Ceramic glassy phase

Finally, one should take note of the fact that in all Si3N4 ceramics a residual, continuous glassy phase remains between the crystalline phases, even after prolonged annealing time at temperatures above 1300 K. This amorphous phase is neglected in all phase diagrams (Sect. 3), yet strongly affects the properties. Therefore much effort has been made to characterise the amorphous phase which will be treated in more detail in Sect. 6.1.4.3. 

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... 

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. 

For ceramics with secondary glassy phases, the accommodation processes are governed by these phases. Although diffusion may occur, the glassy phase viscosity controls accommodation mainly in different ways ... 

Clarke, D.R., Shaw, T.M., Philipse, A. and Horn, R.G., (1993), Possible electrical doublelayer contribution to the equilibrium thickness of intergranular glassy phases in polycrystalline ceramics , J. Am. Ceram. Soc., 76 (5), 1201-1204. 

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