The Science and Art of Dental Ceramics - Volume I

Monograph I

The Nature of Dental Ceramics

The word "ceramic" is derived from the Greek "keramikos" which means "earthen". A ceramic is therefore an earthy material, usually of a silicate nature and may be defined as a combination of one or more metals with a non-metallic element, usually oxygen (Gilman, 1967). The larger oxygen atoms serve as a matrix, with the smaller metal atoms (or semi-metal atoms such as silicon) tucked into the spaces between the oxygens (Fig. 1-1).

The atomic bonds in a ceramic crystal have both a covalent and ionic character. These strong bonds are responsible for the great stability of ceramics and impart very useful properties, such as hardness, high modulus of elasticity and resistance to heat and chemical attack. On the other hand, the nature of this bonding creates difficulties for the dental ceramist since all ceramic materials are brittle.

Crystalline Ceramics

Regular dental porcelain, being of a glassy nature, is largely non-crystalline, and exhibits only a short range order in atomic arrangement. It is therefore more appropriate to consider this material in the second part of this chapter, "The Nature of Glasses".

The only true crystalline ceramic used at present in restorative dentistry is Alumina (Al2O3) which is the hardest and probably the strongest oxide known. The hardness and strength of alumina makes it difficult to cleave because of the "interlocking" nature of the structure. Binns (1970) considers that when a plane intersects a crystal, the force holding the two halves of the crystal together depends upon the number of chemical bonds cut and the strength of the individual bonds. The number of bonds is dependent upon the packing of the solid and is given by the number of gram-atoms per unit volume. The bond strength is less easy to assess, but probably the most important feature is the extent to which bonding is covalent. Ionic potential is defined as charge/ionic radius and Pauling (1945) has reported values. The greater the ionic potential of a cation the greater its polarizing power on the anion. According to Fajans (Weyl and Marboe, 1962) the more the anion is polarized by the cation the greater is the degree of covalency of the bond. Thus the degree of covalency, and hence the bond strength, increases with the valency. In the series of oxides Na2O, MgO, Al2O3, SiO2, where the ionic potential of the cation increases, the bonding becomes increasingly covalent. In the case of Na2O the bonding is almost completely ionic, whereas SiO2 would have about 50 percent covalent bonding. If the number of gram-atoms is multiplied by the valency of the metal, a quantity is obtained which, for some of the harder materials, compares reasonably well with the hardness. This is a simplified explanation which applies only to perfect single crystals; in practice, the conclusions are modified by dislocations, slip, grain boundary effects, and flaws.

Fig. 1-1a Diagram of a silicate unit with each SiO tetrahedra sharing an oxygen atom.

Fig. 1-1b Three dimensional drawing of a silicate unit in which the silicon atom Si is surrounded by four oxygen atoms.

Fig. 1-1c Three dimensional drawing of linked silicate units which form the continuous network in glass.

Extraction of Alumina

Alumina is the oxide of aluminium (Al2O3), commonly extracted from the mineral bauxite, which is mainly a hydrated aluminium oxide. According to normal practice, the ore is crushed and ground to - 10 mesh and is digested in a concentrated solution of caustic soda. The aluminium-bearing liquor recovered from this process is clarified, and the alumina is precipitated in the form of alumina trihydrate crystals which are then washed and dried without removal of the chemically combined water. The alumina trihydrate is converted to alumina by calcination, usually in a rotary kiln at a temperature of 600°C which drives off the chemically combined water in the hydrate to form gamma-alumina. Further calcination at 1250°C converts it to alpha-alumina (gamma-alumina is predominently required by American metal producers while European users mainly require the alpha-form). For ceramic applications the alpha-form is employed and is generally ball milled and commercially supplied as a fine powder, usually below 10 to 20 microns in size.

Fig. 1-2 Sintered high alumina profiles used in restorative dentistry.

Fabrication of Alumina Components

Sintered alumina is used in restorative dentistry in the form of prefabricated profiles or reinforcements for the construction of crowns, bridges, or individual pontics (McLean and Hughes, 1965).

These reinforcements are made by mixing the fine calcined alumina powder with a binder such as methyl cellulose and a release agent. The plastic alumina mass is then extruded through tungsten carbide nozzles to the desired shape or form; for dental purposes rods, tubes or sheets are most commonly used (Fig. 1-2).

The moulded profiles are then placed on refractory trays and fired in a standard industrial tunnel kiln. Very slow oven drying is used to prevent warping and the alumina is finally sintered or recrystallised at temperatures of up to 1650°C. The resulting product is a hard, impermeable ceramic of very high strength and chemical resistance.

Sintering of Alumina

Many theories exist as to the exact processes which occur during the firing cycle of alumina, although none are entirely certain; the terms sintered, fused and recrystallised are widely used to describe the alumina end-product.

Fig. 1-3 Photomicrograph showing the surface of sintered alumina (95 percent Al2O3). Relief polished. Normaski interference contrast. Mag x 850. Courtesy of D. B. Binns. British Ceramic Research Association.

Burke (1958) defined the term "recrystallisation" as changes in microstructure that occur in crystalline or largely crystalline bodies when atoms move to positions of greater stability. By contrast metallurgists use the term recrystallisation in a much more restricted sense: the nucleation and subsequent growth of a new generation of strain-free grains into the deformed matrix of a cold-worked material.

It appears that during the firing of alumina, the following steps occur. Firstly, a welding occurs at points of contact between adjacent oxide particles, giving rise to a lensing effect as normally occurs in sintering processes, i.e. partial fusion. Migration of atoms then leads to growth of the lens areas, movement of grain boundaries and reduction in porosity. During sintering, the shift in grain boundaries results in the formation of a closely interlocking crystalline structure of considerable strength (Fig. 1-3). This improved packing of the oxide particles results in shrinkage of the ceramic body and compensatory mould design is required similar to the oversize tooth moulds used in a tooth factory.

The driving force for the shrinkage in alumina ceramics is surface tension. The surface tension of free surfaces (pores etc.) in a porous ceramic body will always try to make the piece shrink to reduce surface energy. It has been suggested by Nabarro (1948) and Herring (1950) that the lattice vacancies formed at the surface of a pore can be discharged at grain boundaries as well as at the free surface of the piece, and this might explain the relative independence of sintering rates upon specimen size. Sintered alumina will exhibit this phenomenon and it can be shown that pores near the grain boundaries have disappeared, whereas pores near the centre of the grains remain (Fig. 1-4). The latter pores account for the opacity of alumina where only 2 to 10 percent light transmission is obtainable on 1 mm thick discs (McLean, 1966). This opacity will therefore make alumina suitable for use only in the anchorage areas of artificial teeth or crowns.

Translucent Alumina

More recently methods of firing high purity aluminas containing up to 0.2 percent MgO have been devised in which a spinel is formed at the grain boundaries, slowing down grain growth and allowing the diffusion of porosity along grain boundaries. These aluminas (G.E.C. Lucalox) are probably fired in hydrogen or oxygen atmosphere at high temperature (1800°C) which increases rate of diffusion of porosity. The resultant materials are almost pore-free which in turn produces a highly translucent body, through which it is possible to read newsprint. It is conceivable that these materials might have possibilities in a veneer crown technique where the alumina is used as the main reinforcing shell.

Fig. 1-4 Photomicrograph of pores trapped in the centre of sintered alumina crystals (99.5 percent Al2O3). Thermal etch. Normaski interference contrast. Mag × 775. Courtesy of D. B. Binns. British Ceramic Research Association.

Effect of Debasing Alumina

The function of a debasing or fluxing agent, added to high purity alumina, is to lower the sintering temperature. It may do this either by forming a liquid phase, or by going into solid solution in the alumina lattice and increasing the diffusion rate.

Debasing agents must be selected so that they do not appreciably affect the mechanical properties of the fired specimens; for example, by causing excessive grain...