The place of powder metallurgy in contemporary material science and technology

Powder metallurgy is a very attractive technology because it is possible to produce near- net-shape parts by rapid consolidation of fine, pure powders. Unfortunately defects (such as unfilled pores, cracks and distortion) readily form so that near-net-shapes become more of a goal than a readily achieveable actuality. Castings require attainment of a fluid (molten) state so that molds can restrain the molten state long enough to solidify (or at least mainly solidify) the shape so that the near-net-shape is approached. Thus neither pouring and casting satisfies all requirements or (on the other hand) powder metallurgy compaction meets the goals of nearly complete (99.9 % theoretical density). Powders for powder metal compaction need to be fine, spherical and deformable (as much as possible), requirements that are more difficult to meet than would seem at first glance, so it is much simpler to cast than to compact. Certainly powder metal preparation and compaction demand great mastery of the processes. Permanent magnet material shapes are compatable with powder metallurgy fabrication because single domain sizes are readily achieved and fine grained microstructures are much easier to prepare than castings (which tend to grain growth). Powder metallurgy (or other process) can produce a composite materials with superior mechanical properties (such as thermal cycling, impact resistance, bending strength). Examples of these promising materials include alumina/filimentary mullite, zirconia /filimentary mullite and titania/filimentary mullite. These materials are extremely challenging to fabricate, but rewarding benefits are available to those who can master the art. Nitride ceramics (aluminum nitride, silicon nitride and boron nitride) are superior to oxide ceramics (alumina, zirconia, magnesia and titania) composite ceramics (as discussed and described in detail in this paper).