Effects of TiC, Si, and Al on Combustion Synthesis of Ti3SiC2/TiC/Ti5Si3 Composites

The fabrication of Ti3SiC2 from TiC-containing reactant compacts was investigated by combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS). The initial sample composition was formulated based on (3 − x)Ti + ySi + (2 − x)C + xTiC + zAl, with stoichiometric parameters of x from 0 to 0.7, y = 1.0 and 1.2, and z = 0 and 0.1. For all samples studied, combustion was sufficiently exothermic to sustain the reaction in the SHS manner. Due to the dilution effect of TiC, combustion wave velocity and reaction temperature substantially decreased with TiC content. When compared with the TiC-free sample, the TiC-containing sample facilitated the formation of Ti3SiC2 and the TiC content of x = 0.5 produced the highest yield. Excess Si (y = 1.2) to compensate for the evaporation loss of Si during combustion and the addition of Al (z = 0.1) to promote the phase conversion were effective in improving the evolution of Ti3SiC2. All final products were composed of Ti3SiC2, TiC, and Ti5Si3. For the TiC-containing samples of x = 0.5, the weight fraction of Ti3SiC2 increased from 67 wt.% in the sample without extra Si and Al to 72 wt.% in the Si-rich sample of y = 1.2 and further up to 85 wt.% in the Si-rich/Al-added sample of y = 1.2 and z = 0.1. As-synthesized Ti3SiC2 grains were in a thin plate-like shape with a thickness of 0.5–1.0 μm and length of about 10 μm. Ti3SiC2 platelets were closely stacked into a layered structure.


Introduction
MAX phases belong to a family of hexagonal-structure layered ceramics with stoichiometry of M n+1 AX n (usually n = 1, 2, or 3), where M is an early transition metal (Ti, Zr, Nb, Ta, Cr, etc.), A is an element of the IIIA or IVA group (Al, Si, Sn, Ge, etc.), and X is C or N [1].Typical MAX phases such as Ti 2 AlC, Cr 2 AlC, Ti 3 SiC 2 , Ti 3 AlC 2 , Nb 4 AlC 3 , and Ti 4 AlN 3 as well as their associated solid solutions have been broadly investigated [1][2][3][4][5].Properties of the MAX phases combine merits of both metals and ceramics.Like metals, they are excellent thermal and electrical conductors, readily machinable, and highly resistant to thermal shock.Like ceramics, they possess high stiffness, low density, low thermal expansion coefficients, and good corrosion and oxidation resistances [6,7].In addition, higher-order MAX phases with n > 3 such as Ta 5 AlC 4 , Ta 6 AlC 5 , Ti 6 AlC 5 , and Ti 7 SnC 6 have been discovered [8][9][10].
For the improvement of Ti 3 SiC 2 formation, most of the studies have focused on different reactant mixtures, off-stoichiometric compositions, and Al additions.For example, Radhakrishnan et al. [30] synthesized Ti 3 SiC 2 from element powders with a Si-excess composition of Ti:Si:C = 3:1.2:2by reactive sintering at 1800 • C for 10 h.Liu et al. [31] employed TiH 2 , Si, and graphite powders under a Si-rich stoichiometry of Ti:Si:C = 3:1.2:2 to prepare porous Ti 3 SiC 2 through a multi-stage sintering process, within which TiH 2 decomposed to form Ti at 600 • C, followed by the formation of TiC and Ti 5 Si 3 at about 1100 • C, and the production of Ti 3 SiC 2 at 1350 • C. Li et al. [28] indicated that an increase in Si increased the fraction of Ti 3 SiC 2 in a Ti 3 SiC 2 -Ti 5 Si 3 -TiC composite fabricated by reactive plasma spraying.It was believed that excess Si compensated for the evaporation loss of Si at high temperatures and, therefore, led to a higher yield of Ti 3 SiC 2 .With the aid of Al as a reaction promoter in the SPS process, Zhang et al. [32] prepared Ti 3 SiC 2 from an element powder mixture of 3Ti/Si/2C/xAl (x = 0.1-0.3) at 1280 • C, and Sun et al. [33] obtained Ti 3 SiC 2 from the reactant mixtures of 3Ti/SiC/C/xAl (x = 0.15 and 0.2) at 1200 • C. The effect of Al additions on enhancing the formation of Ti 3 SiC 2 was also confirmed by pressureless sintering of reactant mixtures composed of 4Ti/TiC/2SiC/0.2Alat 1380 • C for 1 h [34] and 3Ti/1.5Si/1.9C/0.5Alat 1400 • C for 2 h [35].Moreover, Gubarevich et al. [36] utilized an Al-containing element powder mixture of 3Ti/Si/2C/0.1Alto produce Ti 3 SiC 2 by combustion synthesis.It has been proposed that Al could suppress the evaporation of Si at high temperatures and form a liquid phase in the Ti-Si-C-Al quaternary system, which could facilitate the diffusion of Ti and Si atoms and promote the interactions between TiC and Ti 5 Si 3 or Ti-Si melt [34][35][36].
However, the influence of excess Si and Al on the fabrication of Ti 3 SiC 2 by combustion synthesis with raw materials containing TiC has been little studied.As a first attempt, this study aimed to investigate the effects TiC, Si, and Al on the formation of Ti 3 SiC 2 by solid-phase combustion synthesis in the SHS mode.When compared with other processing methods, the SHS technique takes advantage of highly exothermic reactions to produce MAX phases and has benefits of energy effectiveness, time-saving, simplicity, low cost, scalability, and high-purity products [37][38][39].In this study, starting mixtures comprising different amounts of TiC were prepared with an exact Ti:Si:C = 3:1:2 stoichiometry and Si-rich and Al-added compositions.The effects of TiC content and excess Si and Al were studied on the combustion wave velocity and reaction temperature as well as on the enhancement of Ti 3 SiC 2 formation.

Materials and Methods
The starting materials adopted by this study included Ti (Alfa Aesar, Ward Hill, MA, USA, <45 µm, and 99.8%), Si (Strem Chemicals, Newburyport, MA, USA, <45 µm, and 99.5%), carbon black (Showa Chemical Co., Tokyo, Japan), TiC (Aldrich Chemical, Burlington, MA, USA, <45 µm, and 99%), and Al (Showa Chemical Co., Tokyo, Japan, <45 µm, and 99%).The reactant mixture was formulated based on the following equation: where the parameters x, y, and z signify the numbers of moles of TiC, Si, and Al in the reactant mixture, respectively.In this study, the content of TiC was studied with x = 0, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7.The sample of x = 0 was TiC-free and composed only of elemental Ti, Si, C, and Al powders.Two mole numbers of y = 1.0 and 1.2 were examined for Si; y = 1.0 denoted a sample without extra Si and y = 1.2 represented a sample with Si in excess of 20 mol.%.The value of z defined a sample without (z = 0) or with Al (z = 0.1) additions.The amount of Al included in the reactant mixture was very small.In summary, three sample compositions with different contents of TiC were involved; namely, the exact stoichiometry of Ti:Si:C = 3:1:2, a Si-rich configuration of Ti:Si:C = 3:1.2:2,and a Si-rich/Al-added composition of Ti:Si:C:Al = 3:1.2:2:0.1.Reactant powders were dry-mixed in a ball mill and uniaxially compressed into cylindrical samples with a diameter of 7 mm, a height of 12 mm, and a relative density of 50%.The combustion synthesis experiment was conducted in a windowed stainless-steel chamber under an Ar environment at a pressure of 0.25 MPa.A tungsten coil with an electric voltage of 60 V and a current of 1.5 A was mounted at 1 mm above the sample top surface and was utilized to ignite the sample by thermal radiation.The combustion wave propagation velocity (V f ) was determined from the flame-front trajectory, based on the time sequence of recorded pictures.The exposure time of each recorded image was set at 0.1 ms.To facilitate the accurate measurement of instantaneous locations of the combustion front, a beam splitter with a mirror characteristic of 75% transmission and 25% reflection was used to optically superimpose a scale onto the image of the sample.The combustion temperature was measured by a bare-wire thermocouple (Pt/Pt-13%Rh) with a bead size of 125 µm.Details of the experimental setup and approach have been reported elsewhere [40].The phase composition and microstructure of the final product were analyzed using an X-ray diffractometer (XRD, Bruker D2 Phaser, Karlsruhe, Germany) with CuK α radiation and a scanning electron microscope (SEM, Hitachi, S3000H, Tokyo, Japan), respectively.Based on the XRD peak intensity, the weight fractions of Ti 3 SiC 2 , Ti 5 Si 3 , and TiC in the product were calculated by the following equations [25,41]:  [25,41].The equations derived by Zou et al. [41] were based on the theory of X-ray diffraction, in that the diffraction intensity of ith phase is a function of the mass fraction of the ith phase in the sample and the overall absorption coefficient of the sample.The equations were specifically applied for a quantitative evaluation of the contents of three coexisting phases; namely, Ti 3 SiC 2 , Ti 5 Si 3 , and TiC.The constants of the equations were experimentally determined from the XRD patterns of the powder mixture samples, which were blended from pure Ti 3 SiC 2 , Ti 5 Si 3 , and TiC at weight ratios of 2:4:4, 3:2:5, 4:3:3, and 5:4:1.Representative reflections of Ti 3 SiC 2 (104), Ti 5 Si 3 (102), and TiC (111) were chosen because they did not overlap with other peaks in the XRD patterns [41].

Combustion Wave Kinetics
Experimental observations of this study indicated that combustion wave behavior of the sample compact apparently varied with the TiC content in the starting mixture.For the 3Ti-Si-2C stoichiometric sample with TiC of x = 0.2, Figure 1a shows that upon ignition, a distinct combustion front formed and propagated downward in a self-sustaining manner.As revealed in Figure 1a, the combustion wave traversed the entire sample in about 1.83 s and a slightly bent deformation of the burned sample was observed, perhaps due to a liquid phase being formed during the SHS process.With an increase in TiC content to x = 0.7, as presented in Figure 1b, the combustion wave exhibited a spinning propagation mode.According to Ivleva and Merzhanov [42], this is because the heat flux liberated from self-sustaining combustion is no longer sufficient to maintain the steady propagation of a planar front.There are both thermodynamic and kinetic reasons that provide the basis for departure from a steady condition [43].Thermodynamic considerations arise from the degree of exothermicity of the reaction.Kinetic reasons are largely attributed to insufficient reactivity due to the presence of diffusion barriers.The spinning combustion wave observed in Figure 1b could have been primarily caused by the dilution effect of TiC on reaction exothermicity, which resulted in a longer combustion spreading time of about 4.10 s.It should be noted that the addition of extra Si (y = 1.2) and Al (z = 0.1) had almost no effect on the combustion behavior.
Materials 2023, 16, 6142 4 of 13 the 3Ti-Si-2C stoichiometric sample with TiC of x = 0.2, Figure 1a shows that upon ignition, a distinct combustion front formed and propagated downward in a self-sustaining manner.As revealed in Figure 1a, the combustion wave traversed the entire sample in about 1.83 s and a slightly bent deformation of the burned sample was observed, perhaps due to a liquid phase being formed during the SHS process.With an increase in TiC content to x = 0.7, as presented in Figure 1b, the combustion wave exhibited a spinning propagation mode.According to Ivleva and Merzhanov [42], this is because the heat flux liberated from self-sustaining combustion is no longer sufficient to maintain the steady propagation of a planar front.There are both thermodynamic and kinetic reasons that provide the basis for departure from a steady condition The enthalpy of reaction (ΔHr) and adiabatic combustion temperature (Tad) of Equation (1) with y = 1 and z = 0 were calculated as a function of the TiC content (i.e., x = 0-0.7)from the energy balance equation with thermochemical data [44,45] and plotted in Figure 2.For the TiC-free sample (x = 0), Figure 2 shows that the reaction had the largest ΔHr of −448 kJ and the highest Tad of about 2860 K.The increase in TiC content from x = 0.1 to 0.7 led to a decrease in ΔHr from −430 to −320 kJ and a considerable decline in Tad, from 2754 K to 2182 K.The calculated results of Figure 2 provide an explanation for the observed combustion behavior.The enthalpy of reaction (∆H r ) and adiabatic combustion temperature (T ad ) of Equation ( 1) with y = 1 and z = 0 were calculated as a function of the TiC content (i.e., x = 0-0.7)from the energy balance equation with thermochemical data [44,45] and plotted in Figure 2.For the TiC-free sample (x = 0), Figure 2 shows that the reaction had the largest ∆H r of −448 kJ and the highest T ad of about 2860 K.The increase in TiC content from x = 0.1 to 0.7 led to a decrease in ∆H r from −430 to −320 kJ and a considerable decline in T ad , from 2754 K to 2182 K.The calculated results of Figure 2 provide an explanation for the observed combustion behavior.Figure 3 plots the temperature profiles measured from the samples without extra Si and Al, but containing different amounts of TiC.A temperature profile detected from a TiC-free element powder compact (x = 0) was included for comparison.All profiles exhibited a steep temperature rise followed by a rapid descent, which is typical of the SHS reaction that features a fast combustion wave and a thin reaction zone.The peak value was considered as the combustion front temperature (Tc).Due to the dilution effect of TiC on combustion, Figure 3 shows a decrease in peak combustion temperature from 1464 °C for the TiC-free sample to about 1200 °C for the TiC-containing sample of x = 0.7.The descending trend was consistent with the calculated adiabatic temperature.On the other hand, the addition of extra Si and a small amount of Al essentially had little influence on the combustion temperature.Experimental measurements indicated that the temperature variation induced by adding extra Si and Al was within ± 20 °C of the value detected from the 3Ti-Si-2C stoichiometric sample.Figure 3 plots the temperature profiles measured from the samples without extra Si and Al, but containing different amounts of TiC.A temperature profile detected from a TiC-free element powder compact (x = 0) was included for comparison.All profiles exhibited a steep temperature rise followed by a rapid descent, which is typical of the SHS reaction that features a fast combustion wave and a thin reaction zone.The peak value was considered as the combustion front temperature (T c ). Due to the dilution effect of TiC on combustion, Figure 3 shows a decrease in peak combustion temperature from 1464 • C for the TiC-free sample to about 1200 • C for the TiC-containing sample of x = 0.7.The descending trend was consistent with the calculated adiabatic temperature.On the other hand, the addition of extra Si and a small amount of Al essentially had little influence on the combustion temperature.Experimental measurements indicated that the temperature variation induced by adding extra Si and Al was within ±20 • C of the value detected from the 3Ti-Si-2C stoichiometric sample.Figure 3 plots the temperature profiles measured from the samples without extra Si and Al, but containing different amounts of TiC.A temperature profile detected from a TiC-free element powder compact (x = 0) was included for comparison.All profiles exhibited a steep temperature rise followed by a rapid descent, which is typical of the SHS reaction that features a fast combustion wave and a thin reaction zone.The peak value was considered as the combustion front temperature (Tc).Due to the dilution effect of TiC on combustion, Figure 3 shows a decrease in peak combustion temperature from 1464 °C for the TiC-free sample to about 1200 °C for the TiC-containing sample of x = 0.7.The descending trend was consistent with the calculated adiabatic temperature.On the other hand, the addition of extra Si and a small amount of Al essentially had little influence on the combustion temperature.Experimental measurements indicated that the temperature variation induced by adding extra Si and Al was within ± 20 °C of the value detected from the 3Ti-Si-2C stoichiometric sample.It was found that the reaction front temperatures of the samples with x = 0, 0.2, and 0.4 were above the lowest eutectic point (1330 • C) of the Ti-Si mixture.This justified the reaction mechanism proposed by Gauthier et al. [46], which suggested that Ti 3 SiC 2 is formed through the interaction of a Ti-Si liquid phase with solid TiC x .However, the peak combustion temperatures of the TiC-added samples of x = 0.5, 0.6, and 0.7 varied between 1200 and 1300 • C; therefore, the formation of Ti 3 SiC 2 could have been dominated by solidphase reactions.For a mechanism involving no liquid phases [47,48], Ti 3 SiC 2 is proposed to form through two intermediates, TiC x and Ti 5 Si 3 C x , reacting with free silicon and carbon, respectively.This implies that an increase in TiC content in the starting composition could lead to a transition of the governing reaction mechanism from a liquid-solid scenario to a solid-state mode.
It was proposed that the reaction mechanism of Ti 3 SiC 2 formation in the Ti-Si-C system could be initiated by the reaction of Ti with C as Equation ( 2), followed by the interaction between Ti and Si as Equation (3).These two reactions are highly exothermic and produce TiC and Ti 5 Si 3 as the precursors.Finally, the formation of Ti 3 SiC 2 is via a reaction involving TiC, Ti 5 Si 3 , and Si, as shown in Equation ( 4).
The effects of TiC, Si, and Al on the combustion front propagation velocity are presented in Figure 4, indicating a substantial decrease in the flame-front speed with TiC content from 6.7 mm/s for the elemental 3Ti-Si-2C sample to 2.3 mm/s for the TiC-containing sample of x = 0.7.As the layer-by-layer heat transfer from the reaction zone to an unburned region plays an important role in establishing combustion wave behavior, the propagation speed is strongly affected by the combustion front temperature.Such a great decrease in the combustion wave velocity could be a consequence of the dilution effect of TiC on combustion and the change in the reaction mechanism from a liquid-solid reaction mode to one dominated by solid-phase species, as discussed above.In particular, the combustion wave of the sample with x = 0.7 propagated in a spinning manner, which prolonged the total reaction time and reduced the propagation rate.
Materials 2023, 16, 6142 6 of 13 reaction mechanism proposed by Gauthier et al. [46], which suggested that Ti3SiC2 is formed through the interaction of a Ti-Si liquid phase with solid TiCx.However, the peak combustion temperatures of the TiC-added samples of x = 0.5, 0.6, and 0.7 varied between 1200 and 1300 °C; therefore, the formation of Ti3SiC2 could have been dominated by solidphase reactions.For a mechanism involving no liquid phases [47,48], Ti3SiC2 is proposed to form through two intermediates, TiCx and Ti5Si3Cx, reacting with free silicon and carbon, respectively.This implies that an increase in TiC content in the starting composition could lead to a transition of the governing reaction mechanism from a liquid-solid scenario to a solid-state mode.
It was proposed that the reaction mechanism of Ti3SiC2 formation in the Ti-Si-C system could be initiated by the reaction of Ti with C as Equation ( 2), followed by the interaction between Ti and Si as Equation (3).These two reactions are highly exothermic and produce TiC and Ti5Si3 as the precursors.Finally, the formation of Ti3SiC2 is via a reaction involving TiC, Ti5Si3, and Si, as shown in Equation ( 4). (3) The effects of TiC, Si, and Al on the combustion front propagation velocity are presented in Figure 4, indicating a substantial decrease in the flame-front speed with TiC content from 6.7 mm/s for the elemental 3Ti-Si-2C sample to 2.3 mm/s for the TiC-containing sample of x = 0.7.As the layer-by-layer heat transfer from the reaction zone to an unburned region plays an important role in establishing combustion wave behavior, the propagation speed is strongly affected by the combustion front temperature.Such a great decrease in the combustion wave velocity could be a consequence of the dilution effect of TiC on combustion and the change in the reaction mechanism from a liquid-solid reaction mode to one dominated by solid-phase species, as discussed above.In particular, the combustion wave of the sample with x = 0.7 propagated in a spinning manner, which prolonged the total reaction time and reduced the propagation rate.In addition, Figure 4 reveals that the addition of extra Si (y = 1.2) and Al (z = 0.1) had relatively minor effects on combustion velocity.The combustion velocity of the sample with extra Si of 20% was very close to that with stoichiometric Si.The addition of Al generally increased the combustion velocity by about 5%, possibly due to the formation of a liquid phase in the Ti-Si-C-Al system [32].It should be noted that extra Si and Al also had little influence on the combustion temperature.3Ti-Si-C sample and the TiC-containing sample of x = 0.5, respectively.No extra Si and A were employed in the samples of Figure 5.The final products were composed of thre constituent phases, Ti3SiC2, TiC, and Ti5Si3.The diffraction peak intensity of Ti3SiC2 rela tive to that of either TiC or Ti5Si3 is noticeably amplified in Figure 5b when compared with that of Figure 5a.This justified the role of TiC as the reactant in enhancing the formation of Ti3SiC2 by the SHS process because TiC is one of two major intermediates to form Ti3SiC2.

Composition and Microstructure Analyses of Synthesized Products
The effects of Si and Al on the formation of Ti3SiC2 can be seen in Figure 6, which presents XRD spectra for TiC-containing samples of x = 0.5 with excess Si and the addition of Al. Figure 6a reveals that for the Si-rich sample without Al additions, the dominancy o Ti3SiC2 over TiC and Ti5Si3 was stronger than that observed in Figure 5b.This indicated that the sample with Si in excess of 20 mol.% improved the yield of Ti3SiC2 as extra S compensated for the evaporation loss of Si at high temperatures.For the Si-rich/Al-added sample, Figure 6b indicates that the addition of a small amount of Al further augmented the XRD peak intensity of Ti3SiC2.According to Zhang et al. [32], Al can aid the formation of a liquid phase in the Ti-Si-C-Al system, and the liquid facilitates the diffusion of Ti and Si atoms and promotes the evolution of Ti3SiC2.As a reaction promoter, Sun et al. [33 pointed out that Al of a small quantity could evaporate from the grain boundary, rathe than forming a solid solution in Ti3SiC2.The effects of Si and Al on the formation of Ti 3 SiC 2 can be seen in Figure 6, which presents XRD spectra for TiC-containing samples of x = 0.5 with excess Si and the addition of Al. Figure 6a reveals that for the Si-rich sample without Al additions, the dominancy of Ti 3 SiC 2 over TiC and Ti 5 Si 3 was stronger than that observed in Figure 5b.This indicated that the sample with Si in excess of 20 mol.% improved the yield of Ti 3 SiC 2 as extra Si compensated for the evaporation loss of Si at high temperatures.For the Si-rich/Al-added sample, Figure 6b indicates that the addition of a small amount of Al further augmented the XRD peak intensity of Ti 3 SiC 2 .According to Zhang et al. [32], Al can aid the formation of a liquid phase in the Ti-Si-C-Al system, and the liquid facilitates the diffusion of Ti and Si atoms and promotes the evolution of Ti 3 SiC 2 .As a reaction promoter, Sun et al. [33] pointed out that Al of a small quantity could evaporate from the grain boundary, rather than forming a solid solution in Ti 3 SiC 2 .The weight percentages of Ti3SiC2, TiC, and Ti5Si3 in the products associated with the three initial sample compositions were calculated and are presented in Figure 7a-c.Fo the stoichiometric samples with Ti:Si:C = 3:1:2 (i.e., y = 1 and z = 0), Figure 7a shows tha for the TiC-free sample, the resulting product was composed of 50 wt.%Ti3SiC2, 41 wt.% TiC, and 9 wt.%Ti5Si3.The TiC-containing sample was found to increase the formation o Ti3SiC2; a maximum yield of 67 wt.% was detected at x = 0.5, in which the fractions of the other two phases were 26 wt.%TiC and 7 wt.%Ti5Si3.However, a further increase in the TiC content to x = 0.6 and 0.7 decreased the yield percentage of Ti3SiC2, possibly due to a reduced reaction temperature.
For the Si-rich samples of y = 1.2 and z = 0, the variations in Ti3SiC2, TiC, and Ti5Si fractions in the final products with the number of moles of TiC in the reactants are pre sented in Figure 7b.The contribution of excess Si to the production of Ti3SiC2 was justified and the fraction of Ti3SiC2 reached between 66 and 72 wt.% for the TiC-containing sam ples.In addition, the content of TiC was about 25-27 wt.% and that of Ti5Si3 was around 5-7 wt.% for the products of the Si-rich samples.
The formation of Ti3SiC2 was further improved by the Si-rich/Al-added samples (y = 1.2 and z = 0.1).As unveiled in Figure 7c, the yield fraction of Ti3SiC2 exceeded 80 wt.% for the samples initially containing TiC between x = 0.3 and 0.6.The highest yield o Ti3SiC2, reaching 85 wt.%, was obtained from the sample of x = 0.5 and the product also contained 11 wt.%TiC and 4 wt.%Ti5Si3.Even for the TiC-free sample, as shown in Figure 7c, the fraction of Ti3SiC2 increased up to 78 wt.%.
Figure 8 makes a comparison of the yield fractions of Ti3SiC2 between the samples o three different compositions.It was evident that both extra Si and Al contributed to the formation of Ti3SiC2, and a larger increase in the yield percentage was achieved by the addition of Al.In this study, the highest yield of Ti3SiC2 was produced by the sample o 2.5Ti + 1.2Si + 1.5C + 0.5TiC + 0.1Al.The weight percentages of Ti 3 SiC 2 , TiC, and Ti 5 Si 3 in the products associated with the three initial sample compositions were calculated and are presented in Figure 7a-c.For the stoichiometric samples with Ti:Si:C = 3:1:2 (i.e., y = 1 and z = 0), Figure 7a shows that for the TiC-free sample, the resulting product was composed of 50 wt.%Ti 3 SiC 2 , 41 wt.% TiC, and 9 wt.%Ti 5 Si 3 .The TiC-containing sample was found to increase the formation of Ti 3 SiC 2 ; a maximum yield of 67 wt.% was detected at x = 0.5, in which the fractions of the other two phases were 26 wt.%TiC and 7 wt.%Ti 5 Si 3 .However, a further increase in the TiC content to x = 0.6 and 0.7 decreased the yield percentage of Ti 3 SiC 2 , possibly due to a reduced reaction temperature.
For the Si-rich samples of y = 1.2 and z = 0, the variations in Ti 3 SiC 2 , TiC, and Ti 5 Si 3 fractions in the final products with the number of moles of TiC in the reactants are presented in Figure 7b.The contribution of excess Si to the production of Ti 3 SiC 2 was justified, and the fraction of Ti 3 SiC 2 reached between 66 and 72 wt.% for the TiC-containing samples.In addition, the content of TiC was about 25-27 wt.% and that of Ti 5 Si 3 was around 5-7 wt.% for the products of the Si-rich samples.
The formation of Ti 3 SiC 2 was further improved by the Si-rich/Al-added samples (y = 1.2 and z = 0.1).As unveiled in Figure 7c, the yield fraction of Ti 3 SiC 2 exceeded 80 wt.% for the samples initially containing TiC between x = 0.3 and 0.6.The highest yield of Ti 3 SiC 2 , reaching 85 wt.%, was obtained from the sample of x = 0.5 and the product also contained 11 wt.%TiC and 4 wt.%Ti 5 Si 3 .Even for the TiC-free sample, as shown in Figure 7c, the fraction of Ti 3 SiC 2 increased up to 78 wt.%.
Figure 8 makes a comparison of the yield fractions of Ti 3 SiC 2 between the samples of three different compositions.It was evident that both extra Si and Al contributed to the formation of Ti 3 SiC 2 , and a larger increase in the yield percentage was achieved by the addition of Al.In this study, the highest yield of Ti 3 SiC 2 was produced by the sample of 2.5Ti + 1.2Si + 1.5C + 0.5TiC + 0.1Al.The SEM image and EDS spectrum illustrated in Figure 9 display the fracture surface microstructure and atomic ratio of the product synthesized from the TiC-containing and Si-rich samples of x = 0.5, y = 1.2, and z = 0. Plate-like grains forming a layered microstructure, which is typical of the MAX ternary carbide, are clearly seen; the Ti3SiC2 platelets  The SEM image and EDS spectrum illustrated in Figure 9 display the fracture surface microstructure and atomic ratio of the product synthesized from the TiC-containing and Si-rich samples of x = 0.5, y = 1.2, and z = 0. Plate-like grains forming a layered microstructure, which is typical of the MAX ternary carbide, are clearly seen; the Ti3SiC2 platelets The SEM image and EDS spectrum illustrated in Figure 9 display the fracture surface microstructure and atomic ratio of the product synthesized from the TiC-containing and Sirich samples of x = 0.5, y = 1.2, and z = 0. Plate-like grains forming a layered microstructure, which is typical of the MAX ternary carbide, are clearly seen; the Ti 3 SiC 2 platelets were 0.5-1.0µm in thickness and about 10 µm in length.An atomic ratio of Ti:Si:C = 52.6:13.8:33.6 matching well with Ti 3 SiC 2 was obtained from the EDS analysis.The microstructure and EDS element spectrum presented in Figure 10    Table 1 summarizes the Ti3SiC2-related products fabricated by various methods with different starting reactant mixtures.For the formation of single-phase Ti3SiC2, SPS and reactive sintering are suitable methods.These two methods require high operating temperatures (about 1300-1500 °C) for 20-30 min or even 2-3 h.On the other hand, the SHS technique is an energy-efficient and time-saving fabrication route, and is suitable for the synthesis of Ti3SiC2/TiC/Ti5Si3 composites.As listed in Table 1, the addition of a small amount of Al has frequently been applied to improve the yield of Ti3SiC2 and slightly Sirich mixtures have been adopted.Moreover, TiC and SiC have also been considered as the reactants in different fabrication methods.Table 1 summarizes the Ti3SiC2-related products fabricated by various methods with different starting reactant mixtures.For the formation of single-phase Ti3SiC2, SPS and reactive sintering are suitable methods.These two methods require high operating temperatures (about 1300-1500 °C) for 20-30 min or even 2-3 h.On the other hand, the SHS technique is an energy-efficient and time-saving fabrication route, and is suitable for the synthesis of Ti3SiC2/TiC/Ti5Si3 composites.As listed in Table 1, the addition of a small amount of Al has frequently been applied to improve the yield of Ti3SiC2 and slightly Sirich mixtures have been adopted.Moreover, TiC and SiC have also been considered as the reactants in different fabrication methods.Table 1 summarizes the Ti 3 SiC 2 -related products fabricated by various methods with different starting reactant mixtures.For the formation of single-phase Ti 3 SiC 2 , SPS and reactive sintering are suitable methods.These two methods require high operating temperatures (about 1300-1500 • C) for 20-30 min or even 2-3 h.On the other hand, the SHS technique is an energy-efficient and time-saving fabrication route, and is suitable for the synthesis of Ti 3 SiC 2 /TiC/Ti 5 Si 3 composites.As listed in Table 1, the addition of a small amount of Al has frequently been applied to improve the yield of Ti 3 SiC 2 and slightly Si-rich mixtures have been adopted.Moreover, TiC and SiC have also been considered as the reactants in different fabrication methods.
Experimental evidence indicated that solid-state combustion was highly exothermic and a self-sustaining combustion synthesis process was readily achieved upon ignition.Combustion wave velocity and reaction temperatures substantially decreased from 6.7 to 2.3 mm/s and 1464 to 1200 • C, respectively, as the TiC content in the green samples increased from x = 0 to x = 0.7.This was mainly attributed to the dilution effect of TiC on combustion.The XRD analysis identified that final products were composed of Ti 3 SiC 2 and two intermediate phases, TiC and Ti 5 Si 3 .When compared with TiC-free samples (x = 0), the TiC-containing samples benefited the formation of Ti 3 SiC 2 .The optimum content of TiC was found to be x = 0.5, beyond which the yield of Ti 3 SiC 2 declined due to a decrease in combustion temperature.Even though extra Si (y = 1.2) and Al (z = 0.1) had almost no effect on combustion wave velocity and temperature, they enhanced the formation of Ti 3 SiC 2 to a great extent.Considering the TiC-containing samples of x = 0.5, the weight fraction of Ti 3 SiC 2 formed in the final products increased from 67 wt.% for the sample of Ti:Si:C = 3:1:2 to 72 wt.% for the Si-rich sample, and further to 85 wt.% for the Si-rich/Aladded sample.This justified the contribution of excess Si to subsidize for its loss and the role of Al as an effective reaction promoter.The as-synthesized Ti 3 SiC 2 grains were in a plate-like shape with a thickness of 0.5-1.0µm and length of about 10 µm.The Ti 3 SiC 2 grains were closely stacked into a laminated configuration, which is typical of a MAX microstructure.
One of main limitations to further commercialize MAX phases is the development of a suitable synthesis process to produce large quantities of highly pure MAX powders at an affordable cost [11].Synthesis of MAX powders by the SHS method was shown to successfully produce industrial quantities at reasonable prices.More efforts and attention are required to synthesize and scale-up MAX phase powders of sufficient purities or desired compositions.This study represents a substantial advance on the formation of Ti 3 SiC 2 /TiC/Ti 5 Si 3 composites with controllable compositions by SHS.

Figure 1 .
Figure 1.Time sequences of recorded SHS images illustrating the propagation of self-sustaining combustion waves of Al-free samples with (a) TiC of x = 0.2 and Si of y = 1 and (b) TiC of x = 0.7 and Si of y = 1.The increase in TiC reduced the combustion intensity and decelerated the combustion wave.

Figure 1 .
Figure 1.Time sequences of recorded SHS images illustrating the propagation of self-sustaining combustion waves of Al-free samples with (a) TiC of x = 0.2 and Si of y = 1 and (b) TiC of x = 0.7 and Si of y = 1.The increase in TiC reduced the combustion intensity and decelerated the combustion wave.

Figure 2 .
Figure 2. Enthalpies of reaction (ΔHr) and adiabatic combustion temperatures (Tad) of Equation (1) as a function of number of moles of TiC.

Figure 3 . 4 x = 0. 7 Figure 2 .
Figure 3.Effect of TiC content on the combustion temperature of reactant compacts for synthesis of Ti3SiC2.It was found that the reaction front temperatures of the samples with x = 0, 0.2, and 0.4 were above the lowest eutectic point (1330 °C) of the Ti-Si mixture.This justified the

Figure 3 . 7 Figure 3 .
Figure 3.Effect of TiC content on the combustion temperature of reactant compacts for synthesis of Ti3SiC2.It was found that the reaction front temperatures of the samples with x = 0, 0.2, and 0.4 were above the lowest eutectic point (1330 °C) of the Ti-Si mixture.This justified the

Figure 4 . 1 SampleFigure 4 .
Figure 4. Effects of TiC, Si, and Al on the combustion front velocity of reactant compacts for synthe sis of Ti3SiC2.Combustion velocity was strongly affected by TiC content, but little influenced by extra Si and Al.Number of Mole of TiC, x 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Figure
Figure 5a,b show XRD patterns of the final products synthesized from the elemental 3Ti-Si-C sample and the TiC-containing sample of x = 0.5, respectively.No extra Si and Al were employed in the samples of Figure 5.The final products were composed of three constituent phases, Ti 3 SiC 2 , TiC, and Ti 5 Si 3 .The diffraction peak intensity of Ti 3 SiC 2 relative to that of either TiC or Ti 5 Si 3 is noticeably amplified in Figure 5b when compared with that of Figure 5a.This justified the role of TiC as the reactant in enhancing the formation of Ti 3 SiC 2 by the SHS process because TiC is one of two major intermediates to form Ti 3 SiC 2 .

Figure 7 .
Figure 7. Weight fractions of Ti3SiC2, Ti5Si3, and TiC in the products synthesized from (a) stoichiometric samples of y = 1 and z = 0, (b) Si-rich samples of y = 1.2 and z = 0, and (c) Si-rich/Al-added samples of y = 1.2 and z = 0.1.

Figure 8 .
Figure 8. Variations in weight percentage of Ti3SiC2 formed in the final products with initial TiC content, excess Si, and Al additions.

Figure 8 .
Figure 8. Variations in weight percentage of Ti3SiC2 formed in the final products with initial TiC content, excess Si, and Al additions.

1 SampleFigure 8 .
Figure 8. Variations in weight percentage of Ti 3 SiC 2 formed in the final products with initial TiC content, excess Si, and Al additions.
were associated with the product of the Si-rich/Al-added samples of x = 0.5, y = 1.2, and z = 0.1.Similarly, plate-like Ti 3 SiC 2 grains closely packed into a laminated configuration were observed.The atomic ratio of Ti:Si:C = 51.6:15.0:33.4provided by EDS confirmed the formation of Ti 3 SiC 2 .5-1.0 µm in thickness and about 10 µm in length.An atomic ratio of Ti:Si:C = 52.6:13.8:33.6 matching well with Ti3SiC2 was obtained from the EDS analysis.The microstructure and EDS element spectrum presented in Figure 10 were associated with the product of the Si-rich/Al-added samples of x = 0.5, y = 1.2, and z = 0.1.Similarly, plate-like Ti3SiC2 grains closely packed into a laminated configuration were observed.The atomic ratio of Ti:Si:C = 51.6:15.0:33.4provided by EDS confirmed the formation of Ti3SiC2.

Figure 9 .
Figure 9. SEM image and EDS spectrum of the product synthesized from the Si-rich sample with composition parameters of x = 0.5, y = 1.2, and z = 0.

Figure 10 .
Figure 10.SEM image and EDS spectrum of the product synthesized from the Si-rich/Al-added sample with composition parameters of x = 0.5, y = 1.2, and z = 0.1.

Figure 9 .
Figure 9. SEM image and EDS spectrum of the product synthesized from the Si-rich sample with composition parameters of x = 0.5, y = 1.2, and z = 0.

Figure 9 .
Figure 9. SEM image and EDS spectrum of the product synthesized from the Si-rich sample with composition parameters of x = 0.5, y = 1.2, and z = 0.

Figure 10 .
Figure 10.SEM image and EDS spectrum of the product synthesized from the Si-rich/Al-added sample with composition parameters of x = 0.5, y = 1.2, and z = 0.1.

Figure 10 .
Figure 10.SEM image and EDS spectrum of the product synthesized from the Si-rich/Al-added sample with composition parameters of x = 0.5, y = 1.2, and z = 0.1.