Effects of SPS Mold on the Properties of Sintered and Simulated SiC-ZrB₂ Composites

Abstract

Silicon carbide (SiC)-zirconium diboride ($ZrB_2$) composites were prepared by subjecting a 60:40 vol% mixture of ${\beta}$-SiC powder and $ZrB_2$ matrix to spark plasma sintering (SPS) in 15 $mm{\Phi}$ and 20 $mm{\Phi}$ molds. The 15 $mm{\Phi}$ and 20 $mm{\Phi}$ compacts were sintered for 60 sec at $1500^{\circ}C$ under a uniaxial pressure of 50 MPa and argon atmosphere. Similar composites were simulated using $Flux^{(R)}$ 3D computer simulation software. The current and power densities of the specimen sections of the simulated SiC-$ZrB_2$ composites were higher than those of the mold sections of the 15 $mm{\Phi}$ and 20 $mm{\Phi}$ mold simulated specimens. Toward the centers of the specimen sections, the current densities in the simulated SiC-$ZrB_2$ composites increased. The power density patterns of the specimen sections of the simulated SiC-$ZrB_2$ composites were nearly identical to their current density patterns. The current densities of the 15 $mm{\Phi}$ mold of the simulated SiC-$ZrB_2$ composites were higher than those of the 20 $mm{\Phi}$ mold in the center of the specimen section. The volume electrical resistivity of the simulated SiC-$ZrB_2$ composite was about 7.72 times lower than those of the graphite mold and the punch section. The power density, 1.4604 $GW/m^3$, of the 15 $mm{\Phi}$ mold of the simulated SiC-$ZrB_2$ composite was higher than that of the 20 $mm{\Phi}$ mold, 1.3832 $GW/m^3$. The $ZrB_2$ distributions in the 20 $mm{\Phi}$ mold in the sintered SiC-$ZrB_2$ composites were more uniform than those of the 15 $mm{\Phi}$ mold on the basis of energy-dispersive spectroscopy (EDS) mapping. The volume electrical resistivity of the 20 $mm{\Phi}$ mold of the sintered SiC-$ZrB_2$ composite, $6.17{\times}10^{-4}{\Omega}cm$, was lower than that of the 15 $mm{\Phi}$ mold, $9.37{\times}10^{-4}{\Omega}{\cdot}cm$, at room temperature.

1. Introduction

The field-assisted sintering technique (FAST) and pulsed electric current sintering (PECS) belong to a class of novel sintering methods that employs a pulsed direct current (DC) to enhance consolidation, in order to produce sintered parts from metallic/ceramic powders. In the last decade, several variants of FAST/PECS techniques have been developed.

One example is spark plasma sintering (SPS), which is quite practical and may be applicable in various industries to reduce the thermal stresses of joints if suitable technological parameters are adopted. In particular, to better exemplify the advantages of this technique, it is simple and convenient to design various graded material interlayers in order to improve the compatibility of different materials and to reduce the thermal stresses of each layer.

The SPS technique has been used to fabricate various materials, including metals and alloys, compounds, ceramics, composites, bulk amorphous and nanomaterials, multi-scaled structures, and functionally graded materials [1, 2]. SPS is known as a single-step processing technique, which combines the electric field sintering technique and the uniaxial pressing forming technique. Some important technological benefits of SPS, such as higher heating rates, fewer processing steps, elimination of the need for sintering aids, and near net shape capacity, facilitate the control of grain growth and improve the mechanical, chemical, and physical properties of power materials [3].

The electrical resistivity of SiC is considered to be a negative temperature coefficient of resistance (NTCR) below 1000 °C. This causes increases in temperature and uncontrollable electrical current in SiC [4], resulting in overheating. However, the addition of SiC to ZrB2 results in the formation of a SiC-ZrB2 composite with high electrical conductivity, superior oxidation resistance, and high mechanical strength. These properties make SiC-ZrB2 composites suitable conductive materials and Ohmic-contact electrode materials, able to function satisfactorily at both high and low temperatures [5-7].

The dispersion of ZrB2 or other metal borides in SiCbased matrices was found to be useful in the production of materials for heaters and igniters on the basis of their adequate resistivities, positive temperature coefficients of resistance (PTCR), greater strength and toughness compared to matrix SiC, and good oxidation resistance at temperatures up to approximately 1200 °C [8].

This study was performed to determine whether the current and power densities of a simulated SiC-ZrB2 composite correlated with the experimentally measured properties of sintered SiC-ZrB2 composites according to the mold size used during SPS.

The densification, mechanical, and electrical properties of the sintered SiC-ZrB2 composites were evaluated by conducting apparent density and flexural strength measurements, high resolution x-ray diffraction (HR-XRD) and field-emission scanning electron microscopy (FESEM) studies, energy-dispersive spectroscopy (EDS) mapping, and volume electrical resistivity measurements. The current and power densities of a simulated SiC-ZrB2 composite were analyzed using Flux® 3D computer simulation software.

 

2. Experimental Procedure

2.1 Powder preparation

High-purity β-SiC (Grade BF12, H. C. Starck, Germany) and ZrB2 (Grade B, H. C. Starck, Germany) were combined in a 60:40 vol% ratio. The measured materials were mixed with distilled water in a polyurethane bowl (volume: 1583.4 mL). The mixture was then subjected to planetary ball mill processing with high-purity SiC balls (a 1:5 mixture of 10 mmΦ and 20 mmΦ diameter SiC balls) for 24 h. Next, the powders were dehydrated by heating for 12 h at 100 °C, and sieved through a 60-mesh screen.

2.2 Sintering process

The dried powders were respectively placed in a graphite die with an inner diameter of 15 mmΦ or 20 mmΦ, enclosed with graphite foil, and sintered using a PASH3000 apparatus (ElTek Co., Ltd, Korea) at a sintering temperature of 1500 °C and uniaxial pressure of 50 MPa under argon atmosphere. The sintered composites and the simulated SiC-ZrB2 composites were classified according to the diameter of the mold (15 mmΦ and 20 mmΦ).

2.3 Physical characteristics

Theoretical densities of the sintered SiC-ZrB2 composites were calculated on the basis of the rule of mixtures (3.217g/cm3 for β-SiC and 6.085 g/cm3 for ZrB2).

The final sintered SiC-ZrB2 composites were ground with a diamond wheel and shaped into disks. Then, the relative density of each of the sintered composite specimens was measured 10 times by the Archimedes method.

The disks were machined to obtain sintered SiC-ZrB2 composite bars with dimensions of 1.0×0.7×10 mm3. The bars were polished using 1 μm diamond paste and beveled at 45° for mechanical testing (ASTM F394-78).

The three-point flexural strength of each sintered compact was measured approximately 5-6 times at room temperature using a material testing apparatus (Model 4204, Instron, USA) under the following conditions: outer span, 10 mm; inner span, 8 mm; and crosshead speed, 0.07 mm/ min.

The microstructures of the fracture surfaces of the flexural strength test specimens were observed using FESEM (S-4800, Hitachi, Japan). EDS mapping (S-4800, Horiba, Japan) was carried out to analyze the atom distribution in the sintered SiC-ZrB2 composites. The sintered SiC-ZrB2 compacts were cut by wire electrical discharge machining (WEDM, α-OPiB, FANAC, Japan) to produce specimens for volume electrical resistivity measurements. The volume electrical resistivity of each specimen processed by WEDM was measured 200 times by the van der Pauw method [9].

2.4 Computer simulation

The Flux® 3D (CEDRAT, France) computer software package was used to analyze the effect of the SPS mold size on the electric field of the simulated SiC-ZrB2 composites. Details of the simulations for the SiC-ZrB2 composites follow in Sections 2.4.1 and 2.4.2, for the computer-simulated SiC-ZrB2 composite. The electric field of the simulated SiC-ZrB2 composite was analyzed by applying Flux® 3D computer software [10].

2.4.1 Geometric designs of the SPS molds

The area sections for the quantitative analysis of the current and power densities of the simulated SiC-ZrB2 composites are designated by horizontal and vertical dotted lines, respectively, as shown in Fig. 1. The horizontal specimen sections of the 15 mmΦ and 20 mmΦ molds were 7.5-22.5 mm and 5-25 mm, respectively. The horizontal mold sections for the 15 mmΦ mold were 0-7.5 mm and 22.5-30 mm; those of the 20 mmΦ mold were 0-5 mm and 25-30 mm, as shown Fig. 1. The vertical punch sections of the 15 mmΦ specimen were 27.5-47.5 mm and 52.5-72.5 mm, and those of the 20 mmΦ specimen were 25.5-47.5 mm and 52.5-74.5 mm. The vertical specimen sections of the 15 mmΦ and the 20 mmΦ molds were 47.5- 52.5 mm, as shown in Fig. 1.

Fig. 1.Geometric designs of the SPS molds

2.4.2 Parameters for Computer Simulation

The input parameters of the physical properties for the computer simulation of the simulated SiC-ZrB2 composites were the applied voltage, the volume electrical resistivity, and the relative permittivity. The applied voltage was DC 5 V; the volume electrical resistivity was the average value (7.77×10−4 Ω·cm) [11] of the volume electrical resistivities of the sintered SiC-ZrB2 composites; and the relative permittivity of the sintered SiC-ZrB2 composite was obtained by applying following equation.

relative permittivity of a SiC-ZrB2 composite

where, ε SiC2 , the relative permittivity of SiC, and εZrB2, the relative permittivity of ZrB2, were 10.0455 and 0.5498, respectively [12]. θ SiC2 , the volume percentage of SiC, and θ ZrB2 , the volume percentage of ZrB2, were 60 vol% and 40 vol%, respectively.

 

3. Experimental Results and Discussion

3.1 Computer-simulated current and power densities

Fig. 2 shows the vertical cross sections for the quantitative analyses of the current densities of the simulated SiC-ZrB2 composites. The arrows indicate the flow of current density and the colors of the arrows reflect the intensity of the current density. In Fig. 2, it can be seen that the current densities were the highest in the specimen sections of the simulated SiC-ZrB2 composites. The volume electrical resistivity of the SiC-ZrB2 composite, 7.77×10−4 Ω·cm, was found to be lower than that (6.0×10−3 Ω·cm) of the graphite mold. Fig. 2 also shows that the current density by Joule heating due to thermal conduction and convection phenomena was higher in the punch section of the graphite mold than elsewhere.

Fig. 2.Current density distributions of the vertical cross sections of the graphite molds

Although the conditions for SPS were the same, the sintering properties of the simulated SiC-ZrB2 composites were different according to mold size. The Joule heating in the punch section of the graphite mold is produced by itself, and that of the specimen section of the SiC-ZrB2 composite is produced by itself and spark plasma phenomena [13-15]. The distribution section of the current density between the graphite mold and the specimen in the simulated SiC-ZrB2 composite was different because of differing resistances.

Figs. 3 and 4 show the horizontal and vertical specimen sections for the quantitative analyses of the current densities of the simulated SiC-ZrB2 composites. In these figures, the specimen section current densities were higher than those of the 15 mmΦ and 20 mmΦ mold sections. With a closer approach to the center of the horizontal specimen section, the current densities of the simulated SiC-ZrB2 composites grew larger, as shown in Fig. 3. Also, the current density of the 15 mmΦ mold of the simulated SiC-ZrB2 composite was higher than that of the 20 mmΦ mold in the center of the specimen section (Fig. 3). Thus, the volume electrical resistivity of the simulated SiC-ZrB2 composite was about 7.72 times lower than that of the graphite mold and punch. Also, the flow of current density in the simulated SiC-ZrB2 composite was confirmed to be concentrated in the 15 mmΦ mold.

Fig. 3.Current densities in horizontal sections of 15 mmΦ and 20 mmΦ specimens

Fig. 4.Current densities in vertical sections of 15 mmΦ and 20 mmΦ specimens

Figs. 5 and 6 show the horizontal and vertical specimen sections for the quantitative analyses of the power densities of the simulated SiC-ZrB2 composites. The power density patterns of the simulated composite specimen sections were nearly identical to those of the current density patterns. Fig. 5 reveals that the power density, 1.4604 GW/m3, of the 15 mmΦ mold of the simulated SiC-ZrB2 composite was higher than that of the 20 mmΦ mold (1.3832 GW/m3) in the center of horizontal specimen section.

Fig. 5.Power densities in horizontal sections of 15 mmΦ and 20 mmΦ specimens

Fig. 6.power densities in vertical sections of 15 mmΦ and 20 mmΦ specimens

3.2 Properties of the sintered SiC-ZrB2 composites

As shown in Table 1, the flexural strength of the 20 mmΦ mold of the sintered SiC-ZrB2 composites was lower than that of the 15 mmΦ mold at a sintering temperature of 1500 °C and uniaxial pressure of 50 MPa under argon atmosphere. The flexural strength of a sintered SiC-ZrB2 composite is dependent on its relative density. It was confirmed that varying the SPS mold size affected the properties of the sintered and simulated SiC-ZrB2 composites. The HR-XRD analyses of the sintered SiCZrB2 composites are shown in Fig. 7. All the peak values appeared on International Centre for Diffraction Data (ICDD) cards, with the following code numbers: 00-029- 1128 (SiC), 01-074-1302 (SiC), 00-042-1091 (SiC), and 01-075-0964 (ZrB2). No evidence of reaction between β-SiC and ZrB2 in the sintered SiC-ZrB2 composites under argon atmosphere was observed in the HR-XRD analysis.

Table 1.Properties of the sintered SiC-ZrB2 composites [11]

Fig. 7.High-resolution x-ray diffraction patterns of the sintered SiC-ZrB2 composites

Fig. 8 shows that the ZrB2 distribution in the 20 mmΦ mold of the sintered SiC-ZrB2 composite was more uniform than that of the 15 mmΦ mold on the basis of EDS mapping. Table 1 revealed that the volume electrical resistivity of the 15 mmΦ mold specimen was higher than that of the 20 mmΦ mold at room temperature, because the ZrB2 chain formations were broken by the frequent occurrence of spark plasmas in the 15 mmΦ molded specimen. Thus, the experimental values of the volume electrical resistivities of the sintered SiC-ZrB2 composites were dependent on the ZrB2 distribution, because electrical current flows predominantly by the ZrB2 chain formations with their low electrical resistivities. This also explains why the current and power densities of the 15 mmΦ mold of the simulated SiC-ZrB2 composites were higher than those of the 20 mmΦ mold in the center of the specimen section, as the ZrB2 chain formations are broken by the frequent occurrence of spark plasmas in the 15 mmΦ mold of the sintered SiC-ZrB2 composites.

Fig. 8.Field-emission scanning electron microscopy and energy-dispersive spectroscopy mapping of the sintered SiC-ZrB2 composites

Fig. 9 shows the temperature coefficient of resistance (TCR) of the sintered SiC-ZrB2 composites, calculated to be 3.53×10-5/°C using the rule of mixtures, and the experimental values, which were measured by the van der Pauw method [8, 16]. The TCRs of the 15 mmΦ and 20 mmΦ molds were 3.169×10−3/°C and 3.785×10−3/°C, respectively, indicating that all the sintered SiC-ZrB2 composites had a PTCR. The 15 mmΦ mold had a lower PTCR than the 20 mmΦ mold because chain formations of ZrB2 in the 15 mmΦ mold of the sintered SiC-ZrB2 composite were broken by the frequent occurrence of spark plasmas under the high applied pulse voltage.

Fig. 9.Temperature coefficient of resistance of the sintered SiC-ZrB2 composites

 

4. Conclusion

Sintered SiC-ZrB2 composites were produced according to mold size at a sintering temperature of 1500 °C and uniaxial pressure of 50 MPa under argon atmosphere by subjecting a 60:40 vol% mixture of β-SiC powder and ZrB2 matrix to spark plasma sintering. The current and power densities of the simulated SiC-ZrB2 composites according to mold size were calculated using the Flux® 3D computer simulation software. The following results were obtained:

1. The current densities of the specimen sections of the simulated SiC-ZrB2 composites were higher than those of the mold sections in both the 15 mmΦ and the 20 mmΦ molds. Toward the centers of the horizontal specimen sections, the current densities of the simulated SiC-ZrB2 composites became large. 2. The power density patterns of the specimen sections of the simulated SiC-ZrB2 composites were nearly identical to those of the current density patterns. 3. The power density, 1.4604 GW/m3, of the 15 mmΦ mold of the simulated SiC-ZrB2 composite was higher than that of the 20 mmΦ mold, 1.3832 GW/m3, in the center of the horizontal specimen section. 4. No evidence of reaction between β-SiC and ZrB2 in the sintered SiC-ZrB2 composites under argon atmosphere was observed in the HR-XRD analysis. 5. The ZrB2 distributions in the 20 mmΦ mold of the sintered SiC-ZrB2 composites were more uniform than those of the 15 mmΦ mold on the basis of EDS mapping. 6. The volume electrical resistivity of the 20 mmΦ mold of the sintered SiC-ZrB2 composite was lower than that of the 15 mmΦ mold at room temperature, because the ZrB2 chain formations were broken by the frequent occurrence of spark plasmas in the 15 mmΦ mold.

The current densities of the specimen sections of the simulated SiC-ZrB2 composites were the highest. The volume electrical resistivity of SiC-ZrB2 composite, 7.77 × 10−4 Ω·cm, was lower than that of the graphite mold (6.0 × 10−3 Ω·cm). The current density of the punch section of the graphite mold was higher than those of the others. It is considered that current density by Joule heating due to the phenomenon of thermal conduction and convection was higher in the punch section than those of the others.

Finally, the current and power densities of the 15 mmΦ mold of the simulated SiC-ZrB2 composites were higher than those of the 20 mmΦ mold in the center of the specimen section, as the chain formations of ZrB2 were broken by the frequent occurrences of spark plasmas in the 15 mmΦ mold of the sintered SiC-ZrB2 composite.