Silicon metal powders, used as a starting material for manufacturing silicon nitride ceramics, typically contain trace impurities and/or compounds of Fe, Al, Mn, Ni, Co, Cr, and Li. In addition, powder agents are added to the silicon to function as binders or pressing aids during subsequent processing; these agents, often carrying oxygen, contain trace compounds of Ca, Cr, Fe, Mn, and Na. These compounds are useful and, in some cases, generally required to promote efficient nitriding. Reaction bonded Si.sub.3 N.sub.4 technology may require oxygen carrying agents as part of the starting mixture to facilitate formation of a second phase and densification. It is, however, the impurities that reside in the nitrided body that are significant here because they become the impurities in the final product. The presence of such impurities in the final ceramic product may be detrimental to the development of optimum physical properties. The impurities can produce random flaws which lower the strength of the resulting material and other related properties.
The impurities in the nitrided reaction bonded silicon nitride body typically comprise iron, iron oxide, free silicon, unreacted Y.sub.2 O.sub.3, and other trace elements, all of which can form silicides, silicates, or other compounds detrimental to optimum physical properties. There is little if any SiO.sub.2 present after nitriding and this is a natural result of proper nitriding. The presence of unreacted Y.sub.2 O.sub.3 is undesirable because it affects the distribution and formation of second phase yttrium containing compounds in the final hot pressed product. The formation of silicides from free iron and/or iron oxide, or other trace elements, is particularly detrimental because during hot pressing, Si.sub.3 N.sub.4 is readily dissolved in the silicide liquid and precipitates as large grained beta Si.sub.3 N.sub.4 to define local pockets of silicide, large grains and porosity. These pockets are weak spots or flaws in the material when used as a cutting tool and restrict the wear life of the tool. There may also be produced a concentration of yttrium compounds which affect development of desirable oxynitrides as suitable crystalline binders.
Leaching agents have been used to strip silica from starting powders used for making alumina or silicon carbide, the agents being hydrofluoric or hydrochloric acid (see U.S. Pat. Nos. 4,177,235 and 4,238,434). However, in the process of making Si.sub.3 N.sub.4 by nitriding, SiO.sub.2 is an essential compound in producing the desired secondary phases in the nitrided body; it must not be removed. More importantly, however, even if it were assumed the nitrided body contained SiO.sub.2 and such acids were used on such body (which such references do not suggest), the hydrofluoric acid would be too strong and would dissolve necessary intergranular second phases and also silicon nitride, while the hydrochloric acid would be too weak to dissolve SiO.sub.2 in such nitrided form. Silicon dioxide is soluble in hydrofluoric acid, and insoluble in hydrochloric acid (G. G. Hawley, The Condensed Chemical Dictionery, page 773 (1977) Van Nostrand Reinhold Company, NY. Thus, the teaching of such references fails to recognize and remove the impurities with which this invention is concerned and has come to understand. Such impurities must be removed at a stage of processing very different than that taught by these references.
It would be desirable if some mode were available that obviated the need for purifying the starting materials and permitted the impurities to serve a useful function during nitriding and other related steps. Such mode would selectively eliminate the impurities at a later stage of the process so that they would not be present in the final product after they have served their useful fabricating function, such as facilitating full or partial nitriding (the latter sufficient to encapsulate the silicon particles). In this manner, the impurities would not be available to promote oxidation or cracking in the final product.