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

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·m1/2, 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.


Introduction 
In recent years, MAX phases of ternary layered com- † Yunhui Niu and Shuai Fu contributed equally to this work.
In this paper, high purity Mo 2 TiAlC 2 powders were synthesized firstly and its reaction mechanism was systematically investigated. Dense Mo 2 TiAlC 2 ceramics were consolidated by SPS and their physical and mechanical properties, as well as oxidation resistance, were studied.

Experimental
The preparation of high purity dense Mo 2 TiAlC 2 ceramics was divided into two steps: One step was to explore the reaction mechanisms to synthesize high purity Mo 2 TiAlC 2 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 Mo 2 TiAlC 2 powders, the reaction mechanisms of Mo 2 TiAlC 2 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 Mo 2 TiAlC 2 powder could be successfully prepared at 1873 K.
For fabricating the dense Mo 2 TiAlC 2 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 www.springer.com/journal/40145 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 Mo 2 TiAlC 2 ceramics using the acid solution consisting of HNO 3 , HF, and H 2 O 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 Mo 2 TiAlC 2 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 Mo 2 TiAlC 2 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 Mo 2 TiAlC 2 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.

1 Synthesis and microstructure characterization
In order to explore the reaction mechanisms of Mo 2 TiAlC 2 and synthesize pure Mo 2 TiAlC 2 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 AlMo 3 ( Fig. 1(c)). In addition, a small amount of AlTi 3 and Mo 2 C were detected. The chemical reaction equations could be expressed as Al + 3Mo = AlMo 3 (1) Al + 3Ti = AlTi 3 (2) 2Mo + C = Mo 2 C As the temperature reached 1573 K, the diffraction peak intensity of AlMo 3 decreased a lot and the AlTi 3 phase disappeared, whereas the diffraction peak intensity of Mo 2 C continued to increase to be the main phase, indicating the continuous reaction between Mo and C ( Fig. 1(d)). Additionally, plenty of Mo 3 Al 2 C, a small amount of TiC, and a few Mo 2 TiAlC 2 were examined. At the same time, graphite was also reacted. It is suspected that the consumption of AlMo 3 , AlTi 3 , and C probably contributed to the formation of Mo 3 Al 2 C and TiC. When further increasing the temperature to be 1873 K, all phases of C, AlMo 3 , Mo 2 C, Mo 3 Al 2 C, and  TiC disappeared and only Mo 2 TiAlC 2 could be examined ( Fig. 1(e)). So, the newly formed phases of Mo 3 Al 2 C and TiC possibly reacted with Mo 2 C to form Mo 2 TiAlC 2 . Therefore, the chemical reaction equations could be expressed as 8AlMo 3 + 4AlTi 3 + 21C = 6Mo 3 Al 2 C + 12TiC + 3Mo 2 C (4) 2Mo 3 Al 2 C + 4TiC + Mo 2 C + C = 4Mo 2 TiAlC 2 (5) As a result, it could be determined that high purity Mo 2 TiAlC 2 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 Mo 2 TiAlC 2 bulks and pure Mo 2 TiAlC 2 powder. It is detected that after consolidation by SPS, the obtained dense Mo 2 TiAlC 2 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 Mo 2 TiAlC 2 particles. It is concluded that during the sintering process, the growth of Mo 2 TiAlC 2 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 Ti 2 AlN ceramic densified by SPS, which was ascribed to the mechanical oriented deformation [50].
The microstructure characterization of dense Mo 2 TiAlC 2 ceramics was shown in Fig. 3. In the etched surface of Mo 2 TiAlC 2 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 Mo 2 Ti 2 AlC 3 (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 Mo 2 TiAlC 2 ceramics. It is seen that the sample is very dense and the porosity is extremely rare. The relative density of Mo 2 TiAlC 2 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.

2 Physical and mechanical property evaluation
In order to analyze and describe the properties of Mo 2 TiAlC 2 ceramics systematically, the physical and mechanical properties of dense Mo 2 TiAlC 2 ceramics were compared to those of Ti 3 AlC 2 and Mo 2 Ti 2 AlC 3 . In comparison with the property of Ti 3 AlC 2 , 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 Mo 2 TiAlC 2 and Mo 2 Ti 2 AlC 3 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 Mo 2 Ti 2 AlC 3 and Ti 3 AlC 2 , including physical properties and mechanical properties [40,51]. Also, the properties of reported Mo 2 TiAlC 2 ceramic were added to compare the difference of properties affected by the microstructure [38]. Figure 4 displays the electrical conductivity and electrical resistivity of Mo 2 TiAlC 2 at the temperature range of 2-300 K. The electrical conductivity of Mo 2 TiAlC 2 measured at room temperature was 0.77 × 10 6 Ω -1 ·m -1 , which is higher than that of Mo 2 Ti 2 AlC 3 (0.41 × 10 6 Ω -1 ·m -1 ) [40], but lower than that of Ti 3 AlC 2 (3.48 × 10 6 Ω -1 ·m -1 ) [51]. In addition, the electrical conductivity of Mo 2 TiAlC 2 measured by Anasori et al. [38] at 10 K was 0.67 × 10 6 Ω -1 ·m -1 , lower than the present 0.95 × 10 6 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 × 10 6 to 0.77 × 10 6 Ω -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 where ρ 0 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 × 10 3 K -1 , which is higher than that of Mo 2 Ti 2 AlC 3 (0.324 × 10 3 K -1 ) [40], but lower than that of Ti 3 AlC 2 (3.10 × 10 3 K -1 ) [51].  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 Mo 2 Ti 2 AlC 3 (6.82 W·(m·K) -1 ) [40], and greatly lower than that of Ti 3 AlC 2 (40 W·(m·K) -1 ) [51]. A least-square fitting of the temperature dependent thermal conductivity (λ) for Mo 2 TiAlC 2 ceramics is described as where the coefficient of determination r 2 is 0.99. The trend is similar to those of Mo 2 Ti 2 AlC 3 and Ti 3 AlC 2 , 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 = L 0 σT, where σ is the electrical conductivity at temperature T, and L 0 = 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.
The TEC of Mo 2 TiAlC 2 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 Ti 3 AlC 2 ceramic (9.0 × 10 -6 K -1 ) [51]. Whereas, in comparison with that of the similar Mo 2 Ti 2 AlC 3 ceramic (11.3 × 10 -6 K -1 ) [40], the TEC of Mo 2 TiAlC 2 exhibits a lower value, which is probably associated with the stronger bonds in the Mo 2 TiAlC 2 crystal structure [38,53].  The Vickers hardness of Mo 2 TiAlC 2 ceramics at the load of 10 N was measured as 5.2 ± 0.1 GPa, higher than both Mo 2 Ti 2 AlC 3 of 4.81 GPa [40] and Ti 3 AlC 2 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 Mo 2 TiAlC 2 ceramics, resulting in the high Vickers hardness. In addition, the Vickers hardness of Mo 2 TiAlC 2 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 Mo 2 TiAlC 2 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.
The fracture toughness of Mo 2 TiAlC 2 ceramic was tested to be 6.5 ± 0.4 MPa·m 1/2 , lower than both Mo 2 Ti 2 AlC 3 of 8.4 MPa·m 1/2 [40] and Ti 3 AlC 2 of 7.2 MPa·m 1/2 [51]. Also, Mo 2 TiAlC 2 has a high compressive strength up to 1079 ± 17 MPa, a little lower than Mo 2 Ti 2 AlC 3 of 1145 MPa [40], but much higher than Ti 3 AlC 2 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 Mo 2 TiAlC 2 is 407.9 ± 29 MPa, higher than that of Ti 3 AlC 2 (340 MPa) [51], and a little lower than that of Mo 2 Ti 2 AlC 3 (452 MPa) [40]. In addition, the damage tolerance of Mo 2 TiAlC 2 reflected by the indentation load dependence of residual flexural strength is displayed in Fig. 8

3 Oxidation resistance evaluation
Furthermore, to examine the possible application field of Mo 2 TiAlC 2 ceramic at high temperatures, the dynamic oxidation process of Mo 2 TiAlC 2 ceramics was characterized, as shown in Fig. 9. Figure 9 presents the TG and DSC results of Mo 2 TiAlC 2 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 MoO 3 because its boiling point is only 1423 K.
To further investigate the oxidation resistance of Mo 2 TiAlC 2 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 -Al 2 O 3 and R-TiO 2 , and no molybdenum oxide MoO 3 peaks were detected. The result is matching the test of TG-DSC well that the MoO 3 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 Al 2 O 3 and TiO 2 ( Fig. 11(a)). In the magnified image, the hexagonal Al 2 O 3 plates and TiO 2 rods could be clearly distinguished ( Fig. 11(b)). It seems that different from the oxidized surface of Ti 3 AlC 2 , no dense oxide layer could be formed on the surface of Mo 2 TiAlC 2 ceramic during the oxidation process [28], which presents the poor oxidation resistance above 1473 K. Therefore, it is determined that dense Mo 2 TiAlC 2 ceramic could at least endure the high temperature up to 1200 K in the air.

Conclusions
In this study, high purity Mo 2 TiAlC 2 powder was successfully synthesized by in situ reaction and the reaction mechanisms were carefully investigated. Dense bulk Mo 2 TiAlC 2 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 Mo 2 TiAlC 2 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 Mo 2 TiAlC 2 was ascribed to the reaction among Mo 3 Al 2 C, Mo 2 C, TiC, and C.
2) The SPSed dense Mo 2 TiAlC 2 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 × 10 6 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 × 10 6 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 Mo 2 TiAlC 2 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 Mo 2 TiAlC 2 ceramics could retain up to 1200 K in the air, presenting the potential application in the high-temperature fields.