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9/27/03

3:21 PM

Page 2671

Quantitative Characterization of the Microstructure of Two-Phase TiB2
Al2O3 Ceramics Using Mean Integral Curvature LOUIS FERRANTI, Jr. and NARESH N. THADHANI Two-phase TiB2
Al2O3 ceramics with an interconnected or dispersed TiB2 (minor)–phase microstructure can be produced by variations in processing parameters. A standard method of quantitative characterization of the microstructural bias, i.e., the degree of TiB2 phase connectivity relative to its dispersion, is necessary to comprehend the mechanism(s) controlling the evolution of microstructure during processing. In this work, techniques derived from stereology were used to quantitatively characterize the microstructural bias on the basis of the connectivity and dispersion of the minor phase (TiB2), in addition to the size of the TiB2- and Al2O3-phase regions. The mean integral curvature calculated using the area particle-count and area tangent-count methods was determined to quantitatively describe the connectivity of the TiB2 minor phase around the Al2O3 major phase. The results illustrate that, in spite of partial and mixed bias, integral curvature measurements (particularly those based on the area tangent-count method) provide a reliable and reproducible means for quantitative characterization of the two-phase biased microstructure.

I. INTRODUCTION

MOST traditional or advanced ceramics contain more than a single constituent phase. The second phase may either be intentionally added or be picked up during processing. For example, silicon carbide,[1] titanium carbide,[2] and boron carbide[3] have previously been added to an alumina matrix to inhibit the growth of cracks by providing a plastic “energyabsorbing” barrier and, thereby, enhancing material properties. The addition of a second phase can also assist in ceramic fabrication. For example, sintering aids such as B4C assist in the pressureless sintering of TiB2 ceramics, resulting in enhanced density and grain-growth suppression.[4] On the other hand, impurities such as Fe (
0.5 wt pct) cause abnormal grain growth and also yield a low sintered density. Likewise, the presence of an unintentionally formed glassy second phase can weaken the ceramic and deteriorate its properties. Efforts to design ceramics with unique microstructures that provide improved material properties are receiving great interest. Multiphase ceramics are being developed to supress the inherent brittle nature of these materials and attain a combination of high strength and toughness and wear and impact resistance. For these materials, while it may be required to have a certain desired distribution of the respective phases, the control in forming the complex microstructure is often limited. The properties of two-phase ceramics are influenced by microstructural characteristics, including the type of phases present, their size, morphology, and chemical composition. Mechanical properties establish material limitations for specific engineering applications. Many of these properties are dictated not only by t