Enhancing Ablation Resistance of TaB2-Based Ultra-High Temperature Ceramics by Mixing Fine TaC Particles and Dispersed Multi-Walled Carbon Nanotubes

Ultra-high temperature ceramics (UHTCs) have been widely applied in many fields. In order to enhance the comprehensive properties of TaB2-based UHTCs, the first collaborative use of fine TaC particles and dispersed multi-walled carbon nanotubes (MWCNTs) was employed via spark plasma sintering (SPS) at 1700 °C. The derived UHTCs exhibited an average grain size of 1.3 μm, a relative density of 98.6%, an elastic modulus of 386.3 GPa, and a nano hardness of 21.7 GPa, leading to a greatly improved oxidation resistance with a lower linear ablation rate at −3.3 × 10−2 μm/s, and a markedly reinforced ablation resistance with mass ablation rate of −1.3 × 10−3 mg/(s·cm2). The enhanced ablation resistance was attributable to the physical pinning effect, sealing effect and self-healing effect. Thus, this study provides a potential strategy for preparation of UHTCs with bettered ablation resistance and physical properties.


Introduction
Ultra-high temperature ceramics (UHTCs) have been widely applied in thermochemically harsh environments, such as hypersonic flights, rocket propulsion, and atmospheric re-entry, because of their high hardness, high melting point, outstanding chemical stability, excellent thermal stability, and sufficient elastic modulus [1,2].The raw materials of UHTCs generally include carbides, nitrides, and diborides of early transition metals [3,4].
As a crucial part of UHTCs, transition metal diborides have been widely concerned with physical properties, oxidation resistance, ablation resistance, acid and alkali corrosion resistance, etc. Neuman et al. [5] and Xu et al. [6], respectively, reported dense ZrB 2 ceramics by adding sintering aids (including carbon and boron carbide nanopowder) and refined particles to improve the mechanical and oxidation-resistant properties.Mattia et al. [7] performed oxidation studies of an aluminum nitride-hafnium diboride ceramic composite, and discovered that its anti-oxidant properties were enhanced by the formation of a protective oxide scale containing hafnia (HfO 2 ) and aluminum borate (Al 18 B 4 O 33 ) phases.Monticelli et al. [8] synthesized an HfB 2 -2.5 wt% Si 3 N 4 ceramic via hot pressing sintering, and further exhibited a good corrosion behavior in acid and alkaline solutions of chlorides and sulfates.Hassan et al. [9] fabricated SiC reinforced HfB 2 + ZrB 2 composites with or without CNTs using spark plasma sintering (SPS), and revealed that CNTs' reinforcement increased the heterogeneity and consequently gave rise to the formation of a wide spectrum of solid solutions of varying composition.

Powder Processing
Pure TaB 2 powder: 12 g of TaB 2 and 200 mL of anhydrous ethanol were added into a 500 mL glass beaker, and then the beaker was placed on a magnetic stirrer with 400 rpm for 5 h at 30 • C.Then, the reagents in the beaker were pumped into a filter device.The filtrated reagents were transferred into a new 500 mL glass beaker, then 200 mL of anhydrous ethanol was added into the glass beaker, and then the beaker was placed in an ultrasonic bath for 3 h of ultrasound.The reagents in the beaker were poured into a 500 mL singlemouth flask, which was then placed in a rotary evaporator to remove anhydrous ethanol and obtain dry TaB 2 .The dried TaB 2 was placed into an agate mortar for grinding the apparent lumps into powders.During the rotary evaporation, the collected anhydrous ethanol was used for the processing of the next powder.95TaB 2 -5TaC powder: The mass of TaB 2 was 11.4 g and that of TaC was 0.6 g.The powder processing method followed the same procedure as that of the pure TaB 2 powder.95TaB 2 -5TaC-0.5MWCNTspowder: The mass of TaB 2 was 11.4 g, that of TaC was 0.6 g, and that of MWCNTs was 0.06 g.The powder processing method was the same as that of the pure TaB 2 powder.95TaB 2 -5TaC-0.5MWCNTs-0.2PAApowder: The mass of TaB 2 was 11.4 g, that of TaC was 0.6 g, that of MWCNTs was 0.06 g, and the mass of PAA was 0.024 g.The powder processing method was also as the same as the pure TaB 2 powder.

Spark Plasma Sintering (UHTCs Preparation)
A graphite punch with a diameter of 20 mm was inserted into a graphite die with a 20 mm in inner diameter, and a circular carbon paper with a diameter of 20 mm was placed on the upper surface of the graphite punch.In total, 12 g of pure TaB 2 powder was poured into the graphite die.A mechanical force of 8 MPa was applied to the graphite die with graphite punches inserted above and below to the compact pure TaB 2 powder.
Graphite felt with 5 mm thick was wrapped around the outer surface of the graphite die to evenly distribute temperature and reduce heat loss from radiation.The graphite die wrapped with the graphite felt was placed in a spark plasma sintering (SPS) device (LABOX-1575, Sinter Land, Nagaoka, Japan) equipped with a thermometer (emissivity 0.96).Axial pressure of 20 MPa was applied to ensure adequate electrical contact between the pure TaB 2 powder and the graphite die.When the vacuum degree was less than 30 Pa, the pure TaB 2 powder was heated from room temperature with a heating rate at 100 • C/min, and the axial pressure was increased with 2.5 MPa/min.When the temperature reached 800 • C, the axial pressure was 40 MPa, and the SPS device was kept off the heat for 1 min.Then, the axial pressure remained at 40 MPa and the graphite die was continued to be heated to 1700 • C with a heating rate of 100 • C/min.After being kept at 1700 • C for 5 min, the vacuum degree and axial pressure were released, and the graphite die was cooled to room temperature rapidly.The graphite felt, the graphite die, and the carbon papers were removed to obtain the ultra-high temperature ceramic (UHTC), which was named pure TaB 2 .95TaB 2 -5TaC, 95TaB 2 -5TaC-0.5MWCNTs,and 95TaB 2 -5TaC-0.5MWCNT-0.2PAAwere, respectively, obtained as the same as that of the pure TaB 2 .The four UHTCs were about 20 mm in diameter and 4 mm in thickness.
The SPS curves of the four UHTCs are shown in Figure S1.The top view and main view of the four UHTCs are present in Figure S2.
Elastic modulus and nano-scale hardness of all UHTCs were, respectively, tested using an Agilent Nano Indenter (G200, Keysight Technologies, Santa Rosa, CA, USA) with an 8000 µN applied load and Berkovich tip [3].
The FSME images of the cross-sections of the UHTCs and the surfaces and crosssections of the ablative UHTCs were, respectively, explored using FESEM (Nova NanoSEM 450, FEI Company, The Netherlands) assembled with an energy dispersive X-ray spec-troscopy (EDXS).All samples were, respectively, sprayed with a thin layer of platinum before being examined using FESEM.
Grain size analysis of all UHTCs was conducted using "Image J Fiji 2.13.0" software.The UHTCs were polished using an integrated automatic grinding and polishing machine with a programming control system (Tegramin-25, Struers, Ballerup, Denmark).Then, X-ray diffraction (XRD) data of the polished surface of the above UHTCs and the surface of the ablative UHTCs were, respectively, collected using XRD with the equipment and conditions described above.
Thermal gravimetric analysis (TGA) of the UHTCs were tested using a thermal analyzer (STA449F3, Neotchi Instrument Manufacturing Co. Ltd., Selb, Germany) with a heating rate of 20 • C/min up to 1450 • C in air.
Ablative testing of the polished UHTCs were conducted for 60 s using a welding torch (JH-3VA, Xiamen Jiarui Electronics Co., Ltd., Xiamen, China) equipped with a gas cylinder with gasses including methyl acetylene propylene propane (MAPP gas, Shanghai Chenmai Industrial Co., Ltd., Shanghai, China) that can shoot 1450 • C of flame in air.The samples were kept 7 cm away from the nozzle of the welding torch.
The linear ablation rate (R l ) of the UHTCs was calculated according to the following formula [10]: R l = (l 0 − l 1 )/t, among which, l 0 and l 1 are the thickness of the samples at the center region before and after ablation, respectively; and t is an ablation time, which is 60 s.
The mass ablation rate (R m ) of the UHTCs was calculated according to the following formula [15]: R m = (m 0 − m 1 )/(A•t), among which, m 0 and m 1 are the mass of the samples before and after ablation, respectively; A is the area of the front face of the samples; and t is an ablation time, which is 60 s.

Raw Material Microstructure
The microstructure of the pure TaB 2 powder, pure TaC powder, 95TaB 2 -5TaC powder, 95TaB 2 -5TaC-0.5MWCNTspowder, and 95TaB 2 -5TaC-0.5MWCNTs-0.2PAApowder were, respectively, observed via FESEM, and the corresponding images are shown in Figure 1.From Figure 1a, the pure TaB 2 powder is mostly irregular particles distributed in 1.0-3.0µm range.The pure TaC powder is also mostly irregular particles with an average size of 300 nm (Figure 1b).From Figure 1(d 1 ,d 2 ), the aggregate MWCNTs with 5.0-10.0µm are found in the 95TaB 2 -5TaC-0.5MWCNTspowder, while tiny aggregated and dispersed MWCNTs are observed in the 95TaB 2 -5TaC-0.5MWCNTs-0.2PAApowder (Figure 1(e 1 ,e 2 )), disclosing that the use of PAA is beneficial to the dispersion of MWCNTs in the powders.

UHTCs' XRD Characterization
XRD curves of the polished UHTCs are shown in Figure 3.Only the peaks of TaB 2 were detected in all UHTCs' XRD curves, while no peak of TaC was found.The reason for this could be that loose powders were more easily detected using XRD than dense ceramics at the low content (5.0 wt%) of TaC, which was consistent with the work of Akarsu [18].

UHTCs' Cross-Sectional Microstructure
Before observing UHTCs' cross-sectional microstructure using FESEM, the UHTCs were broken using a hammer.The corresponding FESEM images of their cross-sectional microstructure are presented in Figure 4.As shown in Figure 4(a 1 ,a 2 ), large holes and long cracks are found in the pure TaB 2 , owing to its relatively low sinterability.The average grain size of the pure TaB 2 was 1.5 ± 0.2 µm, calculated using the "Image J Fiji" software.Compared to the microstructure of the cross-section of the pure TaB 2 , that of the 95TaB 2 -5TaC (Figure 4(b 1 ,b 2 )) has fewer and smaller holes and cracks, and a finer average grain size (1.3 ± 0.2 µm), leading to an increased relative density for the 95TaB 2 -5TaC.The refinement of grain sizes was probably caused by the physical pinning effect of the small TaC particles.The physical pinning effect means that the distribution of the small TaC particles on the grain boundaries of TaB 2 would slow down the rapid growth of the grains; similar phenomena also were reported in Refs.[16,[19][20][21].For example, Liu et al. [19] reported that the increasing amount of Si 3 N 4 continuously reduced the grain size of TaC via physical pinning effect of Si 3 N 4 .Meanwhile, TaC was obviously discovered via EDXS, suggesting that TaC did not dissolve into TaB 2 during the SPS processing of UHTCs.Previous researchers also reported similar results.Zhang et al. [22] found that TaC and TaB 2 were virtually insoluble in each other below 2200 • C, which was assigned to the difference in the crystal structures.Particularly, TaC formed a cubic structure (B1); nevertheless, TaB 2 crystallized in a hexagonal structure (AlB 2 type) [18,22].From Figure 4(c 1 ,c 2 ), the agglomerate MWCNTs are clearly observed, and the existence of holes and cracks in the ceramic matrix below MWCNTs are likely to be attributed to the threedimensional structure of the agglomerate MWCNTs, which makes the powders difficult to compact via SPS.Therefore, the average grain size of the 95TaB 2 -5TaC-0.5MWCNTs is 1.4 ± 0.2 µm, a little larger than that of 1.3 ± 0.2 µm of the 95TaB 2 -5TaC.Fortunately, the holes of the 95TaB 2 -5TaC-0.5MWCNTs-0.2PAA are also very small, and the cracks could not be observed; meanwhile, the dispersed MWCNTs are seen on the boundaries of the grains or within the grains (seen Figure 4(d 1 ,d 2 )).Owing to the physical pinning effect of smaller TaC particles and the sealing effect of the dispersed MWCNTs, the average gain size of the 95TaB 2 -5TaC-0.5MWCNTs-0.2PAA is further reduced to 1.3 ± 0.1 µm.The sealing effect of the dispersed MWCNTs referred to that MWCNTs wrapped around the grains of TaB 2 and acted as barriers for mass diffusion, thereby inhibiting the grain growth during preparation of the TaB 2 -based UHTCs.The sealing effect was also reported in the previous studies.Li et al. [23] and Nieto et al. [24], respectively, reported that graphene platelet reduced the grains of the TaC-based UHTCs via the grain wrapping mechanism or the sealing effect.The microstructure refinement and the sealing effect led to the higher relative density, which was apt to enhance the mechanical, anti-oxidant, and anti-ablative properties for the UHTCs [19,25].

UHTCs' XRD Characterization
XRD curves of the polished UHTCs are shown in Figure 3.Only the peaks of TaB2 were detected in all UHTCs' XRD curves, while no peak of TaC was found.The reason for this could be that loose powders were more easily detected using XRD than dense ceramics at the low content (5.0 wt%) of TaC, which was consistent with the work of Akarsu [18].

UHTCs' Cross-Sectional Microstructure
Before observing UHTCs' cross-sectional microstructure using FESEM, the UHTCs were broken using a hammer.The corresponding FESEM images of their cross-sectional microstructure are presented in Figure 4.As shown in Figure 4(a1,a2), large holes and long cracks are found in the pure TaB2, owing to its relatively low sinterability.The average

UHTCs' TGA Characterization
The oxidation behavior of the UHTCs was examined via TGA from room temperature up to 1450 • C in air with a heating rate of 20 • C/min.The possible reaction equations involved in the oxidation processes of the UHTCs are displayed in Equations ( 1)- (8).During the course of oxidation, TaC was oxidized to Ta 2 O 5 and CO 2 ; meanwhile, TaB 2 was oxidized to Ta 2 O 5 and B 2 O 3 .B 2 O 3 melted at 450 • C and then vaporized above 1100 • C [10,26].Although both CO 2 and B 2 O 3 were volatilized from the samples, the mass of the introduced oxygen element was greater than the added weight of both CO 2 and B 2 O 3 , causing a gradual mass increase in the samples during the oxidation process.The TGA and DTG curves of the UHTCs are, respectively, displayed in Figure 5a,b.As shown in Figure 5a, the weight of the pure TaB 2 , 95TaB 2 -5TaC, 95TaB 2 -5TaC-0.5MWCNTs,and 95TaB 2 -5TaC-0.5MWCNTs-0.2PAA,respectively, accounts for 111.4%, 109.1%,107.7%, and 105.8% after the oxidation procedure.From Figure 5b, the derivative weight of the pure TaB 2 , 95TaB 2 -5TaC, 95TaB 2 -5TaC-0.5MWCNTs,and 95TaB 2 -5TaC-0.5MWCNTs-0.2PAAis, respectively, 0.16%/min, 0.13%/min, 0.12%/min, and 0.08%/min.The results of TGA and DTG reveal that the 95TaB 2 -5TaC-0.5MWCNTs-0.2PAA has the best antioxidation property at higher temperature.The outstanding oxidation resistance for the 95TaB 2 -5TaC-0.5MWCNTs-0.2PAA is attributed to its high relative density and good sealing effect, as demonstrated in Figure 4

Linear Ablation Rate and Mass Ablation Rate
Linear ablation rates (Rl) and mass ablation rate (Rm) are two vital indices for evaluating the ablation resistance of materials.The Rl and Rm of the polished UHTCs using a welding torch for 60 s are listed in Table 2. Rl of the pure TaB2, 95TaB2-5TaC, 95TaB2-5TaC-0.5MWCNTs, and 95TaB2-5TaC-0.5MWCNTs-0.2PAAwere, respectively, −8.The values of Rl and Rm for the ablative UHTCs were both negative, indicating that the mass of the UHTCs were increased during the ablation process due to the more introduced mass than the lost one, which was consistent with the results of TGA.The results of Rl and Rm revealed that the high relative density and good sealing effect were both beneficial to gaining the ablative resistance at high temperature for the 95TaB2-5TaC-0.5MWCNTs-0.2PAA.

XRD Characterization for Ablative UHTCs
The above ablative UHTCs were detected using XRD, and the corresponding XRD data are shown in Figure 6.From Figure 6, the peaks of both Ta 2 O 5 and TaB 2 are detected in all UHCTs.There were two possible reasons why the peak of TaB 2 was detected.On the one hand, the shedding of the oxide layers resulted in the exposure of the un-oxidized ceramic matrix.On the other hand, the oxide layers were not thick enough (the thickness of oxide layers for all ablative UHTCs is shown in Figure 6), causing the ceramic substrate underneath the oxide layers to also not be detected.Notably, the detected oxide in the samples is Ta 2 O 5 instead of other tantalum oxides because Ta 2 O 5 has a higher melting point than other tantalum oxides [10].As far as the peaks of B 2 O 3 are not obviously detected, the reason is because the intensity of its peaks is much lower than that of Ta 2 O 5 , causing its peaks to be covered up, or there is not much B 2 O 3 left, resulting in its peaks failing to be detected.

XRD Characterization for Ablative UHTCs
The above ablative UHTCs were detected using XRD, and the corresponding XRD data are shown in Figure 6.From Figure 6, the peaks of both Ta2O5 and TaB2 are detected in all UHCTs.There were two possible reasons why the peak of TaB2 was detected.On the one hand, the shedding of the oxide layers resulted in the exposure of the un-oxidized ceramic matrix.On the other hand, the oxide layers were not thick enough (the thickness of oxide layers for all ablative UHTCs is shown in Figure 6), causing the ceramic substrate underneath the oxide layers to also not be detected.Notably, the detected oxide in the samples is Ta2O5 instead of other tantalum oxides because Ta2O5 has a higher melting point than other tantalum oxides [10].As far as the peaks of B2O3 are not obviously detected, the reason is because the intensity of its peaks is much lower than that of Ta2O5, causing its peaks to be covered up, or there is not much B2O3 left, resulting in its peaks failing to be detected.
Compared to the pure TaB2, the maximum peak intensity ratio of Ta2O5 to TaB2 for 95TaB2-5TaC was decreased, indicating that 95TaB2-5TaC had a better ablation resistance due to its high relative density caused by the use of the finer TaC particles.Among all ablative UHCTs, the maximum peak intensity ratio of Ta2O5 and TaB2 for 95TaB2-5TaC-0.5MWCNTs-0.2PAA was the lowest, revealing that 95TaB2-5TaC-0.5MWCNTs-0.2PAApossessed the strongest ablation resistance because of its highest relative density and best sealing effect.Compared to the pure TaB 2 , the maximum peak intensity ratio of Ta 2 O 5 to TaB 2 for 95TaB 2 -5TaC was decreased, indicating that 95TaB 2 -5TaC had a better ablation resistance due to its high relative density caused by the use of the finer TaC particles.Among all ablative UHCTs, the maximum peak intensity ratio of Ta 2 O 5 and TaB 2 for 95TaB 2 -5TaC-0.5MWCNTs-0.2PAA was the lowest, revealing that 95TaB 2 -5TaC-0.5MWCNTs-0.2PAApossessed the strongest ablation resistance because of its highest relative density and best sealing effect.

Ablative UHTCs' Surface Microstructure
The ablative surface of the polished UHTCs was observed via an FESM equipped with an EDXS.The corresponding FESEM images and EDXS images of the ablative UHTCs' surface microstructure are, respectively, shown in Figures 7 and 8.As can be seen in Figure 7(a 1 ,a 2 ), a big crack and many small cracks, abundant scattered oxide layers, and a number of holes are observed in the surface of the ablative pure TaB 2 .The cracks and holes appeared for the following two reasons: one is because the cracks and holes are caused by the non-densification of the pure TaB 2 matrix (see Figure 4(a 1 ,a 2 )), and the other is because the volatilization of volatiles (such as CO, CO 2 , B 2 O 3 , etc., seen in Equations ( 1)-( 8) above) resulted in the cracks and holes during the ablative process of the pure TaB 2 .The appearance of holes and cracks, especially cracks, were extremely unfavorable to the ablation resistance of ceramic matrix in the ablative process [27] because heat and oxygen could penetrate into the pure TaB 2 matrix through holes and cracks, thus accelerating the oxidation of the pure TaB 2 .In comparison with Figure 7(a 1 ,a 2 ), no large cracks or holes are found in Figure 7(b 1 ,b 2 ), and the oxidation layers increased significantly and nearly became integral, but there still remained a few scattered oxide layers.In addition, there was an interesting phenomenon, being that TaC was found on the oxidized surface of the polished 95TaB 2 -5TaC.In general, the oxidation resistance of borides is better than that of carbides.Similarly, Zhang et al. [22] reported that the oxidation of TaC was faster than that of TaB 2 using TGA in air up to 1500 • C. In order to further compare the high temperature oxidation resistance of TaC and TaB 2 by calculating the free energy change per mole of oxygen (∆G) and partial pressure of oxygen (p O2 ) of Equations ( 1) and ( 2), among which, ∆G reflects the feasibility of chemical reactions [28].∆G of the Equations ( 1) and ( 2) can be calculated by the following relationship (Equation ( 9)): where R is the ideal gas constant, T is the absolute temperature, and K eq is the equilibrium constant.The values of K eq (with reaction numbers as subscripts) are presented as Equations ( 10) and ( 11), as follows: where a is the activity of the species and p O 2 is the equilibrium partial pressure.The values of ∆G and K eq for 1450 • C (1723 K) calculated via Equation ( 9), as well as p O 2 calculated via considering the activities of pure substances as unity from Equations ( 10) and (11), are listed in Table 3. Table 3 shows the following: (i) ∆G 1 < ∆G 2 , and (ii) the value of p O 2 required for the oxidation of TaB 2 is tremendously less than that of TaC, displaying that the driving force of the oxidation at 1450 • C increases in the following order: TaC < TaB 2 .Via analyzing of thermodynamic data, TaC is more likely to be oxidized at higher temperature than TaB 2 .Hence, because of the probable reason that TaC existed in the oxide layers of the polished 95TaB 2 -5TaC, it is speculated that although TaC is easier to be oxidized thermodynamically, but due to the much lower content (5.0 wt%) of TaC and the extremely higher content (95.0 wt%) of TaB 2 , TaB 2 is easily oxidized in exposure to air kinetically, resulting in that some TaC was wrapped by Ta 2 O 5 products before being oxidized in the course of the crystallization growth of Ta 2 O 5 .
ceramic substrates continue to be oxidized after the oxide layers fall off during the continuous ablation.From Figure 7(c 1 ,c 2 ), in addition to cracks and holes, the agglomerated MWCNTs are also observed.As presented in Figure 7(d 1 ,d 2 ), the oxide layers are continuous, complete, and dense, and nearly no cracks and holes are found, disclosing that 95TaB 2 -5TaC-0.5MWCNTs-0.2PAApossesses a superior ablation resistance.
Figure 8 shows the EDXS images, including EDXS elemental mapping and EDXS patterns of the selected regions for the ablative UHTCs.According to Figure 8d, the retained oxide layers have higher oxygen content than that of the exposed substrates of 95TaB 2 -5TaC.Combined with Figure 8k-n, the oxygen element is found in both the oxide layers and the substrates, but its content in the substrates is relatively low, indicating that the ceramic substrates continue to be oxidized after the oxide layers fall off during the continuous ablation.

Ablative UHTCs' Vertical Cross-Sectional Microstructure
After the ablation tests, the oxide layers of the ablative UHTCs' vertical cross-sectional microstructure were also observed using FESEM, and the corresponding images are presented in Figure 9. From Figure 9(a1,a2), the oxide layer thickness of the ablative pure TaB2 is about 3.0 µm, while the penetration thickness of XRD equipment generally is up to tens of microns, which is the reason why the XRD curve of TaB2 presented both peaks of TaB2 and Ta2O5.The oxide layer of the ablative pure TaB2 appeared to have large cracks and even the separation between oxide layers, which consisted of many smaller grains of Ta2O5, were filled or covered by glassy B2O3.Compared to the surface of the ablative pure TaB2, the glassy B2O3 was detected in its vertical cross-section, because the temperature of UHTCs was dropped rapidly after the torch left and a certain amount of B2O3 remained in the oxide layers of the UHTCs before being volatilized [6].It is noteworthy that a hole caused by the low sinterability of TaB2 was found in the pure TaB2 ceramic matrix, and a bright brim around the hole was detected due to higher second electron yield at the edge of the hole [19].From Figure 9(b1,b2), the oxide layer about 2.0 µm thick, small cracks, as well as glassy B2O3, are found, finding that 95TaB2-5TaC possessed a better ablation resistance than that of pure TaB2.As shown in Figure 9(c1,c2), the oxide layer thickness of the ablative 95TaB2-5TaC-0.5MWCNTs is about 2.5 µm; meanwhile, a few MWCNTs, glassy B2O3, are also observed.The oxide layer thickness of the ablative 95TaB2-5TaC-0.5MWCNTs-0.2PAA is about 1.2 µm (see Figure 9d1), and the oxide layer is relatively dense, and MWCNTs are scarcely found in the oxide layer (see Figure 9d2).Figure 10 exhibits FESEM image and EDXS elemental mapping of the vertical cross-sectional microstructure of the ablative 95TaB2-5TaC-0.5MWCNTs-0.2PAA and the EDXS patterns of the selected regions in Figure 9, confirming that the oxygen content of oxide layers is higher than those of the non-oxide layers.By comparing the oxidation layers of all ablative UHTCs' vertical cross-sections, it was found that 95TaB2-5TaC-0.5MWCNTs-0.2PAA had the best ablation resistance, which was consistent with the results of the ablative UHTCs' surface microstructure.The excellent ablative resistance for 95TaB2-5TaC-0.5MWCNTs-0.2PAA was due to the dense oxide layer caused by the synergistic effect of the physical

Ablative UHTCs' Vertical Cross-Sectional Microstructure
After the ablation tests, the oxide layers of the ablative UHTCs' vertical cross-sectional microstructure were also observed using FESEM, and the corresponding images are presented in Figure 9. From Figure 9(a 1 ,a 2 ), the oxide layer thickness of the ablative pure TaB 2 is about 3.0 µm, while the penetration thickness of XRD equipment generally is up to tens of microns, which is the reason why the XRD curve of TaB 2 presented both peaks of TaB 2 and Ta 2 O 5 .The oxide layer of the ablative pure TaB 2 appeared to have large cracks and even the separation between oxide layers, which consisted of many smaller grains of Ta 2 O 5 , were filled or covered by glassy B 2 O 3 .Compared to the surface of the ablative pure TaB 2 , the glassy B 2 O 3 was detected in its vertical cross-section, because the temperature of UHTCs was dropped rapidly after the torch left and a certain amount of B 2 O 3 remained in the oxide layers of the UHTCs before being volatilized [6].It is noteworthy that a hole caused by the low sinterability of TaB 2 was found in the pure TaB 2 ceramic matrix, and a bright brim around the hole was detected due to higher second electron yield at the edge of the hole [19].From Figure 9(b 1 ,b 2 ), the oxide layer about 2.0 µm thick, small cracks, as well as glassy B 2 O 3 , are found, finding that 95TaB 2 -5TaC possessed a better ablation resistance than that of pure TaB 2 .As shown in Figure 9(c 1 ,c 2 ), the oxide layer thickness of the ablative 95TaB 2 -5TaC-0.5MWCNTs is about 2.5 µm; meanwhile, a few MWCNTs, glassy B 2 O 3 , are also observed.The oxide layer thickness of the ablative 95TaB 2 -5TaC-0.5MWCNTs-0.2PAA is about 1.2 µm (see Figure 9d 1 ), and the oxide layer is relatively dense, and MWCNTs are scarcely found in the oxide layer (see Figure 9d 2 ). Figure 10 exhibits FESEM image and EDXS elemental mapping of the vertical cross-sectional microstructure of the ablative 95TaB 2 -5TaC-0.5MWCNTs-0.2PAA and the EDXS patterns of the selected regions in Figure 9, confirming that the oxygen content of oxide layers is higher than those of the non-oxide layers.By comparing the oxidation layers of all ablative UHTCs' vertical cross-sections, it was found that 95TaB 2 -5TaC-0.5MWCNTs-0.2PAA had the best ablation resistance, which was consistent with the results of the ablative UHTCs' surface microstructure.The excellent ablative resistance for 95TaB 2 -5TaC-0.5MWCNTs-0.2PAA was due to the dense oxide layer caused by the synergistic effect of the physical pinning of small TaC particles, the sealing of the dispersed MWCNTs, and the flowing B 2 O 3 filled in the spaces of the grains of Ta 2 O 5 .The sealing of the dispersed MWCNTs mainly acted in three ways, namely, surface adsorption, melt wrapping, and reaction fusion (see Equations ( 5) and ( 8)) [29].
Materials 2024, 17, x FOR PEER REVIEW 16 of 20 pinning of small TaC particles, the sealing of the dispersed MWCNTs, and the flowing B2O3 filled in the spaces of the grains of Ta2O5.The sealing of the dispersed MWCNTs mainly acted in three ways, namely, surface adsorption, melt wrapping, and reaction fusion (see Equations ( 5) and ( 8)) [29].

Ablation Mechanism of TaC and MWCNTs Reinforced TaB2-Based UHTCs
The ablation mechanism regarding transition metal-based UHTCs or transition metal-based ceramic composites was reported in the literature [7,14,25,[30][31][32].Generally speaking, the probable ablation mechanism can be summarized into three categories.The first is that small particles are utilized to fill the spaces between large raw material particles for preventing the excessive growth of ceramic grains through physical pinning for improving the relative density of ceramics in the ceramic preparation process, so as to play the role of ablative resistance.The second is that transition metal-based ceramics themselves or added secondary phases are oxidized into the flowing liquid oxides (such as B2O3, SiO2, and so on) for filling in the spaces of ceramic grains under the environment of high temperature and oxygen, in order to actively form the dense oxide layers retarding the transfer of heat and oxygen into the interior of the ceramic.The third is to use sealing materials, such as carbon nanotubes or graphene oxide, to seal the holes between ceramic grains and then promote the formation of relatively dense oxide layers for improving the anti-ablative performance of ceramics.
Scheme 1 presents the ablation mechanism of the 95TaB2-5TaC-0.5MWCNTs-0.2PAA,which includes all three ways to improve the ablation resistance.First, TaB2 was oxidized into forming liquid B2O3 and solid Ta2O5 at high temperature, and liquid B2O3 filled the spaces of solid Ta2O5 grains, generating the dense oxide layers.Second, fine TaC particles filled the spaces between TaB2 particles to promote the formation of the smaller ceramic grains and higher relative density, so as to enhance the ablation resistance.Third, MWCNTs were well dispersed in TaB2 and TaC powders by the dispersion of PAA to form 95TaB2-5TaC-0.5MWCNTs-0.2PAA with a good sealing for the ceramic holes and cracks, and then the good sealing impeded the diffusion of oxygen and heat into the interior of the ceramic matrix, thus resulting in further improving the ablation resistance.Hence, in addition to the flowing B2O3 filling in the spaces of the grains of Ta2O5, the fine TaC particles and the dispersed MWCNTs collaboratively further improved the ablation resistance of the TaB2-based HUTCs.

Ablation Mechanism of TaC and MWCNTs Reinforced TaB 2 -Based UHTCs
The ablation mechanism regarding transition metal-based UHTCs or transition metalbased ceramic composites was reported in the literature [7,14,25,[30][31][32].Generally speaking, the probable ablation mechanism can be summarized into three categories.The first is that small particles are utilized to fill the spaces between large raw material particles for preventing the excessive growth of ceramic grains through physical pinning for improving the relative density of ceramics in the ceramic preparation process, so as to play the role of ablative resistance.The second is that transition metal-based ceramics themselves or added secondary phases are oxidized into the flowing liquid oxides (such as B 2 O 3 , SiO 2 , and so on) for filling in the spaces of ceramic grains under the environment of high temperature and oxygen, in order to actively form the dense oxide layers retarding the transfer of heat and oxygen into the interior of the ceramic.The third is to use sealing materials, such as carbon nanotubes or graphene oxide, to seal the holes between ceramic grains and then promote the formation of relatively dense oxide layers for improving the anti-ablative performance of ceramics.
Scheme 1 presents the ablation mechanism of the 95TaB 2 -5TaC-0.5MWCNTs-0.2PAA,which includes all three ways to improve the ablation resistance.First, TaB 2 was oxidized into forming liquid B 2 O 3 and solid Ta 2 O 5 at high temperature, and liquid B 2 O 3 filled the spaces of solid Ta 2 O 5 grains, generating the dense oxide layers.Second, fine TaC particles filled the spaces between TaB 2 particles to promote the formation of the smaller ceramic grains and higher relative density, so as to enhance the ablation resistance.Third, MWCNTs were well dispersed in TaB 2 and TaC powders by the dispersion of PAA to form 95TaB 2 -5TaC-0.5MWCNTs-0.2PAA with a good sealing for the ceramic holes and cracks, and then the good sealing impeded the diffusion of oxygen and heat into the interior of the ceramic matrix, thus resulting in further improving the ablation resistance.Hence, in addition to the flowing B 2 O 3 filling in the spaces of the grains of Ta 2 O 5 , the fine TaC particles and the dispersed MWCNTs collaboratively further improved the ablation resistance of the TaB 2 -based HUTCs.

Figure 5 .
Figure 5. TGA curves (a) and DTG curves (b) of the UHTCs under air.

Figure 10 .
Figure 10.FESEM image (a) and EDXS elemental mapping (b-e) of the vertical cross-sectional microstructure of the ablative 95TaB2-5TaC-0.5MWCNTs-0.2PAA, with the EDXS patterns of the selected regions in Figure 9 (f,g).

Table 1 .
A summary of average grain size, relative density, elastics modulus, and nano hardness of UHTCs.

Table 2 .
Linear ablation rate (R l ) and mass ablation rate (R m ) of UHTCs.