Lecture 8

• 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

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

• 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

they reduce the young’s modulus and the flexural strength. Pores act as crack initiators, and cracks propagate fast in ceramics

beta-tricalcium phosphate : beta-Ca3(PO4)2 hydroxyapatite Ca5(PO4)3OH

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)

HA : hexagonal crystal structure LOOK ON THE INTERNET

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

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.

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

titanium core for strength, and ceramic coating for low wear rate

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

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

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

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)

smoother transition between substrate and coating to remove the stress peak and jump in properties Ex : gradient of composition of alumina and zirconia -> microstructure gradient -> higher crack propagation resistance

oxide : higher hardness + wear resistance, higher thermal conductibility, higher T stability CaP : more biocompatible

+) compressive strength, hardness and wear resistance, inert = biocompatible, thermal insulation -) brittle, low toughness, low flexibility (not possible for certain shapes), processing challenges (porosity)