Method for the Manufacture of a Molded Member From a Ceramic Material

1. Field of the Invention

The present invention relates to a method for the manufacture of a molded member from a ceramic material, for instance of silicon nitride or silicon carbide, wherein the molded member, which is arranged in an evacuated capsule of a high temperature-resistant glass, is compressed into a non-porous condition under a pressure acting on all sides thereof. The starting material of the glass capsule is applied onto the molded members in a non-vitrified state and exposed under a vacuum to its reaction temperature until there is formed a molded member-encompassing, cohesive glass layer which constitutes the capsule. This method is generally and summarily designated as high-temperature isostatic pressing. The object of the method is to compress a ceramic member having a porosity of up to about 30% under a pressure uniformly acting thereon on all of its sides up to non-porosity in order to increase its strength.

2. Discussion of the Prior Art

A prerequisite for being able to increase the density of the ceramic member lies in the evacuation of its pores. The molded members are produced in the form of pressings or briquettes, which are surrounded by a capsule or envelope. At present, known are the following approaches to the manufacturing of such pressings:

1. Encapsulation of ceramic powder in glass molds.

2. Encasing of reaction-sintered molded members by dipping them into liquefied glass.

Accordingly, it is a primary object of the present invention to provide a simple, uncomplicated method for the high-temperature isostatic pressing of a molded ceramic member.

A more specific object of the present invention resides in manufacturing a molded ceramic member of the type described, preferably from silicon nitride or silicon carbide, in which a work material for a glass capsule is applied in a non-vitrified condition onto the molded member and subjected to its reaction temperature under a vacuum for so long until there is produced a cohesive molded member-encompassing glass layer which forms the glass capsule.

A particular aspect of the invention provides for a single-step method of producing a molded ceramic member of the above-described type which is particularly suited for mass production due to its being relatively inexpensive. Although subject to the limitation that the glass envelope does not evidence an ideal degree of homogeneity, inasmuch as the glass envelope is, as a rule, again removed after the compressing of the molded member, this limitation need only be considered when it would be disruptive of the maintenance of the vacuum interiorly of the pressing, which can be avoided by refinement of the method.

In another preferred aspect of the present invention, there is contemplated a two-step method of producing the molded ceramic member which provides the advantage of imparting an ideal homogeneous structure to the glass envelope although it is of a somewhat greater complexity than the above-mentioned single-step method of the invention.

In a further aspect of the present invention it is contemplated that the constituents of the enveloping or coating layer be constituted of oxides of the rare earths (or lanthanide series) and silicon oxide, and that the conversion temperature be about 1200.degree. C., with the treatment period consisting of up to a few hours. Finally, the constituents of the coating layer may generally also be constituted of oxides of the elements of the first to fourth groups of the periodic system and of silicon oxide.

The coating of a molded ceramic member with glass is known per se, having reference to German Patent No. 25 10 938. However, in that instance, the glass coating is only for increasing the resistance to oxidation of the ceramic component. The glass layer prevents the entry of air into the pores of the molded ceramic member beneath the glass layer and, thereby, oxidation at the pore edges in the molded ceramic member. In essence, this approach takes the pores and seals them with respect to the exterior by means of a durable glass layer or coating. In contrast therewith, the present invention has as its precise purpose to avoid the pores by means of compression, initially requiring their evacuation, rendered possible through the intermediary of a temporarily applied glass layer; and wherein the invention also concerns itself with a suitable, while concurrently effected, formation and evacuation of the glass envelope.

Described hereinbelow are two preferred approaches, having been found to be valuable approaches in experimentation. When reaction-sintered silicon nitride is specified, the assumption is that the numerously mentioned experimentations are carried out with silicon nitride. However, it may be assumed that other types of ceramic materials, when not a poor ceramic, will correspondingly perform, so that silicon nitride (Si.sub.3 N.sub.4) or the similarly investigated silicon carbide (SiC) should be merely considered as safe forms of equivalent ceramics.

Approach 1: (two-step heat treatment process)

Initially produced was a porous molded member of reaction-sintered silicon nitride. Vapor deposited thereon was metallic aluminum. In a first heat treatment step (annealing in air for about 12 hours at about 580.degree. C.) the aluminum was converted into aluminum oxide. Concurrently, silicon oxide is formed on the surface of the molded member. In a second heat treatment step (at about 1200.degree. C. for about 0.5 hour under a vacuum) silicon oxide and aluminum oxide formed a high temperature-resistant glass layer on the surface of the porous silicon nitride member. After the heat treatment, the vacuum remained intact interiorly of the pressing as a result of the glass layer.

Approach 2: (single-step heat treatment process)

Initially again produced was a porous molded member of reaction-sintered silicon nitride. Deposited on the surface thereof were the constituents of a high temperature-resistant glass such as, for example, oxides of the rare earths and silicon oxide, in the finest possible distribution. In a heat treatment operation at about 1200.degree. C. under a vacuum, the oxides were converted into a high temperature-resistant glass which covered the entire surface of the molded member and thus maintained intact the vacuum in the interior of the pressing.

Resulting from both approaches is a pressing or briquette with an envelope of a high temperature-resistant glass which can be further processed through high-temperature isostatic pressing into a dense molded member. During the compression of the molded member, the glass envelope will soften to such an extent that it no longer hinders the volumetric reduction, however, without allowing for the entry of gas into the evacuated envelope.