High Performance Materials Flashcards
Definition
materials which through their intrinsic properties, and/or processing, show much better performance than their competitors for a particular application.
resistance to corrosion
strength
stiffness
loghtness
heaviness
elastic energy storage
wear resistance
resistance to creep at a high temp
toughness
magnetic
Electrical
Wetability
Toughened Glass
Ultra-strong glass
Normal glass is mainly Silica with additional Na and Ca as network modifiers which reduce the softening temperature and allow the glass to be processed more easily.
Material toughness is controlled by the size of the largest surface flaw. There is a flaw size distribution which leads to a distribution of fracture stress
Reduce stress
reducing the flaw/crack sizes
by generating an internal compressive stress which opposes the applied tensile stress in the surface (the tensile stress to which the flaws are exposed is then reduced and the fracture stress is increased)
How it’s made
surfaces of hot glass sheet are rapidly cooled and become strong and unable to deform
core of sheet cools more slowly and its contraction is restricted by the cooler surface
result (at room temperature) is a sheet with compressive stress in the surface and tensile stress in the core
treat in molten salt bath, diffusion of Na+ ions outwards and K + ions inwards
replacement of small ions by larger ions result in surface compressive stress
Advantages over thermal strengthening
much less distortion of the sheet
can be used for much thinner sheets (down to 0.4 mm)
sheet can be cut after strengthening
can achieve high compressive stress in a thicker surface layer
Roller Bearings
High precision of manufacture
Rollers are subject to high compressive forces
At the contacts with the tracks there are high compressive stresses (e.g. 2 GPa or more) which generate high shear stresses beneath the surface
Non-metallic inclusions in the steel (e.g. oxides or sulfides) act as stress concentrators and lead to fatigue crack initiation. These cracks then grow under cyclic loading and eventually reach the surface causing pitting and rapid bearing failure. In extreme cases fatigue cracks can propagate right through the bearing race causing catastrophic failure
Ultra-low oxygen content achieved by secondary processing
Vacuum arc remelting: metal melted from the bottom of the electrode in an electric arc furnace under vacuum. The resulting melt and ingot have much lower oxygen content than the starting material
vacuum arc remelting, are employed to reduce the oxygen content in the steel. In this process, the metal is melted under vacuum conditions, resulting in a lower oxygen content in the final product compared to the starting material.
CNT Composites
Electrostatically dissipative material
No compromise on strength
Excellent fracture toughness
Minimum weight, maximum strength
Improved tyre durability
No free nanoparticle release
They are used in batteries as a conductive additive
Challenges
Control of length
tube number
properties
maintaining control during scaling
dispersing
need energy to mix surfactants
Inspiration from Nature
Biomimetics: Examining biological phenomenology to gain insight and inspiration for developing physical or composite bio-physical systems
Lotus Effect
Changes the contact angle between a liquid and a solid
Normally the water will want to spread out to minimise its energy and minimise the contact with the air, rather than the solid base
For surfaces that have favourable wetting properties for the liquid (small contact angles), roughness promotes wetting.
The surface that is not wetted by a liquid roughness promotes dewetting.
As the liquid will have a contact angle of 180o with air and the surface fraction of the solid is reduced → 0, we reach superhydrophobicity
Can fabricate surfaces coated in pillars
Superhydrophobic behaviour can give a self-cleaning effect as the water will roll off and any dirt picked up by the water will be picked up
Use pulsed laser ablation surface structuring to create this effect
Nacre
Biomineralization: “The process by which living forms influence the precipitation of mineral materials”
Result is heterogeneous composites with both organic and inorganic components, rarely perfectly pure mineral
Inorganic materials produced include:
Carbonates
Phosphates
Silicas
Iron
Calcium carbonates: CaCO3 (most common!)
Calcite
Corals and mollusks
Eggshells of birds
Aragonite
Same composition as calcite but different crystal structure–metastable
Found in nacre
New Glass?
Soft organic phase ensures cohesion of the small tablets over large separation distances, by forming tough ligaments.
Nanoroughness on the tablets, providing frictional resistance to sliding
Some mineralisation crossing tablets also reinforces the structure.
The ‘waviness’ of the tablets drives a locking mechanism also
