partial denture alloys Flashcards
what are the mechanical properties of partial denture alloys like
- good
what are the important mechanical properties to consider for partial denture alloys
- stress, rigidity, elastic limit, hardness and ductility
what are the alloy processes of hardening
- work hardening
- solution hardening
- order hardening
- precipitation hardening
what are the alloy processes of annealing
- homogenisation annealing
- stress relied annealing
what are the ideal properties of partial denture alloys
- rigid = YM
- strong = UTS, EL
- hard
- ductile
- precise casting = shrinkage
- melting point
- density
why is melting point important for partial denture alloys
- for the investment material
- can be issues with the melting point = determines what investment material to use
why is density important for partial denture alloys
- want as lightweight as possible for patient comfort
what are the types of partial denture alloys
- ADA type IV gold
- white gold = Ag-Pd
- Co-Cr
- titanium
what is the problem with one piece casting for partial denture alloys
- base and clasps need different properties
- base = want a high YM to maintain shape in use, and what a high EL to avoid plastic deformation
- clasp = want a lower YM to allow flexure over tooth, and a high EL to maintain elasticity over wider range of movement/strian
how are the issues of one piece casting resolved
- compromise
- use thick section of alloy for base = rigid
- use thin section of alloy for clasp = flexible
what is the carat for pure gold
- 24 carat
what is the fineness for 100% gold
- 1000
what is pure gold used for
- class III and class IV cavities
what are the 4 types of gold alloys used for
- type 1 = simple alloys
- type 2 = larger (2-3 surface) inlays
- type 3 = crown and bridge alloys
- type 4 = partial dentures = has particular composition
how many other metals are in type IV gold
- 5 other metals
what is the composition of type IV gold
- Au = 65% but can be between 60-70%
- Zn = 2% but can be between 1-2%
- Cu = 14% but can be between 11-16%
- Ag = 14% but can be between 4-20%
- Pd = 3% but can be between 0-5%
- Pt = 2% but can be between 0-4%
what does the phase diagram of order hardening of Au-Cu show
- liquidus and solidus are continuous so means can have lots of different compositions = solid solution in all properties
how can Au-Cu benefit from solidus and liquidus being close together
- as if it undergoes some type of heat treatment it forms an ordered solution
what are the effects of using copper as an alloying element with gold
- solution hardening
- solid solution in all proportions
- order hardening = if 40-80% gold and correct heat treatment
- reduced melting point
- no coring = solidus close to liquidus
- can impart a red colour = not ideal
- reduces density = good for patient comfort
- base metal = can corrode if too much
what does the phase diagram of complete solubility of Au-Ag show
- liquidus and solidus are close tougher so any combo of Au and Ag can co-exist together
what are the effects of using silver as an alloying element with gold
- solid solution in all proportions
- solution hardening
- precipitation hardening with copper and heat treatment = related to Ag and Cu in combo and if heat treat it can undergo precipitation hardening
- can allow tarnishing
what happens if silver and copper in type IV gold
- molten silver absorbs gas such as CO2
- whitens alloy = compensated for red form copper
what does the phase diagram of partial solubility of Ag-Cu in type IV gold show
- there are 2 solid phases = alpha and beta
- can only have material in these areas
- with partial solid, get benefit of precipitation hardening
what does the phase diagram of complete solubility of AuPt show
- get a degree of separation between liquidus and solidus = coring occurs
what are the effects of using platinum as an alloying element with gold
- solid solution with gold
- solution hardening
- fine grain structure = better mechanical properties
- coring can occur = wide liquidus to solidus gap
what are the effects of using palladium as an alloying element
- similar to platinum but less coring occurs
- less expensive than platinum
- coarser grains than platinum
- absorbs gases when molten = means porous casting = weaker material
what are the effect of using zinc as an alloying element
- scavenger
what are the effects of using nickel as an alloying element
- increases hardness and strength = wrought alloys
what are the effects of using indium as an alloying element
- fine grain structure
what is the process of heat treatment for type IV gold
- quench after casting = pour molten alloy into cast then cool quickly which makes it under of coring so then have to undergo annealing
- homogenising anneal = raise temp to 700oC for 10 mins after quenching
- if need to cold work to manipulate to fit into patient then need to do stress-relief annealing
- heat harden = order and precipitation hardening = raise temp to 450oC then cool slowly (15-30mins) down to 200oC then quench
what does quenching after casting produce
- fine grains
what are the heat treatment process done for
- improve partial denture or reduce negative consequences
what is CoCr used for
- wires
- surgical implants
- cast partial dentures = connectors - want high EL and high YM for thick sections, and high EL and low YM for thin sections
what is the composition of CoCr alloy
- Co = 54% but can be between 35-65%
- Cr = 25% but can be between 25-30%
- Ni = 15% but can be between 0-30%
- Mo = 5% but can be between 5-6%
- C = 0.4% but can be between 0.2-0.4%
what is Mo
- molibdium
what are the effects of using cobalt as an alloy
- forms solid solution with Cr = 2 metals co-exist across lattices
- increased strength, hardness, rigidity
- coring is possible
what are the effects of using chromium as an alloy
- forms a solid solution with Co
- increased strength, hardness, rigidity
- coring possible
- forms passive layer = key role in corrosion resistance
what are the effects of using nickel as an alloy
- replaces some Co = improves CoCr
- improves ductility
- slight reduction in strength
- sensitivity for patients = 6% in women and 2% in males
what is the effect of carbon on CoCr alloy
- undesirable, carbide grain boundaries are hard and brittle
- ideally don’t want any but it is inevitable
what is the effect of Mo on CoCr alloy
- reduces grain size, hence increases strength
what is the effect of W (tungsten) on CoCr alloy
- increases strength
- Al = increases Pl
- others = scavengers
what is the investment material for CoCr
- needs to be high temperature = 1200-1400oC
- so need to use silica or phosphate bonded instead of gypsum
how is CoCr melted
- electric induction preferred
- oxyacetylene = avoid carbon pickup
how is CoCr casted
- centrifugal force required
- avoid overheating = coarse grains
- cooling too fast or slow = carbides = brittle
- challenging material to produce casts of
how is CoCr finished
- sandblast
- electroplate
- abrasive wheel
- polishing buff
is CoCr easy to polish
- no
- difficult material to polish, but means as it shard to polish it is hard to roughen up again after polishing
what is the hardness of CoCr
- BHN = 370
- much harder then gold = lasts longer
- wear in mouth better
- finishing/polishing is time consuming
what is the elongation of CoCr
- means ductility
- low ductility
- 4%
- work hardens rapidly
- adjustment difficult
- precision casting = if casting is not precise then 4% is not enough ductility
how much pure metal is titanium
- 99.5%
what are the used of titanium
- uses of pure titanium have developed recently and show promise
- several commercial labs provide titanium casts
- implants, partial dentures, crown and bridge, maxilla-facial skull plates
what are the properties of titanium
- good biocompatibility
- good corrosion resistance = passive oxide layer
how are parts of titanium joined together
- laser welding
- electric arc melting
what is done because titanium absorbs gas
- specialised investment and casting
when comparing partial denture alloys which has best elongation
- titanium but it is expensive so often go for type IV gold or CoCr
when comparing partial denture alloys which has the best UTS (MPa)
- stainless steel
why is UTS important
- because don’t want partial denture to fracture
when comparing partial denture alloys which has the best density
- important for patient comfort
- CoCr is better then Au as it is lower
when comparing partial denture alloys which has the best rigidity
- also known as Young’s modulus
- CoCr is best by quite a lot
when comparing partial denture alloys which has the best hardness
- gold is easier to polish but means it will wear easier
- CoCr highest
when comparing partial denture alloys which has the best shrinkage
- CoCr is highest so need to use different investment material to reduce it
why do you need precision casting for CoCr
- work hardens quickly
what are the properties to know about for type IV gold and CoCr
- shrink = gold is 1.3 %, CoCr is 2.3%
- density = gold is 15, CoCr is 8
- Young’s modulus = gold is 100, CoCr is 250
- UTS = gold is 790, CoCr is 690
- elongation = gold is 15 or 10, CoCr is 4
- hardness = gold is 210-230, CoCr is 370
