Fabrication of Zr₃AlC₂ by Spark Plasma Sintering

Abstract

This study reports the synthesis of the Zr₃AlC₂ MAX phase from ZrH₂, Al, and ZrC as raw materials by means of spark plasma sintering (SPS). The crystal structure of the resulting product was analyzed by X-ray diffraction (XRD), and the microstructure was examined using scanning electron microscopy (SEM). We investigated the influence of sintering temperature and process parameters on the Zr₃AlC₂ content within the sintered product. We also introduce a novel two-layer powder filling approach that significantly enhances the purity of Zr₃AlC₂ in the final product. This method leverages a specific stacking mechanism to control phase distribution, achieving a Zr₃AlC₂ content as high as 93%.

Fig. 11 SEM image of fracture surface of sample No. 4. (online color)

1. Introduction

The MAX phase is a ternary layered compound characterized by a close-packed hexagonal lattice structure [1–3]. The chemical formula for MAX phase materials can be expressed as M_n+1AX_n, where M represents transition elements (such as Ti, V, Cr, Zr, etc.), A represents main group elements (such as Al, Si, S, Cd, etc.), and X denotes either carbon or nitrogen [4–9]. In this formula, n is typically 1, 2, or 3, allowing MAX phase materials to be classified into 211-type, 312-type, and 413-type structures, depending on the value of n [10–12]. Initially discovered in 1960 [13], MAX phase materials have attracted widespread attention in recent years due to their combination of both ceramic and metallic properties [14]. These materials exhibit high flexural strength, high fracture toughness, low density, high electrical and thermal conductivity, good machinability, high-temperature stability, and excellent corrosion resistance, among other advantageous properties [15]. Researchers attribute these properties to their unique crystal structure, in which two MX ceramic layers are separated by an A-element atomic layer, resulting in a layered structure [16].

Common sintering methods for MAX phase materials include pressureless sintering [17], hot press sintering [18], self-propagating high-temperature synthesis (SHS) [19], and spark plasma sintering (SPS) [20, 21]. Among these, SPS offers several advantages, such as rapid heating, uniform temperature distribution, fast cooling, and lower sintering temperatures [22]. For instance, Yang et al. [23] successfully synthesized Ti₃AlC₂ at 1050°C by combining mechanical alloying with SPS, even though this material typically requires synthesis temperatures above 1200°C. Additionally, SPS significantly reduces sintering time; Tian et al. [24] prepared pure Cr₂AlC in only 5 minutes at 1400°C, while hot pressing at the same temperature requires nearly 1 hour of heating.

Since the Fukushima nuclear accident, public attention has focused on the safety of nuclear power plants [25]. To enhance the performance of zirconium alloys under accident conditions, the concept of accident-tolerant fuel (ATF) materials has been introduced [26]. One approach is to coat zirconium alloys with materials that are both corrosion-resistant and radiation-tolerant [27–29]. Due to their mechanical robustness and high resistance to corrosion and radiation, MAX phase materials are promising candidates for ATF applications [30]. Among them, Ti₃AlC₂ was successfully fabricated as a 40 nm-thick coating on a sapphire substrate by Wang et al. [31], who studied its irradiation resistance. The results indicated excellent radiation tolerance. In another study, Tang et al. [32] demonstrated that Cr₂AlC significantly enhances the high-temperature corrosion resistance of Zr alloys, suggesting its suitability as an ATF material.

Compared with conventional MAX phase materials containing elements such as Ti and Cr, Zr has a lower thermal neutron absorption cross-section [33, 34], leading many scientists to regard Zr-based MAX phases as highly promising for nuclear applications [35]. Previously known Zr-based MAX phases include Zr₂TlC, Zr₂PbC, Zr₂InN, Zr₂SC, Zr₂InC, Zr₂SnC, and Zr₂TlN. In 2016, Lappaw and colleagues [36] synthesized Zr₃AlC₂, and later successfully produced Zr₂AlC [37]. To date, various research teams have employed different sintering techniques to fabricate Zr₃AlC₂, but the product purity generally remains low. A common strategy to improve MAX phase purity is to add alloying elements, which can enhance the MAX phase content in the final product. For example, Zapata-Solvas et al. [38, 39] synthesized Zr₃(Al_x,Si_1−x)C₂ and (Zr_1−x,Ti_x)₃AlC₂ by incorporating Si and Ti into the raw materials, while Lappaw et al. [40] prepared (Zr,Nb)₂(Al,Sn)C by adding Nb and Sn. This study aims to produce higher-purity Zr₃AlC₂ using a specific two-layer powder filling approach combined with SPS, without introducing additional alloying elements.

2. Experimental Procedure

The raw materials used in this study are ZrH₂ (purity >99%), ZrC (purity >99%, diameter <3 µm), and Al (purity >99%, diameter <45 µm). ZrC and Al were purchased from the Nilaco corporation. ZrH₂ was synthesized in our laboratory by heating Zr in a hydrogen atmosphere at 350°C for 2 hours. Subsequently, the ZrH₂ blocks were crushed in a ceramic mortar and sieved through a 2000-mesh sieve to obtain ZrH₂ powder. The selection of ZrH₂ as a raw material is based on two main considerations. Firstly, in comparison to Zr, ZrH₂ exhibits greater brittleness and is easier to crush, thereby facilitating uniform mixing of the raw materials. Furthermore, as reported by Lappaw [41], ZrH₂ offers advantages over zirconium as a raw material for synthesizing MAX phases. During sintering, ZrH₂ releases hydrogen, which helps prevent oxidation.

The target composition for the MAX phase is Zr₃AlC₂ (Zr:Al:C = 3:1:2). The composition of the raw materials is presented in Table 1. A slightly higher proportion of Al was used in the raw materials to compensate for Al loss during the sintering process, aiding the formation of Zr₃AlC₂. The carbon content was set slightly lower than the target to account for additional carbon from the graphite mold during sintering.

Table 1 Ingredient composition of the raw materials.

Raw materials were mixed in a stainless steel ball mill jar with grinding balls of two diameters: 30 mm and 10 mm. The ball-to-material ratio was 1:1, with a mass ratio of large to small balls of 2:1. Milling was conducted under argon protection at 200 rpm for 2 hours. To prevent overheating, the milling process included 30-minute intervals with 15-minute pauses. After milling, the powder was collected, ground with a ceramic mortar, and sieved through a 3000-mesh sieve, all conducted in an argon atmosphere.

This study employed two methods to prepare Zr₃AlC₂. The first method utilized a conventional spark plasma sintering (SPS) technique, as illustrated in Fig. 1(a). The powder filling method involved the use of a previously prepared mixed powder with a composition of Zr:Al:C = 3:1.2:1.8. The second method, depicted in Fig. 2, employed a two-layer powder filling approach. The lower layer consisted of the aforementioned mixed powder, while the upper layer comprised pure ZrC powder. During the filling process, the mixed powder was first loaded and compacted, followed by the uniform placement and compaction of the pure ZrC powder. The powder filling was performed in an argon atmosphere, followed by cold pressing at a pressure of 20 MPa.

Fig. 1

Powder filling mode: (a) Conventional SPS, the powder in the mold is mixed raw material powder Zr:Al:C = 3:1.2:1.8, (b) The upper layer is pure ZrC, and the lower layer is mixed raw material powder. (online color)

Fig. 2

Relative density of samples.

Sintering was conducted using SPS equipment, where the samples were heated to 1400°C and 1350°C within 10 minutes (with temperature monitored using an infrared thermometer). Each set of samples was held at the target temperature for 1 to 5 minutes, with a pressure of 30 MPa applied. A mechanical pump maintained the vacuum level inside the furnace below 0.1 Pa. After the designated holding time, the samples were rapidly cooled to 600°C with the assistance of the SPS cooling system. Subsequently, they were then further cooled to room temperature over 1 hour before removal from the furnace. Throughout the cooling process, a constant pressure of 30 MPa was maintained, along with a vacuum level inside the furnace below 0.1 Pa. The sintering parameters and filling modes for the samples used in this experimental study are summarized in Table 2.

Table 2 Powder filling modes and sintering parameters of each group of samples.

After sintering, the bulk samples were polished using sandpaper to remove the surface graphite. The crystalline structure of the samples was analyzed using X-ray diffraction (XRD) with CuK radiation at 40 kV and 40 mA on a Rigaku SmartLab instrument. XRD analysis was conducted in the range of 5° to 90° with a scanning rate of 4°/min. The obtained XRD patterns of the samples were then compared with the calculated XRD patterns of Zr₃AlC₂. Meanwhile, the content of Zr₃AlC₂ in the samples was determined using the following eq. (1) [42], where I_ZrC and I_Zr3AlC2 represent the integrated intensities of the diffraction peaks (2 0 0) for ZrC and (1 0 4) for Zr₃AlC₂, respectively:

  

\begin{equation} \text{W}_{\text{Zr${_{3}}$AlC${_{2}}$}}\ (\text{Vol%}) = \frac{1.80}{1.80 + (I_{\text{ZrC}}/I_{\text{Zr${_{3}}$AlC${_{2}}$}})} * 100\% \end{equation} (1)

The surface and cross-sectional morphology of the samples were observed using scanning electron microscopy (SEM), while the microstructural distribution of the samples was examined through backscattered electron imaging. The observation surfaces were initially polished with 400, 800, 2000, and 5000 grit sandpapers, followed by polishing with 3 µm and 1 µm Al polishing pastes. Energy-dispersive X-ray spectroscopy (EDS) was employed to quantitatively measure the proportions of Zr, Al, and C in different phases present in the sample. Simultaneously, elemental distribution maps for Zr, Al, and C were generated to visually illustrate the spatial variation of elements within these phases. The acceleration voltage used during the experimental process was 25 kV.

The density of the samples was determined using the Archimedes method. For samples prepared using the two-layer powder filling approach, the regions with higher Zr₃AlC₂ content were mainly located in the surface layer of the sintered block (the ZrC powder layer before sintering). The sample preparation process for density testing is briefly described as follows: firstly, the sample surface was polished with sandpaper to remove impurities. Subsequently, a grinding wheel was used to grind the sample from the bottom, gradually thinning the sample to eliminate regions with lower Zr₃AlC₂ content. X-ray diffraction (XRD) was employed to monitor the composition on both sides of the sample until the Zr₃AlC₂ content on both sides exceeded 90%. Finally, the density of the sample was measured using the Archimedes method. Figure 2 is the hydrostatic density test result of sample No. 1–4. The relative density of sample No. 1 is 83%, and the relative density of sample No. 2–4 is 98.24, 98.76 and 99.31. The theoretical density of Zr₃AlC₂ used is 5.62 g/cm³ [42].

3. Results and Discussion

Figure 3 presents the X-ray diffraction (XRD) patterns of the samples. The powder filling approach and the sintering parameters for each group of samples are summarized in Table 2. The XRD test positions for samples No. 2–7 correspond to the ZrC end surfaces where the powder was loaded. The Zr₃AlC₂ content in sample No. 1 is approximately 55 wt%, which is consistent with the literature. The content of Zr₃AlC₂ in samples No. 2–4 is significantly higher than that in sample No. 1. Figure 4 illustrates the relationship between the Zr₃AlC₂ content and the SPS time for samples prepared using the two-layer powder filling approach. As the sintering time increases from 1 minute to 5 minutes, the Zr₃AlC₂ content rises from 85 wt% to 93 wt%. However, when the sintering time is extended to 10 minutes, the sample no longer contains Zr₃AlC₂ after sintering, which may be attributed to the decomposition of Zr₃AlC₂ caused by prolonged SPS sintering. XRD pattern No. 7 indicates that after 10 minutes of sintering, only the diffraction peak corresponding to ZrC is present in the sample. Concurrently, a significant amount of liquid phase material is observed to overflow from the mold surface post-sintering, accompanied by the formation of a considerable amount of white powder, as shown in Fig. 5(A). This liquid phase induces adhesion between the mold and the sample, resulting in difficulties in sample extraction, as depicted in Fig. 5(B). The diffraction peaks of ZrC were observed in all samples. Figure 6 is a comparison of the diffraction peaks of sample No. 4 and the calculated Zr₃AlC₂ [36]. It can be seen that the positions of the diffraction peaks of the two match well. At the same time, the diffraction peak at 2θ = 8.7° corresponds to the (0 0 2) crystal plane of the MAX phase crystal. The low intensity of this diffraction peak may be due to the orientation of the product generated under pressure.

Fig. 3

XRD patterns of the samples: the raw materials of sample No. 1 are loaded using the pattern of Fig. 1(a), and the raw materials of Sample No. 2–7 are loaded using the pattern of Fig. 1(b). (online color)

Fig. 4

Change trend of Zr₃AlC₂ content in the sample after sintering with sintering time. (online color)

Fig. 5

(A) White powder observed on the surface of the mold after sintering; (B) Liquid phase solidification leading to adhesion between the punch and the mold. (online color)

Fig. 6

Comparison of the x-ray diffraction pattern of sample No. 4 and the calculated x-ray diffraction pattern of Zr₃AlC₂. (online color)

Figure 7 shows SEM images of the four samples (Corresponding to XRD samples No. 1 to No. 4). Zr₃AlC₂ exhibits an elongated structure, ZrC presents a granular structure, and darker regions correspond to ZrAl_x. Table 3 shows the elemental proportions of different microstructures in each sample. The long strip structure Zr:Al:C is close to 3:1:2, indicating that the structure is most likely Zr₃AlC₂, which is consistent with the results of XRD.

Fig. 7

Backscattered electron images of the samples. (online color)

Table 3 The proportion of elements in each region in the sample.

The composition of the residual Zr-Al intermediate phase material varies depending on the powder loading mode. The powder filling method for Sample No. 1 is as shown in Fig. 1(a). The dark region exhibits a Zr:Al ratio of 2:3, consistent with XRD results. However, in Samples No. 2–4 (Fill the powder as shown in Fig. 1(b), Zr:Al ratio of the dark region is 1:1. According to the Zr-Al phase diagram, ZrAl is not a high-temperature stable phase. This substance might act as an intermediate product in special reactions, preserved due to the rapid cooling characteristics of SPS, the low content of ZrAl makes it undetectable by XRD.

Lappouw [37] pointed out that Zr₃AlC₂ is synthesized through the following process:

  

\begin{equation} \text{1/3Zr$_{2}$Al$_{3}$}+\text{4/3ZrC$_{0.75}$} = \text{Zr$_{2}$AlC} \end{equation} (2)

  

\begin{equation} \text{Zr$_{2}$AlC}+\text{ZrC} = \text{Zr$_{3}$AlC$_{2}$} \end{equation} (3)

In this experiment, no Zr₂AlC was found during the entire sintering process, and Zr₃AlC₂ was more likely to be directly synthesized from Zr₂Al₃ and ZrC.

  

\begin{equation} \text{Zr$_{2}$Al$_{3}$}+\text{ZrC$_{\text{x}}$} \leftrightarrow \text{Zr$_{3}$AlC$_{2}$} \end{equation} (4)

In the experiment, it was found that when the sintering temperature reaches 1400°C, a large amount of Zr₂Al₃ forms at the interface between the top ZrC layer and the mixed raw material layer, thus forming a layer of Al-rich area of about 50–100 µm at the interface (Fig. 8). When the temperature is slightly lower than 1400°C, such as 1350°C, no large amount of Al enrichment is found at the junction (Fig. 9). At the same time, it can be seen that when the sintering temperature is 1350°C, the structure at the top is a loose granular structure, EDS shows that this area does not contain Al at this temperature, and the composition of the substance is ZrC.

Fig. 8

Element distribution diagram of sample No. 4. (online color)

Fig. 9

SEM and element distribution map of longitudinal section of sample sintered at 1350°C for 5 min. (online color)

ZrC_x x can vary in the range of 0.5–1. Figure 10 shows that when the ZrC on the top changes to Zr_1.1C (ZrC_0.91), the top of the sample still has a loose granular structure after sintering at 1400°C for 5 minutes. EDS and XRD show that the top area does not contain Al, and the composition of the material is still ZrC. This indicates that the synthesis of Zr₃AlC₂ is highly sensitive to the composition of ZrC_x. The higher the carbon content in ZrC_x, the easier it is to generate Zr₃AlC₂. When preparing materials using the method shown in Fig. 1(a), the raw materials ZrH₂, Al, and ZrC are evenly mixed. During the heating stage, part of ZrH₂ reacts with Al to form a zirconium-Al compound, while part of ZrH₂ may react with ZrC, which may cause the C content in ZrC to decrease. When the temperature reaches 1400°C, some ZrC_x with lower C content fails to react with Zr₂Al₃ to form MAX phase materials.

Fig. 10

SEM of the sample with top composition Zr_1.1C sintered at 1400°C for 5 minutes. (online color)

Figure 11 shows the SEM image of the fracture surface of sample No. 4. The long strips of Zr₃AlC₂ dominate the sample, and the partially broken Zr₃AlC₂ shows a typical layered structure of MAX phase materials. Granular ZrC can also be seen along the way. The large pieces of C are caused by part of the graphite contaminating the fracture surface during sample preparation. Figure 12 shows the SEM image of the surface and element distribution map of sample No. 4. The distribution of C is relatively uniform, and the dark areas in the SEM correspond to higher Al content. Granular structures have lower Al content.

Fig. 11

SEM image of fracture surface of sample No. 4. (online color)

Fig. 12

SEM image and element distribution map of sample No. 4. (online color)

Higher carbon content in ZrC promotes the synthesis of Zr₃AlC₂. In the conventional powder filling approach, all raw materials, including Al, ZrH₂, and ZrC, are uniformly mixed and loaded into the mold. During the heating stage, Zr interacts with ZrC, reducing the carbon content of ZrC (as shown in Fig. 13(A)). This decrease can render some ZrC particles unable to synthesize Zr₃AlC₂. However, when the two-layer powder filling approach is used, the ZrC on the upper layer does not come into contact with Zr, ensuring that all particles maintain a high carbon content (as shown in Fig. 13(D)).

Fig. 13

Mechanism of Zr₃AlC₂ Formation. (online color)

The synthesis of Zr₃AlC₂ is reversible, with the liquid phase Zr₂Al₃ crucial in determining the reaction direction. In the conventional powder filling method, as sintering progresses, the liquid Zr₂Al₃ in the sample gradually diminishes (as shown in Fig. 13(B)), shifting the reaction towards MAX phase decomposition. Prolonged sintering ultimately leaves only zirconium carbide (as shown in Fig. 13(C)).

Conversely, in the two-layer powder filling method, the upper ZrC receives a significant influx of liquid Zr₂Al₃ from the lower layer. The surplus Zr₂Al₃ and the abundant zirconium carbide facilitate the synthesis of Zr₃AlC₂ (As shown in Fig. 13(F)). However, some liquid Zr₂Al₃ in the upper layer is still lost during sintering (as shown in Fig. 13(G)). As the lower layer depletes its liquid Zr₂Al₃, the amount available in the upper layer also diminishes. This depletion eventually shifts the reaction towards Zr₃AlC₂ decomposition, leaving only zirconium carbide after 10 minutes of sintering.

4. Conclusion

The purpose of this article is to prepare Zr₃AlC₂ with high purity. Add ZrC to one end of the raw material powder with Zr:Al:C = 3:1.2:1.8. After sintering at 1400°C for 5 minutes, the Zr₃AlC₂ content in the original ZrC position can reach up to 93%. This phenomenon is sensitive to temperature. Only when the temperature reaches 1400°C will the loose granular ZrC turn into dense bulk Zr₃AlC₂. Along with the generation of Zr₃AlC₂, a large amount of Al migrates outward, and a layer of high-Al area appears at the interface between ZrC and the raw material layer.

Acknowledgments

The author would like to express sincere gratitude to the Hokkaido University DX Doctoral Fellowship for their financial support throughout this research.

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