What Are Ceramics?

Dental ceramics consist of silicate glasses, porcelains, glass-ceramics, or highly crystalline solids. They exhibit chemical, mechanical, physical, and thermal properties that distinguish them from metals, acrylic resins, and resin-based composites. Silicate glasses differ from nonsilicate glasses in that silicon is the central divalent cation that is bound to four relatively large oxygen anions that link in a random order to other tetrahedra to form polymeric-type (SiO₂)_n chains. In silica-based ceramics, each oxygen atom invariably connects two (SiO₄) tetrahedra only at the corners, and no edges and faces of tetrahedra are shared. The properties of ceramics are customized for dental applications by precisely controlling the types and amounts of the components used in their production. Ceramics are more resistant to corrosion than plastics. Ceramics do not react readily with most liquids, gases, alkalis, and weak acids. They also remain stable over long time periods. They exhibit good to excellent strength and fracture toughness. One of the strongest and toughest ceramics, zirconium dioxide, has a flexural strength similar to that of steel, but its fracture toughness is much lower than that of steel. However, metals are tougher than either ceramics or plastics. Although ceramics are strong, temperature-resistant, and resilient, these materials are brittle and may fracture without warning when flexed excessively or when quickly heated and cooled (i.e., under thermal shock conditions). Most dental ceramics are compounds of oxygen with metals or semimetals (metalloids) that have some properties of both metals and nonmetals, but, all ceramic products are nonmetallic in nature.

Most ceramics are characterized by their biocompatibility, esthetic potential, refractory nature, high hardness, low to moderate fracture toughness, excellent wear resistance, susceptibility to tensile fracture, and chemical inertness. For dental applications a ceramic with a hardness less than that of tooth enamel and an easily polishable surface are desirable to minimize the wear damage that can be produced on enamel by the ceramic surface. However, adequate fracture resistance is an important requirement of any dental ceramic. In this regard, the fracture toughness is the most critically important property because it is a measure of the resistance to crack growth under a state of tensile stress.

The susceptibility to tensile fracture is a drawback, particularly when flaws and tensile stress coexist in the same location within a ceramic prosthesis. However, the tensile strength of ceramics is not a useful parameter to describe their fracture resistance for several reasons. Strength is not an inherent property of ceramics because it varies with specimen size, specimen length, specimen shape, loading rate, surface preparation methods, and the environment. For example, the strength of silica-based ceramics increases with a decrease in specimen thickness, an increase in stressing rate, and a smoother surface. Such variability indicates that strength is not necessarily a bulk property since surface conditions can significantly alter the mean strength as well as the spread of experimental values, as indicated by the Weibull modulus and the coefficient of variation. In comparison, fracture toughness is a true material property that is a measure of the resistance to crack propagation.

It is generally believed that dental ceramics fail primarily because of their brittleness, which is inversely related to its ductility or percent elongation. In general, the major reason for fracture of ceramics is their inability to suppress crack growth by deforming plastically in areas adjacent to crack tips that are subjected to tensile stress. Since there is no other property that directly expresses brittleness, except perhaps for the reciprocal of percent elongation, two fracture mechanics properties may better explain this behavior. Fracture toughness or plane-strain fracture toughness, which is designated as K_Ic, describes the critical stress intensity factor based on a mode I (tensile loading perpendicular to the crack plane) crack opening under tensile stress. For dental ceramics, K_Ic varies between 0.75 MPa•m^1/2 for feldspathic porcelain to 8 MPa•m^1/2 or more for yttria-stabilized zirconia, or ceria-stabilized alumina-zirconia ceramic. In comparison, the K_Ic values for enamel, dentin, and resin composites are 0.7 to 1.3, 3.1, and 0.8 to 2.5 MPa•m^1/2, respectively.

Another property that is used to describe the brittle fracture resistance of ceramics is the critical strain energy release rate, which is designated as G. The energy release rate failure criterion is that a crack will grow when the energy release rate is greater than or equal to a critical value, Gc, which is referred to as the fracture energy. This property is a measure of the unit strain energy that is released per unit increase in crack area as a ceramic with flaws or cracks is loaded progressively. Comparative values of K_Ic (MPa•m^1/2) and G_c (kJ/m²) for different types of materials are (1) glass: 0.7 versus 0.007; (2) PMMA: 1.1 versus 0.5; and (3)steel: 150 versus 107, respectively.

Chemical inertness is an important characteristic because it ensures that the chemically stable surface of dental restorations does not release potentially harmful elements, and it reduces the risk for surface roughening and increased abrasiveness or increased susceptibility to bacterial adhesion over time. Other important attributes of dental ceramics are their potential for matching the appearance of natural teeth, their thermal insulating properties (low thermal conductivity and low thermal diffusivity), and their freedom from galvanic effects (low electrical conductivity). Because the metal atoms transfer their outermost electrons to the nonmetallic atoms and stabilize their highly mobile electrons, ceramics are excellent thermal and electrical insulators.

Dental ceramics are nonmetallic, inorganic structures, primarily containing compounds of oxygen with one or more metallic or semi-metallic elements (aluminum, boron, calcium, cerium, lithium, magnesium, phosphorus, potassium, silicon, sodium, titanium, and zirconium). Many dental ceramics contain a crystal phase and a silicate glass matrix phase. Their structures are characterized by chains of (SiO₄)⁴⁻ tetrahedra in which Si⁴⁺ cations are positioned at the center of each tetrahedron with O⁻ anions at each of the four corners (Figure 18-1). The resulting structure is not close-packed and it exhibits both covalent and ionic bonds. The SiO₄ tetrahedra are linked by sharing their corners. They are arranged as linked chains of tetrahedra, each of which contains two oxygen atoms for every silicon atom. The primary structural unit in all silicate structures is the negatively charged silicon-oxygen tetrahedron (SiO₄)⁴⁻. It is composed of a central silicon cation (Si⁴⁺) bonded covalently to four oxygen anions located at the corners of a regular tetrahedron. For feldspathic veneering porcelains, alkali ions such as sodium or potassium occupy sites that allow them to bond to electrons from unbalanced oxygen ions (see Figure 18-1). Alkali cations such as potassium or sodium tend to disrupt silicate chains and increase the thermal expansion of these glasses. The expansion coefficient (TEC) can be further increased by including crystalline particles such as tetragonal leucite (K₂O•Al₂O₃•·4SiO₂ or KAlSi₂O₆), whose TEC ranges from 22 to 30 × 10⁻⁶/K. Two of the primary phase fields (potash feldspar and leucite) that are found in commercial feldspathic veneering ceramics are shown in a ternary section of the K₂O•Al₂O₃•SiO₂ phase diagram in Figure 18-2. Sanidine (KAlSi₃O₈), a potassium aluminosilicate phase, exists at high temperatures although it may be retained on cooling in the form of monoclinic crystals.

VITABLOCS Mk II is the only known dental ceramic with sanidine as the primary crystal phase. The glass matrix phase in these porcelains is formulated from one or more forms of the mineral feldspar (KAlSi₃O₈, NaAlSi₃O₈, and CaAl₂Si₂O₈). Many of the veneering ceramics (also called porcelains) are derived from potash feldspar (K₂O•Al₂O₃•6SiO₂ or KAlSi₃O₈), although some may be based on soda feldspar or a combination of both types. Compositions of some dental ceramics are listed in Table 18-1. In industry, the term porcelain is generally associated with ceramics produced with a significant amount of kaolinite [Al₂Si₂O₅(OH)₄ or Al₂O₃•2SiO₂•2H₂O]. Kaolinite is a form of kaolin, which is a type of clay. None of the modern low-fusing or ultralow-fusing porcelains contains any clay product such as kaolinite. However, it may be used in the formulations for high-fusing porcelain and ceramic denture teeth (see Figure 18-2). Thus, these ceramics are technically not porcelains and they can be considered a type of glass (e.g., leucite glass, fluorapatite glass, or feldspathic glass). However, until the international community sees the need to change our terminology from porcelain to glass, we will continue to use the term porcelain.

Ceramics are composed of metallic and nonmetallic elements that form crystalline and/or noncrystalline compounds. They may form binary compounds such as alumina (Al₂O₃) and zirconia (ZrO₂) by the bonding of metals, which release their positive valence electrons to nonmetals that can accept or share electrons (negative ions). The free energy for bonding of positive metal ions to negative nonmetal ions must be sufficiently low so that the metallic ions preferentially attract nonmetallic ions rather than their own ions or other positive ions. Molecules with one oxygen atom (such as Na₂O, K₂O, or CaO) are useful in dental porcelain as fluxes. They may also act as opacifiers. Molecules that contain three oxygen atoms for every two other atoms (such as Al₂O₃) are used as stabilizers. They are also added as crack blockers or toughening crystals. Silica (SiO₂) is the main glass-forming structure used in all dental veneering ceramics. All silicates of dental interest are derived from silica tetrahedral structures that can be linked as chains, double chains, or three-dimensional distributions of tetrahedra. Fluxing cations such as K⁺ or Na⁺ neutralize the negative charges of the silicate backbones and disrupt the continuity of silicate networks (see Figure 18-1), leading to lower sintering temperatures and increased coefficients of thermal expansion.

Zirconia is of major dental importance because of its high fracture toughness. However, pure ZrO₂ is not useful because cracks occur during sintering as a result of transformation from the tetragonal to the monoclinic phase. This transformation can be fully or partially suppressed by the addition of certain oxides such as MgO, Y₂O₃, CaO, or CeO. Most of the current zirconia ceramic materials for dental prostheses are based on tetragonal zirconia particles (TZP) that are fully stabilized with yttria (Y₂O₃). The addition of yttria to pure zirconia replaces some of the Zr⁴⁺ ions in the zirconia lattice with Y³⁺ ions. This produces oxygen vacancies, since three O²⁻ ions replace four O²⁻ ions. For several dental zirconia products, approximately 0.25% of Al₂O₃ is added to prevent the leaching of yttria (Y₂O₃).

Multicomponent or mixed oxide structures may also be useful for dental applications. Three examples of this class of ceramics include MgO•Al₂O₃ (spinel), 3Al₂O₃•2SiO₂ (mullite, which is located along the right-side border of Figure 18-2), and Al₂TiO₅ or Al₂O₃•TiO₂ (aluminum titanate). The spinel structure is used in a glass-infiltrated ceramic (In-Ceram Spinell) for applications in which greater translucency is required. Most nonoxide ceramics are not of practical use in dentistry either because of their high processing temperatures, complex processing methods, or their unesthetic color and opacity. Such ceramics include borides (TiB₂, ZrB₂), carbides (B₄C, SiC, TiC, WC), nitrides (AIN, BN, Si₃N₄, TiN), selenide (ZnSe), silicide (MoSi₂), sialon (Si₃N₄ with Al₂O₃), and syalon (Si₃N₄ with Al₂O₃ and Y₂O₃)_.

Glass-ceramics are partially crystallized glasses that are produced by nucleation and growth of crystals in the glass matrix phase. An example of such a product that has been used in dentistry is Dicor glass-ceramic, which was based on the growth of tetrasilicic fluormica crystals in a glass matrix. This material was originally supplied in a glass ingot form. The glass ingots, which contained a nucleating agent, were melted and cast into a refractory mold and subsequently processed thermally to produce the crystal phase of the glass-ceramic core material. Casting of glass forms is no longer performed for producing dental prostheses. Hot-pressing and CAD-CAM processing are now used to produce glass-ceramic core frameworks for crowns and bridges.

Dental ceramics are formed into prosthetic shapes and configurations through a variety of processes including sintering, casting, hot isostatic pressing, copy milling, and CAD-CAM machining. The components of the ceramics are based on metal oxides and glasses that are formulated for each particular application. For example, ceramics made as veneering structures on metal frameworks must be capable of bonding to the surface oxide of the metal substructure or to other veneering ceramic layers. The coefficient of thermal expansion (CTE) and coefficient of thermal contraction (CTC) of the ceramic layers must be matched to that of the metal framework in such a way that crack-initiating tensile stresses in the ceramic are avoided or minimized during cooling. The CTEs and CTCs of the veneering ceramics are controlled by varying their composition. In this chapter the calculated TEC differences between two bonded materials are used to describe thermal compatibility between adjacent structural materials. Because of “trapped excess volume” of glass-phase ceramics on cooling after a dilatometry specimen is produced, the subsequent TEC may be significantly lower than the true expansion coefficient because of structural relaxation during the subsequent expansion measurement. This discrepancy can provide misleading information on whether or not a ceramic veneer is thermally matched to its metal or ceramic substructure.

Other component oxides are included to modify the esthetic or appearance characteristics of the final prosthesis, including the hue, value, chroma, opacity, and luminescence. Dental ceramic prostheses may also be used without a metal substructure when ceramic teeth are used in dentures or when all-ceramic crowns and fixed dental prostheses are used to satisfy optimal esthetic or biocompatibility requirements. However, because the brittle glass-phase veneering ceramics are susceptible to crack formation at sites of surface and subsurface flaws in the presence of tensile stresses, these materials must be processed carefully and designed in such a way that the magnitudes of these stresses are minimized.

This chapter describes the composition, microstructure, properties, manipulation procedures, and performance characteristics of ceramic materials used for producing metal-ceramic and all-ceramic prostheses cemented on prepared teeth, on implant abutments, or on other material substrates used in the construction process. Although ceramics made from kaolin, feldspar, and quartz have been used for many centuries, major improvements in dental ceramics have occurred only within the past century. To understand the current state of materials science and technology, dental students, graduate students, and practicing dentists should be familiar with the terminology. The Key Terms section provides the definitions required to facilitate an understanding of ceramic science and technology as they relate to dental prosthetic applications and the clinical performance of metal-ceramic and ceramic-ceramic restorations.

Applications of Ceramics in Dentistry

Dental ceramic science and technology represent the fastest growing areas of dental materials research and development. During the past two decades numerous types of ceramics and processing methods have been introduced. Some of these materials can be formed into inlays, onlays, veneers, crowns, and more complex fixed dental prostheses (FDPs). Several of the core ceramics can be resin-bonded micromechanically to tooth structure. The future of dental ceramics is very promising because the increased demand for tooth-colored restorations will lead to an increased acceptance of ceramic-based and polymer-based restorations and the reduced use of amalgam and cast metals.

This chapter describes the ceramics used for metal-ceramic prostheses and the new generation of ceramic products, such as Cercon, Lava, IPS e.max ZirCAD, In-Ceram Zirconia, Denzir, IPS e.max CAD, Denzir, BruxZir, Procera AllCeram, and Procera Zirconia, which are being used currently for ceramic prostheses in dental practices. Previous glass-ceramics were produced either by crystallization of tetrasilic fluormica or leucite crystals. However, further increases in strength and fracture toughness occurred subsequently with lithium disilicate–based products such as IPS Empress 2 and IPS e.max Press, among others. Introduced later were products such as In-Ceram Alumina, a glass-infiltrated alumina core ceramic; In-Ceram Zirconia, a glass-infiltrated zirconia-alumina core ceramic; Lava, a partially sintered or fully sintered zirconia core that is formed by a true CAD-CAM process (by scanning dies without the need for a wax pattern); and Cercon, a partially-sintered zirconia ceramic that is milled to an enlarged size in the green state based on scanning of a wax pattern. It is also possible to scan prepared teeth and mill a prosthesis using the Cerec system (Sirona Corp., Germany). The Cerec 1 system was introduced in the mid-1980s and improvements in software and hardware led to the Cerec 2 and Cerec 3 systems for production of ceramic inlays, onlays, and veneers, and the commercial InLab system for large-scale processing of ceramic copings, frameworks, and monolithic ceramic prostheses.

Dental ceramics are used primarily for the production of fixed dental prostheses. Although ceramics such as zirconia can be used for endodontic posts and implant abutments, their primary applications are for crowns and bridges. Because of its lack of translucency, zirconia is better suited for applications involving posterior teeth or for elderly patients whose teeth have lost much of their original translucency. Each family of dental ceramics has specific indications and contraindications for which it is best suited or not suited. The characteristics and uses of the range of dental ceramics are described in this chapter.

History of Dental Ceramics

Ceramic-like tools have been used by humans since the end of the Old Stone Age around 10,000 B.C. to support the lifestyles and needs of fisher-hunter-gatherer civilizations. During the Middle Stone Age (10,000 to 5500 B.C.) ceramics were important materials, and they have retained their importance in human societies ever since. Stone Age craftsmen took rocks and shaped them into tools and artifacts by a process called flaking and flintknapping, whereby stone chips could be fractured away from the surfaces of hard, fine-grained, or amorphous materials including chert, flint, ignimbrite, indurated shale, lava, obsidian, quartz, and silicified limestone. For example, craftsmen of the Paleo-Indian culture, from 10,000 to 6000 B.C., made arrowheads, spear points, and flaking tools from a variety of natural rock materials. Such rocks contained mineral phases such as feldspar, mica, and quartz, which have since been used in dental ceramics.

Because natural minerals are not tooth-colored, subsequent civilizations used a variety of materials to produce simulated teeth. In approximately 700 B.C., the Etruscans made artificial teeth of ivory and bone, human teeth, and animal teeth (possibly oxen) that were held in place by gold wires or flat bands and rivets (Figure 18-3). Animal bone and ivory from hippopotami and elephants were used for many years thereafter.

Human teeth that were sold by the poor and teeth obtained from the dead were also used for centuries thereafter, but dentists generally avoided this option. One of the first sets of dentures made for U.S. President George Washington contained extracted teeth (Figure 18-4, center) but later his dentures were made of hippopotamus ivory (Figure 18-4, left). The ivory tooth forms were supported in the maxillary denture by a gold palatal plate and the dentures were retained by pressure applied by coiled springs attached to the sides of the denture bases. President Washington was inaugurated in 1789 with one remaining tooth. He suffered from poor oral health (although it is reported that he had brushed his teeth regularly with tooth powder). His poor-fitting dentures caused him much discomfort during his presidency (1789−1797) and until his death, in 1799, at the age of 67. None of his dentures were ever made of wood, contrary to erroneous reports circulated since his death.

B, Portrait of President Washington, who died on December 14, 1799. Technology for producing porcelain denture teeth was not available until 1825, although these prostheses were not refined until vulcanized rubber denture bases were developed in 1839.

The first porcelain tooth material was patented in 1789 by de Chemant, a French dentist in collaboration with Duchateau, a French pharmacist. This product, an improved version of the “mineral paste teeth” produced in 1774 by Duchateau, was introduced in England soon thereafter by de Chemant. However, this baked compound was not used to produce individual teeth, since there was no effective way at that time to attach the teeth to a denture base material.

In 1808, Fonzi, an Italian dentist, invented a “terrometallic” porcelain tooth held in place by a platinum pin or frame. Planteau, a French dentist, introduced porcelain teeth to the United States in 1817, and Peale, an artist, developed a baking process in Philadelphia for these teeth in 1822. Commercial production of these teeth by Stockton began in 1825. In England, Ash developed an improved version of the porcelain tooth in 1837. In Germany, Pfaff in 1756 developed a technique to make impressions of the mouth using plaster of Paris, but it was not until 1839 that the invention of vulcanized rubber allowed porcelain denture teeth to be used effectively in a denture base. In 1844, the nephew of Stockton founded the S.S. White Company, which became active in the further refinement of the design and mass production of porcelain denture teeth.

Charles Land introduced one of the first ceramic crowns to dentistry in 1903. Land, the grandfather of aviator Charles Lindbergh, published in the Independent Practitioner in 1886 and 1887 a technique for preparing the tooth cavity for an inlay, making a platinum foil matrix, and fabricating a ceramic inlay using high-fusing feldspathic porcelain. The sintering temperature was empirically determined as reported by Gilbert (“Notes on Dental Porcelain,” Harvard University Dental School, 1906), who described the temperature required to “set” the inlay as one that was between the melting point of pure gold (1084 °C) and the maximum temperature recommended for the porcelain muffle (1370 °C).

These crowns exhibited excellent esthetics, but the low flexural strength of porcelain resulted in a high incidence of fracture. Since the 1960s, feldspathic porcelains with reliable chemical bonding have been used in metal-ceramic prostheses. However, feldspathic porcelains have been considered too weak to be used reliably in the construction of all-ceramic crowns without a tougher ceramic core, a cast-metal core, or metal-foil coping. Furthermore, their firing shrinkage results in significant discrepancies in fit and adaptation of margins unless correction firings are performed.

Two of the most important breakthroughs responsible for the long-standing superb esthetic performance and clinical survival probabilities of metal-ceramic restorations are described in the patents of Weinstein and Weinstein (1962) and Weinstein et al. (1962). One of these patents identified the formulations of feldspathic porcelain that enabled the systematic control of the sintering temperature and coefficient of thermal expansion. The other patent described the components that could be used to produce alloys that bond chemically to and that are thermally compatible with the feldspathic porcelains. The first commercial porcelain was developed by VITA Zahnfabrik in about 1963. Although the first VITA porcelain products were known for their esthetic properties, the subsequent introduction of the more versatile Ceramco porcelain led to thermal expansion behavior that allowed this porcelain to be used safely with a wider variety of alloys.

A significant improvement in the fracture resistance of all-porcelain crowns was reported by McLean and Hughes in 1965, when they introduced a dental aluminous core ceramic consisting of a glass matrix containing between 40% and 50% Al₂O₃ by weight. Because of the inadequate translucency (opaque, chalky-white appearance) of the aluminous porcelain core materials, a feldspathic porcelain veneer was required to achieve acceptable esthetics. The flexural strength (modulus of rupture) of the core material was approximately 131 MPa. McLean (1979) reported a low 5-year fracture rate of only 2% for anterior crowns but an unacceptably high fracture rate of 15% when aluminous porcelain was used for molar crowns. Another deficiency of the aluminous porcelain crown was the large sintering shrinkage (approximately 15% to 20%) of the core material at its high firing temperature. Because of their relatively high fracture rate in posterior sites, the principal indication for the use of aluminous porcelain crowns was the restoration of maxillary anterior crowns where esthetics was of paramount importance. Since the introduction of aluminous porcelain crowns in the early 1900s and methods to produce durable metal/>