Ceramic materials are of critical importance for a number of high temperature, high performance applications such as gas turbines. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness. Design reliability and the need for economical fabrication of complex shapes, however, have prevented ceramic materials from fulfilling their potential in these critical high temperature, high performance applications.
The design reliability problems with ceramics, and the resultant failure under stress, are due largely to the relatively brittle nature of ceramics. This, in combination with the high cost of fabricating complex shapes, has limited the usage of ceramics.
Ceramics made from organometallic polymers such as organosilicon polymers have the potential to overcome these problems. To this end, polymers based on silicon, carbon and/or nitrogen and oxygen have been developed. See, for example, "Siloxanes, Silanes and Silazanes in the Preparation of Ceramics and Glasses" by Wills et al, and "Special Heat-Resisting Materials from Organometallic Polymers" by Yajima, in Ceramic Bulletin, Vol. 62, No. 8, pp. 893-915 (1983), and the references cited therein.
The major and most critical application for ceramics based on polymer processing is high strength, high modulus, reinforcing fibers. Such fibers are spun from organosilicon preceramic polymers and are subsequently converted to ceramic materials, in particular, silicon carbide/silicon nitride bearing fibers by a two-step process of curing to render the preceramic polymer fiber insoluble followed by a routine pyrolyzation schedule comprising heating the fiber up to about 1,200.degree. C. where upon the fiber is converted to the ceramic form.
Unfortunately, great difficulty has been experienced in melt spinning these organometallic preceramic polymers into fibers utilizing conventional extrusion equipment. Melt spinning is a preferred method of forming fibers and simply involves melting the polymer and extruding the polymer through a spinneret. In melt spinning there is no need for compatible, yet inert, solvents to dissolve the polymer and/or for use as coagulating baths as in solvent spinning processes nor, is there a need for a drying atmosphere as in solvent spinning using the dry spinning technique. Thus, a useful melt spinning process for producing preceramic fibers from organometallic polymers would be very advantageous.
Heretofore, in the melt extrusion of preceramic organometallic polymers, conventional fiber forming equipment has been utilized. A typical operation for melt spinning the organometallic polymers into fibers involves screw-feeding the preceramic polymer in powder form from a hopper into a heated extruder barrel. An extruder screw carries the powder along the heated barrel of the extruder which causes the polymer to melt. A polymer metering pump feeds the molten polymer to a filter pack upstream of the spinneret to remove contaminates and any agglomerated unmelted polymer. The polymer emerges from the spinneret in fiber form.
While conventional melt spinning processes have been found useful in forming fibers from high molecular weight synthetic polymers, melt spinning has not yet been able to produce sufficiently long fibers from organometallic preceramic polymers. One reason for the difficulty is the low molecular weight of these organometallic preceramic polymers. The molecular weight of these preceramic polymers typically does not exceed 20,000 (M.sub.n) and most usually falls within a range of from about 500 to about 2,500. The low molecular weight manifests itself into several problems. For example, the preceramic polymer is crushed into a very fine powder when fed from a conventional hopper apparatus which contains a metering screw and as well by the extruder screw as the polymer travels through the extruder barrel. The powder which is formed tends to pack and form agglomerates at the hopper exit adversely affecting the uniformity of the amount of polymer fed to the extruder. Moreover, packing and formation of agglomerates of polymer powder takes place in the extruder barrel. Mixing and melting into a uniformly molten polymer cannot effectively be obtained. These problems related to the polymer flow consequently result in nonuniform flow from the metering pump to the spinneret and ultimately in the inability to produce adequate fibers from the polymer. One method thought to control the packing problem was to starve feed the extruder, thus, lowering the amount of polymer fed to the extruder barrel for melting. However, using this technique, the full extruder barrel is not filled and as such a uniform screw pressure cannot be maintained. The lack of a uniform pressure again adversely effects the polymer metering pump which cannot uniformly feed the spinneret. Another difficulty has been the tendency of these preceramic polymeric compositions to bubble. The bubbles also interfere with the capacity of the pump to meter the polymer uniformly to the spinneret.
Attempts have been made to solve the problem of bubble formation during the formation of films or fibers from polymeric materials such as nylons and polyesters which exhibit excessive bubble formation during extrusion. Thus U.S. Pat. No. 2,278,875 discloses feeding the polymer to the metering pump from a reservoir or pool of the molten polymer instead of from an extrusion device. U.S. Pat. No. 3,474,773 also utilizes a melt pool feeding system for forming polymeric yarns. Neither of these patents suggests spinning organometallic preceramic polymers into fibers utilizing the melt pool technique in order to solve the unique problems inherent in spinning these low molecular weight materials.