Chinese Chemical Letters  2019, Vol. 30 Issue (2): 502-504   PDF    
Preparation and tribological properties of potassium titanate-Ti3C2Tx nanocomposites as additives in base oil
Xuefeng Zhanga, Yu Guoa, Yijia Lib, Yong Liua,*, Shangli Donga     
a School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China;
b Harbin No.3 High School, Harbin 150001, China
Abstract: A potassium titanate-Ti3C2Tx nanocomposite was synthesized using a facile, low-temperature and environmentally friendly process. The morphology and microstructure of the as-prepared samples were analyzed by means of XRD, SEM, TEM, and XPS. The tribological property of the nanocomposite as additive in base oil was evaluated. The results reveal that the in-situ formed potassium titanate nanowires with average diameter of 30 nm and the uniform distribution on the surface of Ti3C2Tx nanosheets. Compare to Ti3C2Tx nanosheets, the average friction coefficient and wear scar width of the oil containing potassium titanate-Ti3C2Tx nanocomposites are decreased by 4.9% and 22% under the same conditions. The excellent tribological performance is attributed to the synergic effects of the two components in the nanocomposite structure.
Keywords: Potassium titanate     Mxenes     Nanocomposites     Wear and tribology     Oil lubrication    

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 Ti3C2Tx 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 Ti3C2Tx with other nanomaterials. For example, the TiO2-Ti3C2Tx 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)-Ti3C2Tx nanocomposites. The tribological behaviors of the KTO-Ti3C2Tx nanocomposites as additives in base oil were investigated aiming to evaluate the enhancement of friction and wear for the lubricant oil.

The Ti3C2Tx nanosheets were synthesized by etching Al from Ti3AlC2 in HF at 65 ℃, as previously reported [8]. The KTO-Ti3C2Tx nanocomposites were synthesized by the following procedures. Firstly, 500 mg Ti3C2Tx 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-Ti3C2Tx nanocomposites and Ti3C2Tx 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, Ti3C2Tx, and KTO-Ti3C2Tx 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 Ti3C2Tx nanosheets [11, 12]. The diffraction patterns of nanocomposite products contain the Ti3C2Tx 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 Ti3C2Tx, KOH, and the oxygen from air during the treatment [15].

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 Ti3C2Tx 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 Ti3C2Tx nanosheet. Those positive potassium ions in aqueous solution are easy to intercalate into the interlayer of Ti3C2Tx nanosheets under electrostatic force, which will enlarge the interlayer spacing [16-18]. The full XPS spectrum of the Ti3C2Tx 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-Ti3C2Tx nanocomposites. The Ti 2p core level is fitted four double (Ti 2p3/2-Ti 2p1/2) 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 2p3/2 at the binding energies of 458.8 eV, 457.2 eV, 455.9 eV, and 454.8 eV, corresponding to tetravalent Ti, TixOy, TiTx (x < 1), and Ti-C, respectively [21, 22]. In the Ti 2p spectrum of KTO-Ti3C2Tx nanocomposites (Fig. 1d), the intensity of Ti 2p3/2 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 Ti3C2Tx and KTO-Ti3C2Tx nanocomposites are shown in Fig. 2. The HF-treated Ti3C2Tx is composed of exfoliated layered nanosheets that similar to graphene [23] as shown in Figs. 2a and c. Fig. 2b shows that the KTO-Ti3C2Tx nanocomposites are composed of the KTO nanowires and the Ti3C2Tx nanosheets. The curved KTO nanowires have uniform distribution on the Ti3C2Tx 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 Ti3C2Tx 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 Ti3C2Tx nanosheets with uniform distribution leading to formation of KTO-Ti3C2Tx nanocomposites.

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% Ti3C2Tx decreases to 0.081. In contrast, the base oil containing 1.0 wt% KTO-Ti3C2Tx nanocomposites exhibits further reduced friction coefficient of 0.077. Such value of average friction coefficient is lower than that reported on the Ti3AlC2, Ti3C2Tx and TiO2-Ti3C2Tx at the optimal additive amount [7, 8, 10, 24]. Furthermore, the wear scar width of the oil containing KTO-Ti3C2Tx is much less than that of the pure oil and/or the oil containing Ti3C2Tx, as shown in Fig. 3b, indicating the load capacity of the oil with additive is enhanced. Those results show that KTO-Ti3C2Tx 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-Ti3C2Tx 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 Ti3C2Tx 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 Ti3C2Tx 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].

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-Ti3C2Tx nanocomposite. The KTO nanowires with average diameter of 30 nm were in-situ synthesized on the surface of Ti3C2Tx nanosheets with uniform distribution. Under the same tribolocigal condition and adding concentration, the KTO-Ti3C2Tx nanocomposites as additive in base oil exhibit lower friction coefficient and wear scar width comparing to that of the Ti3C2Tx nanosheets. The improvement of tribological property is attributed to the enlarged interlayer spacing of Ti3C2Tx nanosheets by the intercalation treatment, the enhanced load capacity of the base oil by adding KTO-Ti3C2Tx nanocompsites, and the protection of KTOTi3C2Tx nanocomposites particles on the friction surface.


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

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