Lecture 8 Flashcards
describe the manufacturing process of ceramics
- powder
- forming (isostatic pressing, injection molding) -> green body
- sintering : hot isostatic pressing -> a lot of shrinking, try to get the most pores out
- finishing and quality control
present the physical properties of ceramics in relation to polymers and metals
polymers : low strength and toughness, but high ductility
metals : high toughness, high strength, ductile
ceramics : high strength (compressive) but low toughness and brittle (no plastic def), high T resistant
provide info about mechanical behaviour and its analysis : what test do we do, distribution in batches ?
- brittle : cracks propagate fast -> not good with tensile tests / forces
- bending test for flexural strength : 4 point test is better (constant bending moment)
- best at compressive forces
ceramics have a very high property (strength) distribution within batches -> low Weibell modulus
explain the influence of defects and pores
they reduce the young’s modulus and the flexural strength.
Pores act as crack initiators, and cracks propagate fast in ceramics
name the most important representatives of the CaP family
beta-tricalcium phosphate : beta-Ca3(PO4)2
hydroxyapatite Ca5(PO4)3OH
give applications for CaP ceramics
HA : not degradable, non load-bearing, bone substitute, coating on implants
beta-TCP : resorbable (replaced by real bone), pores promote rapid bone integration (fill bone defects)
represent the chemical structure of HA and beta-TCP
HA : hexagonal crystal structure
LOOK ON THE INTERNET
describe typical physiochemical properties of HA and beta-TCP
HA : not very strong but no formation of fibrous tissue and promotes bone growth (connection between implant and tissue), does not degrade
beta : strength comparable to cancellous bone, osteoconductive, gradually resorbed
explain the plasma spray process for the production of HA layers
powder goes in torch, melts in the plasma flame before hitting the surface and cooling immediatly. It creates multiple layers, there is shrinking and enclosed gas can create pores.
describe the typical physiochemical properties of Al2O3 and ZrO2 (one application for each)
alumina : fine grain structure, high compressive strength, resist high T, insulator, low wear rate -> hip implants
zirconia : white (-> dental implants), lower compressive strength than alumina but higher bending strength
what would be the best combo fir hip ball for strength and low wear rate ?
titanium core for strength, and ceramic coating for low wear rate
explain transition toughening
we stabilize the tetragonal phase of zirconia by adding yttria -> higher crack propagation resistance.
When applying a load : stress field at crack tip -> tetragonal to monoclinic -> volume increases which compresses the crack zone -> transition toughening
describe aging and surface degradation of ZrO2
tetragonal is only metastable -> if we wait a long time, everything will be back to monoclinic
By steam sterilizing, this process is accelerated.
When trsnformation to monoclinic : microcracking, water penetrates -> surface uplifts or crevasses
name the idea of Al2O3 and ZrO2 composite ceramics, two toughening mechanisms
properties will depend on the mixing ratio -> better strength, Kic, E
1) low ZrO2 % : tetragonal zirconia particles in alumina matrix -> grains prevent initiation and propagation of cracks
2) higher ZrO2% : platelets -> dissipate energy by deflecting cracks
justify the most important standardized test criteria of Al2O3 and ZrO2 for medical use (6 things)
1) chemical composition
2) check radioactivity (uranium where zirconium is)
3) physical properties : density, low porosity, small grain size, phases
4) mechanical prop.
5) biocompatibility
6) sterilization (no steam, check prop. after sterilization)
