DMS Flashcards
What are the constituents of stainless steel and their proportions
- Iron – 72%
- Chromium – 18%
- Nickel – 8%
- Titanium – 1.7%
- Carbon – 0.3%
What is work hardening
- Work done at temperatures below recrystallisation temperature (such as bending, swaging or rolling)
- This causes slip so any dislocations in the lattice collect at grain boundaries
- This results in a stronger, and harder material
what is sintering (in regards to fabrication of a ceramic crown)
when the particles melt and fuse together as one mass
define springiness
the ability to undergo large deflections without permanent deformation
what are some disadvantages of self-cure PMMA
- water absorption can cause expansion
- poorer mechanical properties
- unreacted monomer can act as irritant
- poorer colour stability
what are some ideal properties of acrylic denture base
-high softening temp
- unaffected by oral fluids
- radiopaque
- non toxic, non irritant
- low density
- high strength, stiffness, hardness, toughness
why is mould liner used in fabrication of an acrylic denture
to prevent the penetration and adhesion of resin into the surface of the plaster mould
- it is usually a solution of sodium alginate
what is gaseous porosity and why does it happen?
when the temp exceeds boiling temp of PMMA before the polymerisation process is complete
–> should be carried out slowly to avoid
how is porcelain prepared in the lab to improve bonding
etched to hydrofluoric acid
what bonds porcelain to composite resin luting cement?
silane coupling agent
how does silane coupling agent act chemically
- one end forms strong bonds b/w the oxide groups on the porcelain
- the other end has C=C that bonds to the composite resin luting agent
when would youse use dual cured composite as a luting agent
when bonding to a metal, as light will not penetrate
i.e. when cementing most crowns, bridges, posts…
why would you do to a metal in a lab to prepare?
sandblast the metal surface
what chemical agent is used to bond metal?
MDP and 4-META
what metal can be use for an acid-etch retained bridge
non precious metal alloys e.g. CoCr, NiCr
What are the two main differences in composition between decorative and dental ceramic
Decorative ceramics contain Kaolin - makes it opaque in appearance
Dental ceramics need to be translucent so Kaolin is removed and feldspar and silica replace it
dental ceramics are glasses
Explain what the following terms mean with regard to optical properties of the materials
a. Translucency
b. Opalescence
Translucency is the relative amount of light transmission or diffuse reflection from a substrate surface through a turbid medium
The opalescence of dental materials is defined as the difference in chroma between the reflected and transmitted colours.
how does an alumina reinforced feldspathic core increase the strength of a ceramic crown
the alumina particles act as crack stoppers preventing cracks propagating through the material and causing fracture
what is a problem with alumina cored crowns
lacks flexural strength
what are the strongest crown types
monolithic block crowns - they are milled from a single block of material
key comparison between crowns with layered and monolithic block crowns
layered =better aesthetics BUT more likely to chip due to stresses between core and crown
For the same material a milled crown will be stronger than a built up or pressed crown. WHY?
the block will have been subjected to the ideal heat treatments to maximise tits properties and all blocks will be consistent
what are some problems with zirconia cored crowns
expensive equipment required
Potential for veneering porcelain to debond from core
what types of crowns are typically used in posterior teeth
Monolithic Zirconia
what types of crowns are typically used in anterior teeth (single crown)
LiDiSi (up to premolar)
what types of crowns are typically used for longer span or heavier occlusion
Zirconia cored with zirconia where occlusal contacts will meet
what are the characteristics of PORCELAIN
Rigid - large stress to cause strain
Hard - withstands abrasion
Strong - high compressive strength
BUT
Tendency to form surface defects –> leads to fractures at low stress
Brittle
what helps eliminate defects/cracks on porcelain surface
bonding to metal oxide - it acts as a support and limits the strain that porcelain experiences
what are the required properties when choosing alloy for porcelain to fuse with
- Good wetting
- similar thermal expansion coefficient
-discolouration - Mechanical = bond strength, hardness, elastic modulus
- melting/recrystallisation temp of allow MUST be HIGHER than fusion temp otherwise CREEP
define wetting
the ability of a liquid to form an interface with a solid surface
define CREEP
gradual increase in STRAIN (permanent) experienced under prolonged application of STRESS
what are WROUGHT ALLOYS
an alloy which can be manipulated by cold working
what is an ALLOY
TWO metals that form a COMMON LATTICE structure are soluble in one another = Form a SOLID SOLUTION
what are the 2 types of substitutional solid solutions
RANDOM - where both types of atoms in the lattice structure are arranged in random fashion.
ORDERED - can predict the type of atom based on its location.
what is an interstitial solid solution
where the two atoms are markedly different in size
define welding
a fabrication process whereby two or more parts are fused together by means of heat, pressure or both forming a join as the parts cool
what are ways to mitigate the risks during the process of welding stainless steel
use of:
- low carbon steel - expensive
- stabilised SS (with small amounts of Ti and niobium)
how are elastomers formed?
by polymerisation with cross-linking of polymer chains.
what properties affect the ACCURACY by which the SURFACE features are recorded in elastomers
surface detail
flow/viscosity
contact angle/wettability
what properties affect the ACCURACY by which the DIMENSIONS and SHAPE of final impression
elastic recovery
stiffness
tear strength
define wettability and contact angles
determines how well the material envelops hard/soft tissue surface.
The contact angle indicates how readily the IM “wets” the tooth surface.
ie how closely the IM envelops the tooth surface.
what does viscosity determine
a material’s potential for making close contact with hard/soft tissue surfaces
what happens if an IM exhibits LARGE contact angles
it means the IM is more likely to have gaps between each globule of material. This means the IM won’t replicate the whole tooth surface.
what is the term to describe an IM that has been stretched/compressed and it fails to return to its original dimensions/shape
permanent deformation
what types of materials are temporary materials
resins
what are some problems with ZINC PHOSPHATE cement as a luting agent
- low initial pH = can cause pulpal irritation
- exothermic setting reaction
- not adhesive to tooth/restoration (just fills spaces)
- not cariostatic
- brittle
- final set takes 24nrs
advantages with using ZINC POLYCARBOXYLATE cement
- bonds to tooth surface
- less heat during reaction
- pH low to begin but returns to neutral more quickly
- cheap
disadvantages of using ZINC POLYCARBOXYLATE cement
- difficult to mix and manipulate
- soluble in oral environment at lower pH
- lower modulus and compressive strength than zinc phospahte
problems with RMGI (as a luting agent)
HEMA is cytotoxic
HEMA expands in wet environment
no bond to indirect restoration
