Marcel Schweiger

A comparison of the microstructure and properties of the IPS Empress®2 and the IPS Empress® glass‐ceramics

The aim of this report is to analyze the microstructures of glass-ceramics of the IPS Empress 2 and IPS Empress systems by scanning electron microscopy. The main properties of the glass-ceramics were determined and compared to each other. The flexural strength of the pressed glass-ceramic (core material) was improved by a factor of more than three for IPS Empress 2 (lithium disilicate glass-ceramic) in comparison with IPS Empress (leucite glass-ceramic). For the fracture toughness, the K(IC) value was measured as 3.3 +/- 0.3 MPa. m(0.5) for IPS Empress 2 and 1.3 +/- 0.1 MPa. m(0.5) for IPS Empress. Abrasion behavior, chemical durability, and optical properties such as translucency of all glass-ceramics fulfill the dental standards. The authors concluded that IPS Empress 2 can be used to fabricate 3-unit bridges up to the second premolar.

Control of nucleation in glass ceramics

Glass ceramics are advanced materials composed of one or more glass and crystal phases. By developing base glasses with appropriate compositions and by controlling crystal nucleation and growth in these glasses, glass ceramics with tailor–made properties can be fabricated. The key to developing this type of material is control of the nucleation processes. Both volume and surface nucleation can be exploited. Heterogeneous volume nucleation has been used to develop glass ceramics showing minimal thermal expansion and high strength. Two nucleation mechanisms can be combined and the precipitation of two crystal phases can be controlled. That the nucleation processes can be controlled by nano– and microscale immiscibility is a special feature, allowing selective nanophase formation or the development of needle–like apatite phases demonstrating a natural morphology. This represents a biomimetic process. The control of nucleation has enabled the development of biomaterials for dental applications.

The effect of P2O5 on the crystallization and microstructure of glass-ceramics in the SiO2–Li2O–K2O–ZnO–P2O5 system

Abstract Six glasses in the SiO 2 –Li 2 O–K 2 O–ZnO–P 2 O 5 system were prepared, varying the P 2 O 5 content from 0.0 to 2.5 mol% with the molar ratios of the other components being invariant. Crystallization processes of these glasses were investigated using high temperature XRD and differential scanning calorimetry. Morphology and microstructure of the glass-ceramics were studied using scanning electron microscopy. Crystallization of these glasses occurred in the temperature range 500–750°C. The main crystalline phase was lithium disilicate (Li 2 Si 2 O 5 ). Lithium metasilicate, lithium orthophosphate and cristobalite were secondary phases. In glasses with a P 2 O 5 content of more than 1.0 mol%, precipitation of cristobalite took place prior to the main crystallization of lithium disilicate. Surface crystallization occurred in glasses with a P 2 O 5 content of 0.0 and 0.5 mol%. Furthermore, a spherulitic morphology of lithium disilicate crystals in the interior of the samples was observed. Lithium disilicate crystals in glasses with a P 2 O 5 content of 1.0–2.5 mol% had an elongated shape along the c -axis, a crystal length ranging from 1 to 5 μm and aspect ratios between 2 and 6. Rod-shaped crystals were randomly oriented in the volume of the glass-ceramics and were responsible for the mechanical strength of up to 440 MPa.

Ceramics as biomaterials for dental restoration

Sintered ceramics and glass–ceramics are widely used as biomaterials for dental restoration, especially as dental inlays, onlays, veneers, crowns or bridges. Biomaterials were developed either to veneer metal frameworks or to produce metal-free dental restorations. Different types of glass–ceramics and ceramics are available and necessary today to fulfill customers’ needs (patients, dentists and dental technicians) regarding the properties of the biomaterials and the processing of the products. All of these different types of biomaterials already cover the entire range of indications of dental restorations. Today, patients are increasingly interested in metal-free restoration. Glass–ceramics are particularly suitable for fabricating inlays, crowns and small bridges, as these materials achieve very strong, esthetic results. High-strength ceramics are preferred in situations where the material is exposed to high masticatory forces.

Bioceramics and their application for dental restoration

Abstract This review article covers the historical development of ceramics, from the beginnings to the present. Feldspar based ceramic biomaterials for veneering metal frameworks, which are based on the jacket porcelain crown, have firmly established themselves in restorative dentistry since the 1970s. Currently, the development of restorative dental materials that can be used to replace metal represents a major challenge. As a result, this review will focus on the latest materials in this field. These materials include glass ceramics as well as high performance sintered ceramics. Glass ceramics exhibit more favourable optical properties, such as translucency and colour, compared with high performance ceramics, while the latter demonstrate high flexural strength and toughness. Both groups of materials have specialised applications in restorative dentistry and are capable of covering all the indications of dental restorations. The two types of materials, that is, glass ceramics and ceramics, have to be processed in accordance with their properties. As a result, the processing techniques, such as moulding, sintering and machining, will be discussed in detail in addition to the properties of the materials. Additional development possibilities for the materials will be presented on the basis of customer/patient needs and the successful long term use of glass ceramics and ceramics. In this context, it is clear that high performance ceramics and layered composites (consisting of high performance ceramics veneered with glass ceramics) offer the best possible solution for indications in the posterior region of the mouth. In contrast, glass ceramics are used to fabricate inlays and onlays for all parts of the jaw. In addition, glass ceramics can be used to fabricate crowns and small bridges to replace anterior dentition.

Microstructure and properties of a composite system for dental applications composed of glass-ceramics in the SiO2–Li2O–ZrO2–P2O5 system and ZrO2-ceramic (TZP)

The objective of the study was to develop a biocompatible composite system which was composed of TZP-ceramic (tetragonal zirconia polycrystals, ZrO2 stabilized with 3 mol % Y2O3) and two glass-ceramics of the SiO2–Li2O–ZrO2–P2O5 type. The metal-free composite system would satisfy the translucency, the biocompatibility and the strength requirements of dentistry. The two glass-ceramics of the SiO2–Li2O–ZrO2–P2O5 type with a content of 15 and 20 wt % ZrO2 respectively, were chemically and physically adapted to TZP-ceramic. The glass-ceramics were used as a dentin buildup material. The TZP-ceramic had the function of a root post. The shape of the post was cylindrical with a conical tip. The composite system was easy to process through viscous flow of the glass-ceramic at 900 and 1000°C, respectively. The microstructure and the mechanical properties of two glass-ceramics of the SiO2–Li2O–ZrO2–P2O5 type were examined therefore.

Nucleation and Crystallization Phenomena in Glass‐Ceramics

Glass-ceramics are modern multiphase materials. Different types of glass-ceramics can be fabricated and their properties tailored to meet specific requirements. It is possible, therefore, to produce glasses that combine desirable optical properties such as transparency and mechanical properties such as strength. Furthermore, this controlled development of glass-ceramics also enables the fabrication of materials exhibiting properties that are unknown in conventional glasses and ceramics. In this article, the authors discuss nucleation and crystallization phenomena in the controlled development of various glass-ceramics on the basis of their own work. The reaction mechanisms in the formation of glass-ceramics and their microstructures are illustrated. In addition, the resulting properties of the glass-ceramics and their particular applications are presented.

Radiopaque Strontium Fluoroapatite Glass-Ceramics.

The controlled precipitation of strontium fluoroapatite crystals was studied in four base glass compositions derived from the SiO2–Al2O3–Y2O3–SrO–Na2O–K2O/Rb2O/Cs2O–P2O5–F system. The crystal phase formation of these glasses and the main properties of the glass-ceramics, such as thermal and optical properties and radiopacity were compared with a fifth, a reference glass-ceramic. The reference glass-ceramic was characterized as Ca-fluoroapatite glass-ceramic. The four strontium fluoroapatite glass-ceramics showed the following crystal phases: (a) Sr5(PO4)3F – leucite, KAlSi2O6, (b) Sr5(PO4)3F – leucite, KAlSi2O6, and nano-sized NaSrPO4, (c) Sr5(PO4)3F – pollucite, CsAlSi2O6, and nano-sized NaSrPO4, and (d) Sr5(PO4)3F – Rb-leucite, RbAlSi2O6, and nano-sized NaSrPO4. The proof of crystal phase formation was possible by X-ray diffraction. The microstructures, which were studied using scanning electron microscopy, demonstrated a uniform distribution of the crystals in the glass matrix. The Sr-fluoroapatites were precipitated based on an internal crystallization process, and the crystals demonstrated a needle-like morphology. The study of the crystal growth of needle-like Sr-fluoroapatites gave a clear evidence of an Ostwald ripening mechanism. The formation of leucite, pollucite, and Rb-leucite was based on a surface crystallization mechanism. Therefore, a twofold crystallization mechanism was successfully applied to develop these types of glass-ceramics. The main focus of this study was the controlled development of glass-ceramics exhibiting high radiopacity in comparison to the reference glass-ceramic. This goal could be achieved with all four glass-ceramics with the preferred development of the Sr-fluoroapatite – pollucite-type glass-ceramic. In addition to this main development, it was possible to control the thermal properties. Especially the Rb-leucite containing glass-ceramic showed the highest coefficient of thermal expansion (CTE). These glass-ceramics allow optical properties, especially the translucency and color, to be tailored to the needs of biomaterials for dental applications. The authors conclude that it is possible to use twofold crystallization processes to develop glass-ceramic biomaterials featuring different properties, such as specific radiopacity values, CTEs, and optical characteristics.

Properties and Crystallization Phenomena in Li2Si2O5–Ca5(PO4)3F and Li2Si2O5–Sr5(PO4)3F Glass–Ceramics Via Twofold Internal Crystallization

The combination of specific mechanical, esthetic, and chemical properties is decisive for the application of materials in prosthodontics. Controlled twofold crystallization provides a powerful tool to produce special property combinations for glass–ceramic materials. The present study outlines the potential of precipitating Ca5(PO4)3F as well as Sr5(PO4)3F as minor crystal phases in Li2Si2O5 glass–ceramics. Base glasses with different contents of CaO/SrO, P2O5, and F− were prepared within the glasses of the SiO2–Li2O–K2O–CaO/SrO–Al2O3–P2O5–F system. Preliminary studies of nucleation by means of XRD and scanning electron microscopy (SEM) of the nucleated base glasses revealed X-ray amorphous phase separation phenomena. Qualitative and quantitative crystal phase analyses after crystallization were conducted using XRD in combination with Rietveld refinement. As a main result, a direct proportional relationship between the content of apatite-forming components in the base glasses and the content of apatite in the glass–ceramics was established. The microstructures of the glass–ceramics were investigated using SEM. Microstructural and mechanical properties were found to be dominated by Li2Si2O5 crystals and quite independent of the content of the apatite present in the glass–ceramics. Biaxial strengths of up to 540 MPa were detected. Ca5(PO4)3F and Sr5(PO4)3F influence the translucency of the glass–ceramics and, hence, help to precisely tailor the properties of Li2Si2O5 glass–ceramics. The authors conclude that the twofold crystallization of Li2Si2O5–Ca5(PO4)3F or Li2Si2O5–Sr5(PO4)3F glass–ceramics involves independent solid-state reactions, which can be controlled via the chemical composition of the base glasses. The influence of the minor apatite phase on the optical properties helps to achieve new combinations of features of the glass–ceramics and, hence, displays new potential for dental applications.