Ceramics intergranular films

The corrosion resistance of selected Si3N4 ceramics in NaOH is given in Fig. 40. The attack by bases is less pronounced than by acids. The extent of corrosion increases with increasing temperature and concentration of the bases [524], Also, in bases the intergranular films are not attacked as strongly as the triple points. In NaOH solutions often a linear dependence of the weight loss on time is reported. Materials which are less stable in acids are more stable in bases. This can be explained by the stability of the grain boundary phase [510],... 

Yoshiya M, Tanaka I, Adachi H (2000) Atomic Structure and Chemical Bonding of Intergranular film in Si3N4-Si02 Ceramics. In Sajgalik P, Lences Z (eds) Engineering Ceramics Multifunctional Properties-New Perspectives. Trans Tech Publications Ltd, Switzerland, p 107... 

Becher PF, Sun EY, Hsueh CH, Painter GS, More KL (2000) Role of Intergranular Films in Toughened Ceramics. In Sajgalic P, Luences Z (eds) Engineering Ceramics Multifunctional Properties - New Perspectives, Key Eng Mat 175-176. Trans Tech Publications, Switzerland, p 97... 

Detailed HRTEM characterization of the specimens showed that all metal-ceramic interfaces in the two different nanocomposites had thin ( 1 nm thick) amorphous films (see Fig. 11.5). In addition, occluded particles were found inside the alumina grains which also had thin amorphous films at their interfaces with alumina. Analytical microscopy showed the films to contain Ca, Si, and Al.41 Hamaker coefficients were calculated for metal-ceramic interfaces in the presence of a Si02-based film, which indicated that a stronger attractive force is expected for intergranular films at metal-alumina interfaces,... 

The ability to modify the metal-ceramic interface in nanocomposites by the formation of intergranular films holds exciting prospects. From a thermodynamic point of view, the existence of a film at equilibrium indicates a lower interface energy than an interface without a film. This indicates the potential to increase the adhesion of interfaces, although experimental investigations are required to fully evaluate this effect. However, the promotion of particle occlusion due to the presence of the films has been shown,28 and this means that a new method to modify and control the microstructural evolution of nanocomposites is available, as discussed in the next section. 

Avishai, A. and Kaplan, W.D., Intergranular films in metal-ceramic composites and the promotion of metal particle occlusion , Zeitschrift fur Metallkunde, 2004 95 266-270. 

However, when HRTEM was employed on the SiC samples, which showed a similar contrast variation across SiC grain boundaries in the SEM, the presence of residual intergranular films was not detected even at the triple junctions. Hence, Kleebe concluded that SEM imaging and Fresnel fringe TEM imaging alone do not enable a safe conclusion to be drawn about interface wetting in ceramic polycrystals. 

For a suitably high critical value of A, this theoretical model predicts a lower limit on the equilibrium thickness that can be observed. This lower limit on Z, Z n, is defined by the conditions F= 0 and <1F/<1L = 0 since for a stable film F= 0 and dF/dL > 0 (Clarke, 1987). Various solutions to these conditions have been examined by Knowles and Turan (2000). In the absence of capillary pressure and external pressure, Zmin = 2.58. Using reasonable estimates for Knowles and Turan estimate Zmin to be >6.50 A. That in practice the observed intergranular film thicknesses are typically of the order of 1-2 nm in non-oxide engineering ceramics indicates that the relevant Hamaker constants for ceramics are significantly lower than the critical value. 

Susnitzky, D.W. and Carter, C.B., (1990), Structure of alumina grain boundaries prepared with and without a thin amorphous intergranular film , J. Am. Ceram. Soc., 73 (8), 2485-2493. 

Cinibulk, M.K., Kleebe, H.-J., Schneider, G.A. and Ruhle, M., (1993b), Amorphous intergranular films in silicon nitride ceramics quenched from high temperatures , J. Am. Ceram. Soc., 76 (11), 2801-2808. 

Garofalini, S.H., and Luo, W.W., (2003), Molecular dynamics simulations of calcium silicate intergranular films between silicon nitride crystals ,/. Am. Ceram. Soc., 86 (10), 1741-1752. 

Observations of microcracks and thin intergranular films in ceramics by transmission electron microscopy. J. Amer. Ceramic Soc., 63, 104-6. 

P. F. Becher, G S. Painter, E.Y. Sun, C. H. Hsueh, M. J. Lance, The importance of amorphous intergranular films in self-reinforced Si3N4 ceramics, Acta Mater., 48,4493-99 (2000). 

Glass is a bulk material. This is true unless we grow it as a thin film or it is present as an intergranular film (IGF) or pocket in, or on, a crystalline ceramic. 

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