The process of heating a powder compact at a high temperature below the melting point so that the particles bond and the compact densifies. The driving force is the reduction in surface energy.
Japanese name: 焼結
Sintering appears in 6 practice questions across 4 topics, including Materials processing and joining, Ceramics and glasses, Battery and energy materials.
Field: Ceramics and glasses
Sintering can be divided roughly into three stages by how it proceeds. In the initial stage, necks form and thicken at the contacts between particles, and shrinkage is a few percent. In the intermediate stage, the pores form connected tunnels along grain boundaries, and most of the densification occurs here. When the relative density exceeds about 92%, the final stage begins, in which the pores become isolated closed pores and the remaining pores disappear by diffusion to grain boundaries. Once pores close, gas cannot easily escape, so the choice of atmosphere also affects densification.
The driving force for sintering is the reduction of surface energy, and a curved surface experiences a pressure of about 2γ/r. With a surface energy γ of 1 J/m² and a particle radius of 0.5 µm, this is about 4 MPa. The finer the particles, the greater the driving force and the shorter the diffusion distance over which material moves, so sintering proceeds at lower temperatures and in shorter times. For the same mechanism, Herring's scaling law states that the time needed for sintering is proportional to a power of particle size: the cube for volume diffusion and the fourth power for grain boundary diffusion.
Holding at high temperature for a long time also advances grain growth. If grain boundaries move too fast, they leave pores behind and trap them inside grains, and pores away from the boundaries that serve as diffusion paths hardly disappear. It is well known that adding a little MgO to alumina suppresses boundary migration, eliminating pores and producing translucent alumina. Hot pressing, HIP and spark plasma sintering, which applies pressure while passing current, are also methods that supplement the driving force with pressure and promote densification while suppressing grain growth.
Ceramic green bodies have relative densities of about 50–60%, so they shrink greatly on densification. Going from 55% relative density to full density gives an isotropic linear shrinkage of about 18%, and die dimensions are set to allow for this. Silicon nitride is densified by liquid-phase sintering with Y₂O₃ and Al₂O₃ added to form a liquid, and cemented carbides also use a Co liquid. The density of sintered bodies is measured by the Archimedes method, and some products, such as oil-impregnated sintered bearings, deliberately retain pores to soak up oil.
Answer: A. The driving force for sintering is the reduction in surface energy as the surface area of the particles decreases
A powder compact has a large surface area and carries excess surface energy. At high temperature, material moves, the necks between particles grow, and the reduction in surface area lowers the energy of the system; this is the driving force for sintering. Of the transport mechanisms, those that carry material from the grain boundaries by grain boundary diffusion or volume diffusion bring particle centers closer together and so produce densification (shrinkage). Surface diffusion and evaporation–condensation, on the other hand, only thicken the necks without bringing particle centers closer, and contribute little to densification. Finer particles have a larger specific surface area and shorter diffusion distances, so sintering proceeds at lower temperatures.
Answer: A. About 9.7%
Since the mass does not change, the volume is inversely proportional to the density, so V/V₀ = 0.70/0.95 ≈ 0.737. Because the shrinkage is isotropic, the length ratio is the cube root of the volume ratio, (0.737)^(1/3) ≈ 0.903, and the linear shrinkage is 1 − 0.903 ≈ 9.7%. About 26% is the volume shrinkage 1 − 0.737, and about 36% is the increase in relative density (0.95 − 0.70)/0.70; neither is the fractional decrease in length. When making sintered parts, the die dimensions are made larger to allow for this shrinkage. If the green density varies from place to place, because of friction with the die wall and the like, the shrinkage also varies, causing warping and dimensional scatter after sintering.
Sintering bonds particles together at temperatures below the melting point, mainly by atomic diffusion. Because the whole body is not melted, even materials with very high melting points can be densified while keeping their formed shape. Even in liquid-phase sintering, which forms some liquid, the main skeleton remains solid.
It varies widely with the material, particle size and additives, so there is no single value. For example, alumina, which melts at about 2072 °C, is sintered at about 1500–1600 °C, or about 0.75–0.8 times its melting point in kelvin.
The Archimedes method, which finds the volume from the difference between mass in air and in water, is common. Because water penetrates open pores, the pores are sealed before measurement, or the water-saturated mass is used to separate open porosity from apparent density.
At high temperature, material moves, the necks between particles grow, and the reduction in surface area lowers the energy of the system; this is the driving force for sintering.
Which statement about sintering of ceramic powders is corre…If the green density varies from place to place, because of friction with the die wall and the like, the shrinkage also varies, causing warping and dimensional scatter after sintering.
A green compact at 70% relative density is sintered to 95% …At the sintering temperature the Co-rich liquid wets the WC particles well, capillary forces rearrange and pack the particles, and small WC particles dissolve and reprecipitate onto larger ones, so the body densifies to nearly full density.
To make WC–Co cemented carbide, WC powder is mixed with Co …Requiring high-temperature sintering to densify is characteristic of oxide electrolytes such as garnet-type Li₇La₃Zr₂O₁₂ (LLZO).
Which statement about sulfide solid electrolytes used in al…The coating is not meant to lower the sintering temperature.
In a ceramic matrix composite (CMC) reinforced with SiC fib…Specific surface area Eutectic reaction Solid electrolyte Ceramic matrix composite Hot isostatic pressing Creep Perovskite structure Piezoelectric effect Transformation toughening Weibull modulus