E ﬀ ect of SiC on Microstructure, Phase Evolution, and Mechanical Properties of Spark-Plasma-Sintered High-Entropy Ceramic Composite

: Ultra-high temperature ceramic composites have been widely investigated due to their improved sinterability and superior mechanical properties compared to monolithic ceramics. In this work, high-entropy boron-carbide ceramic / SiC composites with di ﬀ erent SiC content were synthesized from multicomponent carbides HfC, Mo 2 C, TaC, TiC, B 4 C, and SiC in spark plasma sintering (SPS) from 1600 ◦ C to 2000 ◦ C. It was found that the SiC addition tailors the phase formation and mechanical properties of the high-entropy ceramic (HEC) composites. The microhardness and fracture toughness of the HEC composites sintered at 2000 ◦ C were improved from 20.3 GPa and 3.14 MPa · m 1 / 2 to 26.9 GPa and 5.95 MPa · m 1 / 2 , with increasing SiC content from HEC-(SiC)0 (0 vol. %) to HEC-(SiC)3.0 (37 vol. %). The addition of SiC (37 vol. %) to the carbide precursors resulted in the formation of two high-entropy ceramic phases with two di ﬀ erent crystal structures, face-centered cubic (FCC) structure, and hexagonal structure. The volume fraction ratio between the hexagonal and FCC high-entropy phases increased from 0.36 to 0.76 when SiC volume fraction was increased in the composites from HEC-(SiC)0 to HEC-(SiC)3.0, suggesting the stabilization of the hexagonal high-entropy phase over the FCC phase with SiC addition. phase composition, and mechanical properties of the HECs composite on SiC content were investigated. The sintered HEC–(SiC)x composite showed promoted densiﬁcation compared to HECs without SiC addition. With increasing the SiC content from HEC-(SiC)0 (0 vol. %) to HEC-(SiC)3.0 (37 vol. %), the HECs composites showed remarkable enhancement of the mechanical properties, including higher Vickers hardness from 20.3 to 26.9 GPa, under the load of 1000 g, and improved fracture toughness from 3.14 to 5.95 MPa · m 1 / 2 , attributed to the crack bridging and deﬂecting mechanism. The investigation on the phase composition in the HEC-(SiC)x composite showed that four di ﬀ erent high-entropy phases could be formed, including three cubic crystal-structured phases and one hexagonal phase. However, only two high-entropy phases were retained when the SiC content was increased to 37 vol. % (HEC-(SiC)3.0). The two retained high-entropy phases contained all metal elements, with the face-centered cubic (FCC) structure having relatively higher content of Hf, Ta, and C, and the hexagonal crystal structure containing boron and a relatively higher amount of Mo and Ti,. Composites with higher SiC content showed an increased volume formation of the hexagonal-structured phase, suggesting a favored formation of hexagonal-structured high-entropy phase due to SiC addition and the formation of free carbon.


Introduction
High-entropy ceramics (HECs) that are manufactured from multiple ceramic precursors, such as transition metal carbides and borides, have shown great potential to replace conventional ceramic in certain application fields due to their superior properties including excellent mechanical performance, oxidation resistance, and thermal-electrical properties [1][2][3]. Inherited from the high-entropy alloys (HEAs) [4], HECs can reveal simple solid solution crystal structures attributed to the high-entropy structure stabilization [5]. In the field of refractory HECs, high-entropy carbides and high-entropy borides synthesized from group IV, V, and VI Metal-C and Metal-B ceramics are the most studied systems to date. Gild et al. [3] and Castle et al. [2] reported the first high-entropy diborides in 2016 and high-entropy carbide in 2018, respectively. The synthesized HECs showed simple solid solution structure with the same crystal structure as the constituent ceramic compounds, i.e., face-centered cubic (FCC) B1 structure for the HE-carbides and hexagonal AlB 2 structure for the HE-diborides. In the stated crystal structures, the principal metal atoms occupy the cationic sites randomly while the non-metals occupy the anionic sites. Both of the HE-carbides and HE-borides showed enhancement of mechanical properties as compared to monolithic ceramic compounds [2,3]. Since then, the research on refractory HECs was intensively stimulated, and superior mechanical properties including enhanced fracture toughness [6,7] and elastic modulus [8,9] were discovered. Apart from the improvement of mechanical containing both carbon and boron, as the new class of UHTCs have superior mechanical properties than conventional UHTCs. The incorporation of SiC with HECs is aiming at achieving further improvement of the mechanical performance of UHTCs composites. SiC-reinforced high-entropy carbides have been reported to show remarkable oxidation resistance by Wang et al. [35] and mechanical properties by Lu et al. [36]. Our previous studies on HECs-SiC composite showed that a SiC-reinforced high-entropy B 4 (HfMo 2 TaTi)C/SiC ceramic composite reveals superior mechanical properties and remarkable oxidation resistance under different temperature conditions. In addition, SiC was found to play an essential role in the phase formation during the sintering of high-entropy boron-carbide/SiC ceramic composite [17]. In this study, high-entropy boron-carbide ceramic composites with different SiC content (from 0 to 37 vol. %) were synthesized via spark plasma sintering (SPS). The densification process was investigated by comparing the relative densities of the composites sintered at different temperatures. The sintered high-entropy boron-carbide/SiC composites, HEC-(SiC)x, exhibited improved densification and superior hardness and fracture toughness. The composition, microstructure, and phase evolution were investigated using electron microscopy, energy dispersive spectroscopy (EDS), and X-ray diffraction analysis.
The sintering of the powders was conducted in SPS-530ET (Dr. Sinter Spark Plasma Sintering System, Fuji electronic industrial Co., Ltd., Tsurugashima, Japan) installed in a glovebox. The powders were filled into a graphite die with a diameter of 8 mm and sintered at 1600 • C, 1800 • C, and 2000 • C with a heating rate of 200 • C/min and a holding time of 10 min. The whole sintering process was conducted under vacuum with a constant uniaxial pressure of 60 MPa being applied to the graphite die. All the specimens were free-cooled to room temperature in an ultra-high vacuum. The sintered HEC compositions are denoted as HEC-(SiC)0, HEC-(SiC)0.5, HEC-(SiC)1.0, HEC-(SiC)2.0, and HEC-(SiC)3.0 in the discussion section.

Characterization
The sintered composites were polished using SiC grinding papers up to 600 grit for the measurement of densities. The density of the sintered specimens was measured using the Archimedes water immersion method. The theoretical densities of the HEC composites was calculated using rule of mixtures. The sintered specimens were further polished following the standard metallurgical sample preparation procedure, using SiC grinding paper and a diamond polishing agent for a microstructural study. The microstructural investigation and elemental composition analysis were conducted on the Scanning Electron Microscope JSM-IT300 (JEOL, Tokyo, Japan) equipped with an energy dispersive spectrometer (EDS), calibrated with Cobalt. Based on the contrast of each phase in the micrographs recorded using the backscattered electrons, the volume fraction of each phase was evaluated by applying image thresholding in the image analysis software ImageJ. For each specimen, five images at the same magnification (×250) were recorded from different locations for image analysis. X-ray diffraction (XRD) was conducted using an X-ray diffractometer (Empyrean, PANalytical, Malvern, UK) with Cu-Kα radiation (wavelength 0.154 nm). The XRD data was investigated using the software PANalytical X'Pert HighscorePlus with the PDF−4 + database.
The microhardness of the sintered composites was measured using the Matsuzawa MXT-CX microhardness tester (Matsuzawa Co., Japan) at different loading conditions, 200 g, 300 g, 500 g, and 1000 g. For each indentation load, at least eleven indentations were performed to obtain the average Vickers hardness. The indentation fracture toughness of the composites was measured at the indentation load of 1000 g. The indentation cracks on the sample surface were observed and measured in JSM-IT300 (JEOL, Tokyo, Japan). The fracture toughness K IC was calculated by using the formula proposed by Evans and Charles [37,38]: where c is the length of the cracks, a is the half length of the diagonal of the Vickers indentation, and H is the Vickers hardness. All the characterizations were performed at room temperature.

SiC Effect on Densification and Mechanical Properties
The relative densities as a function of the sintering temperature in Figure 1a show that the relative density of HEC-(SiC)x composites increases with the increase in the SPS sintering temperature. Moreover, the HEC-(SiC)x composite reveals higher densification with increasing SiC content at 1600 • C, with a relative density increasing from 72.5% for HEC-(SiC)0 to 81% for the HEC-(SiC)3.0 composite. As the sintering temperature increases to 1800 • C, the composites containing SiC whiskers show a relative density above 97%, while the HEC-(SiC)0 composite without SiC whiskers shows a comparatively lower relative density of 92%. The composites sintered at 2000 • C are densified to near theoretical density. For each of the HEC-(SiC)x composites, the increase of sintering temperature resulted in higher relative density due to the promoted diffusive mass-transport at higher sintering temperature [39].
In general, the addition of a constituent in form of a whisker to a powder body can hinder the densification of a ceramic powder body during sintering [40]. Therefore, liquid phase sintering [41,42] or pressure-assisted sintering is required to achieve a dense whisker-reinforced ceramic composite [43]. In our case, the sintering at 1800 • C and 2000 • C at 60 MPa resulted in relative densities above 99% of theoretical density, suggesting that the pressure assisted sintering resulted in enhanced densification of HEC-(SiC)x composites containing SiC in form of whiskers. Moreover, the experimentally observed higher densification of the HEC-(SiC)x composites with the increase in the volume fraction of SiC during pressure-assisted sintering, Figure 1a, is likely to be the result of the lower melting point of the SiC (2730 • C) compared to other UHTC constituents such as TaC (3880 • C), HfC (3900 • C), and TiC (3160 • C). The increase in the volume fraction of a low-melting-point component SiC in the HEC-(SiC)x composites, which has relatively higher plasticity and lower resistance to grain boundary sliding at high temperatures [44], can effectively enhance the densification. Lu et al. [36] has reported the promoted densification of high-entropy carbide (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2 )C by adding 20 vol. % SiC. A similar promotion of the densification of other UHTCs by adding SiC has also been reported, such as the relative density of hot pressed TiB 2 -SiC composites could increase from 4.33 to 4.39 g/cm 3 when the SiC w content was increased from 0 to 15 vol. % [45]. Shahedi Asl et al. reported that the relative density of the TiC-SiC composite increased from 99.23% to >100% when the SiC w content was increased from 0 to 30 vol. %, sintered by SPS [46].
(SiC)3.0 composite. As the sintering temperature increases to 1800 °C, the composites containing SiC whiskers show a relative density above 97%, while the HEC-(SiC)0 composite without SiC whiskers shows a comparatively lower relative density of 92%. The composites sintered at 2000 °C are densified to near theoretical density. For each of the HEC-(SiC)x composites, the increase of sintering temperature resulted in higher relative density due to the promoted diffusive mass-transport at higher sintering temperature [39]. In general, the addition of a constituent in form of a whisker to a powder body can hinder the densification of a ceramic powder body during sintering [40]. Therefore, liquid phase sintering [41,42] Figure 1c) varies with the applied load, especially for the composite with lower relative density (HEC-(SiC)0), because of the residual porosity. The enhancement of the hardness with increasing SiC content was observed in both 1800 and 2000 • C composites, possibly due to the hindered grain growth of HECs phases, which can consequently increase the composite hardness by a grain boundary strengthening effect [47]. By comparing Figure 1c,d, it is evident that the hardness of the HEC-(SiC)x composites increases with higher sintering temperature, which can be correlated to the higher densification, i.e., reduction of residual porosity in the composites sintered at 2000 • C compared to 1800 • C. The fracture toughness of HEC-(SiC)x composites sintered at 2000 • C were calculated by analyzing the indentation cracks on the sample surface. Shown in Figure 1b, the fracture toughness of the HECs composites increased 89%, from 3.14 MPa·m 1/2 for HEC-(SiC)0 to 5.95 MPa·m 1/2 for HEC-(SiC)3.0, which is comparable to the 20 vol. % SiC-reinforced high-entropy carbide (5.24 MPa·m 1/2 ) as reported by Lu et al. [36]. The correlated crack morphologies of HEC-(SiC)0, 1.0, and 3.0 are shown in Figure 2. It can be seen that the indentation cracks in the HEC-(SiC)0 composite are propagating through the bright phase (transgranular) and along the boundary of the gray phase (intergranular), producing a continuing straight crack path on the sample surface. With the SiC addition to the composite, SiC grains interact with the crack propagation and enhance fracture toughness by toughening mechanisms such as crack bridging shown in the HEC-(SiC)1.0 composite and crack deflection shown in the HEC-(SiC)3.0 composite (marked with white circles) in Figure 2.  Figure 3 shows the backscattered electron (BSE) micrographs of the sintered HECs with different SiC content from x = 0 to 3.0. Each contrast in the microstructure refers to a region with a constant density due to the high sensitivity of the BSE detector to the atomic number, which is correlated directly to the material density [49]. The microstructures of HECs containing SiC (x = 0.5, 1.0, 2.0, and 3.0) consist of three distinct phases-bright, gray, and dark (marked as 1, 2, and 3 in the microstructure of HEC-(SiC)0.5 in Figure 3). Whereas the microstructure of HEC without addition of SiC (HEC-(SiC)0) show four different contrasts (marked as 1 to 4 in HEC-(SiC)0). According to energy-dispersive X-ray spectroscopy (EDS) analysis, the darkest region in all of the four HEC-(SiC)x compositions (x = 0.5, 1.0, 2.0, and 3.0) refers to SiC and a trace amount of porosity present at the interface between the SiC phase and the adjacent phase, which was created during the grinding and polishing process. The SiC phase appears in different irregular shapes with lower aspect ratio than the original fiber-shaped whiskers, which could be a result of the morphology transformation of SiCw under the effect of elevated temperature. A similar morphology change of SiCw from a fiber-to particle-shaped structure at high temperatures has been observed by Cheng et al. [50] and Zhou et al. [51]. The volume fraction of the phases with a certain contrast are analyzed using the image analysis. In the bar chart in Figure 3, each individual column refers to the 100 vol. % of the corresponding specimen with the contribution of each phase to it, demonstrated by the length of the sub-columns marked with different grayscales. For the HEC-(SiC)0 composite that has four different contrasts, the two brightest phases are summarized in the bright sub-column. From x = 0 to 3.0 in the HEC-(SiC)x system, regardless of the increase in the amount of SiC, the ratio of the volume fraction between the gray and the bright phase rises from 0.36 to 0.76. As the sintering parameters for all these five samples are the same, the experimental results suggest that the addition of SiC facilitated the formation of the gray phase over the formation of the bright phase.  Figure 3 shows the backscattered electron (BSE) micrographs of the sintered HECs with different SiC content from x = 0 to 3.0. Each contrast in the microstructure refers to a region with a constant density due to the high sensitivity of the BSE detector to the atomic number, which is correlated directly to the material density [49]. The microstructures of HECs containing SiC (x = 0.5, 1.0, 2.0, and 3.0) consist of three distinct phases-bright, gray, and dark (marked as 1, 2, and 3 in the microstructure of HEC-(SiC)0.5 in Figure 3). Whereas the microstructure of HEC without addition of SiC (HEC-(SiC)0) show four different contrasts (marked as 1 to 4 in HEC-(SiC)0). According to energy-dispersive X-ray spectroscopy (EDS) analysis, the darkest region in all of the four HEC-(SiC)x compositions (x = 0.5, 1.0, 2.0, and 3.0) refers to SiC and a trace amount of porosity present at the interface between the SiC phase and the adjacent phase, which was created during the grinding and polishing process. The SiC phase appears in different irregular shapes with lower aspect ratio than the original fiber-shaped whiskers, which could be a result of the morphology transformation of SiC w under the effect of elevated temperature. A similar morphology change of SiC w from a fiber-to particle-shaped structure at high temperatures has been observed by Cheng et al. [50] and Zhou et al. [51]. The volume fraction of the phases with a certain contrast are analyzed using the image analysis. In the bar chart in Figure 3, each individual column refers to the 100 vol. % of the corresponding specimen with the contribution of each phase to it, demonstrated by the length of the sub-columns marked with different grayscales. For the HEC-(SiC)0 composite that has four different contrasts, the two brightest phases are summarized in the bright sub-column. From x = 0 to 3.0 in the HEC-(SiC)x system, regardless of the increase in the amount of SiC, the ratio of the volume fraction between the gray and the bright phase rises from 0.36 to 0.76. As the sintering parameters for all these five samples are the same, the experimental results suggest that the addition of SiC facilitated the formation of the gray phase over the formation of the bright phase.

SiC effect on Phase Fraction
corresponding specimen with the contribution of each phase to it, demonstrated by the length of the sub-columns marked with different grayscales. For the HEC-(SiC)0 composite that has four different contrasts, the two brightest phases are summarized in the bright sub-column. From x = 0 to 3.0 in the HEC-(SiC)x system, regardless of the increase in the amount of SiC, the ratio of the volume fraction between the gray and the bright phase rises from 0.36 to 0.76. As the sintering parameters for all these five samples are the same, the experimental results suggest that the addition of SiC facilitated the formation of the gray phase over the formation of the bright phase. According to the EDS results, both the gray and bright regions observed in all five composites contain four constituent metal elements-Hf, Mo, Ta, and Ti-however, with different atomic According to the EDS results, both the gray and bright regions observed in all five composites contain four constituent metal elements-Hf, Mo, Ta, and Ti-however, with different atomic compositions. For SiC-containing composites where only two visible contrasts, except for SiC, were observed, carbon was detected in both of the phases while boron was only observed in the gray phase. EDS spectra collected from these gray and bright phases are similar among the HEC-(SiC)x composite, as shown in Figure 4, which were obtained from HEC-(SiC)1.0. It can be seen from the EDS spectra that the boron X-ray emission peak is absent in the bright phase yet displayed beside the carbon peak in the gray phase. The quantitative analysis shows that the gray phase has higher abundance of Mo and Ti than the bright phase, displaying significantly higher intensities of the correlated EDS peaks of Mo and Ti in the spectrum. The higher content of Mo and Ti in the gray phase can be confirmed in the EDS elemental maps in Figure 4. These results are in agreement with the darker contrast of the gray phase than the bright in the BSE microstructure, due to smaller atomic numbers of Mo and Ti compared to Hf and Ta. For the composition without SiC addition, HEC-(SiC)0, the spectra collected from the two brightest regions are similar with similar quantitative atomic compositions. The HECs synthesized at high temperatures could be meta-stable at room temperature, the microstructure and composition heterogeneity was possibly caused by spinodal decomposition during the cooling [15]. A similar decomposition phenomenon due to a different cooling rate throughout the specimen was observed in the high-entropy carbides by Castle et al. [2].
Ceramics 2020, 3 FOR PEER REVIEW 7 compositions. For SiC-containing composites where only two visible contrasts, except for SiC, were observed, carbon was detected in both of the phases while boron was only observed in the gray phase. EDS spectra collected from these gray and bright phases are similar among the HEC-(SiC)x composite, as shown in Figure 4, which were obtained from HEC-(SiC)1.0. It can be seen from the EDS spectra that the boron X-ray emission peak is absent in the bright phase yet displayed beside the carbon peak in the gray phase. The quantitative analysis shows that the gray phase has higher abundance of Mo and Ti than the bright phase, displaying significantly higher intensities of the correlated EDS peaks of Mo and Ti in the spectrum. The higher content of Mo and Ti in the gray phase can be confirmed in the EDS elemental maps in Figure 4. These results are in agreement with the darker contrast of the gray phase than the bright in the BSE microstructure, due to smaller atomic numbers of Mo and Ti compared to Hf and Ta. For the composition without SiC addition, HEC-(SiC)0, the spectra collected from the two brightest regions are similar with similar quantitative atomic compositions. The HECs synthesized at high temperatures could be meta-stable at room temperature, the microstructure and composition heterogeneity was possibly caused by spinodal decomposition during the cooling [15]. A similar decomposition phenomenon due to a different cooling rate throughout the specimen was observed in the high-entropy carbides by Castle et al. [2].  Figure 5 shows the X-ray diffraction (XRD) patterns of the HEC ceramic composites sintered at 2000 °C. It can be seen from Figure 5(a) that all HEC-(SiC)x composites contain an FCC phase (space group 225) and a hexagonal phase (space group 191). Due to the low diffraction intensities of several peaks, the XRD diffractograms were plotted in a logarithmical scale in Figure 5(b) for clearer observation of the minor phases. A SiC diffraction peak at 2θ = 35.5° was observed in HEC-(SiC)x (x = 0.5, 1, 2, 3) composites. The XRD patterns show that, except for the major FCC and hexagonal phase  Figure 5 shows the X-ray diffraction (XRD) patterns of the HEC ceramic composites sintered at 2000 • C. It can be seen from Figure 5a that all HEC-(SiC)x composites contain an FCC phase (space group 225) and a hexagonal phase (space group 191). Due to the low diffraction intensities of several peaks, the XRD diffractograms were plotted in a logarithmical scale in Figure 5b for clearer observation of the minor phases. A SiC diffraction peak at 2θ = 35.5 • was observed in HEC-(SiC)x (x = 0.5, 1, 2, 3) composites. The XRD patterns show that, except for the major FCC and hexagonal phase (marked as hexagonal (191)), the HEC-(SiC)0 composite contains one minor hexagonal phase (space group 189), one cubic phase (primitive cubic), and trace amounts of a tetrahedral phase. By increasing the SiC content in the HEC, major FCC and hexagonal (191) phases are retained, while the diffraction intensities of the tetrahedral, the hexagonal (189), and the primitive cubic phase gradually reduce and eventually become absent in the HEC-(SiC)3.0 composite. According to the discussion of the microstructures in Figure 3, the BSE micrographs of HEC-(SiC)x (x = 0.5, 1.0, 2.0, 3.0) composites show only two distinct contrasts along with SiC, while the XRD results reveal more than two phases for HEC-(SiC)x (x = 0.5, 1.0, 2.0). This could be due to the small amount of the tetrahedral phase to be observed in SEM, and secondly, the similar elemental compositions between the major FCC phase and the primitive cubic phase and between the two hexagonal phases, which made them indistinguishable based on the contrast difference in the BSE image. The hexagonal (191) phase shows lattice parameters of a = 0.3051 nm and c = 0.3237 nm, while the FCC phase in all composites shows a lattice constant of approximately 0.447 nm, which is close to the lattice parameter of the constituent metal carbides TaC (0.446 nm, ICDD 03-065-8792, 50.3 at% C). The phase formation mechanism discussed in our previous work on high-entropy boron-carbides [16,17] suggested that TaC was the host FCC lattice in the current carbide system so that other atoms migrate in and occupy the corresponding cationic and anionic sites in the TaC rocksalt structure, due to the lowest metal vacancy formation energy of TaC among the FCC carbides.
Ceramics 2020, 3 FOR PEER REVIEW 8 discussed in our previous work on high-entropy boron-carbides [16,17] suggested that TaC was the host FCC lattice in the current carbide system so that other atoms migrate in and occupy the corresponding cationic and anionic sites in the TaC rocksalt structure, due to the lowest metal vacancy formation energy of TaC among the FCC carbides.  Figure 6 shows the comparison of the XRD patterns of the as-mixed precursors and the HEC-(SiC)x composites sintered at different temperatures. It can be seen that phase transformation has not taken place at 1600 °C. The FCC peaks start to converge at 1800 °C, indicating the initial formation of the FCC-structured high-entropy ceramic phase. The strongest diffraction peak of the hexagonal (191) structure at 2θ = 44° (marked with the red arrow) started to be visible at 1800 °C, suggesting the initial formation of hexagonal phase at 1800 °C. The composition of HEC-(SiC)x composites at 2000 °C is similar to what was achieved in our previous study on the B4(HfMo2TaTi)C/SiC composite, however at lower temperature (1800 °C) [17], attributed to a higher heating rate applied in this study (200 °C/min compare to 100 °C/min). Similar dependence of the sintering temperature on the heating rate in SPS has been reported [52,53].  Figure 6 shows the comparison of the XRD patterns of the as-mixed precursors and the HEC-(SiC)x composites sintered at different temperatures. It can be seen that phase transformation has not taken place at 1600 • C. The FCC peaks start to converge at 1800 • C, indicating the initial formation of the FCC-structured high-entropy ceramic phase. The strongest diffraction peak of the hexagonal (191) structure at 2θ = 44 • (marked with the red arrow) started to be visible at 1800 • C, suggesting the initial formation of hexagonal phase at 1800 • C. The composition of HEC-(SiC)x composites at 2000 • C is similar to what was achieved in our previous study on the B 4 (HfMo 2 TaTi)C/SiC composite, however at lower temperature (1800 • C) [17], attributed to a higher heating rate applied in this study (200 • C/min compare to 100 • C/min). Similar dependence of the sintering temperature on the heating rate in SPS has been reported [52,53]. Figure 6 shows the comparison of the XRD patterns of the as-mixed precursors and the HEC-(SiC)x composites sintered at different temperatures. It can be seen that phase transformation has not taken place at 1600 °C. The FCC peaks start to converge at 1800 °C, indicating the initial formation of the FCC-structured high-entropy ceramic phase. The strongest diffraction peak of the hexagonal (191) structure at 2θ = 44° (marked with the red arrow) started to be visible at 1800 °C, suggesting the initial formation of hexagonal phase at 1800 °C. The composition of HEC-(SiC)x composites at 2000 °C is similar to what was achieved in our previous study on the B4(HfMo2TaTi)C/SiC composite, however at lower temperature (1800 °C) [17], attributed to a higher heating rate applied in this study (200 °C/min compare to 100 °C/min). Similar dependence of the sintering temperature on the heating rate in SPS has been reported [52,53].  In a multiphase material, the diffracted intensity in XRD is determined by several factors, including the scattering factor of the elements, the content of the phase in the mixture, and the nature of the correlated crystal structure [54]. In principle, high volume fraction and high scattering factor of the constituent elements lead to high-diffracted intensity in the XRD diffractogram. According to the microstructure and EDS elemental analysis on HEC-(SiC)3.0, the bright phase dominates over the gray in volume fraction and shows a higher content of heavy elements such as Hf and Ta, which have greater scattering factors. Moreover, our work on the same high-entropy boron-carbide system has discovered that the high-entropy solid solution with the presence of boron showed hexagonal structure [17] as the gray phase in this study. It is therefore reasonable to conclude that the bright phase corresponds to the FCC and the gray phase containing a higher content of B has hexagonal crystal structure. Castle et al. [2] have reported that the incorporation of smaller atoms like Ti into TaC leads to the reduction of the lattices parameter. Hence, it is expected that during the phase transformation, higher Ti/Mo to Ta content in the gray phase (shown in Figure 4) can cause severe lattice distortion and may lead to the crystal structure change from a FCC to a hexagonal structure. On the other hand, the formation of a FCC-structured carbon-rich phase and a hexagonal-structured boron-rich phase with different elemental compositions can be explained by the formation energy difference of the corresponding metal carbides (MC) and diborides (MB 2 ) calculated by the density functional theory [55]. The formation energy difference E f (MB 2 ) − E f (MC) for Mo and Ti is more negative than that of Hf and Ta [56]. Therefore, Mo and Ti are more preferable to be dissolved in the hexagonal boride structure, which is in agreement with the EDS results shown in Figure 4 where the gray/hexagonal phase contains a higher content of Mo and Ti.
Thus, the effect on phase composition in the composites by increasing SiC content should be referred to the stabilization of a hexagonal-structured high-entropy phase (the gray region in the microstructure). However, it is interesting that the experimental results showed strong evidence of SiC facilitating the formation of the hexagonal phase but not being involved in the formation of high-entropy phases per se. Considering that the hexagonal and the FCC high-entropy phase is rich in boron and carbon, respectively, the favored formation of the boron-rich hexagon phase with increasing SiC content in the HEC-(SiC)x composites could lead to the formation of free carbon in the composite, which is related to the diffraction peak of graphite at 2θ = 26.6 • in Figure 5b. A slight increase of the intensity of the carbon diffraction peak in Figure 5b can be observed with increased SiC content, suggesting a higher amount of free carbon formation during the phase transformation.
Carbon, as a commonly used sintering additive for UHTCs such as ZrB 2 [57], B 4 C [58], and SiC [59], improves the sinterability by reacting with the surface oxide and enhancing grain boundary diffusion. The incorporation of SiC into UHTCs, such as metal diborides, as a sintering additive has also been reported to promote the grain-boundary diffusion and results in the high densification and strength of UHTCs [60]. Thus, in our case, the combination of SiC and the free carbon formation during the phase transformation from a FCC to a hexagonal high-entropy phase with increasing content of SiC contribute together to the promoted densification of the HEC-(SiC)x composites, as shown in Figure 1a.
All of the constituent metal elements in the present study have lower formation energy of MB 2 than MC, which suggests that diboride instead of carbide is the energetically preferred phase in the current system [56]. Moreover, the diffusion rate of solute metal atoms into the TaC lattice has been discussed previously to be Ti > Mo > Hf, thus the hexagonal structured high-entropy solid solution represents the energetical favorite phase in the high-entropy boron-carbide system due to the corresponding atomic composition (Ti > Mo > Hf, in Figure 4) with the diffusion priorities [17]. Hence, the stabilization of the hexagonal-structured high-entropy phase could be a result of promoted diffusion by SiC and free carbon. On the other hand, SiC addition limited the grain growth and thereby hindered the mass transport in HEC composites during the cooling process. As the formation of multiple cubic phases might be a result of spinodal decomposition of the major FCC phase during cooling, the hindering of mass transport by SiC can contribute to the stabilization of the high-entropy solid solution so that only one FCC phase was observed in the HEC-(SiC)3.0 composite.

Conclusions
High-entropy ceramic composites from multiple refractory carbide HfC, Mo 2 C, TaC, TiC, B 4 C, and SiC were synthesized. The dependence of densification, microstructure, phase composition, and mechanical properties of the HECs composite on SiC content were investigated. The sintered HEC-(SiC)x composite showed promoted densification compared to HECs without SiC addition. With increasing the SiC content from HEC-(SiC)0 (0 vol. %) to HEC-(SiC)3.0 (37 vol. %), the HECs composites showed remarkable enhancement of the mechanical properties, including higher Vickers hardness from 20.3 to 26.9 GPa, under the load of 1000 g, and improved fracture toughness from 3.14 to 5.95 MPa·m 1/2 , attributed to the crack bridging and deflecting mechanism. The investigation on the phase composition in the HEC-(SiC)x composite showed that four different high-entropy phases could be formed, including three cubic crystal-structured phases and one hexagonal phase. However, only two high-entropy phases were retained when the SiC content was increased to 37 vol. % (HEC-(SiC)3.0). The two retained high-entropy phases contained all metal elements, with the face-centered cubic (FCC) structure having relatively higher content of Hf, Ta, and C, and the hexagonal crystal structure containing boron and a relatively higher amount of Mo and Ti,. Composites with higher SiC content showed an increased volume formation of the hexagonal-structured phase, suggesting a favored formation of hexagonal-structured high-entropy phase due to SiC addition and the formation of free carbon.