Microstructures in Sic and Si 3 N 4 Implanted with Ti and Heat Treated
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MICROSTRUCTURES IN SiC AND Si3N4 IMPLANTED WITH Ti AND HEAT TREATED R. G. Vardiman Naval Research Laboratory Washington, DC 20375-5000
USA
ABSTRACT Bulk SiC and Si 3 N4 have been implanted with Ti at room temperature, and subsequently vacuum heat treated between 8000 and 1100 0 C. All specimens were backthinned by ion milling and examined in TEM. SiC becomes amorphous on implantion, and develops a fine dispersion of TiC precipitates up to 800'C. 0 At 900 C recrystallization has begun, possibly nucleated by the TiC particles. Si 3 N4 shows fine TiN particles in an amorphous matrix even as implanted. This structure is 0 retained up to 900 C. At 10000 C, regrowth of the Si 3 N4 apparently from the substrate begins, and the TiN particles also grow as large as 200nm. INTRODUCTION Considerable interest has developed in recent years in the use of silicon carbide and silicon nitride as structural materials, particularly for high temperature applications. Although these materials possess many attractive properties, it has been felt desirable to improve their friction and wear behavior, as well as their propensity for surface cracking. To this end a number of studies [1-15] have explored the effect of ion implantation on the microstructure and mechanical properties of Si and Si 3 N4 . Much more extensive work has been done on SiC than on Si3N4. It has been found that SiC is made amorphous by relatively low ion doses [1-10]. This effect appears to depend on the buildup of damage in the implanted layer [3,8], and is not strongly dependent on the type of ion (aside from the varying efficiency of different ions in creating damage). Si 3 N4 is expected to be comparably easy to amorphize. Surface mechanical properties are distinctly altered in the implanted amorphized material [4,6,11-14]. A number of studies have looked at the recrystallization behavior of ion amorphized SiC [1,5,10,15], with no good agreement found on the recrystallization temperature, which varied from 750 0 C to 1500 0 C or higher. In most cases Rutherford backscattering-channelling (RBS-C) was used to determine the occurrence of recrystallization, with some use of Raman scattering and transmission electron microscopy (TEM). Recrystallization temperatures determined by different techniques in the same study were found to differ substantially [10]. No work on recrystallization of ion amorphized Si 3 N4 has been found in the literature. The present study was primarily undertaken with the hope of developing a unique surface microstructure-a dispersion of TiN or TiC precipitates in an amorphous matrix. This has indeed been found. Determination has also been made of the approximate recrystallization temperature for both materials.
Mat. Res. Soc. Symp. Proc. Vol. 128.
1989 Materials Research Society
370
EXPERIMENTAL METHODS The silicon carbide used here was sintered high density a-phase, the so-called 6H hexagonal polytype. The silicon nitride was hot pressed NC132 a-Si 3 N4, also with a hexagonal crystal structure, with about 1% impurities, principally MgO.
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