Reading Flashcards
(21 cards)
What can a mismatch of thermal expansivity lead to?
Internal residual stresses -> strong effects of mechanical behaviour
What is hardness dependent on that stiffness and density is not?
Microstructure
Name common reinforcements for PMC, MMC and CMC
PMC - Carbon, glass, aramid
MMC - ceramics
CMC - metallic + ceramic
Why is tensile strength data v.approximate espec in ceramic materials?
Ceramics are sensitive to presence of flaws ∴ higher values of strength measured = due to fibres containing few large flaws
Why is axial stiffness and strength high in CF
Graphene has a very strong bonding within the plane so alignment in carbon fibres -> stiffness and strength
Describe CF anisotropic behaviours in terms of thermal expansivity and conductivity
Higher expansivity in transverse
Higher conductivity in axial direction
Why are composites usually bad under transverse loading?
Uniaxial nature means that in transverse loading, the matrix which is usually weak is taking most of the load
Describe how processing temperature affects the property of CF
- lower temp —> less dense, less stiff but strong
- higher temp —> more dense, stiff
Describe the effects that adding ‘network-modifying’ (lower valence) oxides e.g. Na2O to a quartz will have
1) reduces Tg = easier to deform/draw at low temps
BUT reduces max use temp?
2) stiffness is affected
What is a quartz?
Pure silica glass
Where are flaws predominantly located in brittle materials?
Surface
How can dimensions of brittle fibres affect strength?
Reduction in diameter —> stronger ∵ finer flaws
But practicality difficulties can arise if D is too small
Does the toughness of a composite depend on the toughness of the fibre and/or the matrix?
No
What are some limitations of polymeric fibres?
- relatively low stiffness
- limited tolerance of heat
Describe attractive properties of SiC
- low density
- High stiffness
- good thermal conductivity + stability
Describe oxide fibres generally (alumina and silica)
- good resistance to oxidative degradation (problem for other fibres when used @ High temp)
- used for High temp insulation @ 50/50
- alumina fibres w/lower silica content = greater resistance to high temp, higher stiffness + strength
What are whiskers?
Elongated single crystals
Why do whiskers have v. High strengths?
∵ there are soo fine and have no GB
What are some disadvantages to whiskers?
- expensive to produce
- difficult to handle ∵ = readily become airborne = health hazard
- difficult to disperse within a matrix
- despite high tensile strength, toughness likely to be v.low
What’s the difference between particles and whiskers?
- cheaper
- easier to process and handle
- can be used as a density reduction
What are the main mechanisms of interfacial bonding
1) wetting (van der Waals forces)
2) diffusional processes
3) chemical reactions
4) mechanical keying
