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Yorinobu Takigawab) and Jian-Wu Caoc) Japan Fine Ceramics Center, Nagoya, Aichi 456-8587, Japan (Received 8 March 2005; accepted 9 May 2005)

We studied tensile creep behavior in two silicon nitride ceramics, i.e., 4.8 mol% Lu2O3 (SN48) and 1.2 mol% Lu2O3 (SN12), at 1400–1500 °C under applied stress of 137–300 MPa. Time to failure of SN48 increased with decreasing applied stress and minimum strain rate. The stress–rupture parameter was 10.7 at 1400 °C and 11.4 at 1500 °C. Pore formation was confirmed in a creep-tested specimen of SN48 by transmission electron microscopy. These results suggest that SN48 was fractured by creep rupture. The minimum strain rate of SN12 was almost below the measurement system limitation at temperatures below 1500 °C. Time to failure tended to increase with decreasing applied stress. The stress–rupture parameter was 41 at 1400 °C and 73 at 1500 °C. These results suggest that SN12 was fractured by subcritical crack growth.

I. INTRODUCTION

Silicon nitride ceramics were developed for application to gas turbine components. A project for developing heat-resistant materials, High-Temperature Materials 21 Project (HTM21), was implemented by the National Institute for Materials Science (NIMS). One of aims of the project was to develop ceramics that did not fracture at 1500 °C under tensile stress of 137 MPa for 1000 h. Therefore, we tried to develop silicon nitride ceramics by controlling chemical composition and grain boundary crystallization. Based on Si3N4–Yb2O3–SiO2 phase equilibrium at 1750 °C, we fabricated a silicon nitride ceramic with Yb 2 O 3 and SiO 2 . The Yb-J-phase, Yb4Si2O7N2, coexists with Si3N4 in equilibrium and was a candidate for the grain boundary phase because of its high melting point, 1870 °C.1 It crystallized during sintering, although post-sintering heat treatment for grain boundary crystallization was usually needed.2 The hightemperature strength of Si3N4 with the Yb-J-phase decreased with increasing testing temperature, but it broke and remained brittle even at 1500 °C.2 Its oxidation resistance was roughly equal to silicon nitride ceramics

a)

Address all correspondence to this author. e-mail: [email protected] b) Present address: Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan. c) Present address: National Engineering Research Center for Nanotechnology, Huashan Road 2018, Shanghai, China. DOI: 10.1557/JMR.2005.0278 J. Mater. Res., Vol. 20, No. 8, Aug 2005

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with RE2Si2O7, reported as a heat-resistant silicon nitride (rare earth element; RE).3 Tensile creep test results suggest that it sustained creep life at 1300 °C for 1000 h at tensile stress of 137 MPa,4 so we attempted to develop more heat-resistant Si3N4. Andersson and Barton reported the effects of rare-earth elements on the flexural strength of Si3N4 ceramics prepared from powder mixtures of Si3N4–Si2N2O–RE2Si2O7. Flexural strength increased with decreasing ionic radii of rare-earth elements.5 These results sugges