Dental Material Science Flashcards
What is porcelain prone to?
Mechanical failure due to microcracks formed at fitting surface
What is elastic modulus also called?
Young’s modulus
what is elastic modulus a measure of?
rigidity
what ratio is a measure of elastic modulus?
stress/strain ratio
stress required is to cause a change in what?
shape
what is brittleness/ductility?
dimensional change experienced before fracture
what is hardness?
resistance of surface to indentation or abrasion
What are the porcelain characteristics?
Rigid, Hard, Strong, tendency to form surface defects, brittle
what does tendency to form surface defects mean?
Leads to fracture at low stress
For porcelain-metal restorations bonding of what to porcelain? and what does it help eliminate?
bonding of metal oxide to porcelain helps eliminate defects/cracks on porcelain surface
what is function of alloy in a porcelain metal restoration?
Alloy acts as support and limits the strain that porcelain experiences
when forming a porcelain fused alloy restoration what does it have to be fired in?
furnace
after raising the temp of a porcelain fused alloy restoration what do you have to do?
cooling them without developing any thermal stresses that could cause either material or the metal oxide layer to develop defects or micro-cracks
what must porcelain and alloy have similar of?
thermal expansion coefficents
what is indium and tin role in high gold alloy’s
they enable a metal oxide layer to firm which enables bonding to porcelain
what has better mechanical properties high or low gold?
low gold
what are properties of silver palladium?
high mp, care needed in casting
what are properties of nickel chromium?
high melting point, high young modulus, high casting shrinkage, low-ish bond strength
what are advantage of cobalt chromium in porcelain fused metal restorations
high elastic modulus, hard, high tensile strength
what is the only alloy satisfactory in each criteria for porcelain fused to metal alloys?
low gold
what is the big disadvantage of high gold?
tendency to undergo creep
which is the only alloy to have a bicompatibility concern for porcelain fused to metal alloys?
nickel chromium due to allergic responses attributed to nickel
what are the required properties of a porcelain fused to metal alloy?
form good bond to porcelain, similar thermal expansion coefficient, avoid discolouration of porcelain, adequate material props (bond strength, hardness, elastic modulus), good melting recrystallisation temp of alloy
what is form good bond to porcelain also known as?
good wetting
why is similar thermal expansion coefficient imporantant?
To avoid setting up stresses during fusion of porcelain on to alloy
what is not used in high gold alloy as it can cause discolouration of porcelain?
copper
why is high elastic modulus important
to support porcelain and prevent fracture
what is the definition of creep?
gradual increase in strain experience under prolonged application of stress
what is the stressed skin effecgt?
slight differences in thermal contraction coefficients lead to compressive forces which aid bonding
what is the chemical mechanism in procelain metal bond?
The CHEMICAL mechanism is explained by oxides in the metal oxide coating on the alloy migrating with oxides within the porcelain itself
what is the chemical mechanism also described as?
electron sharing mechanism
when does the stressed skin effect occur
during production process, after furnace stage when alloy contracts slightly more on cooling and generates compressive forces on porcelain
when does the chemical mechanism occur
during firing stage
what are the modes of failure in a porcelain fused restoration?
oxide layer itself fracturing, oxide layer delaminating from alloy, porcelain detaching from oxide layer
what are wrought alloys manipulated by?
cold working
what are uses of wrought alloys?
wires for ortho, partial denture clasps
composition for steel?
iron > 98 percent, carbon <2 percent
what are other constituents of steel?
chromium and maganese
what is the purpose of chromium?
improve tarnish resistance
what are the uses of steel?
cutting instruments and forceps
what is allotropic?
undergoes 2 solid state phase changes with temp
where does austenite exist?
exists at high temp
where does ferrite exist?
at low temp
where does cementite exist?
at low temp
what is pearlite?
eutectoid mixture of ferrite and cementite
what is an alloy?
2 metal that form a common lattice structure - are soluble in one another and form a solid solution
what are substitional solid solution
Random and ordered
what does quenching of austenite lead to?
martensite not supersaturated austenite solution
why does martensite have a distorted lattice structure?
carbon being unable to diffuse normally within the array of iron atoms in each grain
slow cooling austenite leads to what?
pearlite
fast cooling of austenite leads to?
martensite
how do you get the conversion of martensite to pearlite
tempering
what is tempering?
heating followed by quenching
what does temp and during affect conversion to?
ferrite (soft, ductile) and cementite (hard, brittle)
what must stainless steel contain if it is to be considered stainless?
if contain >12 percent chromium
what are some properties of chromium?
lowers austentie to martensite temp and rate, decreases % of carbon at which eutectoid formed, corrosion resistant due to chromium oxide layer
what can attack chromium?
chlorides
what are some properties of nickel?
lowers austenite to martensite transition temp, improves fracture strength, improves corrosion resistance
what are the 2 types of stainless steel?
Martensitic and Austenitic
what are properties of martensitic steel?
contains 12-13 percent chromium and little carbon, can be tempered to produce hard materials
how do you get austenite instead of martensite
by having the right proportions of chromium and nickel specifically either 18:8 or 12:12 ratio
uses of austenitic steel?
dental equipment, corrosion resistant, wires, sheet forms for denture bases (swaged)
what are properties of 18-8 stainless steel?
does not heat harden, soft when cast but work hardens rapidly
what is cold working?
It’s work done on metal/alloy at LOW TEMPERATURE - below recrystallisation temperature
what do cold working processes cause?
SLIP
what is slip?
dislocations at grain boundaries
what does cold working produce?
stronger harder material
what does the grade of stainless steel wire depend on?
degree of bending required
what are degrees of bending required?
soft - half hard, hard - spring temper
what is definition of springiness?
ability of a material to undergo large deflections (so as to form an arc) without permanent deformation – that is, the material will subsequently return to its original shape.
what are the requirements of wires?
high springiness, stiffness, high ductitlity, easily joined without impairing properties, corrosion resistant
what is ductility?
bending without fracture
what do you use to solder s/steel?
gold or silver
how do you stop s/steel grains from recrystallising?
quenching the alloy
when does weld decay occur?
between 500-900 celsius
what happens when weld decay occurs?
alloy becomes brittle, less chromium in central region of solid solution so more susceptible to corrosion
how to minimise weld decay?
low carbon steels - expensive, stabilised s/steel
what does stabilised s/steel contain?
titanium or niobium
what is stress relief annealing of s/steel which is needed?
process requires the temperature of s/steel needs to be held at around 450C for a minute or two
what are advantages of stainless steel denture base?
Thin 0.11m – acrylic 1.52mm
Light
Fracture resistant
Corrosion resistant
High polish obtainable
High thermal conductivity
High impact strength
High abrasion resistance
what are disadvantages of s/steel denture base?
Possible dimensional inaccuracy (contraction of die not marches by model expansion)
Elastic recovery of steel – inaccuracy
Damage of due under hydraulic pressure
Loss of dine detail during the many stages
Difficult to ensure uniform thickness
Uneven pressure on die and counter die wrinkling of steel
what are some ideal properties of denture base materials?
Replaces function of natural teeth
Dimensionally accurate and stable in use
High softening temp
Unaffected by oral fluids
Thermal expansion
Low density
High thermal conductivity
Radiopaque
Non toxic, non irritant
Colour/ translucency
Easy and inexpensive to manufacture
Easy to repair
what are the mechanical properties of a denture base material?
High young’s (elastic modulus)
High proportional limit
High transverse strength
High fatigue strength
High impact strength
High hardness/abrasion resistance
what is transverse strength?
How well does upper denture cope with stresses that cause deflection?
what is free radical addition polymerisation?
Chemical union of 2 molecules either the same or diff to form. Larger molecule without elimination of smaller molecule. involves molecules with C=C bonds
what are components of acrylic polymerisation?
Activation – of initiator to provide free radicals
Initiation – free radicals break C=C bond in monomer and transfer free radical
Propagation – growing polymer chain
Termination – of polymerisation
what do you need for acrylic heat curing?
Need efficient polymerisation to give high molecular weight polymer. so high temp but gaseous porosity limits
what are the properties of acrylic?
non toxic, non-irritant if no monomer released, unaffected by oral fluid, poor mechanical properties but increase in bulk to compensate, fatigue strength - fairly resistant, high abrasion resistance, low thermal conductivity, low density, high softening temp, ok - linear contraction
why use self curing acrylic?
lower temp - less thermal contraction hence better dimensional accuracy
what is there more of in self curing acrylic that has a worse effect?
unreacted monomer, softening denture base, reducing transverse strength, potential tissue irritant which comprises its biocompatibility
what is dimensional accuracy of self curing acrylic?
fits original cast better than heat cured but water absorption so expands hence self cured over size (heat cured under size - better tolerated)
what is stronger heat or self cure acyrlic
heat - stronger, will last longer
why is self cure more risk of being an irritant?
more monomer uncured
what are alternative polymers?
nylons and vinyl polymers and polycarbonates
what happens in nylon dentures?
water absorption leads to swelling and softening
what are advantages of polycarbonates over acrylic?
able to withstand large temps more than acrylic
what are weakness of polycarbonates over acrylic?
they develop internal stresses during use that causes distortion and so a poor fit
what is needed to produce, inlays, onlays, crown and bridges etc
casting process
what does casting process require?
an investment material of the required shape, to contain the molten alloy and withstand the high temps involved and ensure the alloy’s dimensions are sustained
outline stages involved with making inlays, outlays crowns etc
1) A wax pattern of the required prosthesis – crown, inlay – is created (ie a positive replica)
2) An InvMater is placed around this wax pattern and allowed to set. It forms a mould (a negative replica)
3) The wax is then removed – by burning or with boiling water. So now we have a cavity of the required shape surrounded by the InvMater
4) Next, the molten alloy is poured into the mould cavity – and this is done via the sprue – hollow tubes that allow the alloy to flow in.
known as lost wax technique
what are types of investment materials?
dental stone or plaster (acrylic dentures), gypsum bonded materials (gold casting alloys), phosphate bonded materials (base metals, cast ceramics), silica bonded materials (base metal alloys)
what are the requirements for an investment material?
expand, porous, strong, smooth surface, chemically stable, easy removal from cast, easy handling, inexpensive
why does an Investment material need to be able to expand?
to compensate for cooling shrinkage
why must an investment material be porous?
to allow escape of trapped gases on casting - back pressure effect
what are the 2 componenets of invmater and what are their purposes?
Binder - to form a coherent mass
refractory - usually type of silica, withstands high temps and undergoes expansion
what is meaning of hygroscopic?
absorb moisture from air