Grain glassy phase

No wetting of pure Mg at 850-950 °C in vacuum or air 90-150° [482, 484] Wetting, 10° at melting point [484] Reaction of the oxides with the glassy grain boundary phase... 

CuO, and PbO react at temperatures >600-700 °C quite strongly with the grain boundary phase and accelerate the oxidation and degradation. At temperatures below the transition temperature Tg of the glassy phase this interaction can be neglected because of the low ion diffusion into the grain boundary. 

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 consolidation mechanism of many of these ceramic composites involves a thin glassy phase linking the matrix grains, the reinforcement and the crystalline portion of the sintering additive.14 The thermal and mechanical properties of all these phases are quite different, which will therefore influence the behaviour of the composite. Although the coefficient of thermal expansion... 

Another method is to apply isostatic pressure and hot-isostatic pressing (HIP), now being another established technique. This technique is a very attractive because it offers possibilities of making dense SiAION ceramics with a negligible residual glassy grain boundary phase and hence better high-temperature properties. However, HP and HIP techniques are very costly. 

As mentioned above, several mechanisms can be responsible for the grain boundary sliding accommodation however, so far there is no consensus on a general single mechanism to accommodate GBS, nor one concerning a particular ceramic. In this section the different mechanisms for accommodation will be analysed. For the sake of clarity, the accommodation process will be described for each type of ceramic, whether monolithic, with secondary glassy phases or composite. 

These glassy phases may act as a lubricant for grain boundary sliding. In this case, the accommodation mechanism is the viscous motion of these secondary phases. 

The secondary glassy phases can improve the diffusivity pathways throughout the grain boundaries. Accommodation is controlled by diffusion along them. 

At high temperatures the glassy phase may become less viscous and even liquid and as a consequence may account for the plastic deformation. However, viscous flow creep is not regarded as a viable creep mechanism for superplasticity due to its limited deformation, which corresponds to the redistribution of the glassy phase and therefore to the squeeze of these secondary phases from grain boundaries subjected to compression.8... 

This model postulates that the glassy phase in compression is able to support normal stresses because of the existence of islands, thus avoiding the complete squeeze of this intergranular liquid (Fig. 16.3). However, it has been shown that these islands are not necessary for the grain boundaries to support normal stresses. Several modifications and revisions of this first model have been made.8... 

The nature of the grain boundaries when segregation or glassy phases exist... 

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