Physical properties of dental materials

PHYSICAL PROPERTIES OF
DENTAL MATERIALS
SUBMITTED BY
AKSHARA.E.S
FIRST YEAR PG
DEPARTMENT OF CONSERVATIVE DENTISTRY AND
ENDODONTICS
MALABAR DENTAL COLLEGE OF SCIENCE AND
RESEARCH
1

CONTENTS
 INTRODUCTION
 STRUCTURE OF MATTER
 MECHANICAL PROPERTIES
 ADHESION
 WETTING
 THERMAL PROPERTIES
 ELECTROCHEMICAL PROPERTIES
 ELECTRICAL PROPERTIES
 CONCLUSION
 REFERENCES
2

3
1. Discuss dental adhesion and various bonding systems related to
restorative dentistry ? 2011 RGUHS
2. Tarnish and Corrosion?
PREVIOUS YEAR QUESTIONS

• Physical properties are based on the law of mechanics, acoustics, optics,
thermodynamics, electricity, magnetism, radiation, atomic structure, or
nuclear phenomena.
• Based on law of optics – Hue, value, and chroma.
• Based on the laws of Thermodynamics - Thermal conductivity and
Coefficient of thermal expansion.
INTRODUCTION4
Anusavice KJ, Shen C, Rawls HR, editors. Phillips' science of dental materials. Elsevier Health Sciences

• All matter is composed of indivisible particles called
atoms.
• An atom consists of a nucleus surrounded by a cloud of negatively charged
electrons.
• The electrons of an atom exist in different clouds at the various energy levels.
• An atom becomes a negative ion when it gains electron or a positive ion when it
loses electron.
STRUCTURE OF MATTER5

• Two or more atoms can form an electrically neutral entity called a molecule.
• Attraction between atoms and between molecules result in materials we can see
and touch.
• The transformation between vapor, liquid, and solid is called the change of
state.
• A change from the solid to the liquid state will require additional energy—
kinetic energy— to break loose from the force of attraction.
• This additional energy is called the latent heat of fusion. The temperature at
which this change occurs is known as the melting temperature.
6

INTERATOMIC BONDS
• The forces that hold atoms together are called cohesive forces.
• Classifieds as:
1. Primary bonds
2. Secondary bonds
• Primary bonds
Ionic bonds
Covalent bonds
Metallic bonds
• Secondary bonds
Hydrogen bonding
Vander Waals bonding
7

• Ionic bonding
 PRIMARY BONDS8

• Covalent bonding9

• A metallic bond is produced when electron orbitals overlap and all electrons are
shared is responsible for the high conductivity, reflectivity, malleability, and
ductility of metals. Metallic bonding is restricted to atoms of a single element.
• Metallic bonding10

 SECONDARY BONDS
• Hydrogen bonding
11

• Vander Waals force12

BOND DISTANCE AND BOND ENERGY
• Bond distance or bond length is the average distance between the centers of
the nuclei of two bonded atoms in a molecule.
• Shorter the bond length, the larger the value of bond energy,
• It is expressed in Angstrom units (A0) or picometers (pm).
13

STRUCTURE OF ATOMS
 CRYSTALLINE STRUCTURE
• A space lattice can be defined as any arrangement of atoms in space in
which every atom is situated similarly to every other atom.
• The type of space lattice is defined by the length of each of three unit cell
edges (called the axes) and the angles between the edges.
• The simplest and most regular lattice is a cubic.
14

• It is characterized by axes that are all of equal length and meet at 90-degree
angles.
• Each sphere represents the positions of the atoms. Their positions are located
at the points of intersection of three planes, each plane (surface of the cube)
being perpendicular to the other two planes. These planes
are often referred to as crystal planes.
Primitive Cubic Unit Cell
15

• An atom at each corner of the cube and
another atom at the body center of the
cube. This crystal form is called a body
centered cubic cell.
• Atoms at the center of each face of the
unit cell but none at the center of the
cube. This form is called a face-
centered cubic cell.
16

• Substances that do not exhibit the orderly symmetry in their molecular
structure as found in crystalline substances. Often referred to as supercooled
liquids, they possess mechanical properties of both a solid and a liquid
• The temperature at which there is an abrupt increase in thermal expansion
coefficient, indicating increased molecular mobility is called glass transition
temperature.
 NONCRYSTALLINE STRUCTURE
17

• Amorphous noncrystalline substances such as silicate glasses, synthetic
rubber, polystyrene, and other polymers soften into liquids when heated above
their glass transition temperatures, at which the molecules become mobile.
18

ADHESION
• When the molecules of one substrate adhere or are
attracted to molecules of the other substrate, the force
of attraction is called adhesion when unlike molecules
are attracted and cohesion when the molecules
involved are of the same kind.
• The material or film used to cause adhesion is known
as the adhesive, the material to which it is applied is
called the adherend
19

WETTING
 To produce adhesion on any targeted surface, the liquid must flow easily over the
entire surface and adhere to the solid. This characteristic is known as wetting.
 Good wetting results in better capillary penetration and the adhesion, indicating
stronger attraction between the solid and the liquid.
 The ability of an adhesive to wet the surface of the adherend is influenced by a
number of factor
• The cleanliness of the surface
• Surface energy
• Impurity-free metal surfaces.
20

CONTACT ANGLE OF WETTING
 When a solid and liquid make contact, the angle between the liquid surface and
the solid surface is known as contact angle.
 It is dependent on the:
• Surface tension of the liquid
• Surface energy of the solid
21

22

RHEOLOGY
 Rheology is the study of the flow or deformation of matter. It is concerned
with the time-dependent deformation of bodies under the influence of applied
stresses (both magnitude and rate of the stresses), whether the bodies are
solid, liquid, or gases.
 Rheological properties include:
• Flow
• Creep
• Viscosity
• Viscoelasticity
• Thixotropy
23

• The term flow, rather than creep, has generally been used in dentistry to describe
the rheology of amorphous materials such as waxes and resins.
• The flow of wax is a measure of its potential to deform under a small static load
even that associated with its own mass.
 FLOW24

 CREEP
 Creep is defined as a time dependent plastic strain of a material under a static load
or constant stress.
 Because of its low melting range, dental amalgam can slowly creep from a restored
tooth site under periodic sustained stress, such as would be imposed by patients who
clench their teeth.
25

Clinical significance of creep in dental amalgam:
 Creep causes the amalgam to flow such that the unsupported amalgam protrudes
out from margins of the cavity.
 Those unsupported edges are weak and may further weaken by corrosion.
 This causes the formation of a ditch around the margins of amalgam restoration.
26

 VISCOSITY
• Viscosity is the resistance of a liquid to flow.
• Cements & impression material undergoes a liquid- solid transformation in the
mouth.
• Gypsum products used in the laboratory are transformed from fluid slurries
into solid structure & they solidify outside the mouth.
• Amorphous materials such as waxes and resins appear solid but actually are
supercooled liquids that can flow plastically (irreversibly) under sustained
loading or deform elastically (reversibly).
27

 Shear stress (τ)= force / area
 Shear strain rate (ε) = V / d
28

 Viscosity = shear stress
shear rate
 Eg: extrusion of a fluid material from syringe. When the material is
extruded at a constant rate the shear stress is related to pressure required
and shear rate is the flow rate
 To explain viscous nature of some materials ,a shear stress v/s shear
strain rate curve can be plotted.
29

 Rheological behaviors of Four types of fluids
30

NEWTONIAN:
• An ideal fluid demonstrate a shear stress that is proportional to strain rate.
• The plot is a straight line.
• Newtonian fluid has a constant viscosity & exhibits a constant slope of shear
stress plotted against strain rate
PSEUDOPLASTIC :
• Viscosity decreases with increasing strain rate, until it reaches a nearly
constant value.
31

DILATANT:
• Liquid that shows the opposite tendency as of pseudoplastic.
• Liquid become more rigid as rate of deformation increases (shear strain rate)
PLASTIC:
• These fluids exhibit rigid behavior initially & then attain a constant viscosity.
• They behave like rigid body until some minimum value of shear stress is reached.
Eg; ketchup- sharp blow to bottle is usually necessary to produce an initial flow.
32

 THIXOTROPIC BEHAVIOUR:
• A liquid of this type that become less viscous and more fluid under repeated
applications of pressure - Thixotropic
• When low shear rate is applied during spatulation or while an impression is
positioned in tray – material is highly viscous but these materials can also be
used in syringe because at higher shear rate -pass through syringe tip –
viscosity decreases
• Ex. Dental prophylaxis pastes, plaster of Paris, resin cements, and some
impression materials
33

 VISCOELASTIC:
 Materials that have properties dependent on loading rate and exhibit both elastic
and viscous behaviour
 Eg : Alginate, a viscoelastic material, does not exhibit permanent deformation
when loaded quickly but shows a great deal of permanent deformation when
loaded slowly.
34

MECHANICAL PROPERTIES
 Mechanical properties are measured responses, both elastic (reversible on force
removal) and plastic (irreversible on force removal), of materials under an
applied force or distribution of forces.
 These properties are expressed most often in units of stress and strain.
35

 The properties can explained under these categories;
Elastic deformation (reversible)
• Proportional limit
• Resilience
• Modulus of elasticity
Plastic deformation (irreversible)
• Hardness
• Percentage of elongation
Combination of both
• Toughness
• Yield strength
36

• The force per unit area acting on millions of atoms or molecules in a given
plane of a material.
• Force acting per unit area.
STRESS = FORCE
AREA
• Unit of measurement is Megapascal (Mpa).
• Stress is the internal resistance of a material to an external load applied on that
material.
 STRESS37

 By means of the direction of force, stresses can be classified as:
• Tensile stress
• Compressive stress
• Shear stress
• Flexural stress
38

• Tensile stress occurs when 2 sets of forces are directed away from each other in
the same straight line.
• Also when one end is constrained and the other end is subjected to a force away
from the constraint.
• It is caused by a load that tends to stretch or elongate a body.
 TENSILE STRESS39

• Compressive stress occurs when 2 sets of forces are directed towards each other
in the same straight line.
• Also when one end is constrained and the other end is subjected to a force
towards the constraint.
• It is caused by a load that tends to compress or shorten a body.
 COMPRESSIVE STRESS40

 SHEAR STRESS
• Shear stress occurs when 2 sets of forces are directed parallel to each other but
not along the same straight line.
• A shear stress tends to resist the sliding of one portion of a body over another.
• Shear stress can also be produced by a twisting or torsional action on a
material.
41

 FLEXURAL STRESS
• Force per unit area of a material that is subjected to flexural loading
(bending).
• A shear stress tends to resist the sliding of one portion of a body over
another.
• A flexural force can produce all the three types of stresses in a structure, but
in most cases fracture occurs due to the tensile component.
42

 Defined as the change in length per unit original length.
 Strain of a material is reported as percentage(%).
 Strain may be either elastic, plastic or a combination of both elastic and
plastic.
 Elastic strain is reversible. i.e. it disappears when force is removed.
 Plastic strain represents permanent deformation of the material which never
recovers when the force is removed.
 STRAIN
43

 STRESS-STRAIN GRAPH44

YOUNGS MODULUS
• Elastic modulus describes the relative stiffness of a material which is
measured by the slope of the elastic region of the stress strain graph.
• It is the stiffness of a material that is calculated as the ratio of the elastic stress
to elastic strain.
• i.e.. a stiff material will have a high modulus of elasticity while a flexible
material will have a low modulus of elasticity
45

HOOKE`S LAW
• According to this law, within the limits of elasticity the strain produced by a
stress (of any one kind) is proportional to the stress.
• The stress at which a material ceases to obey Hooke's Law is known as the
limit of proportionality.
• The value of the constant depends on the material and the type of stress
46

 POISSON`S RATIO
• During axial loading in tension or compression, there is a simultaneous strain
in the axial and transverse or lateral directions.
• Under tensile loading, as a material elongates in the direction of the load,
there is a reduction in cross- section.
• Under compressive loading, there is an increase in the cross-section.
• Within the elastic range, the ratio of the lateral to the axial strain is called the
Poisson’s Ratio.
47

• Poisson’s ratio is a unit-less value since it is a ratio of 2 strains.
• Most rigid materials such as enamel, dentin, amalgam, composite, etc. exhibit a
poisson’s ratio of about 0.3
• More ductile materials such as soft gold alloys show a higher degree of
reduction in cross-sectional area and higher poisson’s ratio
48

 RESILIENCE
• It is the amount of energy per unit volume that is sustained on loading and
released upon unloading of a test specimen.
• Term resilience is associated with springiness of a material but it means
precisely the amount of energy absorbed within a unit volume of a structure
when it is stressed to its proportional limit.
49

 TOUGHNESS
• It is the ability of a material to absorb elastic energy and to deform plastically before
fracturing. Measured as the total area under a plot of the tensile stress v/s strain.
• It can be defined as the amount of elastic and plastic deformation energy required to
fracture a material.
• Toughness increases with increase in strength and ductility.
• i.e. Greater the strength and higher the
ductility, the greater is the toughness.
50

• It is the mechanical property that describes the resistance of brittle materials to
the propagation of flaws under an applied stress.
• The longer the flaw, the lower is the stress needed to cause fracture. This is
because the stress which would normally be supported by the material are now
concentrated at the tip of the flaw.
• The ability of a flaw to cause fracture depends on the fracture toughness of the
material.
• Fracture toughness is a material property and is proportional to the energy
consumed in plastic deformation.
 FRACTURE TOUGHNESS51

52

• Defined as the magnitude of elastic stress above which plastic deformation occurs.
• As stress is increased, the strain is also increased.
• The material is elastic in nature below the proportional limit.
• The region of the stress – strain curve before the proportional limit is called the
elastic region and the region beyond is called the plastic region.
• When a material is said to have high value of proportional limit, it indicates that
the material is more likely to withstand applied stress without permanent
deformation
PROPOTIONAL LIMIT53

ELASTIC LIMIT
• Defined as the maximum stress that a material will withstand without
permanent deformation.
• For linearly elastic materials, the proportional limit and the elastic limit
represents the same stress within the structure.
55

YIELD STRENGTH
• Defined as the stress at which a test specimen exhibits a specific amount of
plastic strain.
• It is a property that represents the stress value at which a small amount of
(0.1% - 0.2%) plastic strain has occurred.
• It is a property often used to describe the stress at which the material begins to
function in a plastic manner.
• The point at which at the parallel line intersect the stress-strain curve is the
yield strength.
56

• Elastic limit, proportional limit & yield strength are defined differently but their
values are fairly close to each other in many cases.
57

• Ultimate tensile strength/stress (UTS) is defined as the maximum stress that a
material can withstand before failure in tension.
• Ultimate compressive strength/stress (UCS) is the maximum stress that a material
can withstand in compression.
• The ultimate strength / stress is determined by dividing the maximum load in
tension (or compression) by the original cross-sectional area of the test specimen.
ULTIMATE STRENGTH58

• It is the relative inability of a material to sustain plastic deformation before
fracture of a material occurs.
• E.g. amalgam, ceramics, composites are brittle at oral temperatures.
• They sustain no/little plastic strain before they fracture.
• i.e. a brittle material fractures at or near its proportional limit.
• Dental materials with low or 0% elongation such as composite, ceramics, etc will
have little or no burnishability because they have no plastic deformation potential.
 BRITTLENESS59

• It is the ability of a material to sustain a large permanent deformation under a
tensile load up to the point of fracture.
• E.g. a metal can be drawn readily into long thin wire is said to be ductile.
• Ductility is the relative ability of a material to be stretched plastically at room
temperature without fracturing.
• Its magnitude can be assessed by the amount of permanent deformation
indicated by the stress-strain curve.
 DUCTILITY60

• It is the ability of a material to sustain considerable permanent deformation
without rupture under compression, as in hammering or rolling into a sheet.
• Gold is the most ductile and malleable pure metal, followed by silver.
 MALLEABILITY61

• It is the resistance of a material to plastic deformation which is typically
produced by an indentation force.
• In mineralogy, the relative hardness of a material is based on its ability to
resist scratching.
 HARDNESS62

 Various hardness tests include:
• Brinell Test
• Rockwell Test
• Vicker’s Test
• Knoop’s Test
 Selection of the test should be done on the basis of the material being tested.
63

• Used extensively for determining the hardness of metals and metallic materials
used in dentistry.
• Related to the proportional limit and ultimate tensile strength.
BRINELL TEST
• The methods depends on the resistance to the
penetration of a small steel ball, typically
1.6mm diameter when subjected to a specified
load.
64

• This test was developed as a rapid method for hardness determinations.
• Here, instead of a steel ball, a conical diamond point is used.
• The depth of the penetration is directly measured by a dial gauge on the
instrument.
• This test is not suitable for testing brittle materials
ROCKWELL HARDNESS TEST
65

• This test uses a square based pyramidal indenter.
• The impression obtained on the material is a square.
• The method is similar to Knoop’s and Brinell tests.
• The load value divided by the projected area of indentation gives the Vicker’s
Hardness Number (VHN).
• The lengths of the diagonals of the indentations are measured and averaged.
 VICKER’S HARDNESS TEST66

• This test was developed mainly to fulfill the needs of a micro-indentation test
method.
• Suitable for testing thin plastic or metal sheets or brittle materials where the
applied force does not exceed 35N.
• This test is designed so that varying loads may be applied to the indenting
instrument.
• Therefore the resulting indentation varies according
to the load applied and the nature of the material
tested.
KNOOP’S HARDNESS TEST67

70
ATOMIC STRUCTURE ADHESIVE PROPERTIES

71

72

THERMAL PROPERTIES
 Thermal conductivity (κ) is the physical property that governs heat transfer
through a material by conductive flow.
 It is defined as the quantity of heat in calories per second passing through a
material l cm thick with a cross section of 1 cm2 having a temperature difference
of 1 °C and is measured under steady-state conditions.
THERMAL CONDUCTIVITY
73

 The International System (SI) unit or measure for thermal conductivity is watts
per meter per kelvin (W × m−1 × K−1 ).
 In general, thermal conductivities increase in the following order:
polymers < ceramics < metals.
 Materials that have a high thermal conductivity are called conductors, whereas
materials of low thermal conductivity are called insulators.
 The higher its thermal conductivity, the greater the ability of a substance to
transmit thermal energy.
74

 THERMAL DIFFUSIVITY
 Thermal diffusivity measures the rate of transfer of thermal energy within a
substance of nonuniform temperature as it attains thermal equilibrium.
 It is calculated from the thermal conductivity divided by the product of density
and heat capacity.
 In the oral environment, temperatures are not constant during the ingestion of
foods and liquids. For these unsteady state conditions, heat transfer through the
material decreases the thermal gradient. Under such conditions, thermal
diffusivity is important.
75

 Thus for a patient drinking ice water, the low specific heat of amalgam and its
high thermal conductivity suggest that the higher thermal diffusivity favors a
thermal shock situation more than that is likely to occur when only natural tooth
structure is exposed to the cold liquid.
 The low thermal conductivity of enamel and dentin aids in reducing thermal
shock and pulpal pain when hot or cold foods are taken into the mouth.
76

 Defined as change in length per unit of the original length of a material when its
temperature is raised by 1ºC.
 It is denoted by the symbol α and expressed in ppm/0k (10-6/0K).
 A thermal property that is important to the dentist is the coefficient of thermal
expansion (CTE)/ contraction, which relates to the behaviour of a restorative
material relative to the tooth structure during changes in the temperature of the
oral cavity.
 COEFFICIENT OF THERMAL EXPANSION:
77

Significance:
 Close matching of the coefficient of thermal expansion (α) is important between
the tooth and the restorative materials to prevent marginal leakage.
 Opening and closing of gap results in breakage of marginal seal between the
filling and the cavity wall, this breakage of seal leads to marginal leakage,
discoloration & hypersensitivity.
78

ELECTROCHEMICAL PROPERTIES
 Tarnish
 corrosion
ELECTRICAL PROPERTIES
 Electrical conductivity
 Resistivity
 Dielectric constant
79

• The ability to conduct electric current may be stated either as “specific
conductance” or “conductivity” and conversely as the “specific Resistance” or
“resistivity”
• The sensitivity of the tooth structure depends upon the electrical resistance of the
tooth structure. Carious teeth shows less resistance than normal
• Zinc Oxide Eugenol has the highest electrical resistivity.
• Conductivity is important in case of restorative materials ,with GIC being the
most conductive.
 RESISTIVITY & CONDUCTIVITY:80

• A material that provides electrical insulation is known as dielectric.
• The dielectric property of a dental cement increases as it hardens.
• Electrical insulation by cement bases is especially important in case of galvanism.
 DIELECTRIC81

 Is observable as a surface discoloration on a metal or a slight loss or alteration of
the surface finish or luster
 Is often the forerunner of corrosion.
 In oral environment , tarnish often occurs from the formation of hard & soft
deposits on the surface of the restoration.
 TARNISH
ELECTROCHEMICAL PROPERTIES
82

 It is a process in which deterioration of a metal is caused by reaction with its
environment.
 Disintegration of a metal by the action of corrosion may occur due to moisture,
atmosphere, acid or alkaline solutions, & certain chemicals.
 Corrosion may cause mechanical failure of a structure
 CORROSION83

Classification of corrosion
Chemical / dry corrosion
Metal and non-metals
Electrochemical /wet corrosion
Dissimilar metals
Heterogeneous surface
composition
Stress corrosion
Concentration cell
corrosion
84

 Direct combination of metallic and non metallic elements to yield a chemical
compound through oxidation reactions
 Eg: discoloration of silver by sulfur – silver sulfide
 Called dry corrosion – absence of water/ electrolyte
CHEMICAL CORROSION:85

 Requires water or electrolyte and a pathway for the transport of electrons
 Also called wet corrosion
 GALVANIC CORROSION / ELECTROCHEMICAL CORROSION:
86

Mechanism:
 Electrochemical cell- 3 essential component
 An anode, a cathode & an electrolyte.
 An apparatus is employed to measure the voltage & current between 2 electrode.
 Anode: is the surface or site where positive ions are formed, i.e. metal surface
undergoing an oxidation reaction & corroding with the production of free
electrons.
M0 M+ + e-
87

88

 Cathode: a reduction reaction , consume free electrons.
 Numerous possibilities exist:
M+ + e- M0
2H+ + 2e- H2
2H2o + O2 + 4e- 4(OH)-
 Electrolyte supplies the ions needed at the cathode & carries away the
corrosion products at the anode.
 External circuit serves as a conduction path to carry electrons from anode to
cathode.
89

 An electrical potential difference that is voltage (V) can be measured –
importance capable of producing a physiological sensation – pain
 For electrochemical corrosion to be an ongoing process, the production of
electrons at the anode must be exactly balanced by the consumption of e- at the
cathode.
 Electromotive series of metal, which classifies the metal by their equilibrium
values of electrode potential, thereby arranging them in the order of their
dissolution tendencies in water
 If two metals form a galvanic cell , the one with the lower electrode potential in
series becomes the anode & undergoes oxidation.
90

 DISSIMILIAR METALS:
 An important type of electrochemical corrosion occurs when combination of
dissimilar metals are in direct physical contact
 Effect of galvanic shock is well known in dentistry
 Eg amalgam restoration is placed on the occlusal surface of a lower tooth
directly opposing a gold inlay in an upper tooth
 Both restorations are wet with saliva , an electrical circuit exists, with a
difference in potential between the dissimilar restorations.
91

 When the two restorations are brought
into contact , there is a sudden short –
circuit through the two alloys, which
result in patient experiencing a sharp
pain – galvanic shock
92

 A current is present even in a single isolated metallic restoration, although it is
less intense.
 In this situation two electrolytes, saliva & tissue fluids provides the means for
completing the circuit.
 Coating with varnish tends to eliminate galvanic shock.
93

• Commercial dental alloys generally contain more than 3 elements , they have
complex microstructures - that result in even more heterogeneous surface
composition
• Corrosion resistance of multiphase alloys is generally less than that of a single-
phase solid solution.
• Eg alloy containing a 2 phase eutectic microstructural constituent is immersed in
an electrolyte, the lamellae of the phase with the lower electrode potential are
attacked & corrosion results.
HETEROGENOUS SURFACE COMPOSITION94

• Homogenized solid solution is also susceptible to corrosion at the grain
boundaries ,because atomic arrangements at grain boundaries are less regular
& have higher energies
• Solder joints between dental alloys also corrode – difference in composition
• Impurities in alloys enhance corrosion – segregated at grain boundaries
95

 STRESS CORROSION
 Imposition of stress increases the internal energy of an alloy , either through
the elastic displacements of atoms or the creation of microstrain fields
associated with dislocations, the tendency to undergo corrosion will be
increased.
 For metallic dental appliances , deleterious effects of stress & corrosion called
stress corrosion are most likely to occur during fatigue or cyclic loading in the
oral environment.
96

 Small surface irregularities ( notches or pits) act as sites of stress concentration
 Any cold working of an alloy by bending, burnishing causes localized
permanent deformation in some parts of the alloys
 Electrochemical cell consisting of the more deformed metal regions (anodic),
saliva & undeformed /less deformed (cathodic) regions, the deformed region
experience corrosion attack.
97

 CONCENTRATION CELL CORROSION
 It occurs whenever there are variations in the electrolytes or in the composition
of the given electrolyte within the system
 Eg accumulation of food debris in the interproximal areas between the teeth,
produces an electrolyte in that area which is different from the electrolyte that is
produced by normal saliva at occlusal surface.
 Electrochemical corrosion of the alloy surface underneath the food debris will
occur.
98

 A similar type of attack occurs from differences in the oxygen concentration
between parts of the same restorations, with the greatest attack at the areas
containing least oxygen.
 Irregularities (pits), region at the bottom of such a concavity has a much
lower oxygen concentration , alloy at the bottom of pit becomes the anode,
alloy surface around the rim becomes the cathode
 Consequently , metal atoms at the base of the pit ionize & go into the solution
,causing pit to deepen .
100

 For this reason ,all metallic dental restorative materials should be polished
 Crevice corrosion an important category of concentration cell corrosion.
 Preferential attack occurs at crevices in dental prostheses or at margins
between tooth structure & restoration from the same causes i.e. changes
in electrolyte & oxygen concentration by food debris & other deposits.
101

PROTECTION
 Certain metals develop a thin ,adherent , highly protective film by reaction
with the environment , such a metal is said to be passive.
 A thin surface oxide forms on chromium , a good e.g.: of a passivating metal,
& stainless steel contain sufficient amounts of chromium to passivate the alloy.
 Tensile stress & certain ions (chloride ions) can disrupt the protective oxide
film leading to corrosion. Titanium also forms a passivating titanium oxide
film.
102

CONCLUSION
 It is very important to know the properties of the materials we use in dentistry,
especially as restorative materials. This will enable us to select a material that
will have properties close to that of natural tooth structure.
103

REFERENCES
1. Kenneth. J. Anusavice, Philips science of dental materials, 11th edition. 2009
elsevier, st louis, Missouri
2. John M. Powers, Ronald L. Sakaguchi, Craig’s restorative dental materials. 13th
edition. 2006 by Elsevier
3. William J. O’Brien, Dental Materials and Their Selection- 3rd edition.
4. Craig’s Restorative Dental Materials – 13th edition
5. Materials Used in Dentistry: S. Mahalaxmi – 2nd Edition.
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