Sintering fuses powders into solid parts without ever melting them
Squeeze loose snow into a hard snowball and you have performed sintering. The same principle, where particles bond under heat or pressure while staying solid, lets factories shape tungsten and molybdenum, metals whose melting points are so high that casting them is barely practical. Pottery, bearings and turbine blades all depend on it.
Atoms in neighbouring grains migrate across the boundaries between them, first forming small necks where particles touch, then gradually closing the gaps. The push comes from surface energy: replacing solid-to-air surfaces with lower-energy solid-to-solid contacts lowers the total energy. That effect grows dramatically when particles are only a few micrometres across, which is a major reason ceramic manufacturing relies on very fine powders. Ice cubes sticking together in a drink and snow compacting into glacier ice are everyday examples.
In ceramics the process typically starts at 50 to 80 percent of a material's melting temperature. A factory mixes powder with water and binder into a slurry, spray-dries it, presses it into a mould to make an unfired piece called a green body, burns off the binder at low heat, then fires it hot enough to fuse the grains. Once enough bonding has happened the piece no longer crumbles in water, and further heating shrinks the pores and adds strength. The product is called sinter, a word related to the English cinder.
Porosity is not always the enemy. Sintered bronze bearings hold lubricant in their tiny pores, sintered copper wicks liquid by capillary action inside heat pipes, and sintered bronze filters serve in aircraft hydraulics. Cemented tungsten carbide needs a variant in which only some ingredients turn liquid. Jewellers sinter silver and gold powders into small pieces, and silver nanocubes have been used to form electrical joints below 200 degrees Celsius.
The technique also yields nearly finished shapes with little machining, keeps starting materials very pure, and can create substances no other method produces. Leftover powder from pure elements stays pure and can be recycled.
Source: Sintering
