1 Introduction

In the last few decades efforts have been made to develop structural cermets for industrial applications and scientific research. Silicon carbide (SiC) and Boron carbide (B4C) are mainly used for the preparation of ceramic armor. Boron Carbide (B4C) with a hardness of about 30 GPa is the third hardest material after diamond and cBN. SiC and B4C have been used in structural applications such as valve components and bearings, seal rings, high-speed machine operations, cutting tools at high temperatures, aerospace industry, heat exchangers, fusion reactors, wear resistant parts and armour plates.[1–5] The SiC-B4C mixture offers a better combination of light weight high-temperature-resistant materials.

Due to their high covalent bonding, low plasticity, low diffusivity, and very low fracture toughness (2–4 MPa m1/2), densification of SiC and B4C ceramics is extremely difficult.[6–8] High-temperature sintering is required to obtain the dense cermet materials. To improve mechanical properties and lower sintering temperatures, different ductile metals, such as Si, Al, etc., have been mixed with SiC and B4C as second phase additives. These metals act as binders and impart a certain degree of toughness into the ceramic without impairing the hardness of the matrix. When a metal is added into the ceramics during high-temperature sintering, there is a possibility of chemical reaction and/or oxidation leading to metal depletion and formation of undesirable phases between ceramic and metal. The metal addition may also result in poor wetting of metal and ceramic interface.[9] Most ceramic materials processed by conventional sintering do not achieve theoretical density more than 85 pct; however, it is possible to acquire density higher than 98 pct by spark plasma sintering (SPS) technique.[10]

Prochazka et al.,[11] first used boron (B) and carbon (C) elements in SiC as sintering aids at a temperature of 2273 K (2000 °C) for better densification, to control the phase transformation, grain growth and enhancing mechanical properties. It has been shown that the fabrication of B4C and SiC cermets with incorporation of Al and Si metal results in improved physical and mechanical properties like density, strength, and toughness. These metal additions also form substitution compounds with boron carbide.[12–18]

As compared to conventional sintering methods, SPS is regarded as a rapid sintering method that generates highly sintered products in less sintering time with finer grain sizes.[19,20] In the SPS process, the graphite mould and stacked powder experience direct heating by the large pulsed current. This gives very high thermal efficiency compared to pressureless conventional sintering. The ON–OFF DC pulse voltage and current produce spark plasma, spark impact pressure, joule heating, and an electric field diffusion effect.[21] Also high-temperature sputtering phenomena are generated by spark plasma and spark impact pressure and are responsible for eliminating adsorptive gas and impurities existing on the surface of powder particles.[21] This enables the powder particles on the surface to be purified and activated than in conventional sintering process. The electrical power generates joule heating; the applied pressure promotes plastic flow in materials, and the action of the electric field gives high-speed ion migration between contacts particles. These are believed to be the main factors promoting sintering. It creates high-speed diffusion, high-speed material transfer at micro- and macro-level.

According to Zhang et al.,[22] the addition of Si reinforcement into SiCw-B4C (w stands for whiskers) composite fabricated by ball milling followed by reactive hot pressing increases the relative density of B4C. The addition of SiC whiskers and Si also improved the flexural strength (487 MPa) and fracture toughness (5.78 MPa m1/2) of this material. Magnani et al.[23] depicted that pressure-less sintering of α-SiC-B4C composite at 2423 K (2150 °C) improved the density up to 96 pct with increasing hardness, fracture toughness and flexural strength. Hulbert et al.[24] stated that SPS is a promising potential technique to synthesize refractory hard materials. Hayun et al.[25] used SPS of B4C without any sintering aids and obtained sufficiently dense B4C with improved mechanical properties. According to Sahin et al.,[26] an addition of 5 wt pct Y2O3 into B4C-SiC composites sintered by SPS at 1973 K and 2023 K (1700 °C and 1750 °C) under a pressure of 40 MPa increased the density of B4C-SiC composites to 99 pct. Hence, the SPS method has been used for the successful fabrication of poorly sintering materials such as borides, carbides, and nitrides.

Three conditions are required to produce dense products. The mixture of SiC and B4C powders is preferred over reaction sintering of SiB6 and graphite to produce compact SiC + B4C, since the reaction of SiB6 and graphite yields porous SiC and B4C.[27] Secondly, the grain size must be as small as possible because powder size greater than 10 µm cannot be sintered properly even at high temperatures.[27] Thirdly, high-temperature sintering near the melting point of SiC and B4C [>2550 K (2277 °C)] is required. Recrystallization of B4C starts above 2073 K (1800 °C) and grains grow rapidly beyond 2273 K (2000 °C).[27] As per the literature, β-SiC powder is prone to a phase transformation and grain growth above 2023 K (1750 °C), whereas α-SiC powders have shown much less grain growth as a function of temperature.[28,29]

Addition of Si in B4C and SiC is very useful in improving the sinterability and properties of sintered products.[30] Boron carbide can act as an alternative source of carbon in the presence of Si to form secondary SiC particles and was beneficial for the improvement in properties.[30–33] Telle et al.[34] studied the B4C-B-Si system and reported that the solid solubility of Si in boron carbide was 2.5 at. pct after pressureless sintering at 2523 K (2250 °C). Higher amounts of Si reinforcement increased the residual Si contents in the sintered product and deteriorated the mechanical properties of the final product.[35,36]

There is a scarcity of information on pressureless sintering and also on SPS of SiC-B4C-based cermets. We have prepared SiC-B4C-based cermets with varying amount of Si (5, 10, and 20 wt pct) by using planetary milling followed by pressure-less sintering at 2223 K (1950 °C). In another set of experiments, sintering was carried out by SPS at 1623 K (1350 °C). The effect of planetary milling on cermet powder before sintering as well as microstructure, density, microhardness, and fracture toughness of sintered samples was studied. These results are compared to understand the effect of Si additions on the processing and properties between these two fabrication methods.

2 Experimental

2.1 Fabrication

2.1.1 Milling

The elemental powders of SiC, B4C, and Si (purity >99 pct, and an initial average particle size <40 µm) were selected as starting materials. The raw materials are of Analar grade with high purity (impurity <1 pct). For SiC and Si, the common impurity is SiO2 (<1 pct). For B4C, impurities are B2C, B8C, and a trace amount of graphite (<1 pct). SiC, B4C, and Si were blended to obtain nominal compositions of SiC60(B4C)35Si5, SiC60(B4C)30Si10, and SiC60(B4C)20Si20 (in wt pct). These compositions were milled in a Fritsch planetary (P5) mill at a speed of 300 rpm for 10 hours. 300 gm of high-carbon chrome steel balls (AISI 52100) of diameter 10 mm were filled with 40 vol pct in each steel jar (volume 500 mL). In the jar, ball-to-powder weight ratio was maintained at 6:1 and milling was carried out with toluene to prevent oxidation. From Figure 1 it has been observed that, as-received powders of SiC, B4C, and Si are irregular in shape with sharp edges. After 10 hours of milling, the initial powder size (around 25–40 µm, Figure 1) was reduced to nearly ~3 to 4 µm characterized by SEM and particle size analysis.

Fig. 1
figure 1

SEM micrographs of the as-received powders: (a) SiC, (b) B4C, and (c) Si, respectively

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2.1.2 Compaction

The milled powder compositions of SiC-B4C with varying Si at 5, 10, and 20 wt pct, respectively, were compacted into pellets in a hydraulic press with applied pressure of 1500 MPa for 3 minutes.

2.2 Sintering

2.2.1 Conventional sintering

The as-milled powders were compacted into pellets in a hydraulic press followed by conventional sintering in a tubular furnace at 2223 K (1950 °C) for 30 minutes in a commercially flowing argon gas atmosphere.

2.2.2 Spark plasma sintering

In another set of experiment, the as-milled powders were loaded into a graphite die. Spark plasma sintering (SPS) was carried out at 1623 K (1350 °C) at a heating rate of 100 K/minute and held for 5 minutes under 50 MPa pressure in a DR SINTER 1050 equipment (Sumitomo Coal Mining Co. Ltd. (SCM), Japan).

2.3 Characterization

The milled powder samples were collected at intervals of 0, 2, 5, and 10 hours of milling. The structure, surface morphology, and particle size distribution of the milled powder samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and particle size analyzer. Particle size distribution analysis for powders milled for different times was conducted by using Malvern, Mastersizer laser scattering particle size analyzer. Milled powders were dispersed in a solution of 500 ml distilled water and 25 ml of sodium hexametaphosphate (1 L distilled water + 2 gm of Sodium hexametaphosphate powder) was added before particle size measurement.

The phase evolutions of the milled powder, as well as sintered pellets, were analyzed by PANalytical model: DY-1656 X-ray diffractometer using Cu-K α (λ=1.54Å) radiation. An optical microscopy with Axio-vision software, JEOL JSM-6480 LV scanning electron microscopy (SEM), and field emission scanning electron microscopy (FESEM) of NOVA NANOSEM 450 was used for structural and morphological analysis of the samples. The microstructure was revealed using Murakami’s etching by attacking the grain boundaries.[14] Murakami etchant was prepared by mixing 10 gm KNO3 and 10 gm K3Fe(CN)6 into a solution of 100 mL distilled water and 10 mL HNO3.

Microhardness of sintered samples was measured by Vickers microhardness tester (Leco Microhardness tester LM248AT) at an indentation load of 300 gmf for a dwell time of 10 seconds. Indentation fracture toughness of sintered samples was calculated by Vickers indentation method using Niihara equation.[37]

3 Results

3.1 Mechanical Milling

3.1.1 Phase analysis

Figure 2 shows the typical XRD patterns of powder with 10 wt pct Si milled up to 10 hours. It can be seen from the figure that SiC, B4C, and Si phases are intact after milling. It was also observed that no reaction took place between SiC, B4C, and Si after 10 hours of milling, neither was any contamination observed.

Fig. 2
figure 2

XRD spectra of cermet containing 10 wt pct Si powder sample at different milling times (0, 2, 5, and 10 h)

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3.1.2 Microstructure analysis

The size and morphology of milled powder at different intervals are investigated by scanning electron microscopy. Figure 3 shows the SEM micrographs that reveal a gradual refinement of powder particles for the cermet containing 10 wt pct of Si with milling. During milling, the particles get fragmented, and the particle size reduced continuously. The initial powder size was around 25–40 µm and after 10 hours of milling, it was reduced to nearly ~3 to 4 µm.

Fig. 3
figure 3

SEM micrographs of the sample with 10 wt pct Si milled for 0 to 10 h in planetary ball mill which shows gradual refinement of particles size

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3.1.3 Particle size analysis

The progress of mechanical milling process continuously changes the size and morphology of powder samples. Figure 4(a) shows the particle size distribution of milled powder containing 10 wt pct Si; with progress of milling time from 0 to 10 hours the particle size gradually decreased. A variation of average particle size is shown in Figure 4(b). The median size (d50) was reduced from 10 to ~ 3 µm after 10 hours of milling which is also corroborated by SEM analysis as shown in Figure 3. The presence of harder and brittle SiC and B4C particles get fragmented and refined the size of the particle by a repetitive collision of powder particles in between balls and vial during the milling process.

Fig. 4
figure 4

(a) Particle size distribution of sample containing 10 wt pct Si at different intervals (b) variation of average particle size of Si 10 wt pct with progress of milling time

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3.2 Consolidation of Milled Powder (Conventional Pressureless Sintering and SPS)

3.2.1 Phase analysis

Figure 5 shows the XRD patterns of 5, 10, and 20 wt pct of Si added compositions prepared by (a) conventional sintering and (b) spark plasma sintering. XRD analysis has supported the presence of SiC, B4C, and Si product phases for conventional as well as SPS samples. During high-temperature conventional sintering at 2223 K (1950 °C), free carbon is released from B4C, which reacts with molten Si and maintains the SiC content in the product. When Si content increases to 20 wt pct in the SiC-B4C composite, the SiC phase increases and minor residual Si phase is also observed.

Fig. 5
figure 5

XRD spectra of 5, 10, and 20 wt pct of Si containing sintered samples (a) by conventional sintering, and (b) by SPS

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Feng et al.[33] studied the solid solubility of Si in B4C with increasing Si content. They reported the dissolution of Si in B4C and found that the solid solubility increased from 1.38 to 1.8 at. pct with increasing the amount of Si from 4 to 8 wt pct. High Si content in the composition increased the residual Si amount and the SiC content increased in the microstructure. In the present study, XRD analysis confirmed the presence of minor SiO2 and B2O3 in conventional-sintered SiC-B4C-Si samples. During sintering process, the SiO2 peaks were observed only on the surface of the pellets. This is because of some residual air or trace amount of oxygen being present in the argon atmosphere inside the furnace. It forms a glassy layer over the pellet and inhibits further oxidation. In the SPS processing, sintering was done under vacuum. The pulsed current and activated sintering during SPS helps to breakdown the oxide layers on the surface of powder particles. With shorter sintering time and applied pressure during sintering, the surface porosity levels were reduced. There was no oxidation peaks in the Si 20 wt pct composition produced by SPS. XRD analysis confirmed the presence of SiC, B4C, and Si phases of the SPS-sintered samples. The SiB4 phase was identified from XRD pattern analysis in Si 10 wt pct composition sintered by SPS. The reaction product SiB4 may be the results from the interaction between B4C and SiC and/or between Si and B4C.[22]

3.2.2 Microstructural characterization

The optical micrograph of 10 wt pct Si sample sintered at 1623 K (1350 °C) by SPS is shown in Figure 6. In the micrograph, the gray, black, and white regions are marked as SiC, B4C, and Si, respectively. The raw optical micrograph was also processed with Axio-vision image analysis software and these phases were represented with red, blue, and yellow colors. Further, the quantities of different phases were calculated based on the volume fraction of each color using the software. The amount of different phases is also shown in the table and it corresponds to the original composition of cermet.

Fig. 6
figure 6

Optical microscope image of spark plasma-sintered sample of 10 wt pct Si and phase distribution with area percentage calculation

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It was also observed that SiC, B4C, and Si are uniformly distributed. The optical micrograph conveys the compatibility of all the phases with good bonding in the sintered sample. Mechanical milling of the powder particles leads to grain refinement, and the uniform distribution of all the phases present in the sintered product is as shown in Figure 6. Uniform distribution of the particles is essential to obtain uniform physical and mechanical properties of the fabricated cermet samples.

Figures 7(a) through (c) represent the back-scattered SEM images of samples consolidated by SPS, and Figures 7(d) through (f) represent the conventional-sintered samples containing 5, 10, and 20 wt pct Si, respectively. Clearly, SPS samples showed less amount of porosity (3–4 pct) in 10 and 20 wt pct Si contents in the compositions, whereas porosity values of around 8–10 pct are achieved for conventional sintered samples. Conventional pressureless-sintered specimens show comparatively rougher surface than SPS-sintered samples. Large grain size in conventional-sintered sample is observed than SPS samples due to prolonged heating at high temperature. As per literature,[29] α-SiC powders show much less grain growth as a function of temperature and our results are consistent with this study, as also seen from the FESEM micrographs. The presence of Si in the SiC-B4C decreases grain growth.[34] The microstructural observation also depicts the presence of molten Si in the SPS samples, as shown in Figure 7(c).

Fig. 7
figure 7

Back-scattered SEM images of Si 5, 10, and 20 wt pct of SPS samples (a through c) and pressureless-sintered samples of Si 5, 10, and 20 wt pct (d through f)

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3.2.3 Density and microhardness study

The density of all the sintered samples is measured by the Archimedes’ method using distilled water as the immersion liquid. Figure 8(a) shows the percentage of relative density with varying composition for both conventional as well as SPS samples. The densities of sintered samples increases with increasing Si content up to 10 wt pct and then slightly decreased with further addition of Si (20 wt pct) for both cases. The highest density was obtained for samples (10 wt pct Si addition) sintered by conventional (92 pct) as well as SPS (98 pct). It is concluded that microhardness values (Figure 8(b)) are a function of the Si content. It indicates that the hardness of SPS samples is much higher as compared to conventional pressureless sintered samples. There was not much deviation of hardness values for the individual sintered sample when the hardness was taken at different positions; this is suggestive that the phases are uniformly distributed as in agreement with Figures 6 and 7.

Fig. 8
figure 8

(a) shows the bar diagram of the relative sintered density of various compositions sintered by pressureless sintering and SPS, and (b) bar chart showing microhardness of pressureless-sintered and spark plasma-sintered SiC-B4C-Si cermets

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The hardness of the cermets is directly related to the addition of Si in the main constituent and distribution of SiC, B4C, and Si phases. It may be pointed out that B4C is harder than SiC and Si. Increasing Si (softer phase) leads to the reduction in the proportion of B4C. To be more precise, the microhardness value of conventional pressureless-sintered samples varies from 16 to 18 GPa with increasing Si content from 5 to 10 wt pct, but decreased to 15 GPa for Si 20 wt pct. For samples prepared by SPS at 1623 K (1350 °C), the hardness varied from 24.4 GPa in 5 wt pct Si samples to maximum microhardness value of 27.80 GPa for 10 wt pct of Si addition. But lower microhardness value of 23.95 GPa was obtained for 20 wt pct Si sample, because of the increase in SiC content and residual Si in the product phase.[34]

3.2.4 Indentation fracture toughness (IFT) study

Indentation fracture toughness (IFT) was calculated by Vickers indentation method using equation proposed by Niihara:

$$ K_{\text{IC}} = 0.067\left( {\frac{E}{{H_{v} }}} \right)^{0.4} H_{v} a^{0.5} \left( {\frac{c}{a}} \right)^{ - 1.5}, $$
(1)

where E is the elastic modulus (MPa), Hv is the Vickers hardness (MPa), c is the average length of the cracks obtained in the tips of the Vickers marks (micron), and a is the half average length of the diagonal of the Vickers marks (micron).[37] Figures 9(a) through (c) show the schematic diagram and FESEM micrograph of the Vickers indentation mark and crack propagation in SiC60(B4C)30Si10 cermet for conventional and spark plasma-sintered samples. The propagation of crack length is comparatively less in case of SPS-sintered samples than conventional-sintered product. Table I shows the indentation fracture toughness of the various investigated cermets developed by conventional and SPS methods. These data shows that with increasing in Si content in SiC-B4C, the indentation fracture toughness value is also increased. For conventional pressureless sintered samples the indentation toughness value increases from 1.3 to 1.97 MPa m1/2 with the increase in Si content from 5 to 10 wt pct. However, the toughness value is reduced to 1.90 MPa m1/2 for 20 wt pct of Si addition (SiC60(B4C)20Si20). A similar trend of indentation fracture toughness is also obtained for the SPS samples.

Fig. 9
figure 9

(a) Schematic diagram of indentation fracture, (b) Vickers indentation mark and crack propagation in the sample of Si 10 wt pct for conventional-sintered, (c) for SPS-sintered

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Table I Composition, Processing Conditions, Microhardness, and Fracture Toughness of Sic-B4C-Based Cermet Materials
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Indentation fracture toughness improves from 2.9 to 3.8 MPa m1/2 with increasing Si content from 5 to 10 wt pct for SPS samples. The toughness was reduced to 3.5 MPa m1/2 for the Si 20 wt pct. Internal stresses are developed due to the mismatch of thermal expansion coefficient of SiC (2.56 × 10−6/K), Si (2.77 × 10−6/K), and B4C (3.2 × 10−6/K). The difference in thermal expansion coefficient resulted in the generation of micro-cracks within the samples. The stresses that created these cracks were developed during cooling from higher temperature to room temperature because of the mismatch in thermal expansion coefficient among SiC, B4C, and Si sintered compacts. It is believed that the stress during cooling is responsible for lower fracture toughness values of the sample sintered at various temperatures.[38]

Figure 10 shows the crack propagation on the surface of the sample by Vickers indentation method. The cracks generated for pure ceramic materials are deflection free and straight in nature. In the SiC-B4C-Si cermets, cracks are wavy and deflected due to the presence of SiC and re-solidified Si.[39] Trans-granular fracture is the major fracture mode for carbide/ceramic materials. This is because the cohesive strength among carbide materials is lower than that of interfacial strength between carbide and Si. Hence, stronger interfacial bonding is achieved by liquid phase sintering of SiC-B4C-Si cermets. The deflection of cracks occurs when the crack encounters different phases as shown in the circular region in Figure 10. The interaction of indentation cracks in B4C and Si particles leads to deflection and bridging, which in turn increases the toughness of the SiC-B4C-Si cermet materials.

Fig. 10
figure 10

Crack path produced by Vickers indentation on the sample surface of SPS Sample

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Table I lists the sintering conditions, density, microhardness, and indentation fracture toughness of SiC-B4C-based cermet materials with varying Si content. It is clearly observed from Table I that, the SPS is a highly effective method for the preparation of SiC-B4C-based cermet samples. With increasing Si content in the base material up to 10 pct, there is an enhancement of density and hardness. Table II summarizes of processing and mechanical properties of SiC-B4C-based cermets from the literature and compares them with the present work. It is clear that the mechanical properties are improved for SiC-B4C-Si cermets when processed by SPS even at lower temperatures compared to those reported in previous literature.

Table II A Brief Summaries on Processing and Mechanical Properties of SiC-B4C-Si Cermets from the Literatures and Comparison with the Present Work
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4 Discussion

4.1 Effect of Si Metal

As Si has high oxidation resistance at higher temperatures, it limits the oxide phase formation during sintering. In Figure 5, it is observed that with increasing Si content from 10 to 20 wt pct, the oxide phase formation is slightly reduced in the conventional-sintered samples. Addition of Si metal in SiC-B4C composites has improved the density and mechanical properties of the sintered samples. It has been found that the hardness of the cermet containing 5 wt pct Si is lower than 10 wt pct of Si (Figure 8(b)). The reason behind the low hardness is that the amount of molten Si required for proper diffusion and flowing around hard ceramic carbide particles is not sufficient. The deficiency in the Si content resulting in micro voids is shown in the microstructure (Figure 7(a)), which are responsible for increasing stress intensity at the crack tip. At this point cracks begin to form and leads to inferior mechanical properties. The decrease in pore size reduces the stress intensification. Hence, the addition of 10 wt pct Si is sufficient to reduce the pore size and refining the microstructure which accounts for the improvement in mechanical properties. The composition with 20 wt pct Si is not advantageous as the excess Si leads to formation of large amount of residual Si and SiC phases with lower hardness compared to B4C, which in turn reduces the overall hardness of the matrix. Du et al.[30] studied the effect of Si addition on B4C-SiC composites prepared from polycarbosilane-coated B4C powder and found an enhancement of hardness, flexural strength, and fracture toughness value by forming liquid phase at 11.4 wt pct Si. Feng et al.[33] also studied the effect of Si on densification and strengthening of B4C consolidated by SPS. They observed maximum hardness, fracture toughness, and flexural strength at 8 wt pct of Si. The activation of powder particles during spark plasma sintering and the liquid Si phase at the matrix interfaces of SiC-B4C cermets are responsible for improved wetting. On the other hand, in the conventional sintered samples the liquid or molten Si phase only helps in particles rearrangement but not achieving higher densification as compared to the SPS samples. The presence of molten Si at high temperature has improved the liquid phase sintering by flowing liquid Si through particle rearrangement and reducing pores. With increasing Si addition, there is an improvement of interfacial bond strength of the sintered product with higher densification as the cohesive strength of carbides are weaker than that of interfacial bond of Si and carbide. The presence of Si leads to crack deflection, branching and bridging that is responsible for toughening of the cermet materials (Figure 10). The plastic deformation of Si above 1623 K (1350 °C) leads to increased toughness with an increase in Si content. As SiC and Si do not have much difference in thermal coefficient of expansion the additional Si percentage in SiC-B4C cermet is advantageous to increase toughness up to certain level.

4.2 Conventional sintering vs SPS

Figure 7 shows the microstructure of samples obtained by pressureless sintering and SPS techniques. It has been observed that the average size of B4C phase in SPS samples is <5 µm. While in the case of pressureless-sintered samples, the average size of SiC and B4C phases increases to ≥10 µm. Conventional pressureless sintering uses only heat energy, but SPS uses heat energy, electric current, and pressure simultaneously. The benefit of SPS over conventional pressureless sintering is derived from the applied pressure which reduces the sintering time and temperature and inhibits the grain growth. The plasma spark on the contact point or in the gap between the particles generates a local high temperature of several thousand degrees of centigrade. This is the main cause of evaporation and melting of surface powder particles and neck formation around contact area between particles in SPS process. Conventional sintering process involves much higher temperature and longer holding times resulting in higher grain growth, which is the main cause for lower mechanical properties[26,40]. The maximum theoretical density of 98 pct and 27.80 GPa of the microhardness was obtained for SPS sample of Si 10 wt pct. Conventional sintered samples yielded samples of only 92 pct theoretical density and micro hardness of 18 GPa for the same composition. The reason is described in Section IV–A. It should be mentioned here that all the density and hardness values of SPS samples were considerably higher than that of pressureless-sintered samples. The superior density and microhardness were supposed to be due to the finer microstructure of spark plasma sintered cermets.

4.3 Effect of Temperature on the Processing of Sintered Samples

Grain size and morphology significantly depend on the sintering atmosphere, temperature, and technique. Low-temperature SPS samples showed less grain growth compared to conventional-sintered samples, since SPS was carried out for 5 minutes, whereas conventional sintering was conducted for 30 minutes. The difference in grain growth is also observed from the micrographs for the two different sintering techniques. It has been found that low-temperature SPS processed samples exhibit higher density and hardness than conventionally consolidated samples due to simultaneous application of heat and pressure.

5 Conclusions

  1. 1.

    Attempts have been made to synthesize SiC-B4C-based cermets by conventional pressureless sintering at 2223 K (1950 °C) as well as SPS at 1623 K (1350 °C).

  2. 2.

    The highest densities of 92 pct and 98 pct are obtained for the composition containing 10 wt pct Si fabricated by conventional sintering at 2223 K (1950 °C) and SPS at 1623 K (1350 °C), respectively.

  3. 3.

    With increasing the Si content from 5 to 10 wt pct, the density and the hardness values, with further increase of Si to 20 wt pct, the hardness of the composition decreases due to the existence of large amount of residual Si and increase of the SiC content in the composition.

  4. 4.

    In conventional-sintered samples, weak peaks of SiO2 and B2O3 are observed from XRD analysis and may be responsible for comparatively lesser improved properties than SPS samples.

  5. 5.

    There are no glassy phases and reaction phases observed in the sintered products for SPS specimens with Si contents. Hence, better mechanical and physical properties were found.

  6. 6.

    There is an enhancement of Vickers microhardness from 18.1 to 27.80 GPa for the cermet containing 10 wt pct of Si fabricated by conventional sintering at 2223 K (1950 °C) compared to SPS at 1623 K (1350 °C).

  7. 7.

    Spark plasma sintering is more effective for fabrication of SiC-B4C-Si cermet at much lower temperature than pressureless conventional sintering.