Refractory grain boundaries

Stacking faults thereby providing barriers to sHp. If carbides are allowed to precipitate to the point of becoming continuous along the grain boundaries, they often initiate fracture (see Fracture mechanics). A thorough discussion of the mechanical properties of cobalt alloys is given in References 29 and 30 (see also Refractories). 

The behavior of cavities during deformation also depends on the refractoriness of the bonding matrix. In a recent study,33,34 stable cavities were observed to form at the grain boundaries of a grade of silicon nitride containing 4wt.% yttria, even though there was very tittle glass at these boundaries Fig. 4.14a. The cavities observed were reminiscent of Hull-... 

Fig. 4.14 Cavity formation in silicon nitride, (a) Lenticular cavities form at grain boundaries for grades of silicon nitride bonded by highly refractory glasses, (b) Crack-like cavities form at boundaries for grades of silicon nitride that are less refractory, (c) In addition to these interfacial cavities, more irregular, /urn-size cavities are also observed at multigrain junctions in silicon nitride. From Hockey and Wiederhom,94 Luecke et a/.101 and Wiederhom et al.100...

Sintering can be brought about by a variety of reactions. Of these, material transport by viscous flow is important in glasses but less so in metals or ceramics. Evaporation and condensation is important in rather volatile compounds such halides and some oxides. Diffusion - bulk, grain boundary and surface diffusion - is important for refractory materials and, for these materials, the presence of traces... 

The hardness at elevated temperatures depends additionally on the softening of the glassy grain-boundary phase. Materials with Mg0/Al203 as sintering additives show a faster degradation of the hardness than materials with more refractory grainboundary phases (Fig. 20). 

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