Investigation of Microstructural Features and Mechanical Characteristics of the Pressureless Sintered B4C/C(Graphite) Composites and the B4C-SiC-Si Composites Fabricated by the Silicon Infiltration Process

The B4C/C(graphite) composites were fabricated by employing a pressureless sintering process. The pressureless sintered B4C/C(graphite) composites exhibited extremely low mechanical characteristics. The liquid silicon infiltration technique was employed for enhancing the mechanical property of B4C/C(graphite) composites. Since the porosity of the B4C/C(graphite) composites was about 25–38%, the liquid silicon was able to infiltrate into the interior composites, thereby reacting with B4C and graphite to generate silicon carbide. Thus, boron carbide, silicon carbide, and residual silicon were sintered together forming B4C-SiC-Si composites. The pressureless sintered B4C/C(graphite) composites were transformed into the B4C-SiC-Si composites following the silicon infiltration process. This work comprises an investigation of the microstructure, phase composition, and mechanical characteristics of the pressureless sintered B4C/C(graphite) composites and B4C-SiC-Si composites. The XRD data demonstrated that the pressureless sintered bulks were composed of the B4C phase and graphite phase. The pressureless sintered B4C/C(graphite) composites exhibited a porous microstructure, an extremely low mechanical property, and low wear resistance. The XRD data of the B4C-SiC-Si specimens showed that silicon infiltrated specimens comprised a B4C phase, SiC phase, and residual Si. The B4C-SiC-Si composites manifested a compact and homogenous microstructure. The mechanical property of the B4C-SiC-Si composites was substantially enhanced in comparison to the pressureless sintered B4C/C(graphite) composites. The density, relative density, fracture strength, fracture toughness, elastic modulus, and Vickers hardness of the B4C-SiC-Si composites were notably enhanced as compared to the pressureless sintered B4C/C(graphite) composites. The B4C-SiC-Si composites also manifested outstanding resistance to wear as a consequence of silicon infiltration. The B4C-SiC-Si composites demonstrated excellent wear resistance and superior mechanical characteristics.

In particular, there has been much work using first-principles approaches to understand the strength of these and other materials [56]. In particular, these prior studies show that the density functional theory calculations can be used to understand atomistic details of structural mechanics and strength [57]. These approaches can reveal mechanisms for the strength of structures in a more unbiased way. The first principle calculation played a very important role in studying the relationship between microstructure analysis and mechanical property of the composites materials [56]. At the same time, the density functional calculation played a very important role in studying the relationship between microstructure analysis and the mechanical property of the composite's materials [57]. The first principle calculation and the density functional calculation could completely understand the relationship between the microstructure characterization and mechanical property of the materials by using the calculation methods [56]. So, it was very important to use the first principle calculation and the density functional calculation to understand the relationship between the microstructure characterization and the mechanical property of the composites materials [57]. According to the principle of interaction between an atomic nucleus and an electron and its basic motion law, using the principle of quantum mechanics, starting from the specific requirements, the algorithm to directly solve the Schrödinger equation after some approximation processing is customarily called the first principle [56]. The density functional theory (DFT) is a method for studying the electronic structure of multi-electron systems. The density functional theory has a wide range of applications in physics and chemistry, especially for studying the properties of molecules and condensed matter. It is one of the most commonly used methods in the fields of computational materials science and computational chemistry in condensed matter physics [57].

Fabrication of the B 4 C/C(Graphite) Composites by Pressureless Sintering
A pressureless sintering method was used for fabricating B 4 C/C(graphite) composites. The starting materials included B 4 C powder and graphite powder having average particle dimensions of 3.5 µm and 10-15 µm, respectively. The graphite powder contents of the B 4 C/C(graphite) composite powders were kept as 10 wt%, 20 wt%, 30 wt%, and 40 wt%. In total, 10 wt% sintering aids comprising Y 2 O 3 and Al 2 O 3 were then mixed with B 4 C and graphite powders. Subsequently, ethanol was added to the powder mixture and the mixture was ball milled and wet mechanically milled for 24 h and allowed to dry slowly. The B 4 C/C(graphite) composite powders were thus prepared. The powders were then put into the steel moulds and fabricated into the strip samples under 200 MPa pressure, with the holding time being 2 min. The strip sample size was about 5 mm × 6 mm × 50 mm. The strip specimens were put into a graphite crucible and set inside a vacuum sintering furnace. A pressureless sintering process was carried out at 1800 • C for 2 h in vacuum conditions yielding the composite's bulks based on B 4 C/C(graphite). The B 4 C/C(graphite) composite's bulks thus produced were cut into strip specimens, with the size of the sample being 5 mm × 6 mm × 50 mm, which were subsequently processed by silicon infiltration.

Fabrication of the B 4 C Monolith by Pressureless Sintering
The B 4 C monolith was fabricated by a pressureless sintering process. For fabricating B 4 C monolith, the raw materials included B 4 C powders having an average particle dimension of 3.5 µm. In total, 10 wt% sintering aids of Al 2 O 3 and Y 2 O 3 powder were mixed with B 4 C powders. Ethanol was added into the powdered mixture which was subsequently balled milled and wet mechanically milled for 24 h. Following milling, the powder was subjected to slow drying yielding B 4 C, Al 2 O 3 , and Y 2 O 3 composite powders. The composite powders were then poured into a steel mould and fabricated into the stripe samples under 200 MPa. The holding time was 2 min, and the strip sample size was about 5 mm × 6 mm × 50 mm. These stripe samples were then put into a graphite crucible and set within a vacuum sintering furnace. The fabrication of B 4 C monolith bulks was achieved using pressureless sintering for 2 h at 1800 • C under vacuum conditions.

Fabrication Process of the B 4 C-SiC-Si Composites by Silicon Infiltration Process
The B 4 C monolith specimens fabricated by pressureless sintering were cut into the strip samples. These strip specimens were coated with silicon powders, placed within a graphite crucible, and set inside a vacuum sintering furnace. The silicon infiltration process was performed for 2 h at 1550 • C under a vacuum. The solid silicon was completely transformed into liquid silicon at 1550 • C (MP of silicon = 1410 • C) during the silicon infiltration process. Thus, the stripe samples were completely immersed in the liquid silicon. Since the porosity of pressureless sintered specimens was about 20-30%, the liquid silicon thoroughly infiltrated into the interior of the specimens. In the process of infiltration, the liquid silicon underwent a reaction with B 4 C, producing silicon carbide and the boron carbide partially reacted with the liquid silicon generating silicon carbide. The residual boron carbide and silicon were found within the matrix of the composite. The boron carbide, silicon carbide, and residual silicon were sintered together and formed the B 4 C-SiC-Si composites. Following the silicon infiltration process, the silicon infiltrated specimens were ground, polished, and fabricated into strip samples of 5 mm × 6 mm × 50 mm. The strip silicon infiltrated specimens were then processed and fabricated into smaller strips with a size of 3 mm × 4 mm × 30 mm. The microstructure and phase composition of the silicon infiltrated specimens were subsequently examined.
The B 4 C/C(graphite) composite samples were fabricated by a pressureless sintering process and then cut into stripe samples. The B 4 C/C(graphite) composites fabricated by a pressureless sintering process have a graphite content of 10 wt%, 20 wt%, 30 wt%, and 40 wt% and were then subjected to a silicon infiltration process by being placed into a vacuum sintering furnace in a graphite crucible. The specimens were covered with silicon powder and the silicon infiltration process was performed for 2 h at 1550 • C under vacuum conditions. The solid silicon was completely transformed into liquid silicon at 1550 • C (MP of silicon = 1410 • C) during the silicon infiltration process. In this way, the stripe samples were completely immersed in the liquid silicon. Since the porosity of pressureless sintered specimens was about 20-30%, the liquid silicon thoroughly infiltrated the interior of the composites. The liquid silicon reacted with B 4 C and graphite producing silicon carbide, the boron carbide partially reacted with the liquid silicon to produce silicon carbide, and the graphite was completely combined with the liquid silicon to produce silicon carbide. The residual boron carbide and silicon, however, were present in the matrix of the composite and sintered together with silicon carbide forming the B 4 C-SiC-Si composites. Following silicon infiltration, the silicon infiltrated specimens were ground, polished, and fabricated first into strip samples of about 5 mm × 6 mm × 50 mm and then cut into smaller sized specimens of 3 mm × 4 mm × 30 mm. The phase composition and microstructure of the B 4 C-SiC-Si composites specimens yielded via silicon infiltration were then investigated.

The Microstructure and Phase Composition of Pressureless Sintered Specimens and Silicon Infiltrated Specimens
The phase composition of the B 4 C/C(graphite) composites fabricated via a pressureless sintering process and that of silicon infiltrated specimens (B 4 C-SiC-Si composites) was ascertained using X-ray diffraction analysis (XRD, D/MAX-2400, Nippon Rigaku Corporation Limited, Tokyo, Japan). The microstructural features of the B 4 C/C(graphite) composites obtained via the pressureless sintering process and that of the silicon infiltrated specimens (B 4 C-SiC-Si composites) were examined and analyzed by a scanning electron microscope (SEM, JSM-6390 A, JEOL Scanning Microscope, Tokyo, Japan). An energydispersive spectroscopic analysis (EDS, JEOL Scanning Microscope, Tokyo, Japan) of the silicon infiltrated specimens (B 4 C-SiC-Si composites) was conducted using the scanning electron microscope (SEM, JSM-6390 A, JEOL Scanning Microscope, Tokyo, Japan).

The Mechanical Property and Wear Resistance of Pressureless Sintered Specimens and Silicon Infiltrated Specimens
The strip samples of the B 4 C/C(graphite) composites fabricated by pressureless sintering and the B 4 C-SiC-Si composites by silicon infiltration process were evaluated for their mechanical property, density, relative density, fracture toughness, fracture strength, Vickers hardness, elastic modulus, and wear resistance. Archimedes' methods were used to estimate the density and relative density of the composites. The composites were evaluated for their fracture strength by a three-point bending test utilizing specimens of sizes 3 mm × 4 mm × 30 mm. The span was kept at 16 mm and the speed of the crosshead was 0.5 mm/min (Testing machine: Instron1195). The fracture toughness of the composites was estimated using a single edge notch beam (SENB) using samples of 3 mm × 4 mm × 30 mm. The respective depth and width of the notch were 1.5 mm and 0.2 mm, the span was 16 mm, and the crosshead speed was 0.05 mm/min (Testing machine: Instron1195). The elastic modulus of the pressureless sintered B 4 C/C(graphite) composites, as well as B 4 C-SiC-Si composites obtained via silicon infiltration, was estimated using a three-point bending test with the dimensions of the specimen being 3 mm × 4 mm × 30 mm, the span and the crosshead speed being 16 mm and 0.5 mm/min (Testing machine: Instron1195), respectively. A Vickers hardness meter equipped with a load force of 49 N and a holding time of 20 s was employed to test the composites for their Vickers's hardness (HV-5 Vickers hardness meter). The wear experiments were performed at MM-W1 B friction and wear tester. Friction wear tests were carried out to measure the wear resistance of the pressureless sintered B 4 C/C(graphite) composites and B 4 C-SiC-Si composites obtained via silicon infiltration, wherein the load force was 20 N and the rotational speed was 220 rpm. The wear loss rates of pressureless sintered specimens were also measured. The wear loss rate (R) was the ratio of wear weight loss(M) to wear time(t), thus the wear loss rate is computed using the following equation: R = M/t. The mechanical property and wear resistance of the pressureless sintered B 4 C monolith were measured similarly using the methods employed for the pressureless sintered B 4 C/C(graphite) composites. The XRD patterns of the B 4 C monolith bulks produced by pressureless sintering have been shown in Figure 1. For the pressureless sintered B 4 C monolith, Figure 1 illustrates that the diffraction peaks corresponding to the B 4 C phase were visible in the observed XRD data, at 19 mm × 30 mm. The respective depth and width of the notch were 1.5 mm and 0.2 mm, the span was 16 mm, and the crosshead speed was 0.05 mm/min (Testing machine: In-stron1195). The elastic modulus of the pressureless sintered B4 C/C(graphite) composites, as well as B4 C-SiC-Si composites obtained via silicon infiltration, was estimated using a three-point bending test with the dimensions of the specimen being 3 mm × 4 mm × 30 mm, the span and the crosshead speed being 16 mm and 0.5 mm/min (Testing machine: Instron1195), respectively. A Vickers hardness meter equipped with a load force of 49 N and a holding time of 20 s was employed to test the composites for their Vickers's hardness (HV-5 Vickers hardness meter). The wear experiments were performed at MM-W1 B friction and wear tester. Friction wear tests were carried out to measure the wear resistance of the pressureless sintered B4 C/C(graphite) composites and B4 C-SiC-Si composites obtained via silicon infiltration, wherein the load force was 20 N and the rotational speed was 220 rpm. The wear loss rates of pressureless sintered specimens were also measured. The wear loss rate (R) was the ratio of wear weight loss(M) to wear time(t), thus the wear loss rate is computed using the following equation: R = M/t. The mechanical property and wear resistance of the pressureless sintered B4 C monolith were measured similarly using the methods employed for the pressureless sintered B4 C/C(graphite) composites.

The Phase Composition of the B4 C Monolith Fabricated Using a Pressureless Sintering Process
The XRD patterns of the B4 C monolith bulks produced by pressureless sintering have been shown in Figure 1. For the pressureless sintered B4 C monolith, Figure 1 illustrates that the diffraction peaks corresponding to the B4 C phase were visible in the observed XRD data, at 19.71°, 22.02°, 23.50°, 31.90°, 34.95°, and 37.82°. The sintering aids, i.e., Al2 O3 and Y2 O3 reacted and produced the Y3 Al5 O12(YAG) during the pressureless sintering process. The corresponding peaks were also evident in the XRD patterns.

The Phase Composition of the B 4 C/C(Graphite) Composites Produced by Pressureless Sintering
The characteristic XRD pattern of the B 4 C/C(graphite) composite's bulks produced by pressureless sintering have been shown in Figure 2. Figure 2a- composites sintered bulks. The diffraction peaks of the B4 C phase and graphite phase are apparent in the XRD patterns. Whereas the diffractive peaks of the B4 C phase were at 19.71°, 22.02°, 23.50°, 31.90°, 34.95°, and 37.82°, the peaks at 26.60°, 42.71°, 43.45°, 44.67°, 46.31°, 54.79°, 56.66°, 60.02°, and 77.69° arise from the graphite phase. The sintering aids, i.e., Al2 O3 and Y2 O3 upon reaction produced Y3 Al5 O12(YAG) during the pressureless sintering process. The diffractive peaks of the low content of Y3 Al5 O12 (YAG) are also obvious in the XRD patterns.    Figure 3 depicts the SEM photographs of the fracture surface of the B 4 C monolith bulks fabricated by pressureless sintering. The pressureless sintered B 4 C monolith manifested a porous microstructure, with porosity as high as 20-30%. The B 4 C particles were disseminated within the composite's matrix and the average particle dimension of boron carbide was approximately 3-5 µm. The porous microstructure of the B 4 C monolith bulks was presumably because of their fabrication from the pressureless sintering process. The pressureless sintered B 4 C monolith also exhibited an extremely low relative density; precisely about 70-80%. bulks fabricated by pressureless sintering. The pressureless sintered B4 C monolith manifested a porous microstructure, with porosity as high as 20-30%. The B4 C particles were disseminated within the composite's matrix and the average particle dimension of boron carbide was approximately 3-5 µm. The porous microstructure of the B4 C monolith bulks was presumably because of their fabrication from the pressureless sintering process. The pressureless sintered B4 C monolith also exhibited an extremely low relative density; precisely about 70-80%.   Figure 4a-d depict the SEM micrographs of the (10 wt%, 20 wt%, 30 wt%, 40 wt%C(graphite)) B4 C/C(graphite) composites, respectively. It is obvious from the micrographs that the pressureless sintered B4 C/C(graphite) composites have a porous microstructure, ranging between 25 and 38%. The B4 C and graphite particles are distributed within the composite matrix and the graphite particles generated a laminated structure. The average particle dimensions of boron carbide are about 3-5 µm, whereas the mean particle dimensions of graphite were about 10-15 µm. It has also been observed that with the increase in the amount of graphite, the microstructure of pressureless sintered B4 C/C(graphite) composites became increasingly porous. The relative density of the pressureless sintered B4 C/C(graphite) composites was found to be extremely low at approximately 62-75%, and their porosity was extremely high.  Figure 4a-d depict the SEM micrographs of the (10 wt%, 20 wt%, 30 wt%, 40 wt%C(graphite)) B 4 C/C(graphite) composites, respectively. It is obvious from the micrographs that the pressureless sintered B 4 C/C(graphite) composites have a porous microstructure, ranging between 25 and 38%. The B 4 C and graphite particles are distributed within the composite matrix and the graphite particles generated a laminated structure. The average particle dimensions of boron carbide are about 3-5 µm, whereas the mean particle dimensions of graphite were about 10-15 µm. It has also been observed that with the increase in the amount of graphite, the microstructure of pressureless sintered B 4 C/C(graphite) composites became increasingly porous. The relative density of the pressureless sintered B 4 C/C(graphite) composites was found to be extremely low at approximately 62-75%, and their porosity was extremely high. bulks fabricated by pressureless sintering. The pressureless sintered B4 C monolith manifested a porous microstructure, with porosity as high as 20-30%. The B4 C particles were disseminated within the composite's matrix and the average particle dimension of boron carbide was approximately 3-5 µm. The porous microstructure of the B4 C monolith bulks was presumably because of their fabrication from the pressureless sintering process. The pressureless sintered B4 C monolith also exhibited an extremely low relative density; precisely about 70-80%.  Figure 4a-d depict the SEM micrographs of the (10 wt%, 20 wt%, 30 wt%, 40 wt%C(graphite)) B4 C/C(graphite) composites, respectively. It is obvious from the micrographs that the pressureless sintered B4 C/C(graphite) composites have a porous microstructure, ranging between 25 and 38%. The B4 C and graphite particles are distributed within the composite matrix and the graphite particles generated a laminated structure. The average particle dimensions of boron carbide are about 3-5 µm, whereas the mean particle dimensions of graphite were about 10-15 µm. It has also been observed that with the increase in the amount of graphite, the microstructure of pressureless sintered B4 C/C(graphite) composites became increasingly porous. The relative density of the pressureless sintered B4 C/C(graphite) composites was found to be extremely low at approximately 62-75%, and their porosity was extremely high.

The Phase Composition of the B4 C-SiC-Si Composites Fabricated by the Silicon Infiltration Process
3.3.1. The Phase Composition of the B4 C-SiC-Si Composites Fabricated by Silicon Infiltration of Pressureless Sintered B4 C Monolith Figure 5 presents the XRD patterns corresponding to the silicon infiltrated B4 C monolith (B4 C-SiC-Si composites). The pressureless sintered B4 C monolith was transformed into the B4 C-SiC-Si composites through the silicon infiltration process. Figure 5 shows that following the silicon infiltration process, the diffractive peaks of the B4 C phase disappeared partially. Besides, some new diffraction peaks were apparent in the XRD pattern, which presumably arise due to the existence of silicon carbide and silicon phase. As seen into the B 4 C-SiC-Si composites through the silicon infiltration process. Figure 5 shows that following the silicon infiltration process, the diffractive peaks of the B 4 C phase disappeared partially. Besides, some new diffraction peaks were apparent in the XRD pattern, which presumably arise due to the existence of silicon carbide and silicon phase. As seen before, the diffraction peaks of the B 4 C phase were observed at 19 The XRD results have also demonstrated the existence of a boron carbide phase, silicon carbide phase, and residual silicon phase on the surface of the specimen obtained after silicon infiltration. These XRD analysis results also confirmed our assumption that the liquid silicon upon reaction with the boron carbide produced silicon carbide, leaving the residual silicon on the surface of silicon infiltrated specimens. The residual boron carbide also existed in the matrix of the composite. The boron carbide, silicon carbide, and residual silicon were sintered together yielding composites of the B 4 C-SiC-Si type. This way the pressureless sintered B 4 C monolith was transformed into the B 4 C-SiC-Si composites following the silicon infiltration process. The porosity of the pressureless sintered B 4 C monolith was about 20-30%. The apparent decrease in porosity can be ascribed to the silicon carbide and residual silicon (generated following silicon infiltration), filling in the pores of the matrix of the composite.

The Phase Composition of the B4 C-SiC-Si Composites Fabricated by the Silicon Infiltration Process
3.3.1. The Phase Composition of the B4 C-SiC-Si Composites Fabricated by Silicon Infiltration of Pressureless Sintered B4 C Monolith Figure 5 presents the XRD patterns corresponding to the silicon infiltrated B4 C monolith (B4 C-SiC-Si composites). The pressureless sintered B4 C monolith was transformed into the B4 C-SiC-Si composites through the silicon infiltration process. Figure 5 shows that following the silicon infiltration process, the diffractive peaks of the B4 C phase disappeared partially. Besides, some new diffraction peaks were apparent in the XRD pattern, which presumably arise due to the existence of silicon carbide and silicon phase. As seen before, the diffraction peaks of the B4 C phase were observed at 19 The XRD results have also demonstrated the existence of a boron carbide phase, silicon carbide phase, and residual silicon phase on the surface of the specimen obtained after silicon infiltration. These XRD analysis results also confirmed our assumption that the liquid silicon upon reaction with the boron carbide produced silicon carbide, leaving the residual silicon on the surface of silicon infiltrated specimens. The residual boron carbide also existed in the matrix of the composite. The boron carbide, silicon carbide, and residual silicon were sintered together yielding composites of the B4 C-SiC-Si type. This way the pressureless sintered B4 C monolith was transformed into the B4 C-SiC-Si composites following the silicon infiltration process. The porosity of the pressureless sintered B4 C monolith was about 20-30%. The apparent decrease in porosity can be ascribed to the silicon carbide and residual silicon (generated following silicon infiltration), filling in the pores of the matrix of the composite.    The XRD data point towards the existence of the boron carbide phase, silicon carbide phase, and residual silicon phase upon the surface of the specimens obtained after silicon infiltration. The XRD analysis suggests the infiltration of liquid silicon into the inner composite matrix and subsequent reaction with boron carbide and graphite producing silicon carbide. Some residual silicon stayed on the specimen surface following silicon infiltration. The boron carbide, silicon carbide, and residual silicon were sintered together forming the B 4 C-SiC-Si composites. The porosity of the pressureless sintered B 4 C/C(graphite) composites was about 25-38%. The apparent decrease in the porosity is because produced silicon carbide and residual silicon filled the pores in the matrix of the composite during the silicon infiltration process. 31.90°, 34.95°, and 37.82°, the diffraction peaks of the SiC phase can be observed at 20.78°, 26.67°, 35.60°, 41.38°, 59.97°, 71.77°, 75.49°, whereas those of the Si phase are apparent at 28.44°, 47.30°, 56.12°, 69.13°, 76.37°. The XRD data point towards the existence of the boron carbide phase, silicon carbide phase, and residual silicon phase upon the surface of the specimens obtained after silicon infiltration. The XRD analysis suggests the infiltration of liquid silicon into the inner composite matrix and subsequent reaction with boron carbide and graphite producing silicon carbide. Some residual silicon stayed on the specimen surface following silicon infiltration. The boron carbide, silicon carbide, and residual silicon were sintered together forming the B4 C-SiC-Si composites. The porosity of the pressureless sintered B4 C/C(graphite) composites was about 25-38%. The apparent decrease in the porosity is because produced silicon carbide and residual silicon filled the pores in the matrix of the composite during the silicon infiltration process.   Figure 7 presents the SEM photographs representing the fracture surface of the silicon infiltrated B 4 C monolith (B 4 C-SiC-Si composites). The pressureless sintered B 4 C monolith was transformed into the B 4 C-SiC-Si composites through the silicon infiltration process. These SEM images depict that the silicon infiltrated products B 4 C-SiC-Si composites possessed a compact and homogenous microstructure. The B 4 C particles and SiC particles were homogenously dispersed within the composite matrix. The mean particle size of B 4 C particles and SiC particles was about 4-5 µm. The existence of some residual silicon in the matrix of the composite is also evident. Because the pressureless sintered B 4 C monolith was a porous material; the porosity being 20-30%, the liquid silicon could infiltrate into the interior of the composite matrix during the silicon infiltration process. Boron carbide reacted with liquid silicon producing silicon carbide particles, and the residual boron carbide, as well as silicon, existed in the composite's matrix. B 4 C-SiC-Si composites exhibited an exceptionally high relative density (98-99%), whereas their porosity was only about 1-2%. The silicon carbide was produced following silicon infiltration and the residual silicon filled the pores of the composites, so the porosity of the silicon infiltrated specimens decreased remarkably, and the B 4 C-SiC-Si composites became more compact and denser.
Boron carbide reacted with liquid silicon producing silicon carbide particles, and the residual boron carbide, as well as silicon, existed in the composite's matrix. B4 C-SiC-Si composites exhibited an exceptionally high relative density (98-99%), whereas their porosity was only about 1-2%. The silicon carbide was produced following silicon infiltration and the residual silicon filled the pores of the composites, so the porosity of the silicon infiltrated specimens decreased remarkably, and the B4 C-SiC-Si composites became more compact and denser.  Figure 8a-d, respectively, depict the SEM micrographs of the (10 wt%, 20 wt%, 30 wt%, 40 wt%C(graphite)) silicon infiltrated B4 C/C(graphite) composites. These SEM data present a compact and homogenous microstructure of the silicon infiltrated B4 C-SiC-Si composites. The B4 C particles and SiC particles were distributed homogeneously in the composite's matrix, and some residual silicon also existed in the composite's matrix. The mean particle size of B4 C particles and SiC particles was about 4-5 µm. Because the pressureless sintered B4 C/C(graphite) composites were porous (the porosity of the pressureless sintered B4 C/C(graphite) composites was about 25-38%), the liquid silicon could infiltrate into the inner composite's matrix during silicon infiltration. The reaction between boron carbide/graphite and liquid silicon produced silicon carbide particles, and the residual boron carbide and silicon existed in the composite's matrix. The relative density of the B4 C-SiC-Si composites was extremely high (98-99%), whereas the porosity of the B4 C-SiC-Si composites was about 1-2%. The produced silicon carbide and residual silicon filled the pores, so the porosity of the silicon infiltrated specimens decreased remarkably, and the B4 C-SiC-Si composites became more compact and denser.  Figure 9 presents the SEM micrographs and Energy Dispersive Spectroscopy (EDS) spectrum of the B4 C-SiC-Si composites fabricated by the silicon infiltration process. As already discussed, the B4 C-SiC-Si composites manifested compact and homogenous microstructural features as shown in Figure 9a. The boron carbide and silicon carbide par-  Figure 9 presents the SEM micrographs and Energy Dispersive Spectroscopy (EDS) spectrum of the B 4 C-SiC-Si composites fabricated by the silicon infiltration process. As already discussed, the B 4 C-SiC-Si composites manifested compact and homogenous microstructural features as shown in Figure 9a. The boron carbide and silicon carbide particles were dispersed homogeneously within the composite's matrix, and the residual silicon also existed within the matrix. Figure 9b is a presentation of the EDS spectrum of the B 4 C-SiC-Si composites fabricated by the silicon infiltration process. The EDS spectroscopic analysis depicted that the silicon infiltrated composites were composed primarily of boron, silicon, and carbon, thereby demonstrating that the composites obtained following silicon infiltration comprised boron carbide and silicon carbide along with some content of silicon. It can therefore be assumed that the liquid silicon infiltrated into the interior of the composite and produce silicon carbide upon reaction with boron carbide, according to the following reaction equations  The liquid silicon underwent a chemical reaction with graphite producing silicon carbide as shown by the following chemical reactions :

The EDS Spectrum of the B 4 C-SiC-Si Composites Fabricated by the Silicon Infiltration Process
The residual boron carbide and silicon sintered together with the product silicon carbide to give rise to B 4 C-SiC-Si composites. The residual silicon and silicon carbide produced during the process filled the pores in the composite's matrix, hence the porosity of the specimens obtained following silicon infiltration decreased remarkably. Consequently, the B 4 C-SiC-Si composites exhibited an extremely high relative density and low porosity. The relative density of the B 4 C-SiC-Si composites was about 98-99%, and the porosity of the B 4 C-SiC-Si composites was about 1-2%.   Figure 10 represents the impact of the graphite content on the density of the pressureless sintered B4 C/C(graphite) composites and the B4 C-SiC-Si composites. The density of the pressureless sintered B4 C/C(graphite) composites demonstrated a gradual decrease with the increase of graphite content. The density of the pressureless sintered B4 C monolith was relatively high, but the density of the pressureless sintered B4 C/C(graphite) composites was rather low. The underlying reason for this observation is that B4 C materials have a density of 2.52 g/cm 3 , whereas graphite has a density of about  was relatively high, but the density of the pressureless sintered B 4 C/C(graphite) composites was rather low. The underlying reason for this observation is that B 4 C materials have a density of 2.52 g/cm 3 , whereas graphite has a density of about 2.26 g/cm 3 , hence with the increase in the amount of graphite, the density of the pressureless sintered B 4 C/C(graphite) composites demonstrated a gradual decline. The porous microstructure of the pressureless sintered specimens also contributed to the extremely low density. The density of the pressureless sintered B 4 C/C(graphite) composites was about 1.5-1.9 g/cm 3 .  Figure 11 shows the impact of the amount of graphite on the relative density of the pressureless sintered B4 C/C(graphite) composites and the B4 C-SiC-Si composites. Figure  11 illustrates that the relative density of the pressureless sintered B4 C/C(graphite) composites exhibited a gradual decline with the increase of graphite content. The relative density of the pressureless sintered B4 C monolith was relatively high; however, the relative density of the pressureless sintered B4 C/C(graphite) composites was rather low. This is presumably because the B4 C/C(graphite) composites were produced via pressureless sintering and therefore exhibited a porous microstructure, causing their relative density to become extremely low, precisely 62-75%. The porosity of the pressureless sintered B4 C/C(graphite) composites was about 25-38%. Figure 11. The impact of the graphite content on the relative density of the pressureless sintered B4 C/C(graphite) composites and the B4 C-SiC-Si composites. Figure 11 also depicts that the relative density of the B4 C-SiC-Si composites demonstrated a remarkable improvement in comparison with the pressureless sintered B4 C/C(graphite) composites. In the B4 C/C(graphite) composites, a gradual increase in the relative density was observed with the increase of graphite content. With the infiltration of liquid silicon into the interior of pressureless sintered B4 C/C(graphite) composites, the B4 C and graphite reacted with liquid silicon to produce the silicon carbide. The produced silicon carbide as well as the residual silicon filled the pores in the composite's matrix, thereby resulting in a remarkable decrease in the porosity of the silicon infiltrated specimens and a consequent increase in their relative density. The residual silicon, boron carbide, and sili- On the contrary, the density of the B 4 C-SiC-Si composites was phenomenally improved as compared to the pressureless sintered B 4 C/C(graphite) composites as depicted in Figure 10. The density of the B 4 C-SiC-Si composites progressively increased with the increase in the amount of added graphite in the B 4 C/C(graphite) composites. This observation was mainly because the liquid silicon infiltrated into the interior of the pressureless sintered B 4 C/C(graphite) composites and reacted with B 4 C and graphite to produce silicon carbide. Thus, with the increment in the amount of graphite in the B 4 C/C(graphite) composites, the graphite completely reacted with liquid silicon to produce silicon carbide, thus markedly increasing the content of produced silicon carbide in the B 4 C-SiC-Si composites. Since the density of silicon carbide is 3.21 g/cm 3 and that of silicon is 2.32 g/cm 3 , the density of the produced B 4 C-SiC-Si composites increased progressively with the increase in the amount of silicon carbide produced. On the whole, the density of the B 4 C-SiC-Si composites was higher than the density of the corresponding B 4 C/C(graphite) composites. B 4 C-SiC-Si composites demonstrated a density value of 2.5855-2.9288 g/cm 3 which is substantially higher than the pressureless sintered B 4 C/C(graphite) composites. Figure 11 shows the impact of the amount of graphite on the relative density of the pressureless sintered B 4 C/C(graphite) composites and the B 4 C-SiC-Si composites. Figure 11 illustrates that the relative density of the pressureless sintered B 4 C/C(graphite) composites exhibited a gradual decline with the increase of graphite content. The relative density of the pressureless sintered B 4 C monolith was relatively high; however, the relative density of the pressureless sintered B 4 C/C(graphite) composites was rather low. This is presumably because the B 4 C/C(graphite) composites were produced via pressureless sintering and therefore exhibited a porous microstructure, causing their relative density to become extremely low, precisely 62-75%. The porosity of the pressureless sintered B 4 C/C(graphite) composites was about 25-38%.
posites exhibited a gradual decline with the increase of graphite content. The relative density of the pressureless sintered B4 C monolith was relatively high; however, the relative density of the pressureless sintered B4 C/C(graphite) composites was rather low. This is presumably because the B4 C/C(graphite) composites were produced via pressureless sintering and therefore exhibited a porous microstructure, causing their relative density to become extremely low, precisely 62-75%. The porosity of the pressureless sintered B4 C/C(graphite) composites was about 25-38%. Figure 11. The impact of the graphite content on the relative density of the pressureless sintered B4 C/C(graphite) composites and the B4 C-SiC-Si composites. Figure 11 also depicts that the relative density of the B4 C-SiC-Si composites demonstrated a remarkable improvement in comparison with the pressureless sintered B4 C/C(graphite) composites. In the B4 C/C(graphite) composites, a gradual increase in the relative density was observed with the increase of graphite content. With the infiltration of liquid silicon into the interior of pressureless sintered B4 C/C(graphite) composites, the B4 C and graphite reacted with liquid silicon to produce the silicon carbide. The produced silicon carbide as well as the residual silicon filled the pores in the composite's matrix, thereby resulting in a remarkable decrease in the porosity of the silicon infiltrated specimens and a consequent increase in their relative density. The residual silicon, boron carbide, and silicon carbide were sintered into dense and compact bulks. The density of silicon carbide is 3.21 g/cm 3 , hence the relative density of the composites based on B4 C-SiC-Si became higher than the relative density of the B4 C/C(graphite) composites obtained through pressureless Figure 11. The impact of the graphite content on the relative density of the pressureless sintered B 4 C/C(graphite) composites and the B 4 C-SiC-Si composites. Figure 11 also depicts that the relative density of the B 4 C-SiC-Si composites demonstrated a remarkable improvement in comparison with the pressureless sintered B 4 C/ C(graphite) composites. In the B 4 C/C(graphite) composites, a gradual increase in the relative density was observed with the increase of graphite content. With the infiltration of liquid silicon into the interior of pressureless sintered B 4 C/C(graphite) composites, the B 4 C and graphite reacted with liquid silicon to produce the silicon carbide. The produced silicon carbide as well as the residual silicon filled the pores in the composite's matrix, thereby resulting in a remarkable decrease in the porosity of the silicon infiltrated specimens and a consequent increase in their relative density. The residual silicon, boron carbide, and silicon carbide were sintered into dense and compact bulks. The density of silicon carbide is 3.21 g/cm 3 , hence the relative density of the composites based on B 4 C-SiC-Si became higher than the relative density of the B 4 C/C(graphite) composites obtained through pressureless sintering. The silicon infiltrated products B 4 C-SiC-Si composites presented compact and homogenous microstructural features. The relative density of the B 4 C-SiC-Si composites was about 98-99%. The density and relative density of the B 4 C-SiC-Si composites showed a significant improvement in comparison to the pressureless sintered B 4 C/C(graphite) composites.

The Fracture Strength and Fracture Toughness of the Pressureless Sintered B 4 C/C(Graphite) Composites and the B 4 C-SiC-Si Composites
The impact of the amount of graphite on the fracture strength of the pressureless sintered B 4 C/C(graphite) composites and the B 4 C-SiC-Si composites has been shown in Figure 12. A gradual decrease in the fracture strength of the pressureless sintered B 4 C/C(graphite) composites was observed with the increase in the amount of graphite. While the fracture strength of the pressureless sintered B 4 C monolith was quite high, the pressureless sintered B 4 C/C(graphite) composites demonstrated a rather low fracture strength value. This is because the B 4 C/C(graphite) composites were fabricated by a pressureless sintering process, which imparted a porous microstructure, a low relative density (62-75%), and porosity of around 25-38% leaving them with an extremely low fracture strength. The fracture strength of the pressureless sintered B 4 C/C(graphite) composites lay within the 102-155 MPa range and reduced remarkably with the increase of graphite content. Figure 12 depicts that the B 4 C-SiC-Si composites demonstrated a remarkable improvement in their fracture strength as compared to the pressureless sintered B 4 C/C(graphite) composites. The fracture strength of the B 4 C-SiC-Si composites increased progressively with the rise in graphite content. It occurs because liquid silicon infiltrated into the interior of pressureless sintered B 4 C/C(graphite) composites, and B 4 C and graphite reacted with liquid silicon producing silicon carbide. The resultant silicon carbide and the residual silicon and boron carbide were sintered together fabricating B 4 C-SiC-Si composites. The silicon carbide and residual silicon settled within the pores in the composite's matrix, thereby causing a remarkable decrease in the porosity of the silicon infiltrated specimens and a corresponding increase in the relative density of the silicon infiltrated specimens. The relative density of the B 4 C-SiC-Si composites was found to be higher than 98%, the B 4 C-SiC-Si composites thus became denser and more compact and their fracture strength was higher than that of the pressureless sintered B 4 C/C(graphite) composites. B 4 C-SiC-Si composites have a fracture strength equivalent to 385-438 MPa. In addition, the produced silicon carbide could also improve the fracture strength of the B 4 C-SiC-Si composites, with the fracture strength increasing gradually with the increase of silicon carbide content. As an obvious fact, silicon carbide content underwent a gradual increase with the increase of graphite content in the B 4 C/C(graphite) composites. All these factors contribute to a remarkable improvement in the fracture strength of the B 4 C-SiC-Si composites as compared to the pressureless sintered B 4 C/C(graphite) composites.
sintering. The silicon infiltrated products B4 C-SiC-Si composites presented compact and homogenous microstructural features. The relative density of the B4 C-SiC-Si composites was about 98-99%. The density and relative density of the B4 C-SiC-Si composites showed a significant improvement in comparison to the pressureless sintered B4 C/C(graphite) composites.

The Fracture Strength and Fracture Toughness of the Pressureless Sintered B4 C/C(Graphite) Composites and the B4 C-SiC-Si Composites
The impact of the amount of graphite on the fracture strength of the pressureless sintered B4 C/C(graphite) composites and the B4 C-SiC-Si composites has been shown in Figure 12. A gradual decrease in the fracture strength of the pressureless sintered B4 C/C(graphite) composites was observed with the increase in the amount of graphite. While the fracture strength of the pressureless sintered B4 C monolith was quite high, the pressureless sintered B4 C/C(graphite) composites demonstrated a rather low fracture strength value. This is because the B4 C/C(graphite) composites were fabricated by a pressureless sintering process, which imparted a porous microstructure, a low relative density (62-75%), and porosity of around 25-38% leaving them with an extremely low fracture strength. The fracture strength of the pressureless sintered B4 C/C(graphite) composites lay within the 102-155 MPa range and reduced remarkably with the increase of graphite content.  Figure 12 depicts that the B4 C-SiC-Si composites demonstrated a remarkable improvement in their fracture strength as compared to the pressureless sintered B4 C/C(graphite) composites. The fracture strength of the B4 C-SiC-Si composites increased progressively with the rise in graphite content. It occurs because liquid silicon infiltrated into the interior of pressureless sintered B4 C/C(graphite) composites, and B4 C and graphite reacted with liquid silicon producing silicon carbide. The resultant silicon carbide and the residual silicon and boron carbide were sintered together fabricating B4 C-SiC-Si composites. The silicon carbide and residual silicon settled within the pores in the composite's matrix, thereby causing a remarkable decrease in the porosity of the silicon infiltrated specimens and a corresponding increase in the relative density of the silicon infiltrated specimens. The relative density of the B4 C-SiC-Si composites was found to be higher than 98%, the B4 C-SiC-Si composites thus became denser and more compact and their fracture strength was higher than that of the pressureless sintered B4 C/C(graphite) composites. B4 C-SiC-Si composites have a fracture strength equivalent to 385-438 MPa. In addition, the produced silicon carbide could also improve the fracture strength of the B4 C-SiC-Si composites, with the fracture strength increasing gradually with the increase of silicon carbide content. As an obvious fact, silicon carbide content The fracture toughness of the pressureless sintered B 4 C/C(graphite) composites and the B 4 C-SiC-Si composite is also affected by the amount of graphite as shown in Figure 13. As obvious from the figure, there was a gradual decline in the fracture toughness of the pressureless sintered B 4 C/C(graphite) composites with the increment in the amount of graphite. The fracture toughness of the pressureless sintered B 4 C/C(graphite) composites was rather low as compared to that of the pressureless sintered B 4 C monolith, presumably because the B 4 C/C(graphite) composites were produced via pressureless sintering and exhibited a porous microstructure. Additionally, the B 4 C/C(graphite) composites obtained via pressureless sintering had a rather low relative density and the porosity was 25-38%. It was also observed that the pressureless sintered B 4 C/C(graphite) composites demonstrated a remarkable decrease in their fracture toughness with the increase of graphite content. This is an outcome of the inherently low fracture toughness of graphitic materials. The observed fracture toughness of the pressureless sintered B 4 C/C(graphite) composites was about 1.568-2.35 MPa·m 1/2 .
It is also evident from Figure 13 that the fracture toughness of the B 4 C-SiC-Si composites underwent a marked improvement when compared with the pressureless sintered B 4 C/C(graphite) composites, demonstrating a gradual increase with the increase in the amount of graphite. This can be attributed to the infiltration of liquid silicon into the interior pressureless sintered B 4 C/C(graphite) composites and the subsequent reaction with B 4 C and graphite producing silicon carbide. Silicon carbide and residual silicon settled into the pores of the composite's matrix, thereby decreasing its porosity and increasing the relative density up to >98%, rendering the B 4 C-SiC-Si composites even more dense and compact. Consequently, the fracture toughness of the B 4 C-SiC-Si composites was recorded as 6.05-7.55 MPa·m 1/2 which is much higher in comparison to the fracture toughness of the pressureless sintered B 4 C/C(graphite) composites.
tering and exhibited a porous microstructure. Additionally, the B4 C/C(graphite) composites obtained via pressureless sintering had a rather low relative density and the porosity was 25-38%. It was also observed that the pressureless sintered B4 C/C(graphite) composites demonstrated a remarkable decrease in their fracture toughness with the increase of graphite content. This is an outcome of the inherently low fracture toughness of graphitic materials. The observed fracture toughness of the pressureless sintered B4 C/C(graphite) composites was about 1.568-2.35 MPa·m 1/2 . It is also evident from Figure 13 that the fracture toughness of the B4 C-SiC-Si composites underwent a marked improvement when compared with the pressureless sintered B4 C/C(graphite) composites, demonstrating a gradual increase with the increase in the amount of graphite. This can be attributed to the infiltration of liquid silicon into the interior pressureless sintered B4 C/C(graphite) composites and the subsequent reaction with B4 C and graphite producing silicon carbide. Silicon carbide and residual silicon settled into the pores of the composite's matrix, thereby decreasing its porosity and increasing the relative density up to >98%, rendering the B4 C-SiC-Si composites even more dense and compact. Consequently, the fracture toughness of the B4 C-SiC-Si composites was recorded as 6.05-7.55 MPa·m 1/2 which is much higher in comparison to the fracture toughness of the pressureless sintered B4 C/C(graphite) composites.
The silicon carbide produced following silicon infiltration is also assumed to improve the fracture toughness of the B4 C-SiC-Si composites, which underwent a progressive increase progressively with the increase of the silicon carbide content. The content of produced silicon carbide in turn gradually increased with the increase of the amount of graphite in the B4 C/C(graphite) composites. Additionally, the relative density of the B4 C-SiC-Si composites was extremely high, about 98-99%. Thus, the fracture toughness and fracture strength of the B4 C-SiC-Si composites were significantly enhanced as compared to the pressureless sintered B4 C/C(graphite) composites. While the pressureless sintered B4 The silicon carbide produced following silicon infiltration is also assumed to improve the fracture toughness of the B 4 C-SiC-Si composites, which underwent a progressive increase progressively with the increase of the silicon carbide content. The content of produced silicon carbide in turn gradually increased with the increase of the amount of graphite in the B 4 C/C(graphite) composites. Additionally, the relative density of the B 4 C-SiC-Si composites was extremely high, about 98-99%. Thus, the fracture toughness and fracture strength of the B 4 C-SiC-Si composites were significantly enhanced as compared to the pressureless sintered B 4 C/C(graphite) composites. While the pressureless sintered B 4 C/C(graphite) composites were soft and brittle and could be easily broken, the B 4 C-SiC-Si composites were compact and hard and exhibited extremely high fracture strength and high fracture toughness.

The Vickers Hardness and Elastic Modulus of the Pressureless Sintered B 4 C/C(Graphite) Composites and the B 4 C-SiC-Si Composites
The impact of the graphite content on the Vickers hardness of the pressureless sintered B 4 C/C(graphite) composites and the B 4 C-SiC-Si composites has been shown in Figure 14. The Vickers hardness of the pressureless sintered B 4 C/C(graphite) composites demonstrated a progressive decline with the increase of graphite content. While the Vickers hardness of the pressureless sintered B 4 C monolith was relatively high, its value was rather low for the pressureless sintered B 4 C/C(graphite) composites. Owing to their fabrication from the pressureless sintering process, the B 4 C/C(graphite) composites exhibited porous microstructures and have a low relative density (62-75%) and a porosity of about 25-38%. It has been known that the Vickers hardness of graphite is rather low. the laminated structured graphite materials have a softening effect on the ceramic composites. Consequently, the Vickers hardness of the pressureless sintered B 4 C/C(graphite) composites exhibited a gradual decrease with the increase in the amount of graphite. The pressureless sintered B 4 C/C(graphite) composites were soft and very brittle and could be easily broken. The Vickers hardness of the pressureless sintered B 4 C/C(graphite) composites was about 2.52-4.88 GPa. Figure 14 also shows that the Vickers hardness of the B 4 C-SiC-Si composites was markedly enhanced when compared to the pressureless sintered B 4 C/C(graphite) composites and increased progressively with the increase in the amount of graphite. This can be ascribed to the infiltration of liquid silicon into the interior pressureless sintered B 4 C/C(graphite) composites, and the subsequent reaction with B 4 C and graphite, producing silicon carbide. Silicon carbide and residual silicon settle into the pores of the composite's matrix, thereby decreasing its porosity and increasing the relative density up to >98%, rendering the B 4 C-SiC-Si composites even more dense and compact and subsequently increasing their Vickers's hardness. of about 25-38%. It has been known that the Vickers hardness of graphite is rather low. the laminated structured graphite materials have a softening effect on the ceramic composites. Consequently, the Vickers hardness of the pressureless sintered B4 C/C(graphite) composites exhibited a gradual decrease with the increase in the amount of graphite. The pressureless sintered B4 C/C(graphite) composites were soft and very brittle and could be easily broken. The Vickers hardness of the pressureless sintered B4 C/C(graphite) composites was about 2.52-4.88 GPa. Figure 14. The effects of graphite content on the Vickers hardness of the pressureless sintered B4 C/C(graphite) composites and the B4 C-SiC-Si composites. Figure 14 also shows that the Vickers hardness of the B4 C-SiC-Si composites was markedly enhanced when compared to the pressureless sintered B4 C/C(graphite) composites and increased progressively with the increase in the amount of graphite. This can be ascribed to the infiltration of liquid silicon into the interior pressureless sintered B4 C/C(graphite) composites, and the subsequent reaction with B4 C and graphite, producing silicon carbide. Silicon carbide and residual silicon settle into the pores of the composite's matrix, thereby decreasing its porosity and increasing the relative density up to >98%, rendering the B4 C-SiC-Si composites even more dense and compact and subsequently increasing their Vickers's hardness.
Besides, the Vickers hardness of boron carbide and silicon carbide is exceptionally high, 20 GPa to be specific. The specimen surface of the B4 C-SiC-Si composites primarily comprises boron carbide, silicon carbide, and residual silicon. Consequently, the Vickers hardness of the B4 C-SiC-Si composites following silicon infiltration is observed to be higher than pressureless sintered B4 C/C(graphite) composites. B4 C-SiC-Si composites demonstrated a Vickers hardness value as high as 22.885-26.768 GPa. Besides, the Vickers hardness of boron carbide and silicon carbide is exceptionally high, 20 GPa to be specific. The specimen surface of the B 4 C-SiC-Si composites primarily comprises boron carbide, silicon carbide, and residual silicon. Consequently, the Vickers hardness of the B 4 C-SiC-Si composites following silicon infiltration is observed to be higher than pressureless sintered B 4 C/C(graphite) composites. B 4 C-SiC-Si composites demonstrated a Vickers hardness value as high as 22.885-26.768 GPa.
Moreover, the Vickers hardness of the B 4 C-SiC-Si composites demonstrated a gradual increase with the increase in the amount of silicon carbide. The content of silicon carbide increased steadily with the increment in graphite content in the B 4 C/C(graphite) composites, thereby enhancing the Vickers hardness of the B 4 C-SiC-Si composites to a remarkable extent. A high value of the Vickers hardness, in turn, guarantees an enhancement in the resistance of the B 4 C-SiC-Si composites to wear. Thus, the B 4 C-SiC-Si composites exhibit an outstanding wear resistance. Figure 15 is a representation of the variation in elastic modulus of the pressureless sintered B 4 C/C(graphite) composites and the B 4 C-SiC-Si composites as a function of graphite content. The elastic modulus of the pressureless sintered B 4 C/C(graphite) composites reduced gradually with the increase in the amount of graphite. While the elastic modulus of the pressureless sintered B 4 C monolith was relatively high, that of the pressureless sintered B 4 C/C(graphite) composites was rather low. This is a consequence of the porous microstructure, low relative density (62-75%), and high porosity (25-38%) of the pressureless sintered B 4 C/C(graphite) composites. Moreover, the Vickers hardness of the B4 C-SiC-Si composites demonstrated a gradual increase with the increase in the amount of silicon carbide. The content of silicon carbide increased steadily with the increment in graphite content in the B4 C/C(graphite) composites, thereby enhancing the Vickers hardness of the B4 C-SiC-Si composites to a remarkable extent. A high value of the Vickers hardness, in turn, guarantees an enhancement in the resistance of the B4 C-SiC-Si composites to wear. Thus, the B4 C-SiC-Si composites exhibit an outstanding wear resistance. Figure 15 is a representation of the variation in elastic modulus of the pressureless sintered B4 C/C(graphite) composites and the B4 C-SiC-Si composites as a function of graphite content. The elastic modulus of the pressureless sintered B4 C/C(graphite) composites reduced gradually with the increase in the amount of graphite. While the elastic modulus of the pressureless sintered B4 C monolith was relatively high, that of the pressureless sintered B4 C/C(graphite) composites was rather low. This is a consequence of the porous microstructure, low relative density (62-75%), and high porosity (25-38%) of the pressureless sintered B4 C/C(graphite) composites. The elastic modulus of boron carbide is high, and the elastic modulus of graphite was rather low. This leads to the introduction of softness and brittleness into the pressureless sintered B4 C/C(graphite) composites. The pressureless sintered B4 C/C(graphite) composites are highly porous and exhibit a low elastic modulus, with the elastic modulus gradually declining with the increment in graphite content. The exact value of elastic modulus of the pressureless sintered B4 C/C(graphite) composites was about 106-145 GPa as illustrated The elastic modulus of boron carbide is high, and the elastic modulus of graphite was rather low. This leads to the introduction of softness and brittleness into the pressureless sintered B 4 C/C(graphite) composites. The pressureless sintered B 4 C/C(graphite) composites are highly porous and exhibit a low elastic modulus, with the elastic modulus gradually declining with the increment in graphite content. The exact value of elastic modulus of the pressureless sintered B 4 C/C(graphite) composites was about 106-145 GPa as illustrated in Figure 15.
When compared with the pressureless sintered B 4 C/C(graphite) composites, the B 4 C-SiC-Si composites demonstrate an improved value of the elastic modulus. There was a gradual increase in the elastic modulus of the B 4 C-SiC-Si composites with the increase in the amount of graphite in the B 4 C/C(graphite) composites as shown in Figure 15. This, again, is a consequence of liquid silicon seeping into the interior of pressureless sintered B 4 C/C(graphite) composites. Thus, the B 4 C and graphite undergo a chemical reaction with liquid silicon, generating silicon carbide. The silicon carbide thus generated takes up the porous spaces in the composite's matrix along with the residual silicon, decreasing its porosity and increasing the relative density. The B 4 C-SiC-Si composites became denser and more compact with a relative density as high as >98%. Due to their compact and homogenous microstructure, B 4 C-SiC-Si composites have an elastic modulus in the range of 385-410 GPa, which is considerably higher than the elastic modulus of the pressureless sintered B 4 C/C(graphite) composites.
Both boron carbide and silicon carbide have an exceptionally high value of elastic modulus. Thus, the silicon carbide generated following silicon infiltration could also contribute to improving the B 4 C-SiC-Si composites in terms of their elastic modulus. A gradual increase in the elastic modulus of the B 4 C-SiC-Si composites was evident with the increase of the content of silicon carbide, which increases linearly with the increase in graphite content. B 4 C-SiC-Si composites thus demonstrated an extremely high elastic modulus which was substantially enhanced as opposed to the pressureless sintered B 4 C/C(graphite) composites.

The Wear Resistance of the Pressureless Sintered B 4 C/C(Graphite) Composites and the B 4 C-SiC-Si Composites
The pressureless sintered B 4 C/C(graphite) composites and the B 4 C-SiC-Si composites were examined for their wear resistance with the help of wear experiments. The vertical coordinate was wear weight loss, whereas the wear time represents the horizontal coordinate. Figure 16a presents the influence of wear time on the weight loss of the pressureless sintered B 4 C/C(graphite) composites due to wear. The wear weight loss of the pressureless sintered B 4 C/C(graphite) composites manifested a progressive increase with the increment in wear time. Additionally, the extent of wear weight loss increased with the gradual rise in graphite content. The pressureless sintered B 4 C monolith gave a rather low wear weight loss curve owing to the relatively high inherent Vickers hardness of B 4 C ceramics. The composites with the graphite content of 10 wt%, 20 wt%, 30 wt%, and 40 wt% exhibited higher wear weight loss curves which may be attributed to the low value of Vickers hardness of the pressureless sintered B 4 C/C(graphite) composites. The weight loss curves in Figure 16a, illustrate that the highest weight loss existed for sintered B 4 C/C(40 wt% graphite) composites while the lowest was observed for the B 4 C monolith. The findings suggest that for the pressureless sintered B 4 C/C(graphite) composites, the wear resistance was very poor, and the wear weight loss was high. The wear resistance of the pressureless sintered B 4 C/C(graphite) composites reduced gradually with the increase in the amount of graphite presumably on account of the low Vickers hardness of the pressureless sintered B 4 C/C(graphite) composites. The wear weight loss curve was created using line fitness in Figure 16, and the slope of line fitness is the ratio of wear weight loss to wear time. The slope thus represents the wear loss rates. The following equation can be used to illustrate the relationship between wear weight decrease and wear time: R = M/t, where R represents wear loss rates, M represents wear weight loss and t represents wear duration. As a result, the slope of the wear weight loss curve was computed using the formulae R = M/t.
The wear resistance of pressureless sintered specimens and silicon infiltrated specimens were measured by friction wear experiments. A ratio of wear weight loss to wear time is referred to as the wear loss rate. Figure 17 presents the influence of graphite content on the wear loss rates of the pressureless sintered B4 C/C(graphite) composites and the B4 C-SiC-Si composites. It can be seen that the wear loss rates of the pressureless sintered B4 C/C(graphite) composites gradually increased with the increase in the amount of graphite. This indicated that the wear resistance of the pressureless sintered B4 C/C(graphite) composites underwent a gradual decrease with the increase in the amount of graphite. A possible explanation for this observation is that the B4 C/C(graphite) composites were fabricated via a pressureless sintering method and have a low relative density and a porosity ranging between 25-38%. The Vickers hardness of the pressureless sintered B4 C/C(graphite) composites reduced progressively with the increase in the amount of graphite. The surface hardness of the pressureless sintered B4 C/C(graphite) composites was also exceptionally low. All these factors contribute to the reduced wear resistance of the pressureless sintered B4 C/C(graphite) composites, which manifested a noticeable decline with the increase in the amount of graphite.
As regards the B4 C-SiC-Si composites, their wear loss rates underwent a gradual decrease with the rise in graphite content as depicted in Figure 17. The outcome revealed that the B4 C-SiC-Si composites specimen's weight demonstrated a steady decrease with the increase in the amount of graphite in the B4 C/C(graphite) composites. Figure 17 also revealed that the wear resistance of the B4 C-SiC-Si composites was substantially enhanced when compared to the pressureless sintered B4 C/C(graphite) composites, indicating that the silicon infiltrated specimen's weight underwent a gradual decrement with the increment in the amount of graphite in the B4 C/C(graphite) composites. The loss in weight observed for the silicon infiltrated specimens was exceptionally minimal. The B4 C-SiC-Si composites manifested remarkable resistance to wear. The underlying phenomenon, in this case, is the infiltration of liquid silicon into the interior of the composites and subsequent reaction with B4 C and graphite yielding silicon carbide. Silicon carbide and residual silicon ultimately occupy the pores in the composite's matrix, decreasing its porosity and increasing its relative density. B4 C-SiC-Si composites are thus The influence of wear time on the wear weight loss of the B 4 C-SiC-Si composites has been shown in Figure 16b. Contrary to the B 4 C/C(graphite) composites, the B 4 C-SiC-Si composites demonstrated an increase in wear weight loss with the increase in wear time. The wear weight loss of the B 4 C-SiC-Si composites manifested a gradual decrease with the increment in graphite content in the corresponding B 4 C/C(graphite) composites. The B 4 C-SiC-Si composites with the graphite content of 10 wt%, 20 wt%, 30 wt%, and 40 wt% in the B 4 C/C(graphite) composites gave lower wear weight loss curves presumably on account of the high Vickers hardness of the B 4 C-SiC-Si composites. As shown in Figure 16b, the highest weight loss existed for silicon infiltrated B 4 C monolith, whereas the lowest was observed for the silicon infiltrated B 4 C/C(40 wt%graphite) composites. When the wear weight loss was low, the wear resistance was very high. Thus, the wear resistance of the B 4 C-SiC-Si composites was quite high on account of their high Vickers hardness which is equivalent to 22.885-26.768 GPa.
From Figure 16a,b, it could also be deduced that the wear weight loss of the B 4 C-SiC-Si composites was relatively lower in comparison to the wear weight loss of the pressureless sintered B 4 C/C(graphite) composites. This implies that the wear resistance of the B 4 C-SiC-Si composites demonstrated a substantial improvement as compared to pressureless sintered B 4 C/C(graphite) composites which may be attributed to their high Vickers hardness.
The wear weight loss curve was created using line fitness in Figure 16, and the slope of line fitness is the ratio of wear weight loss to wear time. The slope thus represents the wear loss rates. The following equation can be used to illustrate the relationship between wear weight decrease and wear time: R = M/t, where R represents wear loss rates, M represents wear weight loss and t represents wear duration. As a result, the slope of the wear weight loss curve was computed using the formulae R = M/t.
The wear resistance of pressureless sintered specimens and silicon infiltrated specimens were measured by friction wear experiments. A ratio of wear weight loss to wear time is referred to as the wear loss rate. Figure 17 presents the influence of graphite content on the wear loss rates of the pressureless sintered B 4 C/C(graphite) composites and the B 4 C-SiC-Si composites. It can be seen that the wear loss rates of the pressureless sintered B 4 C/C(graphite) composites gradually increased with the increase in the amount of graphite. This indicated that the wear resistance of the pressureless sintered B 4 C/C(graphite) composites underwent a gradual decrease with the increase in the amount of graphite. A possible explanation for this observation is that the B 4 C/C(graphite) composites were fabricated via a pressureless sintering method and have a low relative density and a porosity ranging between 25-38%. The Vickers hardness of the pressureless sintered B 4 C/C(graphite) composites reduced progressively with the increase in the amount of graphite. The surface hardness of the pressureless sintered B 4 C/C(graphite) composites was also exceptionally low. All these factors contribute to the reduced wear resistance of the pressureless sintered B 4 C/C(graphite) composites, which manifested a noticeable decline with the increase in the amount of graphite. also extremely high. Silicon carbide and boron carbide along with the residual silicon existing on the silicon infiltrated specimen's surface also contribute towards the remarkable improvement in the resistance of the B4 C-SiC-Si composites towards wear. Figure 17. The influence of graphite content on the wear loss rates of the pressureless sintered B4 C/C(graphite) composites and the B4 C-SiC-Si composites.

Formation Mechanism of the B4 C-SiC-Si Composites Via Silicon Infiltration Process
During the silicon infiltration, the liquid silicon infiltrated into the interior of the composite's specimens, and subsequently reacted chemically with boron carbide producing silicon carbide according to the following chemical reactions : 5 Si + 3 B4 C = 3 SiC + 2 SiB6; 2 Si + B4 C = SiC + SiB4.
The liquid silicon combined with graphite to produce the silicon carbide according to the following chemical reactions : Si + C = SiC.
The B4 C-SiC-Si composites were primarily composed of boron carbide, resultant silicon carbide, and only a little content of silicon; the net content of boron carbide and silicon carbide was >90%, whereas the residual silicon was found to be <10%.

Conclusions
The B4 C/C(graphite) composites were produced via a pressureless sintering process. At an elevated temperature, liquid silicon infiltration was conducted for improving the mechanical property of the pressureless sintered B4 C/C(graphite) composites. Since the porosity of pressureless sintered B4 C/C(graphite) composites was about 25-38%, the liquid silicon could infiltrate into the interior of B4 C/C(graphite) composites, reacting with B4 C and graphite to produce silicon carbide. The boron carbide, silicon carbide, and residual silicon are then sintered together giving rise to B4 C-SiC-Si composites. This way the pressureless sintered B4 C/C(graphite) composites transformed into the B4 C-SiC-Si composites following silicon infiltration.
This work is based on an investigation of the microstructure, phase composition, and mechanical properties of the B4 C/C(graphite) composites obtained via a pressureless sintering process. The XRD data demonstrated that the pressureless sintered bulks were primarily made up of the B4 C phase and graphite phase. Besides, they have a porous microstructure, with the porosity lying in the range of 25-38%. The relative density of the pressureless sintered B4 C/C(graphite) composites was about 62-75% and their mechanical property underwent a gradual decline with the increase in the amount of graphite. The density, relative density, fracture toughness, fracture strength, Vickers hardness, As regards the B 4 C-SiC-Si composites, their wear loss rates underwent a gradual decrease with the rise in graphite content as depicted in Figure 17. The outcome revealed that the B 4 C-SiC-Si composites specimen's weight demonstrated a steady decrease with the increase in the amount of graphite in the B 4 C/C(graphite) composites. Figure 17 also revealed that the wear resistance of the B 4 C-SiC-Si composites was substantially enhanced when compared to the pressureless sintered B 4 C/C(graphite) composites, indicating that the silicon infiltrated specimen's weight underwent a gradual decrement with the increment in the amount of graphite in the B 4 C/C(graphite) composites. The loss in weight observed for the silicon infiltrated specimens was exceptionally minimal. The B 4 C-SiC-Si composites manifested remarkable resistance to wear. The underlying phenomenon, in this case, is the infiltration of liquid silicon into the interior of the composites and subsequent reaction with B 4 C and graphite yielding silicon carbide. Silicon carbide and residual silicon ultimately occupy the pores in the composite's matrix, decreasing its porosity and increasing its relative density. B 4 C-SiC-Si composites are thus rendered denser and more compact and their surface hardness and Vickers hardness were exceptionally high; hence, the wear resistance of the B 4 C-SiC-Si composites was also extremely high. Silicon carbide and boron carbide along with the residual silicon existing on the silicon infiltrated specimen's surface also contribute towards the remarkable improvement in the resistance of the B 4 C-SiC-Si composites towards wear.

Formation Mechanism of the B 4 C-SiC-Si Composites via Silicon Infiltration Process
During the silicon infiltration, the liquid silicon infiltrated into the interior of the composite's specimens, and subsequently reacted chemically with boron carbide producing silicon carbide according to the following chemical reactions  The liquid silicon combined with graphite to produce the silicon carbide according to the following chemical reactions : Si + C = SiC. The B 4 C-SiC-Si composites were primarily composed of boron carbide, resultant silicon carbide, and only a little content of silicon; the net content of boron carbide and silicon carbide was >90%, whereas the residual silicon was found to be <10%.

Conclusions
The B 4 C/C(graphite) composites were produced via a pressureless sintering process. At an elevated temperature, liquid silicon infiltration was conducted for improving the mechanical property of the pressureless sintered B 4 C/C(graphite) composites. Since the porosity of pressureless sintered B 4 C/C(graphite) composites was about 25-38%, the liquid silicon could infiltrate into the interior of B 4 C/C(graphite) composites, reacting with B 4 C and graphite to produce silicon carbide. The boron carbide, silicon carbide, and residual silicon are then sintered together giving rise to B 4 C-SiC-Si composites. This way the pressureless sintered B 4 C/C(graphite) composites transformed into the B 4 C-SiC-Si composites following silicon infiltration.
This work is based on an investigation of the microstructure, phase composition, and mechanical properties of the B 4 C/C(graphite) composites obtained via a pressureless sintering process. The XRD data demonstrated that the pressureless sintered bulks were primarily made up of the B 4 C phase and graphite phase. Besides, they have a porous microstructure, with the porosity lying in the range of 25-38%. The relative density of the pressureless sintered B 4 C/C(graphite) composites was about 62-75% and their mechanical property underwent a gradual decline with the increase in the amount of graphite. The density, relative density, fracture toughness, fracture strength, Vickers hardness, elastic modulus, and wear resistance of the pressureless sintered B 4 C/C(graphite) composites also manifested a progressive decrease with the increase in the amount of graphite.
Upon silicon infiltration, the pressureless sintered B 4 C/C(graphite) composites reacted with liquid silicon and transformed into B 4 C-SiC-Si composites. The microstructure, phase composition, and mechanical characteristics of the B 4 C-SiC-Si composites fabricated via the silicon infiltration process were then thoroughly investigated. XRD data suggest the existence of a B 4 C phase, a SiC phase, and a residual Si phase on the silicon infiltrated specimen's surface. The silicon infiltrated products, i.e., B 4 C-SiC-Si composites exhibited a homogenous and compact microstructure, an extremely high relative density, and remarkably low porosity. The microstructure investigation results showed that the silicon infiltrated specimens were primarily made up of boron carbide, silicon carbide, and residual silicon. There was a substantial improvement in the B 4 C-SiC-Si composites in terms of their mechanical property when compared with the pressureless sintered B 4 C/C(graphite) composites. The density and relative density, fracture strength and fracture toughness, elastic modulus, Vickers hardness, and wear resistance of the B 4 C-SiC-Si composites were also significantly improved and exhibited a gradual increase with the increase in the amount of graphite in the composites. It can thus be concluded that the silicon infiltration process efficiently improves the mechanical property and wear resistance of the pressureless sintered B 4 C/C(graphite) composites, and the B 4 C-SiC-Si composites thus generated can find potentially useful applications due to their superior physical attributes.

Conflicts of Interest:
The authors declare no conflict of interest.