Grain boundary films

As discussed above, the presence of the liquid layer applies a compressive capillary force to the particles. The liquid layer evolves during the sintering process. Once the densification through the solution-precipitation mechanism starts, the thickness of the liquid layer gradually decreases with time. When the liquid capillary becomes too narrow for the liquid to flow, the solution-precipitation nearly stops. In this case, the rate of reduction in thickness of the liquid layer due to viscous flow will compete with the rate of densification. Therefore, because the flow of the liquid though the capillary is sufficiently slow, there is a thin layer of the liquid to be remained after densification, which has a thickness of 0.5-2 nm for most ceramics. 

The theory for two flat plates separated by a liquid layer has been used to calculate the rate of reduction in thickness of the liquid layer that separates two-sphere particles during the liquid-phase sintering [80]. The rate of approach of two plates separated by a Newtonian viscous liquid is expressed as [81] 

This equation indicates that the time for y = 0 is ff = oo. In other words, there will always be liquid to be remained between the particles. However, when the liquid layer is too thin, e.g., several nm, other effects, such as stmctural and chemical forces and charge interactions, will become dominant instead of viscous flow. The equilibrium thickness of the liquid layer between adjacent grains can be explained in terms of the balance between two forces, i.e., (i) the attractive van der Waals forces of the grains and (ii) the short-range repulsive forces due to the resistance to deformation of the liquid phase [82]. 

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Al-Mg (5000 Series) and Al-Mg-Si (6000 Series) In the binary alloy system strength is obtained mainly by strain hardening. Stress corrosion is thought to be associated with a continuous grain boundary film of Mg,Alg which is anodic to the matrix . Air cooling prevents the immediate formation of such precipitates, but they form slowly at ambient temperatures. Thus only low Mg alloys are non-susceptible (Al-3% Mg). Widespread precipitation arising from plastic deformation with carefully controlled heat-treatment conditions can lower susceptibility. Al-5Mg alloys of relatively low susceptibility are subjected to such treatments. Mn and Cr... 

Kleebe, H.J., Hoffmann, MJ. and Riihle, M., Influence of secondary phase chemistry on grain boundary film thickness in silicon nitride , Zeitschrift fur Metallkunde, 1992 83(8) 610-617. 

Wang, C-M.,Pan,X., Hoffmann, M.J., Cannon, R.M. andRuhle, M., (1996), Grain boundary films in rare-earth-glass-based silicon nitride , J. Am. Ceram. Soc., 79 (3), 788-792. 

Figure 12.4 (a) Intergranular fracture of an alumina sample showing creep cavitation due to compressive creep at 1600°C. Note closely spaced cavities along the two-grain facets. (b) Schematic of cavity formation in viscous grain boundary films as a result of applied tensile stress. 

There is furthermore recent evidence from transmission electron microscopy (TEM) studies for the dissolution of matrix grains in the grain-boundary phase during oxidation at high temperatures, which widens the grain boundary films as a function of depth into the material [157,158]. This type of penetration will be even more difficult to detect and quantify. 

After sintering, the additives are situated at the grain boundary (3-20 vol.%). The majority of the grain-boundary phase is concentrated in the triple junctions. The thickness of the grain boundary films between two grains is in the range of 0.8-1.5nm[41]. 

In the latter case, the SiC and/or additions are reacted to an intermediate liquid which not only provides densification at reduced temperatures but also, as it is consumed in the reaction, yields a SiC-based material without grain boundary films. 

Figure 16.4 Illustration of idealized two-dimensional microstructure ofp-Si3N4 the [210] and [001] directions of the p rains are perfectly oriented to directions designated Para and Perp, respectively the grain-boundary film thickness, 6, is independent of crystallographic...

Fig. 2.52 Representative HRTEM photographs of boundaries oriented a parallel and b perpendicular to the applied load direction, indicating that the grain-boundary film thickness decreased after superplastic deformation, under compression ((->) applied stress direction during deformation) [51], With kind permission of John Wiley and Sons...

One of these concepts, regarding GBS, is associated with the presence of an amorphous grain-boundary film along the boundaries between the grains. More specifically, this film has often been termed a glassy phase and considered... 

Fig. 6.65 High-resolution transmission electron micrograph of as processed ABC-SiC, showing the amorphous grain-boundary film [41], With kind permission of Elsevier...

Figure 10.14ab Amorphous grain boundary film with a constant equilibrium thickness in (a) Si3N4(Y203) (Courtesy of M. Cinibulk) (b) SiC (A1 -I- B -I- C) (Courtesy of L. C. De Jonghe) ...

Grain boundary films in ceramics are not just restricted to the thin equilibrium films discussed so far. In a variety of other ceramics, thicker glass films (10 nm to several microns) are also observed, which have a significant effect on microstructure and properties. These thicker films represent a different regime of behavior and may vary in thickness with the volume fraction of liquid and from one boundary to another in a given sample of material. 

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