Cryst. Res. Technol. 46, No. 2, 178 – 182 (2011) / DOI 10.1002/crat.201000496

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Synthesis and tribological properties of hexagonal titanium silicon

carbide crystals

Qiong Wu, Hua Tang, Changsheng Li*, Xiaofei Yang, Haojie Song, and Kangmin Chen

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

Received 23 September 2010, revised 5 December 2010, accepted 27 December 2010

Published online 14 January 2011

Key words titanium silicon carbide, vacuum sintering, lubrication additive, tribological properties.

Hexagonal titanium silicon carbide (Ti3SiC2) crystals were prepared by vacuum sintering of Ti, Si, and C

powders at 1300 °C. The microstructure and grain deformations of Ti3SiC2 were examined by scanning

electron microscopy and transmission electron microscopy. The tribological properties of hexagonal Ti3SiC2

crystals as lubrication additive in HVI500 base oil were investigated by a UMT-2 ball-on-plate friction and

wear tester. The Ti3SiC2 additives exhibited good friction reduction. Under determinate conditions, the

friction coefficient of base oil containing Ti3SiC2 crystals is lower than that of pure base oil. The base oil with

3.0 wt.% hexagonal Ti3SiC2 crystals presented good anti-wear capability.

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

Titanium silicon carbide (Ti3SiC2, TSC) is a member of the family of ternary ceramics with the general formula Mn+1AXn (MAX). Its microstructure details are investigated by atomic-resolution Z-contrast scanning transmission electron microscopy and convergent beam electron diffraction [1]. TSC is an important and functional ceramic because it possesses a unique combination of ceramic and metallic properties [2]. It exhibits a variety of excellent ceramic properties, such as elastic rigidity [3], low density, strong resistance to chemical corrosion [4], and high melting point [5]. It also has metallic properties, such as relatively high electrical [6] and thermal conductivities [7], thermal shock resistance [8], and high damage tolerance [9]. In addition, TSC can be fully reversible under 1 GPa compression [10, 11] and has zero thermal power within the temperature range 300–850 K [12]. Various synthetic routes for different forms of TSC have been done since the 1960s, such as pulsed laser deposition [13] and mechanical alloying [14]. Although different processing routes to obtain dense Ti3SiC2 have been reported, limited work has been carried out to evaluate their potential in tribological applications. Myhra [15] measured a low-kinetic friction coefficient (m) of 0.002 at 25 nN lateral force for the basal planes of Ti3SiC2 using lateral force microscopy. They also reported a steady-state coefficient of friction (COF) of 0.12 for polycrystalline Ti3SiC2 rubbed against a lightly peened stainless-steel sheet at 0.15–0.9 N load. Barsoum [16] measured a steadystate COF of around 0.8 in a Ti3SiC2/steel tribocouple under 5 N load and observed that the frictional response is independent of grain size (5–100 mm). Zhang [17] reported that the friction of self-mated Ti3SiC2 tribocouple is 1.16–1.43 and that of Ti3SiC2/diamond is around 0.1 under varying loads of 0.98–9.8 N. In their research, the Ti3SiC2 ceramic exhibited a rather low friction coefficient during dry sliding against diamond because of the formation of a self-lubricating film. All the literature results are mainly based on the pin-on-disk measurements of the unlubrication friction behavior of Ti3SiC2. However, to the best of our knowledge, the synthesis of hexagonal TSC has not been reported. Moreover, few studies have focused on the lubrication friction properties of hexagonal TSC, which is similar to hexagonal MoS2 as a lubrication additive. In this paper, we have researched the crystal structure and crystalline grain deformation of hexagonal TSC crystals. We have also investigated the tribological properties of hexagonal TSC as lubrication additives in HVI500 base oil.

2 Experimental

Preparation of hexagonal Ti3SiC2 crystals Hexagonal TSC crystals were synthesized from a mixture of Ti (99.5%, –300 mesh), Si (99.9%, –200 mesh), and graphite (99.97%, diameter < 20 µm). The mixed powders ____________________

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

Cryst. Res. Technol. 46, No. 2 (2011) 179

www.crt-journal.org © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

(molar ratio: Ti:Si:C = 3:1:2) with polyvinyl alcohol (C2nH4nOn, PVA) solution addition were subjected to planet ball-milling for 15 h. The slurry was dried in vacuum, taken out of the steel kettle and then compacted into a steel mold with dimension of Ø15×60 mm. This was followed by a homogeneous heating from 250 to 450 °C in order to remove PVA. Then, the resulting powders were sintered in vacuum (5.0×10-3 Pa) at 1300 oC for 1 h before cooling to room temperature (the heating and cooling rates were 15 and 5 K/min, respectively.). In sintering, a furnace with a graphite heater was used and temperatures were measured by an infrared thermometer. The vacuum was produced by a pump assembly composed of a Roots pump (pumping speed of 60 l/s, vacuum limit of 1.0×10-3 Pa) and a rotational vacuum pump (pumping speed of 15 l/s). Finally, hexagonal TSC crystals were obtained. The samples were characterized using x-ray diffraction (XRD), and their morphologies were determined using a scanning electron microscope (SEM) (JEOL JXA-840A). Transmission electron microscopy (TEM) studies were carried out on a JEM-100CX II transmission electron microscope. All measurements were carried out at room temperature.

Tribological properties of hexagonal Ti3SiC2 crystals used as lubricant additives Hexagonal Ti3SiC2 crystals and dispersing agent, sorbitol monooleate (Span-80), were mixed with HVI500 base oil. The mixture was stirred for 30 min to form a uniform suspension using a T-18 high-speed dispersion machine. The suspension was dispersed again by ultrasonic bath for 1 h. It is found that the addition of 1.0 wt.% dispersing agent sorbitol monooleate to the oil produced the best dispersion stability as judged from the absorbance. Then, a series of suspended oil samples were obtained. Friction and wear testing were carried out on a UMT-2 ball-on-plate friction and wear tester made in the USA. Sketch map of the test rig used in the present experiments is shown in figure 1. The friction and wear resistance tests were conducted for 1 h at rotating speeds of 100–300 rpm with loads of 50–350 N. The grinding tracks were measured using a VEECO WYKO NT1100 non-contact optical profile testing instrument.

Fig. 1 Sketch map of UMT-2 friction and

wear tester.

Fig. 2 X-ray diffraction patterns of the samples synthesized at 1300 °C,

according to the diffraction data from JCPDS card 74-0310. The inset

shows the SEM micrograph of Ti3SiC2 grain with hexagonal geometrical

shape.

3 Results and discussion

Synthesis and characterization Figure 2 shows the XRD pattern of the TSC synthesized at 1300 °C. The strong and sharp diffraction peaks indicate that the product was sufficiently crystallized. All the diffraction peaks in this figure can be indexed to the pure hexagonal structure of TSC. The lattice constants, a = 3.064 Å and c = 17.65 Å, are consistent with the data in the standard card (JCPDS 74-0310). No by-product peaks were found. The SEM micrograph in figure 2 shows the apparent hexagonal geometrical figure. These indicate that phase-pure hexagonal TSC is easily formed from Ti, Si, and C powders by vacuum sintering at 1300 °C.

Figure 3a shows a typical SEM micrograph of the TSC grains with a dimension range of 5-10 µm. One of the salient microstructural characteristics observed is the hexagonal TSC grain distribution in the sample. The

180 Qiong Wu et al.: Hexagonal titanium silicon carbide crystals

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

high-magnification image in figure 3b shows the morphology of a single TSC grain viewed from the front perspective, exhibiting a clear and well-defined hexagonal structure. A careful analysis of the SEM micrograph of the TSC grain viewed from the flank perspective indicates that the hexagonal TSC grains consisted of a number of thin slices of less than 40 nm thickness (Fig. 3c). The thin slice is the typical crystallite shape of the layered ternary TSC compound.

Fig. 3 (a) SEM image of the Ti3SiC2 crystalline grains; (b) A single hexagonal Ti3SiC2 crystal under high

magnification; (c) SEM image of the typical flank perspective of Ti3SiC2 grain.

Fig. 4 (a)–(b) SEM images of Ti3SiC2 grain deformations; (c)–(d) Enlarged SEM images of sharp kinks and voids

between the layers in (b).

Fig. 5 Bright-field TEM image of hexagonal plates

prepared from ground Ti3SiC2, the inset showing the

SAED pattern and interface model of Ti3SiC2 (0001).

Fig. 6 Friction coefficients vary with the rotation speed of

change. (Online color for both figures at www.crt-journal.org)

To examine further the internal structure of TSC, a select, but typical, SEM micrograph of TSC grain deformations is presented in figure 4a, wherein microcracking, grain break up, and grain pull-out are evident. Other deformation phenomenon is also observed, as shown in the field-emission scanning electron microscope image in figure 4b. Here, heavily deformed lamellaes are seen on some TSC grains, and significant amounts of delamination and bending are observed. Enlarged images of sharp kinks and voids between layers are shown in figure 4c and d, where small cleavages are also visible. The observed deformations, which have been reported in different studies [8,9], occur locally throughout the TSC sample and result in plastic behavior. The deformations confer high damage tolerance by preventing the breakdown of the entire sample. The hexagonal morphology of sintered TSC sample has been hypothesized as likely to improve further the plasticity and damage tolerance of the material by allowing increased local deformations.

Cryst. Res. Technol. 46, No. 2 (2011) 181

www.crt-journal.org © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The high-symmetry characteristics of the crystalline product are in the TEM observations. A TEM image of a thin hexagonal plate is presented in figure 5, which clearly shows the hexagonal structure. The inset in figure 5 shows that the selected area electron diffraction (SAED) pattern, which is in accordance with the interface model of TSC (0001) (the small inset in lower left corner of figure 5), contains a special set of the TSC diffraction patterns. The aforementioned diffraction patterns can only be observed from the [0001] crystallographic direction, indicating that the hexagonal plate corresponds to the basal (0001) plane of TSC [18].

Friction and wear properties In order to evaluate the friction and wear properties of Ti3SiC2, fretting experiments were conducted with varying loads and rotation speeds at constant sliding distance and frequency. The obtained steady-state values of the coefficient of friction (COF) are compared with the literature data on Ti3SiC2 (Table 1).

Table 1 Summary of the friction data obtained with Ti3SiC2 under varying test conditions. (LFM, lateral

force microscopy; COF, coefficient of friction. * Along the basal plane of Ti3SiC2).

Purity Density (g/cm3) System Method COF Reference

95% 3.73 lubrication friction Ball on disk 0.036 Present work

96% 4.28 unlubrication friction LFM 0.002* Myhra [15]

91% 4.16 unlubrication friction Point on disk 0.83 El-Raghy [16]

93% 4.27 unlubrication friction Point on flat 0.09 Zhang [17]

Fig. 7 Non-contact optical profile testing instrument images of the grinding tracks at 200 rpm under 50 N loads for 1 h.

(a) For base oil; (b) For 1 wt.% hexagonal Ti3SiC2 crystals + base oil; (c) For 3 wt.% hexagonal Ti3SiC2 crystals + base oil.

(Online color at www.crt-journal.org)

The COF data in the present investigation are shown in figure 6. Here, the friction coefficient is plotted as a function of different concentrations of the hexagonal TSC crystals from 1 wt.% to 3 wt.%, at a load of 100 N and the rotation speed of 200 rpm. At mass percentage lower than 3 wt.%, the friction coefficient of the base oil containing hexagonal TSC crystals is lower than that of the pure base oil. The pure base oil has a friction coefficient that decreases as the mass percentage of the additives increases.

To determine the wear resistance properties of hexagonal TSC crystals, a VEECO WYKO NT1100 non-contact optical profile testing instrument was used to measure the grinding track. The three-dimensional interactive display images are shown in figure 7. Obviously, the grinding track for the base oil is composed of wide grooves and irregular pits along the sliding direction (Fig. 7a), whereas the grinding tracks in figure 7c are shallower and smoother than those in figure 7b. From the images, the depth and width of the grinding tracks for the base oil with 1.0 wt.% hexagonal TSC crystals are about 3.0 and 280 µm, respectively, whereas those for the base oil with 3.0 wt.% hexagonal TSC crystals are about 2.1 and 200 µm, respectively. This proves that the base oil with 3.0 wt.% hexagonal TSC crystals has better anti-wear capability than that with 1.0 wt.% hexagonal TSC crystals. The layers of TSC penetrate more easily into the interface with the base oil and form a continuous film on the concave side of the rubbing face, which can decrease shearing stress and yield low wears [19].

182 Qiong Wu et al.: Hexagonal titanium silicon carbide crystals

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

4 Conclusion

Pure-phase hexagonal Ti3SiC2 crystals were successfully synthesized by combining Ti, Si, and C powders using vacuum sintering method. From the TEM and the SAED patterns, the diffraction patterns can only be observed from the [0001] crystallographic direction, indicating that the hexagonal plate corresponds to the basal (0001) plane of Ti3SiC2. The deformations of Ti3SiC2 crystals confer high damage tolerance by preventing the breakdown of the entire sample. The introduction of hexagonal Ti3SiC2 crystals as lubrication additives improves the tribological properties of the base oil, especially in terms of friction reduction and wear resistance. The base oil has a friction coefficient that decreases as the mass percentage of the Ti3SiC2 additives increases. The base oil with 3.0 wt.% hexagonal Ti3SiC2 crystals presents better anti-wear capability than that with 1.0 wt.% hexagonal Ti3SiC2 crystals. Acknowledgements This work was supported by the Ministry of Science and Technology of China (863) under Grant

No. 2007AA03Z358.

References

[1] Z. J. Lin, M. S. Li, and Y. C. Zhou, J. Mater. Sci. Technol. 23, 145 (2007).

[2] M. W. Barsoum, Prog. Solid State Chem. 28, 201 (2000).

[3] M. W. Barsoum and T. El-Raghy, J. Am. Ceram. Soc. 79, 1953 (1996).

[4] B. Pécz, L. Tóth, M. A. di Forte-Poisson, and J. Vacas, Appl. Surf. Sci. 206, 8 (2003).

[5] R. Pampuch, J. Lis, L. Stobierski, and M. Tymkiewicz, J. Eur. Ceram. Soc. 5, 283 (1989).

[6] Z. M. Sun and Y. C. Zhou, Phys. Rev. B 60, 1441 (1999).

[7] M. W. Barsoum, T. El-Raghy, C. J. Rawn, W. D. Porter, H. Wang, E. A. Payzant, and C. R. Hubbard, J. Phys.

Chem. Solids 60, 429 (1999).

[8] T. El-Raghy, M. W. Barsoum, A. Zavaliangos, and S. R. Kalidindi, J. Am. Ceram. Soc. 82, 2855 (1999).

[9] M. W. Barsoum and T. El-Raghy, Metall. Mater. Trans. A 30, 363 (1999).

[10] M. W. Barsoum, T. Zhen, S. R. Kalidindi, M. Radovic, and A. Murugaiah, Nat. Mater. 2, 107 (2003).

[11] F. Barcelo, S. Doriot, T. Cozzika, M. Le Flem, J. L. Béchade, M. Radovic, and M. W. Barsoum, J. Alloy. Comp.

488, 181 (2009).

[12] H. I. Yoo, M. W. Barsoum, and T. El-Raghy, Nature 407, 581 (2000).

[13] C. Lange, M. W. Barsoum, and P. Schaaf, Appl. Surf. Sci. 254, 1232 (2007).

[14] M. Zakeri, M. R. Rahimipour, and A. Khanmohammadian, Ceram. Int. 35, 1553 (2009).

[15] S. Myhra, J. W. B. Summers, and E. H. Kisi, Mater. Lett. 39, 6 (1999).

[16] T. El-Raghy, P. Blau, and M. W. Barsoum, Wear 238, 125 (2000).

[17] Y. Zhang, G. P. Ding, Y. C. Zhou, and B. C. Cai, Mater. Lett. 55, 285 (2002).

[18] Z. J. Lin, M. J. Zhuo, Y. C. Zhou, M. S. Li, and J. Y. Wang, Scr. Mater. 55, 445 (2006).

[19] H. D. Huang, J. P. Tu, L. P. Gan, and C. Z. Li, Wear 261, 140 (2006).