To form a die part for use in the production of a hot pressed component, boron nitride powder is mixed with a refractory, non-sinterable powder other than boron nitride, and an organic carrier liquid to produce a slurry. The slurry is then caused to flow into the shape of said one die part, and the organic carrier liquid is removed from the slurry to produce a powder compact, which is compressible during hot pressing, the compact being of the shape of said one die part and of substantially uniform density.

Current U.S. Class:	419/48 ; 76/107.1
Current International Class:	C04B 33/32 (20060101); B22F 3/12 (20060101); C04B 35/583 (20060101); C04B 35/584 (20060101); C04B 35/593 (20060101); B22f 003/24 ()
Field of Search:	29/420,420.5,423 264/111,122,219,319,65 76/17R

Holman & Stern

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Claims

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We claim:

1. A method of producing a hot pressed component, comprising the steps of:

i. forming a first, hot pressing die part suitable for only a single hot pressing operation by;

a. mixing boron nitride powder with a refractory, non-sinterable powder other than boron nitride and an organic carrier liquid to produce a slurry,

b. causing the slurry to flow into the shape of said first die part, and

c. removing the organic carrier liquid from the slurry to produce said first die part in the form of a powder compact which is compressible during hot pressing and is of substantially uniform density,

ii. positioning said first die part together with a second, hot pressing die part within a die cavity and with material to be hot pressed being received between the die parts,

iii. applying pressure to the die parts at an elevated temperature to hot press the material into the required hot pressed component and simultaneously hot press said first die part, the ratio of the density of said first die part, after hot pressing, to the density of the first die part, prior to hot pressing, being substantially equal to the ratio of the density of the hot pressed component to the density of said material to be hot pressed, and

iv. separating the die parts from said hot pressed component.

2. A method as claimed in claim 1, wherein the particle size of the boron nitride powder is not less than 50 microns.

3. A method as claimed in claim 1 wherein the refractory, non-sinterable powder is silicon carbide, magnesium oxide, or aluminium oxide.

4. A method as claimed in claim 3 wherein the refractory material is silicon carbide and said material to be hot pressed is silicon nitride, and there is provided between said first die part and the silicon nitride a further material which, at said elevated temperature, forms a substantially non-porous, continuous barrier layer adjacent the surface of the silicon nitride to prevent reaction between the silicon nitride and said first die part.

5. A method as claimed in claim 1 wherein said slurry is moulded around said material to be hot pressed so as to produce, on removal of said organic carrier liquid said first and second die parts with the material to be hot pressed received therebetween, each of said die parts being compressed during step (iii).

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Description

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This invention relates to a method of producing a hot pressed component, the method being of the kind in which a pair of die parts are positioned within a die cavity with material to be hot pressed being received therebetween, and the die parts are used to transmit pressure to the material at an elevated temperature so as to hot press the material into the required hot pressed component.

According to the invention, a method of the kind specified includes the step of forming at least one of the die parts by:

A. MIXING BORON NITRIDE POWDER WITH A REFRACTORY, NON-SINTERABLE POWDER OTHER THAN BORON NITRIDE AND AN ORGANIC CARRIER LIQUID TO PRODUCE A SLURRY,

B. CAUSING THE SLURRY TO FLOW INTO THE SHAPE OF SAID ONE DIE PART, AND

C. REMOVING THE ORGANIC CARRIER LIQUID FROM THE SLURRY TO PRODUCE A POWDER COMPACT, WHICH IS COMPRESSIBLE DURING HOT PRESSING, SAID COMPACT BEING OF THE SHAPE OF SAID ONE DIE PART AND OF SUBSTANTIALLY UNIFORM DENSITY.

Preferably, the ratio of the density of said one die part, prior to hot pressing, to the density of said one die part, after hot pressing, is substantially equal to the ratio of the density of the material to be hot pressed to the final density of the hot pressed component.

Preferably, the particle size of the boron nitride powder is not less than 50 microns, and conveniently the particle size of the refractory powder is also not less than 50 microns.

In a first example of the invention, iso-propyl alcohol was mixed with 57 parts by weight of silicon carbide powder and 43 parts by weight of boron nitride powder to produce a slurry containing 64 percent by weight of solids. The silicon carbide powder used in producing the slurry was that sold by Carborundum Company Limited as type CG4k2 and had a particle size varying between 150 and 200 microns. The boron nitride powder used was that sold by New Metals and Chemicals Limited and had a particle size varying between 50 and 150 microns. The slurry was caused to flow into a mould, defining the shape of a hot pressing die part, by using a vibrator and was then dried to produce a compressible powder compact of the shape of the required die part and of substantially uniform density.

Tests were carried out on the compressibility of a number of powder compacts produced by the above method by applying a pressure of 1.5 tons/square inch to the compacts at temperatures varying between 25.degree.C and 1,720.degree. C. In each case, the ratio of the density of the compact prior to hot pressing to the density of the compact when pressed to full densification was 2.0;1, irrespective of the temperature of the pressing operation. Powder compacts produced according to the above method were therefore found to be particularly suitable for use as die parts in the hot pressing of ceramic materials, such as silicon nitride.

Thus in one practical embodiment a pair of die parts produced according to the above method were successfully employed to hot press compacted silicon nitride powder at a temperature of 1,700.degree.C and a pressure of 1.5 tons/square inch the resultant silicon nitride product being substantially of full theoretical density, that is 3.2 gm/c.c. The compacted silicon nitride powder was preferably arranged to have an initial density of 1.6 gm/c.c., which resulted in the ratio of the density of the compacted silicon nitride to the density of the hot pressed component being the same as the compression ratio of each of the compressible die parts. In this way, it was possible to ensure that the space defined between the die parts, which received the compacted silicon nitride, conformed exactly to the shape of the silicon nitride throughout the hot pressing operation. It was, however, found to be necessary, when employing the die parts according to the above example to hot press silicon nitride, to provide between the silicon nitride and the die parts a material which produced at the hot pressing temperature a substantially non-porous continuous barrier layer at or near the surface of the silicon nitride so as to prevent reaction between the silicon nitride and the silicon carbide in the die parts. A suitable material in this respect was found to be alumina which, at the hot pressing temperature, produced at the surface of the silicon nitride a protective layer which was believed to be silicon aluminium oxynitride. After hot pressing the silicon aluminium oxynitride layer was either removed or more preferably was retained since it was found to improve the high temperature properties of the resultant hot pressed silicon nitride product.

In a second example, 49 parts by weight of the silicon carbide powder used in the previous example and 51 parts by weight of the boron nitride used in that example were mixed with iso-propyl alcohol to produce a slurry containing 66 percent by weight of solids. As before, the slurry was then formed into the required die part which was found to have a compression ratio of 2.25:1 at an applied pressure of 1.5 tons/square inch, the compression ratio remaining constant between 25.degree.C and 1,720.degree.C. Again, the resultant die part was found to be useful for hot pressing ceramic materials, although as in the previous example when used to hot press silicon nitride, it was necessary to provide a barrier layer between the silicon nitride and the die parts at the hot pressing temperature.

In a third example, iso-propyl alcohol was mixed with 56 parts by weight of silicon carbide powder and 44 parts by weight of boron nitride powder to produce a slurry containing 72 percent by weight of solids. In this example, the silicon carbide had a particle size varying between 50 and 75 microns, whereas the particle size of the boron nitride powder varied between 100 and 200 microns. A die part produced from this slurry was found to have a compression ratio of 2.0:1 at an applied pressure of 1.5 tons/square inch, the compression ratio remaining constant as before between 25.degree.C and 1,720.degree.C. The die part behaved in the same way as the die parts produced according to the previous examples when used to hot press ceramic materials.

In a fourth example, 54 parts by weight of magnesium oxide powder and 46 parts by weight of boron nitride powder were mixed with iso-propyl alcohol to produce a slurry containing 70 percent by weight of solids. The boron nitride powder was the same as that used in the previous example, whereas the magnesium oxide powder was that supplied by Thermal Syndicate Limited and had a particle size which varied between 75 and 150 microns. As before, a die part was produced from this slurry and the compression ratio of the die part was measured in the same way as in the previous examples. The compression ratio was found to be 2.0:1 over a wide range of temperatures so that again the die part was useful for hot pressing ceramic materials. In the case of hot pressing silicon nitride, it was found that successful results were obtained without the presence of a protective material between the silicon nitride and the die parts at the hot pressing temperature.

In a fifth example, a slurry was formed by mixing iso-propyl alcohol with 64 parts by weight of magnesium oxide powder and 36 parts by weight of boron nitride powder, both the powders being those used in the fourth example. The slurry so formed contained 72 percent by weight of solids and a die part produced from this slurry was found to have a compression ratio of 1.78:1 at an applied pressure of 1.5 tons/square inch.

In a sixth example, a slurry containing 60 percent by weight of solids was produced by mixing 49 parts by weight of aluminium oxide powder and 51 parts by weight of boron nitride powder with iso-propyl alcohol. The boron nitride powder was that used in each of the third to fifth examples, whereas the aluminium oxide powder was supplied by Universal Abrasives Limited and had a particle size in the region of 230 microns. Again a die part produced from this slurry had a compression ratio of 2.0:1 at an applied pressure of 1.5 tons/square inch and was found to be suitable for the hot pressing of silicon nitride without the provision of an intervening protective layer.

In using the die parts of each of the above examples to hot press a powdered ceramic material, it was found to be desirable to initially compact the powdered ceramic into a preform and then mould the slurry of the particular example around the preform in the hot pressing die cavity so as to produce, after removal of the iso-propyl alcohol, the die parts rquired for the hot pressing.

It is, however, to be appreciated that each of the die parts produced in the above examples was suitable for effecting only a single hot pressing operation since each die part was necessarily compressed and permanently deformed during hot pressing.

It was also found to be preferable to ensure that the particle size of the boron nitride powder, and also of the refractory non-sinterable powder, was not less than 50 microns. The upper limit on the particle sizes on these powders was determined by the surface imperfections which could be tolerated in the die parts produced therefrom.

It is to be appreciated that, although iso-propyl alcohol was used as the organic carrier liquid in all the above examples, it was found that other organic liquids could be successfully employed as the carrier medium. Suitable carrier media were those which were readily available as "dry" liquids, that is containing less than 0.05 percent water, and which evaportated on heating completely without decomposition and without leaving a carbonaceous residue. The carrier medium was, of course, also required to be inert to the boron nitride powder and to the particular refractory, non-sinterable powder employed with the boron nitride, as well as being able to produce a slurry of the required consistency for casting into the die parts. It was preferred to use organic liquids having reasonably high boiling points, since this allowed more time for preparation and casting of the slurry before evaporation occurred. Thus examples of other organic liquids which could have been employed as the carrier medium are acetone, ethyl alcohol and butyl alcohol.

Further, it to be appreciated that the particular refractory, non-sinterable powder employed in each of the above examples was chosen because its rheological properties were such as to enable a slurry to be produced which could be cast into the required die parts. In addition, the particle size of the refractory, non-sinterable powder was in each case arranged so as to be similar to the particle size of the boron nitride powder employed. It was, however, found to be necessary to determine empirically for each slurry the required liquid content and the relative proportions of the boron nitride powder and refractory non-sinterable powder in the slurry, the criterion in each case being that, after casting, the slurry would not undergo any substantial shrinkage when the carrier medium was removed. In this respect, it is to be appreciated that a substantial shrinkage of the slurry on removal of the carrier liquid led to distortion of the die part cast from the slurry. In view of this necessity of producing a substantially non-shrinkable slurry, it was found that for powders of given particle sizes the relative proportions of the powders in the slurry could only be varied within a certain range, which had the effect of limiting the possible compression ratio value for the die parts obtainable from the powders.

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