Fabrication and characterization of Ti-TiB 2 functionally graded material system
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I. INTRODUCTION
DUE to higher technical demand on material properties for devices employed in advanced applications, materials that possess a combination of properties are gaining in importance in the recent decades. Recently, the concept of functionally graded materials (FGMs) has been proposed and widely investigated. The characteristic of FGMs lies in their continuously varying compositions or volume fractions of the constituent materials built in a layered form, hence, providing optimum thermal and mechanical properties without a discrete interface within the component. The FGMs have been applied in many areas including structural[1–4] and biomedical applications.[5,6,7] A number of techniques have been reported in the literature on the fabrication of FGMs.[8–11] It is, however, reported that the powder metallurgy method is the most commonly employed technique due to its flexibility in compositional and microstructural control.[12] Nevertheless, it is to be noted that the powder metallurgy technique involves a sintering process for the densification of the porous powder compact. In a metal-ceramic FGM system, the difference in sintering temperatures between most metals and ceramics has largely restricted the possible systems that may be selected. One of the solutions to this issue is to introduce sintering aids into the ceramic phase to assist densification at a lower sintering temperature.[13,14] In the present work, a Ti-TiB2 system is fabricated using the powder metallurgy route, where Ti is the pure metal layer at one end and 100 pct TiB2 is the full ceramic layer at the other end, with mixed composition inbetween the two faces. A sintering aid was added to the 100 pct TiB2 layer to enhance densification of the TiB2 ceramics. The intermediate mixture layers, despite possessing a variation in microstructure and porosity, could actually exhibit toughness enhancement effect, which will be discussed in Section III. This is mainly attributed to the introduction of J. MA, Assistant Professor, and ZEMING HE, Research Fellow, are with the School of Materials Engineering, Nanyang Technological University, Singapore 639798. G.E.B. TAN, Senior Member of Technical Staff, is with DSO National Laboratories, Singapore 118230. Manuscript submitted March 29, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
the crack deflection toughening mechanisms in the system when a difference in fracture energy is present between the adjacent layers.[15,16,17] II. EXPERIMENTAL PROCEDURE A. Raw Materials The raw metal powder used for the present work was 99.7 pct pure titanium powders (Atlantic Equipment Engineers) with mean starting particle size of 10 m. The ceramic powder used was 99.7 pct pure titanium diboride powders (Atlantic Equipment Engineers, Bergenfield, NJ) with mean starting particle size of 3 m. A 99.5 pct pure silicon carbide powder (Aldrich Chemical Company. Inc., Milwaukee, WI) was used as a sintering aid to enhance the densification of the titanium diboride. B. Processing The FGM includes a total of five layers. In
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