# Cr2AlC and metals reactivity: Sintering and oxidation

Axel Zuber, Véronique Gauthier-Brunet, Sylvain Dubois

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Axel Zuber, Véronique Gauthier-Brunet, Sylvain Dubois. Cr2AlC and metals reactivity: Sintering and oxidation. Open Ceramics, 2024, 17, pp.100561. 10.1016/j.oceram.2024.100561. hal-04514727

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# Cr₂AlC and metals reactivity: Sintering and oxidation

A. Zuber<sup>a, *</sup>, V. Gauthier-Brunet<sup>a</sup>, S. Dubois<sup>a</sup>

<sup>a</sup> Institut PPRIME, CNRS/Université de Poitiers/ENSMA, UPR 3346, Boulevard M. et P. Curie, TSA 41126, 86073, Poitiers Cedex 9, France

## ARTICLE INFO

Handling Editor: Dr P Colombo

Keywords:

Cr₂AlC

MAX phase

Metal reactivity

Oxidation

High-temperature

Kinetic

Blistering

## ABSTRACT

Cr₂AlC-20 wt% (Sn; Cu; Co; Fe; Ag) powder mixtures were hot isostatically pressed at 1000 °C for 4 h under 150 MPa to investigate the possible alloying behaviour of the metals with the MAX phase. The as-synthesized bulk materials were further oxidized at 1000 and 1200 °C for 10 h under dry air flux to study the effect of the alloying on the oxidation performance of the MAX phase. The metallic elements did not dissolve into the MAX phase grains: they mainly acted as oxidizing agents on the MAX phase grains, draining the aluminum from it to form intermetallic phases. The oxidation tests resulted in the formation of an alumina scale for most of the samples. The oxidation kinetic study shows no improvement of the oxidation behaviour of the Cr₂AlC-metal composites as compared to pure bulk Cr₂AlC. Sn spheres embedded in an alumina layer and empty alumina shells are respectively observed on the surface of the oxidized Cr₂AlC-Sn and Cr₂AlC-Ag composites.

## 1. Introduction

MAX phases are a family of nanolayered carbides and nitrides with the general formula $M_{n+1}AX_n$ (n = 1, 2, 3) discovered in late 70's and widely studied since the early 2000's for their unique properties cumulating those of ceramics and metals [1,2]. The oxidation behaviour of these materials has been studied since 2005, Cr₂AlC being considered as one of the most promising MAX phases for its corrosion resistance properties [3]. Oxidation of bulk Cr₂AlC in dry and wet air has been studied from 700 °C to 1400 °C for various oxidation times, the most common observation being the formation of a continuous α-Al₂O₃ layer and a Cr₇C₃ sub-layer [4–8]. During high-temperature phenomena such as the high-temperature oxidation, the activated atomic diffusion can lead to the formation of new phases at interfaces. While MAX phases and metal-MAX composites have been largely considered for high-temperature applications [8,9], the interaction of MAX phases with other materials has been poorly documented. Indeed, Ma et al. recently published a thorough review on the reactivity of MAX phases with metals showing that the literature mainly focused on Ti₈₅₃Al₃ (A = Al, Si) interaction with copper, aluminum, titanium, silver, gold and nickel [10]. Essentially, the aluminum and silicon atoms of the MAX phase diffuse in the basal plane up to the grain boundaries to react with the metal, thus reproducing similar pathway to the oxidation process. Depending on the temperature and time of the thermal treatment, the interfacial phases (chromium carbide, titanium carbide and aluminum-metal intermetallics) which are formed can be beneficial to

tune the mechanical properties of the composites. These metal-MAX combinations can make high-strength [11–15], wear-resistant [16], self-lubricating [17] and self-healing [18] for various applications in aerospace, transportation, electronics or sport industries. Most importantly, the review shows that the literature focuses on Ti₃AlC₂ (A = Al, Si) MAX phases, highlighting a gap in the research relative to Cr₂AlC.

Regarding the reported cases of Cr₂AlC, most of the works, in literature, concern the interaction of the MAX phase with alloys. Smialek et al. [19], Li et al. [20] and Sokol et al. [21] studied the interaction of Cr₂AlC with nickel-based superalloys (LSHR, DD5 and Inconel 718 respectively), showing excellent behaviour after high-temperature reaction between the alloy and the MAX phase. The elements of the Ni-superalloy (mainly Ni and Cr) diffuse into the Cr₂AlC phase whereas Al from the MAX phase counter diffuses into the superalloy resulting in the formation of chromium carbides and β-NiAl. In the study of Sokol et al. the Cr₂AlC/Inconel interface didn't crack after thermal cycling contrary to what happens in the equivalent test for Ti₂AlC, demonstrating the potential use of Cr₂AlC-metal composites in extreme environment [21].

Li et al. synthesized Ni composites reinforced with TiCₓ and Cr₃C₂ particles and a dispersed distribution of γ'-Ni₃(Al,Ti) nanosphere by reactive sintering of pure nickel with Cr₂AlC and Ti₂AlC MAX phases [22]. The authors mention an enhancement of the material's mechanical properties and show that such high-performance composites can be used for structural applications. Chen et al. also observed similar decomposition of Cr₂AlC into Cr₇C₃ and (Cr,Fe)₇C₃ carbides and when the MAX

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: axel.zuber@univ-poitiers.fr (A. Zuber).</footnote>
phase reacts with Fe; it is demonstrated that it is a way to make stainless steel composite with unique properties since carbide particles mainly form a hard continuous skeleton in the Fe matrix [23]. Other works were recently conducted by Bentzel et al. on the interaction of Ti₂AlC, Ti₃AlC₂, Ti₃SiC₂ and Cr₂AlC with palladium, and pyrolytic carbon [24, 25]. The Pd-Cr₂AlC diffusion couple showed Cr₂AlC/Cr₂(Al,Pd) C/AlPd/Pd interface formation but also inward diffusion of palladium atoms in the MAX phase [24]. In the case of the pyrolytic carbon interface, the aluminum of the MAX phase diffuses out to react with carbon atoms, thus forming a layer of chromium carbide (Cr₇C₃ and Cr₃C₂) at the interface [25].

In this pioneering work, the alloying behaviour of various pure transition metals (Ag, Sn, Co, Fe, Cu) with Cr₂AlC powders is investigated and their oxidation resistance in dry air at 1000 and 1200 °C up to 10 h is studied. The objective of this work consists in determining the influence of the metal alloying with the MAX phase onto the oxidation behaviour. For that purpose, metal-MAX phase compounds microstructure, oxidation products and oxidation kinetics are investigated.

## 2. Method

In order to prepare Cr₂AlC powders, high purity 350+ mesh powders of chromium (Alfa Aesar, purity >99%), aluminum (Alfa Aesar, purity >99.5%) and chromium carbide (Alfa Aesar, purity >99.5%) were mixed with a molar ratio Cr:Cr₃C₂:Al = 0.525:0.475:1.2 in a tridimensional Turbula for 10 min. The powder mixture was encapsulated into a glass container under primary vacuum to avoid oxidation during the subsequent heat treatment. A reactive sintering treatment was thus applied at 1400 °C during 8 h under argon gas flux. The sample's external part was machined to remove the glass-induced pollution. The remaining part of the sample was crushed into powders.

The as-prepared Cr₂AlC powder was then mixed with 20 wt% metallic (Sn, Cu, Co, Fe, Ag) powders (see Table 1) in a tridimensional Turbula for 10 min. The powder mixture was then uniaxially cold-pressed (35 MPa) into a cylindrical green pellet (12 mm diameter) and encapsulated into a glass container under primary vacuum. An isostatic pressure of 150 MPa was thus applied at 1000 °C for 4 h using a Hot Isostatic Press. The sample's external part was machined to remove the glass-induced pollution.

After HIP treatment, the polycrystalline Cr₂AlC-20 wt% metal samples were cut into small parallelepiped samples (10x2x2 mm³), polished with silicon carbide abrasive disks and finished on felt disk with diamond paste of 0.25 µm particles. The as-prepared samples were placed into the thermogravimetric analysis chamber (TGA, SETARAM EVO-SYSTEM). They were heated under argon flow at 20 °C/min and isothermally oxidized under dry air flow at T = [1000; 1200] °C during 10 h. For each sample, the weight gain was normalized to the sample's surface before being raised to the power n (whole number) until its shape was linear. The kinetic constants were thus deduced at a given temperature from the slope of the straight line.

Scanning electron microscopy (SEM JEOL 7001-TTLS) observations coupled with Energy Dispersive X-Ray Spectrometry (EDS) analyses were performed to observe the sample surface and determine the elemental composition of the different phases, respectively. X-Ray Diffraction (XRD BRUKER D8), with copper cathod (λ = 1,54 Å, 40 kV), was performed for crystalline structure identification of both

<table><caption>**Table 1**
Metallic powder's characteristics.</caption><thead><tr><th>Nature of the metallic powder</th><th>Powder supplier</th><th>Grain size (µm)</th><th>Purity (%)</th></tr></thead><tbody><tr><td>Sn</td><td>Alfa Aesar</td><td>&lt;50</td><td>&gt;99.8</td></tr><tr><td>Cu</td><td>Alfa Aesar</td><td>&lt;25</td><td>&gt;99.9</td></tr><tr><td>Co</td><td>Alfa Aesar</td><td>&lt;40</td><td>&gt;99.5</td></tr><tr><td>Fe</td><td>Alfa Aesar</td><td>&lt;75</td><td>&gt;99</td></tr><tr><td>Ag</td><td>LIFCO Industry</td><td>&lt;10</td><td>&gt;98</td></tr></tbody></table>

synthesized and oxidized samples. Moreover, using MAUD software [26], Rietveld refinement were systematically performed on XRD data to determine the cell parameters of the Cr₂AlC MAX phase.

## 3. Results and discussion

### 3.1. Cr₂AlC powders

X-Ray diffraction pattern of the in-house powders is shown in Fig. 1. It allows demonstrating that powders produced by pressureless sintering mainly contains Cr₂AlC and a few impurities of chromium carbide Cr₇C₃ (1.4 wt% from Rietveld refinement). Such a small amount of that impurity is not considered as harmful for the oxidation study. Indeed, aluminum atoms of the MAX phase are known to oxidize preferentially during high-temperature oxidation [8]. Moreover, oxidation of chromium carbide impurity produces chromia which is a protective oxide.

Later observations of the sintered Cr₂AlC-Sn composite reveal local chromium and aluminum-rich regions at the grain boundaries together with chromium carbide at the interface between such regions and the Cr₂AlC grain as shown in Fig. 2. These regions are supposed to be Cr-Al intermetallic residue of incomplete reaction during the Cr₂AlC synthesis. Indeed, the formation of these intermediate phases during the synthesis of Cr₂AlC has already been discussed by Panigrahi et al. and Lin et al. in relation with the formation mechanism of Cr₂AlC [27,28]. Lin et al. proposed a mechanism from elemental powders while Panigrahi et al. used a mixture of chromium carbides (Cr₇C₃+Cr₃C₂) and aluminum. Nevertheless, both of them postulate the formation of respectively solid and liquid Cr-Al intermetallic. In our case, the formation of Cr₂AlC from a mixture of chromium, chromium carbide and aluminum powders may obey to the mechanism proposed by Panigrahi et al. [27]. However, it is important to mention that contrary to Ref. [27], no aluminum carbide nor aluminum oxide from its decomposition in air could be clearly evidenced by XRD or SEM-EDS in this study.

### 3.2. Liquid phase sintering

Cr₂AlC-Sn and Cr₂AlC-Ag composites have undergone liquid phase sintering process during the HIP treatment at 1000 °C (Ag melting point = 960 °C, Sn melting point = 230 °C). This is confirmed by the SEM observations shown in Fig. 3. Indeed, one can observe that the metallic phase fills the composite's grain boundaries. However, some porosity is

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** XRD pattern of the in-house powder of Cr₂AlC, synthesized by pressureless sintering of commercial powders (Cr:Cr₃C₂:Al = 0.525:0.475:1.2) at 1400 °C for 8 h under argon flux.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 2. SEM micrograph and EDS maps of Al, Cr and C related to a Cr₂AlC-Sn composite sample produced by HIP at 1000 °C for 4 h under 150 MPa.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. SEM micrographs in back-scattered mode of A) Cr₂AlC-Sn, B) Cr₂AlC-Ag composite surface. Synthesis is performed by HIP under 150 MPa at 1000 °C for 4 h from a mixture of Cr₂AlC and 20 wt% of metallic (Sn or Ag) powders.</figcaption></figure>

still visible possibly due to the quite low amount of metal in the material which was estimated to be 10 vol% by Rietveld refinements [29]; such a quantity being in good agreement with the initial amount of metallic powder used to synthesize the composite.

Despite the very high temperature of the HIP treatment, Sn atoms do not react with Cr₂AlC. Indeed, XRD data (Fig. 4-A) and SEM-EDS observations (Fig. 3-A) do not reveal the presence of any secondary phases.

<figure><img src="image_6.png" /></figure>

<table><caption>Fig. 4. A) XRD patterns of Cr₂AlC-Sn and Cr₂AlC-Ag composites after HIP treatment, B) Lattice parameters of the MAX phase determined using the Rietveld refinement method. The reference cell parameters for Cr₂AlC are a = 2.8660 (5) Å and c = 12.820 (3) Å (Rietveld refinement of XRD data in Fig. 1).</caption><thead><tr><th>Composite</th><th>vol.% of the phases</th><th>a (Å)</th><th>c (Å)</th></tr></thead><tbody><tr><td>Cr<sub>2</sub>AlC + Sn</td><td>88.2% MAX; 11.8% Sn</td><td>2.864 ±0.001</td><td>12.824 ±0.004</td></tr><tr><td>Cr<sub>2</sub>AlC + Ag</td><td>88.7% MAX; 11.3% Ag<sub>2</sub>Al</td><td>2.863 ±0.001</td><td>12.829 ±0.007</td></tr></tbody></table>

Moreover, as shown in Fig. 4-B, Rietveld refinement does not indicate any change in the MAX phase lattice parameters, confirming that Sn does not incorporate into the Cr₂AlC structure. This result is surprising since previous papers report the substitution of Al by Sn in Ti₃AlC₂ MAX phase for tribological and mechanical properties improvement [30,31]. However, Al-Sn is known as an immiscible system and it is often called a soft tribological alloy [32]. Al-Sn chemical affinity is poor which very likely explains that there is no reaction between Sn and Cr₂AlC in the case of Cr₂AlC-Sn composites. Nevertheless, outward diffusion of Al atoms may have occurred since alumina grains are observed into Sn regions.

In the Cr₂AlC-Ag composites, Ag atoms react with Cr₂AlC. By coupling XRD (Fig. 3-A) and SEM-EDS (Fig. 4-A) characterizations, one can deduce that the Ag₂Al intermetallic phase is present at the grain boundaries of the MAX phase. As shown in Fig. 4-B, Rietveld refinement does not report any change in the MAX phase lattice parameters, confirming that Ag atoms do not incorporate into the Cr₂AlC structure. Local depletion of aluminum atoms is not observed in Cr₂AlC and the Cr₇C₃ carbide is not formed. Similar composites were synthesized by Liu et al. with Ti₃AlC₂ and higher vol% of Ag (70-90 %) for sliding contact applications. Based on thermodynamic considerations, they supposed that the interface consists in Ag(Al) solid solution and carbide particles [29]. In a comparable context, Wang et al. observed the formation of Ag [TiAl] solid solution and Ag₃ [TiAl] ordered phase using Ag for brazing Ti₂AlC-Ti₂AlC joint [33]. In the present study, the absence of Cr₇C₃ carbide and the absence of Al depletion in Cr₂AlC may be explained by the presence of Cr-Al rich regions in the original Cr₂AlC powder (see Fig. 2) and/or by the low content of silver (20 wt%) in the composite. Thus, the Ag₂Al intermetallic can be formed without decomposing the Cr₂AlC phase.
### 3.3. Solid-state sintering

For the Cr₂AlC-20 wt% (Cu; Co; Fe) powder mixtures, the metallic elements have a higher melting point than the sintering temperature. Due to the solid-state sintering process, one can observe in this case an incomplete densification of the samples resulting in the appearance of pores, as illustrated in Fig. 5. The presence of pores can be explained by the low amount of metal in the composite and/or by the low temperature of the sintering treatment as compared to the melting point of the components in the composite.

Some alumina can be observed at the grain boundaries and notably in contact with the Cr₇C₃ phase in the Cr₂AlC-Fe composites, resulting from the Al depletion in the MAX phase. Indeed, Al atoms diffuse out of the MAX phase and react either with traces of oxygen to form alumina or with Cu, Co or Fe metals to form intermetallic compounds. After HIP treatment of the Cr₂AlC-20 wt% (Cu; Co; Fe) powder mixtures, XRD patterns shown in Fig. 6A allow identifying Al-based intermetallics such as Al₄Cu₉, AlCo and AlFe in the composites as well as the Cr₇C₃ carbide phase. This is in good agreement with SEM-EDS analyses shown in Fig. 5, where one can observe intermetallic regions between Cr₂AlC grains surrounded by Cr₇C₃. These observations point at a strong depletion of the aluminum leading to the formation of the chromium carbide Cr₇C₃. Such an aluminum depletion, shown in Fig. 5-A, B and C, is consistent with the easy outward diffusion of aluminum atoms in its plane during the oxidation of bulk Cr₂AlC [2,8]. The formation of carbides agrees with the previous observations, by Chen et al. on Cr₂AlC-Fe and Ti₃SiC₂-Fe systems or on equivalent systems such as Ti₃AlC₂-Cu by several authors [34–38]. This decomposition of the MAX phase into carbide and the formation of intermetallic phases (or solid solutions) is also comparable to what was observed when Cr₂AlC interacts with nickel-based alloys, with iron or with palladium [27–32]. The aluminum dissolves in the metallic matrix and induces formation of carbide, making Cr₂AlC interesting for metal matrix strengthening of MAX phase-metal composites.

SEM-EDS observations evidence Al outward diffusion from Cr₂AlC while no inward diffusion of the metallic species in the MAX phase grains is detected. Indeed, the Rietveld refinement performed on the XRD data shows no modification of the structural parameters of Cr₂AlC, confirming that no substitution occurred during the sintering process (Fig. 6-B). Copper, iron, cobalt, tin and silver have an atomic radius similar to the one of aluminum (143 p.m.). One could thus expect substitution of a consistent amount of metal in the MAX phase as observed by Bentzel et al. with Cr₂AlC-Pd or by Nechiche et al. with Ti₃AlC₂-Cu [24,37]. In the case of Cr₂AlC-Cu, Fe, Co, Ag mixtures, experimental data show that the system prefers to form Al-based intermetallics by depleting the aluminum atoms from the MAX phase [10–24,39–41]. Indeed, the outward diffusion of aluminum in the MAX phase is known to be due to the weak interaction between the aluminum planes and the carbide octahedrons Cr₆C. Increasing temperature allows the aluminum atoms to easily diffuse out of the MAX phase and to react with the metal. On the other hand, the inward diffusion of the metal in the MAX phase

<figure><img src="image_7.png" /></figure>

<table><caption>Fig. 6. XRD patterns of the Cr₂AlC-Co, Cr₂AlC-Cu and Cr₂AlC-Fe composites after HIP treatment, B) Lattice parameters of the MAX phase determined using the Rietveld refinement method. The reference cell parameters for Cr₂AlC are a = 2.8660 (5) Å and c = 12.820 (3) Å (Rietveld refinement of XRD data in Fig. 1).</caption><thead><tr><th>Composite</th><th>vol.% of the phases</th><th>a (Å)</th><th>c (Å)</th></tr></thead><tbody><tr><td>Cr<sub>2</sub>AlC + Co</td><td>33.3% MAX; 33.5% AlCo; 33.2% Cr<sub>7</sub>C<sub>3</sub></td><td>2.870 ±0.002</td><td>12.81 ±0.02</td></tr><tr><td>Cr<sub>2</sub>AlC + Cu</td><td>56.0% MAX; 23.6% Al<sub>4</sub>Cu<sub>9</sub>; 20.4% Cr<sub>7</sub>C<sub>3</sub></td><td>2.864 ±0.002</td><td>12.823 ±0.008</td></tr><tr><td>Cr<sub>2</sub>AlC + Fe</td><td>41.2% MAX; 28.4% FeAl; 30.4% Cr<sub>7</sub>C<sub>3</sub></td><td>2.863 ±0.001</td><td>12.819 ±0.009</td></tr></tbody></table>

might not be observed due to the formation of a chromium carbide layer at the MAX/intermetallic interface, as observed in this study and in the work of Bentzel [25].

### 3.4. High-temperature oxidation behaviour

The resulting Cu-, Fe-, Co-Cr₂AlC composites were oxidized at 1000 and 1200 °C for 10 h under dry air, and their surface and cross-section were observed by SEM-EDS as reported in Fig. 7. The composites synthesized by solid-state reaction induce the formation of a buckled alumina layer as reported on the top-view micrographs shown in Fig. 7-A to C. The formation of alumina is typical of the oxidation behaviour of alumina-forming phases such as Al based-intermetallics and Cr₂AlC. Furthermore, the buckling of the alumina layer during Cr₂AlC oxidation was previously observed [10]. One can observe variations in the morphologies of the buckles on the different composites: wrinkles in the Cr₂AlC-Cu and Cr₂AlC-Fe composites, expanded bubbles in the case of the Cr₂AlC-Co one. The corresponding sample's cross-section observations in Fig. 7-D to F confirm the formation of a continuous external alumina layer and reveal intergranular oxidation mostly due to the porosity of the sintered samples. The proportion of Cr₇C₃ increases during the oxidation process due to the Al depletion in the MAX phase: very low amounts of Cr₂AlC are detected in the case of the Cr₂AlC-Fe composite.

More interestingly, the oxidation at 1000 °C for 10 h of Cr₂AlC-Sn

<figure><img src="image_8.png" /><figcaption>Fig. 5. SEM micrographs, in back-scattered mode, of A) Cu-, B) Fe-, C) Co-Cr₂AlC composite surface. Synthesis is performed by HIP under 150 MPa at 1000 °C for 4 h.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 7. Top-view SEM micrographs of A) Cr₂AlC-Cu, B) Cr₂AlC-Fe, C) Cr₂AlC-Co composite's surface after oxidation at 1200 °C for 10 h under dry air and D, E, F) corresponding cross-sections.</figcaption></figure>

and Cr₂AlC-Ag composites leads to the formation of an alumina scale forming prominent spheres at their surface, as shown on the top-view micrographs in Fig. 8-A, B. The sublayer mostly consists in chromium

carbide for the Cr₂AlC-Sn composite (Fig. 9-B) or in chromium carbide and Ag₂Al for the Cr₂AlC-Ag composite (Fig. 8-D). While some hollow spheres are observed on both composites, a nice filled structure

<figure><img src="image_10.png" /><figcaption>Fig. 8. Top-view SEM micrographs of A) Cr₂AlC-Sn, B) Cr₂AlC-Ag composite's surface after oxidation at 1000 °C for 10 h under dry air and C, D) corresponding cross-sections.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** A) SEM micrograph with corresponding EDS elemental map of O, Al and Sn showing the internal structure of the spheres observed on the oxidized Cr₂AlC–Sn composites. B) XRD patterns of the Cr₂AlC–Ag and Cr₂AlC–Sn composites after oxidation at 1000 °C for 10 h under dry air.</figcaption></figure>

composed of a Sn core and an Al₂O₃ shell can be observed in the cross-section micrograph of Fig. 8-C. Such a core-shell structure of the Cr₂AlC–Sn composite has not been directly observed in the Cr₂AlC–Ag composite, which doesn't exclude its existence. Indeed, while only empty spherical shells could be observed on cross-section micrographs, these likely originate from once fully filled spheres (Fig. 8-D). Spheres are formed in the case of Cr₂AlC–Sn and Cr₂AlC–Ag composites for which Sn and Ag₂Al are liquid at 1000 and 1200 °C. Thus, Al₂O₃ shell results from the oxidation of Al atoms which are present in the liquid droplets of Sn or Ag₂Al. In the case of Cr₂AlC–Ag composites, empty spheres are observed and may result from diffusion of silver atoms from the droplets to the Cr₂AlC sublayer to form Ag₂Al and Cr₇C₃ as it is the case during the composite synthesis and as observed in Fig. 8D.

The formation of α-Al₂O₃ is confirmed by XRD for all samples without any trace of another oxide, except for the Cr₂AlC–Sn composite for which some tin oxide is present. However, despite its irrefutable identification by XRD analyses, as shown on the X-ray diffraction pattern of Fig. 9-B, SnO₂ couldn't be observed by SEM. In Fig. 9-A, the light grey contrast in the vicinity of the alumina, which corresponds to an O-rich region, could be related to SnO₂ formation but such a small area cannot be resolved using SEM-EDS. As a consequence, it seems that the tin oxide content is very small as compared to the alumina one. Note that Sn spheres have also been observed in Al–Sn alloy oxidized at low temperatures (300 and 500 °C) [42]. The authors show that Sn precipitates along the grain boundaries and forms a network of tin spheroids on the alloy surface. Al is oxidized in a very short period of time, so a protective alumina layer is formed on the surface of Al–Sn bearing alloy. Such a mechanism likely occurs in our experiments and explains the formation of Sn spheres. Nevertheless, an alumina passivating scale is also observed on the surface of Sn spheres which likely results from precipitation of Al metal at the outer surface of Sn droplets and their subsequent oxidation.

One can note that for Co-, Cu-, Fe-, Ag–Cr₂AlC composites, the corresponding metal oxides are not observed after oxidation. For the Sn–Cr₂AlC composite, few amounts of tin oxide are observed by X-Ray diffraction. Thus, Al atoms, either from Cr₂AlC or from the intermetallic/solid solution, are preferentially oxidized. Moreover, Cr₇C₃ is systematically observed after the oxidation process. The presence of such a carbide results from the diffusion of Al atoms out of the MAX phase [8,43]. These Al atoms can either be oxidized to form alumina and/or replace Al atoms in the intermetallics or in solid solutions as metal oxides (copper oxide, cobalt oxide, iron oxide or silver oxide) are not observed.

For each sample, the mass gain was measured as a function of time during the oxidation process, as shown in Fig. 10.

The resulting parabolic kinetic constants are indicated in Table 2. A parabolic or cubic law does not give account of the experimental mass

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Normalized mass gain for the different Cr₂AlC-metal composites oxidized at 1000 °C for 10 h in dry air.</figcaption></figure>

<caption>**Table 2**</caption>

Parabolic kinetic constants deduced from thermogravimetric analysis after 10 h of oxidation at 1000 and 1200 °C in dry air.

<table><thead><tr><th>Compound</th><th>k<sub>p</sub> at 1000 °C (kg<sup>2</sup> m<sup>-4</sup>.s<sup>-1</sup>)</th><th>k<sub>p</sub> at 1200 °C (kg<sup>2</sup> m<sup>-4</sup>.s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>Cr<sub>2</sub>AlC</td><td>6.4 10<sup>-11</sup></td><td>3.5 10<sup>-9</sup></td></tr><tr><td>Cr<sub>2</sub>AlC + Co</td><td>4.8 10<sup>-10</sup></td><td>1.5 10<sup>-9</sup></td></tr><tr><td>Cr<sub>2</sub>AlC + Cu</td><td>1.4 10<sup>-10</sup></td><td>2.9 10<sup>-8</sup></td></tr><tr><td>Cr<sub>2</sub>AlC + Fe</td><td>–</td><td>7.3 10<sup>-8</sup></td></tr><tr><td>Cr<sub>2</sub>AlC + Ag</td><td>–</td><td>8.8 10<sup>-7</sup></td></tr><tr><td>Cr<sub>2</sub>AlC + Sn</td><td>2.5 10<sup>-8</sup></td><td>–</td></tr></tbody></table>

gain variation at 1000 °C for the Ag- and Fe–Cr₂AlC composites which explains that corresponding kinetic constants are not given. Moreover, kinetic constants cannot be used for activation energy calculation as several phases are oxidized during the process and activation energy are relevant parameters for a single oxidation mechanism. It can still be compared to the parabolic kinetic constant of the pure Cr₂AlC MAX phase determined in previous studies [8,44,45]. Note that Cr₂AlC–Sn composite has not been oxidized at 1200 °C, considering 1000 °C is a temperature far enough from tin melting point to observe the oxidation products.

Comparing the different mass gain curves as a function of time (Fig. 10), the addition of tin, silver and iron to Cr₂AlC drastically accelerates the oxidation process, especially at 1000 °C. However, Co and Cu–Cr₂AlC composites are characterized by quite low values of the
parabolic kinetic constant which are only about one order of magnitude larger than the Cr₂AlC one at 1000 °C.

Concerning oxidation kinetics, it has been demonstrated that oxidation of the Cr₂AlC phase is dominated by the internal diffusion of oxygen anions through the alumina grain boundary [45]. If such a mechanism would also govern the oxidation of metal-Cr₂AlC composites, the kinetic constants, in stationary regime, would be the same which is far from the case for Sn, Ag, Fe-Cr₂AlC composites and in a lesser extent for Cr₂AlC-Co and Cr₂AlC-Cu composites. Such a result is quite surprising as the oxide scale only consists in alumina which is formed by oxidation of Al atoms provided by Cr₂AlC, intermetallics and/or solid solutions. As a result, the metal which is added to Cr₂AlC during the composite synthesis necessary plays a major role during oxidation. Moreover, as the oxidation temperature (1000 and 1200 °C) is equal or higher than the synthesis temperature (1000 °C), Cr₂AlC very likely reacts with the metal and/or the aluminides. Thus, one can assume that Al atoms of the intermetallics or solid solution oxidize first whereas the (Ag, Fe, Cu, Co) metallic elements react with the Al from the Cr₂AlC phase to form aluminides and Cr₇C₃ since metal oxides (copper oxide, cobalt oxide, iron oxide or silver oxide) are not observed. As a consequence, the oxidation kinetics is dominated by the oxidation of Al atoms from the intermetallics or solid solutions and the formation of Cr₇C₃. Nevertheless, it is not possible to prove such a mechanism and it seems difficult to go deeper in the explanation of the drastic difference in oxidation rates of the different composites.

## 4. Conclusion

Cr₂AlC-(Co, Cu, Fe, Sn, Ag) composites were synthesized by hot isostatic pressing at 1000 °C for 4 h under 150 MPa. After heat treatment, no inward diffusion of the metal is observed into the MAX phase; the outward diffusion of aluminum atoms from the MAX phase is preferred. This phenomenon leads to the formation of intermetallic species, CoAl, FeAl, Al₄Cu₉, and chromium carbide Cr₇C₃ in the Cr₂AlC-Co, Cr₂AlC-Fe and Cr₂AlC-Cu composites respectively. For the Cr₂AlC-Ag composite, Ag₂Al is formed but Al depletion is not sufficient to lead to the Cr₇C₃ formation. These results are related to the weak Cr-Al metallic bond in the MAX phase. For the Cr₂AlC-Sn composite, Sn atoms do not react with Al ones and the composite consists in Cr₂AlC and Sn phases. A mechanical alloying step would likely help diffusion of added metal atoms in the Cr₂AlC MAX phase as it is the case in the Ti₃AlC₂-Cu composites [37,38].

Oxidation behaviour of the different composites is investigated at 1000 and 1200 °C for 10 h in dry air. It is demonstrated that, whatever the composite, a continuous alumina layer is formed; such a layer can undergo buckling. In the case of Cr₂AlC-Sn composites, Sn spheres embedded in an alumina layer are formed whereas empty alumina shells are observed on the surface of the Cr₂AlC-Ag composites. Liquid droplets of Sn and Ag₂Al lead to the formation of spheres by oxidation of Al atoms from the liquid. Silver atoms react with Cr₂AlC sublayer to form Cr₇C₃ and Ag₂Al. Finally, it is also demonstrated that parabolic kinetic constants are one order of magnitude larger than the Cr₂AlC one in the case of the oxidation of Cr₂AlC-Co and Cr₂AlC-Cu composites at 1000 °C, but in the same range for Cr₂AlC-Co composites at 1200 °C.

Oxidation rates are much faster in the Cr₂AlC-metal composites than in the Cr₂AlC. It is tentatively proposed that Al atoms of the intermetallics and/or solid solutions are preferentially oxidized whereas metallic elements react with the Al from the Cr₂AlC phase to form aluminides and Cr₇C₃ since metal oxides (copper oxide, cobalt oxide, iron oxide or silver oxide) are not observed. Thus, the higher oxidation rate of the Cr₂AlC-metal composites as compared to the MAX phase one likely results from a different and faster oxidation mechanism [46–50].

The investigation of MAX phases interaction with other materials should be continued to develop new composites with enhanced properties. In the case of Cr₂AlC, its oxidation resistance and its ability to decompose into carbide make it interesting for reinforcing the corrosion

and mechanical properties of alloys.

## 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.

## Acknowledgement

The authors thank the Région Nouvelle-Aquitaine and the Ministère de l'Enseignement Supérieur et de la Recherche for funding this research. This work was supported by the French government program "Investissement d'Avenir" (EUR INTREE, reference ANR-18-EURE-0010). This work pertains to the France 2030 program "Investissements d'Avenir" (LABEX INTERACTIFS, reference ANR-11-LABEX-0017-01).

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