1. DENTAL PORCELAIN Stephen C. Bayne and Jeffrey Thompson
Department of Operative Dentistry
School of Dentistry
University of North Carolina
Chapel Hill, NC
Porcelain has been a routine part of dentistry for more than 200 years, but its brittle nature has severely limited its use. From the 1950s to about the 1990s, the primary method of porcelain use has been in PFM restorations. However, the last 1-2 decades have represented an increasing use of all-ceramic restorations. Those will be discussed in more detail in the next lecture. At the moment, the terminology, structure, properties, and fabrication technologies for porcelain will be considered.
[CLICK] To review any other topics or units, go to the master site for all dental materials lectures.
 2004, Bayne and Thompson, UNC School of Dentistry.

2. Terminology and Structure
REVIEW
Terminology and Structure
Review of Definitions and Terminology:
1. Ceramic = Any compound involving metallic and
non-metallic elements (e.g., MX)
a. Enamel = ceramic coating over a substrate
b. Porcelain = ceramic based on K-Al-Silicate (K2O-Al2O3-SiO2)
(1) Dental Porcelain = narrow range of porcelains
B. Review of Ceramic Structure:
1. Arrangement: mixture of crystalline and non-crystalline phases
2. Bonding: ionic and/or covalent
3. Composition:
a. Silicate Types Used to Produce Porcelain:
(1) Clays (hydrated aluminosilicates)
(2) Feldspars (anhydrous aluminosilicates)
(3) Quartz (silica)
b. Non-Silicate Types (e.g., MgO)
4. Defects: pores and cracks
Quickly. Let’s review the major categories of information about ceramics. [CLICK] A good working definition for a ceramic is any material that is composed primarily of a metal and non-metal composition. Enamel is any ceramic coating over a substrate. Dental enamel is just one type of this case. [CLICK] Porcelain is a special type of ceramic based on a specific dominant composition that includes silica, alumina, and potassium oxide. These three oxides are alloyed and produce a potassium aluminosilicate. Dental porcelain is a very narrow range of these compositions.
[CLICK] The important categories of information for any ceramic (or porcelain or dental porcelain) are the arrangement, bonding, composition, and defects. Most dental porcelains are partially crystalline. Their bonding is mixed and dominated more by covalent than ionic character. The composition includes non-crystalline and crystalline phases. [CLICK] Dental porcelain is created, not by directly mixing the three main oxides, but by mixing clay, feldspar, and quartz that contain the oxides. Dental porcelains are dominated by feldspar and tend to have more silicate matrix in the final microstructure than dispersed crystalline phases. These are called feldspathic porcelains. Feldspathic porcelains are esthetic but not very strong. As the alumina content is increased the amount of crystalline dispersed phase, particularly alumina rich ones, is increased and the material becomes stronger. However, aluminous porcelains are whiter and lack the translucency of feldspathic ones. Aluminous porcelains are good for underlying cores while feldspathic ones are good for esthetic veneers. Dental porcelains are limited severely by their defects, particularly pores which originate cracks.

3. REVIEW Properties C. Review of Ceramic Properties:
1. Physical Properties:
a. Intermediate density ( gms/cc)
b. High melting point (= refractory)
c. Low coefficient of thermal expansion (1-15 ppm/C)
2. Chemical Properties:
a. Low chemical reactivity
b. Low absorption and solubility
3. Mechanical Properties:
a. High modulus
b. Much stronger in compression than tension (~10X)
c. Brittle (low plastic deformation (<0.1%); low fracture toughness
4. Biological Properties:
a. Relatively inert
The categories of properties are grouped as physical, chemical, mechanical, and biological properties. Not all possible properties in each category are of importance for all applications so just a few are reviewed above.
[CLICK] Ceramics (and porcelain) have intermediate densities (between the low densities of polymers and the high densities of metals), typically in the range of about 3 gms/cc. Most have high melting temperatures. High melting materials are called refractories. Compositions with high melting temperatures typically have very low coefficients of thermal expansion.
[CLICK] Ceramics have relatively low chemical reactivity and are almost inert in the oral environment. They can be dissolved in very strong acids, such as HF, but generally are very stable under normal oral conditions.
[CLICK] The mechanical properties of ceramics include a high modulus, very little plastic deformation, and a much higher strength in compression than tension. Thus, all loads should be directed to produce compression to avoid any premature fracture.
[CLICK] Biologically, ceramics are relatively non-reactive and are thus considered biocompatable.

4. Composition and Microstructures
REVIEW
Composition and Microstructures 20 40 60 80
100
1,000
1,200
1,400
1,600
1,800
Composition (% SiO2)
TEMPERATURE (C)
Liquid
K20-Al2O3-4SiO2
SiO2
L +
Leucite
Leucite +
Potash
Feldspar
Cristobalite
L + Potash
Potash Feldspar
+ Tridymite
Tridymite
LIQUIDUS
SOLIDUS
Dental
Porcelains
VITRIFICATION
SINTERING
MELTING,
LIQUIFICATION
The best place to start when considering a ceramic (or porcelain) composition is to examine the composition and microstructure. Identify the phases that are present. Since most are semi-crystalline, the microstructural appearance is quite similar to that of a composite. A typical image is a non-crystalline matrix and one or two crystalline dispersed phases. Pores are distributed in the non-crystalline phase.
The amount of the phases depends on the phase diagram. Remember that we are working with silica dominated compositions that have roughly equal moles of Al2O3 and K2O as shown on the ternary phase diagram above. [CLICK] Examining the pseudo-binary phase diagram, the left-hand edge is K2O-Al2O3-4SiO2 and the right-hand edge is pure SiO2. [CLICK] The typical range for dental porcelains is at K2O-Al2O3-6SiO2 as shown above. At high temperatures (above ~1400 C) porcelains can be completed melted. Between about 1040 C and 1400 C, the compositions are vitrified (e.g., partially melted). Below ~1040 C, the compositions can be sintered under heat and pressure to form a coherent solid.
Note that many of today’s modern ceramics are formed under extremely high pressures at sintering temperatures to produce pore-free materials. These are generally produced by a process called H.I.P. or hot isostatic pressing. These will be discussed in a later lecture.

5. Systems for Classifying Dental Porcelains
CLASSIFICATION
Systems for Classifying Dental Porcelains
A. Classification Systems for Dental Porcelains:
1. Classification based on fusion (vitrification) temperature:
a. High-fusing: to 1371 C (2350 to 2500 F)
b. Medium fusing: to 1260 C (2000 to 2300 F)
c. Low fusing: to 1066 C (1600 to 1950 F)
2. Classification by restoration component:
a. Porcelain core
b. Porcelain inlay
c. Cast porcelain
d. PFM
There are at least three systems for classifying dental porcelains. All are arbitrary. They include (1) consideration of the vitrification (or fusion) temperature, (2) role of the restoration component, and (3) the esthetic role of the porcelain. Each of the following are considered in more detail.
[CLICK] The fusing temperature is the melting range or vitrification temperature. High fusing materials are traditionally used for porcelain teeth that are utilized in dentures. [CLICK] Medium fusing materials are processed for cores. [CLICK] Low fusing materials are part of PFM or porcelain veneered ceramic cores.
[CLICK] You can also classify different types of porcelains by applications. This almost parallels the first system but is not very useful.
[CLICK] Finally, one can classify porcelains in terms of their function in producing an esthetic crown – opaque porcelains to hide metal substructures, body porcelains to mimic dentin and enamel shades, and stains or glazes to seal the surfaces of porcelain.
3. Classification based on esthetic role of porcelain:
a. Opaque porcelain
b. Body porcelain (incisal or enamel; gingival or dentin; modifier)
c. Stains or glazes

6. Examples of Low Fusing Dental Porcelains
CHEMICAL ANALYSIS
Examples of Low Fusing Dental Porcelains
B. Chemical Analyses of Low Fusing Porcelains:
Weight Percent
BIODENT
Opaque B62
CERAMCO
Opaque 60
Dentin BD27
Dentin T69
SiO2 =
52.0
55.0
56.9
62.2
Al2O3=
13.55
11.65
11.80
13.40
K2O =
11.05
9.6
10.0
11.3
Na2O
5.28
4.75
5.42
5.37
TiO2
3.01
0.61
ZrO2
3.22
0.16
1.46
0.34
SnO2
6.4
15.00
0.50
RbO2
0.09
0.05
0.10
0.06
CaO
0.98
BaO
1.09
3.52
ZnO
0.26
B2O3,CO2,H2O
4.31
3.54
9.58
5.85
When we talk about dental porcelains from this point onward, we automatically infer that we are using low-fusing (or low melting) porcelain compositions. Obviously these are the simplest to fabricate in a dental laboratory. [CLICK] Shown above are examples of two opaque and two dentin shade porcelains as examples. [CLICK] As we have already mentioned, the dominant oxides in porcelains are silica, alumina, and potassium oxide. As you can see above, these constitute between 75 and 85% of the composition by weight. In opaque porcelains shown to the left, titanium dioxide, zirconia, or tin oxide is added to create dispersed phases within the microstructure that reflect light and hide any metal substructure or less esthetic ceramic core. Dentin shades have a very light yellow tinge and are moderately transparent.

7. Fabrication Techniques (Vitrification)
DENTAL PORCELAIN
Fabrication Techniques (Vitrification)
A. Definitions and Terminology Related to Manipulation:
1. Condensation = padding or packing of wet porcelain into position
2. Biscuit = cohesive power compact
3. Frit = unfused or partially fused compact
4. Firing (low-med-high bisque) = fusing and eliminating porosity
5. Soaking = holding at high temperature
P,V,T Tricks for Firing:
Minimize number of firings.
Use lower temperatures and longer times.
Always heat very slowly.
Use vacuum furnace.
Use fine grained powders for finer porosity.
Only under the rarest of circumstances will you be fabricating your own porcelain restorations. These are processed in a dental laboratory. Porcelain powder is transferred onto a metal substructure and baked in a furnace to partially melt (e.g., vitrify) the material to generate a coherent coating. The stages and terminology are now quickly reviewed above.
[CLICK] For PFM restorations, porcelain powders of the proper color and translucency are layered onto the cast metal substructure. The first layer for a PFM restoration is always an opaque porcelain to hide the metal substructure. Powder is manipulated by mixing it with a small amount of water and painting a slurry onto the metal. Then the water is removed by padding it away and careful evaporation, allowing the particles become compacted and electrostatically attracted to each other, sufficient to maintain the intended shape. At this point the powder layer is called a biscuit. [CLICK] After some firing to high enough temperatures for vitrificatoin, the particles partially melt and begin to fuse together to create a frit. If it were cooled at this point it would be weak and porous. Continued heating (or firing) continues to partially melt the compact, shrink the mass, and eliminate the intervening porosity. [CLICK] [CLICK] [CLICK] Ideally, the final fired porcelain should be pore free. [CLICK] Most firing processes reduce the initial 32% porosity down to about 2-3% porosity.
[CLICK] [CLICK] Some of the tricks for encouraging this process are to minimize the total number of firings, go slower by using lower temperatures for longer times, heating only very slowly, utilizing a vacuum furnace, and starting with fine grained powders so the porosity is of very small size. The stages of applying an opaque and two body porcelain layers [CLICK] is schematically summarized in the bottom right hand corner with three firings [CLICK] [CLICK] [CLICK] for each of the low-fusing compositions. 1
Porcelain
Application
Vacuum
Furnace 2 3

8. Strengthening Mechanisms for Dental Porcelain
1. Alumina or leucite crystalline phases act as reinforcement
2. Cracks stopped by crystalline phases
A. Alumina Reinforcement:
fracture
COMP
STRESS
TIME
TENSILE + -
Pre-
stress
1. Use high-modulus thick metal
substructures for stiffness
2. Porcelain bonded to metal
B. Porcelain-Fused-to-Metal Bonding:
Prevents porcelain flexure
There are really two major paths for producing porcelain veneered crowns: (1) porcelain veneers over stiff metal substructures (PFM) or (2) porcelain veneers over ceramic cores (aluminous porcelains or other strong ceramic cores). [CLICK] Normal porcelain has some dispersed leucite crystals that strengthen it, [CLICK] but more are needed for good strength. Increasing the crystalline phases by the inclusion of more alumina, generates white and opaque ceramic that must be veneered with feldspathic porcelain to become esthetic.
We will concentrate now on the first path (PFM restorations). [CLICK] To insure that the fragile and brittle porcelain veneer will not crack under function, it must be protected from any opportunity to flex. Stiffening is provided by a metal substructure of sufficient thickness and modulus to stress shield the overlying porcelain. [CLICK] Further stress shielding can be produced by trying to pre-stress the porcelain in compression. [CLICK] Since porcelain is strong in compression and weak in tension, [CLICK] pre-stressing increases the amount of tensile strength required to cause fracture. [CLICK] The small stress-time diagram above demonstrates the process.
1. Make metal CTE slightly greater than porcelain
2. On cooling, the metal places porcelain in compression
C. Porcelain-Fused-to-Metal Bonding:
Porcelain Pre-stressing

9. Dependence on Surface Defects
FLEXURAL STRENGTH
Dependence on Surface Defects
CONDITION Fine Medium Coarse
Pristine 320    14
Sandblasted 120    13
Ground    10
Machined    4
Fatigued 122    6
A constant concern of all porcelain or other ceramic systems is control of surface defects as well. Small surface defects may concentrate stress, initiate a crack, and fracture porcelain. Surface defects are minimized by applying glaze coats over porcelain at the end. However, during intraoral occlusal adjustments, relatively large defects are introduced by the grinding with stones or burs. These defects must be removed by careful and diligent finishing and polishing of those sites. Ideally, the final polish should be less than 1 micron diamond paste.
Potential effects of defects on strength are demonstrated above. This data is from Dianne Rekow and considers three different grain sizes of porcelain but we will focus on the fine grained example. [CLICK] The pristine porcelain has a flexural strength of 320 MPa. [CLICK] Sandblasting, grinding, and machining all dramatically reduce the strength, even more than fatigue cycling the material.
Remember to remove all surface flaws.

10. Thank you.
THANK YOU