Extending the Compositional Limit of Combustion-Synthesized B4C-TiB2 Composites by Field Activation H. XUE and Z.A. MUNIR Composites of BaC-TiB2 with molar ratios B4C/TiB: of up to 8 were synthesized by field activated combustion. The combustion wave velocity depended on this ratio and increased approximately linearly with the applied field. The effect of the field was analyzed and its contribution to the combustion temperature was calculated. X-ray and microscopic analyses showed the synthesis to be a two-step process. Titanium boride forms at or near the leading edge of the combustion zone and B4C forms at or near the trailing edge. I. INTRODUCTION THE feasibility of synthesizing composite materials di- rectly from elemental reactants is one of the primary rea- sons for the current interest in the self-propagating combustion method. The use of this method to synthesize composite materials has been amply demonstrated in nu- merous investigations. These include the synthesis of metal- matrix and ceramic-ceramic composites) ~,2] An example of the latter is the system B4C-TiB2, which has been previ- ously investigated as a preform for liquid metal infiltration in armor applicationsJ 3] In this regard, the specific gravity of the composite is of concern, and hence the molar ratio, y, of B~C/TiB: is an important factor. Using elemental re- actants, such a composite can be synthesized by a self- sustaining combustion method for y < 0.5 when the reactants are at an initial temperature, To, of 298 K. [31 For y > 0.5, no self-sustaining combustion wave can be estab- lished without raising To. For example, for y values of 1.0 and 2.0, TO must be 800 and 1200 K, respectively. How- ever, the preheating of the reactants to such temperatures has been shown to result in the formation of additional phases, in this case, TiC and TiB.I41 The formation of these phases is attributed to diffilsional processes occurring prior to combustion, as has also been observed in the synthesis of MoSi2. [5] The fact that composites of B4C-TiB 2 with y > 0.5 can- not sustain self-propagating combustion when To = 298 K is the consequence of a low adiabatic combustion temper- ature, T~, as seen in Figure 1. This is the same limiting factor for the lack of success of synthesizing B4C and SiC directly by the self-propagating high-temperature synthesis (SHS) method. I6:l For these and similar materials, an SHS process can be attained through thermal activation (pre- heating), as indicated previously, or through the use of an electric field. The earlier use of an electric field focused on providing Joule heating to the reactants to raise their tem- perature up to the ignition point.Is.9I This method, however, is restricted to systems in which the reactant's conductivity is relatively high at room temperature. More recently, a new method has been used in which the reactants are ignited in H. XUE, Research Assistant, and Z.A. MUNIR, Professor and Associate Dean, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Manuscript submitted January 3, 1995. the presence of an electric field. The method of field-acti- vated synthesis was first demonstrated for the synthesis of /3_SiC[m.~.,21 and has since been used to synthesize MoSi2- SiC and other composites/u.r4~ The result of these studies showed that the effect of the field is largely confined to the combustion zone. The wave velocity increased linearly with an increase in field strength. On the basis of a combustion- zone confinement, a model was developed to simulate field activation. Modeling and experimental results were in qual- itative agreementY m21 In this work, we investigate the field activation of the combustion synthesis of B4C-TiB 2 composites with B~C/TiB 2 ratios as high as 8. II. EXPERI[VIENTAL MATERIALS AND METHOD Powders of boron, graphite, and titanium were used in the appropriate stoichiometric ratios to form the reactant 3000 v 2500 E 200O 0 .w -0 -0 < 1500 1000 0 1 2 3 4 5 6 7 B4C/TiB2 mole ratio Fig. 1--The dependence of the adiabatic combustion temperature on the B~C/TiB2 ratio in the product. METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 27B, JUNE 199(~475