Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 216

Owing to the inherently nonperiodic arrangement of atoms in glass, use of scanning probes is of special interest for studying the topography of a sample surface and inspecting, for instance, its roughness after polishing. Besides, information on the structure of a crystal present at the surface can be obtained, especially if some relaxation or reconstruction phenomena have caused a local change of its periodic arrangement.

Among the probe microscopy techniques that have been designed, atomic force microscopy (AFM) is probably the most widespread. Rather than describing its noncontact or tapping modes, we will restrict ourselves to the basic principle of its contact mode, which is sketched in Figure 9. When a cantilever is moved against the sample surface in an x–y raster scan, it is deflected differently depending on the local roughness or composition of the sample surface. Because the deflection can be monitored precisely via the reflection of a laser beam on the cantilever head, it can be kept constant with an appropriate electronic feedback loop (constant‐force AFM). If this force is modulated in the z‐direction with a certain oscillation frequency, the sample surface resists the oscillation, so the cantilever bends according to the stiffness of the probed sample area. With this force‐modulation approach, the information obtained thus goes farther than surface topography since a certain measure of the surface elasticity is also obtained.

Again the MAS sample illustrates the results that can be obtained with AFM in terms of surface topography and hardness (Figure 10). For a polished surface, identification of the dark (glass) and bright (spinel or zirconia) areas apparent in the micrograph has to rely on SEM or (S)TEM experiments, but the force‐modulation micrograph yields information not available by other means on the much greater hardness of the crystalline phases compared with that of the glass matrix.

Figure 9 Working principle of the atomic force microscope in the contact mode.

Figure 10 Topography (left) and hardness (right) contrasts between spinel and the glass matrix of the MAS sample in AFM micrographs. (a) Unetched, polished sample and (b) superficially etched sample. Hardness contrast derived from a mapping of the cantilever amplitude damping.

Because different phases are not attacked by acid solutions at the same rate, etching of glass‐ceramic surfaces by HF or HF–HNO₃ solutions can enhance the microstructure contrast in AFM observations. In the MAS sample, the SiO₂‐rich glass is, for instance, more prone to dissolution upon HF etching than spinel and zirconia, so the dendritic‐ or “finger‐like” shape of the spinel growth front is more clearly seen (Figure 10b). In contrast, the large residual glassy areas that are much less etched have a different composition. Indeed, from the EDXS results described in Section 3.2, it is known that these residual glassy areas are strongly enriched in Y.