Citation

Xuefeng Zhang, Yu Guo, Yijia Li, Yong Liu, Shangli Dong. Preparation and tribological properties of potassium titanate-Ti₃C₂T_x nanocomposites as additives in base oil[J]. Chin. Chem. Lett., 2019, 30(2): 502-504  

The nanoparticles of metals, metal oxide nanoparticles, transition metal sulfide, carbon, and carbide derived carbon can be used as the effective lubricant additives for the mechanical systems [1-5]. Among those nanomaterials, MXenes, a novel group of two-dimensional transition metal carbides or carbonitrides, are considered as the promising lubricant additives due to the weak interlayer interaction and excellent tribological performance [6]. It was reported that the average friction coefficient and the wear volume of the oil containing Ti₃C₂T_x nanosheets were distinctly lower than that of the pure oil at an optimal concentration of 1 wt% [7-9]. The tribological property is expected to be further improved by compositing Ti₃C₂T_x with other nanomaterials. For example, the TiO₂-Ti₃C₂T_x nanocomposites synthesize by a facile hydrothermal technology exhibit great friction reduction under the optimal load. The excellent tribological performance is attributed to the synergic effect of the two components in the nanocomposite structure [10]. In this work, a novel chemical route has been developed to prepare potassium titanate (KTO)-Ti₃C₂T_x nanocomposites. The tribological behaviors of the KTO-Ti₃C₂T_x nanocomposites as additives in base oil were investigated aiming to evaluate the enhancement of friction and wear for the lubricant oil.

The Ti₃C₂T_x nanosheets were synthesized by etching Al from Ti₃AlC₂ in HF at 65 ℃, as previously reported [8]. The KTO-Ti₃C₂T_x nanocomposites were synthesized by the following procedures. Firstly, 500 mg Ti₃C₂T_x powders were added into 40 mL KOH solution (2 mol/L), and the mixture was stirred at 200 rmp in air and maintained at room temperature for 24 h. Subsequently, the products were collected by centrifugation at 4000 rpm for 30 min and washed several times with distilled water until the pH value reaching 7. Thereafter, the obtained powders were dried at 80 ℃ for 24 h in a vacuum oven for further characterization.

The phases constitution of the samples were analyzed by an Xray diffractometer (X'PERT) using Cu Kα radiation (λ = 0.01544 nm) at a scanning range from 4° to 55°. The morphology and microstructure were examined using a field emission scanning electron microscopy (zeisssupra55), and a transmission electron microscopy (JEOL JEM-2100). The chemical composition of the synthesized samples was measured using an X-ray photoelectron spectrometer (ESCALAB 250Xi) with a Al Kα radiation (hν = 1486.6 eV) and the operation voltage of 15 kV. All XPS spectra were corrected using the C 1s line at 284.8 eV.

The as-prepared KTO-Ti₃C₂T_x nanocomposites and Ti₃C₂T_x were ultrasonicly dispersed for 1 h in PAO8 oil with concentration of 1 wt% for obtaining the suspended oil samples. The tribological properties of the oil suspensions were investigated on a UMT-2 ball-on-disc tribometer at a rotating speed of 200 rpm with a load of 5 N, for the test duration of 25 min. The ball is 440-C stainless steel with a diameter of 4 mm, and the counterpart is 32, 100 bearing steels disc with Φ 30 mm × 12 mm in size and hardness of 61±1 HRC. The friction coefficients were recorded automatically and the wear scars widths were measured by a VHX-1000E noncontact 3D optical profiler.

The XRD analysis of Ti₃AlC₂, Ti₃C₂T_x, and KTO-Ti₃C₂T_x nanocomposites are shown in Fig. 1a. After HF treatment, the most intense peak around 39.0° corresponding to (104) planes of the Ti₃AlC₂ (JCPDC No. 01-074-8806) precursor vanishes, while the broadened (002) peaks are shifted toward lower angles, indicating the successful removal of Al layers and formation of the highly exfoliated Ti₃C₂T_x nanosheets [11, 12]. The diffraction patterns of nanocomposite products contain the Ti₃C₂T_x peaks and the new peaks around 11° and 29° corresponding to K₂Ti₈O₁₇ (JCPDF No. 41- 1100) [13, 14], implying KTO is produced by the following reactions of Ti₃C₂T_x, KOH, and the oxygen from air during the treatment [15].

Figure 1

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Fig. 1. XRD and XPS results, a) XRD patterns for Ti3AlC2, Ti3C2Tx and KTO-Ti3C2Tx, b) XPS results of full spectrum, c) High-resolution Ti 2p XPS spectra of Ti3C2Tx and d) KTO-Ti3C2Tx nanocomposites.

It is noted that the (002) peak of the Ti₃C₂T_x in composite is shifted from 8.9° to further lower angles of 7.2°, and the interlayer spacing is increased from 0.99 nm to 1.22 nm, suggesting the intercalation effect of the treatment. Because that there are a large number of —F and —OH terminations resulting in a negative surface of Ti₃C₂T_x nanosheet. Those positive potassium ions in aqueous solution are easy to intercalate into the interlayer of Ti₃C₂T_x nanosheets under electrostatic force, which will enlarge the interlayer spacing [16-18]. The full XPS spectrum of the Ti₃C₂T_x exhibits peaks of Ti 2p, C 1s, O 1s (KLL), and F 1s, as shown in Fig. 1b, indicating the presence of the Ti, C, O, and F elements, as previously reported by [19, 20]. Moreover, the new K 2p peak appears in the XPS survey spectrum of KTO-Ti₃C₂T_x nanocomposites. The Ti 2p core level is fitted four double (Ti 2p_3/2-Ti 2p_1/2) with a fixed area ratio equal to 2:1 and doublet separation of 5.7 V, as shown in Fig. 1c. There are four characteristic peaks of Ti 2p_3/2 at the binding energies of 458.8 eV, 457.2 eV, 455.9 eV, and 454.8 eV, corresponding to tetravalent Ti, Ti_xO_y, TiT_x (x < 1), and Ti-C, respectively [21, 22]. In the Ti 2p spectrum of KTO-Ti₃C₂T_x nanocomposites (Fig. 1d), the intensity of Ti 2p_3/2 peak at 458.8 eV is enhanced, while the other peak intensities disappeared, which is caused by the cleavage of Ti—C bond and the oxidation of Ti elements into potassium titanate [13, 15].

The microstructural morphologies of Ti₃C₂T_x and KTO-Ti₃C₂T_x nanocomposites are shown in Fig. 2. The HF-treated Ti₃C₂T_x is composed of exfoliated layered nanosheets that similar to graphene [23] as shown in Figs. 2a and c. Fig. 2b shows that the KTO-Ti₃C₂T_x nanocomposites are composed of the KTO nanowires and the Ti₃C₂T_x nanosheets. The curved KTO nanowires have uniform distribution on the Ti₃C₂T_x surface. The TEM observation shows that the KTO nanowires with average diameter of 30 nm were in-situ synthesized and uniformly distributed on the surface of Ti₃C₂T_x nanosheets, as shown in Fig. 2d. Those TEM measurements are in accordance with above SEM results. The comprehensive analyses on the above results of XRD, SEM, TEM, and XPS confirm that the KTO nanowires are prepared andanchored on the surfaces of Ti₃C₂T_x nanosheets with uniform distribution leading to formation of KTO-Ti₃C₂T_x nanocomposites.

Figure 2

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Fig. 2. SEM and TEM micrographs showing morphologies of (a), (c) Ti3C2Tx and (b), (d) KTO- Ti3C2Tx nanocomposites.

The friction coefficients of the test samples are showed Fig. 3a. It can be seen that the average friction coefficient of the pure base oil has the highest friction coefficient of about 0.111. The average friction coefficient of the base oil containing 1.0 wt% Ti₃C₂T_x decreases to 0.081. In contrast, the base oil containing 1.0 wt% KTO-Ti₃C₂T_x nanocomposites exhibits further reduced friction coefficient of 0.077. Such value of average friction coefficient is lower than that reported on the Ti₃AlC₂, Ti₃C₂T_x and TiO₂-Ti₃C₂T_x at the optimal additive amount [7, 8, 10, 24]. Furthermore, the wear scar width of the oil containing KTO-Ti₃C₂T_x is much less than that of the pure oil and/or the oil containing Ti₃C₂T_x, as shown in Fig. 3b, indicating the load capacity of the oil with additive is enhanced. Those results show that KTO-Ti₃C₂T_x nanocomposites can efficiently improve the friction and wear properties of base oil under additive amount of 1.0 wt%. The nanoparticles as oil additive can facilitate the formation of tribofilm during friction process due to their huge surface area [3, 6, 7]. The lubrication of the base oil with additive mainly depends on the stability of the formed tribofilm [5, 25]. Fig. 3b implies that the KTO-Ti₃C₂T_x nanocomposites further enhance the stability of the tribofilm and increase the load capacity, leading to the decrease in friction coefficient and wear scar width comparing to using the single Ti₃C₂T_x nanosheets. Moreover, the enhancement of tribological performance is also attributed to the following aspects. First, the nanocomposites particles can fill into the wear scars and protect the surface from the direct contact and the further wear damage [26, 27]. Second, the interlayer spacing of the Ti₃C₂T_x nanosheets of the nanocomposites are enlarged by the intercalation treatment leading to the further weakened shear strength and the decreased friction coefficient of nanocomposite itself [28-30].

Figure 3

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Fig. 3. The tribological properties of the pure oil and the oils respectively containing 1 wt% Ti3C2Tx and 1 wt% KTO-Ti3C2Tx nanocomposites, (a) average friction coefficient and (b) wear width.

In summary, we have developed a facile and scalable method to prepare KTO-Ti₃C₂T_x nanocomposite. The KTO nanowires with average diameter of 30 nm were in-situ synthesized on the surface of Ti₃C₂T_x nanosheets with uniform distribution. Under the same tribolocigal condition and adding concentration, the KTO-Ti₃C₂T_x nanocomposites as additive in base oil exhibit lower friction coefficient and wear scar width comparing to that of the Ti₃C₂T_x nanosheets. The improvement of tribological property is attributed to the enlarged interlayer spacing of Ti₃C₂T_x nanosheets by the intercalation treatment, the enhanced load capacity of the base oil by adding KTO-Ti₃C₂T_x nanocompsites, and the protection of KTOTi₃C₂T_x nanocomposites particles on the friction surface.

This work was supported by the Opening Fund of Key Laboratory of Harbin Institute of Technology.