Zrc ceramics incorporated with different-sized SiC particles

Abstract

Three different SiC powders with average particle sizes of 0.45, 3.5 and 10 µm were used to prepare ZrC-20vol.-% SiC ceramics by hot pressing. The effects of SiC particle size on the densification, microstructure, mechanical properties and thermal properties of ZrC–SiC ceramics were studied. Ceramics prepared from SiC with finer particle sizes exert higher bending strength, hardness and lower thermal conductivity. The ZrC–SiC ceramics with a starting SiC particle size of 3.5 µm has relative high fracture toughness than others. Analysis indicates that SiC grain size and the grain boundaries control the thermal conductivity ZrC–SiC ceramics. Ceramics prepared from SiC with the particle size of 10 µm exhibits the highest thermal conductivity due to the larger grains and less grain boundaries.

Keywords

Zirconium carbide (ZrC)silicon carbide (SiC)densification microstructure mechanical properties

Introduction

Zirconium carbide (ZrC) is considered as one of the possible materials for inert matrix fuels (IMF) in the next generation nuclear reactor systems, due to its high melting point (3540°C), solid-state phase stability, excellent neutronic characteristics and high-temperature mechanical properties as well as resistance to corrosion by fission products [1 -8]. However, monolithic ZrC has poor sinterability due to its strong covalent bond and low self-diffusion coefficient. Even though dense ZrC can be obtained by pressure-assisted sintering method at extremely high temperatures (>2000°C), microstructure coarsening cannot be avoided, which will result in decreased mechanical properties of ZrC ceramics.

The second phase particles, such as MoSi₂ [9], SiC [10], TaSi₂ [11] and TaC [12], were introduced to promote sintering and grain refinement of ZrC ceramics. Among these additives, SiC is the only one which can also be used as IMF in nuclear fuels due to a series of favourable features, especially high corrosion resistance and good irradiation tolerance [13 -15]. SiC is commonly used as a second phase to prepare ZrB₂-based ceramics. SiC can not only enhance the densification process but also improve the mechanical properties and oxidation resistance of ZrB₂-based ceramics [16 18]. For ZrC-based ceramics, there are relatively fewer reports about the addition effects of SiC, compared with ZrB₂-based ceramics. Recently, we reported the preparation of ZrC–SiC ceramics by reactive hot pressing at mild temperature and the products demonstrate good properties [10].

It is known that the particle size of starting materials will affect the microstructure and properties of the final products. In the ZrB₂–SiC system, with the decrease of the average particle size of the starting SiC, the densification processes are enhanced [19,20]; besides, the final products with finer microstructures and higher mechanical properties are obtained. Furthermore, the SiC grain size has a much stronger influence on the strength of ZrB₂–SiC than ZrB₂ grain size despite the fact that ZrB₂ is the major phase. In view of this, in the present study, ZrC-20vol.-%SiC ceramics were hot pressed using three different SiC powders with average particle sizes ranging from 0.45 to 10 µm and the effects of SiC particle size on the densification, microstructure, mechanical properties and thermal conductivity of ZrC–SiC ceramics were investigated.

Experimental Procedure

Self-synthesised ZrC (purity 99%, O 0.88%, by a carbothermal reaction between ZrO₂ and graphite in vacuum, mean diameter of 0.76 μm) [12] and three kinds of commercial SiC powders with different particle sizes were used as the starting powders. ZrC and SiC powders were ball milled with Si₃N₄ balls in alcohol in a fluoropolymer-coated jar for 24 h, dried and sieved, and then hot pressed at 2000°C with a holding time of 60 min in argon. The final ZrC-20vol.-%SiC ceramics with starting SiC particle size of 0.45, 3.5 and 10 µm are noted as ZS0.45, ZS3.5 and ZS10, respectively.

The bulk densities of the as-sintered samples were measured by the Archimedes technique. The microstructure of the samples was characterised using scanning electron microscopy (SEM, Hitachi TM3000, Tokyo, Japan) with energy dispersive spectroscopy (Oxford Instruments Swifted3000, Japan). Four-point bending strength was measured on bars with dimensions of 2 mm × 2.5 mm × 30 mm using 10 mm and 20 mm as inner and outer spans and a crosshead speed of 0.2 mm min⁻¹. Vickers’ hardness was tested by the indentation technique on polished samples using a 1 kg load with a dwell of 10 s. According to the indentation crack lengths, the fracture toughness was calculated according to Evans's equation [21 -23]: (1) where P is the peak load (9.8 N), C₁, C₂ are the measured diagonal crack lengths (m) and β is a constant of angle (68°). Measurements of the thermal conductivities were conducted on the samples (diameter 10 mm, thickness 2 mm) using laser-ﬂash diffusivity instrument (Nano-Flash-Apparatus, LFA 447, NETZSCH).

Results and discussion

Densification process

In order to keep the original particle size of the SiC powders, ball-mill was used to mix ZrC and SiC, in order to get as large particle size as possible. The relative densities of the as-sintered ZS0.45, ZS3.5 and ZS10 are 97.7%, 98.1% and 98.7%, together with the open porosity of 0.008%, 0.016% and 0.028%, respectively. The hot pressing programmes with the calculated relative density were shown in Figure 1. The densification curve of ZS3.5 is more or less the same as ZS10, however, obviously different from ZS0.45. For ZS3.5 and ZS10, the densification process almost completed (relative density above 98%) when the temperature approaching the final soaking temperature. For sample ZS0.45, the shrinking rate was slower than the other two samples before 2000°C, the relative density was increasing gradually during the holding stage at 2000°C.

Microstructure

The polished and fracture surfaces of the three hot pressed samples were shown in Figure 2, and the measured grain sizes are summarised in Table 1. The average grain sizes of both ZrC and SiC increased with the increase of the initial particle size of SiC. Previous researchers have revealed that SiC particles could inhabit the growth of ZrB₂ and HfB₂ grains [19,24]. Based on Zener pinning theory, in two-phase materials, the relationship between the average particle diameter of the matrix phase and the added second phase is [25,26]: (2) where D is the average particle size of the matrix phase, d is the average particle diameter of the second phase, and f represents the volume friction of the second phase. In this work, although the volume per cent of SiC is the same, the sample with a larger starting particle size has less number of SiC grains, resulting in a weaker inhibitory ability to ZrC grain growth, and eventually larger ZrC grain in size.

Table 1

Measured grain sizes of the ZrC–SiC ceramics with different starting SiC powders.

	Average grain size (µm)		Maximum grain size (µm)
Sample	ZrC	SiC	ZrC	SiC
ZS0.45	2.3 ± 0.66	0.78 ± 0.31	4.56	1.98
ZS3.5	7.77 ± 3.02	2.96 ± 1.85	18.17	19.53
ZS10	8.59 ± 3.09	4.49 ± 3.73	18.37	22.47

It should be noted that the statistic average sizes of SiC of the final ceramics were even smaller than the starting particles. The abnormality is caused by that a mass of tiny SiC grains were used to calculate the average grain sizes, as shown in Figures 2 and 3 and Table 1.

Mechanical properties and thermal conductivity

The flexural strength, Vickers’ hardness and fracture toughness as a function of SiC particle sizes are shown in Figure 4. The fracture toughness values of the sintered composites increased at first and then decreased as the initial SiC particle size increased. As shown in Figure 4, the ZS3.5 material had the highest fracture toughness of 3.25 MPa m^1/2 in the present study. The second phase of SiC grains can improve the toughness of ceramics by crack deflection and crack bridging. On the other hand, the grain size of the ZrC matrix incorporated with different SiC particles in diameter should also affect the fracture toughness. The toughing effect could be reduced when the SiC grain size is too small or too large. Because the smaller SiC grains will lead to weak crack deflection, and for larger SiC grains, the cracks will traverse the excessively large grains, as shown in Figure 2(g–i).

From Figure 4, it can be seen that the four-point bending strength of ZrC–SiC ceramics decreased significantly as the SiC starting particle sizes increased. The ZS0.45 sample had the highest flexural strength of 547.2 MPa, while the ZS10 had the lowest value of 419.9 MPa. It has been confirmed that the SiC particle sizes can influence the flexural strength of ZrB₂–SiC [19,27] and HfC–SiC [24] materials. For brittle materials, the relationship of flexural strength, fracture toughness and critical flaw size was described by Griffith equation [28]: (3) where σ_f is the flexural strength, K_Ic is the fracture toughness, Y is a geometric constant related to the fracture origin, and c is the critical flaw size. In the present study, the characteristic grain size was taken as the critical flaw size of the composites due to that no obviously macroscopic defects were identified [29]. Assuming that Y is a constant for all the specimens in this study, therefore, the flexural strength is proportional to the ratio of fracture toughness and the square root of grain sizes of the second phase. Figure 5 is a plot of the strength of the composites as a function of the K _Ic ·d^−1/2, which is in good agreement with the previous conclusion. It indicates that the SiC grain size and fracture toughness have an important influence on the strength of ZrC–SiC ceramics. The residual stress caused by the mismatch in thermal expansion coefficients between ZrC (6.7 × 10⁻⁶/K) [30] and SiC (4.5 × 10⁻⁶/K) [27] also play a significant role in the mechanical properties of the ceramics. During cooling after densification at the elevated temperature, the ZrC matrix tried to shrink faster than SiC, resulting in a compressive stress in the SiC particulate phase. The large SiC grains lead to stress concentration easily, which adversely affects the flexure strength for ZrC–SiC materials [24,27]. The effect of the residual stress on the mechanical properties will be studied in the future study.

The Vickers’ hardness of the samples decreased as the starting SiC particle sizes increased. The hardness of ceramics was influenced by porosity and grain sizes [31]. The sample with finer grain size and lower porosity showed higher hardness. Considering the similar porosity in the samples, values of hardness should be mainly determined by grain sizes.

The thermal conductivities at room temperature of the three composites as a function of SiC particle sizes were shown in Figure 6. The thermal conductivity value for ZrC is from a fully densified ZrC ceramic with a grain size of ∼10 µm. The thermal conductivities of ZrC–SiC ceramics increased with the increase of SiC grain sizes. The ZS10 exhibits the highest thermal conductivity (46 W/m·K) at room temperature. This is because ZS10 ceramics have the least interphase surface area, which can affect the thermal conductivity in two ways. First, the arrangement of atoms in the grain boundaries is much more irregular than internal. The impurities were easy to gather in the boundaries as well. Therefore, the thermal conductivity of grain boundary is lower than the internal grain. Second, the grain boundaries themselves can reduce thermal conductivity by scattering of phonons. Therefore, there is a negative correlation between the thermal resistance and grain sizes. The ZS10 composite has a relatively large grain size, which resulting in a lower thermal resistance at grain boundaries and a relatively higher thermal conductivity. It should be noted, compared with ZrC single phase ceramic, the thermal conductivities of ZrC–SiC ceramics increased more than two times. It was much attributed to high thermal conductivity of SiC.

Conclusions

In the present work, the effect of SiC particle size on the densification, microstructure and mechanical properties of ZrC–SiC ceramics was studied. Microstructure analysis shows that the grain size of ZrC increases with the particle size increase of the starting SiC. Finer SiC grains demonstrated a more powerful inhibitory ability for ZrC grain growth during densification. Vickers’ hardness decreased from 19.77 GPa for ZS0.45 to 18.62 GPa for ZS10. The ZS3.5 samples have the highest fracture toughness of 3.25 MPa m^1/2. The SiC grain sizes had a great influence on the flexural strength of ZrC–SiC ceramics. The four-point bending strength decreased from 547.2 MPa for ZS0.45 to 419.9 MPa for ZS10. The ZS10 performs the highest thermal conductivity (46 W/m·K) due to the sample with largest grains and least grain boundaries.

Footnotes

Acknowledgements

The State Key Laboratory of High Performance Ceramics and Superfine Microstructure of Shanghai Institute of Ceramics are gratefully acknowledged.

Disclosure statement

No potential conflict of interest was reported by the authors.

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