Process for preparing a glass-ceramic body

Abstract

A process for preparing glass-ceramic body including the steps of providing a basic glass body and subjecting the basic glass body to a thermal treatment whereby a crystalline phase embedded in a glass matrix is formed. The basic glass body is made of a composition comprising 65 to 72 wt-% SiO.sub.2, at least 10.1 wt-% Li.sub.2O and at least 10.1 wt-% Al.sub.2O.sub.3 based on the total weight of the composition, the proportion of Li.sub.2O to Al.sub.2O.sub.3 being from 1:1 to 1.5:1. The thermal treatment involves a nucleation step followed by several crystallization steps at different temperatures, whereby at least two different crystalline phases are formed.

Claims

1. A process for preparing a glass-ceramic body comprising providing a basic glass body and subjecting the basic glass body to a thermal treatment, wherein: a crystalline phase embedded in a glass matrix is formed, the basic glass body is made of a composition comprising 65 to 72 wt-% SiO.sub.2, at least 10.1 wt-% Li.sub.2O, and at least 10.1 wt-% Al.sub.2O.sub.3 based on a total weight of the composition, the proportion of Li.sub.2O to Al.sub.2O.sub.3 being from 1:1 to 1.5:1, and the thermal treatment comprises a nucleation step that is carried out at a temperature in a range of from 500 C. to 570 C., followed by a first crystallization step at a first temperature range, and a second crystallization step at a second temperature range different from the first temperature range to form at least two different crystalline phases.

2. The process for preparing a glass-ceramic body according to claim 1, wherein a first region of the glass body is subjected to the first crystallization step and a second region of the glass body different from the first region is subjected to the second crystallization step such that a proportion of the first crystalline phase is higher in the first region than in the second region, and a proportion of the second crystalline phase is higher in the second region than in the first region.

3. The process according to claim 2, wherein the first region and the second region are heated to the first temperature range and the second temperature range, respectively, by means of laser irradiation, electromagnetic radiation, and/or susceptors.

4. The process according to claim 1, wherein the first temperature range is from 620 to 820 C., and the second temperature range starts from 825 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is further illustrated by way of the following examples in combination with the attached figures, of which

(2) FIG. 1 shows a graphical representation of the proportion of different phases (in volume-%) in a glass-ceramic material obtained by subjecting the glass composition according to the present invention to different temperature treatments; and

(3) FIG. 2 shows a purely schematic representation of a preferred glass-ceramic body according to the present invention to be subjected to a CAD/CAM process for preparing a dental restoration, as well as a holder for holding the body.

DETAILED DESCRIPTION

Examples

(4) The following experiments are based on the following (raw) glass composition:

(5) TABLE-US-00006 component amount (wt-%) SiO.sub.2 66.5 Li.sub.2O 10.5 Al.sub.2O.sub.3 10.5 K.sub.2O 0.45 Na.sub.2O 4.0 ZrO.sub.2 3.0 CeO.sub.2 1.5 V.sub.2O.sub.5 0.05 P.sub.2O.sub.5 3.5

(6) Differential Scanning Calorimetry (DSC) and Differential Thermal Analysis (DTA) of the composition has shown three peaks, one at about 655 C., one at about 812 C. and one at about 826 C., indicative of three crystallization steps.

(7) Based on these findings, a first sample of the glass-composition hasafter a nucleation step at 550 C. for three hoursbeen subjected to a crystallisation step at 660 C. for three hours (crystallisation step I). A second and a third sample were subjected to a crystallisation step at 815 C. for three hours (crystallisation step II) following crystallisation step I and a crystallisation step at 830 C. for three hours (crystallisation step III) following crystallisation step I.

(8) X-ray diffraction (XRD) analysis has revealed a formation of Li.sub.2SiO.sub.3 (lithium metasilicate) and lithium aluminosilicate (LAS) at crystallization step I, and a formation of Li.sub.2Si.sub.2O.sub.5 (lithium disilicate) and lithium aluminosilicate at crystallization step II and crystallization step III, with an increased content of lithium aluminosilicate (as spodumen) and a decreased content of lithium disilicate formed in crystallization step III in comparison to crystallization step II.

(9) The content of different phases in the final glass-ceramic in relation to different heat treatments has further been determined. In this regard, the raw glass composition hasafter a nucleation step at 550 C. for three hours and a first crystallization step at 660 C. for three hoursbeen subjected to a second crystallization step at a further temperature for three hours, specifically at a temperature of 760 C. (sample 1), 790 C. (sample 2), 820 C. (sample 3) and 850 C. (sample 4). The results are shown in FIG. 1.

(10) As shown in FIG. 1, the content of the different phases in the final glass-ceramic material is highly dependent on the temperature of the second crystallization step. For example, a decrease in the amorphous phase with an increase in the temperature of the second crystallization step has been detected. For the lithium disilicate phase, the highest content has been detected in samples 2 and 3, for which the second crystallization step has been at a temperature of 790 C. and 820 C., respectively. Lithium aluminosilicate is in sample 1 predominantly present as petalite and in sample 2 almost exclusively present as virgilite. In sample 3, it is present both as virgilite and spodumene, whereas in sample 4 it is exclusively present as spodumene.

(11) The results given in FIG. 1 both illustrate that several crystalline phases can be formed in one and the same glass-ceramic material and that the type of crystalline phase and its content can be controlled by adjusting the temperature treatment.

(12) It has been shown that different crystalline phases resulting in different mechanical and optical properties can be achieved in one and the same glass-ceramic body by applying a temperature gradient for the heat treatment. For example, a temperature gradient can be provided in a furnace in which the temperature gradually decreases with increasing distance from the heating source of the furnace (e.g. located in the middle of the furnace). By appropriately placing the respective body into the furnace, the temperature gradient is established in the material, leading to crystalline phases gradually changing from one region to another.

(13) Specifically, it has been shown that by subjecting the glass composition of the present example to a temperature gradient starting at about 550 C., opalescence starts to form at about 570 C. At about 620 C., a violet shade in reflectance light and a yellow shade in transmittance light can be detected, and at about 670 C. opalescence is marked. An opaque material is achieved starting at about 700 C.

(14) By means of the glass composition of the present example it could, thus, be shown that the invention not only allows for the formation of different crystalline phases in different regions of one and the same body, but also for a gradual change of the crystalline phases from one region to another.

(15) As schematically shown in FIG. 2, the glass-ceramic body 2 of the present invention comprises a first region 4 comprising a high proportion of a first crystalline phase and a second region 6 comprising a high proportion of a second crystalline phase. Depending on the local properties to be achieved in the final restorations 8, the portions to be removed are determined and the body is arranged correspondingly. A holder 10 safeguards that the body is kept in place during the computer-aided machining.

(16) Given the distribution of the crystalline phases, a final restoration can be achieved, the load bearing surfaces 12 having a higher toughness than e.g. the bulk area 14 of the body 2 to be removed. Thus, a dental restoration with high toughness in e.g. the pontics, the cusp supporting areas or the edges can be achieved in a relatively easy manner without undue wear of the machining tools.