More details on these and other analytical methods are presented in our HAMM.
SCANNING ELECTRON MICROSCOPY / ENERGY DISPERSIVE X-RAY SPECTROSCOPY(SEM/EDS)
Variable-pressure, or low vacuum, SEM (VPSEM) is a specialized method using a variable-pressure sample chamber. This technique allows direct evaluation of samples that are nonconductive or vacuum sensitive, which are not readily evaluated with traditional high-vacuum SEMs. Field Emission SEM (FESEM) has a much brighter electron source and smaller beam size, extending the useful magnification up to 500,000X. A second advantage of FESEM is that high resolution imaging can be performed at very low accelerating voltages, which enhances the observation of very fine surface features, electron beam sensitive materials, and non-conductive materials. For more information about the capabilities of VPSEM and FESEM, please see the MEE Handbook on SEM.
The three SEMs at MEE meet the needs of a wide variety of imaging requirements. Each can operate in the traditional high vacuum mode, two also operate in the variable pressure mode (JEOL model s5800LV and 6610LV). The 6610LV instrument is a state of the art SEM with enhanced imaging capabilities and large sample size capacity, along with other new features that allow us to get your work done more quickly with superior results. Our third instrument is a Hitachi S4700 FESEM to meet your needs for high magnification, high resolution imaging. All SEMs have digital image capture and analysis.
Energy dispersive x-ray spectroscopy (EDS) is an elemental microanalysis technique performed in conjunction with SEM. Features or phases as small as about 1 micron can be analyzed. EDS can obtain rapid qualitative chemical information, semi-quantitative composition determinations, maps showing lateral distribution of chemical elements, and profiles of composition across a surface. All stable elements can be detected with the exception of hydrogen, helium, and lithium. Each of the SEMs at MEE have EDS analysis capability. The JEOL 6610LV is equipped with the latest SDDEDS detector, which provides a very high throughput for fast x-ray data acquisition for advanced spectral imaging and fast collection of elemental maps.
Together, SEM and EDS provide an excellent combination for efficient characterization of surfaces and the chemical elements comprising specific features.
EDS Spectrum for alloy MP35N
MICROSECTIONING AND METALLOGRAPHY
Microsectioning is the preparation of a sample to reveal internal features. Typically, a sample is encapsulated in a plastic mount for handling during sample preparation. Sample preparation consists of grinding and then polishing using successively finer abrasives to obtain the desired surface finish at the location of interest. The MEE metallography laboratory specializes in precision sectioning of miniature devices, including microelectronic components and implantable medical devices. MEE has a host of automatic and manual sample preparation equipment to handle large sample volumes, as well as, samples of complex geometries.
Metallography is the study of topographical or microstructural features on specially-prepared microsectioned surfaces - typically polished and chemically etched. The microstructure features observed by metallography are directly related to the physical and mechanical properties of the material studied. Microsectioing and metallography may reveal such information as grain size, characteristics of interfaces, and material anomalies.
Light microscopy in materials analysis generally refers to reflected light microscopy. In this method, light is directed vertically through the microscope objective and reflected back through the objective to an eyepiece, view screen, or camera. Various illumination techniques and light filters/analyzers can be used to reveal and enhance microscopic features. Transmitted light is occasionally used for transparent and translucent materials. For some low-magnification work (stereo microscopy), external, oblique illumination can be reflected off the sample into the objective.
MEE has a variety of light microscopes with magnifications ranging from 5X to 2000X. The microscopes are all connected to digital cameras for image capture and subsequent measurement and analysis. The laboratory’s Reichert MeF3 is a research grade inverted metallographic light microscope with brightfield, darkfield, polarized light, and differential interference contrast imaging capabilities. The laboratory also has a Nikon AZ100 microscope and a range of stereo microscopes for low magnification inspection and imaging.
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CORROSION ANALYSIS AND TESTING
Laboratory analysis of corrosion failures includes visual and microscopic examinations to characterize the physical appearance of the corrosion damage and usually microstructure studies to correlate the corrosion damage to the material's structure. Chemical analysis methods, including spot tests and surface analysis methods, are critical to determine whether unexpected contaminants in the service environment contributed to the failure. Accelerated corrosion tests, such as salt spray or high-humidity exposures, are often useful to simulate a failure mechanism or evaluate potential corrective measures. Microbiological testing can also be performed when microbiologically influenced corrosion (MIC) is suspected to have contributed to the corrosion.
All corrosion is an electrochemical process of oxidation and reduction reactions. Therefore, controlled electrochemical experimental methods are used to characterize the corrosion properties of metals and metal components in combination with various electrolyte solutions. Electrochemical corrosion experiments measure and/or control the potential and current of the oxidation/reduction reactions. Several types of experiments are possible by manipulating and measuring these two variables through the use of a potentiostat. MEE can perform potentiodynamic experiments to provide a variety of data related to the pitting, crevice corrosion, and passivation behavior for specific sample/solution combinations.
Typical Applications for Electrochemical Corrosion Experiments
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Rockwell hardness testing is a general method for measuring the bulk hardness of metallic and polymer materials. Although hardness testing does not give a direct measurement of any performance properties, hardness of a material correlates directly with its strength, wear resistance, and other properties. Hardness testing is widely used for material evaluation becauseof its simplicity and low cost relative to direct measurement of many properties. Specifically, conversion charts from Rockwell hardness to tensile strength are available for some structural alloys, including steel and aluminum. The Wilson R500 model Rockwell tester at MEE can perform testing on the regular and superficial Rockwell hardness scales.
The hardness of soft materials, such as many polymers, may be tested with a durometer instrument. MEE has durometer testers for the Shore A and Shore D hardness scales.
Microindentation hardness testing (or microhardness testing) is a method for measuring the hardness of a material on a microscopic scale. A precision diamond indenter is impressed into the material at loads from less than 1 grams to 2 kilograms. The impression length, measured microscopically, and the test load are used to calculate a hardness value. The hardness values obtained are useful indicators of a material’s properties and expected service behavior. Microhardness testing can provide the following: bulk hardness, localized hardness, hardness surveys through a hardened case or coating. The laboratory has a Micromet 5124 microindentation tester with Vickers and Knoop indentors.
The evaluation of the mechanical behavior of a sample under conditions of tension and compression can be performed to provide basic material property data that is critical for component design and service performance assessment. Testing can be performed on machined material samples or on full-size samples or scale models of actual components. These tests are typically performed using a universal mechanical testing instrument. Data from the tensile test are used to determine tensile strength, yield strength, and modulus ofelasticity. Measurement of the specimen dimensions after testing also provides reduction of area and elongation values to characterize the ductility of the material. Tensile tests can be performed on many materials, including metals, plastics, fibers, adhesives, and rubbers. A compression test is a method for determining the behavior of materials under a compressive load. During the test, the specimen is compressed, and deformation versus the applied load is recorded. The compression test is used to determine elastic limit, proportional limit, yield point, yield strength, and (for some materials) compressive strength. The universal tester at MEE can perform tension and compression experiments at force ranges from less than 5 N (1 lbf) to 5 kN (1000 lbf).
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Surface analysis refers to methods that gather chemical data from the outermost material comprising a solid surface or interface. Other analytical techniques often collect data from a larger volume and depth, and thus the surface material is either not detected at all or contributes insignificantly to the results. However, surface chemistry can be critical for many applications, including passivation and bonding. Therefore, surface analysis techniques such Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS or ESCA) are employed to determine the chemical composition of the surface.
AES and XPS both provide elemental surface composition of the uppermost 20 to 50 Å. XPS also provides some chemical bonding information; indicating the presence of certain organic molecular species or whether a metal is present in a metallic or oxidized form. The principal advantages of AES are excellent spatial resolution (< 1 µm) and detection of light elements. Detection limits formost elements range from about 0.01 to 0.1 at%.
Both Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) can also perform depth profiles. For a depth profile, material is removed from the surface by sputtering with an energetic ion beam concurrent with successive analyses. This process measures the elemental distribution as a function of depth into the sample. Depth resolution of < 100 Å is possible.
Typical Surface Analysis Applications
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ANALYSES FOR ORGANIC MATERIALS
Organic materials may be identified and quantified through a variety of analytical methods. Two of the most common methods are Fourier transform infrared spectroscopy (FTIR) and gas chromatography / mass spectrometry (GC/MS). FTIR is an analytical technique used to identify organic materials. This technique measures the absorption of infrared radiation by the sample material versus wavelength. The infrared absorption bands identify molecular components and structures. The wavelength sthat are absorbed by the sample are characteristic of its molecular structure. Typical applications for FTIR are the identification of unknown organic materials, verification of polymers, and quantitiation of silicones, esters, etc as contamination on various materials. Analysis can be performed on liquids, bulk solid materials, and particles as small as about 20 µm.
Gas chromatography / mass spectrometry (GC/MS) is the marriage of two analytical methods into a versatile technique for the identification of complex volatile organic materials. Gas chromatography (GC) effectively separates the different constituents of the sample for subsequent analysis and identification by mass spectrometry (MS). The GC/MS method allows for the separation, identification, and quantification of various organic components in mixture. This is useful for identifying material contamination, out gassing products, polymer additives and by-products.
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