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John Binner - Week 3 - Coggle Diagram
John Binner - Week 3
Sintering
Used to describe the densification of technical ceramics (firing is used for traditional ceramics). Both are a heat treatment used to convert a green body into a rigid polycrystalline solid that is usually dense.
The process requires diffusion, and the main driving force is the massive reduction in surface energy that occurs when a powder becomes a solid. Note: this is much more true for oxides than non-oxides, so the latter often need the use of external pressure
Sintering variables
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System composition - together with use of additives, this controls whether any lqiuids will be present at sintering temperatures - diffusion occurs much quicker through liquids.
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Application of pressure - much higher diffusion rates, but higher costs
Mechanisms
Vitrification - 20-30 % liquid at firing temperature, which turns to glass on cooling and 'glues' the particles together. Rarely sued for advanced ceramics due to low Tm of glassy phases.
Liquid phase sintering - 5-10% liquid at firing temperature, which acts as a diffusion route. Particle shape change is involved.
Solid state sintering - no liquid involved, hence diffusion is much slower. Particle shape change is again involved.
Sintering in oxides - Pore boundary atoms in tension (further apart than in ideal lattice) so diffusion occurs from high stress regions in corners to low stress regions at sides - leads to pore shrinkage and sintering
Sintering in non-oxides - Atoms along pore boundaries in compression and opposite argument applies. Pores will not shrink and sintering is inhibited - hence the need for pressure to be applied.
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Processing ceramics
Fundamental steps
- Ceramic powders + additives, e.g. binders
- Mixing, shaping, debinding, green body formed, firing and sintering, finishing (machining).
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Shaping - the goal is to produce a green body (unfired and unsintered) that is as dense and strong as possible)
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Die pressing
The simultaneous uniaxial shaping and compaction of a powder, usually with small amounts of binder (often water) during compression in a die.
As pressure is applied
- The granules rearrange to take up less space.
- Deform and finally fracture
Note: if pressure becomes excessive, then elastic compression builds up, leading to die wear and increased likelikood of the green body disintegrating when the stresses relax after removal from the die
Isostatic pressing
Cold isostatic pressing applies pressure from all directions to achieve greater compaction uniformity (and hence higher quality parts) and increased shape capability, compared to uniaxial pressing.
Dry bag offers faster production rates than wet bag but is more expensive due to more challenging tool design.
Wet bag is simpler and cheaper, but cycles take longer. Also has better dimensional tolerances
Wet forming
Requires a stable suspension of ceramic particles suspended in a liquid, usually water - this is known as slip.
Slip viscosity must not be too high (faster but flabby casts) or too low (slower casting and brittle casts). Dispersants are used to control slip viscosity and characteristics
Plastic forming
Important factors
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During extrusion the polymer mix has a low enough viscosity to flow, on entering the mould it cools and forms a solid body.
Cycle times can be very rapid (seconds), but the polymer needs burning out very slowly after the part is made, which can take days.
Griffith Theory
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The high theoretical strengths are attributed to the high stiffness, high surface energy and small lattice spacings
However, in practice, we find that the actual strengths of ceramics is 1-2 orders of magnitude lower than the theoretical stengths
This is due to the presence of cracks that are inherent in ceramics, which act as stress concentrators and lead to premature failure
The actual fracture strength is therefore dependent on not only the Young's modulus and the surface energy, but also c, the radius of the largest flaw
Important point - 1 large crack can prove fatal, compared to lots of little cracks - this is to say that even if the average crack length for 1 sample was higher than another, if the formerly mentioned sample contains even 1 anomalously long crack, then it is more susceptible to brittle fracture than the other sample.
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Toughness
Toughness can be increased by making composites through use of particle (e.g. zirconia) whisker or fibre reinforcement
The stress intensity factor is the factor by which a crack will increase the max stress in a stress field.
The fracture toughness is the critical stress intensity factor at which a crack will propagate and lead to fracture
Fundamentals
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Ceramics are extremely good performers under compressive stresses, but fail readily under tensile stresses
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