A process of compacting a mass of metallic (or ceramic) powder into solid form by heat, with or without pressure, and with or without melting. If intimate contact can be established by the particles, and if the temperature is sufficiently high, they will grow together even in the solid state; that is, there is an actual union and growth of grain structure by diffusion. This phenomenon makes it is possible to make solid blocks of metals with high melting points. Sintering is used extensively to join hard particles of tungsten and tantalum carbides for making tool bits, the cement being metallic cobalt.
The temperature used in sintering may be far below the normal melting point of the metal, and the joining occurs by the action of surface cohesive force of the solid particles and not by partial fusion.
Sintered metals are often porous, but can be made nearly 100% dense compared to the theoretical density of the solid if high pressures are used during solid-phase sintering, or if a liquid phase, even if transient, is formed. The porosity depends strongly on the screen analysis of the powder and the presence or absence of vibration during packing of the mold. The pressure used before or during sintering also affects porosity. Although they may be very hard, sintered masses are usually not very strong, that is, the usual hardness-tensile relationship associated with steel does not hold. See POWDER METALLURGY and FRITTING.