Glass-Ceramics in Dentistry: A Review

Abstract

In this review, we first briefly introduce the general knowledge of glass-ceramics, including the discovery and development, the application, the microstructure, and the manufacturing of glass-ceramics. Second, the review presents a detailed description of glass-ceramics in dentistry. In this part, the history, property requirements, and manufacturing techniques of dental glass-ceramics are reviewed. The review provided a brief description of the most prevalent clinically used examples of dental glass-ceramics, namely, mica, leucite, and lithium disilicate glass-ceramics. In addition, we also introduce the newly developed ZrO₂-SiO₂ nanocrystalline glass-ceramics that show great potential as a new generation of dental glass-ceramics. Traditional strengthening mechanisms of glass-ceramics, including interlocking, ZrO₂-reinforced, and thermal residual stress effects, are discussed. Finally, a perspective and outlook for future directions in developing new dental glass-ceramics is provided to offer inspiration to the dental materials community.

Keywords: dental prostheses; glass–ceramics; strength; strengthening mechanisms; translucency.

Figure 1

An idea of scientific and commercial significance of glass–ceramics. The number of published papers searched from Web of Science with the key words “glass–ceramics”.

Figure 2

Applications of glass–ceramics in a wide range of fields.

Figure 3

Microstructure differences between glass, glass ceramic, and ceramic: schematic microstructures of glass (a), glass–ceramics (b), and ceramic (c). Corresponding examples of glass (d), glass–ceramics (e), and ceramic (f). SEM image of non–annealed Li₂O–SiO₂ glass. Reprinted from ref [22] with permission. (e) SEM images of lithium disilicate glass–ceramic after etching. (f) SEM image of zirconia toughened alumina ceramics, with ZrO₂ showing light contrast and Al₂O₃ showing dark contrast. Reprinted from ref [23] with permission.

Figure 4

The two manufacturing processes of glass–ceramics: (a) The classic melting–casting–annealing process; (b) the concurrent sinter–crystallization process.

Figure 5

An example demonstrating the microstructure evolution during the ceramming process. TEM micrograph of an 80 GeO₂–10ZnO–10Ga₂O₃ (+2.5 Na₂O) (mol%) glass with phase separation (a) and corresponding glass–ceramic after ceramming (b). Reprinted from ref [30] with permission.

Figure 6

Computer–aided design and computer–aided manufacturing (CAD–CAM)–based workflow in dentistry. (a) The Cerec workflow includes three steps: first, a intraoral canner is used to acquire optical images of the prepared teeth; second, raw scanning data is processed with the aid of the chairside software, followed by the design of the restoration; third, CAM technology takes the designed model to a computer numeric control machine to manufacture the restoration. (b) The novel cloud connected digital dentistry system. The full worldwide digital platform is characterized by the separation of design work to form independent design centers from the convention production centers. Reprinted from [45].

Figure 7

Additive manufacturing in dentistry. (a) The process of manufacturing a dental prosthesis through additive manufacturing. Reprinted with permission of Ref [47]. (b) Zirconia framework prepared by direct inkjet printing (DIP) technology. Reprinted with permission of Ref [51]. (c) A tooth model consisting of 35 layers printed by SLM technology. The composition is 25.5Al₂O₃–74.5SiO₂ (wt%). Reprinted with permission of Ref [52].

Figure 8

Typical microstructure of commercially available dental glass–ceramics (SEM images). (a) The mica glass–ceramic exhibited a typical “house of cards” microstructure with randomly interlocked mica platelets. Reprinted with permission of Ref [56]. (b) Leucite–based glass–ceramic (IPS Empress Esthetic) shows lamina–like, irregular–shaped leucite crystals. Reprinted with permission of Ref [58]. (c) Interlocked microstructure of IPS e.max Press. The glass phase has been removed by acid etching, leaving needle–like lithium disilicate crystals.

Figure 9

Scanning transmission electron microscopy (STEM) images of 35%ZrO₂–65%SiO₂ (mol%) (a) and 65%ZrO₂–35%SiO₂ (mol%) (b) glass–ceramics. As indicated in (a), nanoparticles with bright contrast are ZrO₂ nanoparticles. Amorphous SiO₂ matrix show dark contrast. Reprinted from Ref [33] and Ref [67]. (c) Optical images and the average flexural strength of the sintered ZrO₂–SiO₂ nanocrystalline glass–ceramics. The thickness of the glass–ceramics is approximately 1 mm. Reprinted from Ref [67].

Figure 10

A schematic (a) and an example (b) demonstrating interlocking effect in glass–ceramics. The SEM image reveals crack deflection in a mica glass–ceramic. Reprinted with permission of Ref [56].

Figure 11

Schematics showing two traditional strengthening mechanisms of glass–ceramics. (a) Glass–ceramics can be strengthened by the addition of ZrO₂ particles. Transformation toughening of ZrO₂ particles contributes to the improvement of mechanical properties of glass–ceramics; (b) thermal residual stress arise in glass–ceramics upon cooling because of the thermal expansion mismatch between the crystalline and glass phases. The coefficient of thermal expansion (CTE) of the crystalline phase is larger than that of the glass phase. A propagating crack deviates from the crystalline phase, leading to crack deflection.

Figure 12

A new strengthening mechanism of glass–ceramic. (a–c) 3D nanoarchitecture built by ZrO₂ NPs in 65%ZrO₂–35%SiO₂ (mol%) glass–ceramic revealed by electron tomography. Reprinted from Ref [83]. (d) A front view schematic diagram of stress state of the glass–ceramic during the bending test.

Figure 13

A perspective and outlook for future directions in developing new dental glass–ceramics.