1. Field of the Invention
This invention relates to pressureless sintered silicon carbide and more particularly relates to a pressureless sintered thermal shock resistant silicon carbide ceramic body which has high density, good electrical properties and good strength. The invention further relates to the method for manufacturing such a silicon carbide ceramic body.
2. History of the Prior Art
In the prior art silicon carbide ceramic bodies are generally made by one of two processes. One of the processes is hot pressing wherein particulate silicon carbide is pressed into a mold at high temperatures and pressures to form a shaped body. The other process is pressureless sintering wherein silicon carbide is preformed at low temperatures into a shape having the general shape of the finished body. The preforming is usually accomplished by pressing silicon carbide particles together at low temperatures. The preforming can also be accomplished by casting a dispersion of silicon carbide into a mold followed by driving off the dispersing liquid. Subsequent to preforming, the body is heated to an elevated temperature at approximately atmospheric pressure to form a finished silicon carbide ceramic body.
Pressureless sintering has certain advantages over hot pressing since equipment needed for forming the body is less complicated and less costly and since more intricate finished shapes are possible.
Until recently however, pressureless sintering has had serious disadvantages when compared with hot pressing since the resulting ceramic body had much lower strength and density than bodies which ae formed by hot pressing.
It has recently been discovered that pressureless sintered silicon carbide ceramic bodies could be obtained which have good strength and high density when boron carbide (B.sub.4 C) is incorporated into the silicon cabide prior to pressureless sintering. A detailed discussion of the method of making a dense silicon carbide ceramic body by the incorporation of boron carbide is disclosed in U.S. Pat. No. 4,004,934 entitled "Sintered Dense Silicon Carbide." That application further generally discloses that boron containing additives in general are good for increasing the density of pressureless sintered silicon carbide. It has, however, been found, contrary to that general statement, that many of such boron containing additives are not effective in increasing the density and strength of pressureless sintered products. The only two compositions which are disclosed by U.S. Pat. No. 4,004,934 are boron carbide and boron. The use of both of these products has substantial disadvantages since, while good strength and high density are obtained, the thermal shock resistance of the ceramic body is not sufficiently high for many applications and the electrical properties of the resulting ceramic body are undesirable in many applications since the large amounts of boron required through the addition of boron carbide or elemental boron result in positive doping characteristics which results in a very high cold electrical resistance which slows down heat up time in such applications as electrical ignitors and requires high driving voltages to obtain sufficiently high temperatures for these applications. Furthermore, the high driving voltages result in ignitors which are very difficult to control since once the initial cold resistance is overcome, resistance rapidly and sharply drops thus providing instantaneous current surges through the body which can easily result in ignitor burnout.
In order to obtain even acceptable electrical properties for many applications, negative dopants such as nitrogen or phosphorus must be incorporated into the finished ceramic body. For example, one method of incorporating such a dopant to overcome the effects of boron is heating the body in a nitrogen atmosphere either during or subsequent to sintering. Even when negative dopants are subsequently added, the electrical characteristics are not as good as desired since the desirable effects of the negative dopant simply neutralize the undesirable effects of the large amounts of boron present. In addition, the method for making a dense silicon carbide disclosed in U.S. Pat. No. 4,004,934 is not generally desirable for pressureless sintering of all crystalline forms of silicon carbide. In particular, the method seeks to avoid the alpha crystalline form.
It has subsequently been discovered and disclosed that high density silicon carbide ceramic bodies can be formed with boron carbide starting with the more readily obtainable alpha crystalline form of silicon carbide when at least about 1.5 percent of carbon is incorporated into the sintering composition. The method for obtaining pressureless sintered high density silicon carbide from the alpha crystalline form is disclosed and discussed in U.S. Patent Application Ser. No. 584,226 filed June 5, 1975 by J. A. Coppola, L. N. Hailey and C. H. McMurtry.
Boron containing additives other than elemental boron and boron carbide have been utilized in hot pressing operations. For example, boron nitride has been used in obtaining silicon carbide ceramic bodies to improve electrically conducting properties. Such a composition and its method for preparation is disclosed in U.S. Pat. No. 3,960,577. There is, however, no indication that boron nitride would be one of the few boron containing additives which might increase the density of a pressureless sintered silicon carbide body while retaining the desirable electrically conducting properties imparted by the boron nitride in hot pressing methods.