CH5716Processing of Materials

Prof. J.T.S. Irvine

Lecture JI3 – Sintering of Ceramics I

Course Aims

Understand sintering mechanisms and how raw materials

and green body processing influence this. Discuss

engineering of microstructures to achieve desired

functionality.

Understand driving forces for sintering, mass transport

mechanisms, and atomic mobility. Solid-state sintering:

mass transport mechanisms. Discuss and understand stages

of sintering and relation with microstructure development.

Topics

•General aspects

•Grain Growth and Coarsening

•Grain Boundary Migration and Grain Growth

•Pore evolution

•Single Phase Sintering

General Aspects

http://www.slideshare.net/support.xhht/solid-state-sintering-fundamentals-242041

Differential pressure at curved surfaces

Pressure at positively curved surface exceeds that at flat surfaceexceeds that at negatively curved surface

Ideal 120o

Interface between grainsTruncated octahedron

4,6 edges 14 faces

3d random spherepackings

Grain Growth and Coarsening

Most applications prefer uniform grain size

Grain growth is driven by reduction in total grain boundary energy

Some grains must disappear/decrease if others are to increase

Whilst diffusion at GB does involve movement of less bound atomsCrystal structures are not continuous and surfaces need to reconstruct

Grain Boundary Motion

Several factors impede motionof grain boundary

inclusion

Pore distortion Pore agglomerationduring grain growth

Simultaneous grain and pore growth

DiscontinuousGrain growth

Al2O3

BaTiO3 Elongated grainmicrostructure in a highfracture toughness siliconnitride with garnet secondphase

Due to non-uniformities- pores/liquid phase/impurities

Ostwald Ripening-Particle coarsening

•Particles smaller thanaverage size disappear, thoselarger increase.•Due to Surface energy

•If growth is simply diffusionlimited•a3 = Ct

Two particle modelGlass sphere examples

Neck exerts capillary pressure

1 surface Diffusion2 lattice diffusion3 Vapour Transport4 Boundary Diffusion5 GB Lattice diffusion6 Dislocation latticediffusion

xx

Due to variationin grain size

Local packing/agglomeration

Poor pressing/Die friction

Rapid surfaceelimination

Residual pores

Agglomerates in alumina