Grain boundaries etching

An accelerated rate of grain boundary grooving (thermal etching) has been observed in alumina ceramics (Fig. 9), where the etching rate was especially enhanced as the level of impurities increased. The microwave enhancement of the grain boundary etching... 

Figure 2. Geometric pitting and grain boundary etching in anodic dissolution,...

Racks 2 and 3 were disassembled, and the coupons were examined, weighed and reassembled for continued exposure. Pitting was detected only at the grain boundaries of polished coupons. This was thought to be due to grain boundary etching and not to be caused by the storage pool environment. Some crevice corrosion was noted between the crevice coupons, and some impurities... 

Picral is better than nital for revealing annealed microstructures. Does not reveal ferrite grain boundaries. Etch by immersion or swabbing. 

The conditions for color etching and subsequent grain boundary etching of silicon carbide and grain boundary etching of boron carbide are given below as examples. 

SiC Color etching, dependent on grain orientation with 10% oxalic add at IS V Grain boundary etching after removal of colored surface layer with 10-20% hydrofluoric add 30-40 s... 

Figure 70. Sintered aluminum nitride ceramic, (a) DIC. By finishing with a relief pohshing step using colloidal SiOj, the grain structure is made visible. The white components consist of iron and silicon, (b) BF. Grain boundary etching by a chemical method. The phases consisting of iron and silicon have been dissolved away.

Figure 119. After removing the etch coatings with 10% hydrofluoric acid solution, growth phenomena in the silicon carbide layers are revealed by electrolytic grain boundary etching with 10% oxalic acid, BF.

The authors also studied both the diffuse reflection spectrum and the photochemical behavior of the deposited films. The efficiency of cathodic photocurrent production was dependent on the deposition potential. The efficiency peaked at a deposition potential of -0.40 volts. The authors attributed this to the presence of excess Te at potentials more positive than -0.40 volts and to the n-type character of the CdTe at potentials more negative than -0.40 volts. The effects of heat treatment and etching were also studied. Heat treatment improved the photocurrent due to the increased grain size which eliminated grain boundaries. Etching had the effect of removing the surface layer of Te and improving photocurrents. 

Extended defects range from well characterized dislocations to grain boundaries, interfaces, stacking faults, etch pits, D-defects, misfit dislocations (common in epitaxial growth), blisters induced by H or He implantation etc. Microscopic studies of such defects are very difficult, and crystal growers use years of experience and trial-and-error teclmiques to avoid or control them. Some extended defects can change in unpredictable ways upon heat treatments. Others become gettering centres for transition metals, a phenomenon which can be desirable or not, but is always difficult to control. Extended defects are sometimes cleverly used. For example, the smart-cut process relies on the controlled implantation of H followed by heat treatments to create blisters. This allows a thin layer of clean material to be lifted from a bulk wafer [261. 

The sputtering process is frequendy used in both the processing (e.g., ion etching) and characterization of materials. Many materials develop nonuniformities, such as cones and ridges, under ion bombardment. Polycrystalline materials, in particular, have grains and grain boundaries that can sputter at different rates. Impurities can also influence the formation of surface topography. ... 

The corrosion behaviour of different constituents of an alloy is well known, since the etching techniques used in metallography eu e essentially corrosion processes which take advantage of the different corrosion rates of phases as a means of identification, e.g. the grain boundaries are usually etched more rapidly than the rest of the grain owing to the greater reactivity of the disarrayed metal see Sections 1.3 and 20.4). 

Fig. l.S(a) Grain boundary intersecting an etched metallographic surface and (b) etch pit at a dislocation interseaing an etched metallographic surface... 

The common defects arising in processing include etching —preferential attack of grain boundaries —which occurs if the film has not fully formed it may be exploited in certain circumstances because the finish can be artistically attractive and the surface area may be increased. Pitting occurs if the film is disrupted at local sites, either by incorrect balance of film former/contaminant or by gas evolution on the surface. 

Figure 6.12 Grain boundary after polishing (a) and after the subsequent thermal etching (b).

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