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ОглавлениеCeramics 101: What Is This Stuff Anyway?
Chapter Highlights
• Ceramics are oxides of metals such as silicon, aluminum, and zirconium.
• There are three basic categories of ceramics: glassy, particle-filled glasses, and polycrystalline.
• Esthetically, there are only two categories: pretty and ugly. Pretty ceramics are generally weak, while strong ceramics are generally opaque and ugly.
• Ceramics can be thought of as “composites”—that is, they contain two or more distinct phases. Glassy ceramics are filled for color and opacity control, while stronger ceramics are filled for strength.
• Strength is the stress at which something bad, like fracture, happens. Ceramics are much stronger under compression than under tension.
Ceramics as Anti-Metals
While ceramics are built from metal atoms (eg, silicon, aluminum, zirconium), ceramics are certainly not metals. In fact, ceramics are anti-metals. Whereas metals conduct heat and electricity, ceramics are insulators. Metals are shiny and opaque, whereas ceramics can be transparent, translucent, or semi-opaque. Ceramics can exquisitely mimic the optical properties of teeth and are extraordinarily stable chemically and biologically, whereas metals are highly unesthetic and notorious for corrosion. Ceramics are brittle, meaning they have high compressive strength but much lower tensile strength, whereas metals can be ductile and have high tensile strength. In the building of the Golden Gate Bridge between San Francisco and the Marin Headlands, it is no surprise that the suspension systems were made from high–tensile strength steel cabling and that the whole structure sits atop concrete pillars (a ceramic) anchored to bedrock (behaving like and often a source of ceramics). Likewise, in the first successful widespread use of ceramics in fixed prosthodontics, porcelain was supported by cast metal substructures for their high tensile strength (see chapter 5).
The two main cables of the Golden Gate Bridge were fabricated by spinning together individual steel wires over the length of the bridge.
Ceramics are oxides of metals such as silicon (Si), aluminum (Al), and zirconium (Zr). Unlike metallic bonding in which aluminum atoms bond to adjacent aluminum atoms (ie, Al-Al-Al), in metal oxide bonding little oxygen atoms (O) intersperse themselves between every metal atom (ie, Al-O-Al) with covalent and ionic bonds. These oxygen atoms do not allow the flow of electrons that is behind the heat flow and subsequent conduction of electricity. Those particularly interested in metallic versus metal oxide bonding will find an intriguing visual illustration at the link provided.
Adding oxygen atoms changes the nature of bonds and the properties of the material. In metal oxide bonding, atoms share “D shell” orbital electrons.
All of the properties of interest in both ceramics and metals arise from this difference in the nature of their bonds. Arrangements of metal and oxygen atoms in ceramics can be either quite regular (ie, crystalline) or not regular at all (ie, amorphous). In crystalline ceramics (eg, zirconia, alumina), adjacent and even next-neighboring atoms are at exactly the same angle and distance throughout the whole structure in a relatively dense arrangement. In amorphous (or glassy) ceramics, bond angles and distances between atoms are all over the map, resulting in a much less dense arrangement that is easier to drive a crack through. X-rays shining on crystalline materials are diffracted at such regular angles that diffraction patterns can be used to identify the material, whereas x-rays diffracted from glassy materials create a broad and featureless hump. Figure 2-1 illustrates a crystalline structure and an amorphous feldspathic porcelain.
Fig 2-1 (a) In polycrystalline ceramics such as zirconia and alumina, metal and oxygen atoms are arranged in very regular relationships. Planes of atoms diffract x-rays in a manner that allows crystalline materials to be identified. (b) In feldspathic glasses, the three-dimensional network of bridges formed by silicon-oxygen-silicon bonds is broken up occasionally by modifying cations, such as sodium and potassium, that provide charge balance to nonbridging oxygen atoms.
Dental Ceramics Are Composites
There are two concepts that can help to demystify dental ceramics by providing a structure within which to organize thinking. First, there are only three main classes of dental ceramics: (1) predominantly glassy materials, (2) particle-filled glasses, and (3) polycrystalline ceramics containing no glass. Each class has defining characteristics (Fig 2-2). Second, virtually any ceramic within this spectrum can be considered a “composite,” meaning that it has a composition of two or more distinct entities. Reviewed within the framework of these two concepts, seemingly different dental ceramics can be shown to be quite similar or closely related to each other. Additionally, the rationale behind the development of ceramics of both historical and recent interest can be more easily understood. Two examples of the utility of these concepts include these basic statements:
Fig 2-2 Based on their microstructure, dental ceramics fall within three basic classes: (1) predominantly glassy (a), (2) particle-filled glasses (b), and (3) fully polycrystalline (c).
1. Highly esthetic dental ceramics are predominantly glassy, and higher-strength substructure ceramics are generally crystalline.
2. The development of substructure ceramics simply involved an increase in crystalline content from about 50% to fully polycrystalline.
Figure 2-3 provides the basic composition details and commercial examples of many esthetic and substructure dental ceramics organized by class, illustrating this concept of ceramics as composites comprised of a matrix and fillers. While the term composite is often reserved in dentistry for resin-based composites, the concept of composite materials is widely applied to metal-based and ceramic-based materials in engineering materials science. For example, concrete is a composite of a hydraulic cement and “clinker,” which is essentially pebbles and sand. Virtually all materials are composites, meaning that they are made of two or more distinct components (phases) distinguished by differences in chemistry, crystalline habit, melting temperature, or other property. Each of these components is considered to be a “phase,” and components are separated by “phase boundaries.”
Fig 2-3 Composition of dental ceramics based on the concept of them being composites consisting of a “matrix” and “fillers.” (a) Composition of dental ceramics used to veneer substructures. (b) Composition of stronger, nonveneering and structural dental ceramics. CAD/CAM, computer-aided design/computer-assisted manufacturing. (See Link 3 for more information.)
In Fig 2-3, we can see that many different dental ceramics, even those from different manufacturers, are really quite similar. For example, Vita Mark II (now Sirona Vitablocs) computer-aided design/computer-assisted manufacturing (CAD/CAM) blocks (Vita) consist of approximately 40% filler particles (crystalline nephaline and albite, and higher-melting glass particles) in a sea of feldspathic glass, while IPS EmpressCAD (formerly Empress, Ivoclar Vivadent) consists of 40% leucite (the crystalline mineral found in metal-ceramic porcelains) again in a sea of feldspathic glass. Similarly, some ceramics differ only in how they are processed; for example, e.maxCAD and e.maxPress (Ivoclar Vivadent) both contain about 70% lithium disilicate crystals in a sea of residual glass (residual from the ceraming step covered in chapter 5), but the former is processed in the laboratory by firing after CAD/CAM machining, whereas the latter is pressed. In general, higher-volume percentages of crystalline filler provide higher strength and fracture toughness. Therefore, the lithium disilicate (Ivoclar Vivadent) and In-Ceram ceramics (Vita) containing approximately 70% filler are listed as structural ceramics and have broadened clinical indications over Vita Mark II and Empress Esthetic ceramics (see chapter 3).
Slide-by-slide lecture discussing Fig 2-3.
By viewing these materials as composites, it is possible to identify similarities in properties; for example, both Vita Mark II and Empress Esthetic are easily acid etched and have remarkably similar clinical indications and behaviors (see chapter 3). Therefore, new ceramics can be more easily understood when introduced by examining their components (ie, their microstructure) and comparing them to that of known materials with similar properties. (Fracture toughness also tends to correlate with clinical indications.)
Predominantly Glassy Ceramics
Dental ceramics that best mimic the optical properties of enamel and dentin are predominantly glassy materials. Glasses are three-dimensional (3D) networks of atoms having no regular pattern to the spacing (distance and angle) between adjacent or next-neighboring atoms; thus, their structure is amorphous, or without form. Predominantly glassy ceramics, also called porcelains, derived from European porcelains in which the original flux—calcium carbonate or chalk (as in the white cliffs of Dover)—was replaced by feldspathic glass.1 Feldspar is a silicate mineral belonging to a family called aluminosilicate glasses. Glasses based on feldspar are resistant to crystallization (ie, devitrification) during firing, have long firing ranges (ie, they resist slumping if temperatures rise above the optimal), and are extremely biocompatible. In feldspathic glasses, the 3D network of bridges formed by Si-O-Si bonds is broken up occasionally by modifying cations such as sodium and potassium that provide charge balance to nonbridging oxygen atoms (see Fig 2-2). Modifying cations alter important properties of the glass, for example, by lowering firing temperatures or increasing thermal expansion/contraction behavior. The low density of these weaker bonds makes the materials themselves weak.
When crystalline feldspar rock is melted and then cooled quickly, it does not recrystallize but remains as an amorphous glass at room temperature. Link 4 illustrates this melting and rapid cooling step (fritting) during the manufacture of a dental porcelain. The modifying cations in the feldspar lower the firing temperature of the porcelain, and this is how manufacturers have lowered firing temperatures over the years. However, with too much modification, the glass becomes soluble, prompting experts at the International Standards Organization to develop a solubility test for the international ceramics standard (ISO 6872).
Mined mineral feldspars in a Vita factory being melted and quenched.
Lead in Dental Porcelains
Another modifying cation causing much recent consternation is naturally occurring lead. In 2008, lead levels around 210 ppm (parts per million) were found in a fixed dental prosthesis fabricated in China for a dental office in Columbus, Ohio, causing all sorts of negative and alarmist media attention.2 But let’s put this amount into perspective. Lead is found as a trace element in virtually everything that comes into contact with the earth. Plants grown in soil as well as surface water and groundwater all extract tiny amounts of lead from the earth. In 2005, a comprehensive study was published that analyzed the French diet based on 41 different categories of food (including regional, seasonal, and national sources).3 It turns out that the average French person eats 18.4 micrograms of lead each day. This amount may seem alarming, but 1 microgram is 1 millionth of a gram. In monetary terms, with gold at $960 an ounce, 18.4 micrograms of gold would fetch only 6.25 one-hundredths of a penny (0.00625¢). That’s how tiny 18.4 micrograms is.
So back to the lead-laden prosthesis from China, assuming that all of the porcelain from one crown were to be eaten in 10 years (which we know is absurd), the daily increase in lead in the body would be 0.078 micrograms, a whopping increase of 0.000004%. If this crown were eaten over only 1 year, that’s still only an increase of 0.00004%. Clearly, dental porcelains are a really, really lousy source of dietary lead. Far better sources include drinking water, bread, fruits, and soups. These foods will provide you with thousands of times more bioavailable lead each day!
Particle-Filled Glasses
When first produced, feldspathic glass is relatively colorless and transparent, so manufacturers add filler particles to adjust the color, opacity, and opalescence. These fillers yield the highly esthetic, but weak, veneering materials for use as small restorations and veneers or as the esthetic outer coating on stronger substructures such as metals and polycrystalline ceramics. To work well on metal substructures, the porcelain must contract at the same rate as the metal during cooling after firing. So yet another filler particle is included for metal-ceramics, the crystalline mineral leucite. Using the Edisonian research approach, Weinstein, Weinstein, and Katz arrived at what they called “component number 1”—a porcelain frit that had a thermal expansion coefficient of nearly 20 × 10–6/°C, allowing it to be mixed in any ratio with the normal-expansion porcelain frit (thermal expansion of 8 × 10–6/°C), generally in the range of 17 mass% to 25 mass%, to “dial in” expansion/contraction to match any dental alloy of interest.
The patent by Weinstein, Weinstein, and Katz.
For particle-filled glasses in dental ceramics, the filler particles are added to increase the strength and fracture toughness of the ceramic. The first successful strengthened-substructure ceramic was made in 1965 of feldspathic glass filled with particles of aluminum oxide (approximately 55 mass%).4 Today, commonly used fillers include aluminum oxide, leucite, and lithium disilicate. These particles can decrease the natural flaw size of the ceramic and make crack initiation and propagation more difficult. They also generally increase the opacity of the ceramic as well by increasing light scattering. Light scattering is caused by differences in the index of refraction between the matrix glass and the filler particles: The closer the indices of refraction, the more translucent the product (in a very strong relationship, the square of the difference). Leucite’s refractive index is very close to that of feldspathic glass, and hence leucite can be used in relatively high concentrations of 35 mass% to 50 mass%. This is also true of lithium disilicate, which is found at approximately 70 mass% in pressed and CAD/CAM ceramics that are esthetically suitable for monolithic prostheses.
Glass-ceramics
Crystalline filler particles can be added mechanically to the glass, for example, by simply mixing together crystalline and glass powders prior to firing. In a more recent approach, the filler particles are grown inside the glass object (prosthesis or pellet for pressing into a mold) after the object has been formed. After forming, the glass object is given a special heat treatment, causing the precipitation and growth of crystallites within the glass. Because these fillers are derived chemically from atoms of the glass itself, it stands to reason that the composition of the remaining glass is altered as well during this process known as ceraming, which is covered in more detail in chapter 5. Such particle-filled composites are called glass-ceramics. The material Dicor (Dentsply), the first commercial glass-ceramic available for fixed prostheses, contained filler particles of a type of crystalline mica (at approximately 55 vol%). More recently, a glass-ceramic containing 70 vol% crystalline lithium disilicate filler has been commercialized for dental use (Empress 2, now e.maxPress and e.maxCAD). New glass-ceramics based on lithium silicate are becoming available from Vita (Vita Suprinity) and Glidewell (Obsidian). Consumer products made from glass-ceramics include cooking and tableware (Pyroceram, including Corningware and Centura), stove cooktops and fireplaces (Ceran, Eurokera), telescope mirrors (Zerodur), and military applications including missile radomes. Link 6 shows a video of the production of Empress Esthetic at Ivoclar Vivadent, including base and creamed pellets as well as an animation of ceraming.
Empress Esthetic production.
Polycrystalline Ceramics
Polycrystalline ceramics have no glassy phases; all of the atoms are densely packed into regular arrays that are much more difficult to drive a crack through than the loose and irregular networks of atoms found in glasses. Hence, polycrystalline ceramics are generally much tougher and stronger than glassy ceramics. Because of their strength, polycrystalline ceramics are also more difficult to process into complex shapes (eg, a prosthesis) than are glassy ceramics. Well-fitting prostheses made from polycrystalline ceramics were not practical prior to the availability of CAM. In general, these computer-aided systems use a 3D data set representing either the prepared tooth or a wax model of the desired substructure. This 3D data set is used either to create an enlarged die upon which ceramic powder is packed (Procera, Nobel Biocare) or to machine an oversized part for firing by machining blocks of partially fired ceramic powder (Cercon, Dentsply; Lava, 3M ESPE; YZ, Vita). Both of these approaches rely on well-characterized and uniformly packed ceramic powders for which firing shrinkages can be predicted accurately (see chapter 5 for more information).
Polycrystalline ceramics tend to be relatively opaque compared with glassy ceramics, so they generally cannot be used for the entire thickness in esthetic areas of prostheses. These higher-strength ceramics serve as substructure materials upon which glassy ceramics are veneered to achieve pleasing esthetics. Laboratory measures of the relative translucency of commercial substructure ceramics are available for both single-layer and veneering materials (see chapters 3 and 6).
Strength
Strength is defined as the stress that a ceramic can withstand before it breaks, or the applied load (or “internal force”) divided by the area or volume being loaded. Ceramic strengths are measured using bars or discs in bending, for which analytical formulae exist to calculate stress as a function of bar dimensions, support dimensions, and load (Fig 2-4). Failure occurs when the largest or most properly oriented flaw in the surface or volume of highest stress has sufficient energy to create two new surfaces—ping! Dental restorations usually fail from surface flaws. Surface flaws in metals become blunted due to plastic flow of metal at the crack tip; essentially, stress intensity energy at the crack tip is used for ductile flow of metal atoms. Such ductile flow is not energetically feasible at crack tips in ceramics, so further load application eventually causes crack propagation. These concepts are illustrated in Fig 2-5. Critical flaws in ceramics can be inherent to their microstructure or introduced as a function of processing, which includes all the steps used in making a ceramic part from powder fabrication and powder packing to firing, finish machining, and clinical delivery. Common flaws include subsurface damage from airborne-particle abrasion, CAD/CAM machining, and rotary diamond grinding during internal adjustments.
Fig 2-4 Bend testing.
Fig 2-5 Tensile stress acting on a surface flaw creates quite different responses in metals and ceramics. Crack tips in metals are blunted due to the ductile flow of atoms, which reduces the stress intensity. In ceramics, however, ductile flow is not energetically feasible, and there is no mechanism to reduce stress intensity at the crack tip other than by the creation of two new surfaces—ping!
While strength measures provide valuable screening information, they do not directly predict relative clinical behavior for a number of important reasons. First, the failure mechanism operating clinically will generally be different from that generated in the laboratory. Clinical restorations generally withstand millions of low loads in a wet environment instead of one load in laboratory air. All ceramics are susceptible to the slow growth of cracks under these clinical conditions, and this susceptibility is not measured during simple bend strength testing. Thus, ceramics weaken at different rates during intraoral service. Second, the clinical stress state is more complicated than that during bending. As illustrated in Fig 2-6a, crowns are fully supported by dentin or core materials and often bonded by cement. As will be discussed more fully in chapter 4, failure strengths of such a structure are dependent on the stiffness of the support (elastic modulus), the square of the ceramic thickness, the thickness of the cement, the loaded contact area (ie, the wear facet), and the quality of the cement-ceramic bond.5 None of these variables play any role in laboratory bend testing. Unlike crowns, multiple-unit fixed dental prostheses (FDPs) are complex beams often failing from the gingival portion of their connectors (Fig 2-6b). Failure strengths function linearly with connector width, with the square of the connector height, in a complex fashion with their radii of curvature (the smaller the radii, the weaker the connector), and with the mobility of the abutment teeth (mobile teeth increase connector stresses). For connectors veneered with weaker porcelain, failures can initiate in this weaker material or from defects at the core-veneer interface. Chapter 3 discusses the clinical implications of this potential failure in the case of veneered lithium disilicate FDPs, which are not highly successful, whereas full-thickness (nonveneered) prostheses are. Again, these variables (ie, weakness of connectors) are not evaluated during standardized tensile strength testing. But prostheses are complex, multimaterial structures that do not behave as simple beams do in bending!
Fig 2-6 (a) Finite element analysis of blunt loading on the wear facet of an all-ceramic crown. High tensile stresses develop on the cementation surface due to slight bending of the stiffer ceramic on the less stiff substructure like dentin. These stresses cause radial cracks in the cementation surface that propagate to the occlusal surface. For any given load, these stresses increase with the difference in stiffness, or elastic modulus, between the ceramic and the substructure. This has direct clinical implications (see chapter 4.) (b) All-ceramic multiple-unit FDPs often fail at their connectors. It is not the connector surface area that is important but rather the square of the connector height, width of the connector, radii of the curvature, and mobility of the abutment teeth.
Fracture Toughness
Fracture toughness is essentially a measure of how difficult it is to propagate a crack though the ceramic; it describes the ability of a material containing a crack to resist fracture. Fracture toughness is one of the most important properties of any material for design applications. While measured strengths are very sensitive to flaw size, fracture toughness is not; therefore, fracture toughness is a more inherent property of ceramics and can be used to compare ceramics more directly than can strength measures. Fracture toughness measures the critical stress intensity at the crack tip for a crack to propagate by mode 1 opening (simple opening like the jaws of a crocodile). Stress intensity is designated by the letter K, with a subscript 1 for mode 1 and C for critical: K1C. Fracture toughness for dental ceramics ranges from 0.8 to 1.0 for esthetic porcelains, to approximately 3.5 for lithium disilicate, 4.0 for alumina, and 5.0 to 8.0 for zirconia. The units for fracture toughness are a bit crazy: MPa·m½. Values for metals begin at about 20 and can reach 100. Unlike strength, fracture toughness has been observed to generally correlate with clinical indications, leading the Technical Committee 106 (Dentistry) of the International Standards Organization to recommend fracture toughness values in a new classification scheme based on clinical indications from numerous clinical trials. Portions of this classification from ISO 6872:2008 are presented in the Table 2-1.
| Table 2-1 | Recommended fracture toughness values for ceramics based on clinical indication* |
| Class | Clinical indication | Commercial examples | Minimum fracture toughness (MPa·m½) |
|---|---|---|---|
| 1 | a. Monolithic ceramic for single-unit anterior prostheses, veneers, inlays, or onlays (adhesively cemented) b. Ceramic for coverage of a metal framework or a ceramic substructure | Many, including Vita CEREC blocks | 0.7 |
| 2 | a. Monolithic ceramic for single-unit anterior or posterior prostheses (adhesively cemented) b. Fully covered substructure ceramic for single-unit anterior or posterior prostheses (adhesively cemented) | Leucite-reinforced In-Ceram Alumina | 1.0 |
| 3 | a. Monolithic ceramic for single-unit anterior or posterior prostheses and for three-unit prostheses not involving molar restoration (adhesively or nonadhesively cemented) b. Fully covered substructure for single-unit anterior or posterior prostheses and for three-unit prostheses not involving molar restoration (adhesively or nonadhesively cemented) | Lithium disilicate In-Ceram Alumina Leucite-reinforced | 2.0 |
| 4 | a. Monolithic ceramic for three-unit prostheses involving molar restoration b. Fully covered substructure for three-unit prostheses involving molar restoration | Lithium disilicate Zirconia | 3.5 |
| 5 | Monolithic ceramic for prostheses involving four or more units or fully covered substructure for prostheses involving four or more units | Zirconia | 5.0 |
*This table is derived from the classification system and recommended methods for measuring fracture toughness in Annex A of ISO 6872:2008.
Transformation-Toughened Zirconia
Transformation-toughened zirconia is potentially the most interesting polycrystalline ceramic now available for dentistry because its fracture toughness (and hence strength) involves an additional mechanism not found in other polycrystalline ceramics. Unlike alumina, zirconia is transformed from one crystalline state to another during firing. At firing temperature, zirconia is tetragonal, but at room temperature it is monoclinic, with a unit cell of monoclinic occupying about 4.4% more volume than when tetragonal. Unchecked, this transformation leads to crumbling of the material on cooling. In the late 1980s, ceramic engineers learned to stabilize the tetragonal form at room temperature by adding small amounts (approximately 3 mass% to 8 mass%) of calcium and later yttrium or cerium. However, the tetragonal form is really only “metastable,” meaning that trapped energy still exists within the material to drive it back to the monoclinic state. The highly localized stress ahead of a propagating crack is sufficient to trigger grains of the ceramic to transform in the vicinity of the crack tip. In this case, the 4.4% volume increase becomes beneficial, essentially altering material conditions around the crack tip and shielding it from the outside world. In scientific terms, transformation incrementally increases the local fracture toughness as the transformation zone develops around the growing crack.
Monoclinic-tetragonal-monoclinic transformation of zirconia.
Crack growth in alumina and zirconia.
Zirconia substructure issues
With a fracture toughness higher than that of alumina ceramics, transformation-toughened zirconia represents an exciting potential substructure material. However, possible problems with zirconia ceramics may involve long-term instability in the presence of water, porcelain compatibility issues (thermal issues during firing and cooling), and some limitations in case selection due to their opacity. Two major concerns with zirconia are porcelain chipping and low-temperature degradation. There are early reports of significant percentages (ie, 25% to 50%) of single-unit and multi-unit prostheses having porcelain chipping and cracking. And the propensity for partially stabilized zirconia to autocatalytically transform at surface grain boundaries due to an interaction with water may create major structural issues in the long run.
Porcelain chipping
