Naoto HIROSAKI, Toshiyuki NISHIMURA,
Yoshinobu YAMAMOTO, and Yutaka SHINODA
Ceramics Team, High Temperature Materials 21 Projest
National Institute for Research in Inorganic Materials (NIRIM)
1-1, Namiki, Tsukuba-shi, Ibaraki 305-0044,
Japan
Ceramic team in this project proposes new silicon-nitride ceramics having excellent strength, oxidation resistance, and creep resistance at high temperatures.
There are many kinds of engineering ceramics, such as silicon nitride, silicon carbide, alumina, and zilconia. In these ceramics, sintered silicon nitride has been the most attractive materials for the engine applications because of high strength, high fracture toughness, and chemical stability. This material is currently used, however, only at temperatures below 1000 ℃ for mass production applications because its strength is reduced at higher temperatures. The mechanical properties at elevated temperatures must be further improved if the material is to be used in gas-turbine engines operated at higher temperatures.
Silicon nitride is difficult to densify without oxide additives because of the covalent nature of Si-N bonding. Oxide additives such as MgO and Y2O3-Al2O3 are generally used to promote densification by the liquid-phase sintering mechanism. These additives, however, remain as grain boundary glassy phases, which degrade the high-temperature properties such as creep and strength at high temperatures. Several methods to improve the mechanical and chemical properties at elevated temperatures have been proposed: (1) use less additives, (2) crystallize the glassy phase by heat treatment, (3) use an adequate composition through which the additives dissolve in the grain after sintering [1]. With these methods, the high temperature properties can be improved by minimizing the volume of the glassy phase. Another approach is to use refractory additives. NIRIM has proposed Yb4Si2O7N2 as an additive for silicon nitride [2-6]. The melting temperature of Yb4Si2O7N2 is higher than common used additives. So, silicon nitride containing this additive has excellent high-temperature properties. In this project, our team plans to develop further high-temperature properties, by grain boundary engineering and microstructure control for the Yb4Si2O7N2-doped materials.
This project will use silicon nitride as base material and select additives with high melting point, such as Yb4Si2O7N2 and other rare-earth oxinitride. New sintering technique will develop in order to decrease the amount of additives as small as possible. Heat treatment after sintering is effective for crystallization of grain boundary phase, which will improve high temperature properties. Oxidation and creep tests will be carried out with the relation of composition and amount of grain boundary phase. Finally, the creep rupture test will be done for 1000 h at 1500 ℃.
For these purpose and approach, this project will include the following tasks.
-1999: Powder processing to form large size samples.
-2000: Sintering process with new sintering additive.
-2001: Heat treatment for crystallization of grain boundary.
-2002: Grain boundary design for oxidation.
-2003: Grain boundary design for creep.
-2000: Characterization of grain boundary
-2001: Characterization of grain size distribution
-1999: Evaluation of fundamental properties of SYB1 (Y4Si2O7N2-doped Si3N4)
-2000: Design of creep test equipment for evaluation at 1400℃
-2001: Precise measurement of creep deformation
-2002-2004: Creep and creep rupture test
The high-temperature strength and oxidation resistance of Y4Si2O7N2-doped Si3N4 were examined. The strength at 1500 ℃ is 480 MPa, which is one of the most excellent value in the reported high-temperature strength of silicon nitride ceramics. The oxidation resistance properties are now testing.
1) T. Nishimura, M. Mitomo, A. Ishida, H. Gu, "Improvement of High Temperature Strength and Creep of alpha-Sialon by Grain Boundary Crystallization", Key Engineering Materials vols. 171-174 ed. by T. Sakuma, K. Yagi, Trans Tech Publications, Switzerland (2000) pp. 741-746.
2) T.Nishimura, and M.Mitomo, "Phase relationships in the system Si3N4-SiO2-Yb2O3", J.Mater.Res., 10[2] 240-42(1995).
3) T.Nishimura, M.Mitomo, H.Suematsu, "High temperature strength of silicon nitride ceramics with ytterbium silicon oxynitride", J. Mater. Res., 12, 203-209 (1997).
4) T. Nishimura, M. Mitomo, A, Ishida, H. Yoshida, Y. Ikuhara and T. Sakuma, "Effect of Al2O3 on High Temperature Mechanical Properties of Silicon nitride with Yb4Si2O7N2", J. Ceram. Soc. Japan, 105 [9] 801-804 (1997).
5) H. Yoshida, Y. Ikuhara, T. Sakuma, T. Nishimura, M. Mitomo, "High-temperature Creep Resistance in Yb2O3-fluxed Si3N4", Proceedings of the seventh International Conference on Creep and Fracture Engineering Materials and Structures, 653-62 (1997).
6) T. Nishimura, M.Mitomo, A. Ishida, H. Yoshida, Y. Ikuhara, T. Sakuma, "Heat resistant silicon nitride with ytterbium silicon oxynitride", Proceeding of 6th International Symposium on Ceramic Materials and Components for Engines, 632-37 (1997).