3.1. Effect of Pressureless Pre-Sintering Temperature on the Physical Phase and Microstructure of High-Entropy Silicide Ceramics
Figure 2 shows the XRD diagrams of V1, V2, and V3 high-entropy silicide ceramics. When the pressureless pre-sintering temperature was 1200 °C, the ceramic specimens formed (Hf, Zr) Si
2 solid solution, (Ta, Ti, Cr)Si
2 solid solution and (Hf, Zr) solid solution. With the gradual increase in temperature, the unreacted elemental Si diffraction peaks and (Hf, Zr) solid solution diffraction peaks in the gradual weakening, when the temperature reaches 1400 °C, the diffraction peaks of the V3 ceramic specimen relative to the other two ceramic specimens become more, which also formed a certain proportion of high entropy phase, and the diffraction peaks of ZrSi were observed, ZrSi is not fully integrated into the (Hf, Zr)Si
2 solid solution. The diffraction peaks of the V2 ceramic specimen are the smoothest, and compared to the V1 ceramic specimen, the elements in the V2 ceramic specimen were diffused more sufficiently, which exacerbates the integration of the solid solution. The diffraction in the V3 appeared with more peaks, which made the two main solid solution phases in the high-entropy silicide ceramics more distinct and more obvious with the further increase in temperature. From the three diffraction lines, it can be seen that the diffraction peaks of (Hf, Zr)Si
2 solid solution and (Ta, Ti, Cr)Si
2 solid solution occupy the vast majority of the diffraction peaks, and the diffraction peaks are also the strongest.
Figure 3 shows the microscopic morphologies of the V1 ceramic specimen. From
Figure 3a, it can be seen that the overall V1 ceramic specimen is not homogeneous enough, the specimen has a lot of large black holes, and the elemental distribution of the sample is not completely homogeneous; From the EDS mappings of
Figure 3b, the elements such as Ta, Ti, and Cr are basically enriched, and the elements such as Zr, and Hf are enriched, and most of the Si elements are enriched in the elements such as Ta, Ti, and Cr, and a small portion elements such as Zr, Hf, etc., which coincides with the XRD diagrams of high-entropy silicide ceramics; this enrichment can be more clearly found by SEM and EDS in
Figure 3c. It can also be observed from
Figure 3c of the V1 specimen that the specimen shows an overall light gray uniformly enriched Hf, Zr and Si phases and dark gray Ta, Ti, Cr and Si particles; in addition to these two overall phases, the V1 ceramic specimen also has a diffusely distributed white granular phase.
The EDS point-scanning energy spectroscopy results of the light-gray Hf, Zr, and Si elementally homogeneous enriched phases and the dark-gray particles of the Ta, Ti, Cr, and Si elementally enriched phases are shown in
Table 2, where it is inferred that the light-gray homogeneous phases are the solid solutions of ZrSi
2 and HfSi
2 based on the atomic fraction ratios of the individual elements of the point 2 [
21], and that the dark-gray particles Ta, Ti, Cr, and Si element-enriched phases are solid solutions formed by TaSi
2, TiSi
2, and CrSi
2 as well as the unreacted element Si. This speculation is supported by the (Ta, Ti, Cr)Si
2 solid solution [
22] and the Si element on one diffraction peak shown in the XRD of
Figure 2.
Figure 4 shows the microscopic morphologies of the V2 ceramic specimen. As can be seen from the SEM and EDS of
Figure 4a, the sample is very homogeneously fused, and the six elements are uniformly distributed on the surface of the specimen without any obvious aggregation, and the pores of the sample are very small compared to the V1 ceramic specimen. In the V2 ceramic specimen, the light gray and dark gray solid solutions are interspersed with each other to form a whole, and no obvious white particles can be seen under low magnification. According to the SEM and EDS results in
Figure 4b, the white particles gradually appear, but they are still very fine and diffusely distributed on the sample surface, and the silver-gray color, which is intermediate between the light-gray and dark-gray colors, also appears in both main phases, and the silver-gray color is observed in the EDS. In the EDS mappings of
Figure 4b, the six elements in the silver-gray phase sparsely appear as small dots, and it is presumed that the silver-gray phase is a high-entropy phase. The white granular phase appears to be enriched in two elements, Hf and Zr, and it is presumed that the white granular phase is a (Hf, Zr) solid solution phase [
23]. The energy spectrum points scan of point 3, 4, and 5 of
Figure 4c are shown in
Table 3, and the energy spectrum results of point 3 confirm that the white particles are (Hf, Zr) solid solution phase. Point 4 and 5 correspond to the presumed silver-gray phase.
Figure 5 shows the microscopic morphologies of the V3 ceramic specimen. As shown in
Figure 5a, the overall structure of the V3 ceramic specimen becomes rougher, and there are no obvious large pores compared to V2, but the overall number of pores has increased a lot. The two main phases of the V3 ceramic specimen are uniformly distributed shown in
Figure 5b, and the light-gray (Hf, Zr) Si
2 solid solution phase and dark gray particles of (Ta, Ti, Cr)Si
2 solid solution phase are hierarchically and uniformly distributed on the sample surface. Compared with V2, the dark gray particles of (Ta, Ti, Cr)Si
2 solid solution phase are obviously increased, and the overall size of the particles is larger and darker in color. V2 ceramic specimens in
Figure 4b appear to be almost all uniformly light gray (Hf, Zr) Si
2 solid solution phase is diffusely distributed, while the V3 ceramic specimen in
Figure 5b looks obviously granular phase occupies the surface of the sample.
Figure 5c shows that the V3 ceramic specimen has some large white particles, and there are also some finer white particles among the large white particles, and the large white particles are not completely and uniformly fused together, and the particles have a lot of dark-coloured irregularly rounded distributions, as well as some holes.
The EDS point-scan spectras of the large white particle phase and the small white particle phase are shown in
Table 4. The small white particle phase is presumed to be a (Hf, Zr) solid solution phase based on the atomic fraction ratios of each element of the point 8, which is consistent with the point 3 in
Table 3. According to the EDS results of
Figure 5c, it can be found that Ta and Si elements are enriched in the dark irregular circles, and point 6 is its point scan result, which speculates that the dark gray irregular circle is the high entropy phase, while according to the energy spectrum result of point 7, the atomic proportion of metal elements occupies about 50%, and the atomic proportion of Si elements occupies about 50%, which speculates that the large white particles are monosilicides [
24].
3.2. Effect of Holding Time of Pressureless Pre-Sintering on the Physical Phase and Microstructure of High Entropy Silicide Ceramics
Figure 6 shows the XRD patterns of V3, V4 and V5 high entropy silicide ceramics. From the
Figure 6, the physical phases formed in the ceramic specimens under the holding time of 2 h are (Hf, Zr)Si
2 solid solution, (Ta, Ti, Cr)Si
2 solid solution, and a small amount of high entropy phase. As the holding time increases, the diffraction peaks of (Hf, Zr) solid solution, ZrSi, and incompletely reacted elements of Si appear in the XRD diagram, and the intensity of the diffraction peaks of several phases is relatively weak. It can also be observed in the figure that the diffraction peaks of the V4 ceramic specimen with a holding time of 2 h are very few, which is because the holding time of 2 h is too short during the initial sintering, the green body is not sufficiently sintered, and the bonding between the particles is not strong enough, and the formation of the green body is not sufficiently strong because of the short sintering time. The diffraction peak intensity of (Ta, Ti, Cr)Si
2 solid solution of V5 ceramic specimen with holding time of 4 h gradually becomes weaker compared with that of V3, and the diffraction peak intensity of (Hf, Zr)Si
2 solid solution gradually becomes stronger, which can be seen from the SEM image of V3 ceramic specimen in
Section 3.1. The diffraction peaks of the ceramic specimens became more and more numerous as the holding time was extended.
Figure 7 shows the microstructures of the V4 ceramic specimen. As shown in
Figure 7a, the sample has many black holes, and the white particles are almost invisible at low magnification, and the overall color is not uniformly distributed. In
Figure 7b, it can be seen even more that the overall bonding of the V4 ceramic sample is not homogeneous, and the (Hf, Zr)Si
2 solid solution and (Ta, Ti, Cr)Si
2 solid solution are all randomly scattered on the surface of the sample, and the sample possesses a lot of grayish-white particles. However, it is not obvious to be observed in
Figure 7a, which means that the grayish-white particles are of very small sizes, and the above mentioned in
Section 3.1 also explains that the grayish-white color is a part of the high entropy phase. The elemental distribution of the V4 ceramic samples is relatively inhomogeneous, as shown in
Figure 7c, the individual elements are diffusely distributed and disordered, and this phenomenon is related to the retention time of V4. The holding time is too short, and the phases formed in the raw billet do not have time to fuse, and the bonding under SPS sintering is weak, resulting in a non-uniform distribution of the phases.
Figure 8 shows the microscopic morphologies of the V5 ceramic specimen, as shown in
Figure 8a, there are many large white particles on the surface of the V5 specimen. The sizes of the particles can be clearly observed in
Figure 8a, which indicates that the large white particles are very aggregated, and the number of high-entropy phase particles of the V5 ceramic specimen has a significant increase compared to that of the V4 ceramic. The longer the holding time of the pressureless pre-sintering, the more high entropy phase particles are induced to form, so that the aggregation phenomenon is caused by high entropy phase particles. As can be seen from
Figure 8b, the high-entropy phase particles of the ceramic specimens increase, in addition, the light gray uniform phase is more widely distributed compared to
Figure 5b, which confirms that the diffraction peak intensity of (Hf, Zr)Si
2 solid solution becomes stronger with the increase of holding time in
Figure 6. The corresponding EDS results of the markers in
Figure 8c are shown in
Table 5. From
Figure 8c, it can be seen that the sample is almost a light gray homogeneous phase except for the large white granular phase, and according to the EDS results of point 11, the light gray homogeneous phase is basically the (Hf, Zr)Si
2 solid solution, and with the increase of the holding time, the (Hf, Zr)Si
2 solid solution is gradually fused and homogeneous. The (Hf, Zr)Si
2 solid solution described in points 9 and 10 is the silver-gray high-entropy phase.
From the SEM and EDS images of V3, V4, and V5 ceramic specimens, the increase of the holding time of the two-step sintering in the pressureless pre-sintering helps the fusion of the phases in the silicide ceramics, and the holding time of the ceramics will be basically homogeneous fusion of the phases in the ceramics in 3 h or more. The longer the holding time, the more the (Hf, Zr)Si2 solid solution phases and high-entropy phases will be formed, and conversely, the less (Ta, Ti, Cr)Si2 solid solution phases are formed.