Introduction to ceramics

1. INTRODUCTION, CLASSIFICATION ,
HISTORY, MICROSTRUCTURE,
STRUCTURE-PROPERTY RELATION
OF CERAMICS
DONE BY MENNA KORIAM

2. Introduction
Historical Development
Classifications
Micro Structure
Structure- Property Relationship
CONTENTS

3. INTRODUCTION

4. The term ceramics is connected with POTTERY (representing the materials class),
BRITTLENESS (representing the most characteristic property) and household and
construction (representing typical applications).
Ceramics contain a variety of inorganic non-metallic materials that are formed into
shapes from powders at room temperature.
They gain their physical properties through a high temperature firing process

5. They are nonmetallic, inorganic structures, primarily containing compounds of oxygen
with one or more metallic or semi-metallic elements (aluminum, boron, calcium, cerium,
lithium, magnesium, phosphorus, potassium, silicon, sodium, titanium, and zirconium).
They contain a crystal phase and a silicate glass matrix phase.
Its chemical unit is SiO2
It is composed of a central silicon cation (Si 4+ ) bonded covalently to four oxygen
anions located at the corners of a regular tetrahedron
WHAT IS A CERAMIC MATERIAL?

6. The resulting structure is not closely packed and exhibits both covalent and ionic
bonds.
They are arranged as linked chains of tetrahedra, each of which contains two oxygen
atoms for every silicon atom resulting in a negatively charged silicon oxygen tetrahedron
(SiO4)
Silica have very high fusion and melting temperatures up to 1700 degrees which will
have difficulty in fabrication due extremely high viscosity

7. It has four shapes, 3 crystalline and one amorphous depending on its cooling rate:
1. Slow cooling is done till the desired shape is obtained followed by rapid cooling to retain
the amorphous structure through vitrification process
2. Rapid cooling and creation of amorphous glass from crystalline silica
Other components:
FLUXES are added (alkaline metal oxides ) as ALO and KO which will disrupt the covalent
bonds between S and O and from ionic bonds between Na+ and S , K+ and S , and Na with O
and K with O , and covalent bonds between S and O ,, which will decrease melting
temperature and increase coefficient of expansion and contraction.
STABILIZERS (alumina ) AL2O3 are added to stabilize the charge ,strength, decreased
opacity because it dissolves in glass do not remain in glass as it melts, without it ,, it is soluble
in water and humidity
These 3 components combined together are called feldspar.

8. Kaolin: Acts as a binder , imparts opacity
Alumina: Forms a network in conjunction with silica
Metallic Pigments: Help to obtain various shades needed to stimulate natural teeth.
 Brown - Iron or nickel oxide
 Green - Copper oxide
 Yellow brown - Titanium oxide
 Blue - Cobalt oxide
 Pink - Chromium tin
Opacity is achieved by addition of Cerium oxide ,Zirconium oxide, Titanium oxide, Tin
oxide

10. HISTORICAL
DEVELOPMENT

11. Humans discovered that clay could be found and made into objects by mixing with water
then firing , a key industry was born where the oldest known artifact is dated as early as
28,000 years ago
HISTORY OF CERAMICS

12. 1858-1929 was used in light improvement using a mixture of yttria/lanthana and
magnesium/zirconia
1941 invention of nerst lamps and tungsten filaments
1952 tough cermet was introduced which is a mix between metal and ceramics , used
in wire making and later in metal drawing tools.
20th century used in TVs and Radios where they required the need of heat resistant
materials to withstand the high frequency electromagnetic fields.
NON DENTAL USES

13. DENTAL USES

14. Dental ceramics were first used in dentistry in the late 1700s.
Porcelain jacket crowns were developed in the early 1900s.
Development of leucite –containing feldspathic porcelain in 1960s .
At the end of twentieth century saw introduction of all ceramic dental restoration,
The first was castable glass ceramics
Later ,heat treatement was to promote its transformation into glass ceramics

15. CLASSIFICATION

16. Classifications are according to :
• Fabrication methods
• Crystalline phase
• Uses
• Firing temperature
• Composition
• Microstructure
• Translucency
• Fracture resistance
• Abrasiveness

17. CLASSIFICATION BY FABRICATION METHOD
1.Condensation
2.Hot pressing 3. Casting 4. Slip casting
5.Computer
Aided milling of
fully sintered
form
6.Computer
Aided designing
if partially
sintered form
7. Copy milling
8. Grinding of
dry pressed
powder on
enlarged die

18. The process of forming a Slurry mix of powder
ceramic adding liquid distilled water using a glass
spatula
Then applying this mix onto a metal coping , ceramic
coping or a die using a ceramic brush.
Through layering technique , using different shades
accordingly , then firing in furnace and glazing process
finally.
CHEMICAL REACTION ( SINTERING )
Examples:
1. Vita Shade
CONDENSATION

20. Sintering is the process of compacting and forming a solid mass by heat or pressure
without melting it to the point of liquefaction.
Once the ceramic powder has been compacted , the “powder compact” is usually
around 50% of its final theoretical density. Full densification is achieved by
sintering at temperatures up to 1800°C.
The sintering process allows powder particles to bond together to remove the porosity
present.
During the sintering process the “green compact” shrinks by around 40 % in
volume However, its predictable and can be overcomed
SINTERING

21. 1. This technique is the simultaneous application of external pressure and temperature
to enhance densification.
2. Use lost wax technique
3. It is conducted by placing either powder or a compacted preform into a suitable die,
typically graphite, and applying uniaxial pressure while the entire system is held at an
elevated temperature, e.g. 2000°C to allow melting of ceramic
4. After investing and firing , the investment is left to cool to room temp , devesting and
finishing is done to give the ceramic coping where incremental layering of ceramic
continues.
5. Examples: IPS Empress
HOT PRESSING

22. 1. Use lost wax technique
2. Ceramic ingot is fused and casted in a
refractory ( phosphate bonded
investment), and the normal
procedures are carried on.
3. Examples:
1. Dicor
2. Dicor Plus
CASTING

24. Slip casting refers to the filling of a mold, a negative of the desired shape, with a slip
consisting of a suspension of micrometer size ceramic particles in liquid.
is a process of forming ceramic shapes by applying an aqueous slurry of ceramic
particles to a porous substrate such as die material and removing water by capillary
action , this densifies the deposited ceramic powder into green body , which is then
sintered to achieve higher density and strength
SLIP CASTING
Slip casting mold
Assembled Slip
cast mold ready for
pouring
Sintered slip cast component
illustrating the shrinkage from
the original mold dimensions

25. CAD/CAM ceramic materials evolved from traditional feldspathic porcelain, an esthetic but
low-strength, brittle material, to a range of materials with different strength, resilience, and
esthetic properties.
They are clinically successful and are replacing PFM restorations.
Ceramics are rapidly changing as the esthetics of the high-strength materials has improved to
the point where it is possible to make anything from a single unit to full-arch monolithic
ceramic restoration.
Monolithic restorations are less prone to failure as there is no weaker outer layer.
They are easier, quicker, and cheaper to fabricate, as the restoration is made through
CAD/CAM and there is no labor intensive, highly skilled layering process.
CAD/ CAM

26. Milling produces stronger, accurate, and economical restorations
Subtractive techniques where material is removed from a block or disc, leaving the
planned shape, which is usually achieved by milling away excess material
This avoids the defects, stresses, and shrinkage that come with layering and multiple
oven cycles.
Therefore, the same material is stronger and has better properties when processed by
CAD/CAM.
Subtractive processing can be wasteful, as a majority of material is ground away and
discarded. Milling burs wear with use leading to inaccuracy.
For porcelains, the grinding process can introduce stresses and fractures.
MILLING PROCESSES

27. The prepared tooth is impressed and turned into a model die when poured into gypsum
A wax pattern is built over the die and ceramic blocks are carved into the restoration by
the aid of computed copy milling machines
Examples:
1. CELAY
2. LAVA ZIRCONIA
3. CERECON
COPY MILLING

28. The principal crystal phase and/or matrix phase includes silica glass, leucite based
feldspathic porcelain, leucite-based glass-ceramic, Lithia Disilicate based glass-ceramic,
aluminous porcelain, alumina, glass-infused alumina, glass-infused spinel, glass infused
alumina/zirconia, and zirconia
After firing >> composed of a glassy phase and one or more crystalline phases, with various
amounts of porosity.
Depending on the nature and amount of crystalline phase and porosities , the mechanical and
optical properties vary
Increasing the amount of crystalline phase may lead to crystalline reinforcement and increase
the resistance to crack propagation but also can decrease translucency.
Materials for all-ceramic restorations have increased amounts of crystalline phase (between
35% for leucite-reinforced ceramics and up to 99% for polycrystalline zirconia ceramics) for
better mechanical properties, but they are more opaque than dental porcelains.
CLASSIFICATION BY CRYSTALLINE PHASE

29. Diamond grinding is a grinding process that can be applied to produce
precision ceramics.
It takes advantage of the fact that diamond has the highest hardness of
any bulk material and high levels of accuracy and smooth surface finish
can be achieved allowing tolerances of only a few microns.
However, its an expensive process.
GRINDING OF DRY PRESSED POWDER ON ENLARGED DIE

30. The major component is SILICA, which is the basis of the glass matrix phase of
feldspathic porcelains used as a veneering or layering structure of both metal-ceramic
and all-ceramic prosthesis
The most common crystal phases are Leucite, Lithia Disilicate, Alumina, Combinations
Of Alumina And Zirconia, Zirconia And Apatite.
Leucite crystals are included in veneering porcelains for metal-ceramics to opacity and
strengthen these glass matrix materials and to control their thermal expansion and
contraction coefficients.
ACCORDING TO COMPOSITION

31. Traditional ceramics are made by natural silicate and aluminosilicate minerals
composed of aluminum, silicon, and oxygen, and other elements (Ca, Mg, Na, K, etc.), with
clay as a major constituent that allows shaping due to its plasticity,
Traditional ceramics are processed through a conventional procedure starting from the
preparation of ceramic suspensions, and ending with sintering
They are characterized by :
 poorly defined properties
 poor reproducibility and inferior reliability.
TRADITIONALAND ADVANCED CERAMICS

32. Advanced ceramics are referred to as a new family of ceramics with tailor-made
multi-functionalities made by synthetic chemicals of high purity and high performance.
Organic binders are added to assist in shaping.
They are classified based on their compositions :
1. Oxide Ceramics Include Binary Oxides, Aluminates, Ferrites, Titanites, Niobates,
Zirconates Etc.
2. Non-oxide Ceramics Cover Carbides, Nitrides, Borides, Carbon Etc.
3. Ceramic Composites Which Are A Combinations Of Oxides And Non-oxides.

33. 1. Structural ceramics are have superior mechanical behaviors in corrosive environments
and at high temperatures. They are selected to replace metals. Examples such as
zirconia, alumina, and glass-ceramics.
2. Functional ceramics can be tailor-made to achieve different properties. They are
electrical insulators, but by manipulating their composition and microstructure, they can
be turned into semiconductors and superconductors.
3. Uranium oxide and nitride are used as nuclear fuels in the forming of ceramics and are
referred to as nuclear ceramics.
ADVANCED CERAMICS ARE DIVIDED INTO THREE
CATEGORIES:

34. 1. Opaque
2. Superior functional properties and strength
3. Unique electrical properties (superconductivity)
4. Superior mechanical properties as enhanced toughness
PROPERTIES

36. 1. Ceramics for metal-ceramic crowns and fixed prostheses,
2. All-ceramic crowns , inlays, onlays, veneers, and fixed prosthesis.
3. Ceramic orthodontic brackets, implant abutments, and ceramic denture teeth.
4. Lithia Disilicate glass-ceramics, alumina/zirconia, and zirconia are used for the core
structure of the all-ceramic applications
ACCORDING TO USE

37. They are either amorphous glass , crystalline or crystalline particles in a glass matrix
CLASSIFICATION ACCORDING TO MICROSTRUCTURE

38. They are made from materials that contain silicon dioxide or silica and some amounts of
alumina.
A glass ceramic is a multiphase solid containing a residual glass phase with a finely dispersed
crystalline phase.
The controlled crystallization of the glass results in the formation of tiny crystals that are
evenly distributed throughout the glass. The number of crystals, their growth rate and their
size are regulated by the time and temperature of heat treatment.
2 subdivisons:
1. Glass-based systems with added fillers ( porcelain )
2. Glass-based systems with generated fillers (ceramming or devitrification)
They are amorphous , very weak used as a matrix only
Processing technique : sintering
GLASS-BASED SYSTEMS

39. Feldspathic porcelain is used as primary application as veneers , metal ceramic veneers
and anterior laminate veneers
Secondary application as single surface inlays or low stress inlays.
Aluminous porcelain used in core ceramic for anterior teeth and low stress premolar
crowns
Advantages of porcelain: •High abrasion resistance •Chemical inertness •Excellent
thermal and electrical insulators •Excellent esthetics •Translucency •Color stability
•Capacity of pigmentation •Stain resistance •Enhanced polishability •High durable
PORCELAIN

40. Ceramming Is A Heat Treatment Of Processed Ceramics Slowly To Produce Crystals
Used As:
1. Monolithic (Esthetic)
2. Single Crown Anterior And Posterior
3. Short Span Bridge To Premolars (Low To Moderate Stress )
4. Inlays – Onlays –Laminate Veneers
Most Common Is ( E-max ) Lithium Disilicate , ( Bluish In Color , Appearance Of Crystals Is
Indicated By Being Whitish )
Processing Technique Is Pressing Or Milling
GLASS CERAMICS BY CERAMMING OR DEVITRIFICATION

41. Ceramic network infiltrated with either glass or polymeric material
2 SUBDIVISIONS:
1. Crystalline based system with glass fillers(INCERAM)
2. Crystalline based system with polymeric fillers(RESIN OR HYBRID CERAMICS)
CRYSTALLINE BASED SYSTEM (INTERPENETRATING PHASE
CERAMIC)

42. Interpenetrating ceramic network
Inceram ( zirconia – alumina)
Known as glass infiltrated crystalline network ( GICN)
Used as Core for crowns and short span bridges
Processed by slip casting
INCERAM

43. Polymer infiltrated crystalline network ( PICN ) , dual network structure
Processed by : Milling or CAD/CAM procedures
CAD/CAM is a process by which high strength ceramic blocks produced under
meticulous advanced procedures which decrease defects , blocks of different ceramics
can be milled into desired shapes designed by computer process , after milling glass
ceramics can be subjected into desired shapes .
RESIN CERAMICS /HYBRID CERAMICS

44. They are dense monophasic all crystalline materials formed directly by sintering crystals
without any glass intervening , has higher strength and lesser esthetic due to crystalline
effect
Examples :
◦ Zirconia only or alumina only , alumina toughened zirconia , or zirconia toughened
alumina
It used as :
1. Monolithic restorations anteriorly (lesser esthetics than glass ceramics )
2. core material for crowns and long span bridges
3. Posterior crowns and bridges
POLYCRYSTALLINE CERAMICS

45. 4. ACCORDING TO FIRING TEMPERATURE
Class Applications Sintering temperature
range
High fusing Denture teeth and fully
sintered alumina and zirconia
core ceramics
> 1300 degrees Celsius
Medium fusing Denture teeth pre-inserted
zirconia
1101 degrees Celsius
Low fusing Crown and bridge ceramic
veneer
850 degrees Celsius
Ultralow fusing Crown and bridge ceramic
veneer
<850 degrees Celsius

46. 5. According to abrasiveness ( comparison relative to tooth enamel against tooth enamel )
6. According to translucency ( transparent – translucent – opaque )
7. According to fracture resistance (low, medium, high)

47. MICROSTRUCTURE

48. Three critical components of ceramic microstructure are:
1. Phase Boundaries
2. Grains
3. Pores
Phase boundaries are interfaces among crystalline grains and between crystalline grain
and a glassy phase.
Grains are

49. Porosity is defined as the ratio of the total pore volume to the apparent volume of the
particle or powder .
Porosity and pore size are two main parameters for the description of a porous solid
which must be present as its very difficult to prepare truly pore-free ceramics
it can be closed or open
Pore size is the distance between two opposite walls of the pore ,Three main categories
are defined by :
1. micropores have widths smaller than 2 nm
2. mesopores have widths between 2 and 50 nm
3. macropores have widths larger than 50 nm.
However, this classification is not always used properly in practice, especially due to a
wide range of pore sizes over 50 nm.

50. Transmission decreases rapidly with small increases in porosity (0.1% is significant).
SO, if pore size is close to the wavelength of the light, scattering is maximized.
This scattering can be engineered by controlling green body preparation, sintering
conditions, and by selection of processing methods.
EFFECT OF POROSITY ON ESTHETICS

51. A low melting glassy phase is usually used for non-oxide ceramics when liquid phase
sintering is necessary to achieve densification at reasonably low temperature
Oxide ceramics are sintered via solid-state sintering and with a minimum non crystalline
phase present as they melt at relatively low temperatures
LIQUID PHASE SINTERING VS SOLID PHASE SINTERING

52. Structural integrity of ceramics is achieved by sintering in the solid state or in a gaseous
or liquid phase.
A low melting glassy phase is used for non-oxide ceramics when liquid-phase sintering
is necessary to achieve densification at reasonably low temperatures.
Oxide ceramics are sintered via solid-state sintering and with a minimum of non-
crystalline phase present.

53. The type of inter-atomic bond affects the crystal
structure of a material.
 Ionic bonds have strong attraction forces to hold
the solid together, no preferred bonding direction,
and charge neutrality.
Covalent bonds have bond direction, but the
highest atomic packing density is sacrificed for the
direction of the bonds .
ATOMIC BONDING AND ATOMIC LEVEL DEFECTS

54. Electronegativity is the measure of an atom’s strength to attract electrons
Covalent compounds are characterized by low electronegativity differences and also by
high average electronegativity.
Ionic compounds are characterized by high electronegativity differences and require
intermediate average electronegativities.
Metals and metallic compounds have low electronegativity differences and low average
electronegativities
Because atoms exist as charged ions , when defect structures are considered ,
conditions of electro neutrality must be maintained
Electroneutrality : is the state that exists when these are equal numbers of positive
and negative charges from the ions
ELECTRONEGATIVITY

55. Physical properties of crystalline solids are determined by the geometric
arrangement of their constituent atoms.
Visualization of atomic packing in crystalline solids takes each atom as a ‘hard sphere’
then identifies the smallest repeating cluster of atoms, the unit cell.
The unit cell is defined in conventional crystallography by the following rules:
1. The unit cell should have the same symmetry as the crystal
2. The origin of the unit cell is usually a center of symmetry
3. The base vectors should be short, and the cell volume minimized. Exceptions arise
only when the symmetry is increased by enlargement of the cell
4. The angle between the axis should be 90° and eventually >90°.

56. The most common imperfections in atomic arrangements are vacancies, interstitials,
impure atoms, and dislocations.
The imperfections influence the physical and mechanical properties ,the missing atom
forms a vacancy and the dislodged atom forms self-interstitial defects after moving from
its normal side.
Vacancies increase the disorder in the crystal exist in solids at all temperatures, and their
concentration increases with temperature .
Imperfections

57. Impurity atoms can be placed in interstitial or substitutional positions, and the solubility of
impurities is favored when HUME-ROMERY CRITERIA are satisfied which are :
1. Small difference in atomic radii
2. Similar crystal structure
3. Small difference in electronegativity
4. Higher valence dissolves more readily than lower valence
In contrast to substitutional solid solutions determined by HUME-ROMEY CRITERIA,
interstitial solid solutions need small atomic radii to fit into the interstices of the host
lattice.
Small amounts of solute can affect the electrical and physical properties of the solvent.
Strengthening and hardening of materials uses the formed lattice strain to increase
strength and hardness.

58. Dislocations are linear defects in crystals that are formed where a plane of atoms
terminates abruptly in the lattice , they are are formed around atomic planes
All crystalline materials contain dislocations that influence the physical and mechanical
behavior of the material, such as plastic deformation, phase transformation, and thermal
stresses.
Dislocations can be mobile under applied stress, and obstacles lead to strengthening of
the material.

59. Charges of atoms are often diverse :
1. Metallic ions ( cations) positively charged ions
2. Nonmetallic ions ( anions) negatively charges ions
The crystals must be electrically neutral , all the cation positive charges = anion negative
charges
Constituent crystals of ceramics consist of atoms with different sizes where the sizes or
ionic radii of the anions and cations are different
Because the metallic elements give up electrons when ionized , cations are ordinarily
smaller than anions

60. 3. Stable ceramic crystal structures form when those anions surrounding a cation are all in
contact with that cation

61. 1. Number of anions nearest neighbors for a cation
2. The coordination number is related to the cation- anion radius ratio
3. For a specific coordination number, there’s critical or minimum rC/rA ratio which is
cation-anion contact is done
COORDINATION NUMBER

62. Point Defect ( 0 D ) :Vacancies – Self Interstitials – Impurities
Line Defect ( 1 D ) : Dislocations
Plane Defect ( 2 D ) :Surfaces And Interfaces
Extended Defects ( 3 D ) : Pores And Cracks
IMPERFECTIONS

63. STRUCTURE-
PROPERTY
RELATIONSHIP

64. Optical properties are affected by composition, crystal structure and by interferences
It can be transparent, translucent, or opaque depending on its microstructure, in
particular the features that diffuse light and make it difficult to pass through.
Three phenomena are important refraction, deflection, and transmission of light , They
are wavelength dependent.
Ceramics can interact with electromagnetic fields and exhibit changes in fluorescence,
phosphorescence, color tone, photoconductivity and polarization.
Refraction is related to the velocity of light, which can be characterized by the refractive
index (n). This is the ratio of the velocity of light in a vacuum to that in any other medium.
OPTICAL PROPERTIES

65. Thickness of the films and the nature of the reflecting surface are also important.
If the surface is not a smooth plane, some of the light will be scattered diffusely rather
than reflected in a single direction. Transmission of light (transparency) through advanced
ceramics is good and the basic requirement is minimal interaction of the electromagnetic
wave with the material.
Optical transparency in polycrystalline materials is limited by the amount of light that is
scattered by their microstructural features. Since visible light has a wavelength scale in
the order of hundreds of nanometers, scattering centers will have dimensions on a similar
spatial scale or bigger.
Clear transparency requires high in-line transmission, otherwise the resulting material is
translucent
The loss of transparency is due to scattering of the incident beam, which can occur
because of several reasons like residual porosity, precipitates, intergranular films, and
grain boundaries cause incoherent scattering of light.

66. Transmission decreases rapidly with small increases in porosity (0.1% is significant). if pore
size is close to the wavelength of the light, scattering is maximized.
This scattering can be engineered by controlling green body preparation, sintering conditions,
and by selection of processing methods.
Grain boundaries can scatter light when refractive indexes are discontinuous, and is caused
by birefringence, precipitates, or intergranular film. Matching refractive indexes between
different phases can reduce scattering losses. This is why symmetric cubic crystal structure is
easily transparent. When the size of the scattering center (or grain boundary) is reduced well
below the size of the wavelength of the light being scattered, there is no longer significant
light scattering.
Grain size is also an important parameter. Scattering is at a maximum when particle
diameter is close to wavelength (λ). The reason is that small grain sizes have scattering
centers that are too small, and big grains have reduced numbers of scattering centers (grain
boundaries). Most oxide ceramics, such as alumina and associated compounds, are formed
from fine powders. This yields a fine-grained polycrystalline microstructure that is filled with
scattering centers comparable to the wavelength of visible light. The solution for transparency
is preparation of nanoceramics with grain sizes below wavelength. Fabrication of transparent
non-oxide ceramics has proven to be more difficult because of their low sinterability and their
high level of intergranular films and precipitates in the polycrystalline sintered bodies.

68. Mechanical properties such as fracture strength and Young’s modulus can be predicted from
an analysis of the strength of the ionic and covalent bonds.
Young’s modulus is related to inter-atomic bonding forces, when it is the measure of small
changes in the separation of adjacent atoms (the same for both tension and compression).
oxide ceramics such as zirconia and alumina are strong, even stronger atomic bonding can
be found in the group of non-oxide ceramics that comprises carbides, nitrides, and borides.
These chemical compounds have covalent bonding or covalent-metallic which determine
general properties such as high melting points, high chemical resistance, high hardness, and
high stiffness.
To achieve fundamental mechanical strength, the covalent bonding has to be arranged in a
symmetric crystalline structure.
MECHANICAL PROPERTIES

69. Melting temperature is an indicator of atomic bonding strength and there is relationship
with Young’s modulus. The general trend is that a higher melting temperature indicates a
higher modulus and vice versa.
Ordering of the crystalline lattice is favorable and that leads to a more stable
structure(thermally and chemically) than amorphous (glasses).
Alumina, where the structure is hexagonal, closely packed, and stable at high
temperatures of up to 1925 °C in both oxidizing and reducing atmospheres. Its chemical
stability is also excellent.
BIOLOGICALAND THERMAL STABILITY

70. powders contain agglomerates, which are weakly bonded groups of particles and
aggregates (hard agglomerates), which are strongly bonded groups of particles.
Both often yield heterogeneities in particle packing during the shaping process that leads
to differential shrinkage during sintering and the formation of residual pores.
Particle packing during the shaping stage determines the microstructure and the final
properties of sintered ceramics.
PROPERTIES DETERMINED BY PARTICLE-PACKING DEFECTS

71. Grain boundaries are characteristic microstructure features of the polycrystalline
ceramics that modify ,introduces additional properties in ceramics, compared to their
constituent crystalline phase
Inside the grain boundaries, atoms are less ordered in conjunction with the impurities and
the formation of the secondary glassy phase. The grain boundaries can be regarded as
structural-disorder weak interfaces.
PROPERTIES DETERMINED BY GRAIN BOUNDARIES

72. The mechanical properties of ceramics are sensitive to porosity.
Strength and hardness decrease with the increase of porosity.
The decrease of strength with porosity is described by the ryshkewitch equation
Porosity and pore size of advanced ceramics influence their chemical resistivity and
bioactivity which increase with increasing porosity. Ceramics with open porosity and high
surface area are vulnerable.
Advanced ceramics aiming for bioactive applications require designed porosity . micro,
meso, and macro porosities are present, also voids and microchannels are introduced to
achieve the required long-term bioactivity
PROPERTIES DETERMINED BY POROSITY AND PORE SIZE

73. The fracture of ceramics is controlled by the size of the microscopic cracks
This implies that fine grain structure is preferable for ceramics with high mechanical
strength for load-bearing restorations
Nanoceramics, defined as ceramics composed of crystalline grains less than 100 nm.
The phase transformation behavior of partially stabilized zirconia also depends on grain
size. When the grain size of tetragonal zirconia is too small (< 0.3 μm), the tetragonal
zirconia appears very stable and can hardly transform to monoclinic zirconia under
cracking stress.
On the other hand, when the grain size of tetragonal zirconia is too large (≥1 μm), the
tetragonal zirconia grains may spontaneously destabilize towards the monoclinical form.
Grain size is also crucial for functional ceramics. In addition, grain size influences the
surface friction and wear behavior.
PROPERTIES DETERMINED BY GRAIN SIZE

74. Phase transformation is considered for improvement of structural and functional
properties.
The crystalline structure of alumina is temperature-dependent and several metastable
structures exist; however, they all irreversibly transform to the hexagonal α-alumina
beyond 1150 °c.
This transformation is connected with a big volume change, and metastable alumina
can be used for preparation of strong porous α-alumina or as a binder for α-alumina
grains or to directly prepare a strong porous body.
PROPERTIES DETERMINED BY PHASE TRANSFORMATION

75. The crystalline structure of pure zirconia is monoclinic up to 1170 °c, at which point it
transforms to the tetragonal phase and remains stable up to 2370 °c, when cubic
zirconia is formed
Phase transformation of t-zro2 to m-zro2 is accompanied by considerable dimensional
changes. This causes stress that results in fragmentation of the material.
Zirconia parts are usually sintered at temperatures above 1200°c, so pure zirconia
(without stabilizing additives) can be used only in powder form.
Stabilized zirconia is free from phase transformation over the entire required
temperature range, mainly from the sintering temperature to room temperature

76. 1. Advanced Ceramics Textbook
2. Craig’s Restorative Dental Materials Textbook
3. Fundamentals of Materials Science and Engineering 5th Edition
REFERENCES

77. THANK YOU