Introduction

Ultrahigh-temperature ceramics (UHTC) produced from borides of transition metals in group IV of the Periodic Table have melting points above 3000°C, high thermal conductivity (>80 W/m · K) [1,2,3], and excellent corrosion resistance [4,5,6]. Hafnium boride shows high corrosion resistance but also high specific weight. Possessing the lowest specific weight, titanium boride exhibits poor oxidation resistance. Zirconium diboride is characterized by an optimal combination of corrosion resistance and specific weight, allowing it to be used as a UHTC matrix. Hence, efforts are ongoing to develop zirconium boride composite ceramics with sintering activators. The activators should not only improve densification but also promote appropriate mechanical properties. Sintering is activated by SiC, Si3N4, MoSi2, WC, C, B4C, ZrSi2, and other additions [7,8,9,10,11,12,13].

The ZrB2–SiC system is a basic zirconium boride ceramic material that shows an optimal combination of mechanical properties. The maximum strength and corrosion resistance are acquired when 15–20 vol.% SiC is added [7,8,9,10]. However, silicon carbide is no longer an exhaustive addition: its maximum strength at 1800°C can reach only 200 MPa [11]. The highest strength (836 ± 113 MPa) at 1800°C was exhibited by a ZrB2–SiC–WC composite [12].

The oxidation resistance of ZrB2–SiC–WC ceramic composites was studied at 1500°C with a 15-min holding time, the scale being 40 μm thick [13]. The effect of corrosion on the mechanical properties was examined only at 1400°C for the ZrB2–15 vol.% MoSi2 system [14]. After being held in an oxidizing environment for 100 h, the ceramic composites showed 50% lower strength than the initial value [14].

Our objective is to analyze how phases form in the ZrB2–15 vol.% SiC–5 vol.% WC system, determine the bending strength of this ceramic composite at room temperature and 1800°C, study its oxidation resistance at 1500°C with a 50 h holding time and at 1600°C with a 2 h holding time, and find out how oxidation influences its bending strength.

Experimental Procedure

The starting powders (Table 1) produced by H.C. Starck (Germany) were mixed in a SAND-1 planetary-ball mill for 5 h in acetone with WC hardmetal grinding balls. The particle size of the ground powder determined by sedimentation was ~0.8–1.5 μm. Then the powder mixture was compacted in steel dies at 100 MPa to reach ~50% of the theoretical density. A combined process was employed to make ceramic composites: annealing in vacuum at 1600°C for 1 h and further hot pressing with an SPD-120 unit at 2050°C, 32 MPa pressure, and isothermal holding for 15 min. The material showed 99% density versus the theoretical value.

TABLE 1. Characteristics of the H.C. Starck Starting Powders for Producing ZrB2–15 vol.% SiC–5 vol.% WC Ceramics
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The structure of ZrB2–15 vol.% SiC–5 vol.% WC ceramics was examined by scanning electron microscopy with a Tescan VEGA 3 system with a Bruker detector for electron microprobe analysis and by X-ray diffraction with a DRON-3 diffractometer (Cu-Kα radiation) using a lanthanum hexaboride reference sample. The X-ray diffraction patterns were processed with the NewProfile software.

The samples were oxidized in an LHT 01/17 D furnace (Nabertherm, Germany) in anisothermal conditions in air at 1600°C for 2 h and at 1500°C for 50 h. The samples were cooled down with the furnace.

The samples to be tested by three-point bending at room temperature and 1800°C were 3.5 mm × 4.5 mm × × 36 mm in size. Steel supports with a 20 mm base were used for room-temperature tests and graphite supports for high-temperature tests (1800°C, vacuum).

Results and Discussion

Structurization. Figure 1 shows structure of the material produced. According to microscopic studies, the material contains four phases: gray zirconium diboride, black silicon carbide, light gray zirconium carbide solid solution, and white tungsten boride. The presence of these phases was also ascertained by X-ray diffraction (Fig. 2). The amount of silicon carbide was 14 vol.%, that of zirconium carbide solid solution was 4.4 vol.%, and that of tungsten boride 8.8 vol.%, zirconium diboride being the rest.

Fig. 1.
figure 1

Microstructure and phase composition of the ZrB2–15 vol.% SiC–5 vol.% WC ceramic composite

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Fig. 2.
figure 2

X-ray diffraction pattern for the hot-pressed ZrB2–15 vol.% SiC–5 vol.% WC ceramic composite; lanthanum hexaboride used as reference for X-ray diffraction

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As mentioned previously, no components interact to form new phases in the ZrB2–SiC [11] and SiC–WC [15] systems. Nevertheless, components interact in the ZrB2–WC [16] and ZrB2–SiC–WC [17] systems. These systems are common in that zirconium boride solid solutions, zirconium carbide phases, and tungsten boride phases are formed. Interaction of tungsten carbide with oxides present on the starting powder particles is one of the main reactions. The interaction between components can be described by the following reactions:

$$ 3{\mathrm{ZrB}}_2+{\mathrm{ZrO}}_2+6\mathrm{WC}\to 6\mathrm{WB}+4\mathrm{ZrC}+2{\mathrm{CO}}_{\left(\mathrm{g}\right)}\uparrow; $$
(1)
$$ 2\mathrm{WC}+{\mathrm{B}}_2{\mathrm{O}}_3\to 2\mathrm{WB}+3{\mathrm{CO}}_{\left(\mathrm{g}\right)}\uparrow . $$
(2)

Oxygen interacts with carbon to form CO(g)↑ according to reaction (1). Our calculations indicate that reaction (1) is thermodynamically favored, its free energy at 1900°C being –40 kJ/mole. The likelihood of this reaction to proceed was reported in [12, 13, 15,16,17].

On the other hand, reaction (2) is more favored than reaction (1) at 1600°C because the Gibbs free energy is –72 kJ/mole. Therefore, annealing at 1600°C in vacuum increases the amount of tungsten boride and decreases the amount of oxygen through interaction between WC and B2O3 [13]. Chemical analysis data are also indicative of reduction reactions and oxygen decrease: oxygen amount was 1% in the starting mixture, 0.3% after additional heat treatment, and lower than 0.1% following additional heat treatment and hot pressing. Note also that SiO2 [21, 22] and B2O3 [18] oxides start evaporating above 1100°C in vacuum. Oxygen is concentrated in the SiC phase in the ceramic material in accordance with electron microprobe analysis (Fig. 1).

Zirconium boride solid solutions emerge in the hot pressing process, which is indicated by local microprobe analysis (Fig. 1) and by change in the lattice parameter from 0.3170 nm for pure zirconium boride to 0.3158 nm for its solid solution (Fig. 2). The solubility of tungsten boride in zirconium boride can reach 5 mol.% [19]. The general formula of the solid solution may be (Zr0.97W0.03)B2 (Fig. 1). The zirconium diboride solid solution is observed as shells at the grain boundaries and pure zirconium diboride concentrates within the grains (Fig. 1).

Besides the zirconium boride solid solution, a zirconium carbide solid solution is formed. According to [20, 21], the amount of dissolved tungsten in the zirconium carbide phase may exceed 5 mol.%. Hence, zirconium boride and zirconium carbide solid solutions show up when high-temperature ceramic materials are produced. Additional heat treatment in vacuum reduces the amount of oxygen to 0.1%.

The grain size is among the important factors that determine the mechanical properties. The average grain sizes of the ceramics are summarized in Table 2. The average size of the zirconium boride grains was found considering their shells.

TABLE 2. Grain Size and Strength of the Ceramic Composite at Room Temperature and 1800°C
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Tungsten carbide additions more significantly increase the ceramic strength at 1800°C as rigid grain-boundary phases show up and complicate ductile slipping and fracture at high temperatures. As we reported previously [22] for ZrB2 UHTC with tungsten-containing additions, grain-boundary shear processes are important for high-temperature tests. In the materials under study, these processes develop at grain boundaries of oxygen-containing SiO2 phases with low melting points, resulting in microcracks near the boundaries and thus lower strength.

The role of grain-boundary shear processes decreases with smaller areas of grain boundaries (greater grain sizes) and with increasing rigidity of grain-boundary regions, resulting from the presence of grains with solid-solution shells. The solid-solution shells at zirconium boride grain boundaries strengthen the boundary areas of the ceramics. Hence, grain-boundary phases with high melting points prevent grain-boundary shear processes [12, 22]. The emergence of such high-temperature and high-strength phases at grain boundaries allows the ceramics with 800 MPa strength at 1800°C to be produced.

These UHTC structural features are described in [12]. In high-temperature tests of such materials, intercrystalline fracture changes to transcrystalline fracture, including that for grains of other phases (tungsten boride); this increases strain resistance and fracture resistance at 1800°C [23].

A greater amount of tungsten boride increases high-temperature strength [23]. However, the paper [24] indicates that the tungsten-containing component should not exceed 10 vol.% to prevent oxidation resistance from being reduced substantially.

High-Temperature Oxidation. The oxidation of ZrB2–15 vol.% SiC–5 vol.% WC ceramics is described in [13, 25,26,27]. However, holding time at 1500°C was no more than 15 min in those studies. This is not sufficient for kinetic processes to occur. The effect of oxidation on residual strength was not considered either.

First consider the material that was oxidized at 1500°C for 50 h. Figure 3a shows a cross-section of the oxidized ceramics. The upper scale part consists of SiO2–B2O3 and is 64 μm thick (Fig. 3a, c). Below is a zirconium oxide layer up to 20 μm thick, with inclusions of tungsten oxide and silicon oxide (Fig. 3b, d, e). The interface between SiO2–B2O3 and ZrO2 + MexOy is clear and has no cracks or pores (Fig. 3b).

Fig. 3.
figure 3

Cross-sectional microstructure of the ceramic composite oxidized at 1500°C for 50 h: a) scale appearance, b) upper scale part with distribution of elements; SiO2 (c), ZrO2 (d), and WO3 (e) spectra

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We observe somewhat different behavior when oxidation proceeds at 1600°C for 2 h. The cross-section is shown in Fig. 4a. The oxidized layer is no more than 85 μm thick, including a SiO2–B2O3 layer 10 μm thick and a ZrO2 + SiO2 + MexOy layer. Electron microprobe analysis and X-ray diffraction (Fig. 5) demonstrate that zirconium oxide with inclusions of other phases, such as SiO2 and WO3, is the main phase in accordance with the spectra of elements (Fig. 3c, d).

Fig. 4.
figure 4

Cross-sectional microstructure of the ceramic composite oxidized at 1600°C for 2 h: a) scale appearance; b) upper scale part with distribution of elements

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Fig. 5.
figure 5

X-ray diffraction pattern for the ceramic surface oxidized at 1600°C for 2 h against a LaB6 reference sample (a); bending strength versus oxidation time (b) at 1500 (1) and 1600°C (2) for the ZrB2–15 vol.% SiC–5 vol.% WC ceramic composite against ZrB2–15 vol.% SiC after oxidation at 1500°C (3)

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A clear interface between SiO2–B2O3 and ZrO2 + MexOy is difficult to see at an oxidation temperature of 1600°C (Fig. 4a). This is because liquid SiO2–B2O3 is highly viscous at 1500°C and greater temperatures decease its viscosity, leading to complete spreading on the surface. Since SiO2–B2O3 wets ZrO2 well, zirconia penetrates between the grains and exerts a splitting effect and we can thus observe individual zirconium dioxide grains in SiO2–B2O3 (Fig. 4b).

Therefore, oxidation of the composite is described by reactions (3)–(6). At the initial stage (~800°C), the zirconium boride solid solution oxidizes [5]:

$$ {\mathrm{ZrB}}_2+{\mathrm{O}}_{2\left(\mathrm{g}\right)}\to {\mathrm{ZrO}}_2+{\mathrm{B}}_2{\mathrm{O}}_3; $$
(3)
$$ {\mathrm{WB}}_2+3{\mathrm{O}}_{2\left(\mathrm{g}\right)}\to {\mathrm{WO}}_3+{\mathrm{B}}_2{\mathrm{O}}_3. $$
(4)

When temperature increases to 1300°C, silicon carbide oxidizes actively:

$$ \mathrm{SiC}+{\mathrm{O}}_{2\left(\mathrm{g}\right)}\to {\mathrm{SiO}}_2. $$
(5)

Holding at 1500°C results in a relatively thick SiO2–B2O3 scale layer (Fig. 3b):

$$ {\mathrm{SiO}}_2+{\mathrm{B}}_2{\mathrm{O}}_3\to {\mathrm{SiO}}_2\hbox{--} {\mathrm{B}}_2{\mathrm{O}}_3. $$
(6)

At an intermediate stage (1000–1200°C), zirconium carbide oxidizes to zirconium oxide and tungsten boride to tungsten oxide.

On the other hand, there is a eutectic (1230°C) in the ZrO2–WO3 system [28], allowing the material to be sintered at low temperatures. The paper also reports that tungsten carbide additions improve oxidation resistance of the zirconium boride ceramics through the formation of dense scale layers [29].

The residual strength of the UHTC measured after oxidation at 1500°C for 50 h constituted 70% of the initial strength (Fig. 5b). The insignificant decrease in strength is due to the development of a dense scale without pores or cracks. Higher oxidation temperature has a more negative effect on the residual strength: it reduces to less than 50% of the initial strength at 1600°C (holding for 2 h). This sharp decrease in strength is due to fracture of the ZrO2 layer through splitting with the SiO2–B2O3 liquid. Comparison of the ZrB2–15 vol.% SiC–5 vol.% WC material with the basic ZrB2–15 vol.% SiC composite indicates that tungsten carbide additions favorably contribute to the residual strength (Fig. 5b).

Therefore, a dense three-layer scale develops in the oxidation of ZrB2–15 vol.% SiC–5 vol.% WC, allowing this material to be reused following oxidation at 1500°C within 50 h and at 1600°C within 2 h.

Conclusions

Components of the ZrB2–15 vol.% SiC–5 vol.% WC ceramic composite interact in the hot pressing process to form new phases: WB, (Zr, W)C, and (Zr, W)B2. These high-temperature and high-strength phases in the (Zr,W)B2–SiC matrix allow ceramics with 800 MPa strength to be produced at 1800°C.

The scale developed in the oxidation process at 1500 and 1600°C consists of three layers: un upper borosilicate glass layer, a medium layer of zirconium oxide and other oxide phases (WO3 and SiO2), and a lower basic material layer depleted of boron and silicon. These layers become thicker with increasing temperature.

The dense scale retains 70% strength of the ceramics after oxidation at 1500°C (50 h). The residual strength of the ceramics is 50% of the initial strength at an oxidation temperature of 1600°C (2 h).