1. Introduction

Many research papers of the anti-oxidation of superhigh temperature ceramics have been published [1-14]. Thermal degradation of the material properties of super high temperature ceramics such as borides, carbides, nitrides, and oxides is very important because they have been used in high temperatures. Zirconium diboride (ZrB₂) as a representative high temperature material has an excellent electrical conductive and mechanical strength. But has a weakness of oxidation from the surfaces exposed to high temperature. To inhibit the high temperature oxidation behavior, many research groups have conducted of the antioxidation of ZrB₂ by the composition. The representative composite materials are ZrB₂-silicon carbide (SiC) [3-10], and ZrB₂-tungsten carbide (WC) [11-14]. For ZrB₂-SiC, the formation of silicon dioxide (SiO₂) layer from the oxidation of SiC is known to play a role to prohibit the diffusion of oxygen from the surface to the inner side. For ZrB₂-WC, the volume expansion of tungsten oxide (WO₃) layer formed from the oxidation of WC is known to decrease the thermal degradation of the composite property. In this paper, we focus on the antioxidation effects of the addition of SiC and WC to ZrB₂. Until now, the addition of SiC and WC to ZrB₂ ceramic was studied for their chemical reactions and strong mechanical properties [13, 14], but now the anti-oxidation properties are not studied. So, we think that the study of simultaneous SiC and WC addition effects to the anti-oxidation of ZrB₂ ceramic is needed. Parthasarathy et al. [15, 16] suggested the model for the oxidation of ZrB₂ in the temperature range; low temperature range below 1000°C, intermediate temperature range between 1000-1800°C, and high temperature range over 1800°C. Boric oxide (B₂O₃) does not evaporate and exists as a liquid phase in the low temperature range, but begins to evaporate in the intermediate temperature, and zirconium oxide (ZrO₂) can evaporate in the high temperature range. In our study, we try to understand the effective oxygen diffusion in ZrB₂ in the case of the addition of either SiC or WC.

2. Experiment

2.1 Raw materials and the sample powder preparations

The raw materials used were ZrB₂ (99.5%, 1~2 μm, APS powder, Alfa Aesar), SiC (99.8%, 1 μm, S.A. 11.5 m²/g beta-phase, Alfa Aesar), WC (99%, 2 μm, Sigma- Aldrich). Four ZrB₂-based composites with different composition were used for this study. Table 1 summarizes the four batches prepared in this study, which are referred to as Z (ZrB₂), ZS (ZrB₂-SiC), ZW (ZrB₂-WC), ZSW (ZrB₂-SiC-WC), respectively. In Table 1, we presents the weight and volume increase of the samples, supposing that 100% of the samples are oxidized to form ZrO₂, SiO₂ and WO₃ and calculate the weight and volume of the oxidized samples. The reference samples of Z, ZS, and ZW were prepared for the exact analysis of SiC-WC addition effects to the thermal degradation. The compositions of samples were shown in Table 2. It is reported that ZrB₂-SiC shows the best antioxidation properties of dense and optimal viscosity after the thermal oxidation at high temperature for the volume fraction of 76-84%, and so we determined the SiC volume fraction as 20% for the excellent sinterability and antioxidation. The addition of WC at 6 mole% is reported that the volume expansion of WO₃ generated from the oxidation of WC through the phase transformation shows the crackfree and excellent anti-oxidative sintered ZrB₂ composite. So, the 6 mol% of WC addition was determined for the preparation for the ZrB₂-WC composite. Small amount (about 0.5%, respectively) of B₄C and C (graphite powders) were added to remove the oxidized impurities of ZrB₂ surfaces and improve the sinterability. Raw materials of ZrB₂, SiC, WC, B₄C and C were ball-milled in the methanol for 24 h and rotary-evaporated. After that, the mixed powders were sieved using #100 mesh screen and dried in a vacuum oven at 80°C for 24 h.

Table 1

The compositions of the samples, Z, ZS, ZW and ZSW

Table 2

Oxidation layer thickness, measured and calculated weight increases after torch test

2.2 SPS sintering

The prepared powders were packed in the graphite mold and then sintered by using the spark plasma sintering equipment (SPS). The sintering conditions are like these; the temperature rise of 100°C/min and the sintering temperature and pressure at 1800°C and 30 MPa for Z and ZS and at 1900°C and 60 MPa for ZW and ZSW, and the sintering holding time of 5 min. The densities of the samples (the bulk densities) were measured by Archimedes’ method and theoretical densities were calculated for the composition of the samples. The relative densities mean the ratio of measured bulk densities to theoretical densities. The full relative densities of the samples were obtained except for ZS with 99.7% of relative density.

2.3 Anti-oxidation property analysis

The anti-oxidation analyses were conducted through two different methods: torch thermal oxidation and ther- mogravimetric (TGA) method. The torch oxidation was conducted by the exposure of the samples to propane and oxygen (20%) flame at 1600°C measured by optical pyrometer for 15 min (Fig. 1). The TGA method were conducted for the oxidation of the samples at 1500°C for 15 min at the temperature rise of 10°C/min at ambient atmosphere to measure the increase of sample weights (TGA-DTA, DTG-60H and TA-60WS, Shimadzu). The thicknesses of the oxidation layers were observed by using field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDS) mapping images (JSM-6700F, JEOL, Japan).

Fig. 1

Torch oxidation experiment set-up scheme (a), the photographs of samples before (b) and after (c) torch oxidation experiment.

3. Results

3.1 Oxide scale formations

1) ZrO₂ columnar scale formation from B₂O₃ evaporation

FE-SEM images before and after torch oxidation test were observed in Fig. 2. After the torch oxidation test, we analyzed the oxidation behaviors by SEM and EDS mapping image observations (Fig. 2). Z sample without the addition of SiC and WC showed 49 μm of oxidation layer thickness. The oxidation layer thicknesses are 29.4, 33.4, and 16.9 μm for ZS, ZW and ZSW, respectively. For Z, the surface layer is composed of ZrO₂ scale layer, which has the columnar structure. The channels between the columns are considered to be paths for oxygen diffusion and boron evaporation.

Fig. 2

FE-SEM images and their correspondent EDX elementary images after torch oxidation. (red; Zr, green : B, blue: oxygen, purple : Si, cyan : W) of the oxidized samples, (a) Z, (b) ZS, (c) ZW, and (d) ZSW. (The scale bar is 40 μm.)

2) SiO₂ glass outer layer and ZrO₂ oxide inner layer

The oxidation layer of ZS is composed of two layers; the outer layer is a SiO₂ layer with about 5 μm thickness and the inner layer is a porous ZrO₂ layer with about 25 μm of thickness. The anti-oxidation effect of SiC is known to result from the formation of SiO₂ layer by the oxidation of SiC on the surface to inhibit the oxidation of ZrB₂. SiO₂ generated from the oxidation of SiC diffuses out to form amorphous silica layer on the surface of the samples, which was identified by XRD data in Fig. 2. After oxidations of ZS and ZSW, the XRD patterns of the samples showed the broad band at around 20°, which is known to be amorphous silica phase. We also confirmed the formation of amorphous silica phase on the surface of the samples, ZS and ZSW in the FE-SEM microstructures in Fig. 2. We guess that SiO₂ amorphous phase migrates onto the surface although ZrO₂ formation from ZrB₂ occurs at the same place so that B₂O₃ gas evolves out and the oxidized ZrB₂ layer is porous. A. Rezaie et al. [3] suggested that B₂O₃ rich borosilicate scale layer formed on the surface of ZrB₂-SiC ceramics below 1200°C and SiO₂ rich scale layer is generated from the evaporation of B₂O₃ gas above 1200°C. Below the surface scale layer, ZrO₂-SiO₂ layer exist and the below are ZrB₂-SiC bulk layer. It indicates that borosilicate glass layer forms below 1200°C but turns to be SiO₂ amorphous layer from the evaporation of B₂O₃ above 1200°C. Fahrenholtz [9] also investigated a thermodynamic behavior of oxidation of ZrB₂-SiC. He insists that the initial formation of ZrO₂ and B₂O₃ layer on the surface at 1200°C due to the rapid oxidation rate of ZrB₂ rather than SiC. At 1500°C, SiC oxidation rate increases to form SiO₂ layer and SiC depletion layer between the surface SiO₂ layer and SiO₂ and ZrO₂ layer. In our study, the surface layer is thought be SiO₂ amorphous phase, and SiC depletion layer exists below the SiO₂ layer which was thought to be consisted of ZrO₂ or ZrB₂.

3) ZrO₂/WO₃ oxide layer formation

ZW has an oxidation layer with 33.4 μm of thickness, which is thought be composed of ZrO₂ and WO₃ mixtures. It is known that WC addition to ZrB₂ improves the anti-oxidation properties of ZrB₂ ceramics due to its densification of oxidized ZrO₂ scale by the volume increase of WO₃ formed from the oxidation of WC [11, 12]. During the oxidation, WO₃ turns to liquid phase within ZrO₂ scales so that the liquid phase sintering densifies the oxidized ZrO₂/WO₃ scale to result in the inhibition of oxidation. In our study, WC addition formed WB after sintering, which indicates that WC reacts with ZrB₂. We guess that WB is oxidized to form WO₃ liquid phase at 1500°C and a dense oxidized scale of ZrO₂/WO₃ phases. It increases the anti-oxidation effects of ZrB₂ so that there are no cracks on the surface of the samples after the oxidation test, although Z shows the cracks due to the formation of ZrO₂ scale and its phase transformation. In our case, the oxidized thickness of ZW was 33.4μm, 32% decrease against the oxidized thickness with 49 μm for Z.

4) SiO₂ glass phase outer layer and ZrO₂/WO₃ oxide layer formation

The oxidation layer of ZSW is composed of a 5-μmthick dense surface layer and a 12-μm-thick porous inner layer. From the EDS analysis mapping image, the location of ZrO₂ and WO₃ crystal phase and SiO₂ amorphous phase is not so clear. From the EDS mapping images for ZS and ZW and XRD data for ZSW, we guess that the dense surface layer is thought to be a SiO₂ amorphous layer and the inner porous layer is a ZrO₂ and WO₃ crystal phase mixtures.

The anti-oxidation effect by the addition of SiC and WC to ZrB₂ is prominent for our study. The oxidized scale thickness is about 17 μm, which was about one third of that for Z. From XRD and SEM-EDS results, we observed that the oxidized scale is composed of two layers, the outer surface layer is SiO₂ dense layer and the inner surface layer is composed of ZrO₂ and WO₃ layer. The two layers are considered to inhibit the diffusion of oxygen toward the bulk ZSW. After sintering, the major crystal phase of ZSW is ZrB₂ and the secondary phases are SiC and WB. WB crystal phase is known to form from the following chemical reactions [13, 14].

5ZrB2 +4WC = 5ZrC +2W2B5ZrB2 + 2WC = ZrC + 2WB

During the oxidation, ZrB₂ and SiC are oxidized to form ZrO₂ (s) and B₂O₃ (g) and SiO₂ and CO₂ (g) phases. SiO₂ amorphous phase is diffused out to the outer surface area and dense amorphous scale layer, which acts as an oxidation inhibition layer [3-10]. We guess that WB is also oxidized to form WO₃ inner scale layer, which are composed of WO₃ and ZrO₂ inner scale layer [11-14]. This layer is reported to be composed of ZrO₂/WO₃ liquid sintered phase to form a dense layer, which improves the anti-oxidation effect. So, in our study, the simultaneous addition of SiC and WC improves the anti-oxidation effect three times as much as that of ZrB₂ by the formation of SiO₂ amorphous outer scale (1) and ZrO₂/WO₃ inner scale (2).

(1)

SiC(s) + 2 O2(g) = SiO2(s) + CO2(g);SiO2 amorphous inner scale formation

(2)

ZrB2(s) + 2.5 O2(g) = ZrO2(s) +B2O3(g)WB(s) + 2.25 O2(g) = WO3(s) + 0.5 B2O3 (g)ZrO2/WO3 inner scale formation}

3.2 Weight changes analysis of the samples by torch and TGA

The weight changes of the samples by TGA are shown in Fig. 4. The samples were placed in alumina sample holder and heated to 1500°C holding for 15 min. The weight increases of the samples are 106.6% for Z, 101.9% for ZS, 105.2 for ZW and 101.0% for ZSW. When the samples were maintained at 1500°C for 30 min, the weight increase of the samples are 112.0% for Z, 110.8% for ZS, 110.9 for ZW and 102.1% for ZSW. The weight increase of the samples before and after the torch oxidation test at 1600°C for 15 min in an ambient atmosphere were 27.4, 22.6, 21.9, and 13.9 μg/mm² for Z, ZS, ZW and ZSW, respectively.

4. Discussion

4.1 The oxidation reactions and behaviors

The oxidation behavior of ZrB₂ is suggested that ZrB₂ is oxidized to produce ZrO₂ structure and B₂O₃ liquid phase between ZrO₂ columns and from the interface between B₂O₃ interphase and ambient atmosphere is evaporated B₂O₃ gas phase (Fig. 3(a)). Oxygen gas phase permeates through the outer ZrO₂ porous layer and diffuses into the B₂O₃ liquid phase, and then reacts with ZrB₂ in the ZrB₂ and ZrO₂/B₂O₃ interface. For ZS, the outer scale layer is found as SiO₂ amorphous layer (Fig. 3(b)). First, ZrO₂ crystals and B₂O₃ liquid phase are formed from the oxidation of ZrB₂ and B₂O₃ gas phase evaporates onto the surface and SiC is also oxidized to from SiO₂ phase. Borosilicate glass (88% of SiO₂ and 10% of B₂O₃ and less than 2% of ZrO₂) is known to form from the oxidation of ZrB₂ containing 15% of SiC. In our study, amorphous phase of SiO₂ was confirmed from XRD data and EDS data (Fig. 2). The outer SiO₂ dense amorphous layer is considered to inhibit the diffusion of oxygen gas into ZrB₂. The addition of WC, which is converted as WB from the reaction with ZrB₂, plays a role of liquid phase sintering, and so the ZrO₂/WO₃ oxide layer is known to be formed. Compared to ZrB₂, WCadded ZrB₂ shows a dense oxide scale layer due to the volume expansion of WO₃ (Fig. 3(c)). For ZSW, the oxidation of SiC and WC (converted to WB) produces SiO₂ outer layer and ZrO₂/WO₃ liquid phase sintered inner oxide layer. The dense diffusion inhibiting SiO₂ glass phase and the expansion of WO₃ cause the synergistic anti-oxidation effects of ZrB₂. The chemical oxidation reactions are expressed as followed (Fig. 3).

Fig. 3

Schematic diagram of oxidation of Z (a), ZS (b), ZW(c), and ZSW (d).

ZrB2 (s) +5/2O2 (g) = ZrO2 (s) + B2O3 (g)WC(s) + 2O2(g) = WO3(s) + CO(g)SiC(s) + 3/2O2(g) = SiO2(s) + CO(g)

We propose that WC reacts with oxygen to form WO₃ for the simple model calculation, although WC reacts with ZrB₂ to form WB for the samples. From the oxidation reactions, we calculated the weight increase of Z, ZS, ZW, and ZSW, supposing that 100% of ZrB₂, SiC and WC are oxidized to form ZrO₂, SiO₂, and WO₃ and no evaporation of the oxides occur. The weight gains of the samples are 109.19, 113.92, 110.10, and 113.60% for Z, ZS, ZW, and ZSW, respectively. The correspondent volume gains are 116.98, 129.86, 122.80, and 133.39%, respectively (Table 1). We measured the oxidation thickness after torch test by FE-SEM and EDS analysis (Fig. 2) and the weight increase (Table 2). We also converted the thickness date to the weight gain and vice versa as shown in Table 2. The weight increase and thickness changes according to the samples show consistent results.

4.2 Diffusion behavior from TGA

The weight increase of Z, ZS, ZW and ZSW are 8.2, 6.8, 5.3 and 1.2% for the oxidation in TGA test at 1500°C for 30 min in ambient atmosphere (Fig. 4(a)). The oxidation behavior shows parabolic increase behavior with oxidation time. It can be converted to oxidation ratio from the weight increase data of 100% oxidation from Table 1. Then, as is shown in Fig. 4 (b), we can know that 89% of Z, 49% of ZS, 52% of ZW and 8.8% of ZSW are oxidized (Table 3). It is considered that the difference of oxidation rate between torch test and TGA analysis result from the sample dimension differences. For example, the sample dimension is about 6% of torch test sample (φ: 10 mm, t: 1 mm). We also converted the oxidation rate data to the oxide scale thickness data supposing that the sample thickness is 500 μm (Fig. 4(c)). We calculated the effective diffusivity of oxygen gas using Fick’s second law. From that law, we can calculate the effective diffusivity, using the equation of L=Dt, where L is diffusion length (that is, oxide scale thickness), D is oxygen effective diffusivity, and t is diffusion time. From this, we can calculate the effective oxygen diffusivity, D=L²/t. The effective diffusivity is given by the following equation [15, 16].

Fig. 4

Weight increases (a), oxidation rate (b), scale thickness variations (c) and oxygen diffusivity variations (d) of the samples with reaction time at 1500°C in an ambient atmosphere.

Table 3

Weight increases and oxidation rates from oxide thickness after torch test (sample size : φ= 10 mm, t = 1 mm) and from TGA test (sample size : about 6% of torch test sample)

D = (Dk−1+D12−1)−1

Here, D_k is the Knudsen diffusivity, and D₁₂ is The diffusivity of species 1 (oxygen) in 2 (metal oxide layer). It indicates that the effective diffusion is the parallel sum of Knudsen diffusion through porous scale and oxygen gas diffusion through the oxide scale layer. The effective diffusivity is saturated at 1×10⁻¹⁰ m²/s for Z, 2×10⁻¹¹ m²/s for ZS and ZW and 1×10⁻¹² m²/s for ZSW.

5. Conclusions

ZrB₂ ceramics and ZrB₂ ceramics with addition of SiC, WC and SiC/WC were successfully synthesized by spark plasma sintering method. SiC additive forms SiO₂ amorphous outer scale layer and SiC-deplete ZrO₂ scale layer. The amorphous SiO₂ layer is considered to decrease the oxidation rate. WC addition forms WO₃ during oxidation process to produce ZrO₂/WO₃ liquid sintering layer, which is more dense scale layer due to volume expansion effect. The addition of SiC and WC to ZrB₂ has an effect to reduce the oxygen effective diffusivity by one fifth of that for ZrB₂. The addition of both SiC and WC shows the formation of SiO₂ outer dense glass layer and ZrO₂/WO₃ layer so that the anti-oxidation effect is improved three times as much as that of ZrB₂. Therefore, ZSW has two order lower oxygen effective diffusivity than Z.

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Figure & Data

References

Citations

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