As the critical component in low-voltage switching device, Ag-based electrical contacts are widely applied in contactors, breakers and relays, etc. The service life of these devices largely depends on the properties of the electrical contact material^[1]. The conventional material “Ag/CdO” has long been preferred because of its outstanding contacting and arc extinction properties since the middle of last century. However, the toxicity of CdO causes serious pollution problems, restricting its applications^[2]. In the past few decades, Cd-free electrical contact materials, such as Ag/SnO₂, Ag/ZnO, Ag/C, Ag/Ni, have been studied extensively^[3,4,5,6,7]. These substitutes cannot yet emulate Ag/CdO in terms of temperature rise, contact resistivity, machinability, arc erosion resistance, etc. Therefore, environment-friendly alternative with properties matching up CdO is in high demand.

Over the past decades, MAX phase^[8,9,10], combining excellent properties of metal and ceramic, has been widely investigated in various fields^{[11,12,13,14,15,16]}. As a representative member of MAX family, Ti₃AlC₂ has been used to reinforce Ag matrix as the electrical contact material^{[17,18,19,20,21]}. However, the electrical resistivity of the Ag/Ti₃AlC₂ composite is not satisfactory, which is initially attributed to the inter-diffusion between Al layer and Ag matrix^[22]. 2011, Gogotsi and Barsoum^[23,24] jointly obtained a new kind of carbide material (Ti₃C₂T_x) with two-dimensional structure, coined as MXenes, were produced by selectively etching off Al atom planes from its parent Ti₃AlC₂. Up to date, Ti₃C₂T_x has received great attentions of many applications^{[25,26,27,28,29]}. In addition to large specific surface area, Ti₃C₂T_x has good electrical conductivity, thermal-conducting property, and hydrophilicity^[30], and thus it is promising reinforcement for electrical conductive composites. In particular, Ti₃C₂T_x has demonstrated its potential as an additive in composites with polymers (PVA, PAM, PEI, PAN, etc.), ceramics (MoS₂, TiO₂, etc.) and carbon materials (CNT, MWCNT, CNFs, etc.)^[31]. Hence, the electric conductive Ti₃C₂T_x is expected to reinforce Ag matrix as a new electrical contact material.

In this study, the application of MXenes to electrical contact material is explored. Ti₃C₂T_x reinforced Ag-based composite was prepared by powder metallurgy, and its overall properties, such as electrical resistivity, hardness, machinability, tensile strength, and anti-arc erosion were investigated and compared with those of Ag-based composite reinforced by Ti₃AlC₂ ceramic. The mechanism of properties difference of these two kinds of samples were also analyzed and concluded. The research results would provide significant data for the design and preparation of the new generation of environment-friendly silver-ceramic composite electrical contact materials in the future.

Base materials for composites were Ag (99.9%, ~10 μm, Xinshengfeng, China) and Ti₃AlC₂ (99.0%, ~10 μm, in-situ prepared with TiC (99%, ~5 μm, Aladdin, China), Ti (99.99%, ~50 μm, Aladdin, China), Al (99.7%, ~50 μm, Zhongnuo xincai, China). Ti₃C₂T_x was obtained by immersing Ti₃AlC₂ (5 g) into hydrofluoric acid (HF) solution (100 mL, 40% in mass) for 24 h under magnetic stirring (40 ℃)^[23]. Ag/10% (in mass) Ti₃C₂T_x (Ag/Ti₃C₂T_x) and Ag/10% (in mass) Ti₃AlC₂ (Ag/Ti₃AlC₂) mixtures were individually homogenized by ball milling for 0.5 h with a medium of ethyl alcohol (99.7%, Shanghai Titan Scientific Co. Ltd., China). These two mixtures were subsequently cold-pressed into green bodies (15 mm in diameter, 2 mm in thickness) under 800 MPa, and then heat-treated at 700 ℃ for 2 h in argon atmosphere.

Phase composition of the samples was characterized by X-ray diffraction (XRD, Bruker-AXS D8, Germany). The structure change of Ti₃AlC₂ and Ti₃C₂T_x powders were further characterized by Transmission Electron Microscope (TEM) (FEI, Nova Nano 450, America). Vickers hardness of samples was tested under 0.1 MPa by the micro-hardness tester (FM-700, Future-Tech Corp., Japan). Resistivity of samples was measured by the four probe method (Metra HIT 27 I, Gossen Metrawatt, Germany). Microstructure and chemical compositions were characterized by a scanning electron microscope (SEM, FEI/Philips Sirion 2000, Netherlands), equipped with an energy dispersive spectrometer (EDS, AZtes X-MAX 80). The Ag/Ti₃C₂T_x and Ag/Ti₃AlC₂ bulk materials were processed into the dumbbell-shaped samples with length of 40.0 mm, width of 7.5 mm, middle part width of 4 mm and thickness of 2.0 mm. The tensile strength of both samples was tested at a universal test machine (AGS- X5kN, SHIMADZU, Japan) at a speed of 1 mm·min^-1. Finally, the Ti₃AlC₂ or Ti₃C₂T_x reinforced Ag-based composite electrical contact was installed in commercial contactors and tested under the harsh conditions (400V/100A/AC3, GB14048.4-2010) at Low Voltage Switch Testing Center of Shanghai Electrical Appliance Research Institute.

Fig. 1 shows the phase compositions and microstructures of raw powders (Ti₃AlC₂ and Ti₃C₂T_x). Ti₃AlC₂ was characterized by granular morphology with smooth surfaces (Fig. 1(a)), and Ti₃C₂T_x exhibited a multilayered morphology with the layer thickness of 0.15-0.37 μm (Fig. 1(b)). Fig. 1(b) obviously shows that the (002) diffraction peak of Ti₃C₂T_x is tilted towards low angle, which also confirms the expansion of Ti₃C₂T_x layer space.

The microstructures and element distributions of Ag/Ti₃AlC₂ and Ag/Ti₃C₂T_x composites are displayed in Fig. 2. As shown in Fig. 2(a, c), both reinforcements (Ti₃AlC₂ and Ti₃C₂T_x) uniformly distribute in Ag matrices, Ti₃AlC₂ retains the granular morphology while Ti₃C₂T_x takes the stripe-shaped morphology. Fig. 2(b, d) display the element distributions of Ag, Ti and Al in composites, which further confirm that Ti₃AlC₂ and Ti₃C₂T_x take different shapes in Ag matrices. Moreover, slight diffusion of Al with Ag is observed in Ag/Ti₃AlC₂ (Fig. 2(b)), while a few Al elements detected in Ag/Ti₃C₂T_x (Fig. 2(d)), which is consistent with the XRD and TEM results.

Ag/Ti₃C₂T_x composite is further observed at higher magnification SEM image (Fig. 3(a)). It is obvious that the interface between Ti₃C₂T_x and Ag matrices is clear with no trace of cracks and holes, indicating a good physical bonding. However, Ti₃C₂T_x has large contact angle with Ag substrate in the high-temperature wetting experiment (Fig. 3(b)), which confirms the absence of reactive wetting (i.e. chemical bonding) between them.

Contact materials are usually manufactured into various shapes, necessitating excellent machinability. A typical negative case is SnO₂ with high hardness leading to the poor machinability of Ag/SnO₂, which hinders its substitute for CdO^[32]. Fig. 4 shows the Vickers hardness of Ag/Ti₃C₂T_x, Ag/Ti₃AlC₂, and pure Ag (for reference). Ag/Ti₃C₂T_x possesses intermediate hardness (64 HV), and can be cut into different shapes, including rod, rivet, disc and square (the insert in Fig. 4). The good machinability originates from the 2D structure of Ti₃C₂T_x, in which weak van der Waals interaction exists between layers. In addition, contacts usually carry high current density in service, thus a low resistivity is a prerequisite for potential electrical contact materials. As shown in Fig. 4, the Ag/Ti₃C₂T_x and Ag/Ti₃AlC₂ composites own low resistivity (16×10^-3 μΩ·m of Ag for reference). In particular, the resistivity of Ag/Ti₃C₂T_x (30×10^-3 μΩ·m) is 29% lower than that of Ag/Ti₃AlC₂ (42×10^-3 μΩ·m), which is highly meaningful for the practice application.

The improved conductivity of Ag/Ti₃C₂T_x can be explained from three aspects: higher conductivity of Ti₃C₂T_x than that of Ti₃AlC₂, enhanced interface bonding between Ag and Ti₃C₂T_x, deformability of the stripe- shaped Ti₃C₂T_x in Ag matrices.

Firstly, based on the first-principle band structure calculations, in Ti₃AlC₂, Ti3d state contributes to the majority of the total densities of states (DOS) at Fermi level; removal of the Al layers from Ti₃AlC₂ results in the redistribution of Ti3d states from broken Ti-Al bonds into delocalized Ti-Ti metallic-like bonding states, leading to the increase of local DOS maximums at Fermi level^[33]. Thus, in MXene (Ti₃C₂T_x), the electron density of states near Fermi level (N(E_f)) is 1.9-3.2 times higher than that in the corresponding MAX (Ti₃AlC₂)^[34]. Secondly, EDS mapping indicates the existence of O and F elements, which may come from the functional groups (-F, -OH) of Ti₃C₂T_x surface^[35] (Fig. 5). Generally, the hydrophilicity of -F/-OH functional groups is beneficial to the bonding between Ti₃C₂T_x and metal matrices^[34], which avoids the similar phenomenon of poor interface bonding between carbon nanotubes, fibers and metal matrices ^[36]. In addition, the SEM observation also displays the tight bonding between Ti₃C₂T_x and Ag matrices without obvious cracks and holes, as shown in Fig. 2(c) and Fig. 3(a). Hence, the uniform microstructure and good bonding of Ag/Ti₃C₂T_x improved the conductivity. Thirdly, as shown in Fig. 2(b, d), the microstructure of stripe-shaped Ti₃C₂T_x is obviously different from that of granular Ti₃AlC₂ in Ag matrices. The 2D layered structure of Ti₃C₂T_x facilitates its deformability during preparation. The Ti₃C₂T_x was cold compacted into the stripe-like Ti₃C₂T_x (average thickness of ~3 μm), whereas Ti₃AlC₂ retains its original shape (average diameter of ~10 μm). In contrast with granular Ti₃AlC₂, the stripe-shaped Ti₃C₂T_x has smaller cross- sectional area perpendicular to the current direction, minimizing the scattering section for electrons and the resistance to the electron transmission. In summary, the excellent machinability and electrical conductivity makes Ag/Ti₃C₂T_x a promising substitute for Ag/CdO.

However, as shown in Fig. 6, the maximum tensile strength of Ag/Ti₃C₂T_x composite (32.77 MPa) is far less than that of Ag/Ti₃AlC₂ composite (145.52 MPa). The superior tensile strength of Ag/Ti₃AlC₂ composite derives from the interdiffusion between the active Al atomic layer with Ag matrices. On the contrary, the absence of Al layer leads to the weaker interface bonding strength between Ti₃C₂T_x and Ag matrices, which finally deteriorates the mechanical property of the entire Ag/Ti₃C₂T_x composite.

In order to further investigate the property of Ag/Ti₃C₂T_x, electrical arc discharging experiments were carried out on this contact surface under a harsh condition (AC-3, 100 A, 400 V, GB14048.4-2010). The Ag/Ti₃C₂T_x contact failed to make and break after 1233 times arc discharging. The optical image shows that the shape of contact remains well with some dents and protuberances (Fig. 7(a, b)). The surface morphologies of Ag/Ti₃C₂T_x contact after arc erosion are subsequently exhibited in Fig. 7(c), complete edge and flat surface were further confirmed by SEM. Some Ag spheres, solidified Ag blocks, and small cracks were observed at high-magnification SEM image (Fig. 7(d)). Fig. 7(e-h) exhibit the microstructure and chemical composition of Ag/Ti₃C₂T_x contact surface. There are many irregular dark blocks surrounded by little bright particles (Fig. 7(e)). As shown in Fig. 7(f), area 1 (white block) contains large amount of Ti, O, F with trace of Ag and Al, which may be attributed to the Ti-O-F mixture produced by electrical arc erosion to Ti₃C₂T_x. Area 2 (bright particles) is mainly composed of Ag, F, and O. It is deduced that the Ag-O-F mixture was produced by the absorption of O₂ in liquid Ag and reaction with -F function group of Ti₃C₂T_x. Fig. 7(h) displays two types of spheroid particles at magnified SEM image. EDS analysis results showed that both the particles contained vast N element, showing that these two particles were composed of Ag-O-F-N.

The relative mass loss of Ag/Ti₃C₂T_x (54% after 1233 times arc discharging) is considerably more than that of Ag/Ti₃AlC₂ (0.82% after 3000 times arc discharging), which is also attributed to the absence of Al layer in Ti₃AlC₂. As analyzed previously, the lack of Al-Ag interdiffusion leads to the weak bonding strength of Ti₃C₂T_x with Ag, and thus decrease the mechanical property of composite, which accordingly impairs the resistance to electrical arc impact damage. In addition, the absence of Al-Ag interdiffusion also results in the poor wettability of Ti₃C₂T_x with Ag during the electrical arc discharging and consequently decreased viscosity of molten pool, finally deteriorating the resistance to the material transfer of Ti₃C₂T_x and Ag under electrical arc high-temperature. Nonetheless, there is still space for further improvement of the arc erosion resistance of Ag/Ti₃C₂T_x with superior electrical conductivity by the composition design, structure optimization, technique promotion in the following work.

In this work, Ag-based electrical contact materials, reinforced by Ti₃AlC₂ ceramic and its derivative (Ti₃C₂T_x), were successfully prepared by powder metallurgy. The microstructure, chemical composition, hardness, conductivity, machinability, mechanical property and erosion resistance of Ag/Ti₃AlC₂ and Ag/Ti₃C₂T_x composites were investigated and compared. The main conclusions are as follows:

1) Stripe-like Ti₃C₂T_x uniformly distributes in Ag/Ti₃C₂T_x composite.

2) In contrast with Ag/Ti₃AlC₂, Ag/Ti₃C₂T_x has lower resistivity (30×10^-3 μΩ·m), 29% lower than that of Ag/Ti₃AlC₂. The superior conductivity of Ag/Ti₃C₂T_x results from the stronger metallicity of Ti₃C₂T_x, uniform microstructure, and smaller cross-sectional area of Ti₃C₂T_x in the composite.

3) The moderate hardness and excellent machinability of Ag/Ti₃C₂T_x are also satisfactory for electrical contact materials.

4) The tensile strength (32.77 MPa) of Ag/Ti₃C₂T_x composite is inferior to that of Ag/Ti₃AlC₂ (145.52 MPa) due to the lack of Al-Ag interdiffusion.

5) Ag/Ti₃C₂T_x shows moderate arc erosion resistance with production of Ti-O-F, Ag-O-F, and Ag-O-F-N mixture, which is inferior to that of Ag/Ti₃AlC₂, and needs to be further improved.