This invention relates to a process for making silicon carbide whiskers and is particularly concerned with a process for providing relatively high yields of beta silicon carbide whiskers while controlling the size and shape of the whiskers.
Silicon carbide is a high strength ceramic material which has good chemical stability and excellent oxidation resistance to high temperatures due mainly to covalent bonding and a crystal structure related to that of diamond. Silicon carbide whiskers are needle-shaped single crystals of silicon carbide having an aspect ratio, i.e., a length-to-diameter ratio, greater than about 3 and a typical diameter between about 0.1 and 10 microns. The high aspect ratio of whiskers makes them a much more effective reinforcement in composites, especially ceramic matrix composites, than silicon carbide particulates. Also, whiskers exhibit much higher mechanical strength than silicon carbide fibers, which are either polycrystalline or amorphous forms of silicon carbide that typically have a diameter greater than 10 microns. These superior properties of silicon carbide whiskers have led to their use as a reinforcing material for ceramics, metals, polymers and glass composites. Silicon carbide whiskers are particularly suited for use in the reinforcement of all types of engineering ceramics including gas turbine ceramics, automotive ceramics and ceramic cutting tools. It is estimated that the market for the use of whiskers as reinforcements for engineering ceramics alone will be several hundred million dollars per year by the year 2000.
When silicon carbide whiskers are used as reinforcement in composites, it is preferable that the size and shape of the whiskers meet specified requirements. For example, in some instances it may be desirable that the diameter of the whiskers be less than one micron while in other cases whisker diameters of several microns are preferred. Also, straight whiskers are normally used to reinforce ceramic matrix composites because such reinforced composites have a higher fracture toughness than those reinforced with curly whiskers. On the other hand, curly whiskers may be more effective than straight whiskers for other reinforcing applications. For these and other reasons, it is desirable that the size and shape of the whiskers be controlled during synthesis.
The current dominant technology for producing silicon carbide whiskers is the carbothermal reduction of silica by carbon in rice hulls. Rice hulls are composed of about 15 to 20 percent ash that is primarily silica. Thus, when the rice hulls are retorted or pyrolyzed in an inert atmosphere at a temperature in the vicinity of 1800.degree.C. while removing gases and other vapors as they form, carbon in the rice hulls reacts with silica to form silicon carbide in accordance with the overall reaction of EQU 3C+SiO.sub.2 =SiC+2CO(g).
The major problem with using rice hulls to synthesize silicon carbide whiskers is that the resultant product normally contains only between about 10 and 20 weight percent silicon carbide whiskers with the remainder being silicon carbide in the form of particulates, unreacted silica and unreacted carbon. Higher yields of silicon carbide whiskers are usually not possible because the chemical composition of the rice hulls and the degree of mixing of the carbon and silica therein are set by nature and cannot readily be varied. Thus, it is difficult to obtain an intimate mixture of carbon and silica that is sufficiently porous to allow carbon monoxide gas to escape and thereby drive the overall reaction of carbon with silica to form silicon carbide to completion while allowing space for silicon carbide whiskers to grow.
It can be seen from the above that future processes for producing silicon carbide whiskers should give a relatively high yield of whiskers having desired sizes and shapes which can be tailored according to the particular use desired for the whiskers.