Sintering Support

Abstract

An object to be sintered is supported on a bed comprising of rolling or flowable particles made from a material that does nor reacts with, and does not adhere to, the object or the surface it is sliding on or the tray holding the particles. For a metallic object ceramic balls are used, rolling on a second metallic surface made of a refractory metal.

Description

FIELD OF THE INVENTION

The invention relates to the art of sintering, in particular sintering metal and ceramic parts from powders.

BACKGROUND OF THE INVENTION

It is well known that when metal or ceramic powders, preferably compressed, are heated to a temperature close to their melting point, the individual powder particles fuse together into a solid object. Metals have to be heated in an inert or reducing atmosphere. This process is accompanied by shrinkage, since the voids between the powder particles have to disappear. The shrinkage is typically between 15 to 20 percent. At the high temperatures used in sintering, the sintered object tends to stick to the plate supporting it inside the sintering furnace, The object being sintered is quite weak at this high temperature, it will often develop cracks or tears during sintering unless it is completely free to move and shrink. This is shown in FIG. 1, where a crack 5 developed because object 1 could not slide freely while shrinking. Because of the high temperature, no liquid lubrication is possible and most dry lubricants, such as graphite, can react chemically with the object being sintered. It is desired to have the object free to move without chemical interaction and without losing the flatness of the bottom part. One prior art is to place the object on a smooth ceramic plate. However, after several sintering cycles the plate loses its smoothness. Another prior art, disclosed in U.S. Pat. Nos. 5,900,208; 4,227,927 and 8,991,211 fully embeds the object in a “flowable” powder or granules. It was found out that even if the powder is loose, the forces than can be generated by the object at sintering temperature are too weak to flow the powder if the it surrounds the object. In the dental industry zirconia is sometimes sintered sitting on a layer of zirconia balls or granules. It was found out that this method is only suitable for very small or light objects, such as zirconia crowns, as the object tends to sinter to the support if both are made of the same material.

SUMMARY OF THE INVENTION

An object to be sintered is supported on a bed comprising of rolling or flowable particles made from a material that does nor reacts with, and does not adhere to, the object or the surface of the tray holding the particles. For a metallic object ceramic balls are used, rolling on a second metallic surface made of a refractory metal.

DESCRIPTION OF THE DRAWING

FIG. 1 is a cross section of an object placed on a bed of balls that are made of the same material as the object, showing the tears than can be created during sintering.

FIG. 2 is across section of an object placed on a layer of balls made of a material different than the object, with the balls supported by a refractory metal tray made of a third material.

FIG. 3 is across section of an object placed on a bed of flowable particulate made of a material different than the object, with the particulate supported by a refractory metal tray made of a third material.

DETAILED DESCRIPTION

Referring to FIG. 1, object 1 is made from metal or ceramic powder that requires sintering to become a dense object. The powder is typically held together by a small amount of polymer-based binder. The binder evaporates or decomposes during the sintering process. To allow object 1 to shrink during sintering without deformation, it has to be able to move freely relative to its support shelf at the sintering temperature. If the object is placed directly on furnace shelf 2, typically made of alumina, it will have a high coefficient of friction at the sintering temperature. Even when the object is placed on ceramic particulate such as ceramic balls 3 rolling on shelf 2, it was found out that the coefficient of friction is too high as the ceramic balls slightly sinter to the ceramic shelf, causing cracks 5 to appear. It was found out that balls 3 have to be made of a material that does not sinter or react with object 1 but also does not sinter or interact with rolling surface 4. In the preferred embodiment for sintering metallic objects, rolling surface 4 is a tray made from a refractory metal such as Molybdenum, Tantalum, Tungsten or Kanthal (Kanthal is a trademark of the Sandvik corporation). For sintering metal objects, the rolling element are typically ceramic balls made of Alumina or Zirconia having a diameter of 1-10 mm. Other forms of rolling elements such as short cylinders can be used. This is shown in FIG. 2, where balls 3 are made of a ceramic material while object 1 and tray 4 are metallic, with tray 4 made of a refractory metal. Another combination that may be used are ceramic balls rolling on a shelf made of hexagonal boron nitride (known as HBN) as HBN does not tend to sinter or adhere to other ceramics even at high temperatures.

FIG. 3 shows object 1 supported on a bed of flowable granules 3 on a tray 4. While it is preferred that the granules are spherical, any shape of granules that allows object 1 to slide as it shrinks can be used. As before, it is best to have the granules 3 made of a material different from object 1 and tray 4. For sintering a metallic object, it is best to use a bed of ceramic granules (for example, alumina, zirconia or hexagonal boron nitride) on a refractory metal tray. The size of the granules or powder can be from several microns to several millimeters.

A secondary problem during sintering is that dust and small particles 6 from the sintered objects accumulate on tray 4. These particles sinter to each other and sometimes to the tray 4, making the granules 3 less mobile. For sintering metals, it is best to use a tray material that is covered by an oxide layer, such as Kanthal. Such an oxide layer prevents particles 6 from bonding to the tray.

The term “shelf” in this disclosure should be understood to mean any horizontal surface used to support the sintered object. The shelf can comprise of several layers, such as a metal tray on top of a ceramic plate. The word “tray” in this disclosure should be interpreted broadly as any surface supporting the granular matter used to support the object being sintered.

Example #1

The ceramic and metal balls used in all the examples are available from suppliers of blasting and milling media such as www.precisionfinishing.com

An object to be sintered made of type 17-4 stainless steel powder (30 um particle size) and 1% of Poly Vinyl Alcohol binder was placed on a single layer of Zirconia (YSZ type) balls having a diameter of 2 mm. The weight of the object was about 1 Kg. The balls were free to roll on a 1 mm thick Molybdenum sheet. The object was sintered in hydrogen at 1350 degrees C. with very low distortion and no cracks.

Example #2

An object to be sintered was made type 17-4 stainless steel powder (30 um particle size) and 1% of Poly Vinyl Alcohol binder was placed on a single layer of alumina balls having a diameter of 3 mm. The weight of the object was about 1 Kg. The balls were free to roll on a shelf made of HBN. The object was sintered in hydrogen at 1350 degrees C. with very low distortion and no cracks.

Example #3

An object to be sintered was made type 17-4 stainless steel powder (30 um particle size) and 1% of Poly Vinyl Alcohol binder was placed on a bed of Hexagonal Boron Nitride (HBN) powder with average particle size of 50 um. The powder was supported on a tray made of Kanthal. The object was sintered in hydrogen at 1350 degrees C. with very low distortion and no cracks.

Example #4

An object to be sintered was made type 17-4 stainless steel powder (30 um particle size) and 1% of Poly Vinyl Alcohol binder was placed on a bed of zirconia balls having a diameter of 2 mm. The weight of the object was about 1 Kg. The balls were free to roll on a 3 mm thick tray made of Kanthal APMT. The object was sintered in hydrogen at 1350 degrees C. with very low distortion and no cracks.

Example #5

An object to be sintered was made of commercial porcelain paste (available from pottery supplies). It was supported on 2 mm diameter Tungsten balls. The weight of the object was about 0.3 Kg. The balls were free to roll on an alumina shelf. The object was sintered in air at 1200 degrees C. with very low distortion and no cracks.

Comparative Example

An object to be sintered made of type 17-4 stainless steel powder (30 um particle size) and 1% of Poly Vinyl Alcohol binder was placed on a single layer of Zirconia (YSZ type) balls having a diameter of 2 mm. The weight of the object was about 1 Kg. The balls were free to roll on a zirconia shelf. The object was sintered in hydrogen at 1350 degrees C. It had severe distortion and several cracks. After sintering it was found out that the object did not move freely as the ceramic balls partially adhered to the ceramic shelf.

Claims

1. A method of supporting a metallic object being sintered, the method comprising of: supporting the object on a bed of flowable ceramic granules, andsupporting said granules on a tray made of a refractory metal.

2. A method of supporting an object during sintering, the method comprising of placing the object on a horizontal bed of flowable granules made of a material that is different than the material being sintered.

3. A method of supporting a ceramic object being sintered, the method comprising of: supporting the object on a bed of metallic granules, andsupporting said granules on a metallic tray.

4. A method as in claim 1 wherein the bed comprises of a single layer of spherical granules.

5. A method as in claim 1 wherein the bed comprises of multiple layers of spherical granules.

6. A method as in claim 1 wherein the tray is made of Kanthal APMT.

7. A method as in claim 1 wherein the granules are zirconia spheres.

8. A method as in claim 1 wherein the granules are alumina spheres.

9. A method as in claim 1 wherein the granules are Hexagonal Boron Nitride powder.

10. A method as in claim 2 wherein the granules are Hexagonal Boron Nitride powder.

11. A method as in claim 2 wherein the granules are supported by a ceramic tray.

12. A method as in claim 2 wherein the granules are supported by a tray made of Hexagonal Boron Nitride.

13. A method as in claim 3 wherein the granules are Tungsten spheres.

14. A method as in claim 1 wherein the tray material does not form a bond with the metal the object is made off.