Nanostructured TiC–TiB2 composites obtained by adding carbon nanotubes into the self-propagating high-temperature synthesis process

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Abstract

The synthesis of nanostructured TiC–TiB2 by self-propagating high-temperature synthesis (SHS) has been investigated by using carbon nanotubes as precursor materials in partial substitution of graphite according to the following reaction: 6Ti + B4C + (3−x)C + x CNT  4TiC + 2TiB2.

Different amounts of CNTs addition have been studied in order to achieve structural refinement of the SHS products. The CNT molar content was varied in order to define the optimal composition, which allows to obtain nanostructured TiC–TiB2 powders morphologically homogenous.

The optimized composition has been chosen for the further densification step. The Pressure Assisted Fast Electric Sintering (PAFES) technique gave bulk composites with ultrafine grained microstructure. The mechanical characterization showed very high hardness and good fracture toughness values if compared to literature data.

Graphical abstract

The synthesis of nanostructured TiC–TiB2 by self-propagating high-temperature synthesis (SHS) has been investigated by using carbon nanotubes as precursor materials. Different amounts of CNTs addition have been studied in order to achieve structural refinement of the TiC and TiB2 phases. The densified composites showed ultrafine grained microstructure, very high hardness and good fracture toughness values.

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Highlights

► We investigated the effect of the CNT in the SHS synthesis of TiC–TiB2 powders. ► We tested different substitution of graphite with CNT. ► The optimal CNT amount was proved to refine the TiC and TiB2 phases. ► We densified the optimized powder through an ECAS technique. ► The bulk TiC–TiB2 composite showed very high hardness and good fracture toughness.

Introduction

Ceramic-matrix composites have recently found important technological applications. Among them TiC–TiB2 composites exhibit an excellent combination of mechanical properties and chemical stability at both room and high temperatures. These features make TiC–TiB2 composites promising materials for potential applications such as for high temperature structural components, wear resistant parts in cutting tools inserts and nonstructural applications [1]. The traditional processing of these composites is conducted by means of furnaces at high temperatures, involving significant grain growth and high production costs. Among the alternative routes investigated, the self-propagating high-temperature synthesis (SHS) process represents an attractive technique for the preparation of composite materials with complex compositions [2]. The principle of the SHS process is based on highly exothermic reactions which are capable of self-sustaining once ignited and yield high-purity products within very short reaction times [3], [4]. Due to its versatile features, the SHS process has proved to be suitable for obtaining TiC–TiB2 nanocomposite powders. The production of nanostructures is a very effective strategy for improving the up-mentioned properties of the TiC–TiB2 composites, mainly fracture toughness and wear resistance. In particular, TiC–TiB2 nanostructured powders were successfully achieved by Vallauri et al. [5] through the use of the metastability in the SHS route. Furthermore, some authors [6] demonstrated that the CNT addition results in grain refinement of the final products besides the material reinforcement. In fact, in the last years carbon nanotubes have been widely employed as reinforcement material in several kind of composites, due to their remarkable strength and flexibility [7], [8], [9].

However, the TiC–TiB2 composites have been usually considered difficult-to-sinter materials and the nanostructures achieved during the synthesis can be damaged during the densification step by employing traditional techniques. In this perspective, a class of densification techniques known as electric current activated sintering (ECAS) has shown its potential in the consolidation of nanostructured and hard-to-sinter materials. The basic principle lies on coupling a high intensity electric current, that activates the sintering mechanisms, with an uniaxial pressure. By this way it is possible to decrease the sintering temperature and to shorten the sintering time, allowing to fully densify a wide range of materials with very limited grain growth [10], [11]. The traditional SPS plants employ pulsed DC current, even if some alternatives are paying growing attention. The authors have developed a self-assembled apparatus which was named pressure assisted fast electric sintering (PAFES), that employs different types of electric current and has been successfully tested for the densification of nanostructured WC-Co powders [12].

In this paper, the carbon nanotubes addition into the precursor powders for the SHS process has been investigated in order to produce nanostructured TiC–TiB2. Different amounts of CNT addition were adopted. The optimized composition has been densified by the PAFES technique and the bulk composite has been characterized in terms of microstructure and mechanical properties.

Section snippets

Synthesis and characterization of the nanostructured TiC–TiB2 powders

The reactant powders were obtained from titanium powder (William Rowland, particle size – 325 mesh, purity 99.3%), boron carbide powder (H.C. Starck, <10 μm, 99.8%), graphite powder (Sigma–Aldrich, <2 μm, 99.5%), and multi-wall carbon nanotubes (Nanocyl-7000 Commercial grade). Different amounts of carbon nanotubes were introduced into the precursor mixture by partially replacing the graphite powders content. The stoichiometry of the reaction is expressed by the following reaction:6Ti + B4C + (3−x)C + x 

Results and discussion

TiC–TiB2 composites were obtained by means of SHS process for the different CNT contents. The Fig. 2 shows the XRD pattern of the sample obtained with x = 1.5. Pure TiC and TiB2 phases were detected [15], [16] with no peaks of unreacted Ti, B4C or graphite, thus indicating that full conversion of reagents into products was achieved according to reaction (1).

The crystallite size of the products obtained with different CNT additions was calculated by means of Rietveld analysis, as shown in Table 1.

Conclusions

  • The positive effect of the addition of CNT to produce TiC–TiB2 nanostructured powders through the SHS process is here demonstrated.

  • Different CNT molar content from 0.30 to 2.25 were tested. The optimal composition of CNT addition was found in order to obtain a nanometric and homogenous structure.

  • The composition giving the best morphology results was then densified by means of the PAFES technique and the bulk TiC–TiB2 composite showed submicron-sized grains and a significant improvement of the

References (25)

  • D. Vallauri et al.

    J. Eur. Ceram. Soc.

    (2008)
  • L. Contreras et al.

    Acta Mater.

    (2004)
  • J.J. Moore et al.

    Prog. Mater. Sci.

    (1995)
  • J.J. Moore et al.

    Prog. Mater. Sci.

    (1995)
  • K. Kondoh et al.

    Compos. Sci. Technol.

    (2009)
  • R. Orrù et al.

    Mater. Sci. Eng. Res.

    (2009)
  • F.A. Deorsola et al.

    Int. J. Refract. Met. Hard. Mater.

    (2010)
  • T.Y. Huang et al.

    Mater. Sci. Eng. A

    (2007)
  • C. Musa et al.

    Ceram. Int.

    (2009)
  • J.W. Lee et al.

    Mater. Sci. Eng. A

    (2002)
  • S.K. Bhaumik et al.

    Mater. Sci. Eng. A

    (2000)
  • A.M. Locci et al.

    Mater. Sci. Eng. A

    (2006)
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