Grain boundary glassy phases

Figure 7.6 Transmission electron micrograph showing the presence of a grain boundary glassy phase in silicon nitride. (Courtesy of X. G. Ning and D. S. Wilkinson, McMaster University.)...

Figure 7.7 Schematic illustration of a) dissolution-precipitation and b) liquid redistribution mechanisms in a material containing a grain boundary glassy phase.

Microscopy did not reveal any grain boundary glassy phase. b) Determine the activation energies for steady-state ereep. c) Predict the creep rate for stresses of 20 and 52 MPa at 1600 °C, if the ... 

Describe a mechanism by which a grain boundary glassy phase can lead to creep. 

A grain boundary glassy phase has a small effect, particularly on the threshold stress intensity factor Kio and on stage I (fig.6). In this case, it can be argued that the... 

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]. 

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. 

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