Microstructure and properties of two glass-ceramics: a lithium disilicate glass-ceramic nucleated by P2O5 and an apatite glass-ceramic

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1 Microstructure and properties of two glass-ceramics: a lithium disilicate glass-ceramic nucleated by P2O5 and an apatite glass-ceramic W. Hoeland; M. Schweiger, M. Frank, and V. Rheinberger Ivoclar Ltd. FL-9494 Schaan Principality of Liechtenstein Abstract Two glass-ceramics, a lithium disilicate glass-ceramic (sintered and hot-pressed) and an apatite glass-ceramics (sintered) are the materials of the new IPS Empress 2 system. First, the sintered and hot-pressed glass-ceramic consists of lithium disilicate crystals (Li2Si2O5) as the main crystal phase and lithium orthophosphate (Li3PO4) as the secondary phase. The crystalline phases are homogenously embedded in a glassy matrix. The crystallinity of the glass-ceramic is more than 60 volume %. The hot-pressed IPS Empress 2 glass-ceramic has a threepoint flexural strength of 350 } 50 MPa. The fracture toughness measures 3.2 } 0.2 MPa- ãm. Second, the special sintered glass-ceramics have been specifically developed for the layering technique. These materials contain apatite crystals similar to those in natural teeth. The apatite glassceramics are sintered on the hot-pressed IPS Empress(R) 2 frame at 800 Ž. The coefficient of linear thermal expansion of the sintered glass-ceramics is conditioned with that of the pressed material. The scattering of light by the apatite crystals in the restorations is similar to that in natural dentition. INTRODUCTION The first lithium disilicate glass-ceramics were developed as early as in the fifties. This development was the work of Stookey 1. Following his fundamental discovery, lithium disilicate glass-ceramics became the subject of a considerable amount of research. The nucleation mechanism and the kinetics of crystallization of the main lithium disilicate phase received the most attention 2-7. A disadvantage of these lithium disilicate glass-ceramics, however, Phosphorus Research Bulletin Vol. 10 (1999), 628

2 was their poor chemical durability, inadequate translucency, and uncontrolled microcrack formation. Considerable progress in the development of a chemically durable lithiumdisilicate-based glass-ceramic was achieved by Beall8 and Echeverria 9. This lithium disilicate glass-ceramic was distinguished for the following three chemical characteristics: Firstly, a special relationship between the main components SiO2 and Li2O, which contributes to the formation of the lithium disilicate main crystal phase. Secondly, new components which initiate the nucleation of the main crystal phase. Thirdly, new additional components which form the glass matrix. These components were specifically selected to achieve good chemical durability of the glass-ceramic. EXPERIMENTAL PROCEDURE The aim of this research program was to investigate the microstrucure and properties of the lithium disilicate glass-ceramic (sintered and pressed material) and of the apatite containing glass-ceramic (sintered glass-ceramic). P205 was used for nucleation in the lithium disilicate system SiO2-Al2O3- K20-Li20. The chemical composition was: wt-% SiO2, O-5 Al2O3, La2O3, O-5 MgO, O-8 ZnO, 0-13 K2O, Li2O, up to 11 P2O5, and 0-8 additional components. A content of up to 6.5 wt-% P205 was applied in the apatite glass-ceramic from the system Si02-Al203-K2O-Na20-CaO-F. The chemical composition can be characterized as: wt-% Si02, 5-22 Al203, P2O5, 3-9 K2O, 4-13 Na2O, CaO, and F. The lithium disilicate glass-ceramic was prepared as ingots by sintering and additionally hot-pressed at 920 Ž by viscous flow to produce the final glass-ceramic. The sintered apatite glass-ceramic was prepared as powder compacts in a temperature range of Ž. The microstructures of the glass-ceramics were analysed by scanning electron microscopy (SEM). The glass-ceramic samples were acid etched to present the microstructure in a Phosphorus Research Bulletin Vol. 10 (1999), 629

3 suitable fashion. The fracture surface of the material was examined with a scanning electron microscope (LEO, Type DSM 962, Germany). The properties of the glass-ceramics were determined on the basics of ISO dental standards 10. RESULTS AND DISCUSSION The lithium disilicate glass-ceramic shows a typical microstructure after sintering. The glass-ceramic ingot shows elongated crystals measuring 0,5-4 Đm in length (Figure 1). These crystals represent the main crystal phase Phosphorus Research Bulletin Vol. 10 (1999), 630

4 FIGURE 1. Microstructure of lithium disilicate glass-ceramic. SEM, after etching (4% HF, 30 % H2SO4, 10 sec). FIGURE 2. The microstructure of the sintered fluorapatite glassceramics. SEM, after etching (4% HF, 30 % H2SO4, 10 sec). The lithium disilicate glass-ceramic for the fabrication of crowns and bridges is coated with sintered apatite containing glass-ceramics. The microstructure of the sintered apatite containing glass-ceramic is shown in Figure 2. This sintered glass-ceramic is fired on the lithium disilicate glassceramic at a temperature of 800 Ž in the Progamat P80 furnace (Ivociar Ltd.). Figure 2 shows that a specific number of very finely dispersd apatite crystals has been precipitated in the glass matrix of the glass-ceramic. These crystals may enhance the biocompatibility of the glass-ceramic. Furthermore, they also allow the optical properties, such as translucency, brightness, and light scattering of the layering material to be controlled. As a result, the entire dental restoration can be made to resemble the natural tooth very closely. The analysis of the area in which the sintered and the pressed glass-ceramic of a three-unit bridge are joined reveals an interface layer (wash firing) measuring approximately 20 ƒêm. Phosphorus Research Bulletin Vol. 10 (1999), 631

5 The objective of increasing the strength of the new glass-ceramic compared with leucite glass-ceramic has been achieved. in fact, the results have surpassed expectations. The average bending strength of the tested lithium disilicate glass-ceramic after the pressing procedure was 350 } 50 MPa. At the same time, a very high fracture toughness was achieved in this material. expressed by K IC of 3.2 } 0.3 MPa Eme0.5. This material also demonstrates much higher translucency and is more favourable mechanical properties than Al2O3 sintered ceramics, since its modulus of elasticity of approximately 95 GPa considerably lower than that of other sintered ceramics. application of the materials. Another outstanding property of these sintered glass-ceramics is their chemical durability. After treatment with 4% acetic acid, the materials exhibited a very low loss in mass of less than 100 Đg/cm2. The abrasion behaviour of the sintered glass-ceramics is also excellent. The abrasion of the materials is similar to that of the pressed glass-ceramic. Phosphorus Research Bulletin Vol. 10 (1999), 632

6 Therefore, the materials have been shown to be particularly compatible with natural antagonist teeth. SUMMARY Because of their outstanding properties, we concluded to apply the glassceramics of the Empress(R) 2 systems as materials for metal-free three-unit bridges in dentistry. REFERENCES 1. S.D. Stookey, Ind. Eng. Chem., 51, 805 (1959). 2. M.P. Borom, A.M. Turalo, R.H. Doremus, J. Am. Ceram. Soc., 58, 385 (1975). 3. P. James, J. Non-Cryst. Sol., 73, 517 (1985). 4. P.W. McMillan, S.V. Philips, G. Partridge, J. Mat. Sci., 1, 269 (1966). 5. J.M. Barrett, D.E. Clark, L.L. Hench, US Pat (1980). 6. J.M. Wu, W.R. Cannon, C. Panzera, US Pat (1985). 7. J.Y. Thompson, K.J. Anusavice, B. Balasubramaniam, J. Am. Ceram. Soc., 78, 3045 (1995). 8. G.H. Beall, Ceram. Trans., 30, 189 (1993). 9. L.M. Echeverria, Bol. Soc. Esp. Ceram. VID, 5, 183 (1992). 10. ISO Dental Norm 6872 (1995). 11. W. Hoeland, Ivoclar Vivadent Report (1998). 12. F. Liebau, Acta. Cryst. 14, 389 (1961). 13. British Standard BS 5612 (1978). Phosphorus Research Bulletin Vol. 10 (1999), 633