ceramics Flashcards
ceramic/porcelain
all porcelain is ceramic but not all ceramic is porcelain
difference between decorative and dental ceramics
decorative contain kaolin - clay - hydrated aluminium silicate - opaque dental need to be translucent so kaolin removed - feldspar and silica replace it
dental ceramic constituents
kaolin <5% quartz (silica) 12-25% feldspar 70-80% metal oxides 1% glass up to 15%
decorative ceramic constituents
kaolin 50+% quartz (silica) 15-25% feldspar 15-25% metal oxides <1% glass 0
are ceramics glasses?
yes
feldspar
potash feldspar (KAl silicate)
soda feldspar (Na, Al silicate)
flux - melts and embeds other particles, increases viscosity
lowers the fusion and softening temp of the glass
lowest fusing component and flows during firing forming a solid mass around the other components
borax
lowers fusing temp
metallic oxides
strengtheners
colour to ceramic
chromium
green
cobalt
blue
copper
green
iron
brown
manganese
lavender
nickel
brown
ceramic powder
dentine pink
enamel blue
conventional ceramics - making the powder
heat constituents to >1000 degrees
cool rapidly (fritting) in H2O - creates cracks and crazing of the ceramic mass
mill frit - fine powder
add binder - often starch
conventional ceramics - building up the Rx
powder and distilled water built up
apply to die with brush
different porcelains for E and D
no further chemical reaction required - powder melts together to form crown
- crown heated in a furnace to coalesce the powder into ceramic
heating leads to sintering
what do feldspathic ceramics form when heated to 1150-1500 degrees?
leucite (potassium aluminium silicate)
forms around glass phase
gives powder of known physical and thermal properties
overall properties
aesthetics chemical stability biocompatibility thermal properties dimensional stability mechanical properties
sintering
occurs just above the glass transition temp
when the ceramic particles begin to fuse into a single mass
glass phase softens - coalesce
over time controlled diffusion - form solid ceramic mass
20% contraction - skilled technician to judge contraction in 3D
chemical stability
v stable
generally unaffected by the wide pH range in mouth
don’t take up stain from food/drink
biocompatibility
good
minimal adverse effects on biological tissues
thermal properties
similar to tooth TEC similar to dentine - low stresses to Rx in mouth during use thermal diffusivity low - protective of remaining tooth
aesthetics
best of any Rx material colour stable v smooth surface retain surface better - less staining long-term optical properties - reflectance - translucency - opacity - transparency - opalescence
dimensional stability
once fully fired v stable
during fabrication - shrinkage - must be accommodated for by technician
- 20% normal for a conventional feldspathic ceramic crown
compressive strength
high
hardness
too high
can lead to abrasion of opposing teeth esp if not glazed
tensile strength
v low
flexural strength
v low
fracture toughness
v low
result of mechanical properties
lead to failure during loading
static fatigue
cracking over time, but over v long period and less than you would get from hydrolysis of composite
time dependent decrease in strength even in the absence of any applied load
probably due to hydrolysis of Si-O group within material, over time in an aq env
surface microcracks
during manufacture, finishing or due to occlusal wear
areas where fractures can initiate
slow crack growth
cyclic fatigue under occlusal forces in a wet env over time
where can conventional ceramics be used?
only in low stress areas
- anterior crowns - not in all pts - if deep OB/Bruxist
too brittle for use elsewhere
overcoming problems with conventional ceramics
aesthetics good but need to be stronger
produce a strong coping resistant to fracture, and cover it in conventional porcelain
cast/press a block of harder ceramic
mill a lab prepared block of ceramic
strong coping
metal coping
alumina core
zirconia core
why can alumina only be used as a core?
because it is opaque
alumina core increased strength
flex strength double porcelain >120MPa
alumina particles act as crack stoppers - prevent propagation
- necessitates more energy to propagate a crack so more resilient
alumina core uses
prev first choice for anterior crowns, not strong enough for posteriors
aesthetics could be excellent but need enough room for Al core and feldspathic layers
possibly more palatal reduction required than MCC, but less labially
alumina core advantages
cheap
no specialist equipment - just need a furnace
conventional alumina core
max 50% Al
increasing the Al content and so strength
new techniques
- INCERAM
- PROCERA
INCERAM alumina content
around 85%
INCERAM flexural strength
high >400MPa
In-ceram spinel
has Spinel (MgAl2O4) rather than alumina as its core material - better aesthetics but lower flex strength
In-ceram zirconia
33% zirconia replacing alumina in core
- higher strength but poorer aesthetics
INCERAM slip casting (complicated)
ceramic core formed onto a refractory model
apply fine slurry of alumina
heat to 1120 degrees for 10 hours
- below glass transition temp for alumina
- partial sintering
- porous core produced
core infiltrated with lanthanum glass at 1100 degrees
PROCERA
pure alumina core >99% pure
PROCERA properties
high flexural strength >700MPa
possibly better translucency than glass infiltrated core
strong enough for posterior teeth, might be strong enough for bridges
PROCERA manufacture
even more complicated process
core is made centrally not in every lab
a fully densified alumina core is produced at around 1700 degrees
PROCERA and INCERAM clinical uses
both cores can be veneered with conventional feldspathic porcelain to produce the final crown
both probably suitable for single posterior crowns
not often used as bridge materials
zirconia core and bonding
difficult to bond to
most popular ceramic core material
probably zirconia
zirconia naturally occurring mineral
zirconium dioxide
in different forms at diff temps
v hard
when couldn’t zirconia be used as a core until?
CAD-CAM
powder doesn’t sinter unless heated >1600 degrees
what type of zirconia should be used and why?
Yttria-stabilised zirconia
pure zirconia can crack on cooling
% of yttria
<1%
normal zirconia structure
monoclinic crystal at room temp
Yttria structure
tetragonal crystal structure
how does Yttria zirconia stabilisation work?
if a crack begins - when the stress at the crack tip reaches a critical level the crystal structure transforms to the monoclinic structure
causes slight expansion of material and closes crack tip
like a self-healing crack - goes from one crystal structure to a bigger crystal structure which fixes the crack
Yttria stabilised zirconia properties
hard
strong (1000MPa flexural strength)
tough
strong enough to use as a bridge framework
zirconia cored crowns fabrication
impression of prep - send to lab
case model then scan digitally
software unit creates a bridge substructure on virtual prep
min thickness of connectors are determined and fabricated
raw zirconia block selected for milling
pre-sintered block much easier to mill
milling for a 3 unit bridge around 1hr
cut framework heat treated 850 degrees to achieve its final physical properties
causes 20% shrinkage but computer deals with this during milling
stain
veneer it with feldspathic porcelain to produce final Rx
manufacturers of zirconia systems
LAVA from 3m IPS e.max Zir CAD Opalite Zerion Everest ZH
problems with zirconia cored crowns
£££ equipment
potential for veneering porcelain to debond from core
- can inbuild stresses between the 2 materials on manufacture
opaque core - are aesthetics much better than MCC?
inert fitting surface - can’t etch or bond
- use conventional cement
advantages of zirconia cored crowns
once you have the equipment they are cheaper
- cost of metal is increasing
fit generally excellent
- work at least as well as MCCs
milled core crowns and bridges
zirconia lithium disilicate precious metal non-precious metal titanium
all have a sintered surface layer for best aesthetics
- but ones milled from a single block are stronger than if you put the layer of sinter on
milled core crowns and bridges prep
need to round internal angles in prep as sharp shoulder can’t be replicated in a milled Rx
sintered vs milled
for the same material milled stronger than built up/pressed
- block has had ideal heat txs to maximise its properties and all blocks will be consistent
as aesthetics of ‘blocks’ of ceramic improve these will become most commonly used crown
- probably already acceptable in posterior teeth
milled fabrication
cast into scanner scanned image of cast lower cast is scanned and articulated select crown margin adjust crown margin select crown type and place on 'model' adjust shape and size of selected crown save file send to milling machine 30-40mins have crown still requires final finishing
truly digital workflow
don’t need models and impressions
- scan in mouth
- design on CAD machine
- mill
- polish
- cement
lithium disilicate glasses crystals
have unique needle-like crystals
crack propagation v difficult - good flexural strength
?maybe use for bridges
why are cast and pressed becoming a bit redundant?
because of the milled ceramics
cast and pressed ceramics technique
like casting a metal Rx
wax up Rx
invest
cast from a heated ingot of ceramic (1100 degrees)
no sintering - ceramic ingot is already fully condensed prior to firing
once devested and cleaned, Rx heated to improve its crystal structure producing crack inhibiting crystals - ceraming
can stain - comes out as single colour on cast
most often is cut back labially and veneered with feldspathic porcelains
ceramic used for cast and pressed
glass ceramics
- lithium disilicate glass
- leucite reinforced glass
ceraming
heat to improve its crystal structure producing crack inhibiting crystals
stage 1 - crystal formation. max no of crystal nuclei are formed
stage 2 - crystal growth to maximise physical properties
crystal phase of the ceramic can approach 100%
strong cast and pressed ceramics
small crystal size and high vol fraction of crystals
luting silica containing ceramics
etch with HF acid - retentive surface
- can bond to it using a silane coupling agent
- bond to tooth w bonding agent
luting zirconia cored crowns
don’t contain silica - not affected by acid
strong enough to be self-supporting, can be luted with conventional dental cement
difficult to bond to
lithium disilicate ceramic material advantages
better aesthetics than zirconia - translucency
contains silica so can etch surface to provide a reliable bond to composite luting agents, with the use of a silane coupling agent
stronger - crystalline structure
lithium disilicate ceramic material occlusal reduction
min 1.5mm
lithium disilicate ceramic material name
Emax
cementing lithium disilicate ceramic material
silane coupling agent - give chemical bond to onlay surface
dual curing composite resin luting cement
- reduced light penetration through onlay
DBA - to allow composite resin to bond to tooth
reinforcing ceramics
dispersion strengthening - alumina often used
monolithic e.g. milled from ceramic block
core (coping)
