1 Introduction

Fused filament fabrication (FFF), a simple and cost-effective filament extrusion-based additive manufacturing technology, gains more and more attentions owing to its outstanding ability of manufacturing complex geometric samples and mold/machining free characteristic, compared with the traditional manufacturing methods (Ref 1). FFF-based additive manufacturing is one of the free form fabrication technologies, which produces the specimens by building the material melt in a layer-by-layer manner. It has been extensively used in thermoplastic polymer and polymeric composites and multi-materials (Ref 2,3,4). In some cases, 3D printed composites exhibit superior property to the conventional mixed samples. Kumar et al. verified that 3D printed multi-material sample showed better mechanical strength and morphological characteristic than its hybrid-blended counterpart (Ref 5). Beyond the thermoplastic polymers, it is capable of printing non-polymeric powders by compounding with thermoplastic binders that can be removed by solvent/catalytic debinding and subsequent sintering processes to get compact samples (Ref 6,7,8). The sample can be printed above the melting point of the binder in any geometry that was designed by computer-aided software. This filament-based additive manufacturing technology has been applied in manufacturing various kinds of materials. Up till now, steels, hard metals, cermets, ferrites, superalloys, and ceramics filaments with various binders have been successfully printed (Ref 9,10,11). Sadaf et al. have manufactured 316 L stainless steel (316 L SS) sintered parts with a densification of 93% by using single-component binder, low-density polyethylene (LDPE) (Ref 12). A much higher tensile strength (540 MPa) and elongation (60%) of 316 L SS sintered compact were achieved by using hot isostatic pressing (HIP) sintering the printed green parts (Ref 13). These great progresses in additive manufacturing technologies allow us to explore novel fillers to produce complex-shaped functional materials with desirable properties by FFF-based 3D printing technology, satisfying the different requirements of the properties.

In our research, based on the FFF-based printing feasibility of metals and ceramics, a typical Mn+1AXn (MAX) phase Ti3AlC2 was selected to be the candidate for printing the functional parts because of its excellent advantages. MAX phases are a distinctive group of ternary carbide or nitride ceramics with layered crystal structures. The chemical bonds composed of a mixed covalent–ionic–metallic structure in MAX phases result in their unique properties. This kind of MAX phase simultaneously owns excellent properties of metals and ceramics, such as good electrical and thermal conductivity, easy machinability, excellent thermal shock resistance, low frictional coefficient, high thermal stability, and high temperature oxidation resistance (Ref 14, 15). The electrical resistivity of MAX phases is low and in general less than that of the corresponding binary transition metal carbides or nitrides. The electrical conductivity and thermal conductivity of the MAX phases at room temperature are in the order of 106 Ω−1·m−1 and reach around 40 W·m−1·K−1, which are much higher than that of TiC and TiAl (Ref 14). These MAX phases are typically used in high temperature occasions, nuclear engineering, and aerospace industries (Ref 16). The current work on MAX phase is mainly focused on manufacturing products by various sintering technologies and exfoliating MAX phase into 2D thin MXene sheets to fabricate advanced nanocomposites (Ref 17, 18). A few recent publications have applied Ti3AlC2 as strengthening and functional fillers in different matrices. As much as we have known from the literature, Ti3AlC2 powders at low loadings have been incorporated to strengthen metals (Al, Cu) by load transfer through a strong interfacial layer (Ref 19, 20), toughen phthalonitrile (PN) resin (Ref 21), endow electrical conductivity to epoxy resin (Ref 22, 23), and thermal conductivity to polyimide (Ref 24) as well as SiC (Ref 25). Up till now, hardly any publications of printing Ti3AlC2 powders via fused filament fabrication technology have been reported from the knowledge of the authors. More importantly, the low density of Ti3AlC2 (4.2 g·cm−3) also makes it an ideal functional component in polymeric composites. A commercial binder composed of a thermoplastic elastomer and polyolefin grafted with a polar component was selected because it owns good hot working processability, high elongation, and excellent adhesion to polar thermoplastics, metals, and ceramics, which is favorable for fused filament fabrication and 3D printed green parts with improved properties (Ref 26). Therefore, in the present work, Ti3AlC2-based composite with this thermoplastic elastomer as the binder was first extruded into filament and then printed into green part. This green part can not only be used directly as functional materials with designed shapes but also can be further processed by the following removal of the binder and subsequent debinding and sintering procedures on the other hand. To this end, the morphology, thermal/electrical conductivity, and EMI shielding effectiveness were investigated for the first time in our research project. Since the most polymers are electrical and thermal insulator, the content of Ti3AlC2 can be adjusted to obtain the polymeric composites with suitable thermal and electrical conductivities. Such composites with good electrical/thermal properties are promising in antistatic, electrical conductor, electromagnetic interference shielding, and heat dissipating fields.

Recently, Zhang et al. fabricated MAX phase-SiOC composite with complex shapes by mixing pre-polymer with MAX phase and subsequent pressureless heat treatment method (Ref 27). From the knowledge of the authors, we have known that very little work has been focused on the additive manufactured MAX phases (Ti3AlC2) via FFF and 3D printing technologies. Consequently, the multi-functional MAX phase composites have not been explored yet. Meanwhile, there are increasing requirements for simultaneously conducting heat and electricity in some electrical and electronic engineering components. The present research will unravel the novel applications of 3D printed MAX phase composites in conductive/semi-conductive and thermal management fields.

2 Experimental

2.1 Materials

MAX phase (Ti3AlC2) powders were provided by the Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), China. Ti3AlC2 powders was produced into bulk by compact hot pressing TiAl and TiC powders in a molar ratio of 1:1.9 and then crushed into powders by ball milling, as reported in the previous paper (Ref 28). The binder was composed of a commercial binder of a thermoplastic elastomer (KRAIBURG TPE GmbH & Co. KG, Germany) and polyolefin grafted with a polar component to improve the adhesion to different kinds of powders (gPO, BYK-Chemie GmbH, Germany). The ratio of MAX phase and the binder was 1:1 (vol.%).

2.2 Compounding, Filament Fabricating, and 3D Printing

2.2.1 Feedstock Compounding

The Ti3AlC2 powder and the binder were compounded in a single extruder (LSJ20, plastic extruder head, D = 20 mm, L/D = 25:1) attached on torque rheometer (XSS-300, Shanghai Kechuang, China). Prior to compounding, the powders and binder were dried in the oven at 80 °C for 24 h. The temperature was set to 190, 190, 200, and 200 °C for the heating zones and the nozzle die during the extrusion process, respectively. The rotational speed of the screw was set at 60 rpm. The compounding was repeated twice at the same extrusion conditions in order to obtain a uniform mixture. The extruded strands were then cut into pellets for the following fused filament fabrication.

2.2.2 Filament Production

The feedstock was dried at 80 °C for 24 h using a VD 23 vacuum drying chamber (Binder GmbH, Germany) to avoid defects during filament fabricating and printing. Using the extruder for producing filaments was not possible because the amount of feedstock produced was lower than the minimum required amount to fill the chamber. Therefore, the filament production of the feedstock was done in the high-pressure capillary rheometer Rheograph 2002 (Göttfert Werkstoff-Prüfmaschinen GmbH, Germany) with a die of 1.75 mm diameter and 30 mm length. The filaments were produced at a temperature of 200 °C with a constant shear rate of 193.45 1/s and piston (diameter 12 mm) speed of 0.9 mm/s. Finally, the filaments were transported on a conveyor belt made of polytetrafluoroethylene (PTFE) as they were cooled down to room temperature.

2.2.3 3D Printing of Green Parts

3D printing of the green prats was performed via FFF method. A small desktop 3D printer (Wanhao Duplicator i3, China) was modified with an E3D V6 type HotEnd. Printing was performed with a 0.6 mm nozzle at a temperature of 250 °C; the bed temperature was set to 100 °C; and the printing speed was set to a maximum of 30 mm/s. The finished parts were carefully removed from the glass platform and prepared for the next steps.

2.3 Characterizations

The morphology of MAX phase and cross-sectional surface of MAX phase/binder green part were observed on scanning electronic microscope (SEM) at a voltage of 20 kV. The tensile properties were tested on a universal testing machine (WSM-20kN, China) at a loading rate of 3 mm/min. Three dumbbell-shaped samples were tested, and their gauge length was 50 mm. Three cylindrical samples with the diameters of 6 mm (diameter/height > 0.4) were used for the compression test (WSM-20kN, China). In order to ensure the stability of the samples, the loading rate of 0.5 mm/min was selected for the compressive test. The compressive test was interrupted when the load force was not increased any more. Standards of GB/T1040.2-2006 and GB/T 1041-2008 were followed for the tensile and compression tests, respectively. The melting behavior and thermal stability of the samples were analyzed on differential scanning calorimeter (DSC-Q20, TA Instrument, USA) and thermal gravity analyzer (TGA-Q500, TA Instrument, USA) at a heating rate of 10 °C /min under the protection of N2. The electrical conductivity of the MAX phase/binder composites (D = 50 mm) was measured at room temperature using a two-probe measurement with two copper electrodes (Keithley-2000 digital multimeter, USA). The electromagnetic parameters of the composite in frequency range of 8.2-12.4 GHz were collected with a vector network analyzer (VNA) (Agilent N5230A, USA) using waveguide method. The S parameters of the reflected (S11 or S22) and transmitted (S21 or S12) powers were used to calculate the shield effectiveness (SE). The thermal conductivity (k) was tested on the thermal conductivity meter (UNITHERM™ Model 2022 thermal conductivity instrument, Anter Corporation, USA) by the ASTM E1530 guarded heat flow meter method. A round disc with a diameter of 50 mm was prepared for the measurement.

3 Results

3.1 XRD of Ti3AlC2

The characteristic diffraction patterns of Ti3AlC2 powders were detected on X-ray diffractometer and are displayed in Fig. 1. The typical peaks positioned at 9.6°, 34.1°, 39.1°, 41.7°, and 60.1° correspond to the planes of (002), (101), (104), (105), and (110), respectively. As shown in Fig. 1, all those peaks are well matched with those of extracted from JCPDS (52-0875) in Jade 6.5 software.

Fig. 1
figure 1

XRD patterns of as-received Ti3AlC2 powder

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3.2 Morphology

The cross-sectional morphology of Ti3AlC2/binder filament and 3D printed green part is shown in Fig. 2. As shown in Fig. 2(a) and (b), it is clear that Ti3AlC2 powders are well dispersed in the binder system, adhering tightly with the binder. The dense aligned layers of Ti3AlC2 even can be distinguished at higher magnification (Fig. 2b). Moreover, Ti3AlC2 particles distribute uniformly in random orientations, indicating the isotropic characteristic of Ti3AlC2/binder filament. The well-connected interface between Ti3AlC2 and the binder is a good indicator for the following printing process and the final properties of the composites. The printed sample presented in Fig. 2(c) and (d) is stacked layer by layer, whose boundaries between each layer are easily seen. This is the characteristic of filament-based printing method. The layer width is about 0.19 mm. The inset in Fig. 2(c) is the digital images of the printed green parts. Evidently, there is a gap between each layer at the outer surface (Fig. 1c), which disappears in the cross-sectional image of fractured surface in liquid nitrogen (Fig. 1d). The thin Ti3AlC2 sheets can be viewed at the higher magnification of the inset image in Fig. 1(d). The inset samples are the printed green parts, suggesting the good printability of the Ti3AlC2/binder system.

Fig. 2
figure 2

SEM cross-sectional images of Ti3AlC2/binder (a) and (b) filament, (c) and (d) 3D printed green part (Color figure online)

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3.3 Thermal Characterization

The melting point/range of the binder is important for determining the compounding temperature during the melt blending, filament fabrication, and the following 3D printing processes. The binder used is a thermoplastic elastomer with excellent processing properties, showing (see Fig. 3) a wide melting temperature region ranging from 140 to 170 °C. There are no peaks caused by heat absorption/release in pure Ti3AlC2 powders during the heat/cool/heat process. However, obvious difference is seen in the DSC curves of MAX phase/binder composite. The melting temperature range of Ti3AlC2/binder is reduced by 10 °C (140-160 °C) compared to that of pure binder. Furthermore, the melting peak temperature is also decreased down to 153.9 and 154.8 °C for the first and second heating run, respectively. This is probably owing to the disruption of the molecular chains of the binder by the Ti3AlC2 powders.

Fig. 3
figure 3

DSC curves of Ti3AlC2 powder, binder pellets, and as-extruded Ti3AlC2/binder composite (1 h and 2 h represent 1st and 2nd heating run, respectively; Tc: Temperature of solidification/crystallization)

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The thermal stability of Ti3AlC2/binder was evaluated by the weight loss/gain (TGA/DTGA curves) during the heating process. From TGA curves, the decomposition behavior of the binder can be tracked, which is an indicator for the maximum hot processing temperatures (extrusion temperature and printing temperature) and the subsequent debinding temperature in the shaping–debinding–sintering (SDS) process (Ref 29). In the meantime, the content of the binder and the filler can be evaluated from the percentage of weight remaining. The theoretical value against the final experimental value (wt.%) of the feedstock filaments at 600 °C in SS 316L weight (%) after the binder degradation based on the SS 316L vol.% in the feedstock was calculated and compared (Ref 30). Ti3AlC2 is a compound, showing good stability dependence on temperature. Differently, the binder starts to decompose above 250 °C, almost fully degrade at 490 °C (0.42% residue). Therefore, the remained weight of Ti3AlC2/binder composite at 600 °C can be regarded as MAX phase (82.56 wt%) (see Fig. 4). This value is close to our designed composition (81.93wt.%; 50vol.%). Additionally, the maximum degradation rate is remarkably reduced from 1.61%/°C for the pure binder to 0.21%/°C for the composite with stable Ti3AlC2, respectively, meaning that the degradation rate of the binder is decreased due to inclusion of Ti3AlC2. This suggests that there is a good interfacial adhesion between Ti3AlC2 and the binder, as illustrated in Fig. 2(b) and (d).

Fig. 4
figure 4

TGA and DTGA curves of Ti3AlC2, Ti3AlC2/binder, and binder

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3.4 Thermal/Electrical Conductivity and EMI Shield Effectiveness

Ti3AlC2 powders are the preferred functional fillers for thermally and electrically insulated polymers due to their outstanding comprehensive properties (Ref 31). As expected, the Ti3AlC2/binder green part presents good electrical and thermal conductivity, as tabulated in Table 1. As listed in Table 1, the SE value and volume electrical conductivity are around 38.23 dB at the thickness of 2.92 mm and 0.21 S/m, which satisfies the requirements for antistatic and EMI shielding applications (Ref 32). As seen from calculated coefficient of reflectivity and absorption in Fig. 5, the reflection of electromagnetic wave plays a dominant role in the total shielding effectiveness, accounting for over 80% of the total shielding coefficient. This mechanism is in agreement with electrically conductive shielding materials such as metals. In addition, the composites also exhibit a good thermal conductivity value of 1.75 W/m·K at room temperature. These outstanding properties are ascribed to the continuous network of Ti3AlC2 powders, forming a connective thermal and electrical conduction pathway. This property has paramount contribution when the material is used as electric and electronic component, capable of prolonging its service duration by effectively dissipating the heat (Ref 33). Compared with the commonly used metallic fillers for the improvement of thermal/electrical conductivity of the polymer matrix, Ti3AlC2 (4.2 g/cm3) owns the advantages of lower density and stronger environmental resistance (Ref 14, 34).

Table 1 Properties of Ti3AlC2/binder composite
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Fig. 5
figure 5

Thermal conductivity, EMI shield effectiveness and reflective and absorption coefficients of as printed Ti3AlC2 /binder green part

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3.5 Mechanical Property

The mechanical property of the filler/binder affects its following processing, especially the printing ability. The tensile behavior of Ti3AlC2/binder composite is shown by the typical stress–strain curve in Fig. 6(a). The strength gradually increases until the sample breaks under tensile test. Apparently, no expected yielding stress is observed on the stress–strain curve. The ultimate tensile strength and elongation at break of the composites are measured to be 8.29 ± 1.64 MPa and 18.20 ± 2.16%, respectively. The mechanical properties are not only associated with the content and size of the fillers in the composites, but also related to the kind of the fillers. The values of tensile test of Ti3AlC2 /binder fall between those of the ceramic and stainless steel composites, yet higher than the strength and elongation of Ti6Al4V/binder with the binder volume fraction of 55% (Ref 10, 35). To be sure, a better combined mechanical strength and elongation can be achieved by adjusting the size and content of Ti3AlC2 powder, which will be carried out in our following research. Meanwhile, the deformation behavior was also evaluated under the compressive load. Due to the ductile feature of the composite, the sample does not fracture during the compressive test. The values of ultimate compressive strength and elongation were adopted when the load force did not rise any more. As shown in Fig. 6(b), the ultimate compressive strength and elongation reaches 44.90 ± 2.56 MPa and 33.76 ± 1.13%, respectively, which are much higher than those values of tensile test. This ultimate compressive strength is more than 10 times lower than that of the sintered bulk Ti3AlC2 compacts (Ref 36). The fracture surface after tensile test was also examined. As observed from Fig. 6(c) and (d), the fracture surface is rough, with the traces of the ductile behavior of the thermoplastic binder. The high loading of Ti3AlC2 breaks the integrity of the binder, leading to the decreased strength and elongation. The interface between the binder and Ti3AlC2 particles becomes the weak site, where the cracks originate and lead to failure of the composite during tensile process.

Fig. 6
figure 6

Tensile and compressive behavior and cross-sectional morphology of Ti3AlC2/binder composite (a) engineering tensile stress–strain curves, (b) engineering compressive stress–strain curves, (c, d) SEM image of the fracture surface after tensile test

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4 Conclusion

The highly loaded Ti3AlC2/binder composites with good comprehensive properties were manufactured by fused filament fabrication (FFF)-based 3D printing technologies. The following conclusions have been drawn:

  1. 1.

    Ti3AlC2 powders showed good filament fabrication ability and printability. Ti3AlC2 powders were uniformly dispersed in the binder and had good interfacial adhesion with the binder.

  2. 2.

    The electrical conductivity, thermal conductivity, and electromagnetic interference shielding effectiveness of the composite with the volume ratio of 1:1 were around 0.21 S/m, 1.75 W/m·K, and 38.23 dB, respectively.

  3. 3.

    The composite also presented the reasonable mechanical property, showing better compression resistance. The ultimate tensile and compressive strength were around 8.29 and 44.90 MPa, respectively.

  4. 4.

    The combination of MAX phase and FFF-based 3D printing technology provides a new insight into producing novel materials with intricate geometry and multifunctional properties.