K. S. Ravi Chandran, A P. Sanders, J. Du
Department of Metallurgical Engineering, The University of Utah, Salt Lake City, UT 84112
L.S. Walker
ABSTRACT
A relatively new ceramic hard material, nano titanium boride (TiB), containing Fe and Mo in solid solution and which has a very uniform nanostructure, has been synthesized in bulk form by reaction sintering under electric field induced heating. X-ray diffraction and SEM analyses indicate that this material is almost 100% TiB and 100% dense. The titanium boride forms in situ as bundles of whiskers during reaction sintering. It was found that the size, morphology, and distribution of TiB whiskers as well as the residual titanium phase were largely independent of the electrical heating path (powder/die) used in the synthesis. In the sintered material, Fe appears to be non-uniformly distributed within the TiB. However, an interesting finding is that hardness values, measured at random throughout the microstructure do not appear to be greatly affected by the inhomogeneous Fe distribution. This suggestes that the TiB synthesized by reaction-sintering under electric field induced heating is largely similar to the TiB material synthesized using a conventional hot-pressing technique. A comparison of the physical and mechanical properties of this material with the Cerbec silicon nitride indicates that the present nanostructured TiB ceramic is superior in performance and could be useful in engineering applications.
I. INTRODUCTION
The present work demonstrates that rapid synthesis of fully dense and nanostructured titanium boride can be achieved, in a shorter processing time than conventional hot pressing, through electric field activated sintering (EFAS). The process is a relatively new technique for synthesizing ceramics, and it has been increasingly employed in the synthesis of a variety of ceramic materials1. The process consists basically of heating the powdered constituent materials using electrical current directed through the powders or their containment die, and simultaneously pressurizing them via a ram. Three modes of heating are possible: (i) direct heating (DH) where all current is forced through the powder; (ii) indirect heating (IH), where all the current is forced through the dies, and (iii) a combination of these two (CH). At least in the direct powder heating, the heating is more rapid than that provided by the established approaches such as pressureless sintering, hot pressing, and hot isostatic pressing. This increased heating rate could translate into reduced process time and a more refined microstructure, and, because of shorter exposure time at high temperature, cause less grain growth.
Our recent research has shown that titanium boride ceramic with finely distributed nanoscale whiskers can be made in a one step process in a hot press, by reaction sintering a mixture of Ti, TiB2, and Fe-Mo powders. This produced a fully dense ceramic consisting of >99% nanoscale TiB whiskers and <1% residual Ti. Because the starting powders are electrically conductive, it is of high interest to see whether TiB can be synthesized in a short time using EFAS. The titanium boride material exhibits excellent mechanical properties including good fracture toughness andhigh wear resistance, which may be largely attributed to its interesting nanostructure. Using the Ti-B system as a model, several works have demonstrated the feasibility of synthesizing TiB-Ti metal-matrix composites and cermets via EFAS2,3,4,5,6,7. However, the formation of nanostructured TiB and its densification using the Ti and TiB2 source powders has not been demonstrated by EFAS processing.
The commonly known ceramic materials such as alumina8, silicon nitride9, and zirconia10, all of which are insulating, have been sintered to full density and have much refined microstructures, which are attributed to the rapid heating inherent in EFAS. Experimental and analytical studies of EFAS processing have shown that with both insulating and conductive powders, there can exist thermal gradients, which are affected by properties of the powders and the design of the current path,...
