Processing, Microstructure and Mechanical Properties of TiB₂-MoSi₂-C Ceramics

Abstract

Titanium boride (TiB₂) is a material classified as an ultra-high-temperature ceramic. The TiB₂ structure is dominated by covalent bonds, which gives the materials based on TiB₂ very good mechanical and thermal properties, making them difficult to sinter at the same time. Obtaining dense TiB₂ polycrystals requires a chemical or physical sintering activation. Carbon and molybdenum disilicide (MoSi₂) were chosen as sintering activation additives. Three series of samples were made, the first one with carbon additives: 0 to 4 wt.%; the second used 2.5, 5 and 10 wt.% MoSi₂; and the third with both additions of 2 wt.% carbon and 2.5, 5 and 10 wt.% MoSi₂. On the basis of the dilatometric sintering analysis, all additives were found to have a favourable effect on the sinterability of TiB₂, and it was determined that sintering TiB₂ with the addition of carbon can be carried at 2100 °C and with MoSi₂ and both additives at 1800 °C. The polycrystals were sintered using the hot-pressing technique. On the basis of the studies conducted in this work, it was found that the addition of 1 wt.% of carbon allows single-phase TiB₂ polycrystals of high density (>90%) to be obtained. The minimum MoSi₂ addition, required to obtain dense sinters with a cermet-like microstructure, was 5 wt.%. High density was also achieved by the materials containing both additives. The samples with higher MoSi₂ content, i.e., 5 and 10%, showed densities close to 100%. The mechanical properties, such as Young’s modulus, hardness and fracture toughness (K_Ic), of the polycrystals and composites were similar for samples with densities exceeding 95%. The Vickers hardness was 23 to 27 GPa, fracture toughness (K_IC) was 4 to 6 MPa·m^0.5 and the Young’s modulus was 480 to 540 GPa. The resulting TiB₂-based materials showed potential in high-temperature applications.

1. Introduction

Ceramic materials classified as ultra-high-temperature ceramics (UHTCs) are characterised by a high melting point, good mechanical properties at high temperature and high oxidation resistance. As they are increasingly used, more demands are put on them [1,2]. The group of materials classified as UHTCs includes metal borides in the fourth group of the periodic table of chemical elements, i.e., TiB₂, ZrB₂ and HfB₂. These borides have very high melting points (approx. 3000 °C), good thermal and electrical conductivity, high hardness, good mechanical properties and oxidation resistance. These valuable properties of AlB₂-type borides are the result of dominant covalent bonds present in their structure, which, on the other hand, has a negative effect on the sinterability of boride ceramics [1,2,3,4].

Improved sinterability of boride ceramics is achieved by using sintering additives (sintering activators), such as nitrides TiN, AlN, Si₃N₄ or HfN [2,5,6,7,8,9]; carbides TaC, SiC, B₄C [2,10,11,12,13,14,15]; silicides MoSi₂, TiSi₂, TaSi₂ [4,16,17,18,19,20,21,22,23]; or oxides like ZrO₂ [24,25,26,27,28]. MoSi₂ and SiC are the most commonly used sintering activators for boride ceramics including TiB₂ [2,4,13,15,16,17,21]. The effect of silicide additives on the sinterability and properties of TiB₂ was, among others, studied by Raju, Murthy et al. [17,19]. Through the introduction of 2.5% MoSi₂ additive and the use of the hot-pressing technique, the authors of the discussed papers obtained dense composites at 1700 °C. According to the authors, MoSi₂ removes oxide impurities and then deforms the plasticity due to high temperature, filling the spaces between TiB₂ grains. In another paper, Murthy et al. [4] investigated the materials sintered by hot pressing with 0 to 25% MoSi₂ addition, leading to samples with density close to 98%, grain sizes of 2–5 µm, high hardness of 25–27 GPa and fracture toughness of 5.1 MPa·m^0.5. The optimum amount of MoSi₂ was 10%, which resulted in a fine-grained microstructure of the sinters and hardness of 27 GPa.

Although carbon is a well-known additive that activates the sintering of covalent ceramics, it has been only marginally investigated for borides [29,30,31]. Also, the combination of carbon additives with other additives has been rarely studied [32,33,34,35]. Among others, Khoeini et al. [34] studied the effect of SiC and carbon on the sinterability of ZrB₂. The authors found that the addition of SiC effectively activates the sintering, but its simultaneous use with a small amount of carbon (1–2%) led to a lower addition of SiC and density close to 100%.

In the present study, it was decided to extend the state of knowledge on TiB₂ sintering. Investigations related to the influence of a combination of MoSi₂ and carbon additives on the sinterability of TiB₂ were carried out. The resulting high-density sinters were then tested with regard to their mechanical properties.

2. Materials and Experimental Procedure

The samples were prepared from commercial powders: TiB₂, ABCR Company, Germany (GRADE F, cat.no AB 134577); MoSi₂, Morton Thiokol, USA (99%, cat.no 48108). As a carbon precursor, phenol-formaldehyde resin of the NOVOLAK type (Chemical Plants Organika, Organika-Sarzyna, Poland) was used. During sintering, it undergoes pyrolysis, leaving 50 wt.% of amorphous carbon.

A reference sample of pure TiB₂ and three series of samples with various additive additions were made. The first series featured the addition of 1%, 2%, 3% and 4% wt. carbon; the second with the addition of 2.5%, 5% and 10% wt. MoSi₂; and the third one containing a constant amount of 2% wt. of carbon and 2.5%, 5% or 10% wt. of MoSi₂. The sample denominations used in this work are summarised in

Table 1. Denomination of the samples.

The Initial Composition	Name
TiB2	TB_0
TiB2 + 1% C	TB_1C
TiB2 + 2% C	TB_2C
TiB2 + 3% C	TB_3C
TiB2 + 4% C	TB_4C
TiB2 + 2.5% MoSi2	TB_2.5MS
TiB2 + 5% MoSi2	TB_5MS
TiB2 + 10% MoSi2	TB_10MS
TiB2 + 2% C + 2.5% MoSi2	TB_2C_2.5MS
TiB2 + 2% C + 5% MoSi2	TB_2C_5MS
TiB2 + 2% C + 10% MoSi2	TB_2C_10MS

.

The powder mixture components were weighed and then homogenised in ethanol in a ball mill for 12 h, using SiC spherical grinding media. Then, the alcohol was evaporated, and the powder mixtures were granulated by passing through a nylon 6 sieve. Cylindrical samples with a diameter of 12 mm and height 3–4 mm were formed through uniaxial double-ended pressing and then subjected to the dilatometric analysis.

The dilatometric sintering analysis was performed in a high-temperature graphite dilatometer of the authors’ own construction. Sintering in the dilatometer was carried out in an argon flow with a heating rate 10 °C/min. The end of sintering happened when a characteristic plateau appeared on a linear dimensional change as a function of temperature. The dilatometric analysis enabled the final temperature of hot pressing to be determined.

The granulated powders were hot pressed using graphite dies under argon flow using Thermal Technology Inc. press model HP916-G-G. The reference sample and the samples with carbon addition were sintered at 25 MPa at 2100 °C, and the samples with MoSi₂ and MoSi₂ and carbon addition were sintered at 25 MPa at 1800 °C. All samples were kept at the final temperature for one hour. The heating rate in each case was 10 °C/min.

Apparent density of the sintered samples was measured using the Archimedes method. The relative density was calculated using 4.52 g/cm³ as the theoretical density of TiB₂. The surface of the samples was ground and polished using LaboPol (Struers, Champigny, France) polishing machine. Their microstructure was analysed using Scios2 DualBeam (ThermoFisher, Waltham, MA, USA) SEM microscope along with the EDS chemical analysis. The observations were made using the Circular Backscatter Electrons (CBS) detector.

In order to determine the phase composition, XRD analysis was performed with the X’Pert Pro apparatus (PANanlitycal, Malvern, UK). The quantitative phase composition of the sinters was determined using the Rietveld method. Hardness measurements were carried out by the Vickers method, using a FV-810 (Future-Tech, Tokyo, Japan) hardness tester. A standard load of 1 kg and an indenter pressing time of 10 s were used. Fracture toughness (K_Ic) was determined using the indentation method at 3 kg load. The Niihara formula (Equation (1)) was used to calculate the critical stress intensity factor (K_Ic).

$${\mathrm{K}}_{\mathrm{Ic}}=0.018·\frac{{\mathrm{HV}}^{0.6}\cdot {\mathrm{E}}^{0.4}\cdot 0.5\mathrm{d}}{{\mathrm{l}}^{0.5}}$$

(1)

where:

E—Young’s modulus, MPa

HV—Vickers hardness, MPa

d—the indentation diagonal, m

l—mean radial cracks length, m

Young’s modulus measurements were carried out using the ultrasonic method by measuring the velocity of transverse (C_T) and longitudinal (C_L) waves passing through the specimen with EPOCH 3 (Panametrics) ultrasonic defectoscope. The defectoscope, equipped with broadband ultrasonic transducers for longitudinal and transverse waves, was used to measure the transit times of the ultrasonic waves through the specimen. The transducers, characterised by short pulse durations, enable measurements on the specimens with small thicknesses to be carried out, eliminating overlapping of the successive pulses. Young’s modulus was calculated from the velocities of ultrasonic wave propagation in the sample and density of the material (Equation (2)).

$$\mathrm{E}=\mathsf{\rho }\cdot {\mathrm{C}}_{\mathrm{T}}^2\frac{3{\mathrm{C}}_{\mathrm{L}}^2-4{\mathrm{C}}_{\mathrm{T}}^2}{{\mathrm{C}}_{\mathrm{L}}^2-{\mathrm{C}}_{\mathrm{T}}^2}$$

(2)

where:

E—Young’s modulus, GPa

C_L—velocity of the longitudinal ultrasonic wave, km/s

C_T—velocity of the transverse ultrasonic wave, km/s

ρ—apparent density of the material, g/cm³

3. Results and Discussion

3.1. Dilatometric Analysis

Figure 1 shows the dilatometric sintering (sample shrinkage vs. temperature) curves, obtained for a series of samples with carbon addition and the reference sample. Sintering in the dilatometer was carried out up to 2150 °C, and despite such a high sintering temperature, sintering curves of the reference sample and the sample with 1 wt.% carbon addition do not show the plateau characteristic for the end of sintering.

Flattening of the sintering curve can be observed for the samples with a carbon addition higher than 1 wt.%. Sintering of the samples with carbon additions between 2 and 4 wt.% ends at ca. 2050 °C.

Figure 2 presents the sintering curves of the samples with MoSi₂ and MoSi₂ and carbon addition.

The course of curves of the samples with MoSi₂ indicate that this addition effectively activates TiB₂ sintering (Figure 2a). When the MoSi₂ additive is 2.5 and 5 wt.%, the end of sintering occurs near 2100 °C, while a significant reduction in sintering temperature occurs when the additive is 10 wt.% because the temperature is close to 1950 °C. The best sintering results are given by the combination of carbon and MoSi₂ additives. For the samples containing 2 wt.% of carbon and 5 or 10 wt.% of MoSi_2, the characteristic plateau, indicating the end of sintering, occurs around 1700–1750 °C. The sample containing 2 wt.% carbon and 2.5 wt.% MoSi₂ addition shows the lowest linear shrinkage 8%, and its sintering ends near 1800 °C (Figure 2b). The beginning of sintering of the reference sample and the samples with carbon addition is in a range of 1300–1400 °C, while for the samples with MoSi₂ and carbon and MoSi₂ addition, it occurs in a temperature range of 1200–1350 °C. Based on the results of the dilatometric measurements, the temperature of hot pressing for the different samples was established.

3.2. Sintering of TiB₂ with Various Amounts of Carbon

The samples with carbon addition and the reference sample were sintered by hot pressing (HP) at 2100 °C. The relative density of the sinters is shown in Figure 3.

The reference sample shows the lowest relative density of 88%, and the introduction of 1 wt.% of carbon addition significantly increases the density (Figure 3). The highest density, around 98%, is achieved by the sinter with 2 wt.% carbon addition, while the density of the sinters with 3 and 4 wt.% carbon additions slightly decreases.

Figure 4 shows SEM microphotographs of the reference sample, the microstructure of which correlates with the measured density. The sample microstructure is inhomogeneous, and significant porosity is visible (black areas).

Figure 5 shows microstructures of the samples with carbon addition between 1% and 4%. The microstructures are homogeneous and characteristic for dense sinters. Black areas visible in the microstructure may be pores but also carbon or carbide inclusions, as confirmed by the EDS analysis (Figure 6).

In view of the XRD phase composition analysis, all the obtained polycrystals consist of 100% of TiB₂. Carbon and carbides may not be detected, as their amount in the composites may lay below the detection threshold of the XRD method.

3.3. Sintering of TiB₂ with Various Amounts of MoSi₂

TiB₂ samples with MoSi₂ additive were hot pressed at 25 MPa at 1800 °C. Figure 7 shows the dependence of relative density of the sinters on the amount of MoSi₂ additive. The lowest density, slightly lower than that of the reference sample, is shown by the composite with 2.5 wt.% MoSi₂ addition. A significant increase in density is observed for the samples with 5 and 10 wt.% addition of MoSi₂. Their relative density is 100%, which clearly indicates good sintering activation of TiB₂ by MoSi₂ (Figure 7).

Figure 8 shows SEM images of the TiB₂ composites containing MoSi₂. Their microstructures correlate well with the relative density. The highest porosity is found in the sample with the lowest amount, i.e., 2.5 wt.% of MoSi₂. The micrographs of the sample clearly show the presence of pores (the darkest areas) (Figure 8a). The number of black areas indicating the presence of pores in the samples with 5 and 10 wt.% MoSi₂ addition is relatively low (Figure 8b,c). It can be said that pores are absent in the sample with 5 wt.% MoSi₂ addition (Figure 8b).

Based on the XRD phase composition analysis (

Table 2. Quantitative phase composition of TiB₂ + MoSi₂ composites.

ICSInitial Phase Composition, wt.%	Phase Composition of the HP Sinters, wt.%
97.5% TiB2, 2.5% MoSi2	68.8% TiB2 1, 29.7% TiB2 2, 1.5% MoC
95% TiB2, 5.0% MoSi2	69.8% TiB2 1, 28.7% TiB2 2, 1.5% MoC
90% TiB2, 10% MoSi2	83.0% TiB2 1, 9.4% TiB2 2, 1.0% MoC, 6.6% MoSi2

), it can be concluded that TiB₂ dominates in all samples. In the samples containing 2.5 wt.% and 5 wt.% MoSi₂, negligible amounts of MoC are also identified. In contrast, MoSi₂ and MoC can be identified in the sample containing 10 wt.% MoSi₂. The presence of molybdenum carbides is the result of a reaction between MoSi₂, oxide impurities and carbon from the graphite foil and the graphite die (HP).

Furthermore, in the light of the XRD analysis, titanium borides with the same structure but different lattice parameters are present in the composites (

Table 3. Lattice parameters of titanium boride phases identified in TiB₂ + MoSi₂ composites.

Lattice Parameter, Å	Theoretical Unit Cell Parameters of TiB2, [36]	TiB2 + 2.5% MoSi2		TiB2 + 5.0% MoSi2		TiB2 + 10% MoSi2
		TiB2 1	TiB2 2	TiB2 1	TiB2 2	TiB2 1	TiB2 2
a	3.028	3.030	3.028	3.030	3.030	3.029	3.029
b	3.028	3.030	3.028	3.030	3.030	3.029	3.029
c	3.228	3.230	3.230	3.230	3.231	3.231	3.230

TiB₂ 1 and TiB₂ 2—TiB₂ with the same structure but different lattice parameters.

, TiB₂ 1 and TiB₂ 2).

The differences in hues of grey visible in the SEM images indicate differences in the chemical composition of individual areas of the samples. Figure 9 shows the result of a local chemical composition analysis of the TiB₂ composite containing 10 wt.% MoSi₂, and the results of the chemical composition analysis are consistent with the results of the phase composition analyses of the sample (Table 2).

In the sinters, grains cores rich in titanium and boron can be identified, which are most likely titanium diboride (Figure 9a). There are also areas in which, in addition to titanium, molybdenum and carbon (Figure 9b) as well as molybdenum and silicon (Figure 9c) can be found. An increased amount of molybdenum is identified in light-grey areas around TiB₂ grains (Figure 9c). There are also the darkest areas, which, in many cases, are pores but not always, as Figure 9d shows. In many such areas, significant amounts of oxygen and silicon can be identified. The reaction between MoSi₂ and the oxides which passivate the boride grains can result in the formation of an amorphous phase from the Si-O-B system [37,38,39].

3.4. Sintering of TiB₂ with 2 wt.% Carbon Addition and Various Amounts of MoSi₂

TiB₂ samples, containing 2 wt.% carbon and different MoSi₂ additions, were hot pressed at 1800 °C. The relative densities of the resulting polycrystals are shown in Figure 10.

The lowest density ca. 90% was exhibited by the sample with the lowest MoSi₂ content. An increase in MoSi₂ addition to 5 wt.% led to composites with densities higher than 98%. The relative density of 100% was achieved by the composite with the highest MoSi₂ addition, i.e., 10 wt.% (Figure 10).

The combination of carbon and MoSi₂ additions results in significantly increased density of the composite with 2.5 wt.% MoSi₂ addition, as compared to the analogous composite without carbon addition (Figure 7).

Figure 11 shows SEM microstructures of the TiB₂ samples with a simultaneous addition of carbon and MoSi₂. In the sample with 2.5 wt.% MoSi₂ addition, significant porosity is visible (darkest areas). A homogeneous, dense microstructure is presented by the samples with 5 and 10 wt.% MoSi₂ additions (Figure 11b,c). Again, the microstructure of TiB₂ composites with carbon and different amounts of MoSi₂ addition is similar to the one typical for cermets [40], i.e., grains consisting of cores and characteristic shells (Figure 11).

According to the results of XRD phase composition analysis, the composites after sintering are dominated by titanium boride with different elemental cell sizes (

Table 4. Quantitative phase composition of TiB₂ + 2%C + x%MoSi₂ composites.

Initial Phase Composition, wt.%	Phase Composition of the HP Sinters, wt.%
95.5% TiB2, 2.0% C, 2.5% MoSi2	96.8% TiB2 1, 0.1% TiB2 2, 1.7% TiC, 1.4% SiC
93% TiB2, 2.0% C, 5.0% MoSi2	66.3% TiB2 1, 28.9% TiB2 2, 1.8% SiC, 3.0% (Ti,Mo)C2
88% TiB2, 2.0% C, 10% MoSi2	76.1% TiB2 1, 18.6% TiB2 2, 3.1% SiC, 2.2% (Ti,Mo)C2

and

Table 5. Lattice parameters of titanium boride phases identified in TiB₂ + 2% C + x% MoSi₂ composites.

Lattice Parameter, Å	Theoretical Unit Cell Parameters of TiB2, [36]	TiB2 + 2%C +2.5% MoSi2		TiB2 + 2%C +5.0% MoSi2		TiB2 + 2%C +10% MoSi2
		TiB2 1	TiB2 2	TiB2 1	TiB2 2	TiB2 1	TiB2 2
a	3.028	3.027	3.034	3.029	3.027	3.029	3.027
b	3.028	3.027	3.034	3.029	3.027	3.029	3.027
c	3.228	3.233	3.220	3.230	3.231	3.230	3.232

TiB₂ 1 and TiB₂ 2—TiB₂ with the same structure but different lattice parameters.

). The presence of two hexagonal TiB₂ phases (here named TiB₂ 1 and TiB₂ 2) with different lattice parameters may indicate substitutions within the boride cell by the additive-derived elements, i.e., Mo, Si and C. Furthermore, MoSi₂ is not present after sintering. It is likely that during sintering, chemical reactions occur between TiB₂, the oxide impurities, i.e., TiO₂ and B₂O_3, and the additives, i.e., MoSi₂ and carbon. These reactions resulted in the formation of silicon carbide and complex carbide (Ti, Mo)C₂ (Table 4).

The EDS chemical analysis of the selected sample (TB_2C_10MS) is shown in Figure 12. The darkest areas could be pores but also oxide or carbide grains since the areas are rich in oxygen, carbon, silicon and titanium as well as molybdenum. The dark-grey areas (cores) are most likely TiB₂ grains, while the light-coloured grain shells and grain boundary areas are rich in molybdenum, titanium, boron and, in some places, also carbon.

3.5. Mechanical Properties of the TiB₂-MoSi₂-C Ceramics

The composites were tested for Vickers hardness, critical stress intensity factor (K_Ic), which is a measure of fracture toughness, and Young’s modulus. Due to the highly subjective measurement of the critical stress intensity factor using the indentation method, this was carried out on the composites with a relative density higher than 95%. The obtained results are summarised in

Table 6. Relative density and selected mechanical properties of the materials.

Sample	Sintering Temperature (HP), °C	Relative Density, % (*)	Vickers Hardness, GPa	Fracture Toughness, MPa·m0.5	Young’s Modulus, GPa
TB_0	2150	88.2 ± 0.3	19.09 ± 6.30	-	-
TB_1C	2150	94.3 ± 0.1	26.31 ± 5.86	-	526 ± 12
TB_2C	2150	97.8 ± 0.1	25.31 ± 0.77	5.16 ± 0.28	536 ± 9
TB_3C	2150	96.9 ± 0.4	23.34 ± 2.17	5.26 ± 0.47	496 ± 16
TB_4C	2150	95.9 ± 0.3	25.68 ± 5.29	5.52 ± 0.20	542 ± 10
TB_2.5MS	1800	84.3 ± 0.6	16.97 ± 2.86	-	-
TB_5MS	1800	98.5 ± 0.2	26.21 ± 2.25	6.25 ± 0.51	536 ± 11
TB_10MS	1800	100.0 ± 0.4	26.78 ± 3.37	4.86 ± 0.19	504 ± 24
TB_2C_2.5MS	1800	89.1 ± 0.8	17.19 ± 1.87	-	440 ± 14
TB_2C_5MS	1800	98.8 ± 0.4	24.88 ± 2.03	4.79 ± 0.52	543 ± 6
TB_2C_10MS	1800	100.0 ± 0.2	24.41 ± 1.90	4.17 ± 0.31	533 ± 12

(*) the theoretical density of TiB₂ was 4.52 g/cm³.

.

The hardness of the samples with the lowest densities, i.e., the reference sample (TB_0), with 2.5% MoSi₂ addition (TB_2.5MS) and with 2% carbon and 2.5% MoSi₂ addition, is not higher than 20 GPa (Table 6). The introduction of 1% carbon results in a noticeable increase in hardness to 26 GPa. The hardness of composites with 1 to 4% carbon addition is similar and ranges from 23 to 26 GPa. For the composites with MoSi₂ addition, the lowest hardness is shown by TB_2.5MS and TB_2C_2.5MS samples. For other composites with MoSi₂ addition, the hardness ranges from 24 to 26 GPa.

Based on the measured values of the critical stress intensity factor (Table 6), it can be concluded that the composites exhibit high fracture toughness. The lowest K_Ic values, larger than 4 MPa·m^0.5, are shown by the composites with both additives, i.e., carbon and MoSi₂ additives. The K_Ic values, close to 5 MPa·m^0.5, are exhibited by the composites with carbon additions of 2 to 4%. In contrast, K_Ic values between 4.86 and 6.25 MPa·m^0.5 are shown by the composites with the MoSi₂ additive.

The Young’s modulus values of all composites are high, ranging from 496 GPa for the TB_3C composite to 543 GPa for the TB_2C_5MS composite.

4. Results and Discussion

On the basis of the results of these investigations, a favourable effect of all used additives, i.e., carbon, MoSi₂ and MoSi₂+C, on the sinterability of TiB₂ titanium diboride was found. Firstly, a dilatometric sintering analysis was carried out, which showed the validity of the abovementioned sintering activators (Figure 1 and Figure 2). Furthermore, it is clear from the dilatometric measurements that the use of 10% MoSi₂ addition and the combination of both additives significantly reduces the sintering temperature of TiB₂ (Figure 2). The sintering temperature of polycrystals with carbon and MoSi₂ additives does not exceed 1800 °C. For the polycrystals with MoSi₂, only a 10% addition of MoSi₂ reduces the sintering temperature to 1900 °C (Figure 2).

According to the dilatometric sintering analysis, the hot-pressing temperatures of all composites were determined. The composites with carbon as well as the reference sample were hot pressed at 2100 °C, while the composites with MoSi₂ and the composites with carbon and MoSi₂ were hot pressed at 1800 °C. In many cases, polycrystals with a relative density of 100% were obtained. In the case of carbon addition, the highest densities were obtained when the carbon addition was between 2 and 4 wt.%. Dense polycrystals were produced by adding 5 or 10 wt.% MoSi₂ as well as 2 wt.% carbon together with 5 or 10 wt.% MoSi₂ (Figure 3, Figure 7 and Figure 10).

Most of the obtained polycrystals (sinters) were non-porous, as can be seen from the microstructure analysis (Figure 5, Figure 8 and Figure 11), while it cannot be deduced from the apparent density measurements and relative density calculations (Table 6). One of the main reasons for the precise calculation of the theoretical density is the numerous reactions that take place during sintering between TiB₂, MoSi₂, carbon and oxide impurities, resulting in the formation of new phases, as confirmed by the results of the phase composition analysis (Table 2 and Table 4). Therefore, taking the density calculated from the initial compositions as the theoretical density of the material in question appears to be incorrect. The calculation of the theoretical density from the phase composition analysis, due to the presence of solid solutions and amorphous phases, is also erroneous. For the sake of comparison, a single value of the theoretical density was used, and the theoretical density of TiB₂, i.e., 4.52 g/cm³, was taken as the theoretical density. High density of the sinters was indirectly evidenced by water absorption measurements (

Table 7. Water absorbability of the materials.

Sample	Water Asorbability, %
TB_0	0.03 ± 0.01
TB_1C	0.03 ± 0.01
TB_2C	0.01 ± 0.02
TB_3C	0.15 ± 0.04
TB_4C	0.08 ± 0.03
TB_2.5MS	3.69 ± 0.50
TB_5MS	0.05 ± 0.02
TB_10MS	0.04 ± 0.01
TB_2C_2.5MS	1.29 ± 0.20
TB_2C_5MS	0.08 ± 0.02
TB_2C_10MS	0.06 ± 0.01

). Water absorption of many polycrystals reaches only a few hundredths of one percent and is indicative of the lack of open porosity in these materials.

The addition of carbon enables single-phase polycrystals to be obtained, as evidenced by the phase composition analysis, according to which only TiB₂ is present in such sinters. Also, the microstructures shown in Figure 5 are characteristic of single-phase and dense sinters when carbon addition is in a range of 2 to 4 wt.%. Phase composition analyses demonstrate the effectiveness of using carbon as an oxide impurity reducer. According to the literature [41], carbon can react both with B₂O₃ and SiO₂ at the temperatures close to 1000 °C in low vacuum (20 Pa). The local EDS analysis of the chemical composition, carried out during SEM observations, showed the trace presence of fine particles, whose chemical composition suggests that they may be the particles of carbon or boron and titanium carbides (Figure 6).

Carbon in the sintering of covalent ceramics acts as a reducer of oxide impurities [29,30,31,32,33,34,35]. When it was intentionally added, the reaction between oxide impurities, i.e., SiO₂ or TiO₂, resulted in the formation of SiC and TiC and complex titanium-molybdenum carbide, respectively (Table 4). When carbon addition was not intentionally added, the favoured reaction was the formation of MoC. In this case, carbon can be regarded as an impurity from the system with which sintering was carried out, i.e., from the heating element, matrix or graphite film. In addition, in composites with only MoSi₂, oxide impurities are present in the form of an amorphous Si-O-B-Ti-Mo phase, which is not identified by XRD analysis but visible in SEM images (Figure 9d and Figure 12).

The phase composition of the polycrystals sintered with MoSi₂ (Table 2) and with carbon and MoSi₂ additives shows that such composites (Table 4) were obtained, in which two TiB₂ phases with different lattice parameters dominated (Table 3 and Table 5). In addition, molybdenum carbide was identified in all composites sintered with MoSi₂ only, and only the sample with the highest MoSi₂ addition showed the presence of this additive. In contrast, for the composites with both additives, the two titanium boride phases with different lattice parameters were dominant (Table 5, Supplementary Materials). Silicon and titanium carbides were also identified in these composites, and a complex carbide with the formula (Mo,Ti)C₂ was found in the samples with 5 and 10% wt. MoSi₂ additions. Substitutions of titanium (a_r = 140 pm) by Mo (a_r = 145 pm), Si (a_r = 145 pm) cations as well as boron (a_r = 75 pm) by carbon (a_r = 70 pm) can led to the presence of hexagonal titanium boride phases with different lattice parameters or, in other words, to the presence of solid solutions [42]. The presence of (Mo,Ti,Si)B₂ solid solutions can be evidenced by a microstructure similar to that of core–shell cermets [40]. This type of microstructure is often found in composites based on metal borides in the fourth group of the periodic table of chemical elements [21,22,23,43,44,45,46]. SEM photographs of TB_MS and TB_C_MS composites (Figure 8 and Figure 11) show microstructures similar to those typical for cermets. The microstructure of cermets typically consists of particles with “hard cores” and solid solution layers on their surface (“shells”). In typical cermets, such core–shell particles are joined together by bonding metals, e.g., cobalt [22,23,44,46]. With regard to the TiB₂-MoSi₂-C composites studied, it can be assumed that the cores are formed by one of the titanium boride phases with the shell by the titanium boride solid solution with substitutions (Figure 13).

The EDS chemical element distribution maps, made during the SEM observations, showed further that, in both groups of composites with the MoSi₂ additive, there are areas enriched in oxygen and silicon (Figure 9 and Figure 12). During sintering, a variety of reactions can occur in all composites, including the carbothermal reduction of oxides, passivating boride particles (Equations (3) and (4)), and the reactions between MoSi₂ and oxide impurities (Equations (5)–(7)) [4,16,43,47,48], leading, among others, to the formation of silica, monoborides and carbides.

$$Ti{O}_2+3C\to TiC+2CO\uparrow$$

(3)

$$2B_2{O}_3+7C\to B_{4}C+6CO\uparrow$$

(4)

$$2MoS{i}_2+2B_2{O}_3+Ti{O}_2\to Ti{B}_2+4Si{O}_2+2MoB$$

(5)

$$2MoS{i}_2+2B_2{O}_3+2.5C\to 2.5SiC+1.5Si{O}_2+2MoB$$

(6)

$$5MoS{i}_2+7O_2\to M{o}_{5}S{i}_3+7Si{O}_2$$

(7)

As shown by the composite microstructures in Figure 8 and Figure 11, solid solutions can be formed at grain boundaries or, more commonly, at the surface of the boride grains. The literature reports that the formation of the solid solutions in question can be related to the presence of liquid phases with the compositions resulting from the initial chemical composition of the composites [1,17,21,44,47,49,50,51]. The most likely formation of liquid phases is from the Si–B–O system, in which the elements (Ti, Mo), forming the components of the composite, can dissolve. It should be added that in the case of the MoSi₂ additive alone, the passivating oxides do not reduce as readily as under the influence of carbon. During cooling, the epitaxial precipitation from the liquid phase and the formation of solid solutions can occur [4,17,21,44,51]. The occurrence of the solid solutions discussed, as well as silicide, carbide and SiO₂ phases, may be indirect evidence of the presence of liquid phases during the sintering of composites with MoSi₂ addition. Furthermore, oxygen-rich and silicon-rich particles were identified during the chemical composition analysis, even when MoSi₂ and carbon were used as additives (Figure 12).

According to the literature [37,38,39], the occurrence of liquid phases from the Si–B–O system is mainly possible in boride composites with silicide additives. These phases can effectively support sintering under pressure by facilitating, among other things, the movement of grains relative to each other [52]. In this case, if the oxides are not fully reduced, even when a small addition of carbon is introduced, it is possible for a reaction between SiO₂ and B₂O₃ to take place, resulting in the formation of a liquid phase from the Si–B–O system, in which elements present in the initial compounds forming the composites can dissolve.

Furthermore, the sintering temperature of composites with MoSi₂ is 1800 °C and does not exceed the melting point of the silicide (T_m = 2050 °C) [50,53] but, as reported in the literature [52,53,54,55], at a temperature higher than 800 °C, silicides, including MoSi₂, deform plastically and can fill pores during sintering.

The relationship between the density of the composites and the values of the tested mechanical properties is observed. The composites with the highest density, regardless of the additive used, show high Vickers hardness of 23 to 26 Gpa. Also, in terms of fracture toughness, all the composites tested show a high value of the critical stress intensity factor K_Ic (Table 6). The lowest values of K_Ic from 4.18 to 4.79 Mpa·m^0.5 are exhibited by the composites with two additives, fracture toughness of composites with 2 to 4 wt.% carbon addition oscillates around 5 Mpa·m^0.5, while in the composites with MoSi₂, it ranges from 6.25 to 4.86 Mpa·m^0.5 for 5 wt.% MoSi₂ and 10 wt.% MoSi₂ addition, respectively. The typical phenomena leading to the increase in effective fracture energy are observed in composites, such as intergranular cracking (Figure 14a,c), crack deflection as well as crack defragmentation (Figure 14e,f). It is noteworthy that the grain boundaries (bonding phase) in the composites with 5 wt.% MoSi₂ addition are weaker than those in the composites with 10 wt.% MoSi₂ addition (compare Figure 14a with Figure 14b and Figure 14c with Figure 14d). This results in a lower value of the critical stress intensity factor in the composites with 10 wt.% MoSi₂ addition (Table 6). The fracture then runs predominantly through the grains as well as along the TiB₂ grain boundaries (Figure 14b,d).

All high-density composites can be classified as low-deformability materials, as evidenced by the high values of their Young’s modulus (Table 6).

The values of hardness, fracture toughness and Young’s modulus shown for the investigated composites are similar to and often better than those reported in the literature [4,16,47,56].

5. Conclusions

1.

The hot pressing of TiB₂ with MoSi₂ or carbon and MoSi₂ with carbon resulted in single-phase polycrystals and composites with density higher than 95%. Both additives used can be considered as TiB₂ sintering activators.

2.

It is noteworthy that the use of MoSi₂ as a sintering activating additive significantly reduces the sintering temperature of titanium boride down to 1800 °C.

3.

With the addition of carbon, it is possible to obtain single-phase polycrystals, in which only the TiB₂ phase is present. On the other hand, with the addition of MoSi₂ as well as the combined addition of MoSi₂ and carbon, it is possible to obtain solid composites with a core–shell microstructure, characteristic for cermets.

4.

Carbon is an effective reducer of oxide impurities during TiB₂ sintering.

5.

Due to the presence of liquid phases from the Si–B–O–Mo system and plastic deformation of MoSi₂, it becomes possible to obtain dense composites based on TiB₂.

6.

The produced materials have potential in high-temperature applications due to the high melting point of TiB₂ and very good properties, such as high hardness, high fracture toughness and low deformability. Nevertheless, further testing of their thermal and chemical properties is required.