Effect of Reactant Particle Size on the Self-Propagating High-Temperature Synthesis Reaction Behaviors in the Ni-Ti-B 4
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TRODUCTION
SELF-PROPAGATING high-temperature synthesis (SHS) has attracted more attention due to its lower energy consumption, higher time efficiency, and product purity, which is widely used for preparing a variety of intermetallics, ceramics, and composites.[1] Among these ceramics, titanium carbide (TiC) and titanium diboride (TiB2) have been widely investigated due to the desired properties such as low densities, high melting points, good thermal and chemical stability, high hardness, and excellent wear resistance. In particular, it has been shown that the fracture toughness and wear resistance of TiB2/TiC composites are significantly higher than those of TiC and TiB2 single-phase materials, making them attractive for advanced structural applications.[2,3] Generally, TiC and TiB2 can be produced from Ti-B-C, Ti-B4C, and Ti-B4C-C with proper proportions of the reactants during SHS.[2–12] It is well known that the formation of liquid and its subsequent capillary spreading are essential to the ignition and the propagation of the combustion wave, because it substantially increases the contact surface area among reactants and provides an easier route for reactant mass transfer. However, the reactions in these systems are somewhat difficult to initiate due to high melting temperatures of the reactants (Ti). Thus, a possible solution to these problems could be the incorporation of metals with low melting points for the formation of low-melting intermetallics or the eutectic liquid (metal-Ti liquid) at a low temperature (eutectic temperature), as done in Y.F. YANG and R.Y. ZHAO, Doctors, and H.Y. WANG and Q.C. JIANG, Professors, are with the Key Laboratory of Automobile Materials, Department of Materials Science and Engineering, Jilin University, Changchun, 130025, Peopleo´s Republic of China. Contact e-mail: [email protected] Manuscript submitted April 30, 2008. Article published online November 5, 2008 232—VOLUME 40A, JANUARY 2009
the Al-Ti-C and Cu-Ti-C systems.[13,14] Among many metals, liquid Ni features low wetting angles under vacuum at 1450 C with solid TiC and TiB2.[15] In addition to reacting with Ti to form a Ni-Ti liquidphase eutectic, Ni can also react with B4C to form a Ni-B liquid-phase eutectic, which not only promotes the easy occurrence of SHS reaction, but also improves the structural applications. In our previous study,[16] the effect of Ni content on the SHS reaction behaviors in the Ni-Ti-B4C system was investigated primarily. It was found that with the increase of Ni content, combustion temperatures, wave velocities, and ceramic particle sizes decreased, while the ignition time showed a first remarkable decrease and then an increase with the minimum value at 20 wt pct Ni. In addition to the composition of reactants, material parameters such as particle size and packing also play an important role in the SHS reaction behaviors, which have been studied in many research papers.[17–19] Therefore, it can be assumed that the reactant powder particle size can influence the SHS reaction behaviors in the Ni-Ti-B4C
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