Сraig. Dental Materials

552 Chapter 18 CERAMICS

T he term ceramic is defined as any product made essentially from a nonmetallic material by firing at a high temperature to achieve desir-

able properties. The term porcelain refers to a family of ceramic materials composed essentially of kaolin, quartz, and feldspar, also fired at high temperature. Dental ceramics for ceramic-metal restorations belong to this family and are commonly referred to as dental porcelains.

The art of processing dental ceramics has remained, to a great extent, with those persons who have learned the art directly from others. The laboratory portion of a ceramic restoration is usually made in a commercial dental laboratory by a skilled technician working with specialized equipment to the shape and shade specifications provided by the dentist. Skilled technicians and artisans are also employed by the manufacturers of artificial denture teeth to produce the many forms, types, and shades necessary in this application of porcelain.

HISTORICAL BACKGROUND . ':

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Dental ceramics mere filst u5ed ~n d e n t l w ~~n the late 1700s. Porcelain jacket crowns were developed in the early 1900s. They consisted of feldspathic or aluminous porcelain baked on a thin platinum foil and can be considered the ancestors of all-ceramic crowns. Because their low strength, however, porcelain jacket crowns were limited to anterior teeth. In the 1960s, the poor match in thermal expansion (and contraction) between framework alloys and veneering ceramics, which often led to failures and fractures upon cooling, stimulated the development of leucite-containing feldspathic porcelains. The problem was solved by mixing controlled amounts of high-expansion leucite with feldspar glass at the manufacturing stage. This allowed the adjustment of the coefficient of thermal expansion of feldspathic porcelains to very narrow specifications. This invention led to considerable improvement in the reliability of ceramic-metals and allowed ceramic materials to be bonded to a metal framework. During cooling, the thermal contraction of the metal framework is slightly

higher than that of the veneering ceramic, thus placing the internal surface of the ceramic in compression. Because ceramics are stronger in compression than in tension, this property is used to advantage to provide increased resistance to shattering.

The end of the twentieth century saw the introduction of several innovative systems for fabricating all-ceramic dental restorations. The first was a castable glass-ceramic system in which the restoration was cast using the lost-wax technique and later heat-treated to promote its transformation into a glass-ceramic. This castable system was later abandoned due to processing difficulties and the high incidence of fractures. Ceramics for slip-casting, heat-pressing, and machining were developed concurrently within the past fifteen years. New materials for all-ceramic restorations are introduced every year and attest to the increasing popularity of ceramics in dentistry.

CLASSIFICATION OF DEFnaL :I)

CERAMICS . .,

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Dental ceramics can be classified according to their fusion temperature, application, fabrication technique, or crystalline phase (Table 18-1).

FUSION TEMPERATURE

The high-fusing ceramics have a fusing range from 1315" to 1370" C; the medium-fusing ceramics, from 1090"to 1260"C; and the low-fusing ceramics, from 870' to 1065' C. This classification dates back to the early 1940s. It was employed more intensively with the earlier dental ceramic compositions, which belong to the triaxial porcelain compositions. The term triaxial means the composition has three major ingredients: quartz (or flint), feldspar, and clay (or kaolin). The fusion temperature is dictated by the relative amounts of these three ingredients. It is interesting to note that these temperature ranges vary from textbook to textbook and are not continuous. This is related to the fact that the processing of ceramic materials was initially monitored us-

1

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Sintered

Manufactured

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Leucite (KAISi,Od

Feldspar

ing pyrometric ceramic cones of specific composition. These cones are placed in the furnace and bend under their own weight when the temperature is reached. The mediumand high-fusing ceramics are used for denture teeth. Dental ceramics for ceramic-metal or all-ceramic fixed restorations belong to the lowor medium-fusing categories. Ultra-low-fusing dental ceramics with firing temperatures below 870" C have been recently introduced.

The mediumand low-fusing porcelains powders are usually modified by the manufacturer with chemicals or fluxes (boron oxide or alkali carbonates) of low melting temperature. They are melted together (prefused) and reduced to powder form. The addition of fluxing agents results in narrower fusing ranges and increases the tendency for the porcelain to slump during repair or when making additions, staining, or glazing. Prefusing and regrinding, however, increase the homogeneity of the powder, which may be an advantage in the handling and fusing operations. Low firing temperatures are a definite asset in the fusion of porcelain to metal, because the differences in the coefficients of expansion of the porcelain and metal can be tolerated better at lower temperature ranges.

High-fusing porcelains are considered superior in strength, insolubility, translucency, and maintenance of accuracy in form during repeated firings. Recent tests of low-fusing products, however, indicate that they are essentially as strong as the high-fusing types, and their solubility and translucency are adequate. The principal practical advantage of high-fusing porcelain is therefore its ability to be repaired, added to, stained, or glazed without distortion.

APPLICATIONS

Ceramics have three major applications in dentistry: (1) ceramics for metal crowns and fixed partial dentures, (2) all-ceramic crowns, inlays, onlays, and veneers, when esthetics is a priority, and (3) ceramic denture teeth.

FABRICATION TECHNIQUE

This classification is summarized in Table 18-1. One of the most common fabrication techniques for dental ceramics is called sintering. Sintering is the process of heating the ceramic to ensure densification. This occurs by viscous flow when the firing temperature is reached. Ceramics fired

to metals are processed by sintering, whereas all-ceramic materials encompass a wider range of processing techniques, including heat-pressing, machining, and slip-casting.

CRYSTALLINE PHASE

Regardless of their applications or fabrication technique, after firing, dental ceramics are composed of two phases: a glassy (or vitreous) phase surrounding a crystalline phase. Depending on the nature and the amount of crystalline phase present, the mechanical and optical properties of dental ceramics vary widely. Increasing the amount of glassy phase lowers the resistance to crack propagation but increases translucency. Materials for all-ceramic restorations have increased amounts of crystalline phase (between 35% and 90%) for better mechanical properties. Table 18-1 lists the various crystalline phases found in dental ceramics.

Ceramic-metal restorations consist of a cast metallic framework (or core) on which at least two

layers of ceramic are baked. The first layer applied is the opaque layer, consisting of ceramic, rich in opacifying oxides. Its role is to mask the darkness of the oxidized metal framework to achieve adequate esthetics. As the first layer, it also provides ceramic-metal bond. The next step is the buildup of dentin and enamel (most translucent) ceramics to obtain an esthetic appearance similar to that of a natural tooth. Opaque, dentin and enamel ceramics are available in various shades. After building up or modelling the porcelain powders, the ceramic-metal crown is sintered in a porcelain furnace.

Fig. 18-1 illustrates a ceramic-metal restoration, and Fig. 18-2 shows a three-unit ceramicmetal fixed partial denture. When fabricated by skilled technicians, these restorations can provide excellent esthetics, along with good strength because of the alloy framework. The alloys used for casting the substructure are usually gold-based containing tin and indium. Gold-palladium, silver-palladium, and nickelchromium alloys were initially developed as lower-cost alternatives. However, the recent steep increase in the price of palladium have changed the palladium-containing alloys into a higher-cost alternative.

Fig. 18-2 View of ceramic-metal fixed partial denture.

(Courtesy Dr. Julie A. Holloway)

It is essential that the coefficient of thermal expansion of the veneering ceramic be slightly lower than that of the alloy to ensure that the ceramic is in slight compression after cooling. This will establish a better resistance to crack propagation of the ceramic-metal restoration. The ceramic-metal systems are described in detail in Chapter 19.

COMPOSITION AND MAPl

COMPOSITION

The quality of any ceramic depends on the choice of ingredients, correct proportioning of each ingredient, and control of the firing procedure. Only the purest ingredients are used in the manufacture of dental porcelains because of the stringent requirements of color, toughness without brittleness, insolubility, and translucency, as well as the desirable characteristics of strength and thermal expansion. In many instances, the manufacturer must formulate a product that is a compromise.

In its mineral state, feldspar, the main raw ingredient of dental porcelains, is crystalline and opaque with an indefinite color between gray

and pink. Chemically it is designated as potassium aluminum silicate, with a composition of K20 . A1203. 6Si02. The fusion temperature of feldspar varies between 1125" and 1170" C, depending on its purity.

Iron and mica are commonly found as impurities in feldspar. It is particularly important to remove the iron, because metallic oxides act as strong coloring agents in porcelain. To remove iron, each piece of feldspar is broken with a steel hammer, and only the uniformly light-colored pieces are selected for use in the porcelain. These pieces are ground in mills to a fine powder. The final particle size is carefully controlled by screening to remove the coarser particles, and flotation processes are used to remove the excessively fine particles. The dry powder is then slowly vibrated down inclined planes equipped with a series of narrow ledges formed by induction magnets. In this way the remaining iron contaminants are separated and removed, and the feldspar is made ready for use.

Pure quartz (SiO,) crystals are used in dental porcelain and ground to the finest grain size possible. Silica is added, and contributes stability to the mass during heating by providing a glassy framework for the other ingredients.

MANUFACTURE

Feldspathic dental porcelains in recent years are made mainly with potash feldspar ( K 2 0 . A120,. GSiO,) and small additions of quartz (SO,). During the manufacturing process, the ground ingredients are carefully mixed together. Alkali metal carbonates are added as fluxes and the mix is heated to about 1200" C in large crucibles. At high temperature, the feldspar decomposes to form a glassy phase with an amorphous structure, as illustrated in Fig. 18-3,and a crystalline (mineral) phase consisting of leucite (KA1Si,06 or K20 . A1203. 4Si0,). The crystalline structure of tetragonal leucite is illustrated in Fig. 18-4.

The mix of leucite and glassy phase is then cooled very rapidly (quenched) in water which causes the mass to shatter in small fragments. The product obtained, called a frit, is ball-milled to

Fig. 18-3Two-dimensional structure of sodium silicate glass.

(Modified from Warren BE, Biscoe J: J Am Ceram Soc 21:259, 1938.)

achieve proper particle size distribution. Coloring pigments in small quantities are added at this stage, to obtain the delicate shades necessary to mimic natural teeth. The metallic pigments include titanium oxide for yellow-brown shades, manganese oxide for lavender, iron oxide for brown, cobalt oxide for blue, copper or chromium oxides for green, and nickel oxide for brown. In the past, uranium oxide was used to provide fluorescence; however, because of the small amount of radioactivity, lanthanide oxides (such as cerium oxide) are being substituted for this purpose. Tin, titanium, and zirconium oxides are used as opacifiers.

After the manufacturing process is completed, feldspathic dental porcelain consists of two phases. One is the vitreous (or glass) phase, and the other is the crystalline (or mineral) phase. The glass phase formed during the manufacturing process has properties typical of glass, such as brittleness, nondirectional fracture pattern, translucency, and high surface tension in the

fluid state. The crystalline phase is leucite, a potassium alumino-silicatewith high thermal expansion (>20 x C). The amount present (10% to 20%) controls the thermal expansion coefficient of the porcelain. Leucite also contributes strength to the porcelain; high-leucite porcelains are approximately twice as strong as those containing low concentrations. The microstructure of conventional feldspathic porcelain is shown in Fig. 18-5; the glassy phase has been lightly acid-etched to reveal the leucite crystals.

Typical compositions for opaque and dentin porcelain powders are given in Table 18-2.

PROCESSING

The production of a satisfactory porcelain restoration requires careful attention to the principles and details of the operation.

Porcelain Application and Condensation After the tooth has been prepared, an elastomeric impression is made, and a working model or die is formed of a suitable die material. Awax framework is fabricated and the wax is cut back by 1 mm in the esthetic areas to ensure enough space for porcelain application. The framework is cast by the lost wax technique. It is extremely important that all sharp angles be rounded to avoid stress concentrations and wedge effects in the fired porcelain. After careful cleaning of the metal framework, a thin layer of opaque porcelain is applied and baked. Dentin porcelain powder, in the shade selected for the body or dentin portion, is mixed with modeling liquid (mainly distilled water) to a creamy consistency and is applied on the opaque layer, with allowances made for shrinkage. To produce minimum shrinkage and a dense, strong porcelain, it is important to achieve a thorough condensation of the particles at this stage. Various means of condensation may be employed. The vibration method is particularly efficient in driving the excess water towards the surface. The wet porcelain mix is applied with a spatula and vibrated gently until the particles settle together. The excess water is then removed with a clean tissue or an absorbent medium. A cross section of

a ceramic-metal crown is shown in Fig. 18-6, right.

Other methods of condensation include the spatulation and brush techniques. The spatulation method consists of smoothing the wet porcelain with a suitable spatula until the excess water is brought to the surface, where it is absorbed with a tissue. The brush or capillary attraction method depends on the action of dry porcelain powder to remove the excess water by capillary attraction. The dry powder is applied with a brush to a small area of the wet porcelain mass, and as the water is withdrawn toward the dry area, the wet particles are pulled closely together. This process is repeated as dry powder is placed on the opposite side of each new addition of the wet mix.

Drying After the porcelain mix has been applied and condensed, the restoration is placed

in front of an open preheated porcelain furnace to be dried. This drying stage, which lasts between 5 and 8 minutes, is a very important step; it ensures that any remaining excess water is

removed from the porcelain mix. During the drying stage, the water diffuses towards the surface and then evaporates. If the piece is dried too quickly, the water evaporation rate becomes greater than the diffusion rate and the unfired porcelain can undergo spontaneous breakage. After the mass is placed in the furnace, remaining free and combined water is removed in various stages of heating until a temperature of 480" C is reached.

Firing/Sintering Porcelain restorations may be fired either by temperature control alone or by controlled temperature and a specified time. In the first method the furnace temperature is raised at a constant rate until a specified temperature is reached. In the second method the temperature is raised at a given rate until certain levels are reached, after which the temperature is maintained for a measured period until the desired reactions are completed.

Either method gives satisfactory results, but the time and temperature method is generally preferred because it is more likely to produce a

Fig. 18-6 Cross sectional micrograph of a ceramicmetal crown (right). Cross sectional view of an allceramic crown (left).

uniform product. Porcelain is a poor thermal conductor, and thus too rapid heating may overfuse the outer layers before the inner portion is properly sintered.

As the temperature is raised, the particles of porcelain fuse by sintering. Sintering is the process responsible for the fusion of the porcelain particles to form a continuous mass. During this densification, the volume change, A in the early stages is related to the surface tension of the porcelain, y, viscosity, T, the particle radius, r, and sintering time, t, as shown in the following equation:

As this equation indicates, the lower the viscosity and the finer the particle size, the greater the rate of densification. A minimum number of three firing operations are needed in the fabrication of a ceramic-metal restoration: one for the opaque portion, one for the dentin and enamel portion, and another for the stain and glaze. However, due to the shrinkage associated with the sintering process, it is necessary to oversize the porcelain buildup. For this reason, only ex-

Fig, 18-7 Microscopic view of air-fired porcelain, showing porosity.

(Courtesy Semmelman JO, York, International.)

perienced operators can complete a porcelain restoration in only three firings.

During the second firing, the dentin and enamel portion, which is formed approximately 13% oversize, is heated to the biscuit (or bisque) bake. This temperature is 56" C below the fusing temperature of the porcelain. Virtually all the shrinkage takes place during this firing.

The presence of gas bubbles (pores) has always been a problem in the production of dental porcelain. The extent to which pores n u y exist in air-fired porcelain is shown in Fig. 18-7. The specimen shown was ground and polished, and the round black spots are the cross-sectioned pores. It has been calculated that air-fired porcelain contains as much as 6.3% voids. This not only results in undesirable roughness and pits when a porcelain crown must be ground, but also exerts an even more undesirable effect on the strength and optical properties of the porcelain.

Pores are caused by air trapped during the sintering process. Air spaces become spherical under the influence of surface tension and expand with increased temperature. Porcelains for ceramic-metal restorations are fired under vacuum. As the porcelain furnace door closes, the

560 Chapter I 8 CERAMICS

pressure is lowered to 0.1 atmosphere. The temperature is raised until the firing temperature is reached; the vacuum is then released and the furnace pressure returns to 1 atmosphere. The increase in pressure from 0.1 to 1 atmosphere helps compress and close the residual pores. This occurs by viscous flow at the firing temperature. The result is a dense, relatively pore-free porcelain, as illustrated in Fig. 18-8. Studies have shown that sintering under vacuum reduces the amount of porosity to 0.56% (from 5.6%) in air-fired dental porcelains. Vacuum-firing improves the translucency and decreases the surface roughness of feldspathic porcelains, and increases impact strength approximately 50%.

An alternate method uses the principle of diffusion to secure improved density in fused porcelain. A diffusible gas such as helium may be introduced to the furnace at low pressure during the sintering (densification) stage. The helium gas (instead of air) is therefore entrapped in the interstitial spaces, and because its molecular diameter is smaller than the porcelain lattice, it diffuses outward under the pressure of the shrinking porcelain.

Before the development of these improved firing methods, fine texture and translucency were not possible at the same time. Maximum

Fig. 18-8Microscopic view of vacuum-fired porcelain showing minimal porosity.

(Courtesy Semmelman JO, York, Pa, 1959, Dentsply

International.)

translucency was obtained only by use of coarse powders that trapped only a few relatively large pores, but which also produced an undesirable granular appearance. Fine-grained powders developed a better texture but increased the opacity because of the large number of small pores. Vacuum-firing achieved both fine texture and translucency in ceramic-metal restorations; however, another property was still missing. Human enamel exhibits a specific optical property called opalescence: a scattering effect that makes it appears bluish when viewed in reflected light and orange when viewed in transmitted light. Studies of pigment particle size and dispersion have made it possible to produce partially opalescent restorations that compare favorably with natural teeth.

Glazing After the porcelain is cleaned and any necessary stains are applied, it is returned to the furnace for the final glaze firing. Usually, the glazing step is very short; when the glazing temperature is reached, a thin glassy film (glaze) is formed by viscous flow on the porcelain surface. Overglazing is to be avoided, because it gives the restoration an unnatural shiny appearance and causes loss of contour and shade modification. Glazing temperatures and times vary with the type and brand of porcelain employed.

C o o k g It is commonly accepted that the cooling stage is a critical one in the fabrication of ceramic-metal restorations and that extreme (too fast or too slow) cooling rates should be avoided. Too-rapid cooling of the outer layers may result in surface crazing or cracking; this is also called thermal shock. Very slow cooling (e.g., in a furnace) as well as multiple firings, might induce the formation of additional leucite and increase the overall coefficient of thermal expansion of the ceramic, and may also result in surface cracking and crazing. Slow cooling is preferred, and is accomplished by removing the fired restoration from the furnace as soon as the firing is finished and placing it under a glass cover to protect it from air currents and possible contamination by dirt.

Materials for all-ceramic restorations use a wide variety of crystalline phases as reinforcing agents and contain up to 90% by volume of crystalline phase. The nature, amount, and particle size distribution of the crystalline phase directly influence the mechanical and optical properties of the material. The match between the refractive indexes of the crystalline phase and glassy matrix is an important factor for controlling the translucency of the porcelain.

As mentioned earlier, several processing techniques are available for fabricating all-ceramic crowns: sintering, heat-pressing, slip-casting,and machining. Fig. 18-6, left, illustrates the cross section of an all-ceramic crown.

SINTERED ALL-CERAMIC MATERIALS

Two main types of all-ceramic materials are available for the sintering technique: alumina-based ceramic and leucite-reinforced ceramic.

Ceramic Aluminous core ceramic is a typical example of strengthening by dispersion of a crystalline phase. Alumina has a high modulus of elasticity (350 GPa) and high fracture toughness (3.5 to 4 MPa . m0,5). Its dispersion in a glassy matrix of similar thermal expansion coefficient leads to a significant strengthening of the core. It has been proposed that the excellent bond between the alumina and the glass phase is responsible for this increase in strength compared with leucite-containing ceramics. The first aluminous core porcelains contained 40% to 50% alumina by weight. The core was baked on a platinum foil and later veneered with matched-expansion porcelain. Aluminous core ceramic is now baked directly on a refractory die. Aluminous core porcelains have flexural strengths of about 138 MPa and shear strengths of 145 MPa.

Leucite-Reinforced Feldspathic Porcelain A feldspathic porcelain containing up to 45% by volume tetragonal leucite is available for the fabricating all-ceramic sintered restora-

tions. Leucite acts as a reinforcing phase; the greater leucite content (compared with conventional feldspathic porcelain for ceramic-metal applications) leads to higher flexural strength (104 MPa) and compressive strength. The large amount of leucite in the material also contributes to a high thermal contraction coefficient. In addition, the large thermal contraction mismatch between leucite (22 to 25 x lo-"/" C) and the glassy matrix ( 8 x C) results in the development of tangential compressive stresses in the glass around the leucite crystals upon cooling. These stresses can act as crack deflectors and contribute to increased resistance of the weaker glassy phase to crack propagation.

Magnesia-Based Core Porcelain A high-expansion magnesia core material has been developed that is compatible with the same dentin porcelains used for ceramic-metal restorations. The flexural strength of unglazed magnesia core ceramic is twice as high (131 MPa) as that of conventional feldspathic porcelain (70 MPa), with an average coefficient of expansion of 14.5 x C. The main advantage of this core material is that it allows a laboratory to veneer it with the more widely available porcelains for ceramic-metal restorations. The magnesia core material can be significantly strengthened by glazing, thereby placing the surface under residual compressive stresses that have to be overcome before fracture can occur.

Sintered all-ceramic restorations are slowly being replaced by heat-pressed all-ceramic restorations that simplify the processing steps.

Heat-Pressed All-Ceramic Materials

Heat-pressing relies on the application of external pressure to sinter and shape the ceramic at high temperature. Heat-pressing is also called high-temperature injection molding. It is used in dentistry to produce all-ceramic crowns, inlays, onlays, veneers, and more recently, fixed partial dentures. Heat-pressing classically helps avoid large pores and promotes a good dispersion of the crystalline phase within the glassy matrix. The mechanical properties of many ceramic sys-