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Commentary
Recent Developments in the Search for Ultrahard Materials
Lorenzo Martinez
Improving the hardness and toughness of materials has always been a driving force of research and development in materials science and engineering. Hardness and toughness are closely related to
wear resistance, cutting, abrasion, and other properties that are relevant to the materials used to form and shape structural components and for the construction of the moving parts of machinery. The cutting tool industry has evolved by giant steps with its movement from the early-century tool steels to new and fashionable ceramics and cermets.
Wear alone is known to cause enormous economical loses to society because of the frequent need to replace damaged components and because of inefficient machinery performance, interrupted production processes, and accidents. Losses by wear and friction are so great that in developed nations they represent a few units of percent of the gross national product.
For centuries, the majority of the abrasive materials were based on natural diamon,~ corundum, emery, garnet, flint, ana quartz. Emery, which is mentioned in the Bible, is a natural form of aluminum oxide that is still used as an abrasive material. New synthetic abrasivematerialshavebeendevelopedsince the late 19th century. However, it was not until the 1970s that synthetic abrasives came to represent the larger share of the wprld production and use. Silicon carbide~ aluminum oxide, synthetic diamond, boron carbide, and boron nitride are now commonly used manufactured abrasives.
On the following pages, recent work in the area of ultrahard materials is described through the presentation of three processing-oriented articles.
Many attempts have been made to synthesize ultrahard eN compounds. In this issue, B. Park describes efforts in this direction that employ nitrogen ion implantation to transform SiC into the metastable Si1.5C1.SN4 phase. Although Park's results are still preliminary, the studies are important to understanding the synthesis of ultrahard CN compounds.
The next contribution is by Barsoum
et al., who describe a novel processing technique called transient plastic-phase processing (TPPP), which is employed for the fabrication of fully dense, ultrarefractory ceramic/ ceramic composites of exceptional hardness. Transient plastic-phase processing was applied to the systems Ti-B-C and Zr-B-C and revealed such attractive intrinsic characteristics as a relatively low processing temperature (below 0.65 Tm)-enough to reach full densification of the composites. Other significant advantages of TPPP are near-net-shape capability, a stage of machinability prior to the reaction synthesis, elimination of sintering aids, and clean processing.
In the final article, Bowen et al. describe their efforts to develop cermets of alumina, titanium carbonitride, molybdenum carbide, and nickel. The work is directed toward producing a material
that has increased toughness and higher hardness while retaining the advantage of lower density. These are attractive for uses requiring intensive wear resistance or performance in chemically aggressive environments and high temperatures.
In general, a variety of novel and traditional techniques for the synthesis of the new hard and tough materials are presented by the authors featured in this issue. It is also noteworthy to mention the spectra of instruments employed by the researchers for materials characterization; these include recent advances in mechanical properties microprobes.
Lorenzo Martinez is scientific director of the Programa de Corrosion del Golfo de Mexico, U.A. Campeche, and is a researcher at the Instituto de Fisica of the Universidad Nacional Autonoma de Mexico. He is the advisor to JOM from the Physical Metallurgy Committee of the TMS Struc.tural Materials Division.
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