MTE3203 Week 4 Sintering

The basic idea — what is sintering?

Heat a pressed powder compact, but below its melting point (~0.8 Tm). The particles weld together and the gaps between them close up, giving a dense solid. Everything else is why that happens and four ways to speed it up.

Why does sintering happen at all?

Powder has a huge amount of surface, and surface costs energy. Fusing two particles trades two surfaces for one grain boundary, which is cheaper. So the system shrinks its surface area to lower its free energy. Finer powder means more surface per gram, so the system pushes harder.

How do the atoms actually move?

Diffusion:

Convex surfaces are compressed, concave necks are stretched. Stretched material holds more vacancies, so vacancies flow out of the neck and atoms flow in. The neck grows and the piece shrinks.

What is solid state sintering?

Heat alone, no liquid phase. Atoms diffuse along particle surfaces, along grain boundaries, or through the bulk. Only works if the material genuinely lets atoms move at temperature, which rules out highly covalent ceramics.

Solid sintering: what are the three stages?

Initial: necks form between particles, little shrinkage.

Intermediate: pores go from open to closed to isolated, densification is rapid and grains start growing

Final: isolated pores shrink away and grain growth accelerates.

why does solid sintering slow down as it goes?

-Less driving force due to less surface area

• Diffusion distance increases

• Removing trapped gas from pores becomes more difficult

Solid sintering limitations

Removing the last pores needs high temperature and long times, which is exactly when grains coarsen. Grain growth accelerates then because the pores had been pinning the grain boundaries — remove the pores and the boundaries sweep freely. Coarse grains mean a weaker ceramic, so the whole game is density versus grain size.

What is liquid phase sintering?

Add something that melts, giving 1–20 wt% liquid at temperature. The liquid forms a concave lens at each contact, and a concave surface sucks, pulling particles together; it also lubricates repacking and dissolves the solid as a fast transport path. Lower temperature, faster, cheaper, so most commercial ceramics use it.

Liquid phase sintering — 3 requirements

-There is liquid, with the right viscosity to flow.

-Good wetting is required because a wetting liquid pulls the particles together, like water between two glass slides. A non-wetting liquid won't, so you get no densification.

And the solid adequately dissolves in it so that it can reprecipitate in otehr areas

Liquid phase sintering — what are the three stages?

Rearrangement: liquid forms, wets and pulls everything in — fast, big density jump.

Solution–precipitation: solid dissolves at the stressed contacts and redeposits elsewhere, flattening the contacts so centres move closer. This is the step that densifies; Ostwald ripening runs alongside but only coarsens.

Coarsening: grains grow and merge, the last pores vanish.

Liquid phase sintering — how is temperature used as a control knob?

Raising temperature increases the amount of liquid and lowers its viscosity, both improving densification. Liquid fraction can be worked out according to lever rule.

What is reaction sintering and when do you use it?

For materials where diffusion basically does not happen. E.g SiC is 88% covalent, and covalent bonds mean atoms essentially don't move, so there's no diffusion to drive normal sintering. Bond the particles by chemical reaction: compound forms at the grain boundaries and glues the particles together. Used for refractories.

What is pressure sintering and why does it work?

Push while you heat. The natural sintering force from curvature is only a few MPa, so 7–35 MPa applied externally overwhelms it, giving roughly 20 times the kinetics. That means full density at lower temperature and shorter time, so grains never get a chance to coarsen. Hot pressing is uniaxial in a die: simple shapes only, die friction gives uneven density. HIP uses argon at 100–320 MPa from all sides: complex shapes, no die friction, but the part must be sealed in a glass or metal skin so the gas cannot leak into the pores.

The big open question — compare the four routes of sintering.

Every route buys density and pays with something different. Solid state pays with grain size. Liquid phase pays with high temperature properties. Reaction sintering is what you use when diffusion is not available at all. Pressure sintering gets you both density and fine grains, but pays in shape complexity and cost.

Limitations of Liquid phase sintering

The cost is a glassy film left on every grain boundary, which softens when hot, so creep resistance collapses and the glass may conduct ions.