BACKGROUND OF THE INVENTION AND PRIOR ART STATEMENT The present invention is directed to a method of making a more economical and distortion free silicon nitride product suitable for use as a cutting tool by hot pressing with significantly reduced need for subsequent- shaping. Hot pressing of ceramic starting materials has been known for some time (see Refractory-Materials, by Allen M. Alper, Vol. 5, III, "High Temperature Oxides," 1970, pages 184-189, for an explanation of the typical hot pressing process and equipment) . The hot pressing sequence usually involves placing a loose particulate powder mixture or semidense pressed block of the powder mixture into a pressing assembly and heating the assembly while applying pressure to the mass sufficient to densify and fuse the particles to a desired degree. Typically, the pressing assembly is a cylinder die closed by end plungers or pistons, one or both of such end plungers or pistons being forceably moved by platens of a press to apply pressure to the mass within the assembly. The cylinder and end plungers are close fitting and are typically constructed of graphite. A refractory insulation shell is wrapped about the cylindrical die assembly and ^" heat is applied thereto by induction coils or by resistance heating. Hot pressing is typically carried out in the temperature range of 1500-1800°C, the pressures employed usually are in the range of 2000-7000 psi, and the time period usually com¬ prises 5-180 minutes. The resulting density for silicon nitride so hot pressed is usually in the range of 3.0-3.35 gm/cm-3.

If the above conventional hot pressing sequence were to be employed for the simultaneous pressing of a

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plurality of stacked, cold compacted, flat plates (particularly large diameter plates on the order of a 6 inch diameter), several problems would be encountered. First, a temperature gradient is created across the lateral width of the plates which results in a corresponding viscosity gradient. Such viscosity gradient causes the pressing assembly to apply a nonuniform pressure dis¬ tribution across and through the mass of the plates. Secondly, there exists a drag force (a friction force between the pressing assembly walls and the plate slides) which also contributes to a nonuniform pressure distribution across the lateral width of the plates. These two factors together cause material transport under the hot pressing conditions which in turn results in "dishing" or severe distortion of the flat plates in their fully densified condition.

Such problems as above indicated are amplified when economy is sought by the simultaneous production of a greater number of products, such as four or more plates (hereafter called billets) not separated by rigid spacers. A billet is defined herein to mean a mass of material of suitable thickness from which several useful cutting tools can be directly shaped by merely subdividing the billet by cutting through the thickness thereof. Such billets are either formed in a solid, single layer with a thickness to width ratio of 1:3 to 1:40, or they may be formed in a slab which is segregated by scoring to define a multiple number of cutting tools within a single slab, the segregated pieces being hinged together by the ^' unscored membrane. In only one instance has the prior art attempted to simultaneously hot press a plurality of silicon nitride components. In British patent 1,405,171, a number of cold compacted preforms are placed in a single layer within a pressing assembly, each preform having a thickness generally equal to its width. Each preform is separated from all others by a release agent. No greater than two

layers are used. The problems overcome by this invention would not be experienced in the application of this British patent. Side wall drag would be insufficient to promote distortion since there is little difference in movement between layers; the preforms do not contact the die wall and the thickness to width ratio is only 1:1. Material transport cannot take place as a result of pressure and thermal gradients because there is little relative movement between layers, little or no side wall drag, and the thickness to width ratio is only 1:1. The disclosure thus fails to appreciate the need for a unique stacking sequence that would eliminate dishing in the hot pressing of multiples of billets having a thickness to width ratio of 1:3-1:40.

SUMMARY OF THE INVENTION

The invention is a method of making a plurality of dimensionally accurate hot pressed ceramic bodies. The method comprises: (a) preparing a plurality of ceramic billets having a thickness to width ratio in the range of 1:3 to 1:40 and a density of 1.7-2.7 gm/cm ³ ; (b) uniquely stacking the billets into a pressure assembly having walls to support said billets normal to the direction of pressure; and (c) hot pressing said stacked billet groups under pressure and temperature to densify each of said billets to at least 95% of theoretical density with a compression ratio of 1.2:1 to 2:1. The stacking is in groups of progressively decreasing number so that for a billet group residing in a zone of compression that will experience the least movement along the pressing direction, the stacked number of billets is greatest within such group. For a billet group residing in a zone of compression that will experience the most movement along pressure direction, the stacked number of billets within such group is the lowest, each group being separated from adjacent groups by an inert rigid spacer.

It is preferable if (a) the hot pressing is carried out with uniaxial pressure and the number of billets in each of said groups proceeds from a maximum number of 5 to 3 to 2 to 1, the latter group has the lowest number and experiences the most relative movement; or (b) the hot pressing is applied biaxially and the sequence of groups contains the following numbers in order: 1, 2, 3, 10, 3, 2, 1, with the group having the single number experiencing the highest movement during compression and the group with the number 10 experiencing the lowest relative movement during compression.

Advantageously, the billets are prepared by cold compacting a dry milled mixture of silicon powder, 2O3, and AI2O3, and then heating the mixture in a nitrogen atmosphere for a period of time and at a temperature to agglomerate the mass to a density of about 2.3 gm/cm ³ and a chemical content of and flphase Si3 4, silicon yttrium oxynitrides, and some amorphous silicate. The cold com¬ paction is carried out with a pressure of 1500-2000 psi. The spacers are preferably fine grained, high strength graphite with a thickness (.25-1.00") appropriate for withstanding the hot pressing forces and fully trans¬ mitting such forces comparable to a die wall. The thick¬ ness of the spacers is determined by the rule that in the zone of greatest axial movement for the body, the spacer associated therewith is the thickest because the forces encountered will be the greatest.

SUMMARY OF THE DRAWINGS

Figure 1 is a schematic illustration in central sectional view of a pressing assembly showing the unique sequence of stacking in conformity with this invention for uniaxial pressure;

Figure 2 is a view similar to that of Figure 1 used in the environment of biaxial pressing; and

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Figure 3 is a schematic illustration of a pressing assembly after densification using a stacking sequence which shows the disadvantages of using a stacking sequence not in conformity with this invention.

DETAILED DESCRIPTION

A preferred method for making silicon nitride -comprising objects according to this invention is as follows:

1. Compacting A compact is formed from a mixture of powdered silicon and reactive oxygen carrying powder agents. Reactive powder oxygen carrying agents are defined herein to mean powder ingredients that are effective to form oxynitrides and appropriate silicates when reacted with the silicon under a heated nitrogen atmosphere. The powder agents can be advantageously selected from the group con¬ sisting of 2°3f Al2°3' ^si0 2' ^M 9° Ce0 ₂ r Zr0 ₂ , Hf0 ₂ , and rare earths. Use of selected quantities of Y °3 a d Al2^3 will result in the formation of a silicon yttrium oxy- nitride phase which (a) will uniformly be disbursed, and

(b) displace the detrimental glassy silicate phase normally formed except for a controlled and limited amount of the latter. For purposes ^' of the preferred method, a uniform powder mixture is prepared with 2000 grams of silicon powder (86.6 weight percent of the mixture), 278 grams of Y2O3 (12 weight percent of _. the mixture and 13.9 weight percent of silicon powder), and 32 grams of AI2O3 (1.4 weight percent of the mixture and 1.6 weight -percent of the silicon) . Silicon is selected to have 98% or greater purity and a starting average particle size of 8-9.2 microns. The major trace metal contaminants experienced with such impurity include iron, aluminum, Ca and Mn. Nonmetallic

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contaminants include carbon and 0 ₂ . The mixture is com¬ minuted and blended by being charged into an inert milling jar along with grinding media in the form of cylinders, milled for 48 hours, at 64 rpm, then the mixture is separated from the media. The resulting milled mixture will have at least 90% sized to an average particle size of less than 23 microns, the oxygen level after milling in air will be increased to 1.6 weight percent of the silicon, and an oxide coating will be present on the silicon in an amount of 3.0 weight percent. The ratio of the Y2θ3/Siθ2 is controlled to be in the range of 1-7 and preferably about 4.

A measured quantity of the milled mixture is loaded into a cold pressed die arrangement and pressed at ambient conditions by use of 1400-1500 psi to form a com¬ pact of a size about 6 inches in diameter and 1/2 inch in thickness, having a green density of 1.4 gm/cm ³ .

2. Heating to Nitride

The compact is heated in a nitriding atmosphere to produce a silicon nitride comprising body (billet) having at least one dispersed silicon yttrium oxynitride phase, and up to .5% by weight free silicon and unreacted oxygen carrying agents. The body will have a size greater than the object to be subsequently formed and a density less than such object.

To carry out the heating the compact is placed in an enclosed furnace, preferably evacuated to a pressure of- ^" less than 1 micron, and heated at a fast rate to 1200°F (649°C). The furnace is then filled with a gaseous mixture consisting of 72% by weight nitrogen, 3% hydrogen, and 25% helium, at a pressure of about 2.7 psig. The total 0 ₂ and H ₂ 0 content in such gaseous mixture is less than 4 ppm. The temperature of the furnace is then increased to a nitriding temperature of 2000-2500°F (1093-1427°C) at a

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slower rate. Fresh nitrogen is continuously supplied to the furnace to replace the nitrogen consumed in forming Si3N4 and silicon yttrium oxynitrides.

The nitrided body (billet) will preferably con- sist of silicon nitride (at least 60% of which is in the alpha form), silicon yttrium oxynitride in the Y Si0 N phase, and up to .5% of either unreacted silicon or yttria. This body is an intermediate product or commodity that has particular utility as a starting material for the hot pressing technique to follow. The billet has a thickness in the range of .3-.1.0 inches and a width or diameter of 3-12 inches; the thickness/width ratio of 1:3 to 1:40.

3. Hot Pressing

The billets are stacked within a pressing assembly, the assembly having walls to support the series of aligned billets normal to the direction of pressure. As shown in Figure 1, the upper piston is used to apply uniaxial pressure to the stacked series of billets.

The pressing assembly has graphite walls 40-41- 42-43. The walls and nitrided body are both coated with a slurry of boron nitride and dried. The graphite walls are additionally covered by graphite foil and/or molybdenum foil.

The sequence of stacking of the billets is criti- cal to this invention. The billets are stacked in groups in progressively decreasing numbers so that for a billet group residing in a zone of compression that will experi¬ ence the least movement along the pressure direction, the stacked number of billets is highest. For the billet group residing in a zone of compression that will experience the most movement along said pressure direction, the stacked number of billets is lowest. Each billet group is separated from adjacent billet groups by an inert rigid spacer comprised of either graphite, substantially full density boron nitride, or other pressure transferring

material. Preferably the graphite spacers are covered with boron nitride which in turn is coated with graphite foil. The spacers maintain geometry, undergo no material transport, and are not affected by thermal pressure gradients. The thickness of the spacers is determined by the rule that in the zone of greatest axial movement for the body, the spacer associated therewith is the thickest because of the greater forces encountered.

In particular, the stacking sequence for Figure 1 shows that there are five billets, 10-11-12-13-14, placed in contiguous relationship with one another in the zone of movement 15 which is at the lowest portion of the stack, the latter experiencing the least amount of relative axial movement in the direction of compression 16. The next billet group comprises three billets,

18-19-20, and will reside in an intermediate zone of move¬ ment 17 and therefore has a reduced number. The billet group containing the least number has billet 21 and is in the zone of highest movement 22. Other combinations of billet grouping can be en¬ visioned as long as the basic criteria of using decreasing numbers of billets in the billet groupings when progressing in a direction from the smallest amount of billet movement (billet 14) to the largest zone of billet movement (billet 21) is utilized.

In the environment of biaxial pressure, as shown in Figure 2, the stacking sequence is as indicated. In the zone of least movement 23, there are 10 billets (2x5); in the zone of most movement 24 there is one billet. The stacking sequence is 1-2-3-10-3-2-1.

If the stacking sequence of this invention was not employed, the following phenomena would have resulted. As shown in Figure 3, a pressure distribution is present which is brought about in part from side wall drag; that is, the edges of the billets 29-34 will contact the graphite side walls and impart a resistance to compression and promote a

drag. The pressure gradient facilitates material transport from the outside diameter to the center of billets 30-31- 32-33 and from the center to the outside diameter on billets 34, 35 and 36. These effects, upon cooling, yield "dished" hot pressed Si3 4 billets that require substantial amounts of material to be removed (diamond grinding) to produce a flat product.

It is important that the compression ratio for hot pressing be in the range of 1.2:1 to 2:1. The pressure is preferably applied in steps, about 150 psi at room tempera¬ ture before heat-up. Pressure is then increased to 500 psi at 1800°F (982°C), pressure is next increased to 2500 psi as the temperature is increased to 2500°F (1371°C), finally the temperature is increased to 3000°F (1649°C) and pres- sure to 3700 psi. The latter conditions are maintained until full density is achieved. This usually requires 2.0 hours at the ultimate pressing temperature. The object is then cooled at any rate to room temperature.

The resulting object will consist essentially of beta phase silicon .nitride, silicon yttrium oxynitrides (predominantly YιSi0 ₂ ) enveloped by amorphous silicate phase having a thickness of 4-10 angstroms and having no microporosity. The object preferably possesses a hardness of 89.0-91.0 on the 45-N scale, a density of 3.30-3.33 gm/cm ³ , a fracture strength of greater than 85,0000 psi at 1200°C in a four-point bend test, and an oxidation resistance that prevents weight pickup by the object after ^' 450 hours in air at 1000°C.