CA1092325A - Method of forming high density beta silicon nitride - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method of fabricating essentially fully densified .beta.-silicon nitride articles by placing .alpha.-silicon nitride in a mold, heating the .alpha.-silicon nitride to a temperature sufficient to bring about conversion to silken nitride, applying a small but insufficient amount of pressure to cause full densification, holding these temperature and pressure conditions until essentially all of the .alpha.-silicon nitride has been converted to the .beta. form, and finally, increasing the temperature and pressure sufficiently to bring about full densification of the .beta.-silicon nitride.

Description

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BACKGR~UND OF THE INVENTION
Field of the Invention The invention relates to processes for fabricating refractory articles. More specifically, the invention concerns high temperature molding of fully dense silicon nitride.
Prior Art Statement The following patents are representative of the most relevant prior art known to the Applicant at the time of filing of the application:
UNITED STATES_PATENTS
3,4Ç8,992 September 23, 1969 Lubatti et al 3,549,402 December 22, 1970 Whitney et al 3,819,786 June 25, 1974 May 3,830,652 August 20, 1974 Gazza 3,835,211 September 10, 1974 Coe et al - 3,839,540 October 1, 1974 Arrol While the essence of the present invention resides in a novel hot-pressing method of forming silicon nitride products, the other commonly used forming methods, sintering and reaction bonding, are also relevant. The relevance of the latter two forming methods is in the fact that they may be used to preform product that may subsequently be subjected to the process of the present invention, particularly when ` the configuration of the final product is relatively simple.
Lubatti et al discloses a typical sintering method for forming silicon nitride consisting of mixing finely particulate silicon nitride with up to 25% of a binder which may be boric acid, boron anhydride, or boron phosphate;
' compressing the mixture in a mold at room temperature under ;, 30 from 0.5 to 5 tons per square centimeter; and sintering the ~-: .

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, 1~92325 compact in a nitrogenous atmosphere at a temperature of from 850 to 1250C to produce essenti~lly ~-silicon nitride.
A reaction bonding or reaction sinterlng process is described by May involving a mixture of ~00 mesh silicon powder and up to 35~ of a temporary organic binder system e.g.
butyl methacrylate and trichloroethylene. Some of the trichloroethylene solvent i5 removed to produce a mix that is stiff at room temperature but flexible and doughy at 50C; the preformed article is molded at ambient temperature and then ~
10 heat treated up to 130C to remove the remaining solvent. The ~ ~ -preform is then heated to 1000C at a rate of 50C per hour with a continuous flow of nitrogen through the furnace, resulting in decomposition and removal of the temporary organic binder. The preform which is now made up essentially ;~
.j of silicon is further heat treated in a static pressure of - nitrogen at 1 psi to an ultimate temperature of 1450C; the l -final product is reaction bonded ~-silicon nitride with a bulk density of 2.24 g/cc. '-Both Gazza and Whitney et al teach the fabrication of ~ ~
`, 20 silicon nitride products by the hot-pressing of silicon nitride. ;
~! The principal difference between the two references is the compound each adds to the powdered silicon nitride prior to hot-pressing. Gazza adds 1.0 to 3.5% by weight of a yttrium compound, while Whitney et al add a lithium compound. In both references the silicon nitride-additive mixture is placed in : ~. ~ ,-, a graphite mold; Gazza presses between 1750 and 1800C at a - pressure of 6000 to 7000 psi while Whitney et al presses at -~i 1395 to 1900C and a pressure as low as 2000 psi.

The process taught by Arrol involves the concept of forming an initial compact by a first step of reaction bonding .` `' .~ ', followed by a hot-pressin~ step. This dual step approach may also be employed within the scope of the present invention.
Arrol takes 3 micron silicon powder, blends in a magnesium oxide fluxing ayent and a temporary organic binder system.
The mixture is heat treated to remove the vehicle for the organic binder. The resulting molding powder is then compacted to the desired shape but with dimensions slightly greater than those desired in the final product. The silicon preform is then nitrided by heating at a temperature not to exceed 1300C ;~
in an atmosphere of 90~ nitrogen, forming a porous ~-silicon nitride body with a density of 1.7 to 2.7 g/cc. The ~-silicon nitride shape is then placed in a graphite mold and hot-pressed at a pressure of 4000 psi and a temperature of 1750C, to the ;;
articles desired final dimensions and density. In another embodiment, a modified nitriding step is employed wherein the nitriding takes place first at 1250C whereafter the temperature is increased to 1400C and then to 1550C to convert the silicon to predominantly ~-phase silicon nitride. The porous low density article is then coated with a release agent and placed in a graphite mold, and finally hot-pressed at 4,000 psi at a temperature of 1750C to give a finished product with a density - between 2.2 and 2.7 g/cc.
The most relevant reference is the patent to Coe et al -~
which is directed specifically at a method of hot-pressing silicon nitride and the conversion of the silicon nitride from the ~ to the ~ form. Coe et al take ~-silicon nitride powder, into which 2~ magnesium oxide has been blended, place it in a graphite mold set-up of desired internal configuration and hot-press the mixture to full density. The pressing conditions for full densification are a temperature of from .

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1500-1850C and a pressure of from 1000-5000 psi. Once full j- densification is indicated, the temperature and pressure are maintained for at least another 10 minutes to convert the ~-silicon nitride to ~-silicon nitride. Alternatively, the silicon nitride powder may be hot-pressed to full density and heated further to fully convert the ~-silicon nitride to the ' ~ form without pressure.
SUMMARY OF THE INVENTION
Thus, in accordance with the present teachings, a method is provided of forming fully dense ~-silicon nitride products which comprises the steps of placing silicon nitride which contains a substantial amount of ~-silicon nitride in a mold, converting the a-silicon nitride to ~-silicon nitride by heating and pressing the powder at a temperature suf~iciently high to cause conversion of the ~-silicon nitride to ~-silicon nitride but at a temperature and at a pressure low enough to avoid full densification of the silicon nitride, holding the temperature and pressure conditions until essentially all of the ~-silicon nitride has been converted to ~-silicon nitride and fully densifying the ~-silicon nitride by increasing the temperature and/or pressure~
The invention process is a hot-pressing method directed specifically at producing fully dense articles of silicon nitride, in which the silicon nitride is converted from a-silicon nitride to ~-silicon nitride during the hot-pressing process. A molding powder which is predominantly ~-silicon nitride (and which may include any of the known ; sintering aids or fluxing agents) is placed in an appropriately designed graphite hot-pressing mold. The mold and its contents are slowly heated to 1500C at a pressure below that which would bring about full densification of the silicon nitride e.g. atmospheric pressure to 1500 psi, but preferably about _5_ ' '' , 1~9232S
500 psi. The temperature and low pressure conditions are maintained for a time period sufficient to allow essentially complete conversion of the a-silicon nitride to the ~ form.
At this point the temperature is increased to as high as 1850C and the pressure increased to at least 2000 psi, thus bringing about full densification of the ~-silicon nitride.
The principal objective is to heat the a-sllicon nitride, convert it to the ~ form without bringing about full densification, and then finally fully densifying. This can be accomplished with a variety of temperature-pressure conditions. The ~ to ~ conversion step can be carrled out without ; :~
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~092325 significant densification using, up to a maximum pressure of 1500 psi at 1400C, up to 1850C at about atmospheric pressure, or any combination of temperature and pressure therebetween.
Similarly, the step of densifying the ~-silicon nitride can be carried out at from 1500-1850C at a pressure of at least 2000 psi.
; According to the Coe et al hot-pressing method, full ~ densification must take place first, followed by the a to ~
., phase conversion caused to take place under pressure. The major problem attendant the Coe et al process is that during the pressurized ~ to ~ convarsion phase of the process, the - costly graphite mold band fre~uently ruptures, probably due ;
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to the change in volume of the silicon nitride which takes place as a result of the ~ to ~ phase change.
` DESCRIPTION OF THE PREFERRED EMBODIMENTS ` ` -:
While it is not a requirement for the sucessful ;
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practice of the invention, it is preferred that the particle ~~"
size of the powders employed, have an average particle size of about 3 microns or finer. As is well known in the art, the highest strength final product is produced when 3 micron or ` finer powders are used. The starting silicon nitride powder should be at least 50% alpha.
The processing parameters which constitute the best mode of practicing the invention are as follows: for the -~
to conversion step heat at from 1400-1750C at a pressure of from 0 - 1000 psi for 30 to 180 minutes; for full ~;~

densification, increase tha temperature to 1600 - 1850C, increase the pressure to at least 2000 psi and maintain these conditions for 10 - 60 minutes. These parameters of course ;~
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function of the presence of other materials such as sintering aids, the thickness and configuration of the piece being processed, the amount of a versus ~ silicon nitride in the ; original powder, and so on. The following is an example of the best mode of practicing the invention.
EXAMPLE
- . , A mixture of 4000 grams of a-silicon nitride and 120 grams of magnesium carbonate were placed in a tungsten carbide lined milling jar 1/3 full of 1/2" tungsten carbide balls. To this was added 9000 milliliters of isopropanol. The mixture was milled for 16 hours after which it was screened through a 500 mesh screen and dried at 90C. The resulting cake was broken up by tumbling in a plastic jar containing rubber balls, for 2 hours.
- A 2254 gram quantity of the mixture was placed in a boron nitride coated graphite mold set~up having a cavity therein, measuring 6-1/4 x 6-1/4. The mold was fully assembled ` and the contents thereof hot-pressed according to the following schedule to obtain a piece approximately 6-1/4 x -`;; 20 6-1/4 x 1 -- (a) Temperature increased from room temperature to ~ ;
1500C over a period of 100 minutes while ` maintaining a pressure of 500 psi;
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-;i (b) Temperature and pressure maintained at 1500C
and 500 psi for 90 minutes;
(c) Pressure increased 250 psi every 10 minutes until pressure reached 3000 psi while maintaining temperature at 1500C;
(d) Pressure held at 3000 psi while temperature raised from 1500C to 1740C over a period ;
of 40 minutes;

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(e) Temperature held at 1740C and pressure at :
- 3000 psi for about 25 minutes;
: (f) Heat and pressure were terminated ant the mold and its contents allowed to cool.
The resulting piece measured approximately 6-1/4 x 6-1/4 x 1" had a density of 3~22 g/cm3, a cross-bending strength of 130,050 psi under 4 point loading, and was essentially all ~-silicon nitride. ~ ~:

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Claims (3)

The embodiment of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of forming fully dense .beta.-silicon nitride products, comprising the steps of:
placing silicon nitride containing a substantial amount of .alpha.-silicon nitride in a mold;
converting the .alpha.-silicon nitride to .beta.-silicon nitride by heating and pressing said powder at a temperature sufficiently high to cause conversion of the .alpha.-silicon nitride to .beta.-silicon nitride but at a temperature and at a pressure low enough to avoid full densification of said silicon nitride;
holding said temperature and pressure conditions until essentially all of the .alpha.-silicon nitride has converted to .beta.-silicon nitride; and fully densifying the .beta.-silicon nitride by increasing the temperature and/or pressure.

2. The method of Claim 1 wherein said conversion of .alpha.-silicon nitride to .beta.-silicon nitride is accomplished within a temperature-pressure range of from, 1400°C with a maximum pressure of 1000 psi, to 1750°C at atmospheric pressure;
and, said full densification of the .beta.-silicon nitride being carried out at from 1500°C to 1850°C under a pressure of at least 2000 psi.

3. The method of Claim 2 wherein said silicon nitride powder has an average particles size of about 3 microns or finer, and is made up of at least about 50% .alpha.-silicon nitride.