GLASS CERAMIC COMPRISING HIGH PROPORTIONS OF ZIRCONIUM OXIDE AND METHOD OF PRODUCING SAID GLASS CERAMIC

The invention relates to glass ceramics, which show high strength, high translucency, high chemical stability and which are still mechanically processible.

The invention further refers to a method for producing a dental restoration comprising such glass or glass ceramic as well as the dental restoration itself.

In the lithium oxide-silicon dioxide system, lithium disilicate (Li₂0 ' 2 Si0₂ <Li2Si₂0₅))-glass ceramics are well known from the literature and several patents are based on this glass ceramic system. In EP 0 536 479 Bl, self-glazed lithium disilicate glass ceramic objects are thus described for the production of tableware and, in EP 0 536 572 Bl, lithium disilicate glass ceramics which can be used by scattering a fine-particle coloured glass onto the surface thereof as cladding elements for building purposes.

A main focus of the publications about lithium disilicate glass ceramics resides in dental applications. The lithium disilicate system is very suitable here for the production of CAD/CAM-processible glass ceramics since the crystallisation is effected here via the lithium metasilicate phase (see S. D.

Stookey: "Chemical Machining of Photosensitive Glass", Ind. Eng. Chem., 45, 115 - 118 (1993) and S. D. Stookey: "Photosensitively Opacifiable Glass" US-A-2 684 911 (1954)).

These lithium metasilicate glass ceramics have such low strengths in this intermediate stage that they can be readily processed by means of CAD/CAM (M.-P.

Borom, A. M. Turkalo, R. H. Doremus: "Strength and Microstructure in Lithium Disilicate Glass Ceramics", J. Am. Ceram. Soc., 58, No. 9 - 10, 385 - 391 (1975) and DE 24 51 121 Al.

Only by the subseguent conversion to lithium disilicate or the growing of lithium metasilicate crystals, in a second heat treatment, dental materials with high strength are achieved.

The heat treatment which is carried out in a dental laboratory or in the dental practice is a burden for a technician as well as for the patient with respect to time and costs. In particular, during the chairside method, inconvenient waiting times can occur.

In this method, an individually adapted crown/onlay/inlay is milled out of a glass ceramic block after the first crystallisation stage by means of CAD/CAM, in the dental practice this is subjected to the second crystallisation stage in a special oven and used directly in the first and only dentist's visit for the patient (DE 10 2005 028 637).

Such a heat treatment requires an oven and the corresponding acquisition and maintenance costs. Moreover, such a heat treatment can be the source for defects of the final product. Another drawback is the required time for such treatment which is between 30 and 60 minutes. For common CAD/CAM-systems, a maximum strength of 170 MPa is the limit. Thus, the machineable materials cannot be directly used for high quality applications. The machineability is not only dependent on the strength of the material, but also dependent on further properties, such as the hardness, the modulus of elasticity, the fracture toughness as well as the structure and microstructure of the glass ceramic. It has to be differentiated between intercrystalline and trans-crystalline fraction formes.

Starting herefrom, it was the object of the present invention to provide glass ceramics which have improved strength values and also improved translucence and chemical resistance.

This object is achieved by the method for producing a dental restoration having the features of claim 1, the translucent and tooth coloured glass ceramic having the features of claim 11 and the dental restoration of claim 14. The further dependent claims reveal advantageous developments.

Within the scope of the present invention, glass compositions were developed in the basic system SiC>2-Li2θ-Zrθ2, which have lithium metasilicate as only or as main crystal phase {> 50%).

It was surprisingly found that the use of specific lithium metasilicate compositions together with specific heat treatments for its crystallisation can result in finally crystallized glass ceramics with a high strength which can be machined with CAD/CAMtechnigues.

It was shown in addition that up to 20% by weight of Zrθ2 can be incorporated in the glass without the structure being significantly influenced. Contrary to all expectations, the ZrC>2 does not hereby crystallise as a separate crystal phase but remains completely or extensively in the amorphous residual glass phase. Because of the high proportion of Zr0₂, the mechanical and chemical resistances are hugely improved in this amorphous phase, which also leads to improved properties in the entire dental glass ceramic {crystal phase(s) and residual glass phase), such as for example final strength and acid solubility.

The method is also suitable for a two-stage production process from the initial glass, a partial crystallisation of the lithium metasilicate being effected in the first processing stage, which enables good CAD/CAM processing. In the second processing stage, an increase in the crystal phase proportion (primary lithium metasilicate) is effected, which leads to the high strength values. The most important cause of the surprisingly high strengths in the lithium metasilicate system is hereby ascribed to the high zirconium oxide proportion (> 8 MA).

High translucence is ensured via the low crystallite size in the glass ceramics. In addition, good chemical stability is ensured by the high zirconium oxide proportion in the glass phase.

According to the present invention, a method for producing a dental restoration comprising a lithium silicate glass ceramic is provided having the following steps:

a) an amorphous glass is subjected to at least one heat treatment with temperatures from 450 to 1100°C resulting in a translucent and tooth coloured glass ceramic with a strength of at least 250 MPa (measured according to DIN ISO 6872) and with the colour of tooth, wherein during the at least one heat treatment at least a partial crystallisation occurs due to the increased temperatures, and

b) the glass ceramic is formed to a dental restoration for immediate dental application and with a strength of at least 200 MPa (measured according to DIN ISO 6872) by using a material removing process.

In the context of the present invention a translucent glass ceramic is a ceramic which has a transmission of light with a wavelength between 360 nm to 740 nm (measured according to DIN EN 410 with a spectrophotometer Minolta CM-3610d).

The tooth colour is determined in accordance with existing dental shade guides, e.g. Vita classical shade guide, Vita 3D master shade guide).

The stabilizer is preferably ZrO^ and/or Hfθ2- Preferably, the stabiliser is essentially present in an amorphous state.

There may be contained as additives components selected from the group consisting of nucléation agents, fluorescent agents, dyes, in particular glass-colouring oxides, coloured pigments and mixtures thereof, in the glass or in the glass ceramic.

The nucleating agents are preferably selected from the group consisting of phosphorous oxide, titanium oxide, tin oxide, mixtures thereof, and noble metals, preferably in an amount of 1 to 10 wt-%, more preferably 2 to 8 wt-% and most preferably 4 to 8 wt-%.

The fluorescent agents are preferably selected from the group consisting of oxides of bismuth, rare earth elements as neodymium, praseodymium, samarium, erbium, and europium, and mixtures thereof, preferably in an amount of 0,1 to 5 wt-%, more preferably 0,5 to 4 wt-% and most preferably 1 to 3 wt-%.

The glass colouring oxides are preferably selected from the group of oxides of iron, titanium, cerium, copper, chromium, cobalt, nickel, manganese, selenium, silver, indium, gold, vanadium, rare earth elements as neodymium, praseodymium, samarium, europium, terbium, dysprosium, holmium, erbium, yttrium, and mixtures thereof, preferably in an amount of 0,1 to 6 wt-%, more preferably 0,5 to 5 wt-% and most preferably 1 to 4 wt-%.

The coloured pigments can be doped spinels, which are comprised preferably in an amount of 0,1 to 6 wt-%, more preferably 0,5 to 5 wt-% and most preferably 1 to 4 wt-%.

Further additives are preferably selected from the group consisting of boron oxide, phosphorus oxide, fluorine, barium oxide, strontium

oxide, magnesium oxide, zinc oxide, calcium oxide, yttrium oxide, titanium oxide, niobium oxide, tantalum oxide, lanthanum oxide and mixtures thereof, which are comprised preferably in an amount of 0,1 to 5 wt-℅.

In a preferred embodiment, the amorphous glass has the following composition:

55 to 70 wt-℅ Siθ2,

10 to 25 wt-℅ Li2θ,

8to 20 wt-℅ of a stabiliser selected from the group consisting of the oxides of Zr, Hf, Ge, La, Y, Ce, Ti, Zn or its mixtures,

0 to 10 wt-% AI2O3,

0 to 10 wt-% K2O and/or Na2θ, and

0 to 20 wt-℅ additives.

In a further preferred embodiment, the amorphous glass has the following composition:

55 to 70 wt-℅ SiC>2,

10 to 25 wt-% Li2θ,

8 to 20 wt-℅ of a stabiliser from a group selected of ZrC>2, HfO2 or its mixtures,

0 to 10 wt-℅ AI2O3,

0 to 10 wt-% K₂0 and/or Na₂0, and

0 to 20 wt-℅ additives.

In a further preferred embodiment, the amorphous glass has the following composition:

55 to 64 wt-% Siθ2,

15 to 22 wt-% LIςO,

8 to 20 wt-% of a stabiliser from a group selected of ZrOϋ, Hfθ2 or its mixtures,

0,1 to 8 wt-% AI2O3,

0 to 8 wt-% K2O and/or Na?0, and

0 to 15 wt-% additives.

In a further preferred embodiment, the amorphous glass has the following composition:

55 to 64 wt-% SiC>2,

17 to 20 wt-% Li₂0,

8 to 20 wt-% of a stabiliser from a group selected of Zrθ2, HfC>2 or its mixtures,

0,1 to 5 wt-% Al₂θ3,

0,1 to 5 wt-% K2O and/or Na2θ,

2 to 8 wt-% P2θb,and

0 to 10 wt-% additives.

In a further preferred embodiment, the heat treatment is a single-stage treatment with a temperature from 600^βC to 950°C, preferably 780 to 900°C. It is another preferred embodiment that the heat treatment is a double-stage treatment with a first temperature from 600 to 800°C and a second temperature from 780 to 900*0.

The lithium silicate glasses or glass ceramics according to the invention are used as dental material or as component of a dental material.

The material removing process is a subtractive process, preferably selected from the group consisting of milling, grinding, and laser ablation, preferably as a CAM process.

In a further preferred embodiment, the dental restoration is subjected to a finishing process before the dental application. Such a finishing process can be a polishing, a glazing, a sealing, a coating, and a veneering with a veneering ceramic or glaze.

The dental restoration is preferably an inlay, an onlay, a bridge, an abutment, a facing, a veneer, a facet, a crown, a partial crown, a framework or a coping.

According to the present invention, also a translucent and tooth coloured glass ceramic with a strength of at least 250 MPa (measured according to DIN ISO 6872) having the following composition is provided:

55 to 70 wt-℅ Si0₂,

10to 25 wt-℅ Li₂0,

8 to 20 wt-℅ of a stabiliser from a group selected of Zr, Hf, Ge, La, Y, Ce, Ti, Zn or its mixtures,

0 to 10 wt-℅ A1₂0₂,

0 to 10 wt-℅ K₂0 and/or Na₂0, and

0 to 20 wt-℅ additives.

In a preferred embodiment, the glass ceramic has the following composition:

55 to 70 wt-℅ Si0₂,

10 to 25 wt-℅ Li₂0,

8 to 20 wt-℅ of a stabiliser from a group selected of Zr0₂, Hf0₂ or its mixtures,

0 to 10 wt-℅ AI2O3,

0 to 10 wt-℅ K₂0 and/or Na₂0, and

0 to 20 wt-% additives.

Preferably, the glass ceramic has the following composition :

55 to 64 wt-% Si0₂,

15 to 22 wt-% Li2θ,

8 to 20 wt-% of a stabiliser from a group selected of Zrθ2, HfC>2 or its mixtures,

0,1 to 8 wt-% AI2O3,

0 to 8 wt-% K₂0 and/or Na2θ, and

0 to 15 wt-% additives.

In a further preferred embodiment, the glass ceramic has the following composition:

55 to 64 wt-% Si0₂,

17 to 20 wt-% Li₂0,

8 to 20 wt-℅ of a stabiliser from a group selected of Zr0₂, Hf0₂ or its mixtures,

0,1 to 5 wt-% AI2O3,

0,1 to 5 wt-% K₂0 and/or Na₂0,

2 to 8 wt-% P₂0₅,and

0 to 10 wt-% additives.

The glass ceramic has preferably a dimensional stability which allows the machining of the glass ceramic with a material removing process.

According to the present invention, furthermore, a dental restoration is provided which is producible by the above described method.

It is preferred that the dental restoration has a degree of crystallization of at least 5 %, preferably at least 50 %.

It is further preferred that the dental restoration has aof at least 200 MPa,at least 300

The dentalcan have aSuch a finishingpreferably a polishing, aa sealing, aand a veneeringa veneering ceramic orSuch a finished dental restoration hasa strength of at leastMPa, preferably ofleast 300 MPa.

The dentalwith thecompositions areaspects of the present invention:

2 6

Composition 3 7

Composition 4 Composition 8

21

Composition 22

23

Theaccording to the application is intended to bein more detail with reference to the subsequent examples without restricting said subject to these

In Table 1, a fixed compositions given by way of example forstabilizer is mentioned, from which high stabilizer-containing metasilicate glass ceramics can be produced for the dental field.

SX represent compositions of stabilizers SI to S5 (s. table 2)

Table 2 shows stabilizers used by way of example for dental applications with the composition of table 1.

The glasses were melted at l,500^βC and poured into metal moulds to form blocks. The blocks were stressrelieved in the oven at 560°C and cooled down slowly. For the various characterisation processes, the glass blocks were divided up and subjected to a first crystallisation treatment. For this purpose, the glasses were stored for 10 to 120 minutes at 600°C to 750°C.

As a result of this, glass ceramics with strength values of 150 MPa to 220 MPa were produced. Exclusively lithium metasilicate was hereby established as crystal phase. In this state, processing by means of CAD/CAM methods is possible very readily.

In Table 3, compositions which are given by way of example are mentioned, from which high zirconium ox-ide-containing metasilicate glass ceramics can be produced for the dental field.

(Data in % by weight)

The glasses were melted at 1,500°C and poured into metal moulds to form blocks. The blocks were stressrelieved in the oven at 560°C and cooled down slowly.

For the various characterisation processes, the glass blocks were divided up and subjected to a first crystallisation treatment. For this purpose, the glasses were stored for 10 to 120 minutes at 600°C to 750°C.

As a result of this, glass ceramics with strength values of 150 MPa to 220 MPa were produced. Exclusively lithium metasilicate was hereby established as crystal phase. In this state, processing by means of CAD/CAM methods is possible very readily.

A glass melt with a composition of 60wt℅ Si0₂, 19wt℅ LiC>2, 10wt% ZrC>2, 6wt% P2O5, 2wt% AI2O3, 2wt% K2O and 2wt% C℮0₂ is cast into a block form. This block is completely crystallized by a two-step firing process. The heat treatment is carried out at 620^βC and 850°C. After this procedure a block holder (e.g. metal attachment) is glued to the block to fix it in a CAM machine.

For this application a dental milling machine (Sirona inLab MCXL) is used. For a first test the preinstalled parameters for presintered IPS e.max CAD (Software version 3.85) were chosen. A designed anterior crown was milled by using the typical diamond tools. The expected milling time was 17 minutes; the real milling time took 28 min. The chosen burs and the resulting crown showed no problems.

In a second test an identical crown was milled in the same machine by selecting the preinstalled parameters for VITA In-Ceram Spinell. The milling process took also ca. 10 minutes longer than the time calculated. The crown and grinders showed no defects.

After milling the surface of the crown can be optimized by hand. A typical procedure for a dental technician or dentist can be e.g. polishing, glazing, staining and veneering. Bend-bars machined after final crystallizing and treated with a glaze showed fracture values of 370 MPa (3-point-bending strength test conform to DIN EN ISO 6872)

The commercially available product IPS e.max CAD (Ivoclar- Vivadent, color LT A2) was tested in a comparative way. For this reason the blocks were additionally heat treated at 850°C. The holder, which was removed before final heat treatment, was attached again by gluing.

Then the same crown design was loaded again and the parameters for (normally only partially crystallized) IPS e.max CAD were selected. The complete milling process took ca. 90 minutes instead of the calculated 17 minutes. The process had to be restarted four times because four diamond grinders broke during the process. This shows that it is not possible to machine finally crystallized IPS e.max CAD crowns in a commercially successful way.