CH627723A5 - Process for coating the surface of a ceramic with a biologically active glass, product obtained by using this process and use of this product - Google Patents

Description

The present invention relates to a process for coating the surface of a ceramic based on A1203 with a biologically active glass, the product obtained by the implementation of this process, as well as to a use of this product.

The mechanical resistance of Al203-based ceramics and their 65 anti-friction and fatigue resistance properties make them ideal for the production of artificial prostheses and orthopedic instruments. However, the biological inactivity of ceramic surfaces based on A1203 makes it very difficult, if not impossible,

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cementless implantation of these prostheses because the bone tissue does not adhere to them or cannot develop there *

Various techniques have recently been proposed to make ceramic surfaces active, in order to improve the possibilities of adhesion of bone tissues on A1203 prostheses. However, all these techniques are either very expensive, or very time-consuming to implement, or give ceramic structures having properties of mechanical strength, antifriction and resistance to fatigue reduced.

The object of the present invention is to provide an internal prosthesis to replace a bone and not requiring the use of cement comprising a ceramic based on Al 2 O 3 and biologically active, and a process for the preparation thereof which is inexpensive and which does not cause a reduction in the mechanical resistance, anti-friction and fatigue resistance properties of the Al 2 O 3 ceramic.

The present invention therefore has as its first object a method of coating the surface of a compact ceramic with a biologically active glass having a coefficient of thermal expansion different from that of ceramic, said method comprising the following steps:

The glass is brought into contact with the surface of the ceramic at a temperature and for a time sufficient to weld the glass to the surface of the ceramic by ion diffusion,

The coated substrate is cooled to a temperature sufficient to produce interconnected microcracks in the vitreous coating as a consequence of the thermomechanical stresses induced by the difference in the coefficients of thermal expansion of said ceramic and said glass, and

- The vitreous coating comprising the microcracks is covered with at least one additional layer of a biologically active glass.

A second object of the invention consists of the product obtained by implementing the method described above, and which comprises a compact ceramic based on Al 2 O 3, the surface of which is coated with at least two layers of a glass. biologically active with a coefficient of thermal expansion different from that of Al203-based ceramic.

Finally, a third object consists in the use of the above product for the production of an internal prosthesis capable of replacing at least part of a bone and not requiring the use of a cement.

The invention will be illustrated below with reference to particular embodiments given by way of examples. The description also refers to the appended figures, in which:

- in fig. 1, a diagram of the times for firing the ceramic, t representing the times in hours and T the temperatures in degrees centigrade,

- in fig. 2, curves giving the relative amounts (R) of aluminum and sodium as a function of the depth (d) in the ceramic coated with a single layer,

- in fig. 3, curves identical to those of FIG. 2, but in the case where the ceramic is coated with two layers,

- in fig. 4, curves illustrating the breaking stress (Rr) as a function of the speed V of application of the force, and

- in fig. 5a to 5c, drawings illustrating the different stages of formation of the coated ceramic.

It is well known that when a glassy coating having a high coefficient of thermal expansion is produced on a substrate having a low coefficient of thermal expansion, stresses arise during cooling. As these thermal stresses generate a weakening of the entire coated structure, it is common, according to the prior art, to try to make the thermal expansion coefficients of the respective materials as compatible as possible in order to minimize these stresses. This necessarily results in a considerable reduction in the number and variety of coatings which can be applied to a particular substrate.

We rely on extreme dissimilarities between the respective thermal expansion coefficients to induce thermo-mechanical stresses in the biologically active vitreous coating. Under the effect of cooling, the vitreous layer cracks to release the stresses due to thermal dissimilarities. This results in isolated batches of the biologically active vitreous coating separated from each other by slits or microcracks. These cracks have widths which range from approximately 0.05 to 0.8 u. The small islets of biologically active glass are welded to the surface of the compact ceramic based on A1203 by a significant diffusion welding which develops by operating at high temperatures (1100 to 1350 ° C). Diffusion welding is a chemical welding between the Al203-based substrate and the biologically active vitreous coating. This removes a well-defined interface between Al203 and biologically active glass. This results in an improvement in the general mechanical strength characteristics of the ceramic.

Multiple coatings of biologically active glasses can then be applied to the vitreous layer comprising the microcracks, without the risk of introducing thermo-mechanical stresses into the structure. This is due to the fact that the second and subsequent layers are welded to the first biologically active vitreous layer and not to the Al 2 O 3 substrate. Thus, the second glassy layer has physical properties identical to those of the first glassy layer without dissimilarities between the respective thermal expansion coefficients.

The resulting structure has the ability to make, adhere living tissue to the material that constitutes the implant comprising a coated ceramic substrate due to the properties of biologically active glass. In addition, the coating process does not harm the mechanical resistance of the Al203-based ceramic since all the thermo-mechanical stresses are released during the operation for producing the first coating layer and no other stress is required. 'is induced neither by the second step of coating with a glass, nor by the following.

As there is no need to take into account the accounting of coefficients of thermal expansion, a greater variety of vitreous biologically active materials can only be used as a coating on the surface of ceramic than in the case of the techniques which currently prevail according to prior art.

Indeed, by carefully controlling the coating process, the mechanical resistance of compact Al203-based ceramic can really be improved. By controlling the size of the cracks or microcracks below 1 djl, the mechanical resistance and the fatigue resistance of Al203-based ceramics are increased.

Any biologically active glass can be used for the purposes of the invention. Those skilled in the art will understand that any suitable biologically active glass, depending on the end use for which the coated ceramic is intended, can be used. In general, a biologically active glass is a glass capable of adhering to living tissue and contains in composition by weight:

Si02 40-62%

Na20 10-32%.

CaO 10-32% P2Os 3-9%

CaF2 0-18%

B203 0-7.5%

NazO + CaO must represent at least 30% to produce adhesion with living tissue.

Particular suitable glasses include those which have the following compositions:

A. Si02 45.0%

Na20 24.5%

CaO 24.5%

P205 6.0%

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B. Si02

42.94%

NazO

23.37%

CaO

11.69%

p2o5

5.72%

CaF2

16.26%

C. Si02

40.0%

Na20

24.5%

CaO

24.5%

p2o5

6.0%

b2o3

5.0%

To obtain a vitreous coating with microcracks, it is generally preferable to use compact ceramics based on Al203 having a coefficient of thermal expansion, for the temperature range from 0 to 1000 ° C, between 35 x 10-7 and 75 x 10-7 / ° C and a biologically active glass having a coefficient of thermal expansion, for the temperature range from 0 to 1000 ° C, between 95 x 10 ~ 7 and 145 x 10 ~ 7 / ° C.

The biologically active glasses are first of all melted (for example in platinum crucibles) for 3 to 12 hours to obtain homogeneity. The range of melting temperatures is from 1300 to 1500 ° C. After melting, the biologically active glass is soaked in water and ground in a ball mill to obtain a glass powder formed from particles having the desired dimensions. In general, it is best to have particles smaller than about 74 percent in size. The powder is then mixed with an organic binder (for example organic polymers such as a mixture of 20% polyvinyl acetate and 80% polyvinyl alcohol) and with a suitable organic solvent (for example toluene, acetone, xylene, etc.) to form a slurry. The amount of binder used depends on the particle size of the powder used. In general, larger particles require larger amounts of binder to obtain an adequate covering effect. The amount of solvent can be varied to control the viscosity of the mud and the thickness of the final coating.

In general, the mud contains approximately 35 to 80% of glass powder, approximately 1 to 10% of binder and approximately 20 to 65% of organic solvent, all these percentages being by weight.

The ceramic substrate based on Al 2 O 3, which must be coated, is then immersed in the mud or the mud is deposited with a brush, or else sprayed onto the substrate. The coating layer is allowed to dry perfectly.

The substrate provided with the coating layer is then baked following a schedule allowing the elimination of the organic binder, followed by a softening of the glass and the resulting welding of the glass with the substrate by ion diffusion. The high proportion of alkalis in biologically active glass is one of the main factors which allow good diffusion welding between the coating layer and the substrate. The coated glass is then annealed to release the mechanical stresses.

If the composition A of biologically active glass described above and a compact ceramic based on Al 2 O 3 having a coefficient of thermal expansion of 50 to 75 × 10 -7 / ° C. are used, the firing program shown in the figure is implemented. fig. 1. Although fig. 1 corresponds to a structure with two layers of coating, the person skilled in the art readily understands that successive layers of coating of biologically active glass can be applied by following the same baking program, according to the surface properties desired for the system coated as a result. The final coating layer of biologically active glass on the surface of the compact Al203 ceramic contains 0.475 mol of NaaO, 0.525 mol of CaO, 0.050 mol of P2Os and 0.900 mol of Si02 (normal composition with regard to alkalis ).

The combination: 1) of the composition of the glasses (strong in alkalis, weak in silica) which allows a relatively high diffusion rate, and 2) different cooking programs (time and temperatures) which make it possible to fix the importance of the diffusion allows the formation of microcracks in the base layer of biologically active glass coating to be controlled. The control of these two variables also makes it possible to adjust the diffusion welding which is ultimately responsible for the success of the coating process.

The temperature at which the system provided with its coating is capable of producing an ion diffusion welding obviously depends on the compositions of the particular glass and of the ceramic based on Al 2 O 3 which are used. Generally speaking, temperatures above 500 ° C are used, preferably in the range from 900 to 1400 ° C and more preferably in the range from 1100 to 1350 ° C.

The first layer is applied in such a way that its final thickness ranges from about 25 to 100 percent. The following coating layers can be in the range of about 50 to about 400 µx.

In fig. 1, the solid line represents the first layer and the dotted line the second layer. Stage I (for example 2 hours) represents the annealing phase, and region II the cooling cycle. Fig. 2 shows the exploration using an electron microprobe of the amounts of sodium and aluminum in the single-layer structure described above. The degree of diffusion welding appears clearly from the fact that alumina is found in the glass up to a depth of 200 (x (S represents the surface and J the interface).

Fig. 3 shows the exploration, using an electron microprobe, of the amounts of sodium and aluminum in the double-layer structure described above. The fact that the second layer is largely welded to the first vitreous layer appears clearly from the decreasing intensity of the signal corresponding to alumina in the second layer of the vitreous coating.

Fig. 4 shows the dependence between the fatigue rate and the vitreous layer of biologically active coating compared to an uncoated A1203 surface. It is visible that the fatigue resistance of the coated material is high in comparison with that of the uncoated substrate.

As indicated above, the application of a vitreous layer having a higher coefficient of thermal expansion on a substrate having a lower coefficient of expansion produces thermal stresses upon cooling. These constraints can be calculated using the following equation:

agi = E (T0 - T ') (agi - ab) (1 - 3d + 6d2)

in which:

thickness of glass ^ thickness of substrate acted = coefficient of thermal expansion of glass E = Young's modulus ab = coefficient of thermal expansion of substrate T0 = annealing temperature acted = thermal stresses T '= final temperature (room temperature 20 ° C )

In the previous example using composition A:

j = 0.02

E = 56 x 108 daN / m2 acted = 100 x 10-7 / ° C ab = 50 x 10-7 / ° C T ° = 450 ° C T '= 20 ° C

By substituting these parameters in the previous equation, the thermal stress is equal to 5.76 daN / m2. Therefore, it is clear that the rate of microcracks can be calculated. It depends on the particular compositions used and the programs followed for coating and annealing cooking.

Figs. 5 show the ceramic substrate coated during the various stages of its production.

In fig. 5a, for which the temperature is above 500 ° C,

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the ceramic substrate 1 is covered by the first layer 2 of biologically active glass.

In fig. 5b, for which the system has been cooled to room temperature, microcracks 3 appear in the coating layer 2 by forming islands 4 of biologically active glass welded by ion diffusion on the ceramic substrate 1.

Fig. 5c shows a substrate based on Al 2 O 3 coated with a first layer of biologically active glass which comprises microcracks, this coated substrate being covered by a second layer 5 of biologically active glass.

Such ceramic substrates based on Al 2 O 3 and provided with a coating are ideally suited to the production of internal prostheses to replace a bone and not requiring cement, these prostheses having a mechanical strength of an unusual intensity. . In addition, biologically active tissues are able to adhere to these prostheses.

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Claims (17)

627723 2 1. Method for coating the surface of a compact ceramic based on Al 2 O 3 with a biologically active glass, said ceramic and said glass having different coefficients of thermal expansion, the method being characterized in that it comprises the following steps: The glass is brought into contact with the surface of the ceramic at a temperature and for a time sufficient to weld the glass to the surface of the ceramic by ion diffusion, The coated substrate is cooled to a temperature sufficient to produce interconnected microcracks in the vitreous coating as a consequence of the thermomechanical stresses induced by the difference in the coefficients of thermal expansion of said ceramic and said glass, and - The vitreous coating comprising the microcracks is covered with at least one additional layer of a biologically active glass. 2. Method according to claim 1, characterized in that said biologically active glass has the following composition by weight: Si02 Na20 CaO P2Os CaF2 b2o3 40-62% 10-32% 10-32% 3-9% 0-18% 0-7.5% the sum of NazO and CaO being greater than 30%. 3. Method according to claim 1, characterized in that the biologically active glass has the following weight composition: Si02 Na20 CaO P20s 45.0% 24.5% 24.5% 6.0% 4. Method according to claim 1, characterized in that the biologically active glass has the following composition by weight: Si02 Na20 CaO p2os CaF2 42.94% 23.37% 11.69% 5.72% 16.26% 5. Method according to claim 1, characterized in that the biologically active glass has the following composition by weight: Si02 NazO CaO p2o5 b2o3 40.0% 24.5% 24.5% 6.0% 5.0% a solvent, an organic binder and a biologically active glass powder, particles constituting the powder having dimensions less than 74%. 11. Method according to claim 10, characterized in that it 5 includes the steps of drying the ceramic substrate covered by the mud and firing the coated substrate in order to remove the organic binder. 12. Product obtained by implementing the method according to claim 1, characterized in that it comprises a compact ceramic based on A1203, the surface of which is coated with at least two layers of a biologically active glass having a coefficient of thermal expansion different from that of said ceramic. 13. Product according to claim 12, characterized in that the biologically active glass has the following weight composition: 20 Si02 Na20 CaO P20 s CaF2 b2o3 40-62% 10-32% 10-32% 3-9% 0-18% 0-7.5% the sum of Na 2 O and CaO being greater than 30%. 14. Product according to claim 12, characterized in that the biologically active glass has the following composition by weight: 25 Si02 Na20 CaO P2Os 45.0% 24.5% 24.5% 6.0% 15. Product according to claim 12, characterized in that the biologically active glass has the following composition by weight: 35 Si02 Na20 CaO P2Os CaF2 42.94% 23.37% 11.69% 5.72% 16.26% 16. Product according to claim 12, characterized in that the biologically active glass has the following weight composition: 40 Si02 Na20 CaO p2o5 b2o3 40.0% 24.5% 24.5% 6.0% 5.0% 6. Method according to one of claims 1 to 5, characterized in that the surface of the ceramic has a coefficient of thermal expansion from 0 to 1000 ° C included in the range 50 x 10-7 to 75 x 10_7 / ° C and in that said glass has a coefficient of thermal expansion from 0 to 1000 ° C in the range 95 x 10-7 to 145x 10_7 / ° C. 7. Method according to one of claims 1 to 6, characterized in that each coating layer is annealed. 8. Method according to one of claims 1 to 7, characterized in that said glass is welded to the surface of said ceramic at a temperature above 500 ° C. 9. Method according to one of claims 1 to 8, characterized in that said ceramic whose surface is provided with a glassy coating is cooled to produce microcracks in said coating having a width less than 1 (i. 10. Method according to one of claims 1 to 9, characterized in that said glass is brought into contact with the surface of said ceramic by covering the surface of the ceramic with a mud comprising 45 17. Product according to one of claims 12 to 16, characterized in that the surface of said ceramic has a coefficient of thermal expansion from 0 to 1000 ° C included in the range from 50 x 10 ~ 7 to 75 x 10 ~ 7 / ° C and in that said glass has a coefficient of thermal expansion from 0 to 1000 ° C included in the range from 95 x 10 ~ 7 50 to 145 x 10-7 / ° C. 18. Use of the product obtained by the implementation of the method according to claim 1 for the production of an internal prosthesis capable of replacing at least part of a bone and not requiring cement. 55