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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