1. Field of the Invention
This invention relates to a silicon nitride-based sinter which excels as in resistance to chemicals.
2. Description of the Related Art
Ceramic sinters having silicon nitride as a main component thereof possess various ideal properties such as very high heat resistance, a small thermal expansion coefficient, and excellent resistance to thermal shock. Thus, attempts are being made to use these ceramic sinters in engine parts, machine parts made of steel, etc. as high-temperature structural materials in the place of conventional heat-resistant alloys. Further, since they excel in frictional resistance, efforts are being continued to realize the use of these ceramic sinters in sliding members and cutting tools.
Generally, silicon nitride is a ceramic material possessing a very poor sintering property. Thus, the silicon nitride powder prepared as a raw material for sintering has the sintering property thereof improved by having such a sintering auxiliary as a rare earth element oxide or aluminum oxide incorporated in a prescribed amount therein so as to permit production of a ceramic sinter having high density and high strength. As concrete examples of the sintering composition (raw material composition) for silicon nitride-based ceramics, such as the Si.sub.3 N.sub.4 --Y.sub.2 O.sub.3 composition, the Si.sub.3 N.sub.4 --Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --AlN composition, and the Si.sub.3 N.sub.4 --Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 oxide of Ti,Mg or Zr composition have been known heretofore.
In the sintering compositions mentioned above, yttrium oxide and other similar rare earth element oxides and aluminum oxide perform the function of exalting the sintering property of silicon nitride and enhancing the densification of the sinter of silicon nitride. The silicon nitride-based sinters manufactured from raw materials incorporating a rare earth element oxide or aluminum oxide have a dense texture and excel in mechanical strength properties. Measures have been adopted to improve the mechanical properties and the frictional resistance manifested by silicon nitride sinters at elevated temperatures by causing the raw material powder to incorporate therein a combination of a plurality of kinds of compounds such as aluminum nitride and titanium oxide.
The silicon nitride-based sinters which have such sintering compositions as mentioned above, however, are invariably deficient in resistance to corrosion caused by such chemicals as acids and alkalis. When they are adopted as structural materials which are destined to be used in an environment having such chemicals contained therein, therefore, they pose the problem of providing neither expected durability nor reliability. Particularly, the rare earth oxides are at a disadvantage in readily yielding to corrosion by such chemicals as acids or alkalis and consequently inducing serious degradation of the strength of silicon nitride-based sinters, though they manifest a conspicuous effect in densifying silicon nitride.
As concrete examples of the silicon nitride-based sinter using no rare earth element oxide, sinters using 0.5 to 5 parts by weight of spinel and 0.5 to 5 parts by weight of titanium carbonate as sintering auxiliaries (JP-B2; 63-23,153) and silicon nitride-based sinters obtained by using a mixture of silicon oxide with silicon carbide not as sintering auxiliaries but as a starting raw material and subjecting this raw material to reactive sintering in an atmosphere of nitrogen gas (JP-A-59-152,270) have been known to date. Though these silicon nitride-based sinters exhibit high strength at elevated temperatures, they are not intended at all for the improvement of resistance to chemicals (resistance to corrosion) and they actually offer no sufficient resistance to chemicals. Further, composite sinters of silicon nitride and silicon carbide (JP-A-63-159,256) have been known heretofore. Since the manufacture of these composite sinters necessitates use of yttrium oxide and other rare earth element oxides as sintering auxiliaries, the composite sinters naturally offer no sufficient resistance to chemicals.
Sinters made of .beta.-Sialon as the raw material are widely used as materials capable of exhibiting relatively high resistance to chemicals. Since they are deficient in mechanical properties, they have the disadvantage of having only a limited range of utility.
In recent years, the demand has increased for heat resisting members and abrasion resisting members which are usable under an atmosphere attended by chemical substances has been growing. For uses of this nature, the corrosion-proof heat-resistant alloys and heat-resistant hard metals which have been heretofore in popular use are not easily adapted. Thus, the appearance of ceramic sinters which excel metals in resistance to heat and resistance to abrasion and offer excellent resistance to chemicals (proofness against corrosion) as well has been earnestly longed for.