Ceramics have two major applications in dentistry: (1) ceramics for metal-ceramic crowns (Figure 11-1, right) and fixed partial prostheses, (2) all-ceramic crowns (Figure 11-1, left), inlays, onlays, veneers, and fixed partial prostheses. Additionally, ceramic orthodontic brackets, dental implant abutments, and ceramic denture teeth are available.

The classification by fabrication method is summarized in Table 11-1, which also includes examples of commercial ceramics and their manufacturers. The most common fabrication technique for metal-ceramic restorations is called sintering. Sintering is the process of firing the compacted ceramic powder at high temperature to ensure optimal densification. This occurs by pore elimination and viscous flow when the firing temperature is reached. All-ceramic restorations can also be produced by sintering, but they encompass a wider range of processing techniques, including slip-casting, heat-pressing, and CAD/CAM machining. Some of these techniques, such as machining and heat-pressing, can also be combined to produce the final restoration.

Regardless of their applications or fabrication technique, after firing, dental ceramics are composed of a glassy (or vitreous) phase and one or more crystalline phases, together with various amounts of porosity. Depending on the nature and amount of crystalline phase and porosity present, the mechanical and optical properties of dental ceramics vary widely. Increasing the amount of crystalline phase may lead to crystalline reinforcement and increase the resistance to crack propagation but also can decrease translucency. Materials for all-ceramic restorations have increased amounts of crystalline phase (between 35% for leucite-reinforced ceramics and up to 99% for polycrystalline zirconia ceramics such as 3Y-TZP) for better mechanical properties, but they are usually more opaque than dental porcelains for metal-ceramic restorations with low crystallinity. Table 11-1 lists the various combinations of fabrication techniques and crystalline phases found in dental ceramics.

Transformation toughening is obtained, for example, in ceramics containing partially stabilized tetragonal zirconia. Zirconia (ZrO₂) exists under several crystallographic forms. The monoclinic form is stable at all temperatures below 1170° C. The tetragonal form is stable between 1170° and up to 2370° C. The transformation from the tetragonal to the monoclinic form upon cooling is associated with a volume increase of the unit cell. This is the reason compacts of pure unalloyed zirconia cannot be obtained at room temperature; the compact would spontaneously crack upon cooling due to the transformation. The tetragonal form can be partially stabilized to room temperature by doping with various oxides, such as yttria oxide (Y₂O₃) or cerium oxide (CeO₂). Zirconia-based dental ceramics produced by machining followed by sintering at high temperature contain tetragonal zirconia polycrystals, partially stabilized with 3 mole percent yttrium (3Y-TZP). This partial stabilization or metastability of the tetragonal phase allows the transformation from tetragonal to monoclinic to occur under external applied stresses. The transformation is also called stress-induced and is accompanied by a volume increase with associated compressive stresses in the vicinity of the crack tip, eventually leading to a closing of the crack in the transformed zone (Figure 11-2). Transformation toughening is responsible for the excellent mechanical properties of 3Y-TZP. Figure 11-3 shows a Vickers indentation in a 3Y-TZP dental ceramic under a 98.1-N load. Only one short crack can be seen emanating from one corner of the indentation.

Tempering and chemical strengthening are extrinsic strengthening techniques based on the creation of a compressive stress layer at the surface of a glass or a ceramic. Tempering is obtained by using rapid but controlled cooling rates whereas chemical strengthening relies on the replacement of small ions with larger ions by diffusion from a molten salt bath in which the ceramic or glass is immersed. Although widely used in the glass industry, neither of these two techniques is used for dental ceramics.

Glazing is the final step in the fabrication of metal-ceramic restorations. This standard technique, also called self-glazing, does not significantly improve the flexural strength of feldspathic dental porcelains. However, a low-expansion glass called glaze can also be applied at the surface of the ceramic, then fired to high temperature. Upon cooling, this glaze layer is placed under compression from the greater contraction of the underlying ceramic. This layer is also known to reduce depth and width of the surface flaws, thereby improving the overall resistance of the ceramic to crack propagation.

Sometimes, researchers use devices that try to simulate dental morphology. However, the experimental variables can become extremely complex and difficult to reproduce in this type of testing. Finite element analysis (FEA) constitutes another approach to the simulation of clinical conditions. Failure predictions of ceramic inlays by the FEA technique have successfully matched fractographic analyses of clinically failed restorations. Fractography is well established as a failure-analysis technique for glasses and ceramics. It has been recognized as a powerful analytical tool in dentistry. The in vivo failure stress of clinically failed all-ceramic crowns can be determined using fractography (see more discussion in Chapter 5.

Flexural strength data for dental ceramics are summarized in Table 11-2. Feldspathic porcelains for metal-ceramic restorations have a mean flexural strength between 60 and 80 MPa. This value is lower than those listed for all-ceramic materials; however, because metal-ceramic restorations are supported by a metallic framework, their long-term probability of survival is usually higher.

Among the currently available all-ceramic materials, zirconia (3Y-TZP) ceramics exhibit the highest values (800-1300 MPa), followed by slip-cast ceramics (378 to 630 MPa), and lithium disilicate–reinforced ceramics (262 to 306 MPa). The flexural strength of leucite-reinforced ceramics is around 100 MPa. As mentioned previously, the nature and amount of the crystalline phase present in the ceramic material strongly influences the mechanical properties of the final product. The shear strength of feldspathic porcelain is 110 MPa, and the diametral tensile strength is lower at 34 MPa. The compressive strength is about 172 MPa, and the Knoop hardness is 460 kg/mm².

Fracture toughness is also an important property of ceramics; it measures the resistance to brittle fracture when a crack is present. The fracture toughness of conventional feldspathic porcelains is very similar to that of soda lime glass (0.78 MPa • m^0.5). Leucite-reinforced ceramics exhibit slightly higher fracture toughness values (1.2 MPa • m^0.5), followed by lithium disilicate–reinforced ceramics (3.0 MPa • m^0.5). 3Y-TZP ceramics have the highest fracture toughness of all-ceramic materials (greater than 5.0 MPa • m^0.5).

The elastic constants of dental ceramics are needed in the calculations of both flexural strength and fracture toughness. Poisson’s ratio lies between 0.21 and 0.26 for dental ceramics. The modulus of elasticity is about 70 GPa for feldspathic porcelain, 110 GPa for lithium disilicate heat-pressed ceramics, and 210 GPa for 3Y-TZP ceramics and reaches 350 GPa for alumina-based ceramics.

Shrinkage remains an issue for all-ceramic materials with the exception of machined ceramics from fully sintered ceramic blocks and heat-pressed ceramics. Shrinkage of the veneering ceramics applied on all-ceramic cores has to be carefully compensated for during porcelain buildup. The large shrinkage of machined zirconia restorations during the subsequent sintering at very high temperature (about 25%) is compensated for at the design-stage by computerized enlargement of the restorations.

The density of fully sintered feldspathic porcelain is around 2.45 g/cm³ and will vary with the porosity of the material. The density of ceramic materials also depends on the amount and nature of crystalline phase present. The theoretical density of 3Y-TZP dental ceramics is 6.08 g/cm³, assuming that the material is pore free. A density greater than 98.7% of the theoretical density is required for medical grade 3Y-TZP ceramics.

The thermal properties of feldspathic porcelain include a conductivity of 0.0030 cal/sec/cm² (° C/cm), a diffusivity of 0.64 mm²/sec, and a linear coefficient of thermal expansion (CTE) of about 12.0 × 10⁻⁶/° C between 25° and 500° C. The CTE is about 10 × 10⁻⁶/° C for aluminous ceramics and lithium disilicate ceramics, 10.5 × 10⁻⁶/° C for zirconia-based ceramics (3Y-TZP), and 14 to 18 × 10⁻⁶/° C for leucite-reinforced ceramics.