WO2002074714A1 - Dense ceramic body, in particular for dental application - Google Patents

Abstract

A body containing more than 70 % of zirconium, prepared by sintering a powder based on zirconium previously formed such as to have a mechanical resistance greater than 800 MPa, a particle size between 0.3 and 5 mu m and a porosity less than 0.1 %.

Description

 The present invention relates to a dense ceramic body for dental applications, in particular for making crowns, bridges, studs, rings, implants or parts known as "inlay". "and" onlay "for dental restoration to replace amalgam fillings.

The use of ceramic materials is widely known today for dental applications. Patent application FR-A-2 594 029 proposes for example a dental prosthesis entirely made of ceramic materials and preferably of alumina. FR-A-2 781 366 claims a ceramic composition of zirconia stabilized with yttrium oxide and colored by the addition of certain oxides. These ceramic materials have an excellent biocompatibility which makes them advantageous compared to certain metals or amalgams conventionally used in the dental field. Zirconia has the additional advantage of having mechanical properties far superior to those of alumina. The article "Relative flexural strengh of 6 new ceramic materials", Int J Prosthodont 1995; 8: 239-246, offers a classification of different ceramic materials according to their mechanical strength. The alumina-magnesia spinel has a mechanical resistance of 380 MPa while that of alumina is 450 MPa and that of zirconia is approximately 600 MPa, Nevertheless, it is always interesting to further improve the mechanical resistance of the materials used . In fact, higher mechanical strength makes finer and more complex geometries accessible, which is particularly useful for crowns and bridges. This also makes it possible to reduce the risk of embrittlement which is important for the studs intended to hold the crown as well as for the rings which are thin.

According to certain patents such as US 4,758,541 or EP 297,908, a translucent or even transparent zirconia can be obtained. However, this is achieved through the addition of titanium oxide. However, it is also shown that such an addition leads to an increase in the size of the grains, beyond 20 μm and up to 200 μm, as well as a significant reduction in mechanical strength. Furthermore, certain interesting results have already been obtained by carrying out a post-treatment of HIP (Hot Isostatic Pressing) under an oxidizing atmosphere (presence of oxygen). However, this solution is not industrial since the elements of the furnace must then be designed in a particular material (platinum for example) and since the only ovens available are very expensive prototypes. On the other hand, a HIP treatment under oxygen is considered to be dangerous and this solution cannot be used for industrial manufacturing. Furthermore, if the addition of coloring oxides makes it possible to simulate the color of the dental enamel of the other teeth of the user as well as possible, the zircones currently available do not have the same translucency as the enamel of natural teeth which is not satisfactory from an aesthetic point of view. There are many materials such as glass-ceramics (porous ceramic impregnated with a glass) proposed for example in FR-A-2 682 297 which have a satisfactory translucency; however, they do not have sufficient mechanical strength.

There is therefore a need for a ceramic body intended for dental applications having a high mechanical resistance and a transiucidity close to that of natural dental enamel.

The present invention aims to satisfy this need.

More particularly, the invention relates to a dense zirconia ceramic body containing more than 70% of zirconia, remarkable in that it is prepared by sintering a powder based on zirconia previously shaped, so as to have a resistance mechanical greater than 800 MPa, a grain size between 0.3 and 5 μm, and a porosity less than 0.1%. Preferably, this ceramic body has a mechanical resistance greater than 1100 MPa.

More preferably, this ceramic body is made up of more than 80% by mass of zirconia and has a porosity advantageously less than 2%. The invention also provides a process for the preparation of this ceramic body, according to which a volume of a zirconia-based powder is shaped and the part obtained is sintered in air at a temperature between 1200 and 1700 ° C. then, after cooling, a thermal post-treatment is carried out under argon, at a temperature between 1200 and 1700 ° C. and under a pressure of at least 900 bars approximately.

The following description, made in conjunction with the accompanying drawings, will allow a better understanding and appreciation of the advantages of the invention.

In the accompanying drawings:

- Figure 1 shows the graphs of the light intensity transmitted, in visible light, by various samples of ceramic bodies according to the invention, and Figure 2 shows the graphs of Figure 1, extended to the wavelength range included between 0.5 and 10 μm.

By seeking to improve the mechanical resistance of the zirconia parts, we have made tests with a thermal post-treatment process under argon. This type of process is not currently used by dentists since it requires a very specific oven. Surprisingly, we managed to increase the mechanical resistance but we also significantly improved the translucency of the parts.

We measured the mechanical resistance by a bending rupture test according to ISO 6872 standard.

The grain size was measured by the so-called "Mean Linear Intercept" method described in standard ASTM Vol. 03.01, Section 10, Test Methods El 12, based on the analysis of images obtained by scanning microscopy.

We evaluated translucency first qualitatively (visual aspect) as dentists do but also quantitatively by measuring the transmitted light on the spectrum of wavelengths in the visible range. For this, we used a photometric transmission measurement device and tested a sample 14mm side and 0.3mm thick, arranged perpendicular to the axis of a beam of light emitted by a light source forming part of the 'apparatus. The transmitted light was measured using a photosensitive sensor also placed on the axis of this beam.

The following examples illustrate the invention, using a comparative example (Example 1).

Example 1 (outside the invention). The sample was prepared from a substantially pure zirconia powder containing 3% by mole of yttrium oxide as well as pressing binders. The mixing powder was homogenized in a jar and ground in ethyl alcohol then dried and sieved. The dried powder is then pressed in the form of a pellet and then sintered in air at 1450 ° C. The sample thus obtained was then characterized. It has a mechanical resistance of 1200 MPa. The visual appearance of the sample is not similar to that of the enamel of a natural tooth; it does not have the same translucency, in particular on the outer contour where the thickness is less. In FIG. 1, it also appears that the percentage of light transmitted is lower than the detection limit of 0.2%, in visible light. The porosity of this sample is 2%. The grain size is around

0.5 μm.

Example 2.

The sample is prepared in the same way as for Example 1. It is then post-heat treated under argon at 1450 ° C and under a pressure of 2000 bars. At the end of this treatment, the sample has a grayish hue indicating a certain reduction. To find the desired shade, the sample is annealed at 1100 ° C. The sample thus obtained was then characterized. It has a mechanical strength of 1850 MPa. The sample has a visual appearance very close to that of a natural tooth. Figure 1 shows that the percentage of light intensity transmitted, or light transmission, increases regularly from about 0.5% to 2.5% in the visible light range.

The sample has a porosity of 0.004%. The grain size is of the order of 0.8 μm.

Example 3.

The sample was prepared with the same powder as in Example 1 but was shaped by injection. The sintering took place in air at 1600 ° C. Then the part obtained was post-heat treated under argon at 1450 ° C and under a pressure of 2000 bars and then annealed at 1100 ° C. The sample thus obtained has a mechanical strength of 1700 MPa. Its visual appearance very close to that of a natural tooth. Figure 1 shows that the light transmission of the sample is, on average, greater than 0.5% in the visible range.

The porosity of the sample is 0.05%. The grain size is 0.8 μm. Example 4.

The sample was prepared in the same manner as for Example 3 by adding 1% of cerium oxide, 0.02% of iron oxide and 0.2% of erbium oxide to the initial mixture. The sample thus obtained has a mechanical strength of 935 MPa. The sample has a visual appearance very close to that of a natural tooth. In FIG. 1, it appears that the light transmission of the sample is substantially the same as that of the sample of example 3.

The porosity of the sample is 0.04%. The grain size is of the order of 0.8 μm.

The temperature chosen for the sintering step must be greater than 1200 ° C to obtain sufficient sintering but must not be greater than 1700 ° C so as not to lead to excessive granular growth. Indeed, it is known to those skilled in the art that high sintering temperatures lead to an increase in the size of the grains and that large grain sizes are symptomatic of a reduction in mechanical strength. The grain size should therefore not be too large and should remain below

5 μm but it should not be too small either because the light is dispersed in a privileged way in the grain boundaries. It is believed that it is necessary to have a grain size greater than 0.3 μm.

In addition, the pores are also privileged places of dispersion of the light. It is therefore necessary to maintain the porosity at a level of less than 0.1%.

For this, thermal post-treatment under argon, at a temperature between

1200 and 1700 ° C and at a minimum pressure of 900 bars is necessary. The temperature of this post-treatment is such that it does not significantly modify the grain size.

It is interesting to note that, when the sample is shaped by injection, or by casting, it is not necessary to carry out a machining of the part before its use. This is a very important advantage in terms of cost because the parts generally require complex and costly machining before they can be implanted.

Furthermore, the oxides added to the initial mixture gave the sample a dark cream coloration which corresponds well to the coloration expected for such oxides.

The addition of oxides in a larger amount, up to 5% by weight, can be envisaged to constitute the coloring charge.

Surprisingly, ceramic bodies usable in the dental field are thus obtained which have both a mechanical resistance at least as high as the zirconia products currently used and a translucency close to that of natural dental enamel.

Comparison of the various curves in FIG. 1 shows that light is not transmitted in the same way for a conventional product (example 1) and for the examples of the invention. In the visible range (from 0.5 to 0.7 μm) the transmission remains weak but significant and corresponds to a real difference in appearance. Furthermore, it can be seen that in the infrared domain (see FIG. 2), the differences between the examples according to the invention and that of the reference (example 1) are much clearer. Indeed for example 2, the average transmission is greater, or much greater, than 10%, in the range of wavelengths between 1 and 8 μm. In fact, it is believed that analysis in this wavelength range is useful for effectively differentiating materials.

Without wishing to link the invention to any theory, we believe that any process making it possible to reproduce redox conditions, of pressure and of temperature close to those of Examples 2 to 4 could make it possible to obtain the products of the invention. Indeed, we think that these conditions favor a reorganization of the phases and a modification of the morphology of the grain boundaries which allows to obtain a translucent ceramic body. Furthermore, the comparison of examples 3 and 4 shows that the addition of coloring oxides plays the same role as in the products of the prior art and does not affect the advantages provided by the products of the invention.

It is noted that the products according to the invention have a particularly high mechanical resistance and very much greater than the products currently known for having a translucency close to that of natural dental enamel. These good mechanical properties are particularly useful for the production of crowns and bridges since the ceramic body can thus be worked in a larger and more precise manner without risk of breakage or cracking. This property is also specially sought for applications such as the studs on which the crowns are fixed and whose main characteristic must be the solidity.

Of course, the invention is not limited to the embodiments described, which have been given only by way of example. Thus the pressing of powder could be replaced by another means of shaping such as, for example injection or casting. Likewise, the process for manufacturing the ceramic body according to the invention is not limited by the temperature and pressure values given in the examples for the sintering step. The invention extends, on the contrary, to manufacturing processes in which the sintering takes place at a temperature of at least 1200 ° and under a pressure of at least 900 bars, the possible subsequent annealing operation is carried out at a temperature at least equal to 700 ° C.

Claims

1. Ceramic body of dense zirconia containing more than 70% of zirconia, characterized in that it is prepared by sintering a powder based on zirconia previously shaped, so as to have a mechanical resistance greater than 800 MPa, a grain size between 0.3 and 5 μm, and a porosity of less than 0.1%. 2. Ceramic body according to claim 1, characterized in that the zirconia is stabilized with yttrium oxide. 3. Ceramic body according to claim 1, characterized in that it has a mechanical strength greater than 1100 MPa. 4. Ceramic body according to one of claims 1 to 3, characterized in that it has an average light transmission greater than 0.5% in visible light, measured on a sample of 0.3 mm thick. 5. Ceramic body according to claim 4, characterized in that said average light transmission is greater than 10% in the range of wavelengths between 1 and 8 μm. 6. Ceramic body according to one of claims 1 to 3, characterized in that it contains from 0 to 5% by weight of oxides constituting a coloring filler. 7. Use of a ceramic body according to one of claims 1 to 6, for the production of dental crowns or bridges. 8. Use of a ceramic body according to one of claims 1 to 6, for the realization of "inlay" or "onlay" for dental restoration to replace amalgams. 9. Use of a ceramic body according to one of claims 1 to 6, for the production of pins or dental implants. 10. Use of a ceramic body according to one of claims 1 to 6, for the production of dental rings. 11. A method of preparing the ceramic body according to any one of claims 1 to 6, characterized in that a volume of a powder based on zirconia is shaped and the part obtained is sintered in air at a temperature included between 1200 and 1700 ° C then, after cooling, a thermal post-treatment is carried out under argon, at a temperature between 1200 and 1700 ° C and under a pressure of at least about 900 bars. 12. Method according to claim 11, characterized in that one then proceeds to an annealing, at a temperature of at least 700 ° C. 13. Method according to any one of claims 11 and 12, characterized in that said shaping takes place by injection. 14. Method according to any one of claims 11 and 12, characterized in that said shaping takes place by casting. 15. Use of a ceramic body obtained by the method according to any one of claims 13 and 14, characterized in that the body obtained is used directly, without prior machining.