across a set of SEM images in Fig. 5, and further compared with SEM images of desized HR fibers annealed prior to deposition, then coated and post-deposition annealed in the same conditions.

The formation of open blisters in the thin film during annealing in inert atmosphere of coated HR fibers is shown in Fig. 5 (a). Each blister was observed at the center of an oxidation zone, chunking out the underneath fiber and leaving the fiber-imprinted thin film in place. Additional images of the blistering phenomenon and its distribution on HR carbon fibers can be found in Supporting Information (Fig. S7). A cross-section view shown in Fig. 5 (b) further highlights the morphology of the oxidised fiber interior and pinpoint the location of the fiber oxidation onset to the thin film blister. The extent of the oxidation zone, along with the size of the blister were found of narrow distribution along the HR

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Specificities of HR carbon fibers. SEM image of a desized HR carbon fiber showing inter-groove bridges aligned parallel to the fiber surface grooves (a). SEM image of a HR carbon fiber after 100 cycles of alumina ALD showing a string of nanoparticular outgrowths aligned parallel to the fiber surface grooves (b, c). Cross-section SEM image of free volume contained within a HR carbon fiber prior to annealing (d). Orange line in (c) indicate the zone of cross-section of image shown in (d).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Blistering of coated HR fibers after post-deposition annealing. SEM images of an amorphous alumina-silica ALD-coated HR carbon fiber annealed in N₂ at 1100 °C for 3 h showing the fiber surface (a, c) and cross-section (b, d). In the cross-section images, the contrasting layer underneath the coating is a FIB artefact. SEM image of the surface of a HR carbon fiber desized and annealed in N₂ at 1100 °C for 3h (e), followed by alumina-silica coating and post-deposition annealing in the same conditions in N₂ at 1100 °C for 3h (f). Schematics of the outgassing of the HR fiber underneath the amorphous alumina-silica coating provoking a blister and a local oxidation onset from the concomitant dehydroxylation of the coating during the first post-deposition thermal cycle (g).</figcaption></figure>

fibers, of roughly 15 µm and 3.5 µm, respectively. The distribution of blistering events on a HR fiber was estimated to 5 per 100 µm fiber length. Uncracked blisters were also observed, as shown in Fig. 5 (c), showcasing the amplitude of the fiber swelling underneath the coating and the thin film diffusion barrier potential. The absence of variation in contrast indicates the absence of fiber oxidation underneath the coating, albeit around the neat cracks of the thin film, perpendicularly to the fiber axis. A cross-section view presented in Fig. 5 (d) reveals the unaltered surface groove pattern of the fiber beneath the thin film, indicating as well that the concomitant thin film dehydroxylation occurs outwards the fiber. Previous works reported the dehydroxylation of the amorphous alumina layer deposited in the same conditions and subjected to the same thermal cycle towards the conversion in a mixed-phase alumina with minor α-Al₂O₃ [35]. Meanwhile, the low temperature of ALD and the inherent conditions of catalytic ALD implied a significant volume of hydroxyl groups contained within the amorphous

film, as previously reported [28]. In turn, the mild oxidative conditions of thin film deposition enabled for the preservation of the carbon fiber surface [28,35]. A prior-to-deposition annealing of the HR fibers carried out in the same conditions as the post-deposition annealing route in N₂ at 1100 °C for 3h is presented in Fig. 5 (e), to enable for the outgassing prior to thin film deposition. The thermally pretreated fiber showed no decrease in fiber diameter, although its surface was slightly oxidised. While annealed in a inert atmosphere, a limited presence of oxygen in the furnace may have resulted in the surface-oxidised fiber morphology. Following thin film deposition and annealing of the coated fiber, thin film chipping events were observed, as shown in Fig. 5 (f). While the distribution of chipping events was found significantly denser in comparison with blistering events, the average width of a chip in the film was estimated below 2 µm.

It is thus proposed that during the first thermal cycle of the coated fiber, the outgassing from the closed pore volume and residual elements
from the HR fiber migrated to the surface and formed a gas pocket un-
derneath the thin film acting as a diffusion barrier, which blistered from
overpressure. Concomitantly, the outward dehydroxylation of the
amorphously deposited alumina-silica layers generated a local release of
water vapor, which in turn reacted with the outgassed volatile species
from the blister, inducing from the fiber surface an oxidizing chain re-
action, as illustrated in Fig. 5 (g). Indeed, surface-limited pits of carbon
oxidation were observed on the pretreated coated HR fibers after
annealing, in accordance with the lower remaining volume to outgass.

To go further, the impact of prior-to-deposition annealing on HR
carbon fibers was investigated as a function of temperature and dura-
tion, as showcased in Fig. 6. Additional SEM images of varying

annealing pretreatment conditions on HR carbon fibers can be found in
Supporting Information (Fig. S8).

The surface of desized HR fibers annealed in inert atmosphere as a
function of temperature from 400 °C to 1250 °C is presented in Figure 6
(a–d). While no significant variation can be expected at low graphite
annealing temperatures, discrete chipping events were observed at
700 °C, which here corresponds to the oxidation onset of the HR fiber in
air [35]. From 1100 °C, the increase in temperature was shown to
decrease the fiber surface groove width and straighten the alignment of
surface grooves. Indeed, the average surface groove width was estimated
to have decreased by half after 1250 °C annealing compared to the
desized fiber. The thermally pretreated HR fibers were then coated with

<figure><img src="image_8.png" /><figcaption>Fig. 6. Impact of annealing pretreatment conditions of HR fibers on the resulting thin film morphology after annealing. SEM images of the variation in surface morphology of a HR carbon fiber annealed prior to deposition and their resulting morphology following post-deposition annealing, in function of pre-deposition annealing temperature at 400 °C (a, e), 700 °C (b, f), 1100 °C (c, g) for 5 h and 1250 °C (d, h) for 3 h. Fixed post-deposition annealing conditions: 0.3 slm N₂, 10 °C.min⁻¹ heating rate, plateau at 1100 °C for 3 h.</figcaption></figure>
an alumina-silica thin film and further annealed following the same procedure as previous, that is 5 nm of alumina ALD overlayered by 15 nm alumina-silica catalytic ALD, and rising to 1100 °C for 3 h in inert atmosphere, respectively. The surface of post-deposition annealed HR fibers is shown in Figure 6(e–h), as a function of the pre-deposition annealing temperature and duration. Below 1100 °C prior-to-deposition annealing, the HR fiber presented after post-deposition annealing a narrow distribution of gas pockets underneath the coating, growing in size and decreasing in density as the prior-to-deposition annealing temperature increased. Considering 1100 °C for 3 h, prior-to-deposition annealing carried out either at higher temperature and same duration, either at same temperature and longer duration than post-deposition annealing was observed to result in a smooth thin film morphology, concurred by the smoothing of the HR fiber underneath.

In complement of the investigation of the HR coated fiber morphology, Raman scattering was measured for the prior-to-deposition annealed HR fibers, as presented in Fig. 7.

The Raman scattering of annealed HR fibers presented in Fig. 7 (a) revealed a succinct decrease in the position of the D band with increasing annealing temperature, with a Raman shift decreasing from 1355 cm⁻¹ to 1340 cm⁻¹ following annealing at 400 °C and 1250 °C, respectively. The G band scattering position remained at 1590 cm⁻¹, independently of the annealing conditions tested. Annealing plateau duration in the considered range of 1h–5h was found to not hold an impact on the observed scattering. The ratio of intensities between in the D and G bands (I<sub>D</sub>/I<sub>G</sub>) presented in Fig. 7 (b) as a function of annealing temperature shows that the intensity of the D band rises above the intensity of the G band for annealing procedures carried above 1100 °C. The observed growing proportion in graphitic defects is consistent with the findings of the previous morphology study such as the increasing surface groove density with increasing fiber post-treatment temperature.

In all, annealing pretreatment of the HR fibers at a higher thermal budget than the desired temperature of use of the coated HR fibers was found necessary to prevent blistering events. Furthermore, variation in annealing pretreatment conditions led to variation in thin film morphology after annealing of the coated HR fibers. Indeed, a pre-treatment at 1100 °C prior-to-deposition was found necessary to preserve a smooth thin film aspect after annealing at the same temperature. That is, in the case of HR fibers, an interplay between alumina-silica crystallisation and carbon structuration kinetics with increasing annealing temperature could be taking place at the film-fiber interface. To date, little is known of the carbon fiber microstructure exhibited at the fiber surface. For instance, annealing in dry synthetic air at 600 °C of pre-annealed HR fibers at 1250 °C in N₂ resulted in the obtention of an organic shell of the fiber, as shown in Supporting Information (Fig. S9). Aside from a succinct narrowing of the peaks FWHM, Raman studies did

not show significant structural variation of the HR fibers annealed below 1250 °C. This result is consistent with the literature, where 1225 °C corresponds to the conversion of amorphous carbon of mixed bonding into amorphous carbon of sp² bonding exclusively [49]. The succinct narrowing of the D peak with increasing temperature tends to support a decrease in the variety of structural disorder within the HR fiber [48]. Furthermore, TGA analysis of HR fibers annealed at 1250 °C revealed a similar behaviour as of MM/HM fibers, as shown in Supporting Information (Figs. S4 and S9), that is including a mass intake from oxide formation prior to the fiber oxidation onset, which could be attributed to an increasing surface O/C ratio from a widening of the mean lateral extension of sp² carbon crystallites.

For the rest of this study, HR fibers were pretreated by annealing in inert atmosphere at 1100 °C with a 5h plateau duration. In comparison with HR fibers and to confirm the XPS survey of residual elements at the surface of HM fibers, the prior-to-deposition annealing route was transposed on HM fibers, as showcased in Fig. 8. The comparative Raman and XPS scattering of HR and HM carbon fibers as a function of the process route with thermal pretreatment can be found in Supporting Information (Figs. S10–S13, Tables S3–S5).

The surface of desized HM fibers after annealing in N₂ at 1100 °C shown in Fig. 8 (a) revealed the presence of pits of about 1 µm diameter and a depth estimated below 1 µm, with a distribution along the fiber estimated at 5 pits per 10 µm fiber length. An enlarged view of the HM fiber surface shifted away from the suface pit areas shown in Figure 8(b and c) further revealed the presence of irregular surface holes aligned along the fiber grooves, of a varying size capped below 150 nm. Thin film deposition on the prior-to-deposition annealed HM fibers resulted in the deepening of the surface pits caused by carbon thermal oxidation during deposition, as shown in Fig. 8 (d). The surface holes were also found widened and joined, resulting in a morphology of groove disj ointment, as shown in Fig. 8 (e). Furthermore, thin film deposition was observed to result also in the deposition of particles of irregular shape and size, anchored around the edges of the hollow grooves. The subse quent annealing post-deposition resulted in a HM fiber surface pre- senting discontinuous linear outgrowths following the surface groove pattern, as shown in Fig. 8 (f). An enlarged view highlighted the con trasted dotted morphology of the outgrowths and the singularly rough surface texture in-between the outgrowths, as shown in Figure 8(g and h). A cross-sectional view presented in Fig. 8 (i) further showed the amplitude of variation in thin film thickness, as well as the nano-scaled surface roughness of the underneath fiber. In one instance, the ampli tude of thin film deposition smoothing on the HM fiber surface was measured to a thickness of about 75 ± 3 nm for a protocol of deposition of 20 ± 2 nm in film thickness.

With a graphitization treatment carried above 1700 °C, HM fibers exhibit at their surface an ordered structure of graphene sheets stacks [49]. The prior-to-deposition annealing treatment, while enabling for

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Impact of annealing temperature and duration on the structure of HR carbon fibers. Raman scattering of a HR carbon fiber as a function of annealing temperature and duration, from 400 °C to 1250 °C and 1 h–5 h, respectively (a). Corresponding intensity ratio between the scattering of the D and G bands (b).</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Impact of prior-to-deposition annealing on the surface morphology of HM carbon fibers. SEM images of the surface of HM carbon fibers after the steps of prior-to-deposition annealing at 1100 °C/5 h (a–c), alumina-silica thin film deposition on the annealed fibers (d, e), followed by post-deposition annealing at 1100 °C/3 h (f–h). SEM cross-section image of a HM fiber after annealing at 1100 °C/5 h, alumina-silica coating, and annealing at 1100 °C/3 h (i).</figcaption></figure>

the outgassing of residual elements in the fiber, induced the generation of oxidation pits as well as empoverished the quality of thin film deposition. The preferential and accelerated growth of the alumina-silica thin film is possible through the catalytic ALD process. The preferential location along opened surface grooves favors the hypothesis of a significant breakage of the graphene sheets stacks resulting in a wide proportion of edge sites for deposition as well as for migration of alumina crystallites from the sub-surface to the protruding surface region of the HM fibers during post-deposition annealing. For the rest of this study, desized HM fibers were not annealed prior to deposition.

### 3.4. Relationship between carbon fiber grade and thin film microstructure

In comparison with HR fibers shown in Fig. 5(f), the extent of cracks and microvoids formation on the alumina-silica films following post-deposition annealing on desized MM and HM fibers is presented in Fig. 9.

The morphology of alumina-silica-coated MM fibers after annealing in N₂ at 1100 °C is presented in Figure 9(a–d). A partial film retraction event on an estimated 7 µm length of MM fiber can be observed in Fig. 9 (a). While this retraction of the amorphous-like thin film matrix left an area of the MM fiber exposed, the MM fiber was found whole with its surface non-significantly oxidised. Furthermore, crystal structures were still oberved anchored on the MM fiber surface, and had not migrated with the film silica matrix. Smaller film retraction events of width limited below 3 µm, were also observed, as shown in Figure 9(b and c). The shape correspondance along opposite sides of the film rip is particularly noticeable in the image shown in Fig. 9 (c), confirming the retraction motion of the thin film. An enlarged view of the film shown in Fig. 9 (d) confirmed a heterogeneous structure with a narrow size

distribution of spindel and square shaped crystals, both dispersed with no particular alignment in an amorphous-like matrix, respectively. In comparison, the morphology of alumina-silica-coated HM fibers after annealing in N₂ at 1100 °C is presented in Figure 9(e–h). Although of a denser distribution with an estimated 8 holes in the film per 10 µm HM fiber length, the hole average width capped below 500 nm was found significantly lower than that observed on MM fibers, as shown in Figure 9(e and f). An enlarged view shown in Fig. 9 (g) revealed a preferential alignment of these holes parallel to the fiber axis and following an inter-groove alignment. Furthermore, oxidation pits on the exposed HM fiber were observed, as shown in Fig. 9 (h), and consistently with Fig. 8. The film on HM fibers presented a similar microstructure than that on MM fibers, albeit with a significant decrease in crystal size distribution and a higher degree of amorphisation of the film matrix, observed from the impeded image resolution.

To go further, a comparative structural analysis of annealed thin film-coated carbon fibers as a function of the carbon fiber grade is presented in Fig. 10.

The comparative morphology of the annealed thin film on HR, MM and HM fibers is presented in Figure 10(a–c), respectively. Whilst the coated HR fiber still exhibited its surface grooves as previously shown, the fiber topography is barely noticeable underneath the coating of HM fibers, and cannot be noticed underneath the coating of MM fibers. Concomitantly, no crystalline structure could be detected at this scale on HR fibers, whilst the spindel length and width were measured at 200 ± 3 nm and 20 ± 2 nm on HM fibers, and at 570 ± 5 nm and 50 ± 2 nm on MM fibers, respectively. The corresponding cross-section images of the coated and annealed HR, MM and HM fibers are shown in Figure 10 (d–f), respectively. Contrary to post-amorphous deposition, thin film thickness was found smoothed all around the fiber perimeter.
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Comparative extent of crack formation following post-deposition annealing between MM and HM carbon fibers. SEM images of amorphous alumina-silica-coated MM (a–d) and HM (e–h) carbon fibers following post-deposition annealing in 0.3 slm $N_2$ at 10 $^{\circ}$C.min$^{-1}$ up to 1100 $^{\circ}$C for 3 h.</figcaption></figure>

Furthermore, the average film thickness was found of similar values compared to post-amorphous deposition (Fig. 3), with an average of 24, 22 and 19 nm, on HR, MM and HM fibers, respectively. In complement, Raman scattering was acquired on the fibers following post-deposition annealing, as presented in Fig. 10 (g). Raman scattering of the desized fibers can be found in Supporting Information (Fig. S4). The annealing of the HR fibers pre- and post-deposition resulted in the alignment of the D band with MM and HM fibers, at 1348 cm$^{-1}$. Furthermore, a slight shifting trend in the position of the G band could be observed, with the G band peaking at 1592, 1587 and 1582 cm$^{-1}$ for HR, HM and MM fibers, respectively. This indicates a minor variation in the ordered graphitic structure between the fiber grades resulting from the 1250 $^{\circ}$C annealing and supposedly stemming from the different initial fiber thermal processing. Previous Raman studies also reported a G band peak position variation when comparing carbon structures in function of annealing

temperature [18,49,57,58]. Several factors were reported to induce a G band position shift, such as crystallite size or variation in carbon isotopes due to different vibrational energies [18,49,57,58]. Further studies are necessary to conclude about the variation in graphene structural ordonancement between the fiber grades after thermal treatment at 1250 $^{\circ}$C. In addition, surface chemistry of the annealed thin film was investigated by XPS with the $C_{1s}$ region scan presented in Fig. 10 (h). The wide scan survey and other regions core peak spectra can be found in Supporting Information (Fig. S11, Table S4). It is to be noted that the presence of gaps in the coating as previously shown in Fig. 9 enables for a distinct signature from the underneath carbon fiber. A minor scattering contribution peaking at 284.1 eV with a FWHM of 1.1 eV and attributed to metal carbide was detected on all coated fibers, albeit of less significance in the case of MM fiber substrates, accounting for 7 % of the carbon bonds [35]. Another minor scattering contribution
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Comparative morphology of alumina-silica coated HR, MM, and HM carbon fibers following post-deposition annealing. SEM images of the coated and annealed carbon fiber surface and cross-section for HR (a, d), MM (b, e) and HM (c, f) carbon fibers. Comparative Raman scattering (g) and XPS C₁₅ region core peak spectrum (h). In C₁₅ C-M, I, II, III, IV represent the contribution of carbon-metal bonds, ethers, carbonyls, carboxyls, and carbonates, respectively. The HR fiber was pretreated by 1100 °C/5 h annealing prior to deposition.</figcaption></figure>

peaking at 289.0 eV with a FWHM of 1.6 eV and attributed to metal carbonate was detected on HR and HM fiber substrates, respectively accounting for 3.5 % and 6.7 % of the carbon bonds, but could not be detected on MM fiber substrates [28].

Finally, the comparative thin film microstructure deposited on HR and MM fibers was further investigated using TEM, as showcased in Fig. 11. A SEM EDX elemental analysis can be found in Supporting Information (Fig. S14).

The microstructure of the annealed film that was deposited on HR fibers revealed the presence of crystals of no particular shape, albeit of relatively homogeneous size distribution with an average width of 7 nm, as presented in Figure 11(a–c). In comparison, the microstructure of the annealed film that was deposited on MM fibers showed a further advanced stage of ripening with the presence of larger grains attaining 38 nm in width, as presented in Figure 11(d–f). Interestingly, a dense distribution of round-shape porosities of about 2 nm diameter were observed all over the film. Furthermore, an elemental analysis presented in Fig. 11 (g) and performed on the encircled areas shown in Figure 11 (a–d) revealed no presence of silicon in the film that was deposited on MM fibers. Instead, a pure Al/O composition in an atom ratio consistent with Al₂O₃ was obtained. Meanwhile, the EDX performed on SEM shown in Supporting Information (Fig. S12) confirmed the combined presence of aluminum and silicon oxides on all fibers. Thus, the observed porosities in the film that was deposited on MM fibers were ascribed to the departed presence of silica resulting from the TEM sample preparation by ultrasonication. Although the coated HR fibers were subjected to the same ultrasonication procedure, the TEM EDX elemental analysis confirmed the presence of aluminum and silicon oxides, in a proportion in Al:Si:O of 20:18:62 at%, close to an Al₂O₃–SiO₂ composition which

would result in a proportion in Al:Si:O of 25:12.5:62.5 at%. In addition, SAED patterns were acquired over the same area for both films, as presented in Figure 11 (h and i) for annealed films that were deposited on HR and MM fibers, respectively. A polycrystalline structure was observed in both cases, with the measured *d* values indicating a hexagonal alpha-alumina structure exhibiting (104) and (113) plans (JCPDS 46–1212) [59]. With regards to the applied temperature of 1100 °C, those findings are consistent with the formation of alumina crystals within an amorphizing matrix [35].

Carbon fiber physico-chemistry and surface roughness determined by the average surface groove width significantly impacted the kinetics of crystallisation of the alumina-silica film upon annealing. This thus impedes the transposition of thin film protocols developed on HR fibers to MM and HM fibers. With a similar order of initial surface roughness, MM fibers in this case better mimicked the impact of HM fibers on the above-deposited thin film. Furthermore, a threshold in the density of groove at the fiber surface, determinant to the surface O/C ratio, also held a major impact on alumina coalescence kinetics, as can be seen from 65 % crystal size difference between thin films deposited on MM and HM fibers. The film microstructure obtained on MM and HM fibers is consistent in morphology with reports of silicate coatings with high loading of Al [60]. Concomitantly, retraction events from alumina condensation reaction led to the formation of gaps in the film, varying in size according to the underneath fiber grade as a result from variation in alumina in silica crystallisation kinetics [35]. The anchorage of a population of alumina spindels at the fiber surface despite the migration of the film matrix attests of the presence of C–Al bonds at the fiber-film interface, as confirmed by XPS results. The strength of interfacial bonds between alumina and silica was found decreased with the
<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Comparative thin film microstructure following post-deposition annealing for HR and MM carbon fibers. TEM images of the microstructure of the annealed alumina-silica thin film deposited on HR (a–c) and MM (d–f) carbon fibers. For TEM analysis, thin films were separated from the fiber by ultrasonication bath. The circles in images (a) and (d) indicate the zones of acquisition for the EDX spectra shown in (g). SAED patterns for thin films deposited on HR (h) and MM (i) carbon fibers, acquired over a zone encompassing image (a) and (d), respectively.</figcaption></figure>

ripening of alumina crystals as seen from the dissociation of silica from the alumina-silica film on MM fibers. Yet, previous reports of alumina-silica film microstructures reported the obtention of sillimanite following annealing above 1000 °C [61]. Another potential impact of the underneath carbon fiber lies in the intra-coating carbon diffusion, which might affect the kinetics of alumina ALD and alumina-silica catalytic ALD layers interfacial diffusion and prompted a variation in the crystallisation pathway. However, carbothermal reaction with silica within the film is limited in the investigated temperature range, as confirmed by the dense film morphology in the TEM images [62]. Furthermore, albeit differing in size, a similar crystal distribution was observed on all thin films, thus independently of the carbon fiber grade. However, a strong limitation in thermal oxidation performance investigation of protective thin film on carbon fibers stems from the initial fiber grade. Indeed, thermal process engineering can be conducted to prevent from blistering events shown in Fig. 5 and from holes in the coating from alumina-silica volume variation post-annealing shown in Fig. 9, so as to reach a dense and total coverage of the annealed fiber as shown in Fig. 10 for HR fibers. But, the crystallisation of the amorphous and hydroxylated alumina-silica film into a single phase α-Al₂O₃ in a silica matrix here requires a thermal cycle conducted above 1100 °C and below 1300 °C, to avoid for a brittle fully crystalline sillimanite (Al₂O₃ (SiO₄)) or mullite (3 Al₂O₃ · 2 SiO₂) coating [60,63,64]. However, such thermal treatment denaturates the HR fiber as shown in Fig. 7 and in Supporting Information (Fig. S9), having been subjected only to carbonisation or light graphitization processes prior, which in turn impedes the integrity of the fiber surface.

## 4. Conclusions

In all, post-treatment temperature of carbon fibers was found to hold substantial influence on the alumina-silica thin film deposited above by ALD and ALD-derived processes, in terms of uniformity of deposition, film cracking and retraction events, as well as in alumina-silica crystallisation kinetics. The free volume contained within HR fibers incurred the need of a thermal pretreatment of the HR fiber above the desired temperature of operation, thus advancing the turbostratic arrangement of the HR fiber towards a MM structure. Then, residual elements such as boron contained within the HR and HM fibers were found to impede the thin film integrity after a thermal cycle of the coated fibers. Finally, the strong interfacial bonds between fiber surface groove and alumina-silica thin film induced a significant variation in the concomitant kinetics of inside-film crystal ripening and film retraction events, resulting from the carbon fiber initial surface groove density and average width, determinant to the fiber surface O/C ratio and surface roughness, respectively.

### CRediT authorship contribution statement

**Elise des Ligneris:** Writing – original draft, Validation, Methodology, Investigation, Data curation, Conceptualization.
**Diane Samélor:** Validation, Methodology, Data curation.
**Abderrahime Sekkat:** Writing – review & editing, Validation, Methodology, Formal analysis.
**Claudie Josse:** Validation, Investigation, Data curation.
**Jean-Bernard Ledeuil:** Writing – review & editing, Validation, Investigation, Data curation.
**Alessandro Pugliara:** Validation, Investigation, Data curation.
**Constantin Vahlas:** Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition, Conceptualization.
**Brigitte
**Caussat:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Funding acquisition.

## Data availability

Data will be made available on request.

## Declaration of competing interest

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

## Acknowledgements

This work was supported by the French Agency for National Research, through RAPID convention number 2021 29 0911. The authors are grateful for the technical support from the Raimond Castaing Electron Microscopy facilities. The authors are particularly thankful to Oliver Feddersen-Clausen from Modular Flow, Etienne Prévot and Hugues Vergnes from LGC, Daniel Sadowski, Ronan Mainguy, Rémi Garcia, Claire Tendero, Cédric Charvillat, Olivier Marsan, Alexandre Freulon, Francis Maury and Dominique Poquillon from CIRIMAT for scientific and technical discussions.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2025.120235.

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