High temperature creep of SiC densified using a transient liquid phase
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Raymond A. Cutler Ceramatec, Inc., 2425 South 900 West, Salt Lake City, Utah 84119 (Received 14 February 1991; accepted 9 May 1991)
Silicon carbide-based ceramics can be rapidly densified above approximately 1850 °C due to a transient liquid phase resulting from the reaction between alumina and aluminum oxycarbides. The resulting ceramics are fine-grained, dense, and exhibit high strength at room temperature. SiC hot pressed at 1875 °C for 10 min in Ar was subjected to creep deformation in bending at elevated temperatures between 1500 and 1650 °C in Ar. Creep was thermally activated with an activation energy of 743 kJ/mol. Creep rates at 1575 °C were between 10~ 9 /s and 10~ 7 /s at an applied stress between 38 and 200 MPa, respectively, resulting in a stress exponent of ssl.7.
I. INTRODUCTION Silicon carbide is recognized as an important structural ceramic due to its excellent oxidation resistance, strength retention to high temperatures, high wear resistance, good thermal conductivity, and relatively low coefficient of thermal expansion. Alliegro et al.1 showed that SiC densified by hot pressing with 1.0 mol % Al had excellent strength retention to 1400 °C. Prochazka's 2 sintering of SiC in the absence of applied pressure opened up the possibility of mass production of relatively inexpensive SiC components. The use of SiC components, however, has been hampered by the low toughness (2.5-3.0 MPa • m 1/2 ) and strength (300-500 MPa) of SiC compared to Si 3 N 4 (toughness of 4 - 6 MPa • m 1 / 2 and strength of 800-1200 MPa 3 ). Silicon carbide with improved strength has been made by adding TiB 2 as a second phase,4 hot isostatically pressing (HIPing) using containment,5 or using liquid phase sintering.6"8 In all three cases, the strength improvement appears to be the direct result of limiting grain growth, thereby eliminating large grains that act as stress raisers. Lange9 showed that the addition of 10 vol. % A12O3 to SiC resulted in hot pressed SiC with an average grain size of 0.7 ^m. Recent work has shown that rapid densification of SiC is possible using liquid phase sintering.6"8 One approach used was a reaction between A12O3 and A14C3, which produced a transient liquid (due to A1 2 O 3 -A1 4 O 4 C or A1 2 O 3 -A1 2 OC eutectics10 during the process of forming A12OC in Ar or A1N in N2.6-7 Strengths in excess of 600 MPa were achieved by pressureless sintering, with additional strength improvement due to cladless HIPing.7 The formation of 2H SiC, A12OC, and A1N, all having the wurtzite hexagonal structure with similar lattice parameters, suggests that SiCAlON,11 a solid J. Mater. Res., Vol. 6, No. 9, Sep 1991 http://journals.cambridge.org
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solution between SiC, A12OC, and A1N, may form during the reactive sintering process in N 2 . Extensive characterization of hot pressed SiC-AIN compositions has been performed.12"17 The creep of SiC-AIN compositions was thermally activated and the dominant mechanism was diffusional creep. The purpose of this communication is to report on high temperatu
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