Lecture 8: Ceramics & Other Materials

ceramics

an inorganic, nonmetallic solid prepared powdered materials and fabricated into products through the application

primary bonds of ceramics

covalent and ionic

categories of ceramics, glasses, and glass-ceramics

• crystalline (bio-resorbable or inert) ceramics

• glass, amorphous

• glass-ceramics, starts as a glass and ends up as a polycrystalline ceramics possibly with a residue of glassy matrix

general properties of ceramics

• ionic bond → difficult to shear, low ductility, high compressive strength, low tensile strength

• low thermal and electrical conductivity

• refractory and high Tm

• high hardness → dental materials

• aesthetically pleasing appearance

metal vs. ceramic ability to go under dislocation guide (slip)

metal - move one lattice step over at a time (easy to deform metal, elastic deformation)

ceramic - ionic, more difficult, more energy required (less slip → more brittle fracture)

metal vs. ceramic stress/strain curves

ceramic - elastic deformation only (rough breakage)
metal - elastic deformation and then plastic deformation, can stretch (skinny smooth breakage)

nearly inert bioceramics

• aluminum

• sapphire or ruby

• single crystal or polycrystalline

nearly inert bioceramics corrosion and compatibility

excellent corrosion resistance and biocompatibility (very thin fibrous layer)

high strength → structural support such as bone plates, bone screws

nearly inert bioceramics grain size

small grain size, narrow distribution → high hardness and low surface roughness → low friction and wear → joint replacement

joint replacement - aluminum

alumina socket and ball, coefficient of friction decreases with time and approaches

the value of a normal joint → wear 10 times slower than metal-PE surfaces

resorption or biodegradation is caused by…

• physiochem dissolution, depending on the solubility of material and local pH

• physical disintegration into small particles as a result of preferential chemical attack of grain boundaries

• biological factors, such as phagocytosis, which also causes a decrease in local pH

calcium phosphate

the mineral phase of bone and teeth is mainly calcium and phosphate ions (most commonly used ceramics for biomaterials!)

calcium phosphate - solubility and hydrolysis

• Solubility and hydrolysis decrease with increasing Ca/P ratio

• Ca/P ratio <= 1 is not suitable for biological implantation

hydroxyapatite (HA)

hard tissue contain 60% HA (mostly carbonate HA), 25% water and 15% organic materials, HA can be converted from coral or animal bone

HA manufacturing

Ca(NO3)2 + Na H2PO4 → precipitate of HA → drying and filtering → furnace 1150 deg C → grounding → sieving → pressing in a die → sintering

HA elastic modulus (40-117 GPa)

• Enamel: 74GPa

• Dentin: 21GPa

• Compact bone: 12-18 GPa

HA hexagonal rhombic crystal

OH replaced by Fl → chemical stability increases

x-ray diffraction

shows how crystalline and amorphous a material is; peaks

typical FT-IR spectrum of HA

standard spectroscopy technique that uses the absorbance to define the actual chemical groups that are in a sample (HA: phosphate, hydroxyl groups, carbon groups)

rate of degradation of calcium phosphate increases as chemical susceptibility to dissolution….

chemical susceptibility increases

rate of degradation of calcium phosphate increases as surfaces area….

surface area increases

rate of degradation of calcium phosphate increases as crystallinity….

crystallinity decreases

rate of degradation of calcium phosphate increases as crystal perfection….

crystal perfection decreases

rate of degradation of calcium phosphate increases as grain size….

grain size decreases (because more grain boundaries, more water can come in)

rate of degradation of calcium phosphate increases as F substation….

F substitution decreases (because lower chemical stability)

clinical applications of calcium phosphate: advantages

bioactive and osteoconductive

bioactive bonding mechanism

differentiated osteoblasts produce a cellular bone matrix of 3-5um layer at the surface → 0.05 to 0.2 um → normal bone attached through a thin epitaxial bonding layer to the bulk implant

calcium phosphate as dense form

small unloaded implants such as in the middle of ear implant

calcium phosphate as porous form

granules for filling bony defects in orthopedic and dental surgery

calcium phosphate as coatings

with reinforcing metal posts as in dental materials

calcium phosphate as fillers

in composites

bioactive glasses and glass-ceramics composition

specific composition → highly reactive surface in aqueous medium

• SiO2 < 60%

• high Na2O and CaO2

• high CaO/P2O5

bioactive glasses and glass-ceramic bonding interface

surface forms a biological active carbonated HA layer that provides the bonding interface with tissue; the interfacial strength >= bulk strength of both the implant or tissue

clinical applications of bioactive glasses and glass-ceramics

• 45S5, SiO2 45%, Ca/P=5:1

• Ceravital®, middle ear surgery to replace ossicles damaged by chronic infection

• periodontal defect repair, maintenance of the alveolar ridge for denture wearers

• toothpaste ingredient against sensitivity

carbon materials

crystalline: diamond, graphite, fullerene

quasicrystalline: glass carbon

glass carbon, extremely inert, used as electrodes in electrochem or prosthetics

quasicrystalline: pyrolytic carbon

as implant surface coating, has high mechanical strength than glassy and graphite carbon, excellent tissue and blood compatibility, used in heart valves and finger joint implants

composite

consisting of 2 or more chemically distinct parts in the macro-scale, having a distinct interfaces separating them

fiber or particulate composites

usually consists of one or more discontinuous phases (usually stronger, called reinforcing materials) embedded in a continuous phase (called matrix)

property of composite material depends on

• properties of each constituent

• shape of heterogeneities

• volume fraction

• interface

composite application examples

natural tissue such as bone and tendon or vessel

HAPEX

• composite of hydroxyapatite in polyethylene matrix

• stiffness similar to cortical bone

• high toughness

• bone bonding in vivo

• orbital implant and middle ear implants