H - DMS Flashcards
How to from martensite?
Quenching of austenite
What percentage of Cr is there in SS?
13%
Ideal properties of impression material?
Low viscosity Low viscoelasticity Low stiffness Low thermal Expansion Low polymerisation shrinkage High tear resistance High surface detail Dimensionally stable
What is weld decay in SS?
Occurs between 500-900 degrees
Chromium carbide precipitate at the grain boundaries
Alloy brittle
Minimise by low carbon content In steel
What are the pros and cons of self-cured PMMA compared with heat-cured?
Pros Much quicker to make Less thermal contraction Cons irritant to tissues - unreacted monomer Water absorption and expansion
Examples of temp materials are direct/indirect? short/long term?
PMMA - indirect, long
PEMA - direct, short
Bis-arcylic composite - direct, long
Requirement of wires, (desirable properties)
Must be springy - High EL YM
Stiffness
High ductility -
Corrosion resistance - resistance to reactions within their environment
What is sintering in dental ceramics?
When a crown is heated in a furnace above glass transition temp
Ceramic particles fuse to form a solid mass
What is kaolin? and what is it replaced with in dental ceramics?
A soft white clay which makes other ceramics opaque
Replaced with silica and feldspar
The function of alumina core?
High flexural strength, prevents cracks propagating through ceramic material
How does yttria (yttrium oxide) work in zirconia core
Crystal structure changes shape if a crack appears
causes slight expansion to close the crack
Sintered vs milled ceramics?
Which is stronger
Milled
Causes of endodontic instrument fracture?
Torsional stress
Cyclic stress/fatigue - generation of tension/compression forces leads to failure
Properties of NiTi Endo instruments?
Super elasticity
Ideal properties of obturation material?
Dimensionally stable Seals canal non-irritant Bacteriostatic/cidal radiopaque Doesn't discolour tooth
What stops the transition to martensite in austenitic SS?
Sufficient Cr and NI
By what mechanism can porcelain bond to metal?
Chemical - electron sharing in oxides
Stress skin effect - slight differences in thermal contraction leads to compressive forces
mechanical - surface irregularities
Process of lost wax technique?
Wax pattern made
investment material passed around it
Wax then eliminated
Molten alloy forced into mould via channels in investment material
Why do investment materials need to be porous?
As if porous it prevents incomplete casting by allowing escape of gases
Types of luting agents?
GIC
Composite luting agents
Conventional cements e.g zinc phosphate
RMGIC - however, HEMMA expands when wet
