Introduction

To date, the primary concern lies in the development of materials capable of enduring extended periods in oxidizing environments at high temperatures [1–3]. In this regard, a noteworthy focus is on ultra-high-temperature ceramics (UHTC), which consist of a ceramic matrix and a protective structure that shields the surface from oxidation [1; 4; 5]. In order to produce such materials a deliberate selection of additive phases is conducted to facilitate the formation of surface structures that can withstand prolonged exposure to oxidizing environments at high temperatures. One prevalent example of UHTCs is composite materials based on zirconium or hafnium borides, which exhibit high thermal conductivity and thermal shock resistance [6].

Sintering zirconium and hafnium boride powders requires temperatures exceeding 1950 °C due to their strong covalent bonds and low self-diffusion coefficient [7; 8]. The sintering process is typically activated by incorporating sintering additives such as SiC, Si3N4 , La2O3 , LaB6 [9; 10], tantalum, titanium, zirconium, molybdenum silicides, etc. The optimum composition is considered to be the volumetric ratio of 80 % ZrB2 to 20 % SiC [11; 12]. At high temperatures, the oxidation of these ceramics leads to the formation of a protective multi-layer coating on the surface, consisting of ZrO2–SiO2 and borosilicate glass. This coating effectively seals cracks and pores on the UHTC surface, creating a gas-tight film that prevents oxygen diffusion into the material [13; 14].

Spark plasma sintering (SPS) is a relatively recent technique that enables the reduction of sintering temperature and time for certain materials when performed under vacuum or an argon atmosphere [15]. Despite some existing studies in this area, the phase composition, structure, and oxidation properties of boride-based composite materials produced through this process have not been adequately investigated [15–17].

The objective of this study was to examine the impact of La2O3 addition on the oxidation properties of composite ceramics with the following volumetric composition: 80ZrB2–20SiC.

Materials and Methods of Research

The source materials utilized in this study included:

– zirconium diboride (DPTP Vega LLC., Russia);

– lanthanum hydroxide concentrate (Solikamsk Magnesium Plant JSC, Russia), with the following elemental content (wt. %): La – 54,2, Nd – 4,3, Pr – 2,8 and trace amounts of other elements (<0.1);

– grade 63C silicon carbide (Volzhsky Abrasive Works JSC, Russia).

The particle size distribution of the powders was determined using laser light diffraction with the Analysette 22 NanoTec plant (Fritsch GmbH, Germany). The ZrB2 powder exhibited a predominant particle size range of 0.5 to 12 μm, with 97 % of particles measuring less than 11.1 μm, and an average particle size of 4.52 μm. The SiC powder demonstrated a bimodal particle size distribution, with an average particle size of 3.47 μm. The La(OH)3 powder exhibited a wide particle size distribution, with an average particle size of 9.76 μm.

In order to convert lanthanum hydroxide into oxide, the material was subjected to annealing at 600 °C for a duration of 1 h in an air atmosphere. The initial powders were mixed using a SAND planetary mill in ethyl alcohol. The grinding process lasted for 2 h at a speed of 160 rpm, a grinding media to powder ratio of 3:1. The mixtures contained lanthanum oxide in two different volumetric percentages: 0, 2 and 5.

The specimens were consolidated using spark plasma sintering (SPS) on the Dr. Synter SPS-1050b equipment (SPS Syntex, Japan) at a temperature of 1700 °C. Heating was achieved by applying a pulsed direct current at a rate of 50 °C per min. The temperature was monitored using an optical pyrometer placed on the outer side of the graphite matrix. The material was loaded into the system just before the heating process began, while maintaining a constant load at a pressure of 30 MPa. The applied load was removed after the completion of the heating phase. To prevent any undesirable reactions between the sintered powder, matrix, and punches, graphite paper was employed. Additionally, graphite felt was wrapped around the mold to minimize heat losses. The specimens were subjected to an isothermal holding time of 5 min.

The apparent density and open porosity of sintered specimens were analyzed following the guidelines outlined in GOST 2409-2014.

For the oxidation study, the specimens were exposed to air in an electric furnace equipped with silicon carbide heaters. The crucibles containing the specimens were placed in the furnace, preheated to 1200 °C. After a specific duration, the specimens were removed, weighed to record their weight, and then returned to the furnace. The total duration of high-temperature oxidation was 20 h.

The phase composition of the specimens was examined using Raman spectroscopy on a multifunction spectrometer SENTERRA (Bruker, Germany), with a laser wavelength of 532 nm. The acquired data were processed using the OPUS 6.5 software.

Microscopic analysis of the specimens was conducted using an analytical auto emission scanning electron microscope VEGA3 (TESCAN, Czech Republic). In order to determine the elemental composition, an Inca X-Act detector (Oxford Instruments Analytical, Great Britain) was employed for energy-dispersive analysis of the elemental composition of the materials.

Results and Discussion

The investigations were conducted on the materials containing 0, 2 and 5 vol. % of La2O3 . No significant changes in the apparent density and open porosity of the specimens were observed after the SPS process. The open porosity values ranged from 3.5 to 5.5 % across all variations.

The specimens were subjected to oxidation in an air environment for a duration of 20 h. The time intervals for recording the specific weight gain of the specimens increased as the weight gain decreased. The results are presented in Figure 1. The most substantial weight gain occurred within the initial 2–4 h of testing.

Among the specimens, those with 5 vol. %. of La2O3 exhibited the lowest weight gain after the 20-hour exposure. It is worth noting that up to 9 h of exposure, the differences in specific weight gain were minimal. Only with further increases in oxidation time did the impact of La2O3 on the material’s oxidation resistance become more pronounced.

It has been established that the resistance of ZrB2 composite materials to high-temperature oxidation is primarily influenced by the composition of the protective layer formed on the material’s surface [18]. The fractures in the specimens were examined using scanning electron microscopy (SEM) along with energy dispersive analysis. SEM images and maps displaying the distribution of silicon, zirconium, and boron within the specimens were obtained (Figure 2: spectrum 1 represents the surface layer of the specimen; spectrum 2 corresponds to an internal structure displaying significant visual dissimilarity; spectrum 3 pertains to a deeper layer of the material).

In the specimen without the addition of La2O3 , a continuous protective layer composed of silicon-containing phases is formed on its surface (spectrum 1 in Fig. 2). When La2O3 is introduced, the silicon content on the specimen surface is also higher (spectrum 1) compared to the subsequent layers (spectra 2 and 3). However, the thickness of these layers is significantly reduced. Figure 3 presents a histogram depicting the calculated thickness of the oxidized layers, determined through analysis of the material’s microstructure (as shown in Figure 2).

Table 1 displays the outcomes of the energy dispersive analysis, providing the elemental composition of the specimens following a 20-hour oxidation process. The first layer exhibits identical composition across all three cases. Notably, boron is not detected in the spectra, which is consistent with the challenges in accurately capturing elements from the second period of the periodic table using spectral methods [19].

Table 2 presents the results of calculating the atomic composition of the layers, with values rounded to the first decimal place after considering the atomic weights of the elements (and then multiplied by 10).

The assumed chemical composition is derived from the known composition of the initial specimens and the determined element ratios within the layers. The primary stoichiometric phases that are likely to be present are listed in Table 2. It should be noted that the formation of silicon monoxide as a crystalline compound has been previously documented in multiple instances [20–22].

The possibility of non-stoichiometric compound formation, such as borosilicate and borate glasses, should also be taken into consideration [23].

Raman spectroscopy confirmed the presence of zircon (ZrSiO4) and monoclinic zirconium dioxide (ZrO2) as the main phases of the oxidized material (Figure 4) [24]. The imaging was conducted from the surface of the specimens, thereby removing the first layer. One of the obtained spectra is presented, with the others being identical. The only distinction lies in the intensity ratio of the peaks corresponding to the main phases. The absence of silicon oxide lines in the specimen without lanthanum addition can be explained by the fact that well-oxidized phases in the second layer hinder the detection of amorphous or concealed-crystalline silicon oxide phases.

There is a noticeable disparity in the distribution of zirconium on the surface of specimens with and without La2O3 (Figure 5, a–c). In the case without La2O3 , ZrO2 , despite its relatively low overall content (as seen in Table 1), is distributed relatively (Figure 5, a). Presumably, it is integrated within the primary silicon-containing phases. However, the addition of lanthanum oxide results in the growth of zirconium-bearing phase grains and the emergence of agglomerates and large pores between these phases. In this scenario, the formation of silicon oxides is unlikely.

Figure 5, d illustrates a fragment of the boron distribution map in the upper layer of the specimen containing 5 vol. % La2O3 , corresponding to the fragment of the zirconium distribution map in Figure 5, c. Since boron could not be detected during the determination of the mass content of elements (as indicated in Table 1), it can be inferred that boron is present in the form of silicate glasses (refer to Figure 2, f ) that fill the gaps between zircon and zirconium dioxide grains [25].

Consequently, on the surface of specimens without La2O3 additives, phases comprising silicon oxides with traces of boron and zirconium oxides are formed. In the presence of La2O3 additives, the main phases observed are zirconium dioxide and zircon, along with an excess of oxygen. However, neither case effectively acts as a significant barrier against the deep penetration of oxygen into the material. The presence of lanthanum oxide appears to enhance the formation of zircon, a phase that exhibits greater resistance to thermal shock than monoclinic zirconium dioxide and contributes to the deceleration of the oxidation process.

Conclusion

The aim of the study was to investigate the impact of La2O3 addition on the oxidation properties of composite ceramics with a composition of 80 vol. % ZrB2 and 20 vol. % SiC, consolidated through spark plasma sintering. The materials were examined in three variations: without La2O3 , with 2 vol. % La2O3 and 5 vol. % La2O3 . In all cases, SiC acted as a sacrificial material, being the first to undergo oxidation. Specimens without La2O3 addition exhibited a surface layer consisting mainly of SiO2 and SiO. On the other hand, specimens with La2O3 addition showcased surface layers composed primarily of ZrSiO4 and ZrO2 .

Consequently, the introduction of La2O3 intensified the formation of zircon and decelerated the oxidation processes. However, it did not serve as a complete barrier to the deep penetration of oxygen into the material.