material science Flashcards
(27 cards)
dinstinguish between chemical, physical and mechanical properties
chemical
- substance transforms into a new substance with different properties, due to the rearrangement of atoms and bonds [molecular change]
physical
- characteristic of matter that may involve changing the arrangement of matter but NOT molecular structure
mechanical
- materials capacity to cope w range of external applied forces eg shear stress, load, ewather conditions, time
list FOUR important dental properties
- chemical
- physical
- mechanical
chemical
1. bonding
2. wetting
3. stability
4. toxicity
physical
1. dimensional stability
2. melting/boiling point
3. surface tension
4. viscosity
5. thermal expansion
mechanical
1. strength
2. resilience
3. ductility
4. elasticity
5. toughness/hardness
describe chemical bonding and its applications
adhesion two substances via molecular bonding
- can be used to repair, fill, restore, fills gaps, change tooth shape/colour
- mechanical bonding is often used in conjunction w chemical bonding [eg outline/retention/resistance form]
describe chemical wetting
- determines if molecules of different materaisl are compatible for bonding
- modulated by surface characteristics eg chemistry and topography
- good wetting = high strength bond
- can also be used to break surface tension
- eg debubbliser on imp material
describe chemical stability
- material in oral environment must’nt react of change in any way which alters its properties
- should not dissolve in saliva or any other fluid in oral cavity
- should not tarnish or corrode when in contact w oral fluids
describe chemical toxicity
- material used should not cause trauma or tx damage
- IDEALLY dental materials are fully biocampatible [can integrate w/ human tx w/o unfavourable results] –> need to prevent/reduce likelihood of negative health outcome
- all dental materials should be TGA approved [Therapeutic Goods Association]
describe physical:dimensional stability and its ramifications
- materials should not shrink/expand to cause discomfort
- can lead to appliance/dentition/restoration failure
- can diminish usefulness of resto/appliance
- maintaing dimensional stability of imp material = vital for accuracy
define physical:density and the importance of this property
mass/unit volume of a material
- should be considered when selecting materials - controls appliance weight
neg eg
- gold = expensice and weights alot whilst titanium is the opposite
pos eg
- the higher the density of a model = more accurate w harder surface to be worked on
define physical:melting point and the importance of this property
- temp where substance changes from solid –> liquid state
- material should be able to operate in warm temp if going in oral environment
- also important in determining how easy a metal is to cast
eg
- wax will distort w temp
- gold melting point 1064°
- titanium melting point 1668°
define physical:boiling point and the importance of this property
- temp where substance changes from liquid –> gaseous state
- determines material processing + if materials and instruments can be placed in autoclave [eg xray holders, stock trays]
define physical:optical and the importance of this property
- makes aesthetic rehabilitation look natural
- all ceramic restos must have equal optical properties to natural teeth
- eg colour, hue, value, translucency, chroma, opacity, light reflection
define physical:thermal expansion and the importance of this property
- almost all substances expand w temp rise and contract w cooling
- impact on accuracy of metal castings, polymer restos and ceramics
- excessive thermally induced loads between resto materials + dentition can = microleakge and wear problems
- when mixing two differnet materials [eg PFM] the two different rates of expansion/contraction can cause destructive stress –> varying amts of expansion in different materials [coefficient of thermal expansion]
- to compensate - investment materials which expand at same rate during casting can be used
define physical:specific heat and the importance of this property
- materials capacity to absorb energy whilst changing temp
- measured as energy req to ^ temp of one unit of mass of substance by one °
- high temps may cause irreversible damage to surrounding tx and organs
- eg titanium dental imps transferring heat following hot liquid intake
define physical:viscosity and the importance of this property
- resistance to flow of a fluid
- high viscosity = flowing material more difficult
- decreases under shear loading [eg CR = more fluid during adaptation to tooth surface]
define physical:fracture toughness and the importance of this property
- amt of stress required to propagate pre-existing flaw
- v important material property - flaw avoidance is not completely avoidable
- higher value = indicator of superior clinical long term performace [longer lasting material]
flaws incl
- cracks, voids, metallurgical inclusions, weld defects, design discontinuities
eg
resto material needs to withstand fractures esp in load bearing areas
define physical:surface tension and the importance of this property
- amt of force which a material is attracterd to another at their surface
- related to chemical wetting
- liquids will either wet a new surface eg solid an spread accross it OR will not wet/spread and withdraw
- can be seen in surfaces pulling liquids upwards against gravity
eg
- pouring gypsum –> silicon mould/alginate imp
define physical:latent heat and the importance of this property
- amt of energy a substance takes up/gives off when changing state
- materials w high specific heat, high latent heat of fusion and high melting point = need more energy to melt
- effects the cost of producing an item which needs melting eg wax
define physical:stiffness/rigidity and the importance of this property
- measure of elasticity + represents a material’s resistance to permenent deformation
- stiffness = describes how much it is deform elastically for an applied stress
- materials w low stiffness = larger deformation than materials w high stiffness for same applied stress
define mechanical:strength and the importance of this property
- all material needs suffucient strength to withstand stresses involved in oral cav
**tensile strength **
- force needed to STRETCH a material
- measured in Newtons or MPa [megapascals]
- ceramics are weaker in tensile strength than metals
- ultimate tensile stress [U.T.S] measure of how much a material will varry before it breaks
compression strength
- force required to deform a material under a load weight
**yield strength **
- stress at which permanent deformation of structure begins
define mechanical:plasticity/ductility and the importance of this property
measure of amt of deformation a material can withstand **up to its failure **
- brittle materails eg porcelain, ceramic resto materials = brittle + low plasticity
- vs ductile materials eg wax = high plasticity
- internally affected by composition, grain size, cell size etc
- externally affected by hydrostatic pressure, temp, plastic deformation
define mechanical:elasticity and the importance of this property
- amt of force which can be applied and material returns to original length/shape
measured by
- elastic modulus
- Young’s modulus
- modulus of elasticity
elastic stress does **NOT cause permanent/irreversible deformation **
- if ONLY elastic deformation occurs = surface will recover when force is removed
eg
- stainless steel is good for clasps as its elastic
define mechanical:hardness and the importance of this property
- resistance of a substance to indentation
- related to wear resistance [control life of a material]
- tested w Vicker’s Hardness Test
- sufficient hardness ensures that restos are resistant to scratching during mastication and abrasion
