This document discusses the mechanical and physical properties of materials used in clinical and laboratory procedures in prosthodontics. It begins with an introduction to the importance of selecting restorative dental materials based on their properties. It then covers various mechanical properties including stress, strain, modulus of elasticity, Poisson's ratio, strength properties, ductility, malleability and hardness. Various tests for measuring these properties are also described. The document emphasizes the relevance of understanding these properties for applications in prosthodontics.

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Presented by:
Dr. Subuhi Siddiqui
Ist year PG
Department of Prosthodontics 1

2. Contents
• Introduction
• Mechanical properties
• Stress
• Strain
• Modulus of elasticity
• Poisson’s ratio
• Flexibility
• Resilience
• Proportional limit
• Elastic limit
• Yield strength
2
• Diametral tensile strength
• Flexural strength
• Fatigue strength
• Impact strength
• Toughness
• Fracture toughness
• Brittleness
• Ductility
• Malleability
• Hardness

3. • Mastication and stresses
• Physical properties
• Abrasion and abrasion resistance
• Viscosity
• Creep and flow
• Optical properties
• Thermal properties
• Tarnish
• Corrosion
• Electrical properties
• Conclusion
• Bibliography
3

4. Introduction
• The principal goal of dentistry is to maintain &
improve the quality of life of the dental patient.
This requires the placement or alteration of
existing tooth structure; the main challenge has
been the selection & development of good
prosthetic materials that can withstand the
adverse conditions of the oral environment.
• Restorative dental materials are selected by the
dentist on the basis of characteristic physical,
chemical, mechanical, and biological properties
of a material
4

5. 5
Dental Materials and Their Selection, 3rd Ed. (2002),
William J. O'Brien

6. Mechanical
ProPerties
6

7. MECHANICAL PROPERTIES
• Defined by the laws of mechanics i.e. the physical
science that deals with energy and forces and
their effects on the bodies
• It primarily centers around static bodies
7

8. I. Mechanical Properties
• Stress
• Strain
II. Mechanical properties based on elastic
deformation
• Modulus of elasticity
• Poisson’s ratio
• Flexibility
• Resilience
8

9. III. Strength properties
• Proportional limit
• Elastic limit
• Yield strength
• Diametral tensile strength
• Flexural strength
IV. Ductility
V. Malleability
VI. Hardness
• Hardness tests
9
• Fatigue strength
• Impact strength
• Toughness
• Fracture toughness
• Brittleness

10. STRESS
• Force per unit area within a structure subjected
to an external force or pressure
• SI unit: MPa or Psi
10
Stress = Force / Area

11. TYPES
I. Tensile: caused by a load that tends to stretch or
elongate a body. A tensile stress is always
accompanied by tensile strain.
II. Compressive: when a body is subjected to a load
that tends to compress or shorten it.
III.Shear: Produced by a twisting or torsional action
in a material
11

12. 12

13. IV. Flexural (Bending) stress: Force per unit area of
a material subjected to flexural loading.
13
(A) Stresses induced in a three-unit bridge by a flexural force (P). (B) Stresses induced in a two-unit
cantilever bridge. Note that the tensile stress develops on the gingival side of the three-unit bridge and
on the occlusal side of the cantilever bridge.

14. STRAIN
• It is the change in length per unit initial length
• Whenever a stress is present, deformation or strain
is induced
• Stress: Elastic or Plastic
14
Strain = Change in Length/Original
length

15. STRESS-STRAIN CURVE
15

16. Application
• When a clasp arm on a partial denture is
deformed past the elastic limit into the plastic
deformation region, only the elastic strain is
recovered when the force is released
• In fixed prosthodontics, a sticky candy can be
used to remove crowns by means of a tensile
force when patients try to open their mouths.
16

17. • If a force is applied along the surface of tooth
enamel by a sharp edged instrument parallel to
the interface between the enamel and an
orthodontic bracket, the bracket may debond by
shear stress failure of the resin luting agent
17

18. MODULUS OF ELASTICITY
• Also Elastic modulus or Young’s Modulus
• The relative stiffness or rigidity of a material
• Measured by the slope of the elastic region of
the stress-strain graph
• Unit: MPa or GPa ( 1GPa = 1000 MPa)
18
E = Stress / Strain

19. • Material with a steep line – high modulus i.e.
More rigid
• Stiffness of a dental prosthesis can increase by
increasing its thickness, but the elastic modulus
does not change
19

20. • Modulus is a reflection of the strength of the
inter-atomic or inter-molecular bonds
• Stronger the basic attraction forces – greater the
modulus – More rigid/stiff the material
• This property is generally independent of any
heat treatment or mechanical treatment that a
meal or alloy has received but is dependent on
the composition of the material
20

21. • Elastic modulus is a constant
• It is unaffected by the amount of elastic or plastic
stress that is induced in the material.
• It is independent of the ductility of a material
since it is measured in the linear region of the
stress-strain plot, and it is not a measure of its
plasticity or strength.
• Materials with a high elastic modulus can have
either high or low strength values.
21

22. Application
• For inlay or impression material, high modulus of
elasticity is desirable
• A polyether impression material has a greater
stiffness (elastic modulus) than all other
elastomeric impression materials. Thus a greater
force is needed to remove an impression tray
from undercut areas in the mouth.
22

23. DYNAMIC YOUNG’S MODULUS
• Elastic modulus can be measured by a dynamic
method as well as the static techniques
• The velocity of the sound wave and the density of
the material can be used to calculate the Elastic
modulus and Poisson's ratio values
23

24. FLEXIBILITY
• Ability of a material to return to its original form
indicates its elasticity, but the strain taking place
at elastic limit is called flexibility
• Flexibility is bending capacity
• Maximum flexibility is defined as the flexural
strain that occurs when the material is stretched
to its proportional limit.
24

25. RESILIENCE
• Defined as the amount of energy absorbed within
a unit volume of a structure when it is stressed to
its proportional limit
• The resilience of two or more materials can be
compared by observing the areas under the
elastic region of their stress-strain plots
• The material with the larger elastic area has the
higher resilience
25

26. 26
•. As the inter-atomic spacing increases, the
internal energy increases. As long as the stress is
not greater than the proportional limit, this
energy is known as Resilience
• The area bounded by
the elastic region is a
measure of
resilience, and the
total area under the
stress-strain curve is
a measure of
toughness

27. Application
• A proximal inlay might cause excessive
movement of the adjacent tooth if large proximal
strains developed during compressive loading on
the occlusal surface.
27
• Hence, the restorative material should exhibit a
moderately high elastic modulus and relatively
low resilience, thereby limiting the elastic strain
that is produced.

28. POISSON’S RATIO
• Simeon Denis Poisson
• When a tensile force is applied to a cylinder or
rod, the object becomes longer and thinner.
Conversely, a compressive force acts to make the
cylinder or rod shorter and thicker. If an axial
tensile stress, along the long axis of a mutually
perpendicular coordinate system produces an
elastic tensile strain, and accompanying elastic
contractions in the other two directions, the ratio
of the two is called Poisson’s ratio (v)
28

29. 29

30. PROPORTIONAL LIMIT
• It is the maximum stress at which stress is
proportional to strain and above which plastic
deformation occurs
30
• If a straight edge is laid
along the straight-line
portion of the curve from O
to P, the stress value at P,
the point above which the
curved digresses from a
straight line, is known as the
proportional limit.

31. HOOKE’S LAW
• By Robert Hooke (1676)
• Within the elastic limit of a solid material,
the deformation (strain) produced by
a force (stress) of any kind is proportional to the
force. If the elastic limit is not exceeded, the
material returns to it original shape and size after
the force is removed. The force at which the
material exceeds its elastic limit is called 'limit of
proportionality.
31

32. • For a material to satisfy Hooke's law, the elastic
stress must be directly proportional to the elastic
strain. The initial region of the stress-strain plot
must be a straight line
32

33. ELASTIC LIMIT
• Defined as the greatest stress to which a material
can be subjected such that it returns to its
original dimensions when the force is released
• If the load is increased progressively in small
increments and then released after each increase
in stress, a stress value will be reached at which
the wire does not return to its original length
after it is unloaded. At this point the wire has
been stressed beyond its elastic limit
33

34. YIELD STRENGTH
• Defined as the stress at which a material exhibits
a specified limiting deviation from proportionality
of stress to strain
• Used in cases when the proportional limit cannot be
determined with sufficient accuracy
• Yield strength (proof stress) is a the amount of
stress required to produce a predetermined
amount of permanent strain usually 0.1% or 0.2%
which is called the Percent Offset
34

35. Application
• Brittle materials like Ceramics and Composites –
yield strength cannot be measured - the stress-strain
plot is a straight line with no plastic region
• Useful property as it is easier to measure
35

36. DIAMETRAL TENSILE STRENGTH
• Tensile strength is usually determined by
subjecting a load, wire, etc. to tensile loading
(uniaxial tension test)
• For brittle materials – Diametral Compression
test
36

37. METHOD
• A compressive load is placed by a
flat plate against the side of a
short cylindrical specimen or a
disc
• The vertical force produces a
tensile stress along the sides of
the disc that is perpendicular to
the vertical plane that passes
through the centre of the disc
• Fracture occurs along the vertical
plane
• Here the tensile stress is directly
proportional to the compressive
load applied
37

38. FLEXURAL STRENGTH
• Also, Transverse strength or Modulus of rupture
• Ability to bend before it breaks
• It is a measure of how a material behaves when
under multiple stresses
• It is measured by subjecting a beam of the
material to a three or four-point loading which
results in development of compressive stresses
on top of the beam, tensile stresses on the
bottom, and shear stresses on the sides
38

39. • Compressive stresses convert to tensile ones
through the neutral axis along the centre of the
beam
• Usually for dental materials – 3 point bending test
39

40. • For brittle materials such as ceramics, flexure
tests are preferred to the diametral compressive
test
• They more closely simulate the stress
distributions in dental prostheses such as
cantilevered bridges and multiple-unit fixed
partial dentures (FPDs or bridges), and clasp arms
of RPDs
40
Application

41. FATIGUE STRENGTH
• When a stress is repeated a great number of
times, the strength of the material may be
drastically reduced and ultimately cause failure
• Fatigue – defined as progressive fracture under
repeated loading
• Fatigue strength – the stress at which a material
fails under repeated loading
41

42. • Fractures develops progressively over many
stress cycles after initiation of a crack from a
critical flaw and subsequently by propagation of
the crack until a sudden, unexpected fracture
occurs.
• This phenomenon is called Fatigue failure
• Types: Static fatigue failure and Dynamic fatigue
failure
42

43. Failure under repeated/cyclic loading is thus
dependant on:
1.Magnitude of the load
2.Number of loading cycles
• For some materials stress can be loaded infinite
number of times without failure – Endurance
limit
43

44. Application
• A rough brittle material would fail in fewer cycles
of stress than a smooth material
• For a given flaw size, less stress is required to
produce failure if the stress is dynamically cycled
between high and low values
• Aqueous solutions corrosively degrade dental
ceramics by converting surface flaws to one or
more cracks over time in the presence of tensile
stress
44

45. IMPACT STRENGTH
• Impact – describe the reaction of a stationary
object to a collision with a moving object
• Impact strength – defined as the energy required
to fracture a material under an impact force
• A material with low elastic modulus and high
tensile strength is more resistant to impact forces
45

46. Tests:
1. Charpy-type impact tester
2. Izod impact tester
• Greatest resilience to impact is associated with the
composites, followed in decreasing order by the
porcelain, PMMA, amalgam, and alumina
46

47. TOUGHNESS
• Defined as the amount of elastic and plastic
deformation energy required to fracture a
material
• Toughness depends on strength and ductility –
Greater the strength, higher the ductility, greater
the toughness
47

48. 48
• Toughness is indicated as the total area under
the stress-strain graph from zero stress to the
fracture stress
• A tough metal may be strong, but a strong metal
may not be tough

49. • A measure of the resistance of a material to
failure from crack propagation in tension
• Given in units of stress times square root of crack
length
49
Fracture toughness

50. BRITTLENESS
• Relative inability of a material to sustain plastic
deformation before fracture of a material occurs
• Eg. Amalgam, ceramics and composites are
brittle at oral temp. – 5-55 degrees Celsius
• Materials that are brittle usually have a very
ordered atomic structure which does not permit
the easy movement of dislocations. Hence, they
are brittle.
50

51. • Brittle materials fracture at or near their
proportional limit
• Brittle materials are not necessarily weak
• Brittle materials are sensitive to internal
flaws/cracks/voids and do not respond well to
tensile or bending forces because these forces
tend to propagate flaws/voids/cracks
• They do well under compressive forces because
they tend to close cracks
51

52. Application
• Dental materials with low or zero percent
elongation, including amalgams, composites,
ceramics, and non-resin luting agents, will have
little or no burnishability, because they have no
plastic deformation potential.
52

53. DUCTILITY
• Ability of materials to sustain a large permanent
deformation under a tensile load before it
fractures
• Gold is the most ductile metal, followed by Silver
and Platinum
53

54. • Increase in temperature -> decrease in ductility
(material’s strength decreases with increase in
temp)
• Ductility is used in dentistry as a measure of
Burnishability
• Burnishability index is defined as the percentage
elongation divided by yield strength
• Therefore, greater the ductility and lower the
yield strength, greater the burnishability
54

55. MEASUREMENT OF DUCTILITY
Three methods:
1. Reduction in area of tensile stress specimen
2.Maximum number of bends performed in a cold
bend test
3. Percent elongation after fracture
55

56. MALLEABILITY
• Ability of material to sustain permanent
deformation without rupture under compression
as in hammering or rolled into a sheet
• Gold is most malleable, followed by Silver and
Aluminum
• Increase in temperature -> increase in
malleability
– Because malleability is dependent on dislocation
movement, and dislocations generally move more
easily at higher temperatures
56

57. HARDNESS
• In mineralogy, the relative hardness of a
substance is based on its ability to resist
scratching
• In metallurgy, and in most other disciplines, the
concept of hardness is the "resistance to
indentation
• Properties related to the hardness of a material
are compressive strength, proportional limit, and
ductility. 57

58. HARDNESS TESTS
58

59. BRINELL HARDNESS TEST
• Small, hardened steel ball
• Indentation – ROUND
• Diameter of the indentation is measured
• Disadvantage: cant be used for brittle materials
59

60. ROCKWELL HARDNESS TEST
• Ball or a metallic cone indenter
• Depth of indentation is measured
• Disadvantage : cant be used for brittle materials`
60

61. VICKERS HARDNESS TEST
• Pyramid shaped diamond with a square base
• Diagonals of the square-shaped indentation for
measurement
Advantage :
• Testing very small specimens
• Used on both hard and soft materials
61

62. 62

63. KNOOP HARDNESS TEST
• Rhombo-hedral pyramid diamond
• Indentation – Rhombic
• Length of the largest diagonal is measured
• Ideal for elastic materials
Advantages:
• Same as Vicker’s test
63

64. SHORE &BARCOL HARDNESS TEST
• Spring loaded metal indenter point
• Measure the hardness of rubber and plastics
64

65. MASTICATION FORCES AND
STRESSES
• The average maximum sustainable biting force is
approximately 756 N (170 pounds)
• Varies markedly from one area of the mouth to
another and from one individual to another
• Higher for males than for females and is greater in
young adults than in children
65

66. • Enamel is a brittle substance with a comparatively
high modulus of elasticity, low proportional limit in
tension, and low modulus of resilience.
• The modulus of resilience of dentin is greater than
that of enamel thus, it is better able to absorb
impact energy
• Molar region- 400 to 830 N (90 to 200 pounds)
Premolar- 222 to 445 N (50 to 100 pounds)
Cuspid region- 133 to 334 N (30 to 75 pounds)
Incisor region- 89 to 111 N (20 to 55 pounds)
66

67. Physical
ProPerties
67

68. • Physical properties are based on the laws of
mechanics, acoustics, optics, thermodynamics,
electricity, magnetism, radiation, atomic structure,
or nuclear phenomena
• Includes:
1. Abrasion and abrasion resistance
2. Viscosity
3. Creep and flow
4. Optical
5. Thermal
6. Tarnish and corrosion
7. Electrical 68

69. ABRASION AND ABRASION
RESISTANCE
• In oral cavity, abrasion is a complex mechanism,
with interaction of numerous factors
• Hardness can be used to compare similar
materials but invalid for dissimilar materials.
69

70. • For example, abrasion of enamel of a tooth
opposing a ceramic crown is affected by:
• Bite force
• Frequency of chewing
• Abrasiveness of the diet
• Composition of liquids
• Temperature changes
• Surface roughness
• Physical prop. Of materials
• Surface irregularities
70

71. VISCOSITY
• Resistance of a liquid to flow
• Rheology: is the study of deformation and flow
characteristics of matter
• Most liquids when placed in motion, resist
imposed forces that cause them to move because
of internal frictional forces within the liquid
• Thus, viscosity is the measure of consistency of a
fluid and its inability to flow
71

72. • Ex. Liquid occupying space between two plates –
Lower plate fixed and upper plate being moved
to right with a velocity V, Force F is required to
overcome viscosity
72

73. SHEAR STRESS Vs SHEAR STRAIN RATE
CURVE
• Used to explain viscous nature of some material
• Based on rheologic behavior, fluids can be
classified as
• Newtonian
• Pseudoplastic
• Dilatant
• Plastics
73

74. NEWTONIAN BEHAVIOR
• An “ideal” fluid
• Shear stress is proportional to the strain rate and
thus the plot is a straight line
• Slope is constant
• Exhibits constant viscosity
74

75. PSEUDOPLASTIC
• Viscosity decreases with increase in strain rate till
it reaches constant value
• Eg. Rubber base impression materials
75
DILATANT
• Opposite to pseudoplastic behavior
• Viscosity increases with increase in shear strain
rate

76. 76
PLASTICS
• They behave like rigid body until some minimum
value of shear stress is reached
• This minimum value is called offset
• Exhibits rigid behavior initially, and then attains
constant viscosity
• Eg. Ketchup in a bottle

77. THIXOTROPIC MATERIALS
• A liquid that becomes less viscous and more fluid
under repeated application of pressure
• Eg. Dental prophylactic paste, plaster of Paris,
some impression materials
• Thixotropic property is often confused with
pseudoplasticity
• A thixotropic material will not flow until sufficient
energy is applied in the form of impact force to
overcome its yield strength. Beyond this point,
the material becomes very fluid
77

78. • Dental prophylactic paste – will not flow out of
rubber cup until it is rotated against the teeth to
be cleaned
• Impression materials – does not flow out of the
tray until placed over dental tissues which is
beneficial for mandibular impressions
• Plaster or Paris – if stirred rapidly and viscosity is
measured, the value is lower that the value for a
sample that’s left undisturbed, due to thixotropic
property
78
Application

79. • High viscous fluids flow slowly as compared to
low viscous fluids
• Ex. Zinc polycarboxylate cement & resin cements
compared with zinc phosphate cement
Viscosities of materials:
• Tempered agar hydrocolloid impression material
- 281,000 cP at 45 degree Celsius
• Light-body polysulfide -109,000 cP at 36 degrees
Celsius
• Heavy-body - 1,360,000 cP at 36 degrees Celsius
79

80. CREEP AND FLOW
• Creep is defined as time dependent plastic strain
of a material under a static load or a constant
stress
• A metal held at a temp near its melting point ->
subjected to constant stress -> increase in strain
over time
• Eg. Creep of amalgam
80

81. • Flow is generally used in dentistry to describe the
rheology of amorphous materials such as waxes
• Flow of a wax is the measure of its potential to
deform under a small static load
81

82. Application :
• Some amorphous materials such as waxes and
resins appear solid but they are super-cooled
liquids that can flow, plastically (irreversibly) with
sustained loading, or elastically (reversibly) with
small stresses
82

83. MEASUREMENT OF CREEP
• Although creep or flow can be measured under
any type of stress, compressive stress is used in
testing dental materials
• Cylinder of particular dimension subjected to
given compressive stress for a specific time and
temperature - % decrease in length gives creep
and flow
83

84. OPTICAL PROPERTIES
Color
• The perception of color is a physiological
response to a physical stimulus (light).
• Light is electromagnetic radiation that can be
detected by human eye.
• The visible electromagnetic radiation ranges from
400 –700 nm
84

85. Three dimensions of color
• Hue : describes the dominant color of an object.
Ex. Red, green, blue
• Chroma : represents the degree of saturation of a
particular hue. The higher the chroma, the more
intense the color.
Ex. The yellow color of the lemon is more “vivid”
than that of a banana (“dull” yellow)
85

86. • Value : identifies the lightness or darkness of a
color
Ex. Yellow of a lemon is lighter than is the Red of
a cherry
• A tooth of low value appears gray and non–
vital=DEAD, therefore, it is the most important
parameter. Because it is intimately related to the
aspect of vitality in human teeth
86

87. Primary colors:
• Blue, green and red
• Combining suitable proportions of wave lengths of
the three primary colors results in white.
Secondary colors:
• Combination of two primary colors, e.g . green and
red gives yellow, blue and red gives magenta
Complementary colors:
• Two colors are complementary to each other when
their combination results in white e.g. yellow is the
complementary color of blue
87

88. Factors affecting color appearance & selection
1. Pigmentation – Esthetic effects are sometimes
produced in a restoration by incorporating
colored pigments in non-metallic materials such
as resin composites, denture acrylics, silicone
maxillofacial materials, and dental ceramics
• Inorganic pigments used because they are more
permanent
88

89. 2. Metamerism - The color of objects under one
type of light may appear different under another
light source. This phenomenon is called
Metamerism
3. Fluorescence – it is the emission of luminous
energy by a material when a beam of light is
shone on it. Natural tooth structure absorbs light
at wavelengths too short i.e. not visible to the
human eye; between 300 and 400 nm (near-ultraviolet
radiation).
89

90. 4. Opacity - is the property of materials that
prevents the passage of light. An opaque material
either absorbs or reflects all of the light. When all
light is absorbed the material appears black.
• Eg. If red, orange, yellow, blue, and violet are
absorbed, the material appears green in reflected
light
90

91. 5. Translucency - is the property of materials that
allow the passage of some light but disperses
(scatters or reflects) the rest so that objects
cannot be clearly seen through them e.g. dental
porcelain, composite resin and pigmented acrylic
resin
6. Transparency - is the property of materials that
allow the passage of light in such a manner that
objects can be clearly seen through them.
eg. glass and clear acrylic resin
91

92. Propagation of light
• REFLECTION - light is reflected in all directions,
this is called Diffuse reflection. Rough surfaces
undergo diffuse reflection when the roughness
have dimension larger than the wavelength of
the reflected wave.
• Smooth surfaces reflect light in one direction
only, this is called Specular reflection. Highly
polished surfaces (mirrors) reflect all light in one
direction where the angle of incidence equals the
angle of reflection.
92

93. • REFRACTION - As light becomes incident on a
surface separating two different media, a part is
reflected and a part is refracted (i.e. there is a
change in the direction of light as it enters the
other medium). The ratio of the angle of
incidence and sine angle of refraction is a
constant called “Refractive index”
93

94. • SCATTERING - This occurs when light passing
through an optical medium redirects some of the
radiation into direction other than that of the
beam. Thus, the original beam is weakened by
scattering in a direction away from the observer’s
eye.
• As scattering of light increases, the body appears
more dull and opaque.
• TRANSMISSION - light passing through an optical
medium without attenuation is said to be completely
transmitted. Total transmission is undergone by
perfectly transparent material
94

95. THERMAL PROPERTIES
Temperature
• Measured with thermometer or thermocouple
• The increase in temperature during cutting of
tooth structure with various types of carbide
burs, rotary diamond is of importance
95

96. Heat of Fusion (L)
• Is the heat in calories, or joules (J), required to
convert 1g of material from solid to liquid state
at the melting temperature
L= Q/m
where, Q = total heat absorbed
m = mass of the substance melted
• Therefore, the larger the mass of the material
being melted, the more heat required to change
the total mass to liquid
96

97. Thermal conductivity
• Thermal conductivity is a thermophysical
measure of how well heat is transferred through
a material by conductive flow
• Materials with high thermal conductivity are
called conductors
• Materials of low thermal conductivity are called
insulators. (e.g.) Enamel, Dentin, etc
97

98. • SI unit is watt per meter per second per degree
Kelvin (W x m-1 x s-1 x K-1)
• Clinical application: High conductivity of metal
compared to resin composites induces greater
pulpal sensitivity, as a negligible, mild, moderate
or extreme discomfort
98

99. Specific Heat
• The specific heat, Cp, of a substance is the
quantity of heat needed to raise the temperature
of 1gm of the substance 1 degree Celcius.
• Specific heat of liquids is higher than those of
solids
• Application : During melting and casting process
99

100. Thermal Diffusivity
• It is a measure of the rate at which a body at
non-uniform temperature reaches a state of
thermal equilibrium
• 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
• SI unit is square meter per second.
100

101. • For a given volume of material, the heat required
to raise the temperature a given amount
depends on its heat capacity (specific heat) and
the density
• When the product of heat capacity and density is
high, the thermal diffusivity may be low, even
though the thermal conductivity is relatively high.
101

102. Application
• eg,. 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 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
102

103. • Insert thermal insulator between metallic
restorations and tooth structure to prevent
pulpal damage
• Artificial teeth in a denture base are a poor
thermal conductor -> prevents heat exchange
between soft tissues and oral cavity -> Loss of
sensation of hot and cold during eating and
drinking
103

104. Coefficient of Thermal Expansion
• Defined as the change in length per unit of the
original length of a material when its
temperature is raised 1 degree Kelvin
• Important to the dentist
104

105. Application
• A tooth restoration may expand or contract
more than the tooth during a change in temp,
thus there may be marginal microleakage or the
restoration may debond from the tooth
• A wax pattern that fits a prepared tooth
contracts significantly when it is removed from
the tooth or a die in a warmer area and then
stored in a cooler area. This dimensional change
is transferred to a cast restoration that is made
from the lost-wax process
105

106. • Denture teeth set in denture base wax in a
relatively warm laboratory may shift appreciably
in their simulated intraoral positions after the
denture base is moved to a cooler room before
the processing of a denture
• Metal-ceramic restorations : porcelain veneer
fired to a metal substrate (coping). It may
contract to a greater extent than the metal
during cooling and induce tangential tensile
stresses or tensile hoop (circumferential) tensile
stresses in the porcelain causing immediate or
delayed crack formation
106

107. TARNISH
• The process by which a metal surface is dulled or
discolored when a reaction with a sulfide, oxide,
chloride, or other chemical causes a thin film to
form
(or)
• Tarnish is observable as a surface discoloration
on a metal, or as a slight loss or alteration of the
surface finish or luster
“Tarnish is often a forerunner of corrosion”
107

108. • Formation of hard and soft deposits
Soft deposits > plaque, films of bacteria, mucin,
stains from pigment producing bacteria, drugs
containing iron or mercury, absorbed food debris
Hard deposits > Calculus
Also thin films of oxide, sulfides and chlorides
108

109. CORROSION
• Chemical or electrochemical process in which a
solid usually a metal, is attacked by an
environmental agent, resulting in partial or
complete dissolution
(or)
• It is a process in which deterioration of metal is
caused by reaction with its environment and is
not merely a surface deposit
109

110. Disintegration of a metal by corrosion in the mouth
may be due to :
• Warmness and moistness
• Fluctuations in temperature
• Ingested foods with a wide range of pH
• Acids liberated from localized attachment of
debris
110

111. • A tarnish film may in time accumulate elements
and compounds that chemically attack metal. Eg.
Egg (sulfur) corrode Ag, Cu, Hg etc
• Oxygen and chloride cause corrosion of Amalgam
111

112. CLASSIFICATION OF CORROSION
1. Chemical / Dry corrosion
2. Electrochemical / Wet corrosion
1. Galvanic corrosion
2. Stress corrosion
3. Concentration cell corrosion
112

113. Electrochemical corrosion
• Requires presence of water or fluid electrolyte
and also a pathway for transport of electrons
• Eg. Corrosion of alloys like surgical stainless steel
Chemical corrosion
• No fluid or water as electrolyte
113

114. Galvanic corrosion
• Presence of metallic fillings in the mouth cause
galvanic action or galvanism
• Results from a difference in potential between
dissimilar fillings in opposing or adjacent teeth
114

115. 115
Metallic fillings + saliva =
electric cell
Fillings contact
Cell is short-circuited
Flow of current through
pulp; pain; more anodic
restoration corrodes

116. Stress corrosion
• Imposition of stresses increases internal energy
of an alloy through elastic displacement of atoms
or creation of micro strained fields associated
with dislocation then the tendency to undergo
corrosion increases called stress corrosion
• during fatigue or cyclic loading
• Eg. Failure of RPD framework due to cyclic
loading
116

117. Concentration cell corrosion
• Occurs whenever there are variations in the
electrolytes or in the composition of the given
electrolyte within the system
• Eg. difference in electrolyte composition
contacting restoration on proximal and occlusal
surface
117

118. • Similar type of corrosion occurs due to difference
in oxygen concentration between parts of same
restoration
• Eg. pits in restoration
• Deepest portion of pit – Low oxygen conc.
Because of debris -> ANODE
• Alloy surface around rim of the pit -> CATHODE
118

119. • CREVICE CORROSION: corrosion at the junction
of tooth and restoration because of presence of
food debris causing changes in oxygen
concentration and change in electrolyte
119

120. Electromotive force
• The electromotive series is a listing of electrode
potentials of metals according to the order of
their decreasing tendency to oxidize in solution
• Serves as the basis of comparison of the
tendency of metals to oxidize in air
• Metals with large negative electrode potential
are more resistant to tarnish than those with a
high positive electrode potential
• Eg. Metals above copper series (Al, Zn, Ni) oxidize
easily
Below copper series (Ag, Au) resist oxidation
120

121. PROTECTION AGAINST CORROSION
• When dissimilar metals in contact – Painting a
non-conductive film
• Metallic and non-metallic coatings over gold
alloys restorations are ineffective because;
–Were too thin
– Incomplete
– Did not adhere to metal
–Were readily scratched
–Were attacked by oral fluids
121

122. PASSIVATION
• Certain metals develop a thin, adherent, highly
protective film by reaction with environment
called Passivation
• Ex, Passivation of iron with chromium,
Passivation of titanium by titanium oxide
formation
• Chromium passivated metals – susceptible to
stress corrosion and pitting corrosion and certain
ions like chloride will disrupt protective layer
122

123. CORROSION OF DENTAL RESTORATIONS
• Corrosion resistance is very important
consideration in dental alloys because release of
corrosion products affect biocompatibility
• At least 50% atoms should be noble metals
• Palladium – prevents sulfur tarnishing of silver
alloys
• Base metal alloys are susceptible to tarnish with
chloride
123

124. CLINICAL SIGNIFICANCE OF GALVANIC
CURRENTS
• Many base materials below restorations lose the
property of insulation when they become wet
through microleakage or dentinal fluid
• Practical method of eliminating galvanic currents
– application of tarnish
• It has been suggested that galvanic currents may
account for many types of dyscrasias such as
lichenoid reactions, ulcers, leukoplakia, cancer,
and kidney disorders but research has failed to
find correlation
124

125. Electrical properties
Electrical conductivity and resistivity
• The ability of a material to conduct an electric
current is specific conductance or conductivity,
or, conversely, the specific resistance or
resistivity
• Study the alteration in internal structure of
various alloys as a result of heat treatment
• Sound enamel is a relatively poor conductor of
electricity than dentin
125

126. • Resistivity: helps in investigation of pain
perception threshold resulting from applied
electric stimuli
• Less electric resistance offered by carious tooth
as compared to normal tooth
126

127. Dielectric constant
• A material that provides electrical insulation is
known as a dielectric
• Varies with temperature, bonding, crystal
structure, and structural defects of the dielectric
• Dielectric constant of a dental cement decreases
as the material hardens. This decrease reflects a
change from a paste that is relatively ionic and
polar to one that is less so
127

128. CONCLUSION
• With the latest advance in material aspect, there
is emergence of many materials in dentistry.
• The complete understanding of various
properties of the materials can make out the
suitable material of choice. And this will
definitely lead to better quality of treatment.
128

129. Bibliography
• Restorative dental materials, 11th Ed., Robert G. Craig and
John M. Powers
• Phillip’s Science of dental materials, 11th Ed.
• Dental Materials and Their Selection, 3rd Ed. (2002),
William J. O'Brien
• Mechanical properties of dental restorative materials:
relative contribution of laboratory tests; J Appl Oral Sci
2003; 11(3): 162-7
• Dental Materials Science; Dr. Graham Cross 2.11.2007
• Internet
129

130. 130