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John Binner - Week 8 - Coggle Diagram
John Binner - Week 8
The Future
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Hybrid bone where ceramic structures support human cells, encouraging the body to repair itself.
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Ceramic feedthroughs.
Designed to provide excellent reliability and durability. The key is the ceramic to metal seal assembly.
Used for pacemakers, defibs, cochlear implants, hearing devices, drug delivery and neurostimulators.
Form from biocompatible metals (e.g. pure gold) as the conductors and ceramics (e.g. high purity alumina) as the insulator.
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Dental ceramics
Most common = lithium disilicate (Li2Si2O5). Acicular crystals embedded in matrix of silica, alumina, Li2O, K2O and P2O5
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One approach is where the dentist takes a photo of the patient's tooth and feeds the image into a computer-controlled milling machine that creates an exact replica of the tooth and pressed and sintered from blocks. Done in dentists lab (useful)
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Dental porcelain
Based on a glassy aluminosilicate matrix which are embedded in several polycrystalline phases, e.g. quartz, feldspar, mullite and alumina.
Is chemically very stable, does not deteriorate with time and tis thermal conductivity and expansion are similar to that of enamel and dentine. Very high compressive strength, but very low tensile strength. It therefore has a low fracture toughness, so is very sensitive to the presence of surface microcracks.
Zirconia is the strongest and toughest ceramic, so should in theory make for the best dental material. However, it suffers from a process called hydrothermal ageing; it is attacked by moisture.
Zirconia exists in 3 different forms - monoclinic up to 1100 C, tetragonal up to 2370 C, and cubic up to its melting point. The monolithic to tetragonal transformation is accompanied by a 4-5 % volume change (expansion on cooling). This causes cracking on cooling after sintering and prevents pure zirconia from being made. MgO or Y2O3 (Yttria) can be used to stabilise the tetragonal structure down to room temperature.
We can use the volume change of the phase transformation to make it harder for these cracks to propagate however, by allowing some regions to change to the monoclinic structure, expanding and putting the surrounding material into a compressive stress state, making it more difficult for cracks to propagate, making the ceramic tougher.
There is no definitive definition of hydrothermal ageing, but it is thought that water preferentially attacks regions of low stabiliser (MgO, yttria) content. The yttria content can range from 6 mol% at the grain centres to 2 mol% at the grain boundaries. The grain boundaries can therefore be attacked by water, causing the transformation to the monoclinic structure to occur locally and the strength of the ceramic to decrease significantly.
This is a problem, as the most common sterilisation approach is in steam at 140 C. Even if a different route is used, the mouth is still a wet place, so attack will occur over time. Research has shown that engineering a nanostructure (less than 100 nm) can protect against hydrothermal ageing.
Body Armour
Stab resistant vests
Made from Kevlar - has a very high specific tensile strength (5 times greater than steel) - arranged into tightly woven fabric that makes it very difficult for a weapon to penetrate. The weapon becomes caught within the weave, whilst the fibre strength resists too many of the fibres from being cut. May need replacing if too many of the fibres are damaged after an incident.
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Hard armour vests
Soft armour shells with hard armour inserts - inserts can be stacked depending on level of protection needed
Hard armour vests are less pliable and much heavier than soft armour vests. They are less comfortable and cause the wearer to get very hot
Requirements: high hardness, strength, elastic modulus and toughness. Maximise ability stop projectiles and as low a density to minimise mass.
Defeat mechanism
Soft armour 'catches' the projectile, hard armour defeats the projectile by shattering it.
Projectile hits ceramic plate and shatters - fragments are caught by spall shield (to prevent shrapnel from causing further damage)
In depth
Bullet tip gets eroded, becoming blunter. Casing is stripped away and mass lost. This is the dwell time - the longer the better (the more energy absorbed). Cracks initiate in ceramic core of bullet. Ceramic begins to erode and blunten. The backing material starts to bulge from the pressure of coming to an immediate stop. The pressure exerted on the backing material by the eroding core causes the backing material to delaminate, falling off the core like a banana peel. Most of the remaining ceramic is ejected from the core, dissipating energy, and the rest simply becomes embedded in the material.
Multi-hit capability
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Since the defeat mechanism causes the ceramic to crack, it becomes weaker, resulting in the possibility of subsequent hits penetrating the armour system. Armour manufacturers therefore need to stipulate the maximum number of hits their armour can withstand.
Mosaic tile style armour can be used - breaks in tiles prevent damage from propagating between tiles, reducing the damage area resulting from a given impact.
Different types
Alumina
Most common, due to cheapness.
Properties aren't great (relatively high density, mechanical properties are OK but not the best).
Being an oxide it can be sintered easily, but it is usually hot pressed or sintered, as the denser the ceramic, the better the performance.
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SiC
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Hardness, strength and elastic modulus are all higher than alumina, leading to better performance.
However, it is intrinsically more expensive and sintering is more difficult as it is a non-oxide.
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B4C armour
Lowest density and highest hardness- means it should be best, but suffers from 'shatter-gap'. Caused by stress induced amorphisation of local crystal structure, which generates large anisotropic strains and causes the crystal to collapse along certain crystallographic directions.
Still finds use in weight critical components, e.g. in helicopter seats.
Intrinsically expensive, difficult to sinter (non oxide) and limited uses.
TiB2 Armour
Best mechanical properties, but also highest density.
Due to high density, restricted to land-based vehicles, e.g. armoured cars.
Non-oxide, difficult to sinter, and very expensive, with limited uses.