Si.sub.3 N.sub.4 Ceramic Densified Using Sc.sub.2 O.sub.3 and SiO.sub.2

This invention relates to the field of ceramics, and particularly to Si.sub.3 N.sub.4 ceramics.

Silicon nitride ceramics encompass a family of compositions in which Si.sub.3 N.sub.4 constitutes an important ingredient. Considerable effort has been expended and is being expended to develop ceramics which have useful structural properties at high temperatures. In particular, there is a continuing need for Si.sub.3 N.sub.4 ceramics which have high oxidation and creep resistance.

Si.sub.3 N.sub.4 ceramics are produced by compacting Si.sub.3 N.sub.4 powder and then sintering or hot pressing the powder at high temperature. Very pure Si.sub.3 N.sub.4 powder cannot be successfully sintered or hot pressed and consequently a densification aid must be added to the Si.sub.3 N.sub.4 powder in order to obtain dense Si.sub.3 N.sub.4. The densification aid reacts with SiO.sub.2 on the surface of the Si.sub.3 N.sub.4 particles, with the Si.sub.3 N.sub.4, and with impurities in the powder to form a eutectic liquid during sintering. The liquid promotes densification of the powder through a solution-reprecipitation mechanism.

Upon cooling, the liquid freezes and forms secondary phases. The number, chemistry, and content of the secondary phases depend on the constituents in the starting powder and the phase relationship of the composite system. Both non-equilibrium (nitrogen glasses) and crystalline phases are observed. Thus, Si.sub.3 N.sub.4 alloys are polyphase materials. As expected, the secondary phases significantly influence the properties of the ceramic, unfortunately by decreasing the ceramic's creep and oxidation strength at high temperatures.

One approach to overcoming the harmful effect of the secondary phase is to reduce the amount of secondary phase in the ceramic. U.S. patent application Ser. No. 266,244, filed May 22, 1981 by F. F. Lange and D. R. Clarke, describe a method of drawing out some of the glassy phase by a post-fabrication oxidation treatment.

Another approach is to use ingredients which produce a secondary phase which has improved high temperature properties. This second approach is attempted in the method described in U.S. Pat. No. 4,046,580, wherein Y.sub.2 O.sub.3 is used as a densification aid.

Currently, MgO is the densification aid used in most commercial Si.sub.3 N.sub.4 ceramics. Other known aids include ZrO.sub.2, Al.sub.2 O.sub.3, BeO, and the rare earth oxides Y.sub.2 O.sub.3, CeO.sub.2, Ce.sub.2 O.sub.3, Nd.sub.2 O.sub.3, and La.sub.2 O.sub.3. Unfortunately, all these aids form intergranular phases which are either viscous glasses at high temperature (thus reducing the ceramic's creep strength) or are metal-silicon-oxynitride compounds (sometimes causing cracking by expanding during oxidation).

It is an object of the invention to provide an improved Si.sub.3 N.sub.4 ceramic.

It is an object of the invention to provide a Si.sub.3 N.sub.4 ceramic having improved creep strength and improved oxidation resistance.

It is an object of the invention to provide a densification aid for use in fabricating Si.sub.3 N.sub.4 ceramic having improved high temperature properties.

According to the invention, Sc.sub.2 O.sub.3 and SiO.sub.2 are used as a densification aid during the sintering of Si.sub.3 N.sub.4 powder. The amount of Sc.sub.2 O.sub.3 and SiO.sub.2 used is selected to be within the composition defined by a triangle formed by the Si.sub.3 N.sub.4 -Si.sub.2 N.sub.2 O tie line, the Si.sub.3 N.sub.4 -Sc.sub.2 Si.sub.2 O.sub.7 tie line, and the Sc.sub.2 Si.sub.2 O.sub.7 -Si.sub.2 N.sub.2 O tie line in the ternary phase diagram for the Si.sub.3 N.sub.4, SiO.sub.2, Sc.sub.2 O.sub.3 system. No quaternary compound is formed within this composition range so that problems such as cracking caused by the expansion on oxidation of quaternary compounds (metal-silicon-oxynitrides) are avoided.

The powders used to form the ceramic should include 25 to 98.5 mole % Si.sub.3 N.sub.4, 1 to 50 mole % SiO.sub.2, and 0.5 to 25 mole % Sc.sub.2 O.sub.3. The proportion of these powders is selected so that the ceramic, after sintering, will contain at least 50 mole % Si.sub.3 N.sub.4 with the balance being the phases Si.sub.2 N.sub.2 O and Sc.sub.2 Si.sub.2 O.sub.7. It must be recognized that most Si.sub.3 N.sub.4 powders already contain some SiO.sub.2 and this must be taken into account.

FIG. 1 is a ternary phase diagram in mole % for the Si.sub.3 N.sub.4, SiO.sub.2, Sc.sub.2 O.sub.3 system for samples hot-pressed in graphite dies at 1600.degree. to 1800.degree. C.

It has been discovered that Sc.sub.2 O.sub.3 and SiO.sub.2 can be used as a densification aid for sintering Si.sub.3 N.sub.4 powder in such a way that no quaternary compounds (scandium-silicon-oxynitrides) are formed. Such quaternary compounds could cause problems such as cracking caused by expansion on oxidation.

To determine the phases formed during sintering, mixtures of Si.sub.3 N.sub.4, Sc.sub.2 O.sub.3, and SiO.sub.2 powders were hot-pressed in graphite dies at temperatures from 1600.degree. to 1800.degree. C. The sintered samples were ground into powder and x-ray analyzed to identify the constituent phases and to construct the ternary phase diagram shown in FIG. 1. No quaternary compounds were observed. The compounds observed were Si.sub.3 N.sub.4, Si.sub.2 N.sub.2 O, and Sc.sub.2 Si.sub.2 O.sub.7. Their tie lines were the Si.sub.3 N.sub.4 -Si.sub.2 N.sub.2 O tie line 2, the Si.sub.3 N.sub.4 -Sc.sub.2 Si.sub.2 O.sub.7 tie line 4, and the Sc.sub.2 Si.sub.2 O.sub.7 -Si.sub.2 N.sub.2 O tie line 6.

This invention is limited to compositions within tie lines 2, 4, and 6, in order to insure that only three compounds (Si.sub.3 N.sub.4, Sc.sub.2 Si.sub.2 O.sub.7, and Si.sub.2 N.sub.2 O) can exist in the sintered ceramic. These three compounds are compatible with each other, and additionally the Sc.sub.2 Si.sub.2 O.sub.7 is compatible with SiO.sub.2 which is the oxidation product of Si.sub.3 N.sub.4. In order to obtain the intrinsic properties of Si.sub.3 N.sub.4 such as low thermal expansion and moderate elastic modulus, the composition is also limited to a minimum of 50 mole % Si.sub.3 N.sub.4 in the sintered ceramic, as shown by dashed line 8.

A minimum of about 0.5 mole % Sc.sub.2 O.sub.3 is required in the mixture of powders to serve as a densification aid as shown by dashed line 10. This minimum amount of Sc.sub.2 O.sub.3 intersects the Si.sub.3 N.sub.4 -Sc.sub.2 Si.sub.2 O.sub.7 tie line 4 at about 1 mole % SiO.sub.2, thus setting the minimum amount of SiO.sub.2 required as a densification aid in the Si.sub.3 N.sub.4 rich corner of the phase diagram.

When Si.sub.3 N.sub.4, SiO.sub.2, and Sc.sub.2 O.sub.3 powders are mixed and sintered at 1600.degree.-1800.degree. C., they react to form a sintered ceramic according to the following relationship:

Point A in FIG. 1 is a composition using the minimum amount of Sc.sub.2 O.sub.3 powder (0.5 mole %) and providing a sintered ceramic with the minimum amount of Si.sub.3 N.sub.4 (50 mole %). Using the above relationship, the mixture of powders required at Point A is: 74 mole % Si.sub.3 N.sub.4, 25.5 mole % SiO.sub.2, and 0.5 mole % Sc.sub.2 O.sub.3. After sintering this mixture of powders, the sintered ceramic should have a composition of: 50 mole % Si.sub.3 N.sub.4, 49.5 mole % Si.sub.2 N.sub.2 O, and 0.5 mole % Sc.sub.2 Si.sub.2 O.sub.7.

Point B in FIG. 1 is a composition using the minimum amount of Si.sub.3 N.sub.4 powder (25 mole %) and providing a sintered ceramic with the minimum amount of Si.sub.3 N.sub.4 (50 mole %). Using the above relationship, the mixture of powders required at point B is: 25 mole % Si.sub.3 N.sub.4, 50 mole % SiO.sub.2, and 25 mole % Sc.sub.2 O.sub.3. After sintering this mixture of powders, the sintered ceramic should have a composition of: 50 mole % Si.sub.3 N.sub.4 and 50 mole % Sc.sub.2 Si.sub.2 O.sub.7.

Point C in FIG. 1 is a composition using the minimum amounts of Sc.sub.2 O.sub.3 and SiO.sub.2 powders. Using the above relationship, the mixture of powders at point C is: 0.5 mole % Sc.sub.2 O.sub.3, 1.0 mole % SiO.sub.2, and 98.5 mole % Si.sub.3 N.sub.4. After sintering this mixture of powders, the sintered ceramic should have a composition of: 99.5 mole % Si.sub.3 N.sub.4 and 0.5 mole % Sc.sub.2 Si.sub.2 O.sub.7.

In order to determine the properties of a ceramic made according to the present invention, a mixture of powders containing 74 mole % Si.sub.3 N.sub.4, 18 mole % SiO.sub.2, and 8 mole % Sc.sub.2 O.sub.3 (Composition D in FIG. 1) was prepared. The powders were milled in methanol with WC grinding media, dried, and then hot-pressed at 29 M Pa in graphite dies. Table I lists the hot-pressing conditions, densities achieved, and the observed phases, the quantities of which (according to the previously mentioned relationship) should be: 4.8 mole % Si.sub.2 N.sub.2 O, 9.5 mole % Sc.sub.2 Si.sub.2 O.sub.7, and the balance substantially Si.sub.3 N.sub.4. As indicated, the theoretical density of 3.21 g cm.sup.-3 could be achieved in 2 hours at 1800.degree. C., but a 4 hour hold was required to nearly complete the .alpha. to .beta. phase transformation of the Si.sub.3 N.sub.4. The sluggish densification and transformation kinetics of this composition as evidenced by these results relative to other Si.sub.3 N.sub.4 systems would be indicative of slow kinetics in the solution/reprecipitation reaction which would imply good high temperature creep resistance also.

Specimens for preliminary oxidation and compressive creep experiments were diamond cut and ground from the Composition D billet hot pressed at 1800.degree. C. for 4 hours.

After 285 hours of oxidation in air at 1400.degree. C., the specimen had a slight, glassy patina and had gained 4.4.times.10.sup.-3 kg.multidot.m.sup.-2. This weight gain is much lower than for commercially available Si.sub.3 N.sub.4. Sc.sub.2 Si.sub.2 O.sub.7 is compatible with Sc.sub.2 O.sub.3, Si.sub.2 N.sub.2 O, and SiO.sub.2 so that destructive interaction on oxidation is avoided and the protective oxidation surface layers have good integrity and remain thin as confirmed by these oxidation tests.

Compressive creep measurements were conducted in air at 1400.degree. C. using the method described by F. F. Lange, D. R. Clarke, and B. I. Davis in "Compressive Creep of Si.sub.3 N.sub.4 /Magnesium Oxide Alloys, Part I, Effect of Composition", The Journal of Material Science, Vol. 15, (1980). The specimen's creep resistance was 1 to 2 orders of magnitude higher than for the Si.sub.3 N.sub.4 /MgO material examined in the above mentioned article.

The microstructure of the Composition D Si.sub.3 N.sub.4 /Sc.sub.2 O.sub.3 material was examined using high-resolution electron microscopy and analytical electron microscopy. Gains of Si.sub.3 N.sub.4 were surrounded by grains of the secondary phase crystalline Sc.sub.2 Si.sub.2 O.sub.7. There was an extremely thin (approximately 1 nm), continuous intergranular phase. In addition, a few large pockets of glass were seen, suggesting that the volume fraction of noncrystalline phase present was small.

Numerous variations and modifications can be made without departing from the invention. For example, the ceramic can include minor impurities or additives which are included for their known effect. Accordingly, it should be clearly understood that the form of the invention described above and shown in the accompanying drawings. is illustrative only and is not intended to limit the scope of the invention.