page_title: Ceramography
section_title: Etching
hierarchical_section_title: Ceramography / Preparation of ceramographic specimens / Etching
caption_attribution_description: English: Left to right: Ceramographically mounted titanium nitride (TiN) bar and chromia-alumina (Cr2O3-Al2O3) refractory brick, and metallographically mounted steel bicycle hub axle cross-section.
context_page_description: Ceramography is the art and science of preparation, examination and evaluation of ceramic microstructures. Ceramography can be thought of as the metallography of ceramics. The microstructure is the structure level of approximately 0.1 to 100 µm, between the minimum wavelength of visible light and the resolution limit of the naked eye. The microstructure includes most grains, secondary phases, grain boundaries, pores, micro-cracks and hardness microindentions. Most bulk mechanical, optical, thermal, electrical and magnetic properties are significantly affected by the microstructure. The fabrication method and process conditions are generally indicated by the microstructure. The root cause of many ceramic failures is evident in the microstructure. Ceramography is part of the broader field of materialography, which includes all the microscopic techniques of material analysis, such as metallography, petrography and plastography. Ceramography is usually reserved for high-performance ceramics for industrial applications, such as 85–99.9% alumina in Fig. 1, zirconia, silicon carbide, silicon nitride, and ceramic-matrix composites.
context_section_description: Etching reveals and delineates grain boundaries and other microstructural features that are not apparent on the as-polished surface. The two most common types of etching in ceramography are selective chemical corrosion, and a thermal treatment that causes relief.  As an example, alumina can be chemically etched by immersion in boiling concentrated phosphoric acid for 30–60 s, or thermally etched in a furnace for 20–40 min at 1,500 °C (2,730 °F) in air.  The plastic encapsulation must be removed before thermal etching.  The alumina in Fig. 1 was thermally etched.
  Alternatively, non-cubic ceramics can be prepared as thin sections, also known as petrography, for examination by polarized transmitted light microscopy.  In this technique, the specimen is sawed to ~1 mm thick, glued to a microscope slide, and ground or sawed (e.g., by microtome) to a thickness (x) approaching 30 µm.  A cover slip is glued onto the exposed surface.  The adhesives, such as epoxy or Canada balsam resin, must have approximately the same refractive index (η ≈ 1.54) as glass.  Most ceramics have a very small absorption coefficient (α ≈ 0.5 cm ⁻¹ for alumina in Fig. 2) in the Beer–Lambert law below, and can be viewed in transmitted light.  Cubic ceramics, e.g. yttria-stabilized zirconia and spinel, have the same refractive index in all crystallographic directions and appear, therefore, black when the microscope's polarizer is 90° out of phase with its analyzer.
   (Beer–Lambert eqn)
  Ceramographic specimens are electrical insulators in most cases, and must be coated with a conductive ~10-nm layer of metal or carbon for electron microscopy, after polishing and etching.  Gold or Au-Pd alloy from a sputter coater or evaporative coater also improves the reflection of visible light from the polished surface under a microscope, by the Fresnel formula below.  Bare alumina (η ≈ 1.77, k ≈ 10 ⁻⁶) has a negligible extinction coefficient and reflects only 8% of the incident light from the microscope, as in Fig. 1.  Gold-coated (η ≈ 0.82, k ≈ 1.59 @ λ = 500 nm) alumina reflects 44% in air, 39% in immersion oil.
   (Fresnel eqn)
caption: Embedded, polished ceramographic sections.
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