Abstract:
The invention relates to a device ( 10 ) and a method for sintering sinter products, such as oxidation-prone materials, in particular metallic sintered goods ( 46 ), in particular a dental framework, comprising a tray ( 18 ), which accommodates the sinter product and is arranged on a base plate ( 14 ), a pot-shaped cover ( 16 ), which surrounds the tray, with a rim ( 36 ) that is sealed towards the base plate, as well as supply and discharge openings ( 38 ) for protective gas connected to the interior chamber ( 30 ) that is surrounded by the pot-shaped cover. In order to facilitate sintering even at temperatures above 1200° C. without any problems, it is suggested that the tray ( 18 ) be covered by a capping element ( 20 ), whereby when the tray is covered by the capping element, the interior chamber ( 34 ) of the tray is connected in a gas-flow-allowing manner to the interior chamber of the cover.

Description:
This application claims priority to German patent application number 10 2011 056 211.7, filed Dec. 9, 2011, the contents of which are incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a device for sintering sinter products, comprising a tray accepting the sinter products that is arranged on a base plate, an interior chamber accommodating the sinter products as first interior chamber, a pot-shaped cover, which surrounds the tray and possesses a rim that is sealed towards the base plate, with a protective-gas supply and discharge opening connecting to an interior chamber encompassed by the pot-shaped cover as the second interior chamber, as well as a sintering chamber, which surrounds the pot-shaped cover and the base plate, as the third interior chamber. 
     The invention further relates to a method for sintering sinter products, such as materials prone to oxidation, in particular metallic sinter products, in particular in form of a dental framework, using a device that comprises a tray arranged on a base plate, with an interior space that accepts the sinter products as the first interior chamber, a pot-shaped cover, which encompasses the tray and has a rim that is sealed towards the base plate, as well as openings for discharge and supply of protective gas connecting to the interior chamber that is surrounded by the cover as the second interior chamber, whereby protective gas flows through the second interior chamber surrounded by the pot-shaped cover. 
     A device and method of the above-mentioned type are described in DE 20 2010 007 606 U1 and DE 20 2010 002 533 U1. For this purpose, dental frameworks consisting of a silver palladium alloy or chromium cobalt alloy are sintered in a tray, which consists of quartz and is supported at elevation on a fireclay block in a sintering furnace. The fireclay block and the tray are surrounded by a quartz vessel that is sealed by a graphite gasket towards a base plate, on which is arranged the fireclay block. Both the base plate and the fireclay block have bore-hole openings, in order to be able to flush the interior chamber, which contains the quartz tray and is surrounded by the quartz vessel, with a protective gas such as argon. In order to prevent oxidation of the frameworks, the sintering is performed in a protective gas atmosphere. Since an undesirable corrosion of the quartz takes place at temperatures around 1200° C., one coats this in advance with a boron nitride spray. The material to be sintered is placed in the tray of inert pellets of corundum, alumina, or zirconia. 
     This device suffers from the disadvantage that it can not be used at temperatures above 1200° C.; since on the one hand the life expectancy of quartz is highly limited at these temperatures and on the other, direct contact with finger grease as a result of handling the quartz materials leads to spalling. 
     DE 20 2011 106 734 U1 relates to a device for the oxygen-free sintering of metal or ceramics and comprises a base plate, upon which is supported a dome. Within the dome, a sintering crucible, into which sinter products may be placed, is supported via a support plate. Also present are openings for the supply and discharge of protective gas. As suitable materials for the device are listed quartz glass or re-crystallized silicon carbide. 
     WO 94/16642 A1 relates to a furnace, in which is arranged a furnace chamber, which on the fore-side is covered by a lid, through which gas is allowed to flow when protective gas is introduced into the furnace chamber. 
     Subject matter of EP 2 101 133 A1 is a sintering furnace for dental preparations. 
     DE 10 2008 012 578 A1 relates to a dental furnace, which is used to perform various heating cycles. An overshoot of the heating output may occur during this. 
     DE 20 2011 005 465 U1 relates to a device for the oxygen-free sintering of metal or ceramics. In this, the product to be sintered in a sintering container is embedded in sintering granules, which for example may consist of zircon granules. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to further develop a device and a method of the above-mentioned type in such a way that the disadvantages of the state of technology are avoided, and in particular a sintering at temperatures above 1200° C. can be performed without any problems. At the same time it should be ensured that undesired discoloration or oxidation of the sinter products are ruled out. 
     With respect to the device this objective is essentially met by the tray being covered by a capping element, and that the first interior chamber is connected to the second interior chamber in a gas-flow-allowing fashion when the tray is covered by the capping element. 
     The invention further relates to a device for sintering of sinter products goods that comprises a tray, which accepts the sinter product, is arranged on a base plate, and possesses an interior chamber as first interior chamber that accepts the sinter product, a pot-shaped cover encompassing the tray with a rim that is sealing with respect to the base plate, with an opening for the supply and discharge of protective gas connecting to the interior space as second interior chamber surrounded by the pot-shaped cover, as well as the sintering chamber, which surrounds the pot-shaped cover with the base plate, as third interior chamber, which is characterized in that the tray is covered by a capping element, and that the first interior chamber is connected in a gas-flow admitting fashion to the second interior chamber when the tray is covered by the capping element. 
     In accordance with the invention&#39;s teaching, sintering of the sinter products takes place in the interior space surrounded by the tray that is referred to as first interior chamber, whereby the tray is covered by a capping element. Irrespective of this, inert gas can flow through the first interior chamber, but the risk of interference such as oxygen reaching the interior of the tray are reduced to a large degree. 
     It in particular is intended that the cover be supported on the base plate directly on its rim in a gas-tight or form-fitting fashion. The rim of the cover as well as the base plate are surface-ground to such a degree that the cover can rest directly on the base plate without any additional sealing, in order to obtain a sealing to such a degree that ingress of oxygen into the interior space surrounded by the cover and referred to as second interior chamber is ruled out or ruled out to a great extent. 
     These measures serve to ensure that no oxygen or almost no oxygen ingresses from the surroundings into the (second) interior chamber of the cover, that also may be referred to as cone, and consequently none or almost none ingresses into the (first) interior chamber surrounded by the tray. 
     It should also be emphasized that at least the tray and its capping element, such as a lid, but in particular the base plate, the tray, the capping element, and the pot-shaped, hood-like, or cone-shaped cover, consist of a material that is a member of the group made up of SiC, SiN. This choice of material offers the advantage that sintering can be performed at temperatures above 1200° C., in particular up to 1350° C., without the materials suffering any damage. Particularly preferred is the use of SiC, since it possesses a reducing effect with respect to oxygen. 
     The inert gas conducted into the space between the tray and the pot-shaped cover, which in particular should be argon but possibly may be nitrogen, in accordance with a further development of the invention can be conducted from the second interior chamber, which is encompassed by the cover, directly into the sintering chamber surrounding the base plate and the cover, i.e. into the interior space of a sintering furnace referred to as third interior chamber. This results in a further drop of the concentration of oxygen in the sintering chamber (third interior chamber) of the sintering furnace. Also, ingress of oxygen into the (second) interior chamber of the cover is impeded. 
     In a further development of the invention it is intended that the tray be supported on a ring that possesses openings, which in turn is arranged on a base plate, whereby preferably through the base plate inside of the ring passes the protective-gas supply-line opening and possibly the protective-gas discharge-line opening. 
     As an alternative it is suggested that from the tray or its bottom wall originate projections, preferably at least three projections, which are arranged uniformly around a circle and serve to support the tray on the base plate. 
     These measures ensure that the base plate can possess a thickness that is the same anywhere on the entire surface, so that one does not face the risk of fissuring due to the temperature changes occurring during sintering. 
     The invention is further characterized in that the product to be sintered is supported in the tray on bulk material that consists of solid balls of ceramics, in particular zirconium oxide or aluminum oxide. Zirconium dioxide offers the advantage that under the exclusion of oxygen this is partially converted to zirconium monoxide. The voids generated by the lack of oxygen result in a dark discoloration. 
     Surprisingly this initial oxygen release has no negative effect and in the following can be used in a reducing and indicating role, since this effect is reversible in case of an ingress of oxygen. 
     As a result of the invention&#39;s teaching one can ensure that sintering of metal alloys, in particular cobalt chromium alloys such as cobalt-chromium-molybdenum alloys, can be performed at temperatures of 1200° C. and above, in particular 1250° C., without any risk of oxidation or uncontrolled discoloration. In this, the sinter products are arranged in a tray with a capping element capping the latter, whereby it is ensured that the (first) interior chamber of the tray that accepts the sinter product is flushed with a protective gas in order to flush out any oxygen that might be present. The capping element acts in a lid-like manner or is a lid. 
     If in a preferred manner the capping element covering the tray is arranged on the tray in a non-sealing fashion, a sealing contact would also be possible as long as fine bore holes, e.g. created by lasers, are provided, e.g. in the capping element itself, that allow protective gas exchange. 
     A further option is to let protective gas—which was conducted to the outside from the (second) interior chamber of the cover—flow around the pot- or bell-shaped cover, i.e. the so-called cone. 
     It is in particular of advantage that the materials are suitable for high-temperature sintering, whereby silicon carbide is to be emphasized, in particular because of its reducing action. Alternatively one also can use silicon nitride. 
     A method of the above-mentioned type is also characterized in that the tray—after the sinter product has been placed in it—is covered by a capping element, so that through this or at least through an opening in the capping element or through an opening in the tray, protective gas can enter the interior of the tray, i.e. the first interior chamber, and that the (second) interior chamber of the pot-shaped cover is charged with protective gas at an excess pressure that rules out lifting the cover, in particular with an excess pressure p with 1 mbar≦p≦25 mbar, in particular 2 mbar≦p≦mbar above ambient pressure. 
     In particular it is intended that as sinter product be used a larger object, in particular a dental bridge framework, in particular a bridge framework with at least three bridge elements, preferably at least five bridge elements, and that the sinter product be heated in a sintering chamber, which accommodates the device, i.e. surrounds it, from room temperature Tz to a temperature T 1  with 800° C.≦T 1 ≦1100° C. at a heating rate R 1  with 5 K/min≦R 1 ≦100 K/min, in particular 20 K/min≦R 1 ≦80 K/min, after a possible holding time t 1  at the temperature T 1  with 1 min≦t 1 ≦10 min is heated to a temperature T 2  with 1200° C.≦T 2 ≦1350° C. at a heating rate R 2  with 5 K/min≦R 2 ≦30 K/min, the sinter product is held at the temperature T 2  for a time period t 2  with 5 min≦t 2 ≦120 min, in particular 15 min≦t 2 ≦50 min, whereby subsequently, i.e. after the holding time t 2 , the sinter products possibly are heated to a temperature T 3  with T 3 &gt;T 2  to melt the surface of the sinter product, and subsequently from the temperature T 2  or T 3  is cooled to a temperature below 400° C. at a cooling rate R 3 , which preferably at the start of the cool-down amounts to 5 K/min≦R3≦100 K/min. This is followed by cooling to room temperature Tz. 
     The invention also intends that the sinter product be heated in a sintering chamber surrounding the device from room temperature to a temperature T 2  such as 1200° C.≦T 2 ≦1350° C. at a heating rate R 1 , be kept at the temperature T 2  for a time period t 2  with 5 min≦t 2 ≦220 min, in particular 15 min≦t 2 ≦60 min, whereby possibly subsequently, i.e. after the holding time, the sinter products are heated to a temperature T 3  with T 3 &gt;T 2  to melt the surface of the sinter product, in order to be cooled subsequently from the temperature T 2  or T 3  to a temperature above 400° C. at a cooling rate R 3 , whereby the cooling rate R 3  preferably at least at the start of the cool-down is 5 K/min≦R 3 ≦100 K/min. 
     In this, it in particular is intended that the heating rate R 1  be set to a value with 5 K/min≦R 1 ≦100 K/min, in particular 20 K/min≦R 1 ≦80 K/min. 
     The heating to a temperature T 1  and the possibly performed holding at the temperature T 1 , to be subsequently further heated to a temperature T 2 , is the preferred treatment for larger objects such as bridge frameworks. 
     The initially occurring heating to the temperature T 1  and subsequently to the temperature T 2  with a potentially different heating rate can be modified so that heating to the temperature T 2  takes place immediately, if the products to be sintered are smaller objects, such as a framework for a tooth. 
     In this, the short-term melting of the surface, which can be performed independently of the size of the object, represents a suggestion with its own inventive merit that may be applied even if one uses in the sintering a device that deviates from the invention&#39;s teaching. 
     It may be intended additionally that after cooling of the sinter products to the temperature T 1 , the base plate with the tray, the latter&#39;s capping element, and the cover, are removed from the sintering chamber at least in parts but preferably entirely. The sintering chamber may be the interior chamber of a sintering furnace, which is referred to as the third interior chamber. 
     Independently of the temperatures and heating rates specified above, one has to emphasize as a feature with its own independent inventive merit that it is possible to perform an additional heating step after the dense sintering, as a result of which the surface of the sintered product melts and one achieves desired surface characteristics. 
     The high temperatures present in the interior of the tray for a short time period result in a melting of the surface, so that for frameworks the surface will have an appearance that is very similar to that of frameworks subjected to dental polishing. 
     In particular, it is intended that the protective-gas-inlet or protective-gas-discharge opening be connected to a supply or discharge line consisting of aluminum oxide. In this, the line may be connected to the base plate using a high-temperature bonding-agent, in particular on aluminum oxide basis. 
     In the choice of SiC as material for the base plate, the cover, the tray and its capping element one makes use of the good thermal conductivity and the nearly complete leak tightness of the material. As a result of this, temperature differences within the components are minimized, which reduces thermal stresses. Consequently, rapid temperature changes are even feasible for large components, e.g. with a diameter of 100 mm. In addition, SiC exhibits a reducing effect and is able to convert residual oxygen in the atmosphere together with the carbon present to form carbon monoxide. This effect is still in effect. But at the same time, no measurable reduction of the wall thickness can be detected. 
     Placing the sinter products in a sintering tray that is not closed seal-tight improves the sintering results. The reason for this improvement may be the creation of an internal chamber that possesses walls with a reducing action with respect to oxygen. The interference by oxygen is consequently attenuated. Interference of this type is not longer able to reach the sinter product directly. The probability of the interference being flushed out and consequently being moderated is higher. 
     The controlled supply and discharge of inert gas ensures that the internal pressure in the cover can not increase to such a degree that the latter is lifted. In this manner one facilitates the exclusion of oxygen. Instead of one or several protective gas discharge line openings in the base plate, it is possible that at least one opening formed by laser is present in the cover, in order to discharge inert gas in a controlled manner. 
     Deviating form conventional techniques, no hollow balls are used in the support of the sinter products. Hollow balls can store oxygen and consequently at high temperatures contaminate the atmosphere in immediate proximity to the goods to be sintered. The invention employs dense balls that can not store oxygen. In this, zirconium oxide has surprisingly been found to be suitable as material for the balls, even though initially it tends to release oxygen in low-oxygen atmospheres. After the oxygen release is complete, e.g. achieved by a temperature cycle, the corresponding solid balls of zirconium oxide exhibit a reducing effect for oxygen. 
     In order to position the cover and the tray in their proper positions on the base plate, the state of technology usually intends for the presence of steps. The invention breaks away from this and employs a level plate, which in a simple manner can be polished in the regions of the contact areas to the cover, so that sealing takes place to such a degree that an ingress of oxygen on principle is excluded or substantially excluded. Since no steps are present, there will be no thickness variations in the base plate, so that thermal stresses consequently are reduced. 
     Further details, advantages, and features of the invention are not only found in the claims, the characteristic features described therein—individually or in combination—but also in the following description of preferred embodiment examples illustrated in the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a first embodiment variant of a device for sintering sinter products, 
         FIG. 2  shows a second embodiment variant of a corresponding device, and 
         FIG. 3  shows a diagram of temperature as a function of time. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 and 2 , which on principle use identical reference labels for equal elements, show in respective purely schematic illustrations an embodiment variant of a device  10 ,  100  that is used to sinter metallic sinter products, in particular dental frameworks. As is schematically illustrated in  FIG. 1 , the device  10 —and accordingly the device  100 —is located in an interior or sintering chamber  12  of a sintering furnace, of which wall sections  15 ,  17  are schematically illustrated. In the sintering chamber  12 , which is also referred to as third interior chamber, one sets the temperatures that are required to sinter to the required degree the sinter product present in the device  10 ,  100 . 
     The device  10  consists of a base plate  14 , a pot-shaped or dome-shaped cover  16 , to be referred to as cone, a tray  18  with a U-shaped cross-section, as well as a lid  20  referred to as capping element, by means of which the tray  18  on principle is sealed in a less than complete manner. 
     Further, the tray  18  is supported on the base plate  14  via a ring element  22 . The ring element  22  possesses cut-outs  24 ,  26 ,  28 , so that a connection exists between the interior chamber  30  surrounded by the cone  16 —referred to as second interior chamber—and the chamber  32  surrounded by the ring element  22 . The sinter product—not illustrated in  FIG. 1 —is arranged in the interior chamber  34  surrounded by the tray  18  and capped by the lid  20 , referred to as the first interior chamber. 
     According to the invention&#39;s teaching, the base plate  14 , the cone  16 , the tray  18 , the lid  20 , as well as the ring element  22 , preferably are manufactured from SiC, even though SiN can be considered as an alternative material. 
     The base plate  14  and the circumferential rim  36  of the cone  16  have been ground flat to such a degree that one can be sure of a form-fitting contact of the cone  16  on the base plate  14 . This in principle prevents the ingress of oxygen. 
     In accordance with the invention, the lid  20  should not completely seal the (first) interior chamber of the tray  18 , so that a flow-allowing connection exists between the (second) interior chamber  30 , which extends between the tray  18  and the cone  16 , and the first interior chamber  34  that is surrounded by the tray  18 . If the lid  20  rests upon the tray  18  in a sealing manner, then the lid  20  possesses at least one opening, so that a flushing of the interior chamber  34  of the tray  18  can take place. The equivalent would be achieved by an opening in the tray  18 . 
     In order to prevent oxidation and discoloration, a protective gas such as argon or nitrogen is fed into the second interior chamber  30  via an opening  38  that in the embodiment example is provided in the base plate  14 . This protective gas reaches the first interior chamber  34  that is surrounded by the tray  18 , since, as already mentioned, the lid  20  does not close the tray  18  in a sealing manner. Alternatively or additionally the protective gas travels via the at least one opening in the lid  20  and/or in the wall of the tray  18 . 
     The gas admitted to the second interior chamber  30  subsequently flows out via an opening that preferably is also present in the base plate  14 . But it is also possible to create an opening in the circumferential wall of the cone  16 , e.g. by means of a laser, through which gas discharges. In this, the out-flowing gas preferably is conducted into the sintering chamber  12 —i.e. the third interior chamber—in such a manner that the protective gas flows around the cone  16  at least in the region of the latter&#39;s circumferential rim  36 . 
     Due to the fact that the lid  20  rests on the tray  18  in a non-sealing manner, protective gas can flow into the first interior chamber  34 , which is surrounded by the tray  18  and in which the sinter products are located. At the same time, the ingress of oxygen (interference) is reduced. The equivalent applies with respect to the at least one opening. 
     The (second) interior chamber  30  should be at a higher pressure than the surroundings, whereby an excess pressure burden between 1 mbar and 25 mbar, in particular between 2 mbar and 10 mbar is preferred. 
     The embodiment example of  FIG. 2  differs from the one of  FIG. 1  in that the tray  18  is not supported on a ring  22 , but rather via projections  42 ,  44  that rise from the bottom wall  40  of the tray  18 . In this there are provided in particular three projections that are arranged uniformly distributed around a circle. In all other respects the embodiment variant corresponds to that of  FIG. 1 , so that the reader is referred to the respective explanation. 
     Supporting the tray  18  on the ring  22  or via the projections  42 ,  44  realizes the advantage that the base plate  14  possesses a uniform thickness, so that one avoids thickness variations, which consequently reduces internal stresses. 
     In order to facilitate that during the sintering, the sinter product  46  located in the first interior chamber  34  not touch the inner surface of the tray  18 , a spheroidal bulk matter  50  consisting of solid balls, i.e. not of hollow balls is introduced to the interior side  48  of the bottom wall  40 , i.e. on the bottom surface. Materials that are preferred for this are aluminum oxide or zirconium oxide. The solid balls offer the advantage that oxygen can not be stored. This is also true for zirconium oxide balls, even though they initially tend to release oxygen, namely in atmospheres low in oxygen. However, after a completed oxygen release, they exhibit a reducing action. 
     The preferred dimensions of the components, consisting in particular of silicon carbide, are:
     Base plate  14 : Diameter 90 mm to 110 mm, Thickness 2 to 4 mm;   Cone  16 : Outer diameter 95 mm to 105 mm, wall thickness 3 mm to 5 mm, height 50 mm to 55 mm;   Ring element  22 : 4 mm to 8 mm, outer diameter 60 mm to 70 mm, wall thickness 3 mm to 5 mm;   Sintering tray  18 : Height 30 mm to 35 mm, outer diameter 80 mm to 90 mm, wall thickness 3 mm to 5 mm;   Lid  20 : same outer diameter as tray  18 , thickness in the border region: 2 mm to 5 mm, Central thickness 4 mm to 8 mm   

     As is apparent in the drawing, the distance between the outer surface of the tray  18  and the inner surface of the cone  16  can be chosen to be small. This offers the advantage that this additionally inhibits the ingress of oxygen into the (first) interior chamber  34  of the tray  18 , in particular if the protective-gas discharge opening is located in and passes through the edge region of the cone  16 . 
       FIG. 3  shows a heating and cooling curve used in the sintering of the sinter products  46 . The curve of  FIG. 3  applies to an object of larger size that is to be sintered. As example one could mention a dental bridge framework with 4 elements. 
     According to the minimum basic principle, the sintering furnace and consequently the sintering chamber  12 , and consequently also the product to be sintered  46 , initially is heated from room temperature T z  to a temperature T 1  whereby T 1  is between 800° C. and 1100° C. if cobalt-chromium alloys are used as material for the sinter product  46 . The heating rate preferably should be between 20 K/min and 80 K/min. The sinter product  46  is held at the temperature T 1  for a time period between 1 min and 10 min. Next takes place a heating from the temperature T 1  to a temperature T 2  between 1200° C. and 1350° C. at a heating rate between 5 K/min and 30 K/min. The sinter product  46  is held at the temperature T 2  for a time period t 2  between 5 and 120 min and subsequently is cooled to a temperature below 400° C., whereby one must at least choose a cooling rate between 5 K/min and 100 K/min. 
     Subsequently cooling to room temperature takes place, whereby preferably the device  10 ,  100  is removed from the sintering chamber  12 . For this purpose, the device  10 ,  100  can be lowered, as is symbolized by the double arrow  52 . If required, the sinter product can after cooling to the temperature T 1  be held at the temperature T 1  for a time period between 1 min and 10 min. This is followed by a cooling to room temperature Tz. 
     As is evident in the schematic illustration of  FIG. 3 , after reaching the sintering temperature T 2 , the sintered product may for a short time period be heated to a higher T 3  (dashed region), in order to effect a melting of the surface. The short-term deliberate melting can be performed during the holding time t 2 , preferably after the holding time t 2 . The latter is illustrated in  FIG. 3 . 
     Heating to the temperature T 1 , holding at this temperature, and subsequently the further heating to the temperature T 2  with possibly deviating heating rate for the sintering of smaller-size objects, such as a framework for a single tooth, is not required. Rather, one can heat directly from room temperature to the temperature T 2 . Independently hereof, one also has the option after the holding at the temperature T 2  to implement a short-term temperature increase to a temperature T 3 , in order to achieve a melting of the surface of the sintered product.