Transmission electron microscopy of liquid phase densified SiC
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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)
Transmission electron microscopy was used to characterize microstructures of SiC densified using a transient liquid phase (resulting from the reaction of A12O3 with A14C3) by hot pressing at 1875 °C for 10 min in N 2 . High resolution electron microscopy showed that the SiC grain boundaries were free of glassy phases, suggesting that all liquid phases crystallized upon cooling. Phases that might be expected due to reactive sintering (i.e., A1N, A12OC, A12O3, A14O4C, A13O3N, or solid solutions of SiC, A1N, and A12OC) were not observed. However, significant Al, Si, O, and C concentrations were found at all triple junctions of these rapidly densified ceramics.
I. INTRODUCTION Silicon carbide, a high temperature structural ceramic used in a wide variety of applications, is conventionally pressureless sintered by heating to temperatures near 2100 °C for times between 30 and 60 min.1'2 Small additions of boron and carbon are used to promote solid state sintering.3'4 SiC has also been sintered with small additions of Al, 5 Al and C additions,6 or A12O3 and C.7 A number of oxide additives have been proposed as sintering aids.8 Recent results have shown that rapid liquid phase sintering is possible with oxide additives.9 The compounds 2H SiC, A12OC, and A1N, all of which have the hexagonal wurtzite structure with similar lattice parameters, form a solid solution known by the acronym SiCAlON.10 Extensive characterization of hot pressed SiC-AIN compositions has been performed.11"16 SiC-Al 2 OC ceramics densified using a transient liquid phase, based on reactions between A12O3 and A14C3, have recently been studied.17"19 When sintering occurs in Ar, the oxycarbide is formed by the reaction:
Work by Kuo et al.21 in the SiC-AlN-Al 2 OC system has shown that a large amount of 2H solid solution is formed after hot pressing SiC, A1N, A12O3, and A14C3 powders for 30 min at 2000 °C in N 2 . The recent work of Lihrmann et al.22 shows that A12OC decomposes into A14O4C and A14C3 at temperatures below K.1115 °C in the absence of wurtzite compounds of similar lattice parameters (i.e., A1N or SiC). Lihrmann et al. have demonstrated that the presence of A14O4C is dependent on temperature and time. They show liquid formation between A12O3 and A14C3 at temperature in excess of «1800 °C. Silicon carbide hot pressed at 1850-1900 °C for 5 min, taking advantage of the liquid phase formation, has been shown to have fine grain size and good mechanical properties. 1819 The purpose of this paper is to report transmission electron microscopy observations of the microstructure of SiC densified with A12O3 and A14C3 at short times in N 2 . II. EXPERIMENTAL PROCEDURES
A1 2 O 3 + A1 4 C 3 -> 3A12OC
(1)
When processing occurs in nitrogen, the cubic spinel aluminum oxynitride (A13O3N) forms as a solid solution in the pseudobinary A1N-A12O3 system.20 A1N can be formed by reactions such as: A12OC + N 2 -+ 2A1N + CO
(2)
AI4C
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