Table of Contents
Bonding and how it works
This series represents a mini course in dental ceramics for the beginner, and persons seriously interested in gaining a basic working knowledge of dental ceramics are advised to take the time to start at the beginning. If all five pages are read in order, the reader will gain a good understanding of just what dental ceramics really are, how they differ from each other other and how different forms of porcelain are utilized in various applications.
Since dental ceramics contain both glass and refractory structures, many of the terms used on pages devoted to the specialized dental porcelains are defined on this page and the first one in the series. While the material presented on subsequent pages is designed to stand alone, a real understanding relies on knowledge presented on the first two pages.
Recall, from page 1 of this course we discussed the composition of naturally occurring forms of glass called feldspars. On this page, we delve more deeply into glasses in general.
There is no single chemical composition which characterizes all glass. There are thousands of different formulations for different types. However, all glass formulations have three things in common; Glass formers, fluxes and stabilizers (also known as modifiers).
Glass formers are metal oxides which are able to retain the amorphous property of their molten state when setting to a solid. Other minerals return to crystalline state as they set, or soon thereafter. The most common glass former is silicon dioxide (SiO2), also known as silica, which is responsible for the glass in our windows, and in our cupboards, It is also the glass former found in most feldspars. Feldspars account for the glassy phase in pottery and domestic porcelain. Silica is also the basis of the glazes which protect and waterproof the outside of finished pottery. Finally, silica is the glass former which is involved in the bulk of dental porcelain. Therefore, it is the glass former that we will be concentrating on in this series.
To recap from the previous page; Silica has the chemical formula of SiO2. Even though the chemical formula of silica shows only two atoms of oxygen associated with each silicon atom, silicon actually forms bonds with four oxygen atoms when in combination with other silica molecules. It does this by sharing oxygen atoms with two adjacent silica molecules. Thus, it forms tetrahedral crystalline structures. The tetrahedrons are bonded together via shared oxygen atoms at each apex of the tetrahedron, as in the diagram below.
This diagram describes crystalline silica, but the addition of alkaline metallic oxides in the form of fluxes can cause the ordered crystalline structure of quartz to become disordered, as it does in feldspar glasses. Glasses differ from crystalline solids in that glasses do not have distinct melting points while crystalline structures do. Crystalline structures have a specific temperature above which they exist in only liquid form, and below which they are frozen solids.
The chemical bonds holding the atoms together in a regular crystalline structure (like the quartz crystalline structure diagramed above) are identical. When the crystalline solid is heated, all the bonds break at exactly the same temperature. Below this temperature, called the melting point, the material is solid; above the melting point the material is a liquid.
In contrast, the bonds in glass show a range of strengths as a result of their disordered molecular structure. (The amorphous nature of glass will be discussed in detail later). When a glass is heated, these bonds break over a range of temperatures. As a result, a glass softens gradually as it heats, and hardens gradually as it cools. While glasses do not have a definite melting temperature, they do have definite solidus temperatures. The solidus is the lowest temperature at which a non crystalline material shows any characteristics of melting, including a tendency to flow.
Glasses have the mechanical rigidity of crystals, but the random disordered arrangement of the molecules that characterizes liquids. Thus glass is considered a “supercooled liquid”, but since at ordinary temperatures it is well below its solidus, it does NOT flow at room temperature. By the time the glass has cooled to several hundred degrees centigrade, even the most recalcitrant silica tetrahedrons have bonded in some fashion with their neighbors. All those stories you have heard about cathedral windows being thicker at the bottom than at the top are myths.
Silica is the most important glass-former, but it is not the only one. All glass formers are metallic oxides. They include the oxides of boron, phosphorous, antimony, arsenic germanium and selenium. Of these, boron oxide (B2O3) is probably the most useful since small amounts–between 5 and 15%– added to a silicon based glass create a very tough and heat resistant glass. Borosilicate glass is the basis of Corning’s Pyrex cookware and is used in laboratory test ware and sealed beam headlights. Used by itself without silicon, however, a glass based entirely on boron oxide is of little use since it is soluble in water.
In crystalline silica, (quartz, flint, chert and sand), the silica tetrahedrons naturally align themselves in a three dimensional lattice network in which each silicon molecule is covalently bonded to four oxygen atoms, and each oxygen atom is covalently bonded to two silicon atoms. Complex three dimensional lattices are difficult to illustrate so we will work with a simplified diagram in which each silicon atom is linked to only three oxygen atoms. In reality, the hexagonal structure illustrated here is really made of three dimensional silica tetrahedrons. In order to picture the real three dimensional lattice, picture a series of images like the one above stacked, one on top of another. The image of the quartz crystals below shows what the final outcome would look like; a six sided crystal that ends in a six sided pyramid.
In the illustration above, the red balls represent the silicon atoms and the blue balls represent the oxygen atoms. The silicon and oxygen atoms are bonded together with covalent bonds in which each atom shares an electron with its neighbor. Covalent bonds are highly directional, and the orderly lattice you see in the diagram above is representative of the order in a quartz crystal.
When the quartz crystal is heated to its melting temperature, the silicon and oxygen atoms remain covalently bonded, but they begin to trade partners freely disrupting the orderly lattice. As the temperature rises above the melting point, more and more of the lattice disappears and the melt becomes thinner and thinner–i.e. less and less viscous. Now, if the temperature is allowed to drop very slowly, the lattice begins to reform. On the other hand, as the liquid cools, it becomes thicker and thicker–more and more viscous. The increasing viscosity is apparent to the individual atoms in the melt, and as the melt cools, it becomes more difficult for the atoms to rearrange themselves back into the original orderly matrix. The crystalline lattice can reform, but it takes a lot of time. It helps if the mixture is held for a long time within the range of temperatures in which the glass is cool enough for seed crystals to form, but still hot (thin) enough to allow movement of the molecular structures. Once a seed crystal forms, it serves as a template to which neighboring atoms can attach, allowing the crystal to grow larger and larger.
On the other hand, if the melted silica is cooled relatively quickly, especially if it cools quickly through the temperature range in which seed crystals tend to form, then the viscosity of the melt increases too quickly for the atoms to migrate back into the original lattice formation. Thus the atomic structure remains disordered, and as the viscosity increases still further, the original quartz crystal becomes frozen into a shapeless gel of disordered silica tetrahedron chains. This amorphous gel is called glass, and the process that created amorphous glass from the original silica crystal is known as vitrification. (Amorphous means shapeless, and here it refers to the lack of an orderly crystalline matrix at the molecular level.) The opposite of vitrification is called devitrification which can happen if glass is annealed for to long a time.
Fluxes are alkaline metallic oxides that are used to lower the melting point of the glass former. Crystalline silicon in the form of quartz melts at 1713 degrees centigrade. This is an extremely high temperature, even for industrial purposes, and exceeds the melting temperature of iron (1510 °C) and steel (1370 °C). A mixture of finely ground powders of 25% sodium oxide and 75% quartz will begin to melt at 793 degrees centigrade. It is a eutectic, which means that it is a mixture that melts more easily than the materials from which it is made.
On the molecular level, fluxes work by disrupting the covalent bonds that bind the silicon and the oxygen atoms together. Lets assume that we are working with sodium oxide as a flux. Oxygen is a constituent of both silicon dioxide and sodium oxide. Oxygen has two valence electrons in its outer shell (orbit) that it can use either to trade with or donate to whatever partner, sodium or silicon, that it is bonded with. In the silicon dioxide lattice (quartz), the oxygen atom shares an electron with each of two silicon atoms. This type of bond in which atoms are held together by trading electrons back and fourth is known as a covalent bond.
On the other hand, the flux, sodium oxide, is held together by ionic bonds rather than covalent bonds. Instead of trading electrons back and fourth, the two sodium atoms in sodium oxide donate an electron to the oxygen atom creating a positively charged sodium ion–composed of two atoms of sodium that share a net positive charge–and a negatively charged oxygen ion. It is these opposing charges that hold the molecule together.
At low temperatures, the sodium oxide molecule is held tightly together by its ionic bond. But at high temperature, the ionic bonds that bind the sodium ions to their respective oxygen ions begin to break down leaving the mixture full of positively charged sodium ions and negatively charged oxygen ions. These negatively charged oxygen ions then start to trade places with the oxygen in the silica matrix. With an extra electron in its valence shell, the oxygen atom in the silica lattice no longer needs to form a covalent bond with one of its two neighboring silicon atoms, and the lattice becomes disrupted, as in the illustration above. (The green balls represent the positively charged sodium ions and the light blue balls represent the negatively charged oxygen ions.) Because of the extremely energetic nature of the electrically charged flux ions, this disruption of the crystal can happen at a much lower temperature than would normally be required to disrupt the crystalline lattice, and once the lattice has been disrupted in this fashion, the presence of the positive and negative ions makes it impossible for the lattice to reform.
The most commonly used fluxes are sodium oxide–Na2O (soda) and potassium oxide–K2O (potash). There is a long list of other fluxes. Not all the fluxes are alike. For example:
- Some fluxes are active at lower temperatures and volatilize at higher temperatures.
- Some do not start their fluxing action until higher temperatures are reached.
- Some work only in the presence of other fluxes by a process of interaction.
- Sodium oxide is better than potassium oxide at lowering the melting temperature of silica.
- Potassium oxide increases the viscosity of the melt and reduces slumping.
- Some fluxes, like sodium and potassium oxides have high rates of thermal expansion and large amounts will cause a mismatch in fit between an underlying ceramic body (or metal substructure in the case of dental porcelain fused to metal ceramics) and the overlying glass veneer or glaze. This often leads to crazing (cracking) of the glaze.
- Other fluxes with lower rates of thermal expansion (especially boron and lead oxides) may be used to offset this excess expansion.
- Some fluxes are better at adding color or opacity to the glass than they are at fluxing.
- On the other hand, all do disrupt the crystalline structure of the glass former and act as fluxes to one extent or another.
- Lithium oxide – Li2O
- beryllium oxide – BeO
- zinc oxide – ZnO
- lead oxide – PbO
- manganese oxide – MnO
- nickel oxide – NiO
- sodium oxide – Na2O
- magnesium oxide – MgO
- strontium oxide – SrO
- boron oxide – B2O3
- iron oxide – FeO
- copper oxide – CuO
- potassium oxide – K2O
- calcium oxide – CaO
- barium oxide – BaO
- bismuth oxide – Bi2O3
- cobalt oxide – CoO
- copper oxide – Cu2O
Stabilizers are added to make the glass strong and water resistant. The stabilizers used in glass making are also added in oxide form, and include many of the same metal oxides that are included in the flux list above. Stabilizers are integrated into the glass matrix, and do not remain in crystalline form, but dissolve as the glass melts, so they do not add opacity to the glass. Without a stabilizer, glass fluxed with sodium or potassium is soluble in water and humidity. Glass made with soda flux, but without a stabilizer is called “water glass“. Soda glass actually dissolves in water and can be painted on objects. It is used to coat the outside of eggs so they don’t dry out, and is also used in laundry detergent to help protect the washer from corrosion.
Calcium from the addition of calcium oxide (or calcium carbonate–lime–which breaks down to calcium oxide and carbon dioxide at high temperatures) is the most common stabilizer used in the glass industry. Ninety percent of all glass made is soda-lime glass, and if you are bored and looking out the window, chances are that you are looking through this type of glass. Soda-lime glass has the composition of 60-75% silica, 12-18% soda (sodium oxide), 5-12% lime (calcium carbonate). A typical formula for soda-lime glass is, 73% silica, 14% soda, 9% lime, 3.7% magnesia, and 0.3% alumina. Note that the lime and magnesium both act as fluxes in addition to their function as stabilizers.
(Aluminum Oxide–Al2O3) is added to most glasses in order to strengthen it and make it weatherproof. We have run into alumina before, in the form of opaque refractory crystals. However, the alumina in common glass, like other stabilizers, is not in crystalline form but is molecularly integrated into the glass matrix. In this form, it does not cause opacity in the glass. Aluminum atoms can bond with silicon via a shared oxygen atom and can thus be an integral part of the amorphous silicon matrix. Aluminosilicate glass has even higher corrosion resistance than borosilicate glass and can withstand higher temperatures. A typical recipe for aluminosilicate glass is, 64.5% silica, 24.5% alumina, 10.5% magnesia, 0.5% soda.
Lead Oxide, used as a stabilizer makes lead glass, a dense, hard glass with great optical characteristics. It is also a very good electrical insulator. It blocks x-rays quite well and is used as the glass in hospital radiology operatories. A typical formula for lead glass is 57% silica, 31% lead oxide, and12% potassium oxide. Note that the lead oxide also acts as a flux. Lead crystal glassware is still sold and collected (ref. Waterford crystal) and is noted for its clarity and its ability to ring like a bell when struck. The US Food and Drug Administration has recommended that liquid foods not be stored in lead modified glassware as small amounts of lead are known to leach out of the glass and into the liquid. Lead is not used in dental glass applications.
Boron Oxide, used as a stabilizer creates borosilicate glass. Borosilicate glass has at least 5% of boric oxide in its composition. It has high resistance to temperature change, mechanical shock and chemical corrosion. Most adults have seen a hot glass tumbler or pan shatter when cold liquid is poured into it. This happens because the glass on the inside rapidly shrinks while the glass on the outside does not. This differential in the respective volumes of inside and outside walls places stresses on the brittle glass which causes it to crack and shatter. The boron changes the molecular structure of glass so that it expands and contracts very little regardless of the temperature. With less volumetric change between outside and inside walls, the glass is less likely to shatter. Borosilicate glass is used extensively in applications in which thermal or mechanical shock may be a problem, as well as in laboratory applications in which corrosion is a concern. Typical applications include pipelines, light bulbs, photochromic glasses, sealed-beam headlights, laboratory ware, and bake ware. The Hale 200 inch reflecting telescope mirror was made from borosilicate glass. A typical borosilicate formula is, 81% silica, 12% boron oxide, 4% soda, and 3% alumina.
It is NOT impossible to melt pure silica (quartz). When this is done, it can be cooled relatively quickly until it forms a pure fused silica glasswith no flux or modifier molecules in its vitreous structure. This form of glass is very expensive (due to its very high melting temperature), but it has properties which are superior to any of the other forms of glass discussed above. It is not soluble in water because it lacks alkaline flux molecules. It is the most heat resistant of all glasses and can sustain temperatures of 1200 degrees centigrade. It is used in the outer windows of the space shuttle.
In order to form a glass from a molten mass of silica, it is necessary to cool the mixture fairly quickly. If it cools too slowly, crystals form within the glass body which will degrade its optical properties, turning if from a clear glass into a cloudy one. This process is known as devitrification (the opposite of vitrification).
On the other hand, if it is cooled too quickly, stresses build up in the glass. For example if the molten mass is quenched in cold water, it will shatter into billions of tiny particles which form a powder called a frit. Less aggressive cooling may create a glass with small cracks in the structure. Rapid cooling that is only a bit too fast may produce a flawless glass article, but the internal stresses throughout the structure make it prone to cracking or shattering with minimal shock or even only a slight surface scratch.
To reduce the stresses trapped in the glass, it is kept near the glass transition temperature (its solidus) for a long time so that the atoms in the glass can rearrange just enough to relieve the stress. When most of the stress has been eliminated, the finished glass is finally allowed to cool to room temperature. This process is known as annealing.
The image above shows the surface of a glazed piece of pottery that was allowed to remain in the annealing oven for a long time. The crystals you see were produced intentionally for decorative effect. This is a graphic example of devitrification. The idea behind annealing is to allow the glass article to remain just below its solidus for enough time to relieve the stresses in the glass, but not so long that internal crystals form that would affect the optical qualities of the glass.
The opposite of annealing is called tempering. Tempering involves hardening a molten or nearly molten material by quenching it suddenly in cold water instead of letting it cool slowly. This process is generally done with metallic items in order to case harden them, and NOT with glass articles. However, an interesting historical anecdote involves the tempering of molten drops of glass. For about a month in 1663, “exploding “chymical glasses” became an obsession of the British Royal Society. Small glass beads called Prince Rupert’s drops had become a sort of craze all over Europe in the second half of the seventeenth century. King Charles II of England had used the drops as practical jokes after being introduced to them by his nephew, Prince Rupert of Bavaria. These are teardrop-shaped beads formed by dropping molten glass into cold water. The bulbous head can withstand hammering on an anvil (case hardening), but breaking the curved, tapered tail shatters the entire drop into fine powder (called frit). The king would have subjects hold the bulb end in their palms, and then he’d break off the tip, startling the victims with a harmless little explosion. Prince Rupert’s drops are an extreme example of glass that has been tempered, and not annealed. Interestingly, it was not until 1994 that their secret was fully revealed when scientists did high-speed photographic analyses of the drops, observing the cracks accelerating from the drop’s tail towards the head at more than 4,000 miles per hour.