Grain boundaries and intergranular films

Grain boundaries can also be important in the formation and. stabilization of microinclusions. A specific example can be found in the work of Pring and Williams [70] in which TEM studies revealed microinclusions as small as a few nm of galena (PbS) in pyrite (FeS2 - Structures of this type have potential implications for. selective separation of the minerals, and may affect the ultitnate grade of mineral concentrate.s. 

Properties of intergranular films with widths of a few nm, and with composition and structure distinctly different from those of the mineral pha.ses. have been studied by XPS, SIMS, SAM and related techniques. A specific example concerns the presence of relatively thick (i.e., 25 nm) surface coatings of graphitic carbon on iron sulfide fracture faces after grinding a copper/lead-zinc sulfide ore 

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The application of surface analytical techniques to fracture faces, at which grain boundaries and intergranular films are preferentially expo.sed, also has a well-... 

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],... 

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. 

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