Intergranular film

Addition of niobium to austenitic stainless steels inhibits intergranular corrosion by forming niobium carbide with the carbon that is present in the steel. Without the niobium addition, chromium precipitates as a chromium carbide film at the grain boundaries and thus depletes the adjacent areas of chromium and reduces the corrosion resistance. An amount of niobium equal to 10 times the carbon content is necessary to prevent precipitation of the chromium carbide. 

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. 

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

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