Seeds Innovation and Bearing Applications of Silicon Nitride Ceramics

摘要

Silicon nitride (Si_3N_4) is one of the most attractive materials for wear applications because it has excellent wear resistance and offers advantages such as light weight, higher strength and toughness, and good corrosion resistance. In 1984, Materials Div., Toshiba Corp. (today, Toshiba Materials Co., Ltd.) and Koyo Seiko Co. Ltd. (today JTEKT Corp.) successfully utilized high-strength silicon nitride for anti-friction bearings for the first time in the world. This ceramic bearing was a most successful product and has expanded in area and volume through key innovations such as pioneered compositions, further improvement of durability against a steel ball and the development of a conventional fabrication process. Since 1989, Yokohama National University group has investigated new materials development in silicon nitride ceramics, densification/strengthening mechanisms in an optimized sintering aids system, powder processing for reliable components and tribological evaluation for bearing applications. Subsequently it was confirmed that the addition of TiO_2 and A1N to an Si_3N_4-Y_2O_3-Al_2O_3 system promoted densification at low temperatures. During firing, the TiO_2 changed into TiN at the grain boundary, causing grain boundary strengthening. Most recently, it has developed a carbon nanotube (CNT) dispersed silicon nitride with high strength and high electrical conductivity that is expected to open up new applications as a new functional silicon nitride. However, there are many items to be overcome toward the future, which are the development of cost reduction processes with higher material reliability, and the opening up of new applications supported by validated evaluation techniques including tribology, flaw detection and life prediction, raw powder problems related to cost and production volume, and the classification of silicon nitride bearings for various graded applications.