<P> Shock consolidation, or dynamic consolidation, is an experimental technique of consolidating powders using high pressure shock waves . These are commonly produced by impacting the workpiece with an explosively accelerated plate . Despite being researched for a long time, the technique still has some problems in controlability and uniformity . However, it offers some valuable potential advantages . As an example, consolidation occurs so rapidly that metastable microstructures may be retained . </P> <P> These techniques employ electric currents to drive or enhance sintering . Through a combination of electric currents and mechanical pressure powders sinter more rapidly thereby reducing the sintering time compared to conventional thermal solutions . The techniques can be divided into two main categories: resistance sintering, which incorporates spark plasma sintering and hot pressing; and electric discharge sintering, such as capacitor discharge sintering or its derivative, electro sinter forging . Resistance sintering techniques are consolidation methods based on temperature, where heating of the mold and of the powders is accomplished through electric currents, usually with a characteristic processing time of 15 to 30 minutes . On the other hand, electric discharge sintering methods rely on high - density currents (from 0.1 to 1 kA / mm ^ 2) to directly sinter electrically conductive powders, with a characteristic time between tens of microseconds to hundreds of milliseconds . </P> <P> Many special products are possible with powder metallurgy technology . A nonexhaustive list includes Al O whiskers coated with very thin oxide layers for improved refraction; iron compacts with Al O coatings for improved high - temperature creep strength; light bulb filaments made with powder technology; linings for friction brakes; metal glasses for high - strength films and ribbons; heat shields for spacecraft reentry into Earth's atmosphere; electrical contacts for handling large current flows; magnets; microwave ferrites; filters for gases; and bearings which can be infiltrated with lubricants . </P> <P> Extremely thin films and tiny spheres exhibit high strength . One application of this observation is to coat brittle materials in whisker form with a submicrometre film of much softer metal (e.g. cobalt - coated tungsten). The surface strain of the thin layer places the harder metal under compression, so that when the entire composite is sintered the rupture strength increases markedly . With this method, strengths on the order of 2.8 GPa versus 550 MPa have been observed for, respectively, coated (25% cobalt) and uncoated tungsten carbides . </P>

Where was the technique of powder metallurgy invented