2.04 - Processing of Silicon Carbide-Based Ceramics

Abstract

The history of the success of the covalently bonded compound silicon carbide (SiC) started more than a century ago (1893) with the synthesis of raw SiC by Edward Goodrich Acheson. Though even today Acheson's process is still current, other synthesis methods have also been developed for the production of SiC powders, whiskers, platelets, and fibers. Due to its low chemical reactivity, it turned out to be difficult to develop methods and processes for the production of SiC-based ceramics. In the past, many approaches for processing SiC have been attempted with specific benefits and limitations in each case. Today a large spectrum of SiC-based ceramics manufactured by a wide variety of processing techniques exists. In order to get a systematic overview over the complex set of SiC-based ceramics, they can be divided into three groups. The first group includes porous ceramics with relatively coarse SiC particles bonded in a matrix of amorphous aluminum silicate. These SiC-based ceramics may be compared with the well-known traditional silicate ceramics in terms of processing technique via sintering in air and corresponding microstructure evolution. Some grinding tools, refractory bricks and foam filters belong to this silicate bonded SiC-based ceramics. The second group is sintered as well, but the focus of attention is more sharply directed toward keeping the intrinsic characteristics of SiC as a polycrystalline ceramic material. Typically they are sintered in protective atmospheres utilizing solid-state, liquid-phase or vapor-phase (evaporation–condensation) mechanisms. However, some of the desired properties can sometimes only be achieved by using pressure-assisted sintering techniques such as axial hot pressing, hot isostatic pressing or field-assisted sintering. This group of sintered SiC-based ceramics also contains some advanced modifications such as in situ toughened or nanostructured ceramics. The third group uses three reaction bonding processes for consolidation, namely liquid–solid, gas phase, and polymer-derived reaction bonding, in order to reduce the maximum consolidation temperature and to increase densification with the aim to achieve low-to-zero shrinkage. All reaction bonding approaches result in untoughened SiC ceramics like nitride bonded, chemical vapor infiltrated silicon, carbon bonded, and ex situ toughened ceramic matrix composites (i.e. carbon and SiC fiber-reinforced ceramics). Through reaction bonding, paper and even natural products like wood can be converted into SiC-based ceramics.