Material Properties Flashcards
What are the 4 categories of Material properties?
Physical
Mechanical
Chemical
Biologic
Physical properties refer to 5 areas of interest which are:
1
2
3
4
5
1 mass properties
2 thermal properties
3 electrical properties
4 optical properties
5 surface properties
Mechanical properties refer to:
a description of the stresses and strains within a material resulting from an external force.
Chemical properties refer to:
chemical and electrochemical interactions
Biologic properties refer to:
characterization of toxicity or sensitivity reactions.
Materials respond to environment according to their _____________ and _________________
Materials respond to the environment according to their structure and bonding
due to the coefficient of Thermal Expansion what happens during temperature increases or decreases.
When temperature increases atomic motion increases which stretches bonds and produces expansion. The opposite is true when temperatures are decreased (ie: shrinkage, *think george castanza* “I was in the pool!”)
Which materials have low coefficient of thermal expansion and which materials have high coefficient of thermal expansion?
ceramics
metals
polymers
tooth structure
LOW (dont expand much)
• Ceramics: 1-15 ppm/°C
Tooth Structure: 9-11 ppm/°C
Metals: 10-30 ppm/°C
Polymers: 30-600 ppm/°C
HIGH (expand quite a bit)
What is Percolation?
Percolation is where the margins of a restoration become weaker and “leaky” because the restoration undergoes cyclical expansion and contraction at a different rate than the tooth structure around it.
Major cause of loosening restorations and secondary caries
This is an example of PERCOLATION! note the expansion and contraction as temperature changes. This weakens the margins and can result in secondary caries.
What is thermal conductivity?
thermal conductivity is the ability of a material to conduct heat.
Low conductors are also good insulators
What are some examples of materials that are good insulators/poor conductors?
Dentin, Cements, composite resins, cavity liners
High conductivity means the material is a poor insulator and will conduct heat readily. What are some examples of materials that are good conductors/poor insulators?
amalgam and gold
What is HEAT DIFFUSIVITY?
Heat diffusivity is a measurement of heat conductivity per unit time, it takes the density of the material into account.
Dental pulp can withstand __________ temperature changes for __________amounts of time with ___________ damage
dental pulp can withstand small temperature changes (37-42 °C) for short periods of time (30-60 seconds) with no permanent damage.
What is Electrical Conductivity
rate of electron transport through a material
What types of materials are best at conducting electricity?
Metallic restorations may conduct flow through the pulp.
Amalgam + gold = shock
What does specific gravity refer to?
specific gravity refers to the density of a material in reference to water
What are the 4 main categories or Optical Properties
Reflection- light bounces off surface
Refraction- light is bent as it passes through (changes directions at surface)
Absorption (and Fluorescence)- light loses energy as it goes through
Transmission- linked to opacity/translucency
What are the 3 categories in the MUNSELL COLOR SYSTEM
HUE (wavelength)
VALUE (intensity)
CHROMA (purity)
In dentistry we use a __________ to help us match restorative dental materials to tooth structure.
A SHADE GUIDE
What 3 substances are added to composite materials to make them radiopaque on radiographs?
Lithium
Barium
Strontium
What does Contact Angle refer to?
The angle a drop of liquid makes with the surface it is resting upon.
What determines if a material is “Poor/Non Wetting”?
Contact Angle greater than 90 degrees and or/ approaching 180 degrees
What determines if a material is “Good wetting”
contact angle less than 90 degrees
What is the relationship between wetting ability and surface dension?
the greater the surface tension difference between the liquid and solid the greater the contact angle will be and the lesser the wetting ability.
what types of materials have intentionally good wetting properties?
varnishes
liners
cements
bonding agents
All have good wetting properties so that they adapt to microscoptic interstices of the surface.
Good wetting=
Poor wetting=
Good wetting = hydrophilicity
Poor wetting = hydrophobicity
Mechanical properties refer to the materials response to __________
LOADING
What are some types of loading that can by placed on a material?
- compression
- tension
- shear
- torsion (twisting)
- flexion (bending)
Understand the following graph and know the differences between
Plastic deformation
elastic deformation
Define:
Elastic Strain/Deformation:
Plastic Strain/Deformation:
Elastic limit:
Elastic Strain/Deformation- Reversible strain/deformation
- *Plastic Strain/Deformation-** irreversible strain/deformation
- *Elastic limit:** The point after which plastic strain occurs
Continuous plastic strain eventually leads to _______________
Failure by fracture
Ultimate strength refers to
highest amount of stress tolerable before the material fractures
Materials that are resistant to plastic deformation are called:
DUCTILE
Materials that cannot tolerate very much plastic deformation are called:
BRITTLE
In general we want restorative materials that are _____ _____ so that under load elastic deformation will be _____.
In general,we want a restorative material that is very stiff so that under load its’ elastic deformation will be very small
What are the names of the two common hardness tests:
MOHS test: scratch resistance comparison
Rockwell, Brinell, Knoop tests: indentation resistance comparison
What is Resilience?
Resilience is the amount of energy a material can absorb before it starts to undergo PLASTIC DEFORMATION
What is Toughness in regards to materials
Toughness refers to the total energy that can be absorbed before a material fractures
KNOW THIS CHART AND WHAT IT MEANS
What is “CREEP”
deformation over time in response to a constant stress
Weak materials and materials close to their melting point are more prone to “CREEP”
What 2 commonly used dental materials are known to exhibit creep??
Dental waxes- will creep under their own weight over time
Amalgam restorations- especially low copper amalgams in proximal boxes
What is Fatigue
strain that occurs over time through multiple cycling of low stresses.
Over time these cycles cause infinitesimal strain, which can eventually lead to failure after millions of cycles.
What is the standard design limit for failure due to fatigue?
(how many cycles are materials made to last for)
approximately 10 million cycles
What is the difference between primary and secondary bonding?
Primary Bonding- occurs during chemical reactions
Secondary bonding- occurs during adsorption or absorption
for metals in the mouth, primary bonding results from:
1
2
Chemical corrosion (tarnishing)
Electrochemical corrosion
Electrochemical corrosion occurs when these 4 elements are present
1
2
3
4
1 anode (+ charge, site of corrosion)
2 cathode (-charge)
3 circuit
4 electrolyte (saliva)
When amalgam and gold restorations these 4 types of corrosion are possible:
1
2
3
4
1 Galvanic corrosion- macroscopically different electrode sites
2 local Galvanic corrosion- electrochemical differences in single material (alamgam covered in plaque)
3 crevice corrosion- provide environment for concentration cell corrosion
4 stress corrosion- distribution of mechanical energy is not uniform
What 2 materials DO NOT corrode?
Ceramics and Polymers
Ceramics may not corrode but they can be effected by ____________
SOLUBLE DISSOLUTION usually caused by high local acidity
What is the difference between adsorption and absorption?
Adsorption • adding molecules to a surface by secondary bonding
Absorption • penetration of moleclues into a solid by diffusion
The two main issues with BIOLOGICAL PROPERTIES are:
1
2
1 Toxicity
2 Sensitivity
Dental materials interact with a variety of tissues and local reactions to various materials may differ.
these 2 factors determine a materials toxicity:
1
2
1 Exposure time
2 Concentration
