Modulated Structures in SiC‐AIN Ceramics

Abstract

Single-phase solid solutions of 2H crystal type in the Sic-A1N system containing 50 and 75 mol% AlN were fabricated by hot-pressing mixtures of S i c and AlN powders. Dense samples were subjected to thermal treatments over a range of temperatures between 1600" and 1900°C for a period of time up to 320 h. Transmission electron microscopy revealed the existence of modulated structures indicative of spinodal decomposition.ORK by Cutler et a/.' showed that the w2H polymorphs of S i c and AIN form an extensive solid solution. Ruh and Zangvil'.' and Rafaniello ef ~1 .~ fabricated dense specimens in the Sic-AlN pseudobinary by hot-pressing mixtures of S i c and AlN from commercial sources and SiCAlN derived from carbothermal reduction of SiO,-AI,O, mixture in nitrogen, respectively. These authors also examined various physical and mechanical properties of the resulting ceramics. In the work of Ruh and Zangvil,* a complete 2H solid solution over the entire range of compositions was not observed although 2H was the predominant polytype (along with other polytypes) present. By contrast, Rafaniello et ~1 1 .~ observed a complete solid solution over the range of compositions from = 10 to 100 mol% AIN with 2H as the only polytype present. This difference was rationalized on the premise that carbothermal reduction of Si02-A120, in nitrogen yields an intimate mixture of S i c and AIN which facilitates the formation of the solid solution.In later work on annealed samples of Sic-AlN, Rafaniello et noted that twophase microstructures evolved with both phases being of the 2H polymorph. Precipitation occurred by a cellular mechanism as well as by nucleation and growth. Further, preliminary evidence indicated the formation of possible modulated structures.6 These results indicated that perhaps a miscibility gap exists below about 2000°C. Analysis of some of the grain-boundary precipitates indicated the existence of spinel-type structure. Kuo et al.' subsequently showed that excess oxygen may be accommodated by the formation of A1303N spinel. Consequently, precipitation reactions observed were in samples containing oxygen as an impurity.

Induction-field-activated self-propagating high-temperature synthesis of AlN–SiC solid solutions in the Si3N4–Al–C system

“…For example, formation of AlN-SiC ss during densification inhibited grain growth and, as a result, dense materials with fine microstructure were obtained. 11 However, both AlN and SiC have strong covalent bonding and the formation of AlN-SiC ss by conventional methods, such as hot pressing, requires long times (15 h) and high temperatures (1700-2300°C). 12,13 The slow kinetics are the results of low diffusion coefficients associated with such materials.…”

Field Activated Combustion Synthesis of Ceramic and Composite Materials

“…A most interesting observation was that of modulated AIN/SiC structures, implying a spinodal decomposition during the cooling down of the product. Such a decomposition was previously reported to take about 100 hrs at temperatures below the miscibility gap [8].…”

A New Route to Synthesize SiC−AlN Solid Solution Whiskers

“…Because of its good mechanical properties, the 2H solid solution between SiC and AlN has been widely reported. [1][2][3][4][5] SiC-AlN solid solution can be synthesized by a variety of methods such as high-temperature sintering, combustion synthesis, and carbothermal reduction and nitridation. Sintering is the most conventional method to prepare SiC-AlN solid solution, which is generally carried out by directly heating the mixture of AlN and SiC powders at elevated temperatures.…”

“…The lattice parameters of 2H-SiC and 2H-AlN are close, and in this way, the two compounds can form a solid solution in a wide range of compositions. Because of its good mechanical properties, the 2H solid solution between SiC and AlN has been widely reported. – …”