basic properties of materials used in dentistry

1. Basic physical properties of materials.
Test methods for materials in dental
material science.
2nd lecture
Dr. Kinga Turzó
11th of September 2012

2. Subject of the lecture:
Basic properties (bulk and surface) of materials:
Overview of the:
Solid state
Interatomic bonds
Atomic structure
Physical properties of materials:
Mechanical
Thermal Bulk properties
Electrical and electrochemical
Surface properties of materials:
Surface energy
Wetting
Methods of testing materials in dental material science.

3. Bulk properties of materials
Solid state:
Their constituent atoms are held together by strong/primary
interatomic forces/bonds (Pauling, 1960).
Interatomic primary bonds:
Ionic
Covalent
Metallic
Interatomic secondary bonds:
Hydrogen bonding
van der Waals forces
The electronic and atomic structures, and almost all the
physical properties of solids depend on the nature and
strength of the interatomic bonds.

4. Overview of interatomic primary
(strong) bonds
Ionic bonding (A)
• Attraction of + and – charges, characterized by electron
transfer from one element (+, cation, metal) to another (-,
anion, nonmetal)
• Examples: Na+Cl-, NaF, MgCl2, certain crystalline phases
of some dental materials (gypsum and phosphate
cements)
• Poor electrical conductors (tightly bound e-), low
chemical reactivity (low overall energy state)
Covalent bonding (B)
• Characterized by electron sharing and very precise bond
orientations, particular atomic arrangement or crystal
structure
• Examples: H2, diamond, silicon, many organic
compounds (dental resins)
• Poor electrical conductors (localization of the valence
electrons in the covalent bond)
Metallic bonding (C)
• Characterized by electron sharing and formation of
„gas” of electrons that bonds the atoms (which become
positively charged because of the electron gas
formation) together in the lattice.
• Examples: metal crystals, gold, cobalt
• The nonlocalized bonds permit plastic deformation and
the electron gas accounts for the chemical reactivity and
thermal conductivity of metallic systems

5. Overview of interatomic secondary
(weak) bonds
Hydrogen bonding (H...O)
• Shared electrons (covalent bond), forming a permanent dipole, an
asymmetric molecule (e.g.: water molecule)
• Associated with the positive charge of hydrogen caused by polarization
• Due to this polarity: intermolecular reactions in many organic
compounds (sorption of water by synthetic dental resins)
Van der Waals forces
• These forces form the basis of a dipole attraction
• E.g.: in a symmetric molecule (inert gas) the electron field is constantly
fluctuating, its charge becomes momentarily positive and negative
• The fluctuating dipole will attract other similar dipoles
• Quite week
Strength: 3-10% of the primary C_C covalent bond
Significantly influence the properties of some solids, especially
polymers

6. Atomic structure of dental materials
Crystalline structure:
The atoms are bonded by either primary or secondary forces.
In the solid state, they combine in a manner that ensures minimal internal energy
(e.g.: Na+Cl-).
They do not simply form only pairs, all the positively charged ions attract all of the
negatively charged ions.
As a result they form a regularly spaced configuration:
space lattice or crystal (14 possible lattice types).
Noncrystalline structure:
Some of dental materials can occur in this form, for example: waxes may solidify as
amorphous materials such that the molecules are distributed randomly.
Glass is also considered to be a noncrystalline solid, since its atoms tend to develop a
short range order instead of a long-range order characteristic of crystalline solids.
The internal energy is not so low as for crystalline arrangements.
They do not have a definite melting temperature, but rather they gradually soften as
the temperature is raised.
Synthetic dental resins are examples of materials that often have glassy structures.

7. Space lattice types
Single cells of cubic space lattices: Other simple lattice types of dental
interest:
Simple cubic
Body
centered
cubic
Face-
centered
cubic

8. Atomic structure of metals
Metallic bonding in the solid
state B
Alloys: mixtures or solutions
of different metals
A
85% of all metals have one
of the crystal structures
shown in Figure
A: Face-centered cubic (FCC) C D
B: Full size atoms in FCC
C: Hexagonal close packed (HCP)
D: Body-centered cubic (BCC)

9. Atomic structure of ceramics
Combination of ionic or
covalent bonding
Tightly packed
structures, but with
special requirements for
bonding, such as fourfold
coordination for covalent
solids and charge
neutrality for ionic solids
More open and complex
crystal structures
Different physical Carbon, two different crystal structures:
properties: hardness, A – diamond (cubic, tetrahedral coordination)
color, electrical B – graphite (hexagonal, t.arrang. is distorted into
conductivity, density a nearly flat sheet, stacking of these sheets)

10. Atomic structure of polymers
Constituent atoms are usually carbon, joined in a linear chainlike
structure by covalent bonds.
The bonds within the chain require two of the valence electrons of
each atom, leaving the other two bonds available for adding a
great variety of atoms (H, etc.), molecules, functional groups, etc.
Based on this organization: 2 classes of polymers:
Thermoplastic polymers: the basic chains have little or no branching,
can be melted and remelted without a basic change in structure
(amorphous th.p.p: van der Waals and hydrogen bonding holds the
chain together).
Thermosetting polymers: if side chains are present and form links
(covalent) between chains, a three-dimensional network structure is
formed; strong structures and once formed by heating will not melt
uniformly on reheating.

11. Polymer arrangements

12. Physical (bulk) properties of materials
Mechanical properties:
– Strength (tensile, compressive, shear stress)
– Elasticity (elastic modulus, resilience)
– Plasticity (ductility, percent elongation, yield strength)
– Hardness (Brinell, Vickers, Knoop, Rockwell, Shore A)
Thermal properties:
– Heat flow (thermal conductivity, thermal diffusivity)
– Thermal expansion
Electrical and electrochemical properties:
– Electrode potential
– Electrical resistivity

13. Mechanical properties
Strength is one of the most
important properties of a
material.
Elastic behavior, Hooke’s
law (1678), tensile test: Load
– A solid material subjected (Newtons)
to a tensile (distraction)
force would extend in the
direction of traction by an
amount that was
Extension (mm)
proportional to the load.
– Most solids behave like a
spring if the loads are not to
great.

14. Stress and strain.
Tension and compression.
The extension for a given load varies with
geometry of the specimen as well as with its
composition.
To resolve this confusion, the load and
deformation can be normalized:
STRESS = Normalized load
σ = F/A (unit: N/m2 or Pa)
STRAIN = Normalized deformation
ε = ∆l/lo (measured applying reference
marks)
In tension and compression the area (A)
supporting the load is perpendicular to the
loading direction and the change in length is
parallel to the original length.

15. Example:
Stresses induced in a three-unit bridge (A) and in a
two-unit cantilever bridge (B) by a flexural force (P).
• Tensile and compression stress is produced.
• In case of the three-unit bridge the tensile stress develops on the gingival side.
• In case of the cantilever bridge the tensile stress develops on the occlusal side.
• These areas of tension represent potential fracture initiation sites!

16. Shear stress, elastic constants
For cases of shear the applied load is
parallel to the area supporting it - shear
stress, τ − and the dimensional change
is perpendicular to the reference
dimension - shear strain, γ
Quantitative expression of Hooke’s law,
using the definitions of stress and
strain:
σ=E.ε for tension or compression
τ=G.γ for shear
E – tensile constant (Young’s modulus)
G – shear modulus
Represent inherent properties of the
material (since all geometric influences
have been removed)

17. Explanation
σ = F/A
STRESS = Normalized load, force/area
SI unit: [N/m2] or [Pa]
ε = ∆l/lo
STRAIN = Normalized deformation
no SI unit (m/m), usually given in %
If, F is proportional with ∆l (based on Hooke’s law), and A (area) is
constant and also lo (initial length of the solid) is constant, than
σ will be also proportional with ε!
The factor of proportionality is called tensile constant (E).
The same explanation is valid for the shear modulus (G).

18. Atomic model illustrating elastic shear
deformation (A) and plastic deformation (B)
• Bonded two material system: material A – white atoms, material B – shaded atoms
• Figure A: the shear force is applied far from the interface, as it increases produces an
elastic or reversible shear strength, that would return to zero if the shear force is
removed.
• Figure B: if the shear force is closer to the interface and increased sufficiently, a
permanent or plastic deformation will be produced.

19. Stress-strain plot
Measurement of stress and strain
on an object being stretched
Elastic deformation region: stress
is proportional to strain
(proportional limit)
Plastic deformation region: stress
and strain are no longer
proportional
Ultimate tensile strength: the
maximum stress just before the
object breaks
Percent elongation: the total
amount that the object stretches
(the total strain), the sum of the
elastic and the plastic deformation.

20. Ductility, brittle fracture, toughness,
resilience, fatigue
Ductile: materials that experience a large amount
of plastic behavior or permanent deformation.
Brittle: materials that undergo little or no plastic
behavior; in real materials, elastic behavior does
not persist indefinitely; microscopic defects will
begin to grow under the applied stress, and the
specimen will fail suddenly.
Toughness: the entire area under the stress-
strain curve; a measure of the energy required to
fracture the material.
Resilience: the area under only the elastic region
of the stress-strain curve; a measure of the
ability of the material to store elastic energy.
Fatigue: when materials are subjected to cycles
of loading and unloading (e.g.: mastication) they
may fail due to fatigue stresses below the
ultimate tensile strength.

21. Stress-strain plot for a stainless steel orthodontic
wire that has been subjected to tension
Points: measured values
PL: proportional limit
YS: yield strength at a 0.2%
strain offset from the origin (O)
UTS: ultimate tensile strength
E: Young or elastic modulus is
calculated from the slope of
the elastic region

22. Mechanical properties of Ti and its alloys (ASTM
F136, 1992 and Davison et al., 1994)
Properties Grade 1 Grade 2 Grade 3 Grade 4 Ti6Al4V Ti13Nb13Zr
Tensile strength (MPa) 240 345 450 550 860 1030
Yield strength (0.2%
170 275 380 485 795 900
offset) (MPa)
Elongation (%) 24 20 18 15 10 15
Reduction of area (%) 30 30 30 25 25 45

23. Comparison of dentin and enamel
Dentin is capable of sustaining
significant plastic deformation
(ultimate compressive strength
CS = 234 MPa) under
compressive loading before it
fractures.
Dentin is more flexible and
tougher than enamel.
Elastic modulus E for enamel is
~three times greater than that of
dentin.
Enamel is stiffer and more brittle
material than dentin.

24. Elastic modulus of different biomaterials
compared to bone

25. Hardness
Hardness: the resistance of a material to indentation or
penetration (mineralogy: ability to resist scratching).
Properties that are
related to the hardness
of a material:
– strength
– proportional limit
– ductility
Methods for measuring
hardness:
– Brinell, Vickers, Knoop,
Rockwell, Shore A

26. Thermal properties
The rate at which heat flows through a
material is expressed as:
Thermal conductivity: k
Is a measure of the speed at which heat
travels through a given thickness of
material, when one side of the material is
maintained at a constant temperature that
is 1oC higher than the other side.
Unit: [Cal / cm . sec . oC]
Thermal diffusivity: h
Tells us how rapidly the interior of the
material will reach the temperature of the
exterior.
h = k/(Cp . ρ) , [mm2/ sec]
Cp – heat capacity, ρ -density

27. Thermal properties
There are several situations in
dentistry in which the thermal
expansion of materials is important:
Linear coefficient of thermal
expansion = change in length per
unit original length for 1oC
temperature change
Some restorative materials have
coefficient of thermal expansion that
are markedly different from tooth
structure.
In such cases, temperature
fluctuations that occur in the mouth
can cause percolation at the tooth-
restoration interface as the Relative thermal expansions of several
restoration contracts and expands. restorative materials and tooth structure.

28. Electrical and electrochemical
properties
- If there is a difference between the electrode
Electrode potentials: potentials of two metals in contact with the
- An electrochemical series is a listing of same solution (Au, Al), an electrolytic cell may
elements according to their tendency to develop, discomfort in the mouth!
gain or lose electrons in solution.
- The series is referenced versus the
potential of a standard Hydrogen
electrode, which is arbitrarily assigned a
value of 0 Volts.
- If the elements are listed according to
their tendency to lose electrons:
oxidation potentials, if they are listed
according to the tendency of their ions to
gain electrons, reduction potentials.
Standard electrode potential: reduction
potentials at 25oC and 1 atmosphere of
pressure.
Examples:
- metals with a large positive electrode
potential (platinum, gold) are more
resistant to oxidation and corrosion in the
oral cavity.

29. Electrical and electrochemical
properties
Electrical resistivity: ρ
:
measures the resistance of a
material to the flow of an
electrical current.
Relationship: R = ρ . (l/A)
R -resistance, l -length, A -area
Examples: Metallic restorative materials:
low resistivity (Cu: 1.7x10-6 Ω m),
discomfort for the pulp.
Cements: good insulating properties (zinc
phosphate cement: 2x105 Ω m).

30. Surface properties of materials
Important question in biocompatibilty:
- how the device or material „transduces”
its structural makeup to direct or
influence the response of proteins, cells
and organisms?
This transduction occurs through the
surface structure–the body „reads” the
surface structure and responds.
The surface region is known to be
uniquely reactive and is different from the
bulk.
Surface energy or tension: the increase in Adhesion: attraction (secondary
energy per unit area of surface. bonding) across the interface between
The energy at the surface of a solid is unlike molecules (silver, gold adsorbs
greater than that of its interior oxygen).
(interatomic distances are equal, minimal Chemisorption: primary bonding is
energy, while at surface the outermost involved (chemical bond is formed
atoms are not equally attracted in all between the adhesive and the
directions). adherent).

31. Surface properties of materials
Important parameters to Contact angle (θ) of wetting:
biological reaction:
- roughness, wettability, surface - measuring method of the extent to
mobility, chemical composition, which an adhesive wets the surface.
crystallinity and heterogeneity.
Wetting: to produce adhesion - when θ = 0ο , the liquid will spread
between two solid surfaces, a completely over the surface
liquid most flow easily over the - large θ formed by poor wetting
entire surface and adhere to the
solid.
The ability of an adhesive to wet
the surface is influenced by a
number of factors:
– cleanliness of the surface, etc.
Teflon (polytetrafluoroethylene,
commercial synthetic resin) is
often used to prevent the
adhesion of films to a surface.

32. Mechanical requirements
The mechanical requirements of
dental materials can be different,
depending from their function.
Example: the masticatory forces applying on dental
implants is quite high (500-800 N), therefore their
design and mechanical strength has to be adequate
for such loading.

33. Technological requirements
The most important aspects of technological
suitability are:
– easy processing, machining
– precise and good elaboration
– cheep and economic raw material
Other practical aspects:
- easy sterilization
- good radiographic image
- simple surgery/implantation and prosthodontic procedure
- easy extraction if the implant fails

34. Testing methods in dental materials
science
Biomechanical tests
Methods measuring hardness
Finite element analysis
Birefraction analysis
Load compression test
Testing osseointegration between bone and dental implant:
– Pull-out
– Push-out tests
– Torque
Methods characterizing dental material surfaces

35. Methods for measuring hardness
Brinell hardness number:
The indenter is a small hardened steel ball
(Ø1.6 mm), which is forced into the surface
under a specified load (123 N, 30 s). Leaves
a round dent in the material.
Vickers hardness number:
The indenter is a square pyramid-shaped
diamond (136o); hardness is determined by
measuring the diagonals of the square and
Rockwell hardness:
taking the average of the two dimensions.
Used mostly for determining the hardnesses of
Knoop hardness: steels; different hardened steel balls or diamond
cones and different loads.
the indenter is also made of a diamond, but
one diagonal is longer than the other; only Shore A hardness :
the long diagonal is measured. Used to measure the hardness of rubbers and
soft plastics. Scale between 0 and 100 units: 0
complete penetration, 100 no penetration.

36. Finite-element analysis for determination of stresses in
dental implants
The implant is
divided in small,
grid-like
structures/elements.
Elastic (Young) modulus (E):
Al2O3 conical
Implants: 380 000 MPa
and step-form
Cortical bone: 20 000 MPa implant
Trabecular bone: 2 000 MPa

37. Finite element testing of the design of dental
implants with different connections
External hexagonal connection: Ø 3,75 mm Tube in tube connection: Ø 3,5 mm
Stress: 32 N·cm Stress: 15 N·cm
Loading: 300 N/30° Loading: 300 N/30°
Merkle H. et al. 1998

38. Mechanical testing
Diametral compression test:
for materials that exhibit elastic deformation
and little or no plastic deformation.
Controlled load-deflection (stress-strain)
test:
The load frame is much stiffer and stronger than
the specimen; the cross head is moved up or
down by a screw or a hydraulic piston; the load
Compressive force (P) applied along the disk, a cell monitors the force being applied.
tensile fracture is produced. Compression testing: the direction of the
Tensile strength depends from the fracture load (P), cross–head is reversed.
disk diameter (D) and the thickness (t). Torque test, push-out test: for implant tests

39. Push-out and torque tests
Methods for the investigation of
the osseointegration of implants
(LLoyd Instrument)

40. Common Methods of Characterizing
Biomaterial Surfaces
Depth Spatial Analytical
Method Principle Costa
analyzed resolution sensitivity
Low or high
Liquid wetting of surfaces is used to
Contact angles 3-20 Å 1 mm depending on the $
estimate the surface energy
chemistry
X-rays wetting cause the emission
ESCA 10-250 Å 10-150 µm 0.1 Atom % $$$
of electrons of characteristic energy
Auger electron A focused electron beam causes the
50-100 Å 100 Å 0.1 Atom % $$$
spectroscopyb emission of Auger electrons
Ion bombardment leads to the
SIMS 10 Å-1 µmc 100 Å Very high $$$
emission of surface secondary ions
IR radiation is adsorbed in exciting
FTIR-ATR 1-5 µm 10 µm 1 Mole % $$
molecular vibrations
Measurement of the quantum
STM tunneling current between a metal 5Å 1 Å Single atoms $$
tip and a conductive surface
Secondary electron emission
High, but not
SEM caused by a focused electron beam 5Å 40 Å typically $$
quantitative
is measured and spatially imaged

41. ESCA

42. Roughness measurement:
Atomic Force Microscope
AFM

43. Visible difference in Ra:
3D-Analysis ~0.5 nm → ~16 nm
silica
(PLL/PGA)6