Synthesis of Ti2SnC under Optimized Experimental Parameters of Pressureless Spark Plasma Sintering Assisted by Al Addition
As an effective and novel rapid sintering technology with the advantages of fast heating speed and short sintering time, SPS has been applied to the research and development of various materials. After sintering at 1325°C, Ti5Sn3 and Sn occurred as impurities accompanying the synthesis of Ti2SnC with a raw powder mixture of Ti/Sn/C = 2/1/1 (molar ratio). But by addition of 0.2 molar Al, and further optimization of sintering parameters at 1400°C for 10 min, almost fully pure Ti2SnC was obtained with a clear layered microstructure. The reaction mechanism analysis suggests that this beneficial effect of Al could be attributed to the suppression of decomposition of Ti2SnC by formation of Ti2SnxAl1−xC solid solution at a high sintering temperature. The present study reports a novel route to synthesize Ti2SnC by PL-SPS with a self-designed graphite die, and Al was also proposed as a sintering aid to remove impurities.
In the 1960s, the Nowotny team  pioneered the concept of ternary transition metal carbides or carbonitrides and discovered a variety of compounds with similar structures. Almost forty years later, Barsoum et al. classified these materials as Mn+1AXn phases(or MAX phases), wherein M is a transition metal element, A is a family of IIIA or IVA element, X is a C or N element, and n is generally 1, 2 or 3 [2–4]. Among these compounds, Ti2SnC is one of the most attractive materials for its excellent properties of low hardness, eminent electrical conductivity, high modulus elasticity, high fracture toughness, self-lubrication, high chemical resistance, and good thermal stability [5–9]. Therefore, it is promising as a new generation of motor brush materials, heat exchanger materials, and various antifriction wear parts, chemical reactor mixer bearings, fan bearings, and special mechanical seals .
Up to now, Ti2SnC has been synthesized by various methods mostly using the powder mixture of Ti/Sn/C elements or their compounds [10–14]. In consideration of large-scale fabrication possibility, self-propagating high-temperature synthesis (SHS), and pressureless sintering have been paid much attention due to their simplicity and easy operation . However, unexpected impurities, such as TiC, Ti6Sn5, and or Sn, are usually found to accompany the formation of Ti2SnC in these processes. Although some researchers have tried to increase the purity by tailoring the molar ratio of Ti–Sn–C or Ti–Sn–TiC, the almost complete conversion of the raw material to the desired ternary compound is still a big challenge .
Spark plasma sintering (SPS) has been emerging for more than thirty years as a modern method to produce advanced ceramics with great application prospects due to its rapid sintering rate and low sintering temperature [17–19]. Most recently, pressureless spark plasma sintering has been realized as a promising method for special requirements [20–22]. For example, Dudina et al. chose pressureless spark plasma sintering as the treatment method for reactive sintering of porous FeAl which reduces the time of high-temperature exposure thus short ending the sample shrinkage time . By using this method, single-phase FeAl powders can be obtained at 800°C for the reason that electric current can be heated rapidly and uniformly distributed in the whole volume of the powder sample. Through coupling the combined aspect of conventional pressureless sintering with fast heating, Sairam et al. synthesized about 90% density of CrB2 at 1900°C to 2000°C by multistep PL-SPS . To the best of our knowledge, however, the synthesis of MAX by PL-SPS has not been reported yet.
In this study, Ti2SnC was synthesized from mixed powders of Ti/Sn/C using Al as a sintering aid by a PL-SPS process with a specially designed graphite die. Adding Al can prevent the generation of impurities in sintering MAX phases as reported . The effects of molar ratio and other experimental parameters on the formation and morphology Ti2SnC were investigated attentively. The reaction routes and mechanism were proposed through experimental and theoretical analysis.
2. Experimental Details
Powders of Ti (99.9% purity, 20 m), Sn (99.9% purity, 3 m), Al (99.9% purity, 40 m), and C (99.9% purity, 15 m) were mixed using an agate mortar in ethanol for 8 h with different molar ratios. Then, the mixed powders were precompacted in a drying cabinet and put into a self-designed graphite die with two fastigiated T-shape punches (10 mm front diameter and a 8 mm rear side die) for achieving the effect of pressureless sintering on both ends of the sleeve (50 mm in height, 10 mm in inner diameter, and 30 mm in outer diameter), which applied pressure on the punches instead of the powders (Figure 1). The specific sintering current and pressure are adopted to the die by using the upper and lower T-shape punches. It is known that by using Al and Sn with low melting points, the gas releasing and punch sticking to sleeve should be hindered to avoid the composition segregation. Combined with optimized sintering parameters, including temperature, holding time, and so on, such T-shape punches with fastigiated ends are best for demoulding to make the graphite dies reusable. A layer of carbon paper was wound on the graphite mold in order to avoid bonding between powders and graphite die in the sintering process . Then, the graphite die was heated at 100°C/min in SPS furnace (SPS-2040 Japan) filled with Ar, at low temperature about 550°C, in order to promote the formation of Ti–Sn–Al intermetallics; 3 min residence time is in the process of binding. Then the graphite die was held at a target temperature from 800°C to 1500°C for 10 min. The as-synthesized samples were determined by X-ray diffraction (XRD, Bruker D8 Advance) to identify the phase composition, and the microstructure of the product sintered in the 800°C to 1500°C was characterized by field emission scanning electron microscopy (FE-SEM, S4800) equipped with an energy dispersive spectroscopy (EDS).
3. Results and Discussion
3.1. Effect of Molar Ratio on the Formation and Morphology of Ti2SnC
Figure 2 shows the XRD patterns of products at the temperature 1325°C with different molar ratios of Ti : Sn : Al : C = (a) 2 : 1 : 0 : 1; (b) 2 : 1 : 0.05 : 1; (c) 2 : 1 : 0.1 : 1; (d) 2 : 1 : 0.15 : 1; (e) 2 : 1 : 0.2 : 1; (f) 2 : 1 : 0.3 : 1 in which the corresponding SEM images of the fracture surfaces were also embedded for reference. Firstly, it is clear to observe different phase compositions with the addition of Al. When adding Al, the new diffraction peak of Ti2SnC is shown in Figure 2(b) such as β(006), β(105), and β(110). But no Al (Figure 2(a)) favors the formation of Ti5Sn3 and a few impurities. In Figure 2 however with increasing Al ratio to 0.2, the peak height of Ti2SnC increased and a few Ti3Al appears. The typical layered morphology at the Al ratio of 0.2 should also be mentioned here, validating our conjecture for the formation of Ti2SnC. From Figure 3, the diffraction peak of Ti2SnC (β(103)) shifts to a larger angle with an increase in Al from 0.05 to 0.2, indicating that the addition of Al could be reported to benefit the deletion of the impurities in Ti2SnC since the solid solutions were produced.
3.2. Effect of Sintering Temperature and Holding Time on the Formation and Morphology of Ti2SnC
Figure 4(a) shows the effects of (a) sintering temperature and (b) holding time on the peak ratio of Sn/Ti2SnC and Ti5Sn3/Ti2SnC at a molar ratio of 2 : 1 : 0.2 : 1 for Ti : Sn : Al : C. As shown in Figure 4(a), with increasing the sintering temperature from 1325°C to 1500°C, the peak intensity of Ti2SnC(Iβ(103)) increased, whereas the impurity of titanium tin compound (Ti5Sn3) decreased inversely, indicating that the purity of Ti2SnC was elevated. With further increasing the sintering temperature to 1400°C, however, the peak ratio of both Iα(202)/Iβ(103) and Iγ(220)/Iβ(103) increased, suggesting that the impurity content of Ti5Sn3 and Sn unexpectedly decreased due to the decomposition of Ti2SnC at a higher temperature, for which Sn was released to leave TixSny dissociated [27, 28]. Moreover, the result peak ratio of Iα(202)/Iβ(103) is lower than the previous results, indicating that the impurity was decreased effectively . But the effect of holding time is almost negligible since the purity kept almost the same after extending the holding time from 10 to 60 min at 1400°C (Figure 4(b)).
On the other hand, consolidation of Ti2SnC at a pressure of 100 MPa from the same raw powder mixture of 2 : 1 : 0.2 : 1 for Ti : Sn : Al : C was also conducted for comparison with that by PL-SPS. As shown in Figure 5, before the sintering temperature increased near 1400°C (about 1370°C), the graphite die was found to be broken in the SPS furnace, which could be attributed to the release of liquid Sn impurity from the decomposition of Ti2SnC, and the as-obtained sample was identified to be composed by Sn as well as Ti6Sn5 and TiC, inducing the extremely lower content of Ti2SnC . Such comparison validates PL-SPS as more competitive compared with SPS for synthesis of Ti2SnC powders. Compared to conventional pressureless sintering for Ti2SnC MAX phase, pressureless spark plasma sintering (PL-SPS) has the merits of short sintering time at almost 10 minutes and rapid temperature increasing/decreasing rate (100°C/min). By optimization of the experimental parameters, this method is quite available for fabrication of high purity MAX powders with a low cost and high efficiency.
3.3. Reaction Mechanism
Vincent et al. have studied the stability of Ti2SnC in Al liquid and found that Ti2SnC tended to be decomposed completely according to the following formula at a temperature above 1000°C :
However, it should be noticed that the addition of minor Al also facilitates the formation of highly pure Ti2SnC by PL-SPS in our present study, which is an interesting phenomenon that needs explanation. The reason is that Al decreases the residual impurities, which will be elaborated as follows.
The optimized powder mixture of 2 : 1 : 0.2 : 1 for Ti : Sn : Al : C was sintered at 800°C to 1300°C to declare the formation mechanism by SPS, and the XRD patterns of as-obtained samples are shown in Figure 6. At 800°C, Ti6Sn5 appeared in XRD results and then decreased continuously with increasing temperature and turned to Ti5Sn3 at 1000°C due to the reaction with Ti. The XRD patterns of Ti3Al also appear at about 1000°C. It is astonishing that the target product Ti2SnC started to appear at a low temperature of 1000°C, accompanied by Ti5Sn3, Sn, and TiC. Figure 7 shows the TG-DSC-DTG curve of Ti2SnC with ratio Ti : Sn : Al : C = 2 : 1 : 0.2 : 1 heated at a rate of 10°C/min from room temperature to 1200°C. The sharp endothermic peak at 230°C corresponds to the melting of Sn. The other exothermic peak was observed at 560°C, 800°C, and 950°C clarify the reaction route of the Ti/Sn/C system.
However, it should also be mentioned that Al could consume Ti by forming Ti–Al intermetallics . In consideration of minor content of Al, the formation of TimAln (Ti3Al) should be taken into consideration for formation of Ti2SnxAl(1−x)C solid solution.
Figures 9 and 10 show the microstructures of the powders sintering at 800°C, 1000°C, and 1200°C. In Figure 9(c), layered structure starts to appear which is considered as Ti2SnC validated by EDS spectrum. The layered structure with the size between 10 m and 12 m is more obvious at 1200°C. From Figures 6 and 9, the results suggest that the formation of Ti2SnC is in accord with previous work [7, 9, 11, 14]. Therefore, the modified reaction route for the synthesis of Ti2SnC assisted by Al can be clarified based on this consideration, as shown in Figure 11. By increasing the sintering temperature gradually, Sn and Al from the raw powder mixture start to melt successively. Then, intermetallics Ti5Sn3 starts to form as shown in Figure 11(b). When increasing the sintering temperature further, the impurity phase of TiC appears as validated combined with TimAln (most possibly Ti3Al) [34–36], and simultaneously, Ti2SnxAl1−xC formed in Figure 11(c). As the sintering temperature goes near 1400°C, almost fully pure Ti2SnxAl1−xC formed by the whole reaction of TiC, TimAln, and Ti5Sn3, and Ti2SnxAl1−xC promotes the preparation of layer-like Ti2SnC [14, 27].
Ti2SnC was obtained by PL-SPS using Al as a sintering aid. The addition of Al was found to favor the fabrication Ti2SnC by preventing the generation of Sn-like impurities and formation of Ti2SnxAl1−xC solid solution at optimized sintering parameters of 1400°C for 10 min. But with further increasing the temperature, Sn impurity increased due to the incurable decomposition of Ti2SnC. By PL-SPS, we prepared porous compact Ti2SnC. The present study also validates PL-SPS as a promising method to synthesize MAX phase by careful experimental design and parameter optimization.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
The authors would like to acknowledge the financial supports from Fundamental Research Funds for the Central Universities (2015B01914), the National Natural Science Foundation of China (51301059), the Natural Science Foundation of Jiangsu Province (BK20161506), the Opening Project of State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology, 2016-KF-8), and the National 973 Plan Project (2015CB057803).
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