Microstructures/microstructured materials microscopic examination

All commercially available polishing pads are relatively complex composite materials, as evidenced in photomicrographic cross sections of the major pad types illustrated in Figs. 5-7. The signature structural characteristics of each class of pads (Table I) are readily apparent. The impact of manufacturing process on microstructure is sufficiently strong that the manufacturing process used to produce an unknown pad sample can be readily determined from microscopic examination. 

Finally, microscopical examination alone may not provide sufficient answers to the questions of clinker microstructure or a cement s inferior performance. Cement particle size distribution, variations in crystal chemistry, mineral and chemical admixtures, as well as the effectiveness of the set-controlling material (normally gypsum or similar minerals), may have stronger effects on cement hydration than the clinker production problems inferred by routine microscopy. Some clinker and cement problems, however, are simple and easily solved others require the analysis of a tangled set of multiple causes and effects. Microscopy should be one of the first steps in that analysis. 

For ceramic materials that are not extremely hard (e.g., stabilized zirconium oxide or multiphase materials), surface relief can be created within a few minutes by means of final polishing with colloidal silica on a chemicaUy resistant, short-napped fiber cloth. Because removal is dependent on the grain orientation and the type of phase, it is possible to distinguish between grains of a single phase and between different phases during microscopic examination. Application of the DIC method makes it possible to use even poorly defined reUef to display microstructure under the optical microscope. A brief final polishing step with very fine alumina (0.05 pm) will help reveal the spinel phase in aluminum oxide materials, for example. 

Avoidance of these artifacts will make it easier to use microscopic methods to distinguish between pores, pull-outs, phases, and smearing. This requires preparation techniques which are well-matched to material in question, especially with respect to its microstructure and manufacture. It is always necessary to perform microscopic examinations of the section after the individual process steps, so that the preparation... 

Quality control of plastic molded parts can use optical techniques. In this procedure thin slices of the material are cut from the part and microscopically examined under polarized light transmitted through the sample. Study of the microstructure by this technique enables rapid examination of quality-affecting properties. This kind of approach can provide the molder with information for failure analysis, part and mold design, and processing optimization [12]. 

Microscopic examination is an extremely useful tool in the study and characterization of materials. Several important applications of microstructural examinations are as follows to ensure that the associations between the properties and structure (and defects) are properly understood, to predict the properties of materials once these relationships have been established, to design alloys with new property combinations, to determine whether a material has been correctly heat-treated, and to ascertain the mode of mechanical fracture. Several techniques that are commonly used in such investigations are discussed next. 

Typical information that can be obtained firom microscopic examination of refractories are parameters such as grain size, grain distribution, and mineralogy. However, many parameters are subjective in nature and therefore need to be examined in comparison with similar materials. Considerable experience is required if microstructures are to be correctly and objectively interpreted. 

As in the case of corrosion failures, the sequence of steps involved in analyzing wear failures are initial examination of the failed component including service conditions to establish the mode or combination of modes of wear failure, metallographic examination to check if the microstructure of the worn part met the specification, both in the base material and in the hardened case or applied surface coatings, existence of localized phase transformations, shear or cold worked surfaces, macroscopic and microscopic hardness testing to determine the proper heat treatment, X-ray and electron diffraction analysis to determine the composition of abrasives, wear debris, surface elements and microstructural features such as retained austenite, chemical analysis of wear debris surface films and physical properties such as viscosity and infrared spectral determination of the integrity of lubricants and abrasive characteristics of soils or minerals in the cases of wear failures of tillage tools. 

A light microscope for examining material microstructure can use either transmitted or reflected light for illumination. Reflected light microscopes are the most commonly used for metallography, while transmitted light microscopes are typically used to examine transparent... 

The microstructure of a material can only be viewed in a light microscope after a specimen has been properly prepared. Metallurgists have developed extensive techniques and accumulated knowledge of metal specimen preparations for over a century. In principle, we can use these techniques to examine not only metallic materials but also ceramics and polymers in practice, certain modifications are needed and a certain degree of caution must be exercised. The main steps of specimen preparation for light microscopy include the following. 

The EDS type of X-ray spectrometer is commonly included as a part of SEMs and TEMs. The reason for using EDS rather than WDS is simply its compactness. With EDS in an electron microscope, we can obtain elemental analysis while examining the microstructure of materials. The main difference between EDS in an electron microscope and in a stand-alone XRF is the source to excite characteristic X-rays from a specimen. Instead of using the primary X-ray beam, a high energy electron beam (the same beam for image formation) is used by the X-ray spectrometer in the microscopes. EDS in an electron microscope is suitable for analyzing the chemical elements in microscopic volume in the specimen because the electron probe can be focused on a very small area. Thus, the technique is often referred to as microanalysis. 

The microstructure of this type of material was studied as early as 1952 by Fischer and Isenbarth (1). These authors demonstrated by thin section transmission microscopy that a two-phase microstructure was present in a number of materials available commercially for restoring tooth structure. In 1955 Helmcke reported similar findings with electron microscopic studies of fracture replicas (2, 3). Then in 1958, Smith examined at low magnification fracture surfaces in materials made from denture base polymers attention centered on a system of ridges concentric with a mirror region (4). In retrospect, this phenomenon was similar to that observed in one-phase samples of poly (methyl methacrylate) (5). Subsequently, in 1961, Smith showed that the microstruc-... 

Published references cannot substitute for personal examination of the raw materials and all raw feed components are subjects for microscopical analysis. For our purposes, the important data in raw materials analysis are mineralogy, microstructure, and chemistry. 

The POM is a conventional optical microscope (OM) added with a pair of polarizer such that the microstructure of the material or crystallites can be observed as it interacts with the polarized light. It is the most basic and widely used microscopic method to examine the features and internal surface of a material especially polymer and liquid crystals in micrometer level. The POM gives information on the grain size, grain boundary, and also the multiple phases which exist in a material... 

Electron microscopy is a powerful direct experimental technique. Using electron microscopy information can be obtained on the presence of inhomogeneities, and on their shapes, sizes, size dispersion and number density. The experimental and theoretical aspects of this technique have been reviewed by Hirsh a/. (1965). There are two methods of observation. In the first, the topography of the sample surface is replicated and it is this replica, and not the sample which is then examined in the electron microscope. Generally carbon is used as the replicating material and shadowing at an angle with heavy elements (Pt) is used to accentuate the surface reUef. The resolution limit is about 50 A due to a microstructure in the rephca. As the sample itself is not examined, a diffraction pattern is not obtained. The sample surface can be either etched or unetched. An unetched surface wiU reveal cracks, voids, and polyphase microstructures if the various phases... 

Some of the epoxy resins on the market are mixed, or can be mixed, with special dyes that cause the mounting materials to glow as the specimen is examined in an optical microscope under a bright field or polarized light. The same efiect can be achieved by means of fluorescent substances and a filter set intended for fluorescence applications. The illumination of these components has the advantage of making impregnated features of the microstnicture (e.g., pores and cracks) immediately visible. It also causes them to stand out clearly from other microstructural features... 

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