Thermal Si3N4 ceramics, mechanical

In inert atmospheres the mechanical properties of RBSN are constant up to 1200-1400 °C because of the absence of a glassy grain boundary phase, which is also the reason for the excellent thermal shock and creep behaviour. The thermal shock resistance, hardness and elastic constants depend on the microstructural parameters but are much lower than for dense Si3N4 ceramics [539]. 

The sialons are a group of ceramic compounds of increasing technological importance for their thermal, chemical and mechanical properties. These are similar to Si3N4 but can be more readily tailored to specific requirements because of the wide range of possible sialon compositions. 

Mechanical Properties of High-Thermal Conductivity Si3N4 Ceramics... 

Though these materials are thermally very stable and mechanically inert they do only poorly resist mechanical stresses. This drawback can be overcome by reinforcing the monolithic ceramics with fibers. Today SiC-, Si3N4- and siliconcarbonitride fibers are commercially available, which are used in worldwide research activities to develop bulk component parts with composites. 

BN fibers are used for reinforcing ceramic materials (e.g., Al203, Si3N4, SiC) to enhance mechanical properties as well as to extend the range of possible applications. They serve as reinforcement of organic polymers (e.g., epoxides, polyether-polyketones, polyphenylensulfides) which exhibit good thermal conductivity and low thermal expansion. 

The thermodynamics of the above-elucidated SiC/C and SijN Si composites are determined by the decomposition of silicon carbide and silicon nitride, respectively, into their elements. The chemistry of ternary Si-C-N composites is more complex. If producing Si-C-N ceramics for applications at elevated temperature, reactions between carbon and silicon nitride have to be considered. Figure 18.2, which exhibits a ternary phase diagram valid up to 1484°C (1 bar N2) displays the situation. The only stable crystalline phases under these conditions are silicon carbide and silicon nitride. Ceramics with compositions in the three-phase field SiC/Si3N4/N are unknown (this is a consequence of the thermal instability of C-N bonds). Although composites within the three-phase field SiC/Si3N4/Si are thermodynamically stable even above 1500°C, such materials are rare. The reasons are difficulties in the synthesis of the required precursors and silicon melting above 1414°C. The latter aspect is of relevance, since liquid silicon dramatically worsens the mechanical properties of the derived ceramics. 

This chapter discusses the behavior, under thermal shock conditions, of epoxy resins toughened with ceramic particulates. Alumina Al203 and silica Si02, which are usually used as filler for insulation materials, and the new ceramic materials silicon carbide SiC and silicon nitride Si3N4 are employed. For these toughened epoxy resins, the thermal shock resistance is evaluated by using fracture mechanics. The difference between experimental and calculated values of the thermal shock resistance is discussed from a fractographic point of view. 

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