Cryst. Res. Technol. 45, No. 8, 851 – 855 (2010) / DOI 10.1002/crat.201000246

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Synthesis and tribological properties of laminated Ti3SiC2 crystals

Qiong Wu, Changsheng Li*, and Hua Tang

School of Material Science and Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, P. R. China

Received 26 April 2010, revised 11 May 2010, accepted 27 May 2010

Published online 11 June 2010

Key words Ti3SiC2 crystal, growth mechanism, lubrication additive, tribological properties.

Laminated Ti3SiC2 crystals are prepared of Ti, Si, C and Al powders by the method of hot isostatic pressing

with NaCl additive in argon at 1350 °C. The laminated morphology of Ti3SiC2 is presented through the SEM

and TEM observations. The results of high resolution transmission electron microscope (HRTEM) and

selected area electron diffraction (SAED) patterns combined, it can be seen that the layers are of Ti3SiC2

crystals. The growth mechanism of Ti3SiC2 crystals, controlled by two-dimensional nucleation, is also

explained. The tribological properties of Ti3SiC2 crystals as additives in HVI500 base oil are investigated by a

UMT-2 ball-on-plate friction and wear tester. The study shows that under determinate conditions, the friction

coefficient of the base oil containing Ti3SiC2 crystals is lower than that of pure base oil, and it decreases with

the increase of mass percent of Ti3SiC2 nanolayers when its proportion is lower than 5wt.%.

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

Titanium silicon carbide (Ti3SiC2, TSC) belongs to the family of ternary ceramics with a general formula of Mn+1AXn (MAX). The unique combination of ceramic and metallic properties renders TSC a kind of important functional ceramics [1]. TSC exhibits a variety of outstanding ceramic properties, such as elastic rigidity [2], low density, strong resistance to chemical corrosion [3], and high melting point [4]. Simultaneously, TSC also has metallic properties such as relatively high electrical [5] and thermal conductivities [6], lower hardness, thermal shock resistance [7], and good damage tolerance [8]. In addition, TSC can be fully reversible under 1 GPa compression [9,10] and has zero thermal power within a temperature range of 300–850 K [11]. However, to the best of our knowledge, the growth process of TSC has not been reported because of the difficulties in observing the morphologies formed during the initial stages of crystal growth. More over, little work focused on the tribological properties of TSC as lubrication additive. In this thesis, we have presented the growth mechanism of TSC crystals on the basis of observing the crystalline grains morphologies and also investigated the tribological properties of laminated TSC crystals as additives in the HVI500 base oil.

2 Experimental

Preparation of laminated Ti3SiC2 crystals Laminated TSC crystals were synthesized from a mixture of Ti powder (99.5%, –300 mesh), Si powder (99.9%, –200 mesh), Graphite powder (99.97%, diameter < 20 μm), Al powder (99%, –200 mesh) and NaCl powder (99.5%, –300 mesh). The mixed powders (molar ratio: Ti:Si:C:Al:NaCl = 3:1:2:0.1:0.1) were subjected to planet ball-milling for 10 h, then taken out of steel kettle and put into a quartz tube (15 mm in diameter) with one tip sealed. The quartz tube was heated on acetylene flame till molten state and then elongated to become thinner, which was pumped down by vacuum pump and then enveloped. In this way, the closed quartz tube full of reactant was obtained. The quartz tube with powders of Ti, Si, C, Al and NaCl was heated at 1350 °C in argon for 1 h before cooling to room temperature, and finally, the expected laminated TSC crystals were obtained. Afterwards, the samples were characterized by XRD and the morphologies of the samples were characterized by an SEM (JEOL JXA-840A). TEM studies were carried out on a JEM-100CX II transmission electron microscope. All the measurements were carried out at room temperature. ____________________

* Corresponding author: e-mail: wuqiong0511@126.com

852 Qiong Wu et al.: Synthesis and tribological properties of laminated Ti3SiC2 crystals

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

Tribological properties of laminated Ti3SiC2 crystals as lubrication additive Different mass percent of laminated TSC crystals were dispersed in the HVI500 base oil by ultrasonic vibration (1600 W powder, 2 kHz frequency) for 2 h without any active reagent, and then a series of suspended oil samples were obtained. The tribological properties of the base oil containing laminated TSC crystals and pure base oil were investigated on a UMT-2 ball-on-plate friction and wear tester. The friction reduction and wear resistance test was conducted at the rotating speeds from 100 ~ 250 rpm and with loads from 100 ~ 400 N for 1 h. And the grinding crack was measured by the VEECO WYKO NT1100 non-contact optical profile testing instrument.

3 Results and discussion

Synthesis and characterization Among known TSC polymorphs [12] the most studied material is at the α-phase, and here we will have a further study of it. Its structure has been reported [13] as hexagonal in space group P63/mmc (No. 194) and can be described as a stacking of ordinary Ti monocarbide layers interleaved with a single sheet of Si atoms as depicted in figure 1. Thus, this material consists of hexagonal layers stacked in the repeated sequence of Si–Ti2–C–Ti1–C–Ti2, where the unit cell is composed of two formula units. The atoms are located at the following positions: titanium at 2a and 4f (denoted as Ti1 andTi2 respectively), Si at 2b and C at 4f Wyckoff positions. It should be noted that Ti2 has both C and Si neighbors whereas Ti1 has only C neighbors.

Fig. 1 Fragment of crystal structure (left) and the stacking

of the atomic sheets for hexagonal Ti3SiC2 phase (right).

Fig. 2 X-ray diffraction patterns of the samples

synthesized at 1350 °C, in accord with diffraction data from

JCPDS card 74-0310.

Figure 2 shows the XRD pattern of laminated TSC crystals synthesized at 1350 °C. The strong and sharp diffraction peaks indicate that the product was well crystallized. All the diffraction peaks in this figure can be indexed to pure hexagonal structure TSC with lattice constants of a = 3.064 Å and c = 17.65 Å, which are consistent with the data in the standard card (JCPDS 74-0310), and no byproduct peaks were found. This indicates that phase-pure TSC is easily formed of Ti, Si, C and Al powders by the method of hot isostatic pressing with NaCl additive in argon at 1350 °C. The laminated TSC crystals range from 0.5 to 5 μm were successfully synthesized, as presented in figure 3a, a panoramic SEM image of the product obtained by the method of hot isostatic pressing with NaCl additive at 1350 °C for 1 h. Figure 3b shows that as-synthesized TSC is composed of a lot of layers which thickness was less than 40 nm and that the nano-layers of TSC are stacked together to form a mountain-like structure. The high magnification images in figure 3c–e show the morphology of a single pile of layers viewed from front, flank, and tilted front perspectives respectively and which exhibit a clear and well-defined laminated structure of hexagonal growth layers.

The high symmetry characteristic of the crystalline was presented through TEM observations, and the TEM image of thin hexagonal plates is shown in figure 4a, from which we can clearly see the hexagonal structure. The inset in figure 4a shows that the selected area electron diffraction (SAED) pattern which in accord with interface model of Ti3SiC2 (0001) contains a special set of TSC’s diffraction pattern, which can only be observed from the [0001] crystallographic direction, indicating that the hexagonal plates should correspond to

Cryst. Res. Technol. 45, No. 8 (2010) 853

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the basal (0001) plane of TSC. Further structural characterization of the laminated structure was carried out by high resolution transmission electron microscope (HRTEM). The image in figure 4b shows that the clear lattice fringes of TSC are presented in the layer structure, which indicates its crystalline nature. The associated SAED pattern also shows the layer is of pure TSC. We have also found that NaCl is a crucial additive for the formation of laminated TSC crystals. Only nanoparticles can be obtained if the reaction is activated in system without NaCl additive and under identical experimental condition. Further study of the role of NaCl additive in the growth of laminated TSC crystal is currently under way.

Fig. 3 (a) SEM image of the crystalline grains; (b) A

high magnification of laminated Ti3SiC2 crystal; (c–e)

SEM images of layer growth steps of Ti3SiC2 crystal in

different directions (c, front; d, flank; e, tilted front); (f)

SEM image of the single hexagonal Ti3SiC2 crystal.

Fig. 4 TEM image and SAED

pattern of Ti3SiC2 crystals: (a)

Bright-field image of hexagonal

plates prepared from ground

Ti3SiC2, the inset showing the

SAED pattern and interface

model of Ti3SiC2 (0001); (b)

HRTEM image and the

corresponding SAED pattern of

the layer structure. (Online

color at www.crt-journal.org)

Growth mechanism In general, the growth morphology of a crystal is determined by the relative growth rates of all the possible faces. The lower growth rates at which one face is formed, the greater its morphological importance it has. Briefly, the Donnay–Harker method [14-16] defines the surface F, which will grow at the slowest rate and hence will be of greatest morphological importance in crystal morphology. According to the Donnay–Harker theory, the crystalline shape of TSC is determined by the relative growth rate of the (0001), (1010) and (1011) faces. If the growth rate of the (1010) face is the fastest, faces (0001) and

(1011) with relatively slow growth rate are important and they will determine the morphology of TSC. The crystalline shape, simulated according to the Donnay–Harker theory by Zhou et al. [17], is a hexagon. The top face is (0001) and the side faces are {1011}. The detailed surface morphological observations have revealed that the growth steps are on the basal plane (0001) and normal to this plane. So, this is a dominant layer growth mechanism for face (0001).

The morphology of the TSC crystals have proved the typical growth mechanism of the F faces as defined by Donnay and Harker, and suggest that the growth of TSC crystals is controlled by two-dimensional nucleation. The growth process of the layered TSC crystal is controlled by two different steps, i.e., intermittent two-dimensional nucleation and continuous lateral spreading of layers on the growth faces. Once a new nucleus originates, it begins growing in the two-dimensional directions. If the rate of nucleation is very slow,

854 Qiong Wu et al.: Synthesis and tribological properties of laminated Ti3SiC2 crystals

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

the single hexagonal crystal will be produced (see Fig. 3e). Otherwise, the growth pattern of layered steps is formed. The terrace is the (0001) basal plane of the TSC crystal, whose shape clearly reflects the hexagonal symmetry normal to this plane, and its ledges are {1011}. After a long time two-dimensional growth,

hexagonal symmetry characteristics of the terrace still remain, which indicates that the {1011} planes have preferable stability.

Friction and wear properties Figure 5a shows the friction coefficient as a function of different concentration of the laminated TSC crystals from 1 wt.% to 5wt.% with the load of 100 N at the rotating speed of 200 rpm. With any mass percent of it under 5 wt.%, the friction coefficient of the base oil containing laminated TSC crystals is always lower than that of pure base oil, and it decreases with the mass percent of the additives increased. Figure 5b shows the effects of the rotating speed and the load on the friction coefficient when the base oil contains 5 wt.% laminated TSC crystals. At lower rotating speed, the friction coefficient of low load is decreased obviously and stabilized at higher rotating speed. Obviously, the friction coefficient of low load is lower than that of high load. So, the base oil containing the additives has a better performance in the condition of high rotating speed and low load.

Fig. 5 Variation of friction coefficient with the change of rotating speed: (a) for HVI500 base oil and

HVI500 base oil containing 1wt.%, 3wt.%, and 5wt.% laminated Ti3SiC2 crystals at 50N; (b) for HVI500

base oil containing 5wt.% laminated Ti3SiC2 crystals under different loads. (Online color at www.crt-

journal.org)

Fig. 6 Non-contact optical profile testing instrument

images of the grinding cracks at 200 rpm under 400 N

loads for 1 h: (a) for base oil; (b) for 1wt.% laminated

Ti3SiC2 crystals + base oil; (c) for 5wt.% laminated

Ti3SiC2 crystals + base oil. (Online color at www.crt-

journal.org)

To study the wear resistance properties of laminated TSC crystals, VEECO WYKO NT1100 non-contact optical profile testing instrument is used for measuring the grinding crack and the 3-Dimentional Interactive Display images are shown in figure 6. Obviously, the grinding crack for base oil is composed of wide grooves and irregular pits along the sliding direction, as shown in figure 6a, and the grinding crack in figure 6c is shallower and smoother than that in figure 6b. From the images we can see that the depth and width of the grinding crack for base oil with 1.0 wt.% laminated TSC crystals are about 1.5 μm and 230 μm respectively, while those for base oil with 5.0 wt.% laminated TSC crystals are about 1.0 μm and 200 μm. This proves that base oil with 5.0 wt.% laminated TSC crystals presents better anti-wear capability than that with 1.0 wt.%

Cryst. Res. Technol. 45, No. 8 (2010) 855

www.crt-journal.org © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

laminated TSC crystals. The layers of TSC will penetrate more easily into the interface with base oil and form continuous film in concave of rubbing face, which can decrease shearing stress, therefore, give a low wear [18].

4 Conclusion

The laminated TSC crystals were prepared of Ti, Si, C and Al powders by the method of hot isostatic pressing. Phase-pure TSC is easily synthesized with the help of the liquid formed by the addition of NaCl in argon at 1350 °C. The results of HRTEM and SAED patterns combined, we see that the layers are of crystal TSC. The detailed surface morphological observations have revealed that the growth of TSC crystals is controlled by two-dimensional nucleation and the growth steps are on the basal plane (0001) and are normal to this plane. The introduction of laminated TSC crystals has improved the tribological properties of base oil especially in the field of friction reduction and wear resistance. Acknowledgements This work was supported by the Ministry of Science and Technology of China (863) under Grant

No. 2007AA03Z358.

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