EP1268364A1

EP1268364A1 - Sintered shaped body, whose surface comprises a porous layer and a method for the production thereof

Info

Publication number: EP1268364A1
Application number: EP01915331A
Authority: EP
Inventors: Dirk Rogowski; Hans-Georg Pfaff; Alwin Nagel
Original assignee: Ceramtec GmbH
Current assignee: Ceramtec GmbH
Priority date: 2000-03-29
Filing date: 2001-03-14
Publication date: 2003-01-02
Also published as: US7074479B2; DE10015614B4; WO2001072664A1; DE10015614A1; US20030180518A1

Abstract

Porous coatings on high-performance ceramics attempt to combine the mechanical and thermal characteristics, which fulfil stringent demands, of the substrate material with the advantageous properties of coating materials. The subsequent application of layers of this type to the pre-sintered substrate produces unsatisfactory results in several areas of use with regard to possible layer thickness, porosity and adhesion. According to the invention, a shaped body consisting of a sintered, inorganic material, whose surface comprises a porous layer is produced in such a way that the base body is first formed as a green body. A layer in the form of a suspension, also containing an inorganic material, is then applied to the surface or to one section of the surface of the base body. A predetermined fraction of a pore-forming substance is mixed with at least the material of said layer and the green body with its applied layer is subjected to the thermal treatments required for producing a monolithic sintered body.

Description

Sintered molded body with porous layer on the surface as well

Process for its manufacture

The invention relates to a molded article according to the preamble of the first claim and to a method for producing a molded article according to the sixteenth claim.

Coatings serve to improve mechanical, electrical, chemical, optical or other material properties on the surface of a component in order to achieve application advantages or to prevent or delay negative effects on the component during use.

The application of dense layers in the form of glazes to ceramic substrates has been known for a long time. The substrate materials are mostly coarse or refractory ceramics with a correspondingly low level of mechanical properties and structure. The dense coating should essentially cover these disadvantages. The glazes significantly increase chemical resistance, for example.

In the case of coatings on high-performance ceramics, on the other hand, attempts are made to combine the extreme mechanical and thermal properties of the substrate material with the advantageous properties of coating materials.

Layers made of various chemical elements and compounds, which are applied to the substrate by means of CVD, PVD, plasma or similar techniques and also combinations thereof, are used and tested in technology. A disadvantage of these application methods is the way via the gas phase, which severely limits the number of materials that can be used for coating. The layer thicknesses that can be achieved are in the range from a few μm to approximately 25 μm and, due to the coating process, are very cost-intensive. With the methods mentioned it is only possible to To change surface properties. However, it is not possible to significantly influence the structure of the surfaces. In addition, the adhesiveness of the layers depends on the method used. In the case of coatings using the plasma process, the layer adheres only via adhesive forces, which naturally limits long-term adhesion.

Other thermal and chemical coating processes have the disadvantage that the structure of the substrate material can be influenced by the coating process and the material properties can even be deteriorated. Due to the two-stage process for producing a component as a substrate and the subsequent coating, stresses can arise between the layer and the material of the substrate, which impair the adhesive strength of the layer on the substrate.

Sintering together ceramic shaped bodies of different porosity is state of the art, however, complex components cannot be produced due to problems at the interface of the shaped bodies and the residual stresses that occur.

Ceramic molded parts that consist entirely of an open-pore material are state of the art. However, their mechanical strength is greatly reduced.

With the above-mentioned methods, it is therefore not possible to produce a layer with a defined thickness and pore structure on a densely sintered substrate made of an inorganic material.

The invention is therefore based on the object of avoiding the known disadvantages in the production of a porous layer on a sintered body from an inorganic material.

The problem is solved with the aid of a shaped body as claimed in the first claim and a method for producing a shaped body, in particular a molded body according to claims 1 to 15, as claimed in claim 16. Advantageous embodiments of the invention are claimed in the subclaims.

The invention avoids the disadvantages of the prior art in the production of a shaped body with a porous layer on its surface in that first a base body, the substrate, is formed as a green body from an inorganic material and a suspension is formed on the substrate in the state of the green body the same inorganic material from which the substrate is made, or a different material is applied. In addition to the inorganic material, this suspension also contains a pore-forming substance. Only after the layer has been applied is there a common heat treatment of substrate and layer by drying and sintering to produce a monolithic shaped body. The method for producing the substrate does not differ from that which is known from the prior art.

The base body can either be pore-free, densely sintered, or also contain pores. In the latter case, it also contains a portion of a pore-forming substance in its state as a green body. However, the proportion of this substance is then such that the proportion of pores per unit volume in the layer is always larger than in the substrate.

Ceramic materials such as the known oxide ceramics, silicates, phosphates, apatites and related materials as well as nitrides, carbides and silicides are particularly suitable as inorganic materials for the base body, the substrate. It is also possible to produce moldings with a porous surface layer from metals produced by powder metallurgy using the process according to the invention.

The same inorganic materials that are suitable for producing the base body, the substrate, are suitable for producing the layer. However, it is advantageous if, when choosing an inorganic material for the layer, the does not match the inorganic material of the base body, care is taken that the material of the substrate and the material of the layer have an almost equal expansion coefficient and thermal stability in the temperature range provided for the sintering of the molded body. This avoids that due to the different expansion of the different inorganic materials, as well as changes in the lattice structure or the chemical composition of a material, stresses occur when passing through the intended temperature range, in particular in the border area between the two materials, which can lead to detachment or destruction of the layer.

It has an advantageous effect on the thermal behavior of the shaped body during the sintering process if the grain size of the material of the substrate and the grain size of the material of the layer match. In the case of different grain sizes, in particular in the boundary region between the base body, the substrate, and the layer, there is a risk that tensions will occur which can also lead to detachment or destruction of the layer.

So that a porous layer can form on the base body, the substrate, the inorganic material provided for the layer is mixed in a suitable grain size with a suitable liquid and a suitable pore-forming substance to form a suspension and this suspension taking into account the shrinkage during the Heat treatment, drying and sintering, applied in the required layer thickness on the green body. The production of a suspension from an inorganic material in a liquid matched to this material and a substance suitable for the size, shape and number of pores are known from the prior art, for example from DE 44 42 810 A1, DE 44 32 477 C2 or the publication "Influence of Organic Compounds on Ceramic Masses", W. Mann, Ber. DKG, 37 (1960), pp. 11 to 22.

In the latter publication, a number of methods for pore formation are explained. Then there is the burnout process, the - o -

Solution process, the sublimation process, the evaporation process, the swelling process, the gas blowing process and the foam process.

Particularly suitable as pore-forming substances are organic substances, for example starches, cellulose or waxes, and natural and synthetic polymers which evaporate, gasify, consume or burn during the thermal treatment of the substrate and the layer applied to it and thereby form the pores. The number of pores per unit volume, their size, that is to say their diameter, and their shape can advantageously be determined by the selection of a suitable pore-forming substance. In the case of solid substances, the quantity of particles, their size and their shape are the decisive influencing factors. The shape of a solid pore-forming material can, for example, be spherical, globular, platelet-shaped or fibrous.

As a rule, the pore-forming substances are converted into a gas phase during the thermal treatment of the shaped body, which leads to open pores when the gas escapes from the shaped body, that is to say the pores are interconnected. As can be seen from the latter publication, there are also processes, for example the gas blowing and the foam process, in which the pores remain closed. The type of pores depends on the intended use of the shaped body. Open pores are always advantageous if liquids or gases are to flow through the molded body and, for example, additional substances are to be stored in the pores. Shaped bodies with closed pores are suitable, for example, for sound and heat insulation and for electrical insulation.

The porosity, that is to say the proportion of pores per unit volume, can be controlled in the case of solid substances by the amount, in the case of liquid substances optionally by the concentration of the pore-forming substance added, so that the porosity is approximately between 25% and 90%, preferably approximately is between 25% and 70%. In the case of solid substances, the pore size, the diameter of the pores, depends in particular on the particle size of the substance forming the pores and can have values of approximately between 1 μm and 1000 μm, preferably between 20 μm and 500 μm can be set. The prerequisite is that the substances used do not undergo any change in volume during the burning out or gasification.

In an advantageous development of the invention, when the layer is applied to the green body, to the substrate, the moisture content of the suspension can be adapted to the precompression of the material of the substrate. The lower the pre-compaction of the substrate and the higher its moisture content, the more carefully the moisture content of the suspension must be adjusted so that the substrate retains its shape and stability when the layer is applied. In addition, the moisture content of the substrate and suspension must be coordinated with one another in such a way that the shrinkage of the substrate and layer is approximately the same in the subsequent heat treatments, so that cracks, deformations or detachment of the layer do not occur during drying.

The layer materials and the pore-forming substances are suspended in water or in another suitable liquid, which is known from the prior art already mentioned, in such a way that the suspension has a consistency suitable for the application process. In addition, dispersants can be added to produce a suspension, which advantageously achieve a uniform distribution of the solids within the suspension. The viscosity of the suspension can be influenced by adding organic or inorganic auxiliaries. When strongly wetting liquid is added, the adhesion of the suspension to the substrate can be increased in the green state.

What was listed as an advantageous process parameter for the production of the suspension for application to the substrate applies equally to the production of the substrate itself.

The method for applying the layer on the substrate can advantageously be based on the geometry and the surface shape of the substrate and the desired thickness of the Layer to be matched. The layer can be applied to the entire surface of the substrate or only to one or more partial areas.

The immersion method is particularly suitable for complicated surface structures and thin layers of approximately 0.02 mm to approximately 2 mm. The dipping process also enables a layer to be built up in several dipping steps in succession up to the desired overall thickness. After each dipping process that builds up a layer of a certain thickness, this layer is first dried to a degree suitable for building up the new layer before the next layer is built up.

In particular, the suspension can also be spread on flat surfaces and filled with thick layers. Spraying requires a sprayable suspension. Sprayed layers have a rough surface, which can be advantageous for implants or catalysts, for example. The layers can also be easily applied in multiple layers by spraying. With the aid of the methods presented, layers in the range from approximately 0.02 mm to 10 mm, preferably from approximately 0.1 mm to 2 mm, can be applied. By changing the properties of the characteristics listed below and the possible combination of these characteristics with each other, i.e. through different inorganic materials of substrate and layer, through different proportions of pores per unit volume in the substrate and in the layer, through the pore size and the pore shape, through the The thickness of the layer, the arrangement of the layer on the surface of the substrate and the surface shape of the layer itself can be found in a large number of applications for moldings according to the invention, some examples of which are listed below:

The moldings according to the invention can be used, for example, as implants in medical technology. Medical implants, for example socket inserts for hip joints, are made from high-purity aluminum oxide ceramics because of their good compatibility and biocompatibility as well as their very good wear behavior. Through a layer that is also made of aluminum oxide, Al ₂ O ₃ , a few tenths of a millimeter thick with open pores with a diameter of about 200 - σ -

microns to 400 microns, the bone tissue is given the opportunity to grow or ingrowth into the layer and the pan can be anchored directly in the bone. Instead of a porous aluminum oxide layer, the pan as the base body can also be coated with a layer of hydroxylapatite or other calcium phosphate compounds with the same thickness and with the same pore structure. The hydroxyapatite stimulates bone growth and facilitates the ingrowth of the bone tissue in the pores of the layer of the implant. Hydroxyapatite can also be applied in a thin layer on the porous aluminum oxide layer.

The following examples show industrial applications. A further layer of porous silicon nitride is applied to a silicon nitride substrate, Si ₃ N ₄ , of a cutting tool, so that a good adherent, active coating with precursors can then take place.

In process engineering and chemistry, for example, porous layers of silicon carbide, SiC, on substrates, which are also made of silicon carbide, promote the evaporation of liquids due to the enlarged surfaces.

The moldings according to the invention are also suitable as catalyst supports. The porous layer on the high-temperature ceramic materials serves as a carrier for the catalyst material. Such catalysts are used, for example, in motor vehicles or in the chemical industry. Furthermore, the moldings according to the invention are suitable for lining containers, pipelines and channels in metallurgy and in the chemical industry. In order to protect the surfaces that come into contact with metal melts from corrosion, for example in foundry tools, a porous layer of cordierite on dense cordierite or a porous layer of aluminum titanate on dense aluminum titanate is proposed. This increases the surface tension compared to the melt and reduces wetting. The invention is explained using the following exemplary embodiments. Show it:

1 shows a platelet-shaped body with a porous layer,

2 is a sectional view of the porous layer and the adjacent base body in an enlarged view,

Fig. 3 shows the insert shell of a hip joint endoprosthesis with a layer promoting the ingrowth of the bone tissue and

Fig. 4 is a sectional view of the porous layer and the adjacent material of the insert shell in an enlarged view.

The production of a molded article according to the invention from silicon nitride, Si ₃ N ₄ , is described below, as is shown in FIG. 1 and is designated by 1. With process steps known from the prior art, silicon nitride is prepared into a pressable mass by dispersing it in water with the addition of water-soluble binders, grinding and spray drying. The granules obtained by spray drying are pressed into a square plate 1 with an edge length of 17 mm and a height of 7 mm at an axial pressure of 2000 bar. The embodiment is shown in Fig. 1 on an enlarged scale. The density of the green body 2 is 1.9 g / cm ³ , corresponding to 60% of the theoretical density of Si ₃ N ₄ .

A portion of the aqueous Si ₃ N ₄ dispersion is branched off before spray drying. The solids content is about 60 w% (weight percent). 15% by weight of a starch powder with a grain size between 20 μm and 50 μm are added to the dispersion. The viscous dispersion produced in this way is coated as layer 3 on the pressed Si ₃ N ₄ platelets, the substrate 2. The water portion of the spread dispersion is sucked up by the green body 2 and the applied layer 3 solidifies. The thickness 4 of the layer 3 can be adjusted as desired, for example up to 2 mm, by repeated spreading. The moisture content of the _ _{1 Q} _

Substrate 2 as a green body and layer 3 during application are coordinated with one another in such a way that tensions and cracks are avoided during drying and subsequent firing.

The substrates 2 provided with a layer 3, the platelets 1, are dried like conventional moldings made of silicon nitride and sintered at the usual sintering temperature of up to 1800 ° C. Layer 3 sinters monolithically with substrate 2. The burned-out organic components leave open pores 5.

FIG. 2 shows a section through the layer 3 on the plate 1 and the area of the substrate 2 located underneath it. The image shows a 200-fold magnification through a light microscope. The thickness of the porous layer 3 arranged on the right is approximately 0.3 mm. Layer 3 clearly shows an approximately uniform distribution of coherent, spherical pores 5 of approximately the same size, which have a diameter 6 of approximately 20 μm to 30 μm. The proportion of pores per unit volume, the porosity, is approximately 35%.

The edge layer 7 of the substrate 2 also has pores 8, which are sometimes larger and irregularly arranged than the pores in the porous layer 3. This effect, which is generally referred to as sintered skin in ceramic materials, results from reactions of the surface with the sintering atmosphere. The edge layer 7 in the present exemplary embodiment is formed, for example, when silicon nitride is sintered in the presence of substances which, when decomposed, emit gases containing carbon and oxygen, which react with the nitrogen and the silicon and likewise form gaseous phases, for example SiO and N ₂ . This was the case with the sintering of the present exemplary embodiment because the starch powder has decomposed. The gases that have formed have reacted with the material of the surface layer 7 to form pores. The porosity decreases from the surface of the substrate 2 towards the inside. The sinter skin can reach a thickness of up to 3/10 mm. While the so-called sintered skin is usually removed by grinding because its porosity interferes with the otherwise intended purpose of sintered ceramics, it can even be called desirable in the present case because it opens the pores into the base body. In the case of infiltration of these pores, for example, this results in the possibility of firmly anchoring the porous layer to the base body, the substrate 2, via the infiltrated materials.

An exemplary embodiment from medical technology is shown in FIGS. 3 and 4. FIG. 3 shows an insert shell 10 of a hip joint endoprosthesis made of aluminum oxide, Al ₂ O _3. The insert shell 10 shown schematically consists of the base body 11 with the sliding surface 12 and the surface 13 on which a porous layer 14, also made of aluminum oxide, has been applied is. This porous layer 14 is intended to promote the growth and ingrowth of the bone tissue. The layer 14 has a uniform distribution of open pores 15.

Layer 14 is branched off from the material provided for the production of the insert shell. 15% by weight of a polyethylene wax with a grain size between 100 μm and 500 μm are added to this dispersion. The viscous dispersion produced in this way is spread onto the outer surface 13 of the base body 11, the procedure being as described in the previous exemplary embodiment.

FIG. 4 shows the structure of the porous layer 14 and the adjoining base body 11 after sintering in a light microscope image with a microsection magnified fifty times. The pore-free base body 11 and its outer surface 13 can be clearly seen as the boundary between the base body 11 and the porous coating 14. The sample from an insert shell is embedded in a synthetic resin 16 suitable for the preparation of micrographs. The embedding material 16 appears dark in the micrograph. It has filled the pores 15, which is why they are hardly recognizable, particularly in the transition to the surface 17 of the coating 14. The layer 14 has a thickness 19 of approximately 1.5 mm and one Porosity of about 50%. It consists of the same material as the material of the base body 11, Al ₂ O ₃ .

The rounded pores 15 of up to 400 μm in diameter form an essentially coherent structure. As can be seen, this results in a very strongly jagged surface, which advantageously supports the growth and ingrowth of the bone tissue.

Claims

claims

1. Shaped body, made of at least one sintered inorganic material, consisting of a base body, the substrate, and a porous layer located on the substrate, the substrate and layer each having a different proportion of pores per unit volume, characterized in that the shaped body as Green body consists of the substrate brought into its shape, optionally mixed with a pore-forming substance, and a layer applied to the surface or a partial surface of the surface of the substrate in the form of a suspension made of an inorganic material and containing a pore-forming substance.

2. Shaped body according to claim 1, characterized in that after the sintering of the shaped body, the layer is sintered monolithically with the substrate.

3. Shaped body according to claim 1 or 2, characterized in that the substrate has a proportion of less than 1% pores per unit volume.

4. Shaped body according to one of claims 1 to 3, characterized in that the substrate and the layer consist of different inorganic materials.

5. Shaped body according to one of claims 1 to 4, characterized in that the material of the substrate and the material of the layer have an almost equal expansion coefficient and thermal stability in the same size

Have temperature range that is required for the sintering of the molded body.

6. Shaped body according to one of claims 1 to 5, characterized in that the grain size of the material of the substrate and the grain size of the material of the layer match.

7. Shaped body according to one of claims 1 to 6, characterized in that the thickness of the layer on the substrate is approximately between 0.02 mm and 10 mm, preferably between 0.1 mm and 2 mm.

8. Shaped body according to one of claims 1 to 7, characterized in that the proportion of pores per unit volume in the layer approximately between 25% and

90%, preferably between 25% and 70%.

9. Shaped body according to one of claims 1 to 8, characterized in that in the layer the diameter of the pores is approximately between 1 micron and 1000 microns, preferably between 20 microns and 500 microns.

10. Shaped body according to one of claims 1 to 9, characterized in that the shaped body is a medical implant.

11. Shaped body according to one of claims 1 to 9, characterized in that the shaped body is part of a filter.

12. Shaped body according to one of claims 1 to 9, characterized in that the shaped body is part of a catalyst.

13. Shaped body according to one of claims 1 to 9, characterized in that the shaped body is part of a foundry tool.

14. Shaped body according to one of claims 1 to 9, characterized in that the shaped body is part of a cutting tool.

15. Shaped body according to one of claims 1 to 9, characterized in that the shaped body serves as the lining of containers, pipelines and channels in the metallurgy and in the chemical industry.

16. A process for producing a shaped body from at least one sintered inorganic material, the shaped body consisting of a base body, the substrate, and a porous layer located on the substrate, and in that the substrate and the layer each have a different proportion of pores per unit volume, in particular according to one of the claims

1 to 15, characterized in that first the base body is shaped as a green body, that a layer in the form of a suspension is applied to the surface or a partial surface of the surface of the base body, which also contains an inorganic material that at least the material of this layer previously determined share of a

Pore-forming material is admixed and that the green body and the layer applied thereon are jointly subjected to the heat treatments required for producing a monolithic sintered body.

17. The method according to claim 16, characterized in that only the material of the layer to be applied is mixed with a pore-forming substance.

18. The method according to claim 16 or 17, characterized in that a layer of a different material is applied to the substrate than that of which the substrate consists.

19. The method according to any one of claims 16 to 18, characterized in that the layer is applied to the pre-dried substrate.

20. The method according to any one of claims 16 to 19, characterized in that the moisture content of the suspension is adapted to the pre-compression of the material of the substrate still in the green state.

21. The method according to claim 20, characterized in that the viscosity, the wetting and drying behavior and the adhesive strength of the suspension are adapted to the state of the material of the substrate still in the green state.

22. The method according to any one of claims 16 to 21, characterized in that the application of the material of the layer is carried out by dipping.

23. The method according to any one of claims 16 to 21, characterized in that the application of the material of the layer by brushing or filling

5 is done.

24. The method according to any one of claims 16 to 21, characterized in that the application of the material of the layer is carried out by spraying.

25. The method according to any one of claims 22 to 24, characterized in that the layer is applied in several layers.

10. The method according to any one of claims 16 to 25, characterized in that the layer is applied in a thickness in which the shrinkage caused by the heat treatments is taken into account.

27. The method according to any one of claims 16 to 26, characterized in that the material of the layer in a thickness of about 0.02 mm to about 10 mm

15 is applied, preferably in a thickness between 0.1 mm and 2 mm.

28. The method according to any one of claims 16 to 27, characterized in that the pore-forming substance is added to the material of the layer in such an amount or concentration that the intended proportion of pores per unit volume is reached when the shaped body is sintered

20, which is approximately between 25% and 90%, preferably between 25% and

70%.

29. The method according to any one of claims 16 to 28, characterized in that the particle size of the solid substance forming the pores is matched to the desired diameter of the pores to be produced, which is approximately between

25 1 microns and 1000 microns, preferably between 20 microns 500 microns.