Leucite glass-ceramics

This crystal growth mechanism is linked to surface nucleation. The success-fill application of the mechanisms of surface nucleation and crystallization in the development of cordierite (Semar and Pannhorst, 1991), apatite-wollastonite (Kokubo 1991), and leucite glass-ceramics (Holand et al., 1995a) has already been mentioned in Section 1.4. 

These mechanisms are presented by using the examples of cordierite and leucite glass-ceramics. To clearly demonstrate the mechanisms, experiments were conducted with monolithic test specimens rather than the fine-powdered glasses required in the industrial manufacturing of glass-ceramics. The following descriptions, therefore, refer only to the mechanism as demonstrated in monolithic specimens. 

Following the initiation of surface nucleation, the crystal growth mechanism demonstrated at least three extraordinary features in cordierite (Zanotto 1994 Schmelzer et al., 1995 Donald 1995) as well as leucite glass-ceramics (Holand et al., 1995 a,b). 

In comparison to the IPS Empress microstructure, a different type of leucite glass-ceramic was developed by Schweiger et al. (1999). The leucite-reinforced glass-ceramic was developed as a biomaterial for dental restoration. The material can easily be machined by a CAD/CAM system. The mechanical properties are characterized by a flexural strength of 135-160 MPa and a fracture toughness as of 1.3 MPa m (section ProCAD in Section 4.4.2 dental glass-ceramics). 

After nucleation in the apatite-leucite glass-ceramic, however, the apatite grows anisotropicaily in a preferred orientation into needlelike apatite (Holand et al., 1995). Previously, these crystals had only been produced under hydrothermal conditions (Newesely 1972 Jaha et al., 1997). Their morphology corresponds to that of the hydroxyapatite in natural teeth (enamel). 

Non-steady-state leucite glass-ceramics with heat treatment at 1050 C/20 min. 

Figure 2-33 Number (N) and length (L) of needlelike apatite crystals as a function of time for apatite-leucite glass-ceramics.

As reported the mechanisms of the nucleation and crystallization of needlelike apatite in apatite-leucite glass-ceramics is very difficult. Therefore, the mechanism was investigated in detail and the main results can be explained by the following two investigations. 

Figure 2-40 Crystal formation in apatite-leucite glass-ceramic (A NaCaP04, B Na2Ca4(P04)2Si04, C KAISigOg, D unknown phase, E C g PO JF.

Figure 2-41 SEM image (etching 10 sec, 3% HF) of the microstructure of apatite-leucite glass-ceramic for dental restorations. Heat treatment of the glass powder at SSO C/I h and 1050°C/1 h. The apatite crystals measure approximately 0.1-0.5 and 1-2 pm in diameter. The leucite crystals measure approx. 2 pm.

Figure 3-3 SEM overview of the coast-and-island microstructure in opal leucite glass-ceramics.

Figure 4-33 Schematic diagram of a glass-ceramic crown of IPS EMPRESS leucite glass-ceramic. Pressed glass-ceramic (dentin), sintered ceramic with opal glass-ceramic (incisal), and glaze.

Properties of Pressed Glass-Ceramic (Lithium Disilicate) of IPS EMPRESS 2 in Comparison to IPS EMPRESS (Layering Technique) (Leucite Glass-Ceramic) ... 

The materials system containing the apatite-leucite glass-ceramic called IPS d.SIGN is described as a composite from a materials science point of view. This restorative dental product is produced by joining and combining three types of materials, that is, metal... 

A typical apatite-leucite glass-ceramic is presented in Section 2.4.6. This section shows that the characteristic part of phase formation in the glass-ceramic is the controlled crystallization of the leucite and apatite crystal phases. The formation of the crystals and the typical microstructure of the glass-ceramic with the needlelike apatite crystals are described in Sections 2.4.6 and 3.2.12. 

The most important properties of the dentin and incisal materials are shown in Table 4-19. The coefficient of linear thermal expansion plays an important role in the optimal joining of ious types of apatite-leucite glass-ceramics and the Zr02-rich opaquer, which are applied to the different metals. Therefore, CTE of the opaquer has been included as a comparative value in Table 4-19. A comparison of CTE of glass-ceramics and of the opaquer with that of metals clearly shows that the application of the glass-ceramic to the metal framework systematically builds up compressive strain. As a result, the finished dental product demonstrates surface tension and a controlled increase in strength, ensuring retention on the substructure. 

Figure 4-50 SEM image showing the microstructure of apatite-leucite glass-ceramic (incisal). Etched In 3% HF for 10 sec.

In terms of quantity, the incisal and dentin materials comprise the largest part of the apatite-leucite glass-ceramics of the IPS d.SIGN system. Their microstructure and properties are described in Section 2.4.6. The main applications of these glass-ceramics include dental crowns and multi-unit bridges. 

Reprinted, with permission, from W. Holand, V. Rheinberger, S. Wegner, and M. Frank, Needlelike Apatite-Leucite Glass-Ceramic as a Base Material for the Veneering of Metal Restorations in DentistryJournal of Mater. Sci. Mater. Med, 11, 1-7, Plenum Publishing, 2000. 

Szabo I., Barnab S., Volksch G., and Holand W, "Crystallization and Color of Apatite-Leucite Glass-Ceramic," Glastech. Ber. Glass Sci. TechnoL, 73 [Cl] 354—57 (2000). 

Figures 4-8 and Table 2 show the edge chip data of Part 1 is fitted very well with the new quadratic relationship, eq. 8. The dashed veitical lines correspond to

Figure 4. Edge chip results dental restorative materials, (a) is for a feldspathic porcelain (Mark 2) and (b) is for a leucite glass ceramic (Empress CAD).

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