Ceramic phase in sintered silicon nitride containing cerium, aluminum, and iron

Reference Number: N 04-12

Inventors: Edler, James P.; Lisowsky, Bohdan

USPTO Link: 5079198

Invention Summary

In accordance with the invention, sintered silicon nitride is made which has a non-glassy phase which is characterized by significant X-ray diffraction peaks at 2-theta diffraction angles corresponding to d-spacings of about 2.86 Angstroms and 3.59 Angstroms. The sintered silicon nitride may be prepared by a method which begins with comminuting (i) a silicon-containing powder, (ii) water, (iii) about 0.5 to 5 volume percent Fe.sub.2 O.sub.3, (iv) about 0.1 to 5 volume percent Al.sub.2 O.sub.3, and (v) about 0.5 to 10 volume percent CeO.sub.2, wherein all volume percents are based on the volume of the resultant silicon nitride. The composition of the comminuted mixture is such that about a total of 4 to 12 volume percent liquid phase is achieved during sintering, based on the total volume of the resultant silicon nitride. Comminution of the operative chemical compounds with water is conducted for a period of 1 to 5 hours, to produce a silicon based slurry. The nitriding agent and the densification aids may be added to the silicon-water slurry after comminution. Organic additives such as binders, plasticizers, viscosity modifiers, and dispersing agents may be added to the slurry toward the end of the comminution or thereafter. The slurry may then be aged for a period of 12 to 24 hours to allow the reaction of the silicon with the water to substantially reach completion which is believed to provide a silicon oxyhydride coating on the silicon. The aged slurry is then dried by spray drying or other technique and formed into a green body, if desired, such as by compaction of the spray-dried granules, slip casting of the original slip, extrusion, injection molding or other method. In addition, the nitriding agent and the densification aids may be added to the slurry after the aging step, if that is desirable. It may be advantageous to comminute the silicon and water first before deciding which agents and aids are desired.

The green body is then nitrided to form silicon nitride using a nitriding cycle which employs a nitriding gas of constant chemical activity composed of nitrogen, helium, and hydrogen gases, and uses a thermal heat treatment starting nitridation at a temperature of about 1000.degree. C. and increasing the temperature at a constant rate of about 5.degree. C. to 50.degree. C. per hour to an upper temperature of about 1450.degree. C. The green body may either be in the form of powder, or may be compacted into an article before nitriding.

Finally, the silicon nitride is sintered at a temperature of between 1450.degree. C. and 2100.degree. C. for a period of 0.5 to 5 hours in an atmosphere containing at least nitrogen which is at a pressure from ambient pressure to about 50 psig. Other sintering conditions are also contemplated, varying the heating, atmosphere, cooling and pressure conditions and materials included in the heating chamber.

Preferably, the method of preparing the article for sintering is that disclosed in two co-pending U.S. patent applications, the first being U.S. Patent application Ser. No. 135,864, "Process For Making Silicon Nitride" filed on Dec. 21, 1987, and the second being a continuation-in-part of the first, entitled Process For Making Silicon Nitride And Powder and Articles Made Therefrom filed on the same day herewith. Both co-pending applications are commonly assigned to the same assignee as the present application, and both co-pending patent applications are hereby incorporated by reference. In addition, the method of making sintered silicon nitride is disclosed in co-pending U.S. Patent Application entitled, "Process For Making Densified Beta-Phase Silicon Nitride Articles Or Powder" filed on the same day herewith and also hereby incorporated by reference.