In 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.

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). 

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.

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. 

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. 

Heat treatment of glass powders results in the precipitation of leucite from the surface of the glass-ceramics and the volume crystallization of needlelike apatite. 

Figure 3-8 SEM image showing dendritic surface crystallization of leucite in leucite-apatite glass-ceramics at 900°C, heat-treated for 1 h, etched for 10 sec with 3% HF (Haiand et al., 2000c).

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