Characterization of the structure of TiB2/TiC nanocomposite powders fabricated by high-energy ball milling
Introduction
TiB2 and TiC ceramics offer many desirable properties, such as high hardness, low density, high melting point, and high corrosion resistance. Furthermore, the high electrical and thermal conductivities of TiB2 and TiC also suggest the potential utilization of TiB2–TiC composites in high performance electrolytic systems, such as cathodes for Hall-Heroult cells [1], [2], [3]. However, TiB2–TiC composites are very difficult to be fabricated by conventional sintering methods due to their high melting points (2980 °C and 3200 °C, respectively) and low self-diffusion coefficients [4]. Recently, it has caught the attention of many researchers that dense TiB2–TiC composites from Ti–B4C or Ti–B4C–C compact reactants could be fabricated by the SHS reaction with hot isostatic pressing [5]. However, the shapes and size scales of such composites are limited by the hot isostatic pressing moulds, widely limiting their applications. High-energy ball milling can be designed as an intermediate step to promote reactions that can be completed at high temperatures [6]. Furthermore, nanosized powders, which can activate subsequent sintering processing and result in fine microstructures, can be prepared during the milling. The evolution of the microstructure in the processing is governed mainly by the sizes of mixture powders, therefore using powders after high-energy ball milling for the proper time as starting materials might be helpful. In fact, Nihara and many researchers have demonstrated that ceramic materials with fine microstructures, especially nanocomposites, exhibit improved mechanical properties [7], [8]. Thus, the preparation of TiB2–TiC nanocomposite powders via high-energy ball milling merits further study. Lee studied the synthesis of nanocrystalline TiB2–TiC using high-energy ball milling of a mixture of Ti, amorphous B and C and Wu also studied the synthesis of nanocrystalline TiC by mechanical alloying using Ti and C as starting materials which have provided good basis for our studies [9], [10].
In this paper, we used Ti and relatively inexpensive B4C as starting materials to synthesize nanosized TiB2–TiC powder mixtures by high-energy ball milling. The phase evolution with milling time was investigated by X-ray diffraction. The morphologies of the powder mixtures after milling for different time intervals were also observed by scanning electron microscopy and transmission electron microscopy, respectively.
Section snippets
Experimental procedure and methods
The starting materials used in this study were titanium powder (>99%, purity) with a sieve size of 200–300 mesh, and B4C powder (99%, purity) with an average particle size of 10.23 μm. In order to improve the milling efficiency, the starting materials were dried at 80 °C for 24 h inside a vacuum drying oven. They were then blended stoichiometrically according to the following reaction:3Ti + B4C = TiC + 2TiB2
High-energy ball milling experiments were conducted in a QM-WX4 planetary ball mill (Nanjing
XRD analysis of powder mixtures
Changes in the XRD patterns reflecting the evolution with time of the phases during milling of the Ti–B4C powder mixtures are shown in Fig. 1. The nature of the phases present and the general shape of the peaks depended on the milling time. The as-received material mainly contained Ti and B4C, and no other phases existed in the mixture (Fig. 1). The initial sharp peaks of Ti were broadened due to the refinement of the crystalline size and the generation of strain with increasing the milling
Conclusions
TiB2/TiC nanocomposite powders were successfully prepared via high-energy ball milling. The formation of TiC and TiB2 is gradual, in contrast to the conventional synthesis by the SHS reaction. Diffusion played a major role in the formation of the TiC and TiB2 phases. The formation of TiC appeared within 24 h of milling, earlier than that of TiB2 because of C atoms diffuse more rapidly in the Ti matrix than do B atoms. Only a minimal amount of TiB2 was produced even after 48 h of milling.
Acknowledgements
We are grateful for the financial support of the Foundation Committee of the National Nature of China (No. 50904017). Their support enabled us to complete this work.
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