Dental ceramics, structure and propertiesrandwick.sydneyinstitute.wikispaces.net/file/view/cms+structure... · Ceramics have a tradition of being used in dentistry, long before we

Structure of

Dental Ceramic Materials

IntroductionCeramics have a tradition of being used in

dentistry, long before we could use them well.

Why?

Ceramic materials, have significantly different properties from metals and plastics. The first thing we have to do to understand why we use ceramics is to look at their properties. Then we will look at how the properties of ceramics are achieved, so that we can learn to control and use them.

Introduction

Ceramics are named from a Greek word for “fired material” because so many of them are made by heating a powder or a liquid with powder in it until it fuses together sufficiently to form a solid mass. This is an ancient technique and the materials produced have the following typical ceramic properties:

They have high melting, or softening

temperatures.

They are brittle, that is, they have no

capacity for plastic deformation. If you apply a stress to ceramics and increase it, they will eventually break. After fracture, the pieces will fit back together in the original shape.

They are generally hard and wear

resistant

Introduction

Introduction

They do not readily conduct electricity or

heat.

They are much stronger in compression than in

tension; this is connected with their brittleness.

Although they are not fully chemically

inert, they are often resistant to chemicals that would damage metals or polymers.

They can be treated to give a wide range of

colours and aesthetic effects.

The first ceramics used in dentistry in the late eighteenth century were porcelains, that is, they were made from a highly refined fired white clay.

The major advantages of this material were: It was hard and wear resistant (in fact harder

than human teeth).

It was very inert and harmless to human

tissues (bio-compatible).

It was cosmetically similar to human bone or

teeth. (No other restoration material had that ability at the time)

The first ceramics used in dentistry

The major disadvantages were:

It was brittle.

Had low impact toughness.

It appeared opaque and “lifeless” even if it

was the right colour.

It was difficult to manufacture dimensionally

accurate products because of shrinkage during firing.


Modern dental ceramics have changed considerably since then. They are no longer made from clay but from firing a powdered glass mixture, a technique first developed in 1903. Also their toughness is improved by the ceramic being bonded, or ‘enamelled’ onto a metal base. This technique is as old as jewellery but first adopted for dental use in the 1950’s.


This technique is as old as jewellery but first adopted for dental use in the 1950’s. Modern porcelains are stronger and their application techniques have been developed to have a semi-translucent appearance, which makes them cosmetically almost completely undetectable.


It is rather hard to define what a ceramic is. More than one scientist has suggested that they are what are left over when metals and polymers are removed. Most ceramics are made from compounds of a metal and a non-metal. They contain a mixture of covalent and ionic bonding, the proportions of which determine the mechanical properties. The most common ceramics, for example, are metalloid oxides, such as silica (SiO2). The minerals which make up the bulk of the natural materials around us are complex metal silicates hence ceramics.

The Molecular Structure of Ceramics

The molecular structure of ceramics is generally very complex compared to metals. If ceramics are crystalline, their crystal structures are often more complex than those of metals. If ceramics involve covalent bonding, the molecular structures are three-dimensional and complexcompared to polymers.


As a result ceramics cannot undergo atomic like metals or chain realignment like polymers. They are not capable of changing shape without fracturing, unless they are heated to considerably higher temperatures. This is why we can only form ceramic objects by mechanically grinding or wearing them away or by heating them until they become plastic.


The most common ceramics we use are clay-based or argillaceous ceramics. These are made by suspending clay particles in water to make a paste, which we shape, then firing that shaped article. The water is first removed, then the particles gradually fuse together until they become a glass (vitreous) material.


The resultant shaped article is held together by this vitreous phase. The higher the firing temperature, the more vitreous phase there is, and the stronger and more transparent the ceramic becomes. If we fire such a material until it is totally vitreous, we have a glass. Usually the material will become liquid at this stage, although it will be a very viscous liquid.


The Molecular Structure of CeramicsClay ceramics are made from mixtures of clay, sand, and minerals which contain metal oxides of sodium, potassium or calcium. These have the effect of fluxing the mixture or lowering the softening temperature to a useful point. Dental ceramics, however, contain very little clay. Dental ceramics are glasses. Like most glasses, they are made from minerals essentially containing a lot of silica (SiO2), some alumina (Al2O3) and small but important amounts of other metal oxides.


Dental ceramics are essentially made from a mineral called Feldspar, with added silica and fluxing oxides. Feldspar has the approximate composition - K2O.Al2O3.6SiO2 and also Na2O.Al2O3.6SiO2 (albite). Dental ceramics are made from, roughly 80% feldspar, 15% silica (SiO2)and 4% clay.


Dental ceramics are actually fired twice. The first time they are fired until all the ingredients are fused together at so high a temperature that the material becomes liquid. This liquid is then cooled rapidly, making it a solid glass. The glass is crushed to a powder, technically called a frit.

This powder becomes the basis for the additions necessary to make the various special purpose dental ceramics. Dental technicians mix the powder with water to form a formable paste and make the dental restoration. This is then fired a second time, until the softening temperature of the glass is reached. At this point the glass powder particles start to soften on their outside surfaces and bond together at the contact points. This is a process called sintering. Our so-called dental “porcelains” then are not porcelain at all, but sintered glass.


Glass is defined as a material where the loosely arranged mixture of atoms found in the liquid state has been kept down to a temperature low enough that the substance has the mechanical properties of a solid. A short definition is to call glass a “supercooled liquid”. In fact, even window glass is a liquid. Window panes measured after several hundred years in a window frame actually become thicker at the bottom and thinner at the top; all very slowly flowing downwards!

So, what is a glass?

Glass forms when a material is cooled too quickly for the atoms to re-arrange themselves from the random structure of the liquid into the regularly spaced structures of a solid. Theoretically any material can be made into a glass if it can be cooled quickly enough. If metallic gold is heated to 1083˚C, the atoms separate from their fixed positions in the crystal lattice and move freely around in the liquid state.


If this liquid is cooled normally, the atoms go back to their positions in a regular crystal structure and the metal crystals reform. If liquid gold is cooled to room temperature at a rate of about eight million degrees a second, the atoms have no time to form a crystal structure, and remain in their random liquid positions. Gold glass is formed, a glass which is transparent.


A cooling rate of eight million degrees a second is difficult to achieve! However, if the material has a much more complex structure than the simple structures of metals, we don’t have to cool them so fast to keep the liquid structure. Something as complicated as an organic carbohydrate will form a glass if cooled by dropping the liquid in cold water. The result of cooling a sugar based liquid at this rate of about two degrees a second is a glass called toffee. That’s right, you’ve been eating glass!


For something with the complex structure of mineral silicates, the cooling rate necessary to form glass is only a few degrees an hour. In fact, to form the crystal structure of these minerals, some have to be cooled as slowly as a few degrees a year. The easiest glasses to form are made from the oxides of small multivalent atoms such as silicon, boron, germanium or phosphorus. Easily the most common glasses are based on Silica (SiO2) the most common mineral on the surface of the earth. The dental ceramic we use is also a glass made mainly from silica.


The atom arrangement in its structure consists of two covalently bonded atoms, two oxygen to one of silicon. Silicon has four electrons in its outer shell, oxygen has six, so that the resulting structure has each oxygen atom bonded to two silicon atoms, and each silicon atom bonded to four oxygen are double bonds.



This is the basic geometric unit of the resulting structure, a pyramidal shape called a tetrahedron.


Although it shows one silicon atom to four oxygen,

each oxygen in fact bonds to another silicon atom as well. This joins the pyramid units together to make a structure. If all the pyramids are joined at regular intervals and spaces, there will be a crystal structure lattice, and the resulting material is a crystalline compound, quartz, or cristobalite, ortridymite, depending on the actual structure. If the pyramids are joined at irregular intervals and spaces, as would happen if molten silica was rapidly cooled, the compound is a silica glass.

In the diagrams below a triangle represents one silica pyramid


Regular bonded structure of tetrahedra - a crystalline material

Irregular bonded structure of tetrahedra – a glass

There are three types of substance that can take a place in the structure of a glass. These are network formers, network modifiers, and intermediates.

A network former is a substance such as silica that can form the loose network of molecules to become a glass. Other examples are germanium oxide (Ge2O3) and boron oxide (B2O3).

An intermediate is a compound that does not form its own glass network, but can be inserted into the network of another material. Examples are Alumina (Al2O3) and lead oxide (PbO).

Substances which are be incorporated into the glass structure

A network modifier is a most important additive. These are compounds which give large metallic ions. The ions penetrate the glass network, and replace the covalent bonds with ionic bonds. The ionic bonds are weaker, so the resulting glass now has a lower softening temperature, reduced strength and reduced chemical resistance. Examples are compounds of sodium potassium and calcium, such as Na2O or Na2CO3; K2O or K2CO3, and CaO.


An example of the effect of network modifiers is that if enough sodium ions are added to silica glass, it gradually becomes a water soluble crystalline compound, Na2SiO4, sodium silicate. Another example is normal window glass, which has enough sodium (in the form of sodium oxide) and calcium (in the form of calcium oxide, lime) added to it to reduce the softening temperature of the pure silica from about 1800˚C to around 850˚C.


Obviously, we need some modifiers in our dental ceramic glasses, to reduce the softening temperature to a point where it is easy to fire. However, we don’t need so much that the glass will dissolve, or become weak in use in the mouth.


A typical dental ceramic will contain about 50-70% silica; 10-20% alumina; 4-10% sodium oxide, 8-10% potassium oxide and 1 or 2 % calcium oxide. There will also be smaller amounts of many other metal oxides.


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Dental ceramics, structure and propertiesrandwick.sydneyinstitute.wikispaces.net/file/view/cms+structure... · Ceramics have a tradition of being used in dentistry, long before we