Etch pits systems

Low-temperature ductility is rarely observed in ceramics, which are inherently brittle, but some bulk ceramics show plasticity at ambient temperatures. One example of low-temperature plasticity in MgO is considered here. First, consider a single crystal, where i orientation-dependent properties are of interest. Orientation is one of the factors that influence mechanical properties. It was observed (by etch-pit technique) that the flow in MgO occurs on the 110 (110) slip system. However, it was also found [28] that the 110 (110) slip system contributes to deformation above 600 °C. Details on Plastic deformation in MgO single crystals were presented in Sect. 2.2, Figs. 2.33 and 2.38. Consequently, some information on deformation in polycrystalline ceramics may be of interest. 

Unlike the main minerals with the rock salt structure, fluorite cleaves on 111 planes and slips on 100 in the (0 11 direction [189]. Above 200 °C, slip on jl 10 (11 0) is also possible. Above 320 °C, polycrystalline plasticity is observed with the [10 0 planes providing three independent slip systems and the 110 planes giving two more [190]. Dislocations in fluorite have been studied by etch-pitting methods, and by TEM [191] but the latter is not without difficulty, since the mineral rapidly damages under electron and ion irradiation. A cubic symmetry void superlattice can be formed in TEM if the radiation flux is not restricted [192]. 

Among the effects of impurities on selective etching, two deserve particular attention. Impurities present in an etching solution facilitate the revelation of etch pits at dislocation and often change the pit morphology. The actual process of impurity adsorption may be quite complicated for different crystal-etchant systems, but certain features are common for different systems. 

Uniform rates of corrosion such as general etch corrosion seldom occur in steam-water circuits. Rather, pitting, tuberculation, and other complex types of corrosion tend to predominate. These forms of corrosion often result directly or indirectly from reactions occurring in particular areas of the system where fouling and deposition may be present. 

Case II of Fig. 1 shows the cross-section of a pit formed by a preferential etch at a dislocation on a material whose planes form closed figures, such as the cubic system or the pyramidal planes of the hexagonal system. The relative etch rates are Rd > Ra > Ry F°r example, if the dislocation is normal to a 111 surface, triangular pyramidal pits will be formed. These pits have smooth, rather than terraced, sides as shown in Fig. 3. If, however, the dislocation line is at an angle to a ill surface, the triangular pyramidal pit is lopsided (Fig. 

Case in of Fig. 1 illustrates the cross-section of a pit formed in crystal systems, such as hexagonal, that have sets of planes that do not form closed figures. For hexagonal materials, Ryis the etch rate of the 0001 planes and R the rate of a set of prism planes. The relationship between the various etch rates is R(j > %J> Ry planes revealed depend to some extent on the etchant. Fig. 4 shows this type pit produced on a SiC by molten Na202 (17). The pyramidal pits are at the dislocations. Here the sides of the pits run in the... 

Figure 17. Surface morphology of the (111) face of Si etched in gaseous HCI. Transition lines from smooth to bunched structures are for the HJ HCI and Ar/HCI systems the transition line from smooth to pitted structures is for the H /HCI system. From Ref. (83).

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