DE4425209A1 - Process for joining compact sintered ceramic bodies - Google Patents

Abstract

Process for joining compact sintered ceramic bodies comprises: (a) only polishing the joining surfaces to unevenness < 0.5 mu m and roughness < 1 mu m; (b) using as foil when joining the ceramic bodies, a green ceramic foil, whose heat expansion coefft. does not deviate > 1x10<-6>/K from that of the ceramic body; (c) maintaining the foil in a 1st phase of a temp./time regime to gasify the binder and solvent under such a pressure between the joining surfaces so as to prevent movement of the joining surfaces and the foil; and (d) crystallising the foil in 2nd and 3rd phases of a temp./time regime under a pressure of at least 50 kPa so that the pressure remains below the max. pressure for the joining bodies. The green ceramic foil used in the process is also claimed.

Description

The invention relates to a method for the integral joining of compact sintered ceramic parts, especially for gas-tight joining of different ceramics with different ones Coefficient of thermal expansion.

Because of their material properties, ceramic materials are like low specific weight, high wear resistance, favorable tribological Properties, high temperature resistance and in general also chemical Resistance due to the high hardness, very interesting for special applications.

But the molding processes can only be used to a limited extent, so that only certain ones Shapes can be created.

In particular, the manufacture of large, voluminous workpieces as well complicated parts mostly impossible or subject to great difficulties.

For the reasons mentioned, connection methods for ceramic parts are very important.

Processes are known which are called soldering, diffusion welding, Gluing, pressing and sintering for the production of ceramic composites and Ceramic-metal composites are used.

The following considerations are intended to be limited to connection types that are geared towards mechanical strength and material cohesion. In the Patent DE-PS 38 20 459 discloses that the connection through the Introducing a reactive, electrically conductive material between the ground ceramic surfaces is achieved by heating the layer through a brief surge occurs to react with the surrounding Avoid atmosphere largely.

DE-PS 38 42 984 deals with the joining of silicon nitride moldings under Interposition of nitrogen over and with regard to the formation of Si₃N₄ submerged silicon-containing layers and hot pressing of the joint Pressures of more than 10 MPa and temperatures above 1500 ° C.

In DE-OS 36 12 458 a maximum of 1 µm is used for joining silicon carbide brought a thick layer of different metals between the joining surfaces. With inert or reducing atmosphere are preferably at a pressure of 15  up to 45 MPa and a temperature between 1500-1800 ° C welded together. The layer is practically an activation layer and no longer detectable after the process.

The solutions shown either require a metallic bonding layer or require high pressures and temperatures.

In addition, the joining surfaces are of high purity and surface quality (polished) demands.

The invention is therefore based on the object of a novel possibility for cohesive joining of ceramic parts to be found at low pressure and relatively low temperature low stress and mechanically firm Ceramic composites generated. It should be possible to use ceramics at will to combine different coefficients of thermal expansion.

According to the invention the object is achieved in a method for the integral joining of compact sintered ceramic bodies, in which a foil is placed between the two joining surfaces of the ceramic bodies, the joining bodies then being connected under pressure and high temperature in a reaction-supporting atmosphere, in that the joining surfaces be ground only on unevenness of less than 0.5 microns and a surface roughness of less than 1 micron that used as a green ceramic film, the coefficient of thermal expansion of which does not deviate more than 1 · 10 ^-6 / K from the coefficient of thermal expansion of the ceramic body is that the film is held in a first phase of a temperature-time regime for outgassing the binder and solvent at least under such a pressing pressure between the joining surfaces that prevents the joining surfaces and the film from slipping relative to one another, and that the film in egg ner second and drift phase of the temperature-time regime is brought to crystallization under a pressure between 50 and 100 kPa.

Appropriately, in the first phase of the temperature-time regime, there is a Heating to about 350 ° C a holding time of about 2 hours. It turns out however, from an energetic and time point of view, advantageous after heating up approx. 350 ° C a slower heating period to approx. 400 to 500 ° C to perform within 2 hours, the purpose of the first phase in the same Way fulfilled.

In the second phase, a temperature of approximately 750 ° C. is advantageously set and held for about 2 hours to produce nuclei in the film.  

In the third phase, the sintering temperature of the film is expediently set and held for about 3 hours until the film is sintered. It is preferably under sintered oxidizing atmosphere. For joining pure aluminum (<99.6% Al₂O₃) the first phase of the temperature-time regime is advantageous transferred directly to the third phase, since the doped composite film already used has crystallization nuclei.

Although only for the sintering process above 800 ° C (second and third phase) it has to be applied with a pressure of approx. 50 kPa turns out to be expedient, this pressing pressure directly for locking in the first Phase and maintain it throughout the temperature-time regime.

Proven to join ceramic bodies with different heat coefficients it is advantageous to have several foils with graded between the joining surfaces Thermal expansion coefficient and the temperature described above Submit time regime.

According to the invention, the object is achieved with the method according to the invention a green ceramic film solved in that the film of ground powder exists, the powder has a nominal grain size of less than 3 microns.

The film is preferably produced using the doctor blade method. In order to create a gradual transition of the coefficient of thermal expansion from joint body to joint body, the film advantageously has a suitable proportion of the powder of each joint body; the differences in the coefficient of thermal expansion between the film and the joint body are each less than 1 · 10 ^-6 / K.

For larger differences in the coefficient of thermal expansion is appropriate a film package with a gradual transition of the coefficient of thermal expansion compiled by different powder mixtures of Material shares of both joining bodies foils with adaptively adjusted Contains coefficient of thermal expansion.

In many cases, it proves to be advantageous in the individual foils Material proportions of the joining elements in uniform steps from film to film and to change from foil to joint.

In certain cases, however, it is also expedient that the material parts in the individual foils in even stages from foil to foil and compared to one joining body, but to the second joining body Leap in the continuous gradual change in material proportions is present.  

The invention is based on the consideration that cohesive composites of sintered ceramic by welding or melting the surface material the joining surfaces always have a high temperature and / or a high pressure on the Results in joining surfaces.

The reason for this is the need for the joining partners to be adapted to one another in terms of their adhesion behavior and their coefficient of thermal expansion. Exchange processes must also be possible due to the chemical composition. Even with different types of ceramics, these requirements are met by the intermediate storage of raw ceramic foils (green ceramic foils) that hold the proportions of both joining partners. For the adaptation of joining bodies with thermal expansion coefficients which differ by more than 1 · 10 ^-6 / K, foil packages made of several such superimposed foils with thermal expansion coefficients gradually adapted to the joining bodies are suitable.

The production of such foils which impart the coefficient of thermal expansion is provided according to the invention in such a way that the sintered ceramic material is ground is up to the nominal grain size of <3 µm. Then proportions of different ceramics (or glass phases with glass ceramics) with each other mixed and according to known techniques, e.g. B. according to the doctor blade method (see, for example, Keramische Zeitschrift 44 (1992) 1, pp. 23-27), ceramic Raw foils made. According to the proportions Ceramic foils with different coefficients of thermal expansion and different material composition. These slides become one gradual layer structure with regard to expansion coefficient and material The composition of the joining partners is sorted and between the cleaned, ground and sintered ceramic body.

The joining process then runs after locking the shape and size Raw foils between the joining surfaces in three essential phases. In a first Phase, binder and solvent are removed from the raw film by degassing. A second phase with a pronounced holding time ensures the formation of Crystallization nuclei and a third phase leads while the sintering temperature is maintained the ceramic components for complete sintering of the ceramic foils. Sintered is at a pressure of approx. 50 kPa in the normal direction of the joining surfaces and at oxidizing atmosphere.

The method according to the invention allows using green ceramic films produced according to the invention at low pressure on the Joining surfaces and relatively low temperatures a mechanically stable, gas and to produce vacuum-tight ceramic composite.

The invention will now be described with reference to three exemplary embodiments are explained. The drawing shows

Fig. 1 shows the temperature-time regime to be used according to the invention.

The method according to the invention essentially comprises the use of a green ceramic film, which in particular conveys the composition of matter and the thermal expansion coefficients of the joining partners. The temperature-time regime shown in FIG. 1 is used for the composite, the first phase freeing the film from binders and solvents, the second and third phases containing the necessary steps for crystallizing the film.

1st example Joining Bioverit I and Vitronit (manufacturer or distributor: Vitron GmbH Jena)

The expansion coefficient of Bioverit I glass ceramic is 11.6 · 10 ^-6 / K, that of Vitronit 8, 7 · 10 ^-6 / K.

From the glasses used to ceramize Bioverit I and Vitronit have been melted, a powder is produced, which has the nominal grain size Is <3 µm. 3 mixtures are made from this powder.

These mixtures are advantageously milled again (grinding). A film with a thickness of 0.5 mm is z. B. drive according to the Doctor Blade Ver [Ceramic Journal 44 (1992), pp 23-27] produced (green raw film). The mixtures and their grinding ensure that the coefficient of thermal expansion changes less than 1 · 10 ^-6 / K from film to film and to the joining bodies.

Then, depending on the size of the joining surface, foil sections are provided.

The joint surfaces of the Bioverit I and Vitronit joining partners are ground a surface roughness of less than 1.0 µm. The surface unevenness may do not exceed 0.5 µm. Then the joining surfaces of mechanical particles cleaned.  

The piece of film of the 1st mixture is placed on the joint surface of Bioverit I, then that of the 2nd mixture and then that of the 3rd mixture and on top of that Joining part made of Vitronit.

This package is positioned in an oven so that the pairing does not slip can and a pressure can be exerted on the ceramic on top.

First of all, a small minimum pressure is sufficient to prevent the foils and joining surfaces from slipping. This pressure is sufficient at least in the first phase according to FIG. 1 to achieve the outgassing of the solvents and binders from the films.

A temperature of 350 ° C is required for outgassing, which must be maintained for about 2 hours. After a heating process of about 1 hour, point A (see Fig. 1) is reached, which corresponds to this temperature, the dashed line to point B 'indicates the required holding time for outgassing. However, the variant of slow further heating within the necessary holding time is more energetically more favorable, as indicated by the solid line AB in FIG. 1. The binders and solvents continue to outgas at this temperature increase. However, the temperature increase must be limited until the end of the necessary holding time so that processes that are reserved for the second phase do not already start. In the example given, this is easily guaranteed up to approx. 500 ° C. The advantage of this modified "holding time" is that the heating-up time is reduced in the second phase.

In the second phase, regardless of the temperature-time regime chosen for the first phase, a process of nucleation is initiated at a temperature of approximately 750 ° C., which is indicated by the solid line CD in FIG. 1. A two-hour hold time is also provided for this process. Only after its completion (in point D) is heating continued in the third phase.

The third phase is used to ceramize the foils. A pressure of approximately 50 kPa is applied to the composite surfaces in the normal direction and the temperature is increased to the sintering temperature of approximately 1050 ° C. in accordance with FIG. 1. This temperature is held for approx. 3 hours to sinter the foils (line E - F). This is followed by a slow cooling phase, the result of which is a cohesive, low-stress and mechanically strong ceramic composite.

2nd example Joining glass ceramic Bioverit I with aluminum oxide ceramic (KER 710, manufacturer: Boartceramics Auma)

The aluminum oxide ceramic KER 710 has a coefficient of thermal expansion of 7.0 · 10 ^-6 / K and an Al₂O₃ content of more than 95%. The difference in the coefficient of thermal expansion compared to Bioverit I is 4.6 · 10 ^-6 / K.

Mixtures from the granules of Bioverit I and the Al₂O₃ powder are as follows prepared, with a grinding process (as in Example 1) the nominal grain size of Batch of less than 3 µm ensures:

Films are made from these mixtures.

The foils are adapted to the outer dimensions of the joining surfaces. The honed Joining surfaces (surface roughness <1 µm) are made up of mechanical particles cleaned and the foils in the order of the mixture numbering on the feet area of Bioverit I launched. This creates an even layer up to the 3rd film Gradient of the proportions of Bioverit I and the proportions of Al₂O₃. The jump of the gradient of the proportions of both ceramics at the transition to the joint body made of Al₂O₃ is tolerable in this case, since for the composite and Ver The ability to adjust the coefficient of thermal expansion is decisive important role. The installation of small amounts of Al₂O₃ in the films affected the expansion coefficient of Bioverit I to a considerable extent.

According to the same temperature-time regime as in example 1 (solid Line) the composite is ceramized. From approx. 800 ° C a pressure of at least 50 kPa (up to a maximum of 100 kPa) on the joints perpendicular to the Joined surfaces applied.

3rd example Joining of pure aluminum oxide (<99.6% Al₂O₃)

In this example a composite film (as e.g. in Journ. At the. Ceram. Soc. 73 [1990] 1, pp. 3476-3489) from doped Al₂O₃ used.

For a selected connection of ceramic tubes to one another or to any other ceramic part (e.g. a flat plate), rings are punched out of the above-mentioned composite film using a suitable tool. The diameter of these green foil rings should be 1 mm larger than the diameter of the tubes in order to compensate for the shrinkage of the foil rings in the joining process. As already described in the previous examples, the foil rings are securely locked between the joining surfaces, are first heated to 350 ° C. for outgassing the binder and solvent and are kept at this temperature for 2 hours (see 1st phase in FIG. 1, line AB ′ ). Then it is further heated to 1200 ° C and held until the films are completely sintered ( Fig. 1, dashed line B'-CE'-F '). From a temperature of approx. 800 ° C - as already described in the first two examples - a pressure of approx. 50 kPa is exerted perpendicular to the joint surfaces and an oxidizing atmosphere is added.

In this example, as the dashed line in FIG. 1 clearly shows, the second phase of the temperature-time regime is omitted since the formation of nuclei due to the doping of the composite film is not necessary. This in turn results in a cohesive, mechanically strong and low-stress ceramic composite.

Claims (16)

1. A method for the integral joining of compact sintered ceramic bodies, in which a foil is placed between the two joining surfaces of the ceramic bodies, the joining bodies then being connected under pressure and high temperature in a reaction-supporting atmosphere, characterized in that

• - the joining surfaces are only ground to an unevenness of less than 0.5 µm and a surface roughness of less than 1 µm,
• a green ceramic film is used as the film, the coefficient of thermal expansion of which does not deviate from the coefficient of thermal expansion of the ceramic body by more than 1 · 10 ^-6 / K,
• - The film is held in a first phase of a temperature-time regime for outgassing the binders and solvents at least under such a pressure between the joining surfaces that prevents the joining surfaces and the film from slipping relative to one another, and
• - The film is brought to crystallization in a second and third phase of the temperature-time regime under a pressure of at least 50 kPa, the pressure must remain below the maximum pressure that can be absorbed by the joining bodies.

2. The method according to claim 1, characterized in that that in the first phase of the temperature-time regime after a warm-up time a holding time of about 2 hours at a temperature of about 350 ° C follows. 3. The method according to claim 1, characterized in that in the first phase of the temperature-time regime after heating up approx. 350 ° C another slowed down heating period to approx. 400 to 500 ° C done within 2 hours. 4. The method according to claim 1, 2 or 3, characterized in that in the second phase a temperature of about 750 ° C is set and for is held for about 2 hours so that there are crystallization nuclei in the film form.   5. The method according to claim 4, characterized in that in the third phase a temperature which is the sintering temperature of the Ceramic foil corresponds, adjusted and held for about 3 hours, so that the Foil is completely sintered. 6. The method according to claim 1, characterized in that the third phase of the temperature-time regime under oxidizing Atmosphere takes place. 7. The method according to claim 1, 2 or 3, characterized in that that for the assembly of joining bodies made of pure aluminum oxide (<99.6% Al₂O₃) the first phase of the temperature-time regime directly in the third phase is transferred. 8. The method according to claim 1, characterized in that the film in all phases of the temperature-time regime under the same Press pressure is treated. 9. The method according to claim 1, characterized in that several films with graded coefficients of thermal expansion for Adjustment of the coefficient of thermal expansion between the joining surfaces of the Joining body to be placed. 10. The method according to claim 9, characterized in that in a film package consisting of several films, films with a graded different thermal expansion coefficient are placed one above the other, the difference in expansion coefficient between adjacent films not greater than 0.7 · 10 ^-6 / K is set. 11. Green ceramic film for performing the method according to claim 1, characterized featured, that the film consists of ground powder of both joining bodies, the Powder has a nominal grain size of less than 3 µm.   12. Green ceramic film according to claim 11, characterized in that the film is made by the doctor blade process. 13. Green ceramic film according to claim 11, characterized in that to create a gradual transition of the thermal expansion coefficients from joining body to joining body, the film has a suitable proportion of the powder of each joining body, the differences in the coefficient of thermal expansion between film and joining body in each case being less than 1 · 10 ^{- 6} / K are. 14. Green ceramic film according to claim 13, characterized in that the different coefficient of thermal expansion for films one Foil package with gradual transition of the coefficient of thermal expansion due to different powder mixtures of material proportions of both Joining body is adjustable. 15. Green ceramic film according to claim 14, characterized in that the material proportions in the individual foils in almost uniform stages changed from film to film and from joint to film. 16. Green ceramic film according to claim 14, characterized in that the material proportions in the individual foils in uniform stages of foil changed to foil, but a jump to at least one joint body with regard to the continuous gradual change in the material proportions.