Microstructure and tribological property of a MXene derived from Ti3AlC2

Ti3C2 layers with a layered two-dimensional structure are synthesized by immersing a precursor Ti3AlC2 with hydrofluoric acid solution. The microstructure of Ti3C2 layers is analyzed by SEM, XRD, Raman and AFM. Additionally, the tribological behavior of Ti3C2 layers is investigated at various loads and sliding speeds by sliding against Si3N4 balls on a UMT-2 tribometer. In comparison, the tribological behavior of Ti3AlC2 at various loads is tested under identical condition. The results indicate that the Ti3C2 layers have been successfully prepared and their thickness mainly distributes in the range of 2.3–3.1 nm. Moreover, the Ti3C2 layers exhibit better tribological behavior than its precursor -Ti3AlC2 in dry sliding. It is expected that the Ti3C2 layers will be applied as a solid lubricant additive for other materials.


Introduction
MAX phases combine the advantages of both metals and non-metals. They have a big family with about 150 members [1]. As one member of this huge family, Ti 3 AlC 2 inherits the layered hexagonal with P6 3 /mmc symmetry, where the Ti layers are nearly closed packed, and the C atoms fill the octahedral sites. The Ti 3 C 2 layers are, in turn, interleaved with layers of Al atoms [2]. When the Al layers are selectively etched away without disrupting the Ti-C bonds, then Ti 3 C 2 layers with a two-dimensional (2D) character will be obtained. They were first reported in 2011 by M. Naguib et al [3]. Just like other nanosheets, the Ti 3 C 2 layers have rich surface chemistries and high electronic conductivities and exhibit prominent performance in catalysis chemistry [4,5] and energy storage systems [6][7][8][9]. Peng et al [10] have studied the unique lead adsorption behavior of 2D Ti 3 C 2 layers. Additionally, they have the potential to be applied in biomedical [11,12] and sensing [13,14]. In the field of friction, the Ti 3 C 2 layers have been used as additives in base oil and a possible mechanism has been put forward [15]. However, researches on the dry friction of the Ti 3 C 2 layers are still blank.
To shed light on future material application, Ti 3 C 2 layers on Ti 3 AlC 2 were fabricated and characterized. Additionally, the tribological behavior of the Ti 3 C 2 layers sliding against Si 3 N 4 balls in air was investigated using a ball-on-disk tribometer at room temperature.

Material and methods
2.1. Preparation of Ti 3 C 2 Polycrystalline Ti 3 AlC 2 bulk (Φ24 mm × 7.9 mm) was in situ synthesized by a solid-liquid phase reaction using hot press sintering stove in vacuum. A powder mixture of TiC, Ti and Al with a mole ratio of 1 pressed at 1400°C and 25 MPa for 30 minutes in a graphite die. The purity and density of the as-prepared Ti 3 AlC 2 was 98.9 vol.% and 97.4%, respectively. Its average grain size and micro-hardness was 13 um and 4.66 GPa, respectively. After polishing treatment, the surface roughness (Ra) of the Ti 3 AlC 2 bulk was approximate 0.26 um.
The Ti 3 AlC 2 bulk was immersed in a 40% concentrated HF solution at room temperature for 10 h. The product was then washed with ethanol several times, dried in vacuum at 70°C for 12 h and the 2D Ti 3 C 2 layer saturated by suitable ligands (such as F or -OH) covering on Ti 3 AlC 2 was obtained.
In order to facilitate the examination of the microstructure of the Ti 3 C 2 layers, some particles were mechanically obtained from the Ti 3 C 2 layers covering on Ti 3 AlC 2 . They were subsequently washed by using ethanol for several times and dried in vacuum at 70°C for 12 h.

Friction and wear test
The tribological tests were conducted on a reciprocating ball-on-disc UMT-2 tribometer (CETR, USA). The stroke was 5 mm in length. The test was carried out at room temperature with the Ti 3 C 2 layers sliding against Si 3 N 4 balls (Φ3 mm) with a surface roughness of 0.05 um. The sliding speeds were 0.02 and 0.1 m s −1 , respectively. The loads were 0.1, 0.5 and 1 N, respectively. In comparison, the precursor Ti 3 AlC 2 was tested sliding against Si 3 N 4 balls under identical conditions. Before each test, all discs and balls were ultrasonically cleaned in acetone. Each test was repeated at least 3 times. The friction coefficients were recorded automatically and then processed by Origin software.

Characterization
The phase composition of the Ti 3 C 2 layers and original Ti 3 AlC 2 was examined by x-ray diffractometer (XRD, D/ Max-2400, Rigaku) using Cu Ka (λ=0.154 18 nm). XRD patterns were recorded in the 2 θ range of 5-65°with a step size of 0.02°and a scan speed of 1°min −1 . Raman spectra of the Ti 3 C 2 layers was carried out on Raman spectrometer (Renishaw, inVia) with an Ar laser (514.5 nm) as excitation source in the wavelength range of 100-1000 cm −1 . Surface morphology and its worn surface of the Ti 3 C 2 layers was analyzed by using Scanning electron microscopy (SEM, JEOL JSM-5600LV), equipped with energy dispersive x-ray spectroscopy (EDS). The thickness of the 2D Ti 3 C 2 nanocrystal, which was scattered on the surface of silicon wafer (10×10 mm 2 ) cleaned by nitro hydrochloric acid, was measured by atomic force microscopy (AFM, Multimode 8, Bruker).

Results and discussion
3.1. Microstructure XRD patterns of Ti 3 AlC 2 and the Ti 3 C 2 layers are shown in figure 1(a). Compared with Ti 3 AlC 2 , the diffraction peaks of the Ti 3 C 2 layers are obviously broadened and weak. Raman spectra of Ti 3 AlC 2 and the Ti 3 C 2 layers are seen in figure 1(b). Compared with Ti 3 AlC 2 , peaks I, II and III of the Ti 3 C 2 layers vanished, while peaks IV, V and VI merged, broadened, and downshifted. Such downshifting phenomena has been observed in Raman spectra of very thin layers of inorganic layered compounds [16]. The line broadening and the spectral shifts in the Raman spectra are in agreement with the broadened XRD profiles. Similar to Ti3SiC2 [17], peaks I to III in figure 1(b) can be assigned to Al-Ti vibrations, while peaks V and VI involve only Ti-C vibrations. The fact that the latter two peaks merge and exist after etching confirms both the mode assignments and, more importantly, the loss of Al from the structure. The above results are in agreement with [3]. SEM graph of the Ti 3 C 2 layers is shown in figure 1(c). It can be seen from figure 1(c) that the obtained Ti 3 C 2 layers showed an obvious 2D layered structure. The corresponding EDS (see figure 1(d) and table 1) data demonstrates that the Ti 3 C 2 layers are composed of Ti, C, O and F with negligible Al. This finding is in accordance with [3].
Surface image and thickness measurement of Ti 3 C 2 nanoplates analyzed by AFM are shown in figure 2. It can be seen from figure 2(a) that some defects (holes) are formed on Ti 3 C 2 , which may result from the prolonged HF treatment [18]. The thickness of Ti 3 C 2 was manually measured by counting at least 500 steps of the crosssectional profile of the Ti 3 C 2 layers (see figure 2(b)). It was proved that the thickness of the Ti 3 C 2 layers mainly distributed in the range of 2.3-3.1 nm, accounting for 85% (see figure 2(c)).

Tribological properties
Tribological behaviors of Ti 3 AlC 2 and the Ti 3 C 2 layers at various loads sliding against Si 3 N 4 balls with a reciprocating motion are shown in figure 3. When the load is 0.1 N, the friction coefficient of Ti 3 AlC 2 has a large fluctuation with a running-in period of about 60 s. When the load is larger than 0.1 N, the fluctuation of the friction coefficient of Ti 3 AlC 2 are obviously reduced. Moreover, with the increase of load, the average friction coefficient of Ti 3 AlC 2 is slightly reduced. In comparison, the tribological behavior of the Ti 3 C 2 layers is different from that of Ti 3 AlC 2 . When the load is 0.1 N, the friction coefficient of the Ti 3 C 2 layers fluctuates to a certain extent. When the loads are 0.5 and 1 N, the friction coefficient of the Ti 3 C 2 layers are quite stable throughout the sliding. Additionally, the average friction coefficient of the Ti 3 C 2 layers increases with the increase of load. At lower loads, the friction coefficient of the Ti 3 C 2 layers is lower than that of Ti 3 AlC 2 . It is speculated that larger load makes direct contact of Ti 3 AlC 2 with Si3N4 in the end of the sliding, which is demonstrated by the scratch on the wear track of the Ti 3 C 2 layers (see figure 5(a)). Such phenomena may be resulted from the wear out of the Ti 3 C 2 layers on Ti 3 AlC 2 . Therefore, similar friction coefficients in the latter part of the friction curves are observed for Ti 3 AlC 2 and the Ti 3 C 2 layers under the load of 1 N. It is deduced that Ti 3 C 2 is a potential solidlubricating material.
The tribological behavior of the Ti 3 C 2 layers at different sliding speeds is studied, which is shown in figure 4. As shown in figure 4, the friction coefficient of Ti 3 C 2 is only slightly sensitive to the sliding speeds. Compared with the curve (black line) at a sliding speed of 0.02 m s −1 , the curve (red line) at a sliding speed of 0.1 m s −1 ha −1 s −1 a shorter running-in period and lower steady friction coefficient with a similar trend. It is speculated that higher sliding speed promotes Ti 3 C 2 to behave lower friction to a certain extent.
The wear morphology of Ti 3 C 2 layers is shown in figure 5. As seen in figure 5(a), the wear of the Ti 3 C 2 layers during sliding is obvious with a distinct wear track. From the local magnification of the wear track (see   figure 5(b)), some fine debris are found. It is not deniable that the as-synthesized Ti 3 C 2 layers are easy to wear. The unsatisfactory wear resistance of Ti 3 C 2 is perhaps owing to the big volume shrinkage of Ti 3 C 2 (approximately 19%) [3] and the weak interaction between Ti 3 C 2 and its precursor. To sum up, the Ti 3 C 2 layers behave lower friction coefficients than Ti 3 AlC 2 , whereas its wear resistance is unsatisfied. It is more applicable for lubricating additives or bulk materials.

Conclusions
Microstructure and tribological property of the Ti 3 C 2 layers have been investigated in this paper. Its microstructure is examined by SEM, XRD, Raman and AFM. The results show that the Ti 3 C 2 layers have a layered two-dimensional structure with the thickness mainly in the range of 2.3-3.1 nm. The layered structure of  Ti 3 C 2 makes it exhibit lower friction under various loads and sliding speeds. It is predicted that the Ti 3 C 2 layers can be applied as a solid lubricant additive of other materials, which needs further exploration.

Acknowledgments
The author acknowledges the financial support from the Youth Foundation of Changzhou Institute of Technology (YN1711).

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).