en>Rjwilmsi: /* References */Added 1 doi to a journal cite using AWB (10104)

2014-05-08T14:53:14Z

References: Added 1 doi to a journal cite using AWB (10104)

en>Citation bot 1: Citations: [Pu168] added: issue. You can use this bot yourself! Report bugs here.

2010-06-16T04:30:28Z

Citations: [Pu168] added: issue. You can use this bot yourself! Report bugs here.

New page

'''Ceramography''' is the art and science of preparation, examination and evaluation of [[ceramic]] [[microstructure]]s.<ref>R.E. Chinn, ''Ceramography'', ASM International and the American Ceramic Society, 2002, p 1.</ref> Ceramography can be thought of as the [[metallography]] of ceramics. The microstructure is the structure level of approximately 0.1 to 100 [[Micrometre|µm]], between the minimum [[wavelength]] of [[Visible spectrum|visible light]] and the resolution limit of the naked eye. The microstructure includes most grains, secondary phases, [[Grain boundary|grain boundaries]], pores, micro-cracks and hardness microindentions. Most bulk mechanical, optical, thermal, [[Electroceramics|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]] (Al2O3) in Fig. 1, [[zirconia]] (ZrO2), [[silicon carbide]] (SiC), [[silicon nitride]] (Si3N4), and ceramic-matrix [[Composite material|composites]]. It is seldom used on whiteware [[Pottery|ceramics]] such as sanitaryware, wall tiles and dishware.

<gallery caption="Ceramographic microstructures" widths="225px" perrow="2">
Image:Etch_A999.jpg|Fig. 1: Thermally etched 99.9% alumina
Image:Thin_A999.jpg|Fig. 2: Thin section of 99.9% alumina
</gallery>

== A brief history of ceramography ==
Ceramography evolved along with other branches of materialography and [[Ceramic engineering#History of Ceramics in Engineering|ceramic engineering]]. [[Count Alois von Beckh Widmanstätten|Alois de Widmanstätten]] of Austria etched a meteorite in 1808 to reveal pro[[Eutectic point|eutectoid]] [[Ferrite (iron)|ferrite]] bands that grew on prior [[austenite]] grain boundaries. Geologist [[Henry Clifton Sorby]], the "father of metallography," applied petrographic techniques to the steel industry in the 1860s in Sheffield, England.<ref>C.S. Smith, ''A History of Metallography'', University of Chicago Press, 1960, p 169–185.</ref> French geologist [[Auguste Michel-Lévy]] devised a chart that correlated the optical properties of minerals to their transmitted color and thickness in the 1880s. Swedish metallurgist J.A. [[Brinell scale|Brinell]] invented the first quantitative hardness scale in 1900.<ref>V.E. Lysaght, ''Indentation Hardness Testing'', Reinhold Publishing Corp., 1949, p 17–18.</ref> Smith and Sandland developed the first microindention hardness test at [[Vickers]] Ltd. in London in 1922.<ref>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.</ref> Swiss-born microscopist A.I. Buehler started the first metallographic equipment manufacturer near Chicago in 1936. Frederick Knoop and colleagues at the [[National Institute of Standards and Technology|National Bureau of Standards]] developed a less-penetrating (than Vickers) microindention test in 1939.<ref>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.</ref> [[Holger F. Struer|Struers]] A/S of Copenhagen introduced the [[electrolytic]] polisher to metallography in 1943. George Kehl of [[Columbia University]] wrote a book that was considered the bible of materialography until the 1980s.<ref>G.L. Kehl, ''The Principles of Metallographic Laboratory Practice'', McGraw–Hill Book Co., 1939, 1943 & 1949 (three editions).</ref> Kehl co-founded a group within the [[United States Atomic Energy Commission|Atomic Energy Commission]] that became the [[International Metallographic Society]]<ref>[http://www.internationalmetallographicsociety.org/ International Metallographic Society]</ref> in 1967.

==Preparation of ceramographic specimens==
The preparation of ceramic specimens for microstructural analysis consists of five broad steps: [[saw]]ing, embedding, [[Grinding (abrasive cutting)|grinding]], [[polishing]] and [[Industrial etching|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 tool|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 [[Thermosetting plastic|thermosetting]] solid [[resin]], activated by heat and [[Compression molding|compression]], e.g. mineral-filled [[epoxy]], is best for most applications. A castable (liquid) resin such as unfilled epoxy, [[Polymethyl methacrylate|acrylic]] or [[polyester]] may be used for porous [[refractory]] ceramics or [[Microelectronics|microelectronic]] devices. The castable resins are also available with [[fluorescence|fluorescent]] dyes that aid in [[fluorescence microscopy]]. The left and right specimens in Fig. 3 were embedded in mineral-filled epoxy. The center refractory in Fig. 3 was embedded in castable, transparent acrylic.
*'''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-[[Particle size (grain size)|grit]] metal-bonded [[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 [[Nap (textile)|napless]] paper rotating at 240 rpm. The specimen is again washed in an ultrasonic bath after each step. The three sets of specimens in Fig. 3 have been sawed, embedded, ground and polished.
*'''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 {{convert|1500|°C|°F}} in air. The plastic encapsulation must be removed before thermal etching. The alumina in Fig. 1 was thermally etched.

<gallery>
Image:Ceramographic_sections.jpg|Fig. 3: Embedded, polished ceramographic sections.
</gallery>

Alternatively, non-cubic ceramics can be prepared as [[thin section]]s, 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 to a thickness (''x'') approaching 30 µm.<ref>U. Täffner & R. Telle, "Experience Gained in Preparing Thin Sections of High-Performance Ceramics Using the Discoplan-
TS," ''Structure'', '''24''', Feb 1991, p 12–14.</ref><ref>W. Ahmed, "Petrographic Examination Methods," ''Tech-Notes'', Buehler Ltd., Vol 3, Issue 5, 2000.</ref> 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 crystal system|Cubic]] ceramics, e.g. yttria-stabilized zirconia and [[spinel]], have the same refractive index in all [[Crystallography|crystallographic]] directions and are, therefore, opaque when the microscope's [[polarizer]] is 90° out of phase with its [[Optical mineralogy|analyzer]].
: <math>I_t = I_0e^{-\alpha x}</math> (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 equations|Fresnel formula]] below. Bare alumina (η ≈ 1.77, ''k'' ≈ 10 −6) has a negligible [[refractive index#Dispersion and absorption|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]].
: <math>R = \frac{I_r}{I_i} = \frac{(\eta_1 - \eta_2)^2 + k^2}{(\eta_1 + \eta_2)^2 + k^2}</math> (Fresnel eqn)

==Ceramographic analysis==
Ceramic microstructures are most often analyzed by reflected visible-light [[Optical microscope|microscopy]] in [[Bright field microscopy|brightfield]]. [[Dark field microscopy|Darkfield]] is used in limited circumstances, e.g., to reveal cracks. [[Polarized]] transmitted light is used with thin sections, where the contrast between grains comes from [[birefringence]]. Very fine microstructures may require the higher [[magnification]] and [[Optical resolution|resolution]] of a [[scanning electron microscope]] (SEM) or [[confocal laser scanning microscopy|confocal laser scanning microscope]] (CLSM). The [[cathodoluminescence microscope]] (CLM) is useful for distinguishing phases of refractories. The [[Transmission electron microscopy|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 (matter)|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 [[ASTM]] E112. In the line-fraction methods, a statistical grain size is calculated from the number of grains or grain boundaries intersecting a line of known length or circle of known circumference. In the area-fraction method, the grain size is calculated from the number of grains inside a known area. In each case, the measurement is affected by secondary phases, porosity, [[Texture (crystalline)|preferred orientation]], [[exponential distribution]] of sizes, and non-equiaxed grains. [[Image analysis]] can measure the [[shape factor (image analysis and microscopy)|shape factor]]s of individual grains by ASTM E1382.

'''Second-phase content''' and '''porosity''' are measured the same way in a microstructure, such as ASTM E562. Procedure 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 (vision)|contrast]] between phases is adequate, as is usually the case.

Grain size, porosity and second-phase content have all been [[correlate]]d with ceramic properties such as mechanical strength σ by the [[Hall–Petch]] equation. [[Hardness]], [[toughness]], [[dielectric constant]] and many other properties are microstructure-dependent.

== Microindention hardness and toughness==
The '''hardness''' of a material can be measured in many ways. The [[Knoop hardness test]], a method of microindention hardness, is the most reproducible for dense ceramics. The [[Vickers hardness test]] and superficial [[Rockwell scale]]s (e.g., 45N) can also be used, but tend to cause more surface damage than Knoop. The Brinell test is suitable for ductile metals, but not ceramics. In the Knoop test, a diamond indenter in the shape of an elongated pyramid is forced into a polished (but not etched) surface under a predetermined load, typically 500 or 1000 g. The load is held for some amount of time, say 10 s, and the indenter is retracted. The indention long diagonal (''d'', [[Micrometre|μm]], in Fig. 4) is measured under a microscope, and the Knoop hardness (HK) is calculated from the load (P, [[Gram|g]]) and the square of the diagonal length in the equations below. The constants account for the projected area of the indenter and unit conversion factors. Most oxide ceramics have a Knoop hardness in the range of 1000–1500 [[Kilogram-force|kgf]]/mm2 (10 – 15 G[[Pascal (unit)|Pa]]), and many carbides are over 2000 (20 GPa). The method is specified in ASTM C849, C1326 & E384. Microindention hardness is also called micro[[indentation hardness]] or simply microhardness. The hardness of very small particles and thin films of ceramics, on the order of 100 nm, can be measured by [[nanoindentation]] methods that use a [[Berkovich tip|Berkovich]] indenter.
:<math>HK = 14229 \frac{P}{d^2}</math> (kgf/mm2) and <math>HK = 139.54 \frac{P}{d^2}</math> (GPa)

The '''toughness''' of ceramics can be determined from a Vickers test under a load of 10 – 20 kg. [[Fracture toughness|Toughness]] is the ability of a material to resist crack [[Fracture mechanics|propagation]]. Several calculations have been formulated from the load (P), [[elastic modulus]] (E), microindention hardness (H), [[Fracture|crack]] length<ref>G.R. Anstis ''et al.'', "A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: I, Direct Crack Measurements," ''J. Am. Ceram. Soc.'', '''64''' [9] p 533–538 (Sep 1981).</ref> (''c'' in Fig. 5) and [[flexural strength]] (σ).<ref>P. Chantikul ''et al.'', "A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: II, Strength Method," ''J. Am. Ceram. Soc.'', '''64''' [9] p 539–543 (Sep 1981).</ref> 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 carbide]]s are often over 20.<ref>D.W. Richerson, ''Modern Ceramic Engineering'', 2nd Ed., Marcel Dekker Inc., 1992, ISBN 0-8247-8634-3, p 741.</ref> The toughness-by-indention methods have been discredited recently and are being replaced by more rigorous methods that measure crack growth in a notched [[Beam (structure)|beam]] in [[flexure]].<ref>G.D. Quinn & R.C. Bradt, "On the Vickers Indentation Fracture Toughness Test," ''J. Am. Ceram. Soc.'', '''90''' [3] p 673–680 (Mar 2007).</ref>
:<math>K_{icl} = 0.016 \sqrt{\frac{E}{H}}\frac{P}{(c_0)^{1.5}}</math> initial crack length
:<math>K_{isb} = 0.59 \left(\frac{E}{H}\right)^{1/8}[\sigma (P^{1/3})]^{3/4}</math> indention strength in bending

<gallery caption="Indented Microstructures" widths="225px" perrow="2">
Image:Knoop_A995.JPG|Fig. 4: Knoop indention (P=1kg) in 99.5% alumina
Image:Tough_A96.JPG|Fig. 5: Toughness indention (P=10kg) in 96% alumina
</gallery>

==References==
{{reflist|30em}}

== Further reading and external links ==
* ''Metallographic Preparation of Ceramic and Cermet Materials'', [http://www.leco.com/resources/met_tips/met_tip19.pdf Leco Met-Tips No. 19], 2008.
* ''Sample Preparation of Ceramic Material'', [http://www.buehler.com/ Buehler Ltd.], 1990.
* ''Structure'', Volume 33, [http://www.struers.com/default.asp?top_id=5&main_id=43&doc_id=307&target=_self Struers A/S], 1998, p 3–20.
* [http://www.struers.com/default.asp?top_id=5&main_id=19&sub_id=25&doc_id=89 Struers Metalog Guide]
* S. Binkowski, R. Paul & M. Woydt, "Comparing Preparation Techniques Using Microstructural Images of Ceramic Materials," ''Structure'', Vol 39, 2002, p 8–19.
* R.E. Chinn, ''Ceramography'', [[ASM International (society)|ASM International]] and the [[American Ceramic Society]], 2002, ISBN 0-87170-770-5.
* D.J. Clinton, ''A Guide to Polishing and Etching of Technical and Engineering Ceramics'', The Institute of Ceramics, 1987.
* [http://www.udri.udayton.edu/DLCM/Home.asp/ Digital Library of Ceramic Microstructures], University of Dayton, 2003.
* G. Elssner, H. Hoven, G. Kiessler & P. Wellner, translated by R. Wert, ''Ceramics and Ceramic Composites: Materialographic Preparation'', Elsevier Science Inc., 1999, ISBN 978-0-444-10030-6.
* R.M. Fulrath & J.A. Pask, ed., ''Ceramic Microstructures: Their Analysis, Significance, and Production'', Robert E. Krieger Publishing Co., 1968, ISBN 0-88275-262-6.
* K. Geels in collaboration with D.B. Fowler, W-U Kopp & M. Rückert, ''Metallographic and Materialographic Specimen Preparation, Light Microscopy, Image Analysis and Hardness Testing'', ASTM International, 2007, ISBN 978-0-8031-4265-7.
* H. Insley & V.D. Fréchette, ''Microscopy of Ceramics and Cements'', Academic Press Inc., 1955.
* W.E. Lee and W.M. Rainforth, ''Ceramic Microstructures: Property Control by Processing'', Chapman & Hall, 1994.
* I.J. McColm, ''Ceramic Hardness'', Plenum Press, 2000, ISBN 0-306-43287-0.
* [http://products.asminternational.org/mgo/index.jsp Micrograph Center], ASM International, 2005.
* H. Mörtel, "Microstructural Analysis," ''Engineered Materials Handbook, Volume 4: Ceramics and Glasses'', ASM International, 1991, p 570–579, ISBN 0-87170-282-7.
* G. Petzow, ''Metallographic Etching, 2nd Edition'', ASM International, 1999, ISBN 978-0-87170-633-1.
* G.D. Quinn, "Indentation Hardness Testing of Ceramics," ''ASM Handbook, Volume 8: Mechanical Testing and Evaluation'', ASM International, 2000, p 244–251, ISBN 0-87170-389-0.
* A.T. Santhanam, "Metallography of Cemented Carbides," ''ASM Handbook Volume 9: Metallography and Microstructures'', ASM International, 2004, p 1057–1066, ISBN 0-87170-706-3.
* U. Täffner, V. Carle & U. Schäfer, "Preparation and Microstructural Analysis of High-Performance Ceramics," ''ASM Handbook Volume 9: Metallography and Microstructures'', ASM International, 2004, p 1057–1066, ISBN 0-87170-706-3.

[[Category:Ceramics]]
[[Category:Ceramic engineering]]
[[Category:Metallurgy]]
[[Category:Microscopy]]
[[Category:Materials science]]
[[Category:Materials testing]]

Viomycin kinase - Revision history

en>Rjwilmsi: /* References */Added 1 doi to a journal cite using AWB (10104)

en>Citation bot 1: Citations: [Pu168] added: issue. You can use this bot yourself! Report bugs here.