L8 Nano-Ceramics Flashcards
Re Quiz 7 Week 10
- Which of the following best describes the mechanical properties of typical monolithic ceramics?
(A) Weak and ductile
(B) Stiff and brittle
(C) Stiff and tough
(D) Strong and ductile
(B) Stiff and brittle
- Name TWO mineralized biological materials. (2 marks)
- Teeth
- Bone
- Mollusc Shells
- Nacre
These materials are basically mostly hard, rigid nanosized calcium-based ceramic ‘particles’ embedded between proteins
- Which of the following best describes the typical composition of mineralized biological materials?
(A) Protein-based building blocks with mineralized rigid interface
(B) Mineralized rigid building blocks with protein-based interface
(C) Mineralized rigid building blocks with metallic-based interface
(D) Mineralized rigid building blocks with cells occupying the interface region
(B) Mineralized rigid building blocks with protein-based interface
- Name TWO toughening mechanisms present in mineralized biological materials. (2 marks)
The whole point of these mechanisms is to absorb as much energy as possible before material failure
Process Zone Toughening - providing allowances for the region ahead of the crack to deform slightly (this deformation would result in some of the energy being dissipated at the process zone)
Crack bridging - preventing or slowing down crack propagation (and it’s this crack propagation that leads to catastrophic failure) by ‘connecting/bridging’ either sides of the crack with a ‘stretchy’ or ‘weak’ interface material, so that some of the energy is used/dissipated to deform the material that is used to bridge the crack
Crack deflection - making the crack path longer by making the crack propagate ‘zig-zag’ rather than a relatively straight line: longer crack - more energy dissipated, and also at every ‘turn’, energy is dissipated
Interface toughness - (1) make the cracks travel through the interfaces, and (2) make it hard for the cracks to travel through the interface (again, deformation of the interface material would dissipate the energy)
- List the THREE universal design principles in highly mineralized biological materials. (3 marks)
(copy this from native materials to design tough materials)
Nanoscaled building blocks (stiff but strong) - to provide the strength
Weak (loosely speaking, weaker compared to the nanoscale building blocks) bio-polymeric interfaces (tough) to act as areas where cracks can propagate and the energy used to deform the material can dissipate through
Designed Architecture (additional toughening mechanism)
Could be a simple ‘brick and mortar’ structure like those that you see in buildings/houses where you have horizontal layers of ‘bricks’/ceramics and in between you have layers of ‘mortar’/proteins, or you could intricate shapes designed based on your predicted loading profile (most of the mineralized tissues and these complex architecture composites tend to be anisotropic)
Usually what happens when you’re toughening a material, is that you lose a bit of that peak strength (a matter of balancing and trade-off), and the architecture is usually designed in such a way that the direction of the crack propagation is relatively controlled, and this results usually in one or two of the principle directions being toughened.
- Briefly explain why nanoscale architecture in beneficial in achieving both high toughness and high strength in materials? (2 marks)
As materials approach nanometers in size, they approach their theoretical maximum strength (because, going back to fracture mechanics, the probability of a defect that can initiate crack propagation is significantly decreased with decreased size, and these nanoscale particles should only really break when there’s enough force to break the actual atomic bonds apart)
Where the strength of the materials is inversely proportional to its size ← this primarily applies to ceramics (not so much for metals and polymers because they have their own ‘bridging’ mechanisms (metallic bonds and hydrogen bonds respectively) - ceramics (brittle materials in general) don’t have this - ceramics have mostly covalent and ionic bonds, so no really intrinsic bridging mechanism to enhance toughness
- In your own words, briefly describe why hydrated nacre possesses higher toughness compared to dry nacre. (2 marks)
(everything is related to the answers Question 4 and 5; toughening mechanisms and universal design principles)
- The protein interface in dry nacre is brittle (i.e. no/very little energy dissipation) compared to protein interface in hydrated nacre
- Dry protein interface can’t bridge the gap as well as hydrated proteins
- Hydrated protein interfaces are tougher than dry protein interface, allows stretching of proteins
- Water also acts as an agent for energy dissipation
- The attached image (upper) illustrates a simple architectural design of flat 2D materials whereby “suture lines” are engraved or cut with the intention to enhance the toughness of the material in the in-plane direction indicated by the black force arrows (lower figures).
Which of the following statements best describes the effect of increased friction between the ‘locking sites’?
(A) Increasing the friction between the locking site increases the overall toughness of the material in the direction of loading.
(B) Increasing the friction between the locking site can lead to increased chance of fracture at the locking site itself, leading to reduction in overall toughness in the direction of loading.
(C) Increasing the friction between the locking site does not impact the overall toughness of the material in the direction of loading.
(D) Increasing the friction between the locking site decreases the strength of the material (actually, it increases the strength of the material).
(B) Increasing the friction between the locking site can lead to increased chance of fracture at the locking site itself, leading to reduction in overall toughness in the direction of loading.
Regarding:
(A) Increasing the friction between the locking site increases the overall toughness of the material in the direction of loading. - increases strength, but not necessarily toughness if you keep increasing friction. If you increase friction too much, it acts almost like the monolithic version - leads to cracking of the ‘tabs’. (to a certain extent, but only up to the point where the product of the increased strength and the strain at failure is maximum)
(D) Increasing the friction between the locking site decreases the strength of the material (actually, it increases the strength of the material)
- The angle of the curvature of the locking site, labelled θ (image attached), can be altered to investigate its effect on the materials toughness. Which of the following statements best describes the effect of increased θ?
(A) Increased θ increases the fracture toughness of the material by a factor of 100.
(B) Increased θ increases traction force, but results in increasing number of locking site fractures.
(C) Increased θ decreases traction force, resulting in lower fracture toughness and strength.
(D) Increased θ decreases the number of locking site fractures, leading to higher fracture toughness of the material.
(B) Increased θ increases traction force (in essence, friction), but results in increasing number of locking site fractures. Again, balance.
(too high theta - leads to cracks just propagating through tabs, rather than through the interface, effects are similar to increased friction)
- One of the strategies employed to further enhance the toughness of materials with the architectures shown in the attached image is through the infiltration of a soft polymer such as polyurethane in between the locking sites. Briefly explain why this polymer infiltration can further enhance the toughness of such material.
(Hint: consider toughening mechanisms)
(2 marks)
Crack bridging - preventing or slowing down crack propagation (and it’s this crack propagation that leads to catastrophic failure) by ‘connecting/bridging’ either sides of the crack with a ‘stretchy’ or ‘weak’ interface material, so that some of the energy is used/dissipated to deform the material that is used to bridge the crack.
Interface toughness - (1) make the cracks travel through the interfaces, and (2) make it hard for the cracks to travel through the interface (again, deformation of the interface material would dissipate the energy)
Polyurethane interface vs empty/air interface
PU interface - allows for increased crack bridging and increased interface toughness vs empty/air interface
(Not crack ‘deflection’, as this has already been introduced with the design of the interface geometry)
(My answer: A soft polymer reinforces a bridging strategy to prevent crack propagation into the material. The polymer experiences most of the strain at the locking sites an adds a resistive strength to further deformation/separation at these sites.)
