EP2900405B1 - Method for sintering a workpiece - Google Patents

Description

The present invention relates to a method according to the preamble of patent claim 1.

The sintering and heat treatment of, in particular dental, workpieces e.g. made of sintered metal, requires a practically oxide-free atmosphere during the sintering process to prevent oxidation of the workpiece at high temperatures. It is known to displace the oxygen around the workpiece to be sintered by introducing protective gases, preferably argon or nitrogen, into the sintering chamber in order to enable sintering to be as oxide-free as possible. In the prior art, large amounts of protective gas are often required to achieve optimal results without residual oxidation on the workpiece. This is an unfavorable cost factor in the case of generic methods or corresponding manufacturing processes of the workpiece.

The US 5,911,102 A proposes to place the workpieces to be sintered during the sintering process on a plate-like, beam-like or net-like support body or with a plurality of elevations, wherein the support body can or should be ground and polished after each sintering process. A so-called getter material is provided at the entrance of the sintering chamber in order to absorb oxygen. The JP S56-81603 A suggests that the workpiece to be sintered be completely embedded in ceramic powder. From the JP S51-94407 A the basic use of getter material is known. The EP 0 480 107 A1 describes a process for making an injection molded steel product by injection molding kneaded steel powder and binder to form a product, then removing the binder from the product, then sintering and post-processing the product to a final shape and by hardening the product. The JP S60-224702 A teaches that a workpiece a mixture of metallic powder and a synthetic resin is formed as a binder. This workpiece to be sintered is encased in a sheet-like material which consists of a mixture of refractory material and synthetic resin as a binder. The workpiece to be sintered covered by this layer is stored in a material which contains or consists of steel balls.

The object of the invention is to improve methods of the type mentioned above in such a way that the problems mentioned are reduced.

For this, the invention proposes a method according to claim 1.

In other words, a support material is used in the method according to the invention, which on the one hand has ceramic support grains, which ensure the mechanical support or storage of the workpiece in the sintering chamber. On the other hand, the support material also includes an additional material which reduces the oxygen content or residual oxygen content in the sintering chamber. It is favorable here that the additional material enables this reduction in the oxygen content in the sintering chamber by attracting and binding the oxygen or residual oxygen present in the sintering chamber. By oxygen or residual oxygen is meant the free, still unbound oxygen through which an undesired oxidation of the workpiece can occur. Apart from any oxygen radicals that may be present, this is usually present in the air as a molecule with two oxygen atoms.

In this context, the method according to the invention provides that a material or a material mixture which has a higher affinity for oxygen than the workpiece is used as the additional material.

Oxygen affinity is understood to mean the endeavor of a substance or material to bind oxygen to itself, in particular through a chemical reaction. The higher the oxygen affinity of a substance or material, the sooner and the more oxygen is bound to this substance or material.

The invention also provides that a material or a material mixture is used as the additional material which has a higher mass-specific surface than the workpiece. The higher affinity for oxygen can essentially result from the larger surface which is available for a corresponding binding of the oxygen. In this context, it should be mentioned that it can even be sufficient within the meaning of the invention that the additional material consists of the same material as the workpiece, the additional material then having to have a higher mass-specific surface area than the workpiece. The mass-specific surface is understood to mean the totality of all surfaces present in a body or material, both outside and inside in the pore space, this totality of the surfaces being related to the mass of the body or material. The mass-related specific surface can be specified in the unit m ² / kg (square meters per kilogram). In combination with a higher specific surface area, it can also be provided that a material or a material mixture is used as the additional material which has at least one chemical element or at least one chemical compound which has a higher affinity for oxygen than the material of the workpiece. This is the case if the material or at least one chemical element or at least one chemical compound of the same, ie also with the same mass-related specific surface, has a higher affinity for oxygen than the workpiece. In this case, the additional material is a different material or a different material composition than the workpiece.

Generally speaking, the use of the additional material in the support material chemically binds the residual oxygen that is still present in the sintering chamber despite the protective gas atmosphere, so that it is no longer available for oxidation of the workpiece during the sintering process. It is thus achieved by the invention that there is none or at least none, despite a certain residual oxygen content in the protective gas atmosphere substantial oxidation of the workpiece occurs during the sintering process in the sintering chamber. As a result, the amount of protective gas required for the sintering process can be reduced, which generally results in a corresponding cost advantage.

A protective gas atmosphere is understood to mean a gas composition which has as little free oxygen content as possible. As is known per se, appropriate inert gases can be used to generate this protective gas atmosphere in the sintering chamber. This can e.g. Nitrogen or noble gases. Argon and / or nitrogen are particularly preferably introduced into the sintering chamber in order to provide the protective gas atmosphere.

Methods according to the invention are particularly preferably used for sintering dental workpieces. The term dental workpiece encompasses in particular all those artificially produced components which can be used in the denture as a replacement for natural teeth or tooth components, as well as auxiliary bodies for fastening and for producing such tooth replacement parts. The workpieces to be sintered are particularly preferably metals or metal alloys. The workpieces can be sintered in the green or white state.

Particularly preferred embodiments of the method according to the invention provide that the ceramic support grains are arranged between the workpiece and the additional material. This prevents the additional material from adhering to the workpiece during the sintering process.

The support material with ceramic support grains has, in addition to the ceramic support grains, the additional material for reducing the oxygen content of the sintering chamber. This additional material has a higher affinity for oxygen than the ceramic support grains in order to bind the residual content of molecular oxygen in the protective gas atmosphere, so that it is no longer available for oxidation of the workpiece during the sintering process.

The support material can consist exclusively of granular, pourable material. In these embodiments, not only are the ceramic support grains but also the additional material and any other constituents that may be present as grains or powder. The various components of the support material can be present as a homogeneous but also as an inhomogeneous mixture. As already stated, it is particularly advantageous in the area in which the workpiece lies directly on the support material if only or essentially only ceramic support grains are present there. The ceramic support grains are mainly used to support and support the workpiece. Conveniently, at least when viewed with the naked eye, they have a rounded and / or smooth outer surface. As support grains e.g. ceramic sintered beads known per se in the prior art, e.g. made of zirconium dioxide partially stabilized with yttrium. In preferred embodiments, the ceramic support grains have a larger diameter than all other components of the support material. Their diameter is preferably in the range between 0.4 mm and 2 mm. The ceramic support grains can also form the main component of the support material. The support material preferably consists of at least 90% by mass, preferably at least 95% by mass, particularly preferably at least 98% by mass, of the ceramic support grains. The ceramic support grains are preferably highly temperature-resistant, which means a temperature resistance of at least 1,600 ° C.

The additional material for reducing the oxygen content in the sintering chamber is preferably metals or metal alloys. With a correspondingly high mass-related specific surface, they can also be ceramic, for example. The additive material can be in pellet or powder form. However, they can also be larger, preferably sieve-like or lattice-like, preferably porous, bodies, in particular metal bodies. These can also be larger than the diameter of the ceramic support body mentioned. It can be, for example, cobalt chrome molybdenum or titanium or titanium alloys.

In particular, if the additional material for reducing the oxygen content in the sintering chamber is in the form of powder or fine-grained material, preferred embodiments of the support materials provide that the support material additionally has a carrier substance to which the additional material can attach. It can e.g. are ceramic materials such as aluminum oxide. The carrier substance advantageously has a rough and / or abrasive surface. It can be equipped with a grain diameter between 50 and 110 microns. Generally speaking, the grain diameter of the carrier substance is advantageously between the larger grain diameter of the ceramic support grains and the smaller grain diameter of e.g. additional material available in powder form to reduce the oxygen content in the sintering chamber. Due to the rough, abrasive surface of the carrier substance, the additional material adheres particularly well. Due to the smaller grain diameter, the carrier substance, including the additional material attached to it, can slide between the ceramic support grains and its smooth surfaces and, at least essentially, can be deposited under the ceramic support bodies. This achieves the preferred embodiment mentioned above, in which the ceramic support grains are arranged between the workpiece and the additional material. This at least largely prevents the workpiece from coming into direct physical contact with the additional material during the sintering process. The carrier substance, insofar as it is present, advantageously takes up a proportion of 0.1 to 2 percent by mass of the support material. The carrier substance is advantageously also highly temperature-resistant, so it preferably has a temperature resistance of up to at least 1,600 ° C. The proportion of the additional material for reducing the oxygen content in the sintering chamber in the support material in the case of such at least three-component mixtures is advantageously at least 0.1% by mass of the support material.

In the case of an arrangement with a sintering chamber and at least one workpiece arranged in the sintering chamber, the workpiece being supported in the sintering chamber by a support material according to the invention, the support material provides mainly by means of the ceramic support body for good storage and support of the workpiece during the sintering process. The additional material extracts the free residual oxygen from the protective gas atmosphere so that it is no longer available for oxidation of the workpiece during the sintering process. The distribution of the additional material in the support material preferably ensures on the one hand that the additional material does not come into direct contact with the workpiece and therefore cannot adhere to it. On the other hand, the additional material in the support material is in the immediate vicinity of the workpiece during the sintering process, so that in particular in the immediate vicinity of the workpiece there is particularly little free residual oxygen capable of binding for unwanted oxidation of the workpiece during the sintering process. If the support material is in the form of bulk material or a granular mixture with the two or three components mentioned, it can be produced by a mechanical mixing process in that corresponding proportions of additional material and, if appropriate, also carrier substance are added to the ceramic support grains. The addition of the additional material to the carrier substance can take place during the mixing process. The workpiece to be sintered lies in the sintering chamber during the sintering process, preferably directly on the support material and is thus supported by the latter.

• Fig. 1 vergrößert und schematisiert dargestellt, ein Stützmaterial in Form von reinem Schüttgut bzw. einer Körnermischung;
• Fig. 2 eine alternative Ausgestaltungsform eines Stützmaterials, bei der das Zusatzmaterial zur Reduktion des Sauerstoffgehalts in der Sinterkammer als poröser Festkörper ausgebildet ist und
• Fig. 3 beispielhaft eine Anordnung, bei der ein zu sinterndes Werkstück in einer Sinterkammer auf dem Stützmaterial aufliegt.

Preferred embodiments of the invention are explained by way of example in the following description of the figures using schematic representations. Show it:

• Fig. 1 enlarged and shown schematically, a support material in the form of pure bulk material or a mixture of grains;
• Fig. 2 an alternative embodiment of a support material, in which the additional material for reducing the oxygen content in the sintering chamber is designed as a porous solid and
• Fig. 3 an arrangement, for example, in which a workpiece to be sintered rests in a sintering chamber on the support material.

In Fig. 1 the support material 3 is completely a grain mixture or bulk material. The ceramic support grains 4 have a completely rounded outer surface. They form the main component of the support material 3 and also have the largest diameter. The additional material 5, which is preferably attached to the carrier substance 6, is located in the spaces between the ceramic support grains 4. The diameters of the grains of the additional material 5 and the carrier substance 6 are significantly smaller than the diameters of the ceramic support grains 4. In the exemplary embodiment of a support material 3 in Fig. 1 it is a mixture of the three components 4, 5 and 6 mentioned.

Fig. 2 shows another embodiment of a support material 3. Here there are only two components, namely the ceramic support grains 4 and the additional material 5. The portion forming the additional material 5 here is a coherent porous body on which the ceramic support grains 4 rest. In this exemplary embodiment, there are no further components of the support material 3 in the spaces between the ceramic support grains. Of course, mixed forms of the in 1 and 2 shown variants possible.

Fig. 3 shows an example of a very simplified sintering chamber 2. Through the inlets and outlets 7, protective gas can be introduced into the interior 8 shielded from the outside atmosphere. By purging the interior 8 with protective gas via the inlets and outlets 7, the interior 8 can be at least substantially filled with an inert gas atmosphere. In order to keep the amount of protective gas required as low as possible, support material 3 is provided in the interior 8 of the sintering chamber 2 in addition to the workpiece 1 to be sintered, in which the workpiece 1 is embedded during the sintering process or on which it rests. At the in Fig. 3 support material 3 shown is a bulk material-like mixture of grains with the three components 4, 5 and 6, as they are basically in Fig. 1 is shown. However, the additional material 5 bound to the carrier substance 6 is essentially down on the Sedimented bottom of the sintering chamber, so that the workpiece 1 is essentially only in direct contact with the ceramic support grains 4. Nevertheless, the additional material 5 bound to the carrier substance 6 is evenly distributed around the workpiece 1 in relative proximity and can thus bind the residual free oxygen present in the protective gas atmosphere in the immediate vicinity of the workpiece 1, so that it does not oxidize the workpiece 1 during it Sintering process is available.

Notwithstanding the in Fig. 3 The arrangement shown can of course also be used with other sintering chambers 2 known in the prior art. Of course, the in Fig. 3 Specifically illustrated version of the support material 3 by other configurations, in particular by the in Fig. 2 variant shown, to be replaced.

Legend to the reference numbers:

1

workpiece

2nd

Sintering chamber

3rd

Support material

4th

ceramic support grains

5

Additional material

6

Vehicle

7

Inlet and outlet

8th

inner space

Claims (3)

1. A method of sintering at least one, in particular dental, workpiece (1) using a protective atmosphere in a sintering chamber (2), wherein the workpiece (1) in the sintering chamber (2) is supported by a support material (3) during the sintering procedure, wherein ceramic support grains (4) together with at least one additive material (5) to reduce the oxygen content in the sintering chamber (2) are used as support material (3), characterized in that as the additive material (5), use is made of a material or a material mix which has a higher oxygen affinity than the workpiece (1) and which has a higher mass-related specific surface than the workpiece (1).
2. A method according to claim 1, characterized in that as the additive material (5), use is made of a material or a material mix which has at least one chemical element or at least one chemical compound which has a higher oxygen affinity than the material of the workpiece (1).
3. A method according to one of claims 1 and 2, characterized in that the ceramic support grains (4) are arranged between the workpiece (1) and the additive material (5).

Images