Synthesis, microstructure, and properties of high purity Mo2TiAlC2 ceramics fabricated by spark plasma sintering

Abstract: The synthesis, microstructure, and properties of high purity dense bulk Mo₂TiAlC₂ ceramics were studied. High purity Mo₂TiAlC₂ powder was synthesized at 1873 K starting from Mo, Ti, Al, and graphite powders with a molar ratio of 2:1:1.25:2. The synthesis mechanism of Mo₂TiAlC₂ was explored by analyzing the compositions of samples sintered at different temperatures. It was found that the Mo₂TiAlC₂ phase was formed from the reaction among Mo₃Al₂C, Mo₂C, TiC, and C. Dense Mo₂TiAlC₂ bulk sample was prepared by spark plasma sintering (SPS) at 1673 K under a pressure of 40 MPa. The relative density of the dense sample was 98.3%. The mean grain size was 3.5 μm in 
length and 1.5 μm in width. The typical layered structure could be clearly observed. The electrical conductivity of Mo₂TiAlC₂ ceramic measured at the temperature range of 2–300 K decreased from 0.95 × 10⁶ to 0.77 × 10⁶ Ω^–1·m^–1.Thermal conductivity measured at the temperature range of 300–1273 K decreased from 8.0 to 6.4 W·(m·K)–1. The thermal expansion coefficient (TEC) of Mo₂TiAlC₂ measured at the temperature of 350–1100 K was calculated as 9.0 × 10^–6 K^–1. Additionally, the layered structure and fine grain size benefited for excellent mechanical properties of low intrinsic Vickers hardness of 5.2 GPa, high flexural strength of 407.9 MPa, high fracture toughness of 6.5 MPa·m^1/2, and high compressive strength of 1079 MPa. Even at the indentation load of 300 N, the residual flexural strength could hold 84% of the value of undamaged one, indicating remarkable damage tolerance. Furthermore, it was confirmed that Mo2TiAlC2 ceramic had a good oxidation resistance below 1200 K in the air. 

Keywords: MAX phase; Mo₂TiAlC₂; synthesis; microstructure; properties 

1 Introduction

In recent years, MAX phases of ternary layered compounds have attracted much attention from material scientists in the world owing to their excellent combining properties of ceramics and metals. The formula of MAX phases could be presented as M_n+1AX_n, where M is a transition metal, A is an A group element, X is carbon or nitrogen, and n = 1–6 [^1–15]. The research results have shown that the MAX phases possess good electrical and thermal conductivities, low hardness, normal thermal expansion coefficient (TEC), high damage tolerance, and thermal shock resistance, being the candidates applied in the industrial and aerospace fields ^[16–31]. 

In the MAX phase family, there is one kind of solid solutions of (M‘,M’‘)_n+1AX_n where the M site is composed of two elements, and they have the possible multiple properties of M’_n+1AX_n and M‘’_n+1AX_n, triggering the interests of researchers. By controlling the solid solute elements, new MAX phases-compounds could also be synthesized. For example, Sun et al. ^[32–36] successfully prepared (V_0.5Cr_0.5)₃AlC₂, (V_0.5Cr_0.5)₄AlC₃, and (V_0.5Cr_0.5)₅Al₂C₃ MAX phases by using in situ reaction method and found that these phases had the good high-temperature stiffness of V-based MAX phase and the good hot corrosion resistance of Cr-based MAX phase. Zheng et al. ^[37] synthesized (Ti_0.5Nb_0.5)₅AlC₄ compound which is the first layered 514 phase, therefore improving the discovery of more new MAX phases. In recent years, Anasori et al. ^[38,39] identified two new solid solution MAX phases of Mo₂TiAlC₂ and Mo₂Ti₂AlC₃ and determined the order arrangement of Mo atom layer and Ti atom layer in the crystal structure. Also, Fu et al. ^[40] synthesized high purity dense Mo₂Ti₂AlC₃ sample with a relative density of 99.3% by the hot pressing method and systematically characterized its physical and mechanical 
properties. Fu et al. [40] confirmed that Mo₂Ti₂AlC₃ ceramic has the lowest thermal conductivity of 6.82 W·(m·K)^–1 in the known MAX phases. Whereas, until 2020, except for Refs. ^[38,39], the experimental researches on the basic physical and mechanical properties of Mo₂TiAlC₂ were quiet rare. 

Additionally, in order to fabricate the dense Mo₂TiAlC₂ ceramics with fine grains, spark plasma sintering (SPS) was adopted based on the merits of on-off DC pulse current to generate discharge plasma, releasing point impact pressure and Joule heat, and accelerating electric field diffusion ^[41–44]. In previous studies, SPS method has been well utilized to successfully prepare MAX phases of Ti₂AlC ^[45], Ti₂AlN ^[46], Ta₄AlC₃ ^[47], Ti₂SnC ^[48], and Cr₂AlC ^[49]. The good characteristics of fast heating, shortening annealing time, and high production efficiency have been well proved. Especially, under the condition of short annealing time and high pressure, it is convenient to achieve fine grains in the dense bulks, benefiting for the good mechanical properties.

In this paper, high purity Mo₂TiAlC₂ powders were synthesized firstly and its reaction mechanism was systematically investigated. Dense Mo₂TiAlC₂ ceramics were consolidated by SPS and their physical and mechanical properties, as well as oxidation resistance, were studied. 

2 Experimental 

The preparation of high purity dense Mo₂TiAlC₂ ceramics was divided into two steps: One step was to explore the reaction mechanisms to synthesize high purity Mo₂TiAlC₂ powders, and the second step was to consolidate the dense bulks by SPS. For the synthesis of high purity powders, commercial powders of Mo (300 mesh, 99% purity), Ti (200 mesh, 99.5% purity), Al (300 mesh, 99% purity), and graphite (1500 mesh, 99% purity) were used as initial materials. The designed molar ratio of Mo:Ti:Al:C was 2:1:1.25:2. The adding excessive aluminum was to compensate for the evaporation of Al during high-temperature treatment. The raw powders were weighed in an electric balance with an accuracy of 10^–2 g and drily mixed in a resin bottle at 50 rpm in a rotary milling machine for 20 h. After sieving, the mixture was filled in an alumina crucible and sintered in the tube furnace (BR-17MT, Brother Co., Ltd., China) at the aimed temperature in a flowing argon atmosphere (99.9% purity). The heating and cooling rate was 5 K/min and the holding time was 4 h. In order to synthesize pure Mo₂TiAlC₂ powders, the reaction mechanisms of Mo₂TiAlC₂ ceramic were systematically investigated at the temperatures of 300, 973, 1273, 1573, and 1873 K, respectively. After sintering, the powders were drilled from the specimens and sieved using a 200 mesh sieve. The compositions of obtained fine powders were examined by a D8 Bruker X-ray diffractometer (XRD) (PA, Almelo, the Netherlands) with Cu Kα (λ = 1.54178 Å). Finally, it was confirmed that the pure Mo₂TiAlC₂ powder could be successfully prepared at 1873 K. 

For fabricating the dense Mo₂TiAlC₂ ceramics, the high purity powders with the particle size of 200 mesh were put into a graphite mold with a diameter of 20 mm and consolidated at 1673 K in a SPS furnace (SPS-20T-10, Chenhua Co., Ltd., China) in a vacuum (1 Pa) under a pressure of 40 MPa and then the samples were cooled down to the ambient temperature naturally. The heating rate was 20 K/min and the holding time was 10 min. Finally, the dense samples were ground to remove the surface contaminations by a diamond grinding wheel. Phase constituents of the sample were examined by XRD, and the density of the sample was measured by the Archimedes’ method. After etching the polished surface of Mo₂TiAlC₂ ceramics using the acid solution consisting of HNO₃,HF, and H₂O with the volume ratio of 1:1:1, the grains were carefully checked by a scanning electron microscope (FEI, Hillaboro Co., USA) equipped with an energy-dispersive X-ray spectroscopy (EDS). At least 50 grains were measured in length and width. 

The electrical conductivity was measured by physical property measurement system (PPMS DynaCool 9, Quantum Design Co., USA) at the temperature range of 2–300 K. The sample size used was 3 mm × 3 mm × 15 mm. The thermal conductivity was measured by a hot disk method (TPS2500, Hot Disk Co., Sweden) at the temperatures of 300, 473, 673, 873, 1073, and 1273 K, respectively. The sample used for testing thermal conductivity was a cylinder with a 20 mm diameter and 5 mm height. The TEC was measured using a thermal expansion analyzer (DIL402C, Netzsch Co., Germany) at the temperature range of 350–1100 K in flowing argon. The heating rate was set at 1.5 K/min, and the dimension of the sample was 3 mm × 4 mm × 4 mm. 

The Vickers hardness was tested in a micro-hardness tester (HVS-50, Wheng Co., China), and the loads were selected as 1, 2, 3, 5, 10, 50, and 100 N, respectively. An universal testing machine (YC-100KN, Yice Co., China) was used to determinate the mechanical properties, including fracture toughness (crosshead speed, 0.05 mm/min), flexural strength (crosshead speed, 0.5 mm/min), and compressive strength (crosshead speed, 0.5 mm/min). The specimens were machined into bars with the dimensions of 2 mm × 4 mm × 18 mm, 1.5 mm × 2 mm × 18 mm, and 2 mm × 2 mm × 4 mm respectively by an electrical-discharged method and then polished down to 1.0 μm diamond grits. The notch of the samples for the fracture toughness test was 2 mm in length and 0.2 mm in width. The testing span of three-point bending strength and the fracture toughness was 16 mm. Three samples were used for each test. The damage tolerance of Mo₂TiAlC₂ ceramics was characterized by the indentation load dependence of flexural strength, and the indentation loads selected were 0, 10, 50, 100, 200, and 300 N, respectively. The fracture surface was observed by scanning electron microscope (SEM). 

In addition, in order to determine the oxidation resistance of Mo₂TiAlC₂ ceramics, the DSC–TG curve was evaluated by a TG–DSC facility (STA 449F3, Netzsch Corp., Germany) at the temperature range of 313–1473 K. The heating rate was 20 K/min and the flowing rate of air was 20 ml/min. Furthermore, the dense Mo₂TiAlC₂ sample with the dimension of 1.5 mm × 2 mm × 5 mm was heat-treated in the tube furnace in the air from room temperature to 1473 K at the heating rate of 10 K/min. The oxidized surface was examined by XRD and SEM. 

3 Results and discussion 

3. 1 Synthesis and microstructure characterization 

In order to explore the reaction mechanisms of Mo₂TiAlC₂ and synthesize pure Mo₂TiAlC₂ powder, the sintering was processed at different temperatures. The XRD results of the obtained phases are shown in Fig. 1. The compositions in the prepared samples were listed in Table 1. The temperatures corresponding to Figs. 1(a)–1(e) are 300, 973, 1273, 1573, and 1873 K, respectively. In comparison with the initial compositions of mixture powder at 300 K (Fig. 1(a)), no reaction occurred at 973 K, and only element powders could be examined (Fig. 1(b)). When raising the temperature to 1273 K, a large amount of Al and Mo reacted and generated the main phase of AlMo₃ (Fig. 1(c)). In addition, a small amount of AlTi₃ and Mo₂C were detected. The chemical reaction equations could be expressed as 

Al + 3Mo = AlMo₃    (1) 
Al + 3Ti = AlTi₃         (2) 
2Mo + C = Mo₂C      (3) 

As the temperature reached 1573 K, the diffraction peak intensity of AlMo₃ decreased a lot and the AlTi₃ phase disappeared, whereas the diffraction peak intensity of Mo₂C continued to increase to be the main phase, indicating the continuous reaction between Mo and C (Fig. 1(d)). Additionally, plenty of Mo₃Al₂C, a small amount of TiC, and a few Mo₂TiAlC₂ were examined. At the same time, graphite was also reacted. It is suspected that the consumption of AlMo₃, AlTi₃, and C probably contributed to the formation of Mo₃Al₂C and TiC. When further increasing the temperature to be 1873 K, all phases of C, AlMo₃, Mo₂C, Mo₃Al₂C, and TiC disappeared and only Mo₂TiAlC₂ could be examined (Fig. 1(e)). So, the newly formed phases of Mo₃Al₂C and TiC possibly reacted with Mo₂C to form Mo₂TiAlC₂. Therefore, the chemical reaction equations could be expressed as 

8AlMo₃ + 4AlTi₃ + 21C= 6Mo₃Al₂C + 12TiC + 3Mo₂C   (4) 

2Mo₃Al₂C + 4TiC + Mo₂C + C = 4Mo2TiAlC₂                 (5) 

Fig. 1 XRD patterns of synthesized specimens sintered at different temperatures: (a) 300, (b) 973, (c) 1273, (d) 1573, and (e) 1873 K. 

Table 1 Phase compositions of samples sintered at different temperature

As a result, it could be determined that high purity Mo₂TiAlC₂ powder could be successfully synthesized at 1873 K with the molar ratio of Mo:Ti:Al:C of 2:1:1.25:2. 

Figure 2 shows the XRD patterns of the dense Mo₂TiAlC₂ bulks and pure Mo₂TiAlC₂ powder. It is detected that after consolidation by SPS, the obtained dense Mo₂TiAlC₂ ceramics still kept high purity with almost no impurity. The diffraction peaks of (002), (004), (006), and (008) became stronger in the dense specimens in comparison with those of initial fine Mo₂TiAlC₂ particles. It is concluded that during the sintering process, the growth of Mo₂TiAlC₂ grains showed the preferential direction with the c-axis of grains parallel to the loading direction under high pressures. A similar result has been previously reported on Ti₂AlN ceramic densified by SPS, which was ascribed to the mechanical oriented deformation ^[50].

Fig. 2 XRD spectra of (a) dense Mo₂TiAlC₂ bulk sample fabricated by SPS and (b) pure Mo₂TiAlC₂ powder. 

The microstructure characterization of dense Mo₂TiAlC₂ ceramics was shown in Fig. 3. In the etched surface of Mo₂TiAlC₂ ceramics, plate-like grains with random orientation could be clearly observed (Fig. 3(a)). By measuring the length and width of at least 50 grains, the mean grain size was calculated to be 3.5 μm in length and 1.0 μm in width, which is finer than those of Mo₂Ti₂AlC₃ (5 μm in length and 1.3 μm in width) ^[40]. The smaller grain size is probably ascribed to the SPS sintering method which supplies a high heating rate and short annealing time to effectively inhibit the rapid grain growth. Figure 3(b) shows the fracture surface of Mo₂TiAlC₂ ceramics. It is seen that the sample is very dense and the porosity is extremely rare. The relative density of Mo₂TiAlC₂ ceramics measured by the Archimedes’ method was 98.3%, matching well with the micrograph. Transgranular fracture and intergranular fracture are the main damage modes. Like other MAX phases, the layered structure of grains could be easily observed, indicating the excellent mechanical properties. 

Fig. 3 SEM images of (a) etched surface and (b) fracture surface of dense Mo₂TiAlC₂ ceramics. 

3. 2 Physical and mechanical property evaluation 

In order to analyze and describe the properties of Mo₂TiAlC₂ ceramics systematically, the physical and mechanical properties of dense Mo₂TiAlC₂ ceramics were compared to those of Ti₃AlC₂ and Mo₂Ti₂AlC₃. In comparison with the property of Ti₃AlC₂, it is easier to investigate the property variation law of the 312 phase whereas the M position atom was partially replaced, which provides a possibility to prepare a new MAX phase according to the intention. The comparison between Mo₂TiAlC₂ and Mo₂Ti₂AlC₃ could enrich us with the understanding of the properties of the Mo–Ti–Al–C system MAX phases. Table 2 summarizes the property comparison of Mo₂Ti₂AlC₃ and Ti₃AlC₂, including physical properties and mechanical properties ^[40,51]. Also, the properties of reported Mo₂TiAlC₂ ceramic were added to compare the difference of properties affected by the microstructure ^[38]. 

Table 2 Comparative analysis of physical and mechanical properties of Mo₂TiAlC₂, Mo₂Ti₂AlC₃, and Ti₃AlC₂ ceramics

Figure 4 displays the electrical conductivity and electrical resistivity of Mo₂TiAlC₂ at the temperature range of 2–300 K. The electrical conductivity of Mo₂TiAlC₂ measured at room temperature was 0.77 × 10⁶Ω^–1·m^–1, which is higher than that of Mo₂Ti₂AlC₃ (0.41 × 106 Ω^–1·m^–1) ^[40], but lower than that of Ti₃AlC₂ (3.48 × 10⁶ Ω^–1·m^–1) ^[51]. In addition, the electrical conductivity of Mo₂TiAlC₂ measured by Anasori et al. ^[38] at 10 K was 0.67 × 10⁶ Ω^–1·m^–1, lower than the present 0.95 × 106 Ω^–1·m^–1, which is because of the difference of relative density and content of impurities. The decreasing trend of electrical conductivity from 0.95 × 10⁶ to 0.77 × 10⁶Ω^–1·m^–1 can be seen at the temperature range of 2–300 K and indicates a metal-like conductivity from 50 to 300 K with a linear change. Correspondingly, the electrical resistivity increases with the temperature at the range of 2–300 K, similar to other MAX phases. The temperature dependence of electrical (ρ) resistivity can be expressed by

ρ (μΩ·m) = ρ₀ (1–βΔT) = 1.27 [1 – 0.000717(273.15–T)]  (6) 

where ρ₀ is the electrical resistivity value at 273.15 K (μΩ·m), T is the absolute temperature, and β is the temperature coefficient of resistivity (K^–1). The value of β is 0.717 × 10³ K^–1, which is higher than that of Mo₂Ti₂AlC₃ (0.324 × 10³ K^–1) ^[40], but lower than that of Ti₃AlC₂ (3.10 × 10³ K^–1) ^[51]. 

Fig. 4 Electrical conductivity and resistivity of dense Mo₂TiAlC₂ ceramics as a function of temperature. 

The thermal conductivity measured at a temperature range of 300–1273 K is shown in Fig. 5. It is seen that the thermal conductivity changes linearly with the increment of temperature, decreasing from 8.0 to 6.4 W·(m·K)^–1. The thermal conductivity at room temperature is 8.0 W·(m·K)^–1, which is slightly higher than that of Mo₂Ti₂AlC₃ (6.82 W·(m·K)^–1) ^[40], and greatly lower than that of Ti₃AlC₂ (40 W·(m·K)^–1) ^[51]. A least-square fitting of the temperature dependent thermal conductivity (λ) for Mo₂TiAlC₂ ceramics is described as 

λ = 8.4 – 0.00154T (7) 

where the coefficient of determination r² is 0.99. The trend is similar to those of Mo₂Ti₂AlC₃ and Ti3AlC₂, whose thermal conductivities decrease with the increment of temperature. In addition, the total thermal conductivity can be divided into electronic contribution and phonon contribution, and the relation can be expressed as λ_total = λ_electron+ λ_phonon. Based on the Wiedmann–Franz Law, the value of λ_electron can be calculated by the formula of λ_electron = L₀σT, where σ is the electrical conductivity at temperature T, and L₀ = 2.45 × 10^–8 W·(m·K)^{–1 [52]}. The calculated result of λ_electron at 300 K is 5.7 W·(m·K)^–1. Therefore, the contribution of phonon is 2.3 W·(m·K)^–1 at 300 K. 

Fig. 5 Thermal conductivity of dense Mo₂TiAlC₂ ceramic as a function of temperature. 

The TEC of Mo₂TiAlC₂ ceramic measured in the temperature of 350–1100 K is shown in Fig. 6. The simulated line is very fitting to the measured thermal expansion curve. The mean TEC in the temperature range of 350–1100 K was 9.0 × 10^–6 K^–1, which is the same as that of Ti₃AlC₂ ceramic (9.0 × 10^–6 K^–1) ^[51]. Whereas, in comparison with that of the similar Mo₂Ti₂AlC₃ ceramic (11.3 × 10^–6 K^–1) ^[40], the TEC of Mo₂TiAlC₂ exhibits a lower value, which is probably associated with the stronger bonds in the Mo₂TiAlC₂ crystal structure ^[38,53]. 

Fig. 6 TEC of dense Mo₂TiAlC₂ ceramic versus temperature. 

The Vickers hardness of Mo₂TiAlC₂ ceramics at the load of 10 N was measured as 5.2 ± 0.1 GPa, higher than both Mo₂Ti₂AlC₃ of 4.81 GPa ^[40] and Ti3AlC2 of 2.7 GPa ^[51]. The higher Vickers hardness could be probably ascribed to the SPS method. Fast sintering speed and short holding time contributed to the achievement of small grain size of Mo₂TiAlC₂ ceramics, resulting in the high Vickers hardness. In addition, the Vickers hardness of Mo₂TiAlC₂ as a function of indentation load is displayed in Fig. 7. It is shown that the hardness decreases with increasing load and gradually approaches 5.2 GPa. This phenomenon is well matching the indentation size effect (ISE) of higher hardness at lower loads consisting of the larger elastic recovery ^[54]. Therefore, the intrinsic hardness value of Mo₂TiAlC₂ ceramic is close to 5.2 GPa. An SEM image of a Vickers indent under a load of 10 N is shown in the inset of Fig. 7. No cracks propagated at the indent diagonals, and similar to other MAX phases, grains crushing, delamination, transgranular, and intergranular fractures were observed around the indent region. The mechanical energy could be effectively absorbed by these damage modes to prevent stress concentration. As a result, the damage induced by load could be retained in a limited region to keep the integrity of the bulk sample to remain the high strength.  

Fig. 7 Vickers hardness of dense Mo₂TiAlC₂ ceramic as a function of indention load. Inset: SEM image of the indent induced by 10 N. 

The fracture toughness of Mo₂TiAlC₂ ceramic was tested to be 6.5 ± 0.4 MPa·m^1/2, lower than both Mo₂Ti₂AlC₃ of 8.4 MPa·m^{1/2 [40]} and Ti3AlC₂ of 7.2 MPa·m^{1/2 [51]}. Also, Mo₂TiAlC₂ has a high compressive strength up to 1079 ± 17 MPa, a little lower than Mo₂Ti₂AlC₃ of 1145 MPa [40], but much higher than Ti₃AlC₂ of 764 MPa ^[51]. The high compressive strength was contributed by the fine grain size which provides great mechanical properties, including flexural strength. The flexural strength of Mo₂TiAlC₂ is 407.9 ± 29 MPa, higher than that of Ti₃AlC₂ (340 MPa) ^[51], and a little lower than that of Mo₂Ti₂AlC₃ (452 MPa) [40]. In addition, the damage tolerance of Mo₂TiAlC₂ reflected by the indentation load dependence of residual flexural strength is displayed in Fig. 8. At the indentation loads below 200 N, Mo₂TiAlC₂ ceramic can still hold high flexural strength. No obvious strength degradation was observed. When the indentation load was up to 300 N, the residual flexural strength of 343.9 MPa is still about 84% of the strength of undamaged samples. Mo₂TiAlC₂ ceramic possesses a good damage tolerance like Ti₃AlC₂ ^[55].

Fig. 8 Indentation load dependence of flexural strength of dense Mo₂TiAlC₂ ceramic, presenting the excellent damage tolerance. 

3. 3 Oxidation resistance evaluation 

Furthermore, to examine the possible application field of Mo₂TiAlC₂ ceramic at high temperatures, the dynamic oxidation process of Mo2TiAlC2 ceramics was characterized, as shown in Fig. 9. Figure 9 presents the TG and DSC results of Mo₂TiAlC₂ ceramic measured at the temperature range of 313–1473 K inflowing air. Based on the TG results, it is obviously seen that the mass gain starts at 1070 K and continues to increase until 1200 K, indicating that the sample began to be oxidized. The DSC curve also coincides with this result with an endothermic peak appearing from 1070 K and reaching an extreme value at 1200 K. After 1200 K, the TG curve begins to decrease and the DSC curve begins to turn the direction of heat release. The weight loss is probably associated with the evaporation of MoO₃ because its boiling point is only 1423 K. 

Fig. 9 DSC–TG curves of dense Mo₂TiAlC₂ ceramic as a function of temperature in flowing air. 

To further investigate the oxidation resistance of Mo₂TiAlC₂ ceramics, the bulk sample was oxidized in the tube furnace in the air. The XRD analysis of the sample surface after the oxidizing test from room temperature to 1473 K is shown in Fig. 10. It can be seen that the surface phases are α-Al₂O₃ and R-TiO₂, and no molybdenum oxide MoO₃ peaks were detected. The result is matching the test of TG–DSC well that the MoO₃ phase has completely volatilized at high temperatures. Figure 11 shows the micrograph of the surface after oxidation. The surface was covered by a loose layer consisting of Al₂O₃ and TiO₂ (Fig. 11(a)). In the magnified image, the hexagonal Al₂O₃ plates and TiO₂ rods could be clearly distinguished (Fig. 11(b)). It seems that different from the oxidized surface of Ti₃AlC₂, no dense oxide layer could be formed on the surface of Mo₂TiAlC₂ ceramic during the oxidation process ^[28], which presents the poor oxidation resistance above 1473 K. Therefore, it is determined that dense Mo₂TiAlC₂ ceramic could at least endure the high temperature up to 1200 K in the air. 

Fig. 10 XRD pattern of oxidized surface of dense Mo₂TiAlC₂ ceramic heat treated in the tube furnace from room temperature to 1473 K in the air. 

Fig. 11 SEM micrographs of (a) the oxidized surface of dense Mo₂TiAlC₂ ceramic when oxidized from room temperature to 1473 K in the air and (b) the magnified image of the square region in Fig. 11(a).

4 Conclusions 

In this study, high purity Mo₂TiAlC₂ powder was successfully synthesized by in situ reaction and the reaction mechanisms were carefully investigated. Dense bulk Mo₂TiAlC₂ ceramics were consolidated by SPS, and the physical and mechanical properties, as well as oxidation resistance, were systematically studied. The conclusions are listed as follows: 

1) High purity Mo₂TiAlC₂ powder could be prepared at 1873 K using Mo, Ti, Al, and C element powders with the molar ratio of 2:1:1.25:2 in the flowing argon. The formation mechanism of Mo₂TiAlC₂ was ascribed to the reaction among Mo₃Al₂C, Mo₂C, TiC, and C. 

2) The SPSed dense Mo₂TiAlC₂ ceramics with a high relative density of 98.3% had the fine grain size of 3.5 μm in length and 1.0 μm in width. The electrical conductivity measured at room temperature was 0.77 × 10⁶ Ω^–1·m^–1, and it tended to increase with the decreasing temperature. When the temperature reached 2 K, the electrical conductivity increased to be 0.95 × 10⁶ Ω^–1·m^–1. The thermal conductivity measured in the temperature of 300–1273 K decreased from 8.0 to 6.4 W·(m·K)^–1, and TEC measured in the temperature of 350–1100 K was 9.0 × 10^–6 K^–1. 

3) Dense Mo₂TiAlC₂ ceramic had the low intrinsic Vickers hardness of 5.2 GPa, high fracture toughness of 6.5 MPa·m^1/2, high compressive strength of 1079 MPa, and high flexural strength of 407.9 MPa. When the indent load was up to 300 N, the residual flexural strength could still hold 84% of the value of the undamaged sample, presenting excellent damage tolerance. 

4) The oxidation resistance of dense Mo₂TiAlC₂ ceramics could retain up to 1200 K in the air, presenting the potential application in the high-temperature fields.

References: omitted