Effect of Molten Al/Si Impregnation on the Oxidation Resistance of TiB₂ at 1300°C

Abstract

Despite their superior material properties at high temperatures, the poor oxidation resistance of borides such as TiB₂ above 1000°C limits their applications. Herein, we demonstrate the liquid-phase impregnation of Al or Si into the sintered compact of TiB₂ and study their effect on the oxidation behavior at 1300°C. The thermogravimetric curves obtained under oxidation and subsequent crystal phase identification suggest that Al impregnation can prevent the evaporation of boron oxide by forming aluminum borate, which is an unstable protective layer, resulting in a slight increase in the oxidation resistance. By contrast, the Si-impregnated specimens showed lesser mass change due to oxidation than that in the unimpregnated specimens, owing to the formation of a stable protective SiO₂ phase on the sample surface. Hence, molten Si impregnation of sintered borides is a promising new approach for improving high-temperature oxidation resistance.

Fig. 5 Schematics of oxidation processes for TiB₂ without and with the impregnation of Al/Si.

1. Introduction

The ultrahigh temperature ceramics (UHTCs) with melting points >3000°C are used in load-bearing applications in extremely harsh environments, such as in air at temperatures >2000°C [1–11]. Among UHTCs, transition metal diborides (TMB₂) have been extensively studied and are used in various because TMB₂ has a higher melting point and higher thermal and electrical conductivities than other ceramics, making it a promising conductive structural material in high-temperature environments. Specific applications include crucibles for molten metals, cutting tools, wear-resistant materials, electrodes for electrical discharge machines (EDM), Hall-Eroux process electrodes, electronic equipment, armor components, Al deposition boats, neutron shielding in the nuclear energy field, rocket nozzles, fire-resistant materials, and high-temperature structural materials [1–14]. The applications in aerospace, such as the fabrication of the outer shells of atmospheric reentry capsules, wing tips, nose cones of supersonic transport aircraft, and components of scramjet/rocket propulsion mechanisms, are particularly interesting. In these environments, materials are exposed to extremely high temperatures of 1400 to 2000°C as well as to a rapidly flowing atmosphere [1–12, 15, 16].

Although TMB₂ has attracted significant attention as a high-temperature structural material from various perspectives, only a few examples of its practical applications are available. One of the main reasons for this is their low, high-temperature oxidation capability. Improvements in TMB₂ by adding transition metal disilicides (TMSi₂) are exclusively studied as methods to enhance oxidation resistance [17–23]. The addition of ∼10 mass% MoSi₂ or TiSi₂ to TiB₂ decreases the mass increase during oxidation at 1200°C [24]. The improvement in oxidation resistance is attributed to the formation of a SiO₂ film during oxidation owing to the addition of silicide. SiO₂, a more stable phase at higher temperatures than B₂O₃, acts as a film that effectively inhibits the inward diffusion of oxygen into the bulk material. With regard to the methods for improving the high-temperature oxidation resistance of borides using other additives, composites with SiC have been widely studied [25–31]. The ZrB₂–SiC combination has been studied in detail, and the following reactions occur during high-temperature oxidation [25]:

  

\begin{equation*} \text{TMB$_{2}$} + \text{2.5O$_{2}$}\to \text{TMO$_{2}$} + \text{B$_{2}$O$_{3}$ (liq.)} \end{equation*}

  

\begin{equation*} \text{B$_{2}$O$_{3}$ (liq.)}\to \text{B$_{2}$O$_{3}$ (gas) at above 1100${}^{\circ}\text{C}$} \end{equation*}

  

\begin{equation*} \text{SiC} + \text{1.5O$_{2}$}\to \text{SiO$_{2}$ (liq.)} + \text{CO (gas)} \end{equation*}

  

\begin{equation*} \text{B$_{2}$O$_{3}$ (liq.)} + \text{SiO$_{2}$}\to \text{B$_{2}$SiO$_{3}$} + \text{O$_{2}$ at above 1100${}^{\circ}\text{C}$} \end{equation*}

When the ZrB₂–SiC system is oxidized, a mixed layer is formed consisting of (i) a glass phase with SiO₂ and B₂O₃, (ii) a SiC–ZrO₂-rich phase, and (iii) a ZrB₂–SiC-rich phase. In this configuration, even at temperatures >1000°C, where the original boride protective film, B₂O₃, evaporates, the SiO₂-containing phase covering the underlying material suppresses oxygen diffusion and protects the bulk material. Solvas et al. confirmed that not ZrO₂ but SiO₂ serves as the protective coating, and the oxygen diffusion coefficient in ZrO₂ at 1500°C and in SiO₂ at 1550°C are ∼10⁻¹⁰ (m²/s) and ∼10⁻²¹ (m²/s), respectively [27]. However, the degradation of the mechanical strength and electrical and thermal conductivities owing to the addition of SiC are notable.

Surface modification has been widely used as a method with relatively little change in bulk material properties. Herein, we propose liquid-phase impregnation (LPI) treatments using molten Al or Si, which are expected to improve the high-temperature oxidation resistance through formations of stable oxides such as Al₂O₃ and SiO₂ on the surface of TMB₂. In this study, TiB₂ was chosen to investigate the applicability of LIP and its effect on high temperature oxidation properties. Because the high temperature oxidation resistance of TiB₂ is significantly degraded from 1100°C due to the evaporation of B₂O₃, mass change of the LPI-treated TiB₂ was measured at 1300°C.

2. Experimental Procedure

Commercially available TiB₂ powder (99% purity, 2–3 µm particle size, Kojundo Chemical Laboratory Co., Ltd., Japan) was sintered using a spark plasma sintering machine (DR. SINTER Model SPS-1050, Sumitomo Coal Mining Co., Ltd., Japan). The sintered specimens were cut into pieces using EDM (DE-70, Sankyo Engineering, Inc., Japan) and a low-speed saw (IsoMet Low-Speed, Buehler Ltd., USA) and polished to #2000 using water-resistant abrasive paper. The dimensions of each sample were 3 mm × 3 mm × 2 mm.

The LPI treatment using Al was performed as follows: I. 10 g of commercially available aluminum plate (A1050) was cut into pieces, weighed, and placed in an alumina crucible with boron spray (Kaken Tec Co., Ltd., Japan) applied as a release agent on the surface. II. The crucibles were heated to 1000°C using a muffle furnace (KDF 007EX, DENKEN HI-DENTAL Co., Ltd., Japan), and Al was melted in the air. III. TiB₂ samples were immersed and heat-treated at 1000°C for 1 h. IV. After the heat treatment, the molten Al was removed from the crucible and allowed to cool in the air, and the impregnated specimens were removed and polished. LPI using Si was conducted as follows: A. Sintered TiB₂ specimens were vacuum-sealed into quartz tubes (Fujiwara Manufacturing Co., Ltd., Japan) with 1 g of small fragments of silicon wafers (Niraco Corporation, Japan). The ultimate pressures were of the order of ∼10⁻⁴ (Pa) for all samples. B. The vacuum-sealed tubes were heated to 1450°C for 1 h using an electric furnace (KBF524N, Koyo Thermo Systems Co., Japan). C. After impregnation, the specimens were removed from the quartz tubes and polished.

The porosity of the sintered compacts was measured using the Archimedes method before and after impregnation to confirm the effect of impregnation. The cross-sections after Si/Al impregnation were analyzed for elemental mapping using an electron probe microanalyzer (EPMA, JXA-8500FK, and JXA-8530F, JEOL Ltd., Japan). Isothermal thermogravimetric measurements were performed on Al/Si impregnation-treated and untreated TiB₂ using thermogravimetric differential thermal analysis (TG-DTA, Thermo plus EVO2 TG8121, Rigaku Corporation, Japan) at 1300°C for 8 h at 100 mL/min airflow. Crystal-phase identification using X-ray diffraction (XRD; MiniFlex600-C/TH, Rigaku Corporation, Japan) was performed on the surface of each sample after the oxidation test. Subsequently, elemental mapping analysis using EPMA (JXA-8530F, JEOL Ltd.) was performed on the sample cross-sections.

3. Results

Table 1 shows the results of density measurements using the Archimedes method for the as-sintered TiB₂ and LPI-treated TiB₂. The theoretical density of the as-sintered TiB₂ used to calculate the relative density is 4.54 (g/cm³) [32]. For the impregnated sample, only open porosity was measured because quantifying the Al and Si contents and theoretical density after impregnation was difficult. The sintered specimens used in this experiment were sufficiently dense, with a relative density of >95% and an open porosity of ∼1%. Moreover, the open porosity of the samples significantly decreased after impregnation treatment. The open pores on the sample surface were likely filled with Al/Si, reducing the open porosity. Elemental maps of the cross-section of the LPI-treated samples are shown in Figs. 1(a) and (b), which suggest that Al/Si penetrated from the surface of the sintered sample to a depth of several hundred micrometers into the TiB₂ interior. The high-magnification mapping image shows that Al/Si is distributed in a network along the grain boundaries of TiB₂.

Table 1 Relative density and porosity for TiB₂, Al-impregnated TiB₂, and Si-impregnated TiB₂.

Fig. 1

Cross-sectional elemental-mapping images for TiB₂ specimens after impregnation with (a) Al and (b) Si.

To investigate the high-temperature oxidation resistance of TiB₂, isothermal thermogravimetry was performed at 1300°C using TG-DTA. The thermogravimetric curves are shown in Fig. 2. The mass increase was the highest for the Al-impregnated specimens, followed by the unimpregnated TiB₂ and Si-impregnated specimens. The TiB₂ and Al-impregnated specimens exhibited a wavy mass change in the localized mass increase and decrease, whereas the Si-impregnated specimens exhibited a smooth monotonic increase. The XRD results for each sample surface after oxidation tests are shown in Fig. 3. For the unimpregnated specimens after oxidation, only TiO₂ (rutile) was identified. By contrast, Al₁₈B₄O₃₃ and Al₂O₃ (corundum) were identified in the Al-impregnated sample, in addition to TiO₂. For the Si-impregnated sample, SiO₂ was observed in addition to rutile. As shown in the Figs. 4(a) and (b), the post-oxidation cross-sectional elemental mapping images of the TiB₂ and Al-impregnated specimens showed ∼700 µm thick oxide scales consisting of Ti and O. In comparison, the Si-impregnated specimen exhibited a mixed phase of Ti–O and Si–O compounds with a thickness of ∼200 µm. In both cases, a minor amount of B was detected in the oxide scale. From the elemental mapping in Figs. 4(b) and (c), no clear Al/Si depletion zone near the matrix-oxide interface was identified. The unimpregnated and Al-impregnated specimens exhibit interlayer spallation in the oxide scales near the matrix. By contrast, the Si-impregnated specimens showed a continuous adhesive Si–O scale between the Ti–O oxide and matrix. The contrasts observed in the oxidation film in the BSE image of Fig. 4(c) correspond to Si concentration in the dark phase and Ti concentration in the bright phase. According to the results of XRD analysis, it can be determined that they correspond to SiO₂ and TiO₂ phases, respectively.

Fig. 2

Mass change during oxidation at 1300°C.

Fig. 3

X-ray diffraction patterns for the surface of oxidized specimens.

Fig. 4

Cross-sectional elemental-mapping images for oxidized TiB₂ specimens (a) without and with the impregnation of (b) Al or (c) Si.

4. Discussion

4.1 Effects of Al impregnation

As shown in Fig. 2, the oxidation mass gain curves for all samples exhibited an increase consistent with the parabolic law, indicating that oxidation was not saturated. From this, it can be inferred that when the oxide film forms with good adhesion, such as the one observed in the Si-impregnated samples, the rate of oxidation is controlled by the mass transportation within the oxide film. In contrast to the Si-impregnated case, the unimpregnated and Al-impregnated samples showed a characteristic wavy mass gain curve, with repeated local mass increases and decreases in the oxidation test at 1300°C. The weight gain was higher for the Al-impregnated sample than that for the unimpregnated TiB₂. The high-temperature oxidation behavior of pure TiB₂ is generally understood as follows (Fig. 5): 1. Oxidation of TiB₂ starts at ∼700°C. In the initial stages of oxidation, solid phase TiO₂ and liquid phase B₂O₃ are formed on the top surface. Problems associated with oxidation are unlikely to occur up to 1000°C because B₂O₃ (liq.) covers the bulk surface and acts as a protective film against oxygen penetration. 2. When the oxidation temperature exceeds 1000°C, a portion of B₂O₃ begins to evaporate. As the temperature increases, the mass gain due to oxide formation competes with the mass loss due to evaporation of B₂O₃, and above 1800°C, B₂O₃ forms and evaporates immediately. In this state, B₂O₃ no longer functions as a protective coating, and the oxidation resistance of TiB₂ was lost. Based on the oxidation test at 1300°C, as illustrated in the schematic diagram shown in Fig. 5, only B₂O₃ evaporates from the originally mixed-phase oxide film of TiO₂ and B₂O₃. As a result, a porous and non-protective TiO₂ oxide film is formed on the surface of TiB₂. As the oxidation time increases, the waves in the curve become larger, which is speculated to be due to the increase in reaction area as the sample surface becomes porous. In the current study, the oxidation behavior at 1300°C was similar for the unimpregnated specimens and Al-impregnated samples (Fig. 2). This suggests that the Al oxide or reaction product did not serve as a stable, protective coating at 1300°C. The reason for the higher mass increase in the Al-impregnated sample compared to that in the unimpregnated sample may be the immobilization of boron through aluminum borate formation to suppress the evaporation of boron through B₂O₃. Al₁₈B₄O₃₃ identified by XRD, as shown in Fig. 3, was produced by the following reaction:

  

\begin{equation*} \text{2B$_{2}$O$_{3}$} + \text{9Al$_{2}$O$_{3}$}\to \text{Al$_{18}$B$_{4}$O$_{33}$}. \end{equation*}

This indicates that the part of B₂O₃ that had evaporated in the unimpregnated TiB₂ reacted with Al₂O₃ and transformed to a stable phase. In Fig. 2, it can be seen that the rapid increase in oxidation mass gain occurred above 1000°C. Referring to the pseudo-equilibrium phase diagram of Al₂O₃-B₂O₃ [33], it is evident that a phase transformation occurs around ∼1030°C. Below or equal to 1030°C, in the range where the amount of Al₂O₃ to B₂O₃ is ≤66 mol%, Al₄B₂O₉ is formed. Conversely, above 1030°C in the same range, Al₁₈B₄O₃₃ is formed. In the Al-impregnated samples, it is speculated that there was a rapid increase in the fixation of B and O upon exceeding this temperature during the oxidation test, leading to an increase in mass gain. From above, the Al impregnation treatment of TiB₂ suppresses the desorption of B by oxidation, although no significant improvement in the oxidation resistance was observed.

Fig. 5

Schematics of oxidation processes for TiB₂ without and with the impregnation of Al/Si.

4.2 Effects of Si impregnation

In the isothermal oxidation test conducted in this study at 1300°C, the Si-impregnated specimens showed better oxidation resistance than that by the unimpregnated specimens (Fig. 2). The oxide scales observed in the cross-section of the unimpregnated sample after oxidation (Fig. 4(a)) were composed of Ti oxides, all of which were porous layers. These do not function as protective oxide scales, which resulted in a relatively large increase in oxidation. However, in the cross-section of the Si-impregnated sample after oxidation (Fig. 4(c)), in addition to Ti oxides, Si was observed to aggregate at the interface with the matrix phase. Based on the XRD results of the sample surface after the oxidation test (Fig. 3), these phases were identified as SiO₂, which is stable at high temperatures, formed over a wide area of the interface between the matrix phase and the scale, and is highly protective. Besides, no Si-depletion zone below the segregation in Fig. 4(c), implying diffusion of Si was high enough to maintain protectiveness of SiO₂ layer even in the case of damage. Therefore, the Si impregnation treatment of TiB₂ has formed a dense SiO₂ phase on the matrix surface and improved the oxidation resistance (Fig. 5). The improved oxidation resistance on TMB₂ above 1300°C due to silicon oxides had been investigated by M.M. Opeka et al. [34] At 1400°C, SiO₂ and B₂O₃ forms borosilicate glass as a protective oxide layer. On the outer side of the borosilicate, SiO₂ becomes enriched as B₂O₃ evaporates, further reducing O₂ transport. The formation of this glass protective layer can enhance the protection up to at least 1600°C, with the potential to exhibit significantly improved oxidation resistance above 2000°C by further adding other TMB₂ compounds.

Considering the above results and discussion, the LPI treatment of Si is a promising new method to improve high-temperature oxidation resistance while minimizing the effect on high-melting-point borides. However, the mechanism of molten material penetration into the sintered compact by impregnation treatment, its controlling factors, and the effects of molten Si impregnation treatment on various properties of the sintered compact is not clear. When considering practical applications, it is necessary to conduct cyclic oxidation tests in addition to the isothermal oxidation tests performed in this study to verify the delamination resistance of the protective coating. Moreover, it is known that for SiO₂ layers, the amorphous layer crystallizes at temperatures above 1400°C, and for SiO₂-B₂O₃ layers, it crystallizes at temperatures above 1500°C, resulting in significant volume changes and embrittlement. Therefore, in the future, cyclic oxidation testing and oxidation testing at higher temperatures should be conducted to confirm the practicality of the molten Si impregnation treatment for applications in high-temperature environments. In the future, the applicability of the molten Si impregnation treatment to high-melting-point borides needs to be studied from various viewpoints.

5. Conclusion

This study aimed to investigate the applicability of the LPI treatment of Al and Si to enhance the oxidation resistance of TiB₂. The following results were obtained:

1. (1)    The molten Al impregnation process can distribute Al in a network at the grain boundaries near the surface of TiB₂-sintered compacts. Although this process did not improve the oxidation resistance of TiB₂, the evaporation of the boron component associated with oxidation was partially suppressed.
2. (2)    The molten Si impregnation process enabled the distribution of Si in the form of a network on the grain boundaries near the surface of the TiB₂ sintered compacts. This process also markedly improved the oxidation resistance of TiB₂ at 1300°C. This is possibly owing to the forming of a SiO₂ protective phase known as a stable oxide film.

Acknowledgments

The authors thank Dr. Kiyohiro Yabuuchi and Dr. Keisuke Mukai for performing the EPMA and heat treatment, which was supported by the Joint Usage/Research Program on Zero-Emission Energy Research, Institute of Advanced Energy, Kyoto University (ZE2022A-02). This work was supported by GIMRT Proposals of the Cooperative Research and Development Center for Advanced Materials (202112-CRKEQ-0404). The authors thank Mr. Kotaro Seki, Mr. Nozomi Mizumoto, and Mr. Geng Diancheng for their fruitful discussions.

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