Sintering is the extremely useful process of using heat to turn powdered substances into solids without actually melting the material. For most people, knowledge of its existence is one of the casualties of making the "phases of matter" section in science class simple and clear. We're taught that materials melt or boil at very specific temperatures and in clear phases. Solids are solid until they become liquids. What's missing from this explanation is just how mobile and "fluid" the atoms or molecules are at temperatures often well below the melting point.

It turns out that there's considerable mobility between regions, even in completely solid metal objects. What we see as a single material is usually a big clump of very small, crystalline micro-regions called grains. Grains can also have even finer structure and be different materials and sizes. It's not exagerating to say that the majority of metallurgy is really grainology–the study and practice of grain sizing, grain shaping, grain chemistry, grain mixing, grain interaction...the science of poking, prodding and inspecting grains.

Grains can be extremely confusing, especially if you're an atom right on the boundary between two hot grains. When a material is hot its component atoms thermally vibrate in place. As the temperature goes up there can be enough motion that grain boundaries blur and, whoops, the atom's on the other grain. This results in wholesale migration of atoms from one grain to another–especially from small grains to bigger ones. The subsequent grain growth explains how a powder can become a solid; little grains connect and fuse into bigger grains. Sintered materials explicitly start out as powder grains while "regular" metals develop grain by solidifying from molten material. The net result is that powders of most any material can become useful solids at relatively low temperatures. For a substance like tungsten this is handy because it melts at extremely high temperatures (3420°C) but can be made quite solid at 2300°C (a temperature just attained by the gases of a hot flame).

Tungsten Lightbulb Filaments
Sintering is not brand spanking new. Lightbulb filaments have been made by swaging, drawing and sintering since 1904. Here's a link to a history of tungsten wire forming. But wire making isn't the whole story for lightbulb filaments. No, there's something wildly improbable about lightbulbs–they use tungsten wires laced with small bubbles of potassium. Potassium? Potassium as in the element with the low melting point (146°F) and violent reaction with water? The same. You can thank the fractal nature of reality for this one (and I thank this fabulous book on tungsten for being so wonderfully thorough).

Lightbulb filaments have pores 4 millionths of an inch in diameter filled with potassium that keep the wire from sagging or breaking. The way this works is fiendishly clever–in other words it was discovered by accident and then required clever people working over many years to figure it out. The lucky accident was that the Battersea Company in London was found to have lightbulb filaments of above average strength. It turned out that a very small amount of potassium aluminum silicate was taken up by the tungsten from Battersea's clay crucibles.

During processing the tungsten powder is densified and stretched; resulting in a grain structure resembling fibers. At a certain point the aluminum and silicon is driven off by high heat but some potassium can't get out; it's left behind in long, thin tubes because the potassium atoms are too big to squeeze into (or through) tungsten's crystal structure. As the filament material is processed there comes a high heat treatment (2700-3000°C) which causes the potassium tubes to reform into strings of bubbles which stay in place for the life of the material. Without potassium bubbles the high operating temperatures of the lightbulb would cause the tungsten's crystal structure to recrystalize into big grains which would allow the filament to sag and break easily. The potassium interferes with this process so that the tungsten crystal structure stays much longer than wide–a much stronger shape leading to a much stronger, non-sagging lightbulb filament.

Powder metallurgy, one of the hottest and most powerful new materials technologies, is completely dependent on sintering. You've seen how plastics have invaded every aspect of life. That's because of their ability to be injected into molds to form precise and complex parts. Metal injection molding is an honest-to-god real process that is slowly but surely revolutionizing the fabrication of metal parts. Incidently, the most exotic powder metallurgy alloys in existence use a process called mechanical alloying to stir small grains of ceramics into a host alloy powder–exactly the initial process accidentally discovered for non-sag lightbulb filaments.

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