physical and mechanical properties

1. PHYSICAL AND MECHANICAL
PROPERTIES OF DENTAL
MATERIALS
DR. ABHIDHA TRIPATHI
MDS 1ST YEAR

2. CONTENTS
1. INTRODUCTION
2. PHYSICAL PROPERTIES
 ABRASION AND ABRASION RESISTANCE
 CREEP AND FLOW
 THERMAL CONDUCTIVITY AND DIFFUSIVITY
 VISCOSITY
 TARNISH AND CORROSION
 COLOR AND OPTICAL EFFECTS
3. MECHANICAL PROPERTIES
 STRESSES AND STRAIN
 ELASTIC PROPERTIES
 STRENGTH PROPERTIES
 DUCTILITY & MALLEABILITY
 FRACTURE TOUGHNESS AND BRITTLENESS

3. INTRODUCTION
 Properties of materials can be divided into three categories : physical,
mechanical and biological.
 Physical properties are based on laws of physics that describe mass,
energy, force, light, heat, electricity, color and thermal conductivities.
 Mechanical properties are defined as materials ability to resist forces.
Mechanical properties are dependent on the amount of material and on
the size and shape of the object. Examples are strength and stiffness.
 Biological properties of the material are effects the materials have on
living tissue. For example a crown should not irritate the gingiva, tongue
or buccal mucosa.

4. PHYSICAL PROPERTIES
 Physical properties are based on laws of mechanics,
thermodynamics, optics, atomic structure.
 Physical properties based on laws of mechanics are Creep
and Flow, Abrasion and Abrasion Resistance, Viscosity
 Physical properties based on thermodynamics are
thermal conductivity, thermal diffusivity and coefficient of
thermal expansion
 Electrochemical properties are tarnish and corrosion.

5. VISCOSITY
 Viscosity is the resistance of a fluid to flow.
 A force (F) is required to overcome the frictional resistance within the fluid (i.e., the
viscosity) and cause the fluid to flow at a given velocity (V).
 As the shear force F increases, V increases, and a curve can be obtained for force versus
velocity.
 Curves depicting shear stress versus shear strain rate are used to characterize the viscous
behavior of fluids.

6. VISCOSITY
 NEWTONIAN FLUID : Constant fluid
 An ideal fluid
 Shear stress proportional to strain rate
 Straight line on curve
 Eg : water, zinc phosphate
 PSEUDOPLASTIC FLUID :
 Viscosity decreases with increasing strain rate, until
it reaches a nearly constant value.
 Eg : Monophase elastomeric impression materials

7. VISCOSITY
 DILATANT FLUID : Viscosity increase with increasing stress.
 They become more rigid under stresses
 Eg : Acrylic denture base, material, sand in water
 PLASTIC FLUID : Material behaves rigid until a minimum of stress is
applied, then it starts behaving like Newtonian fluid.
 Eg : Clay suspension, composite material

8. ABRASION AND ABRASION
RESISTANCE
 Abrasion is the process of scraping or wearing
 Abrasion resistance: It is the ability of material and structures to withstand
abrasion. It is the method of wearing down or rubbing away by means of a
friction.
 CLINICAL SIGNIFICANCE : Excessive wear of natural teeth that opposes a
denture porcelain teeth will occur.

9. CREEP AND FLOW
 Creep is defined as the time dependent
plastic strain of a material under a static
load or constant stress.
 Dental amalgams because of its low
melting range, can undergo creep at a
restored tooth site under periodic
sustained stress, such as would be
imposed by patients who clench their
teeth.
 The term creep implies a relatively small
deformation produced by a relatively
larger stress over a long period of time.

10. CREEP AND FLOW
 Flow implies a greater deformation produced more rapidly with a smaller applied
stress.
 The term flow, has generally been used in dentistry to describe the rheology of
amorphous materials such as waxes.
 The flow of wax is a measure of its potential to deform under a small static load, even
that associated with its own mass.

11. THERMAL CONDUCTIVITY
 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 l K (1
°C) and is measured under steady-state
conditions in which the temperature gradient
does not change.

12. THERMAL DIFFUSIVITY
 Thermal diffusivity is a measure of the speed with which a temperature
change will spread through an object when one surface is heated.
 A material with a high density and high specific heat will likely have a low
thermal diffusivity. Such a material changes its temperature very slowly.
Low heat capacity and high thermal conductivity lead to high diffusivity,
and temperature changes transmit rapidly through the material.

13. TARNISH AND CORROSION
 Tarnish is a surface discoloration on a
metal or a slight loss or alteration of
the surface finish or luster.
 Tarnish also arises from the formation
of thin films, such as oxides, sulfides,
or chlorides.
 In the oral environment, tarnish often
occurs from the formation of deposits
on the surface of a restoration.

14. TARNISH AND CORROSION
 Corrosion is a process whereby deterioration
of a metal is caused by reaction with its
environment.
 Corrosion can cause severe and catastrophic
disintegration of metals.
 Corrosive disintegration can take place
through the action of moisture, atmosphere,
acid or alkaline solutions, and certain
chemicals.
 Corrosion of a metal is either a chemical or an
electrochemical process, in each of which the
first step is the loss of an electron

15. CLASSIFICATION OF CORROSION

16. DRY CORROSION
 In which there is direct combination of
metallic and non metallic elements.
 Electrolytes are absent
 Examples are Oxidation, Halogenation
reactions

17. WET CORROSION
 Corrosion occurs in presence of water or some
other liquid electrolyte.
 Also known as galvanic corrosion or
electrochemical corrosion.
 Electrochemical corrosion is seldom isolated
and almost invariably is accompanied by
chemical corrosion.

18. COLOR AND OPTICAL EFFECTS
 Esthetics is an important goal to be reached in dentistry i.e. the color and
appearance of natural dentition.
 Light from an object that is incident on the eye is focused in the retina and
is converted into nerve impulses, which are transmitted to the brain. Cone-
shaped cells in the retina are responsible for color vision.
 Electromagnetic radiation in the visible region interacts with an object
through reflection from its surface, absorption, refraction, or transmission.
 These phenomena determine the opacity, translucency, or transparency of
an object

19. NATURE OF AN OBJECT UNDER VIEW
 The nature of the restorative material, or that of any object under view,
determines how that object will appear.
 Electromagnetic radiation in the visible region interacts with an object
through reflection from its surface, absorption, refraction, or transmission
(i.e, by passing through unchanged.) These phenomena determine the
opacity, translucency, or transparency of an object.
 The opacity of a material is related to the amount of light it can absorb
and/or scatter.
 For example, if 1-mm thicknesses of each of two materials absorb 20% and
50%, respectively, of the light passing through them, the former is less
opaque or more translucent than the latter

20. INTERACTION OF EMITTED LIGHT
WITH AN OBJECT
 REFLECTION
 REFRACTION
 ABSORPTION
 TRANSMISSION
 DIFFRACTION
 SCATTERING

21. REFLECTION
 Reflected light rays are those rays which bounce back from the surface of
the object being hit instead of being transmitted or absorbed.
 Two types
 Specular reflection
 Diffuse reflection

22. TYPES OF REFLECTION

23. REFRACTION
 Change in direction of propagation of any wave as a result of its travelling
at different speeds at different points along the wave front

24. ABSORPTION
 Absorption is the process by which a matter takes up the energy of a
electromagnetic radiation
Object
Absorb energy of light
Excite electrons to higher energy
arrangement
Then electrons come back to stable
energy position giving out heat

25. ABSORPTION
 A material will absorb only a particular wavelength

26. THREE DIMENSIONS OF COLOR
 Color perception is described by three objective variables: hue, value, and chroma.
These three parameters constitute the three dimensions of “color space”.
 Hue: The dominant color of an object, for example red, green, or blue. This refers
to the dominant wavelengths present in the spectral distribution.
 Value: Value is also known as the gray scale. It is the vertical, or Z-axis. Value
increases toward the high end (lighter) and decreases toward the low end (darker).
Value is also expressed by the “lightness” factor
 Chroma: Chroma is the degree of saturation of a particular hue. For example, red
can vary from “scarlet” to light pink, where scarlet has a high saturation and pink
has a low saturation.

27. THREE DIMENSIONS OF COLOR
HUE
CHROMA
VALUE
Sikri VK. Color: Implications in dentistry. J Conserv Dent. 2010;13(4):249-255.

28. MEASUREMENT OF COLOR


29. CIELAB SYSTEM (CIE L* A* B*)
 Represents quantitative relationship of colours on
three axis
 L* value indicates lightness
 Represented on a vertical axis with values from
0(black) to 100(white)
 A* value indicates red green component
 +a* (positive) and –a* (negative) indicates red and
green values respectively.
 B* value indicates yellow and blue component
 Centre of the plane is neutral or achromatic
 Distance from thecentral axis represents Chroma
 Angle of chromacity represents Hue

30. MUNSELL COLOUR SYSTEM
 Albert H. Munsell noted that each color has a
logical relationship to all other colors.
 It brought clarity to color communication by
establishing an orderly system for accurately
identifying every color.
 Made a “color wheel” which include the
dimensions of hue, value and chroma.
 Translucency is not addressed in Munsell’s system

33. COLOR MATCHING
 In dental practice, color matching is most often performed with the use of a shade
guide, to select the color of ceramic veneers, inlays, or crowns.
 The VITAPLAN classical shade guide introduced in 1956 still is widely used for shade
matching in dentistry.
 Recently, however, the trend is to arrange shade guides in decreasing order of value
(lightest to darkest: B1, A1, B2, D2, A2, C1, C2, D4, A3, D3, B3, A3.5, B4, C3, A4, C4).
 Matching tooth shades is simplified by the arrangement of tabs by value.

34. MECHANICAL PROPERTIES
 Mechanical properties are defined by the laws of mechanics—that is, the
physical science dealing with forces that act on bodies and the resultant
motion, deformation, or stresses that they experience.
 All mechanical properties are measures of the resistance of a material to
deformation, crack growth, or fracture under an applied force or pressure
and the induced stress.
 Mechanical properties are expressed most often in units of stress and/or
strain.
 Mechanical properties are measured responses of, both elastic (reversible
on force removal) and plastic (irreversible on force removal) materials
under an applied force.

35. STRESS AND STRAIN
 Stress is the force per unit area acting on millions of atoms or molecules in
a given plane of a material.
 It’s the internal resistance of the material to the applied load.
 Stress in terms of mathematical terms can be described as
Stress = Force/ Area
 Three types of “simple” stresses can be classified: tensile, compressive, and
shear.
 Complex stresses, such as those produced by applied forces that cause
flexural or torsional deformation.

36. TENSILE STRESS
 TENSILE STRESS is caused by a load that tends to
stretch or elongate a body.
 Tensile stress occurs when two sets of forces are
directed away from each other in the same
straight line.
 Also when one end is constrained the other end
is subjected to a force away from the constraint.

37. COMPRESSIVE STRESS
 COMPRESSIVE STRESS occurs when two sets of
force are directed towards each other along the
same straight line.
 It is caused by a load that tends to compress or
shorten the body.

38. SHEAR STRESS
 SHEAR STRESS occurs when two 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 the body over another.
 Shear stress can also be produced by a twisting
or torsional action on a material.
 Eg: 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.

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

40. MODULUS OF ELASTICITY
 Elastic modulus describes the relative stiffness
or rigidity 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 elastic stress to elastic strain.
 A stiff material has high modulus of elasticity
while a flexible material will have a low modulus
of elasticity.

41. MODULUS OF ELASTICITY
 Example: Impression Material
 The impression materials should have a low
modulus of elasticity to enable it to be
removed from the undercut areas in the
mouth.
 Modulus of elasticity should not be very low
that the material cannot withstand tearing.

42. DYNAMIC YOUNG’S MODULUS
 It is defined as the ratio of stress to strain for small cyclical deformation at
a given frequency and at a particular point on the stress strain graph.
 Elastic modulus can be measured by a dynamic method as well as the
static techniques.
 Since the velocity at which sound travels through a solid can be readily
measured, the velocity of the sound wave and the density of the material
can be used to calculate the elastic modulus and Poisson’s ratio.

43. FLEXIBILITY
 It is defined as the flexural strain that occurs
when the material is stressed to its proportional
limit.
 For example, in an orthodontic appliance, a
spring is often bent a considerable distance
under the influence of a small stress. In such a
case, the structure is said to be flexible and to
possess the property of flexibility

44. RESILIENCE
 It is the resistance of material to permanent
deformation.
 It indicates the amount of energy absorbed within a
unit volume of a structure when it is stressed to its
proportional limit.
 It is measured as area under the elastic portion of
the stress strain curve.

45. TOUGHNESS
 It is defined as the amount of energy required to
fracture a material.
 It is the measure of the energy required to
propagate critical flaws in the structures.
 Tough material is generally strong although a
strong material is not necessarily tough.
 Toughness is also known as resistance of material
to fracture.

46. POISSON’S RATIO
 During axial loading in tension or
compression there is a simultaneous strain in
the axial and transverse, or lateral, directions.
 Within the elastic range, the ratio of the
lateral to the axial strain is called Poisson’s
ratio.
 Poisson’s ratio is a unitless value because it is
the ratio of two strains.
 Most rigid material exhibit Poisson’s ratio of
0.33 i.e. Enamel.

47. STRENGTH PROPERTIES
 Strength is equal to the degree of stress necessary to cause either fracture
(ultimate strength) or a specified amount of plastic deformation (yield
strength).
 Strength of the material can be described in terms of
 Elastic Limit
 Yield Strength
 Ultimate tensile Strength
 Shear Strength
 Compressive Strength
 Flexural Strength

48. ELASTIC LIMIT
 The elastic limit of a material is defined as the
greatest stress to which the material can be
subjected such that it returns to its original
dimensions when the force is released.
 The region of stress strain curve before the
proportional limit is called the elastic region.

49. YIELD STRENGTH
 Also known as Yield point or Yield stress.
 It is defined the stress at which the material
exhibits a specified limiting deviation from
proportionality.
 It is a property that can be determined readily and
is often used to describe the stresses at which the
material begins to function in a plastic manner.
 It defines the transition from elastic to plastic
material.

50. PERMANENT DEFORMATION
 If the material is deformed by stress at a point above the proportional limit before
fracture, removal of the applied force will reduce the stress to zero, but the plastic
strain (deformation) remains. Thus the object does not return to its original
dimension when the force is removed. It remains bent, stretched, compressed, or
otherwise plastically deformed.
 In stress strain graph the region beyond elastic limit is plastic region.

51. FLEXURAL STRENGTH
 Flexural strength, which is also called transverse strength and modulus of
rupture.
 It is essentially a strength test of a bar supported at each end or a thin disk
supported along a lower support circle under a static load.
 The test is a collective measurement of tensile, compressive and shear
stresses simultaneously.

52. IMPACT STRENGTH
 Impact Strength is the energy required to fracture a material under an
impact force.
 The term impact is used to describe the reaction of a stationary object to a
collision with a moving object.
 The impact resistance of material is defined from the total energy
absorbed before fracture when struck by a sudden blow.
 A material with a low elastic modulus and a high tensile strength is more
resistant to impact forces. A low elastic modulus and a low tensile strength
suggest low-impact resistance

53. DUCTILITY AND MALLAEBILITY
 Ductility represents the ability of a material to
sustain a large permanent deformation under a
tensile load up to the point of fracture. For
example, a metal that can be drawn readily into
a long thin wire is considered to be ductile.
 Malleability 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, and silver is second.

54. HARDNESS
 It maybe broadly defined as the resistance to permanent surface indentation or
penetration.
 It is the measure of resistance to plastic deformation and is measured as force per unit
area of indentation.
 It is indicative of the case of finishing of the structure and its resistance to in service
scratching.
 Some of the most common methods of testing the hardness of restorative materials
 Vickers
 Brinnel
 Knoop
 Rockwell
 Barcol

55. BRINNEL HARDNESS TEST
 The Brinell hardness test has been used
extensively for determining the hardness of
metals and metallic materials used in dentistry.
 It is related to the proportional limit and the
ultimate tensile strength of dental gold alloy
 In this test, a hardened steel ball is pressed
under a specified load into the polished surface
of a material.
 The load is divided by the area of the projected
surface of the indentation, and the quotient is
referred to as the Brinell hardness number

56. ROCKWELL HARDNESS TEST
 The Rockwell hardness test is similar to the Brinell
test in that a steel ball or a conical diamond point
is used.
 Instead of measuring the diameter of the
impression, the depth of penetration is measured
directly by a dial gauge on the instrument.
 The convenience of the Rockwell test, with direct
reading of the depth of the indentation, has led to
its wide usage in industry.
 Both Brinnel and Rockwell hardness test are not
suitable for brittle materials

57. KNOOP HARDNESS TEST
 The Knoop hardness test employs a
diamond-tipped tool that is cut in the
geometric configuration.
 The impression is rhombic in outline, and
the length of the largest diagonal is
measured. The load is divided by the
projected area to give the Knoop hardness
number.
 Knoop and Vickers hardness tests are micro
hardness tests while Rockwell and Brinnel
are macro hardness test.

58. VICKERS HARDNESS TEST
 The Vickers hardness test employs the same
principle of hardness testing that is used in the
Brinell test. However, instead of a steel ball, a
square-based pyramid is used.
 The lengths of the diagonals of the indentation
are measured and averaged.
 The Vickers test is employed in the standard
testing of dental casting gold alloys.
 The test is suitable for determining the
hardness of brittle materials;

59. OTHER HARDNESS TESTS
 Shore and the Barcol tests, are sometimes employed for measuring the
hardness of rubber and plastic types of dental materials.
 These tests use compact portable indenters of the type generally used in
industry for quality control.
 Many material which have micro structural constituents or in case of micro
filled composite, NANO INDENTATION are used.

60. References
 Philips’ Science Of Dental Material by Kenneth J. Anusavice DMD PhD
(Jun 17, 2003)
 Notes on Dental Materials (Dental Series) by E. C. Combe (Nov 1992)
 Craig’s Restorative Dental Materials 13th edition
 Sikri VK. Color: Implications in dentistry. J Conserv Dent. 2010;13(4):249-255.