Ceramography

Ceramography is the art and science of preparation, examination and evaluation of porcelain sanitary fixtures or decorative and commodity ceramics such as dishes and pottery.

A Brief History of Ceramography

Ceramography evolved along with other branches of materialography. Alois de Widmanstätten of Austria etched a meteorite in 1808 to reveal pro Brinell invented the first quantitative hardness scale in 1900. R.L. Smith and G.E. Sandland developed the first microindention hardness test at Vickers Ltd. in London in 1922.^[2] Swiss-born microscopist A.I. Buehler started the first metallographic equipment manufacturer near Chicago in 1936. Frederick Knoop and colleagues at the National Bureau of Standards developed a less-penetrating (than Vickers) microindention test in 1939.^[3] G.L. Kehl of Columbia University wrote The Principles of Metallographic Laboratory Practice in 1949, a book that was considered the bible of materialography until the 1980s. Kehl co-founded a group within the Atomic Energy Commission that became the International Metallographic Society in 1967.

Preparation of Ceramographic Specimens

The preparation of ceramic specimens for microstructural analysis consists of five broad steps: sawing, embedding, grinding, polishing and etching. The tools and consumables for ceramographic preparation are available worldwide from metallography equipment vendors and laboratory supply companies.

Sawing: most ceramics are extremely hard and must be wet-sawed with a circular blade embedded with diamond particles. A metallography or lapidary saw equipped with a low-density diamond blade is usually suitable. The blade must be cooled by a continuous liquid spray.
Embedding: to facilitate further preparation, the sawed specimen is usually embedded (or mounted or encapsulated) in a plastic disc, 25, 30 or 35 mm in diameter. A fluorescence microscopy.
Grinding is abrasion of the surface of interest by abrasive particles, usually diamond, that are bonded to paper or a metal disc. Grinding erases saw marks, coarsely smooths the surface, and removes stock to a desired depth. A typical grinding sequence for ceramics is one minute on a 240-diamond wheel rotating at 240 rpm and lubricated by flowing water, followed by a similar treatment on a 400-grit wheel. The specimen is washed in an ultrasonic bath after each step.
Polishing is abrasion by free abrasives that are suspended in a lubricant and can roll or slide between the specimen and paper. Polishing erases grinding marks and smooths the specimen to a mirror-like finish. Polishing on a bare metallic platen is called lapping. A typical polishing sequence for ceramics is 5-10 minutes each on 15-, 6- and 1-µm diamond paste or slurry on napless paper rotating at 240 rpm. The specimen is again washed in an ultrasonic bath after each step.
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 1500°C in air. The plastic encapsulation must be removed before thermal etching.

Alternatively, non-cubic ceramics can be prepared as analyzer.

I t = I 0 e - α x

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. Gold-coated (η ≈ 0.82, k ≈ 1.59 @ λ = 500 nm) alumina reflects 44% in air, 39% in immersion oil.

$R = \frac{I_r}{I_i} = \frac{(\eta_1 - \eta_2)^2 + k^2}{(\eta_1 + \eta_2)^2 + k^2}$

Ceramographic Analysis

Ceramic microstructures are most often analyzed by reflected visible-light microscope in brightfield. Darkfield is used in limited circumstances, such as to reveal cracks. birefringence. Very fine microstructures may require the higher magnification and resolution of a scanning electron microscope (SEM) or confocal laser scanning microscope (CLSM). The cathodoluminescence microscope (CLM) is useful for distinguishing phases of refractories. The transmission electron microscope (TEM) and scanning acoustic microscope (SAM) have specialty applications in ceramography.

Ceramography is often done qualitatively, for comparison of the microstructure of a component to a standard for quality control or failure analysis purposes. Three common quantitative analyses of microstructures are grain size, second-phase content and porosity. Microstructures are measured by the principles of stereology, in which three-dimensional objects are evaluated in 2-D by projections or cross-sections.

Grain size can be measured by the line-fraction or area-fraction methods of preferred orientation, exponential distribution of sizes, and non-equiaxed grains. Image analysis can measure the diameter(s) and shape factors of individual grains by ASTM E1382.

Second-phase content and porosity are measured the same way in a microstructure, such as ASTM E562. E562 is a point-fraction method based on the stereological principle of point fraction = volume fraction, i.e., P_p = V_v. Second-phase content in ceramics, such as carbide whiskers in an oxide matrix, is usually expressed as a mass fraction. Volume fractions can be converted to mass fractions if the density of each phase is known. Image analysis can measure porosity, pore-size distribution and volume fractions of secondary phases by ASTM E1245. Porosity measurements do not require etching. Multi-phase microstructures do not require etching if the contrast between phases is adequate, as is usually the case.

Grain size, porosity and second-phase content have all been correlated with ceramic properties such as mechanical strength σ by the dielectric constant and many others.

Microindention Hardness and Toughness

The hardness of a material can be measured in many ways. The indentation hardness or simply microhardness.

$HK = 14229 \frac{P}{d^2}$ and $HK_{GPa} = 139.54 \frac{P}{d^2}$

The toughness of ceramics can be determined from a Vickers test under a load of 10 - 20 kg. flexural strength (σ). Modulus of rupture (MOR) bars with a rectangular cross-section are indented in three places on a polished surface. The bars are loaded in 4-point bending with the polished, indented surface in tension, until fracture. The fracture normally originates at one of the indentions. The crack lengths are measured under a microscope. The toughness of most ceramics is 2-4 MPa√m, but toughened zirconia is as much as 13, and cemented carbides are often over 20.^[4]

$K_{icl} = 0.016 \sqrt{\frac{E}{H}\frac{P}{(c_0)^{1.5}}}$ initial crack length

$K_{isb} = 0.59 \left(\frac{E}{H}\right)^{1/8}[\sigma (P^{1/3})]^{3/4}$ indention strength in bending

References

^ R.E. Chinn, Ceramography, ASM International and the American Ceramic Society, 2002, p 1.
^ R.L. Smith and G.E. Sandland, “An Accurate Method of Determining the Hardness of Metals, with Particular Reference to Those of a High Degree of Hardness,” Proceedings of the Institution of Mechanical Engineers, Vol. I, 1922, p 623–641.
^ F. Knoop, C.G. Peters and W.B. Emerson, “A Sensitive Pyramidal-Diamond Tool for Indentation Measurements,” Journal of Research of the National Bureau of Standards, V23#1, July 1939, Research Paper RP1220, p 39–61.
^ K.E. Amin, "Toughness, Hardness and Wear," Engineered Materials Handbook, Vol 4: Ceramics and Glasses, ASM International, 1991, p 599-609.

Categories: Materials science

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