PROCESSING OF NANOCRYSTALLINE CERAMICS FROM ULTRAFINE PARTICLES

The recent interest in the properties of ceramics with grain sizes les s than 100 nm has created a need for processing routes with which to m anufacture such ceramics. This article briefly reviews the variety of techniques currently used to manufacture ultrafine starting powders, c ompact the powders, and sinter them into bulk nanocrystalline ceramics . The unique challenges associated with processing such fine structure s are discussed together with each technique. Major obstacles have inc luded the difficulty (now largely overcome) of producing sufficient qu antities of ultrafine powders; the strong tendency of nanocrystalline powders to agglomerate; the difficulty of manufacturing homogeneous, s tress free compacts from ultrafine powders; and the ever present obsta cle of unwanted grain growth during sintering. Despite numerous techni cal difficulties, researchers have managed to develop techniques for p rocessing ultrafine powders into nanocrystalline ceramics which are bo th fully dense (Or near fully dense) and still retain a grain size les s than 100 nm. Pressureless sintering, sinter-forging, hot pressing, a nd hot isostatic pressing, when carried out under the correct conditio ns, have all been shown to be capable of producing nanocrystalline cer amics. Microwave sintering, rapid rate sintering, plasma activated sin tering, and shock compaction techniques have produced near-nanocrystal line ceramics. At the root of these accomplishments is an improved und erstanding of, and exploitation of, the microstructural events which o ccur during traditional powder processing procedures. (C) 1996 The Ins titute of Materials and ASM International.