Handbook of Analytical Methods for Materials Copyright © 2001 by Materials Evaluation and Engineering, Inc. 21 LIGHT MICROSCOPY DESCRIPTION OF TECHNIQUE 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. 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. Magnification of the sample image is obtained by light refraction through a combination of objective lenses and eyepieces. The minimum feature resolution is approximately 0.2 µm. However, smaller features - as small as about 0.05 µm - can be detected by image contrast enhancement with polarized light, interference contrast, and dark field illuminations. The resulting images can be recorded either on traditional films or as digital files for computer display, analysis, and storage. ANALYTICAL INFORMATION Bright Field Light Microscopy - This method produces true color images at magnifications up to approximately 2000X. The sample surface is uniformly illuminated by incident light rays directed perpendicularly to the sample surface. Light reflected back toward the objective lens is collected and focused on the eyepieces to form the observed image. Surfaces that are reflective and perpendicular to the light rays appear bright. Alternatively, nonreflective or oblique features reflect less light and appear darker. Polarized Light - This method produces enhanced contrast for features that have anisotropic refractive properties. Two polarizing lenses are inserted into the optical path - one in the incoming illumination and one between the sample and eyepieces. When these lenses are rotated 90° to one another, the “crossed polarizers” result in the subtraction of a portion of the light spectrum by destructive interference. Contrast is obtained between sample features that have different reflec- tive properties. Many metals, including beryllium, zirconium and titanium, are anisotropic and exhibit grain contrast with polarized light illumination. Polarized light enhances contrast for many polymer samples and shows variations of internal stress in some clear polymers. Interference Contrast Image of IC Corrosion on the Inside of a Copper Pipe Handbook of Analytical Methods for Materials Copyright © 2001 by Materials Evaluation and Engineering, Inc. 22 Differential Interference Contrast after Nomarski (DIC) - This method produces a 3-dimen- sional image by creating brightness contrast on very minor topographical changes. DIC utilizes crossed polarizers as described for polarized light. A double quartz prism is also inserted into the light path to split the incident light into two separate paths. This results in two slightly shifted images of the sample on the viewing plane, which produces contrast between features with different heights and topographic orientations. The analyzer can be adjusted to obtain various degrees of interference to enhance selected features or create contrast colors in the image. Darkfield - Enhanced contrast from subtle topographic features is produced with this method. An occluding disk is placed in the light path, blocking the direct vertical illumination. Peripheral rays in the illumination are reflected in such a way that light reaches the sample at oblique angles. The absence of incident vertical rays results in bright reflectance only from oblique features, such as ridges, pits, scratches and particles. Thus, subtle features that might be completely invisible in bright field micros- copy are readily observed with this method. Quantitation - Microscope magnification is calibrated against reference standards. Lateral feature dimensions can be measured to an accuracy greater than 0.5 µm. Computer analysis of digitally- acquired images can measure area and volume fractions, particle sizes, grain size, and other features. TYPICAL APPLICATIONS • Small sample inspection • Metal microstructure evaluation • Small feature measurements • Fracture mode identification • Corrosion failure inspection • Surface contamination evaluation SAMPLE REQUIREMENTS Sample size, shape, and condition requirements depend on the configuration of the microscope. Low- magnification stereo microscopes are small and have a long focal length (up to 5 in.), so these can be set up to examine even relatively large samples. Some portable field microscopes can be fixtured directly to large structures. For magnifications of 100X and above, microscopes are not usually amenable to portable use (except specialized field units). Samples for typical high-resolution light microscopes are limited to a few pounds in weight, and examination is limited to readily accessible flat surfaces due to the small depth of field at higher magnifications. LIGHT MICROSCOPY Metallurgical Light Microscope Handbook of Analytical Methods for Materials Copyright © 2001 by Materials Evaluation and Engineering, Inc. 23 METALLOGRAPHIC STUDY DESCRIPTION OF TECHNIQUE Metallographic study, or metallography, is the imaging of topographical or microstructural features on prepared surfaces of materials. The structures studied by metallography are indicative of the proper- ties and performance of materials studied. In this technique, planar surfaces are prepared to obtain a polished finish. Chemical or other etching methods are often used to delineate macrostructure and microstructure features. Once prepared, samples are examined by the unaided eye, light microscopy, and/or electron microscopy. (See sections on Light Microscopy and Scanning Electron Microscopy.) Samples for microstructure evaluation are typically encapsulated in a plastic mount for handling during sample preparation. Large samples or samples for macrostructure evaluation can be pre- pared without mounting. Sample preparation consists of grinding and then polishing using successively finer abrasives to obtain the desired surface finish. For microstructure examination, a mirror finish is needed, but a finely-ground finish is adequate for macrostructure evaluation. Etchants are specially formulated for the specific sample material and evaluation objectives. Sampling for metallography can be a random section to evaluate representative bulk properties or a section in a specific location to characterize localized material conditions. For example, a section through the facture initiation site is often made to assist with a component failure analysis. For micro- electronic components, precision metallographic methods can obtain sections though specific wire bonds, solder pads, or even individual components on an integrated circuit device. ANALYTICAL INFORMATION Macrostructure Evaluation - Deep chemical etching is used to characterize large-scale variations in material composition, structure, density, etc. This method is useful for evaluation of welds, brazes, forgings, and polymer- matrix composites for configuration, defects, and struc- ture. Microstructure Evaluation - Characteristic features provide information about composition, phase distribution, mechanical and physical properties, thermo-mechanical process history, and defects. In failure analysis, the morphology of corrosion or cracks can be characteristic of the failure mode. Stress Corrosion Cracking at SS Weld Microstructure of Welded Titanium Handbook of Analytical Methods for Materials Copyright © 2001 by Materials Evaluation and Engineering, Inc. 24 Quantitative Metallography - Observed features can be analyzed to obtain measurements of microscopic characteristics, including grain size, phase volume fractions, and linear dimensions. Measurements are made manually or by computerized semi-automated methods on digitally- acquired images. Field Metallography - Metallographic examination can be performed in situ for large components or on structures in the field. The selected areas of the sample surface are polished using portable tools. The prepared surface can be examined directly with a portable light microscope. Alter- natively, the surface can be replicated with an acetate tape or castable polymer for examination by light microscopy or electron microscopy in the laboratory. TYPICAL APPLICATIONS • Metal alloy heat treatment verification • Coating thickness measurement • Weld or braze joint evaluation • Case hardening depth determination • Corrosion resistance evaluation • Failure analysis • Microscopic defects in IC devices • In situ evaluation of thermo-mechanical degradation SAMPLE REQUIREMENTS Most samples are sectioned and encapsulated in a metallographic mount to facilitate preparation. The mount sizes range from about 1 in. (25 mm) to 3 in. (75 mm) in diameter. Sections up to approximately 8 in. (200 mm) across can be prepared in the laboratory without mount- ing. Localized areas on large samples or those that cannot be cut are prepared in situ and evaluated using a field microscope or replicas of the prepared surface. METALLOGRAPHIC STUDY Manganese Bronze Microstructure Mis-registration in Multilayered Circuit Board Plating Defect Caused by Chemical Attack Before Plating at Nonmetallic Inclusion Handbook of Analytical Methods for Materials Copyright © 2001 by Materials Evaluation and Engineering, Inc. 25 MICROINDENTATION HARDNESS TESTING DESCRIPTION OF TECHNIQUE 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 a few grams to 1 kilogram. The impres- sion 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. Conversions from microindentation hardness values to tensile strength and other hardness scales (e.g., Rockwell) are available for many metals and alloys. The indentations are typically made using either a square-based pyramid indenter (Vickers hardness scale) or an elongated, rhombohedral-shaped indenter (Knoop hardness scale). The tester applies the selected test load using dead weights. The length of the hardness impressions are precisely measured with a light microscope using either a filar eyepiece or a video image and computer software. A hardness number is then calculated using the test load, the impression length, and a shape factor for the indenter type used for the test. ANALYTICAL INFORMATION Bulk Hardness - Randomly-located impressions measure the representative bulk hardness value of a relatively homogeneous material. Localized Hardness - Impressions are made at specific sites located using the light microscope to determine the hardness at discrete features or phases in the sample. Hardness can be measured for features less than 0.1 mm across. Hardness Survey - A series of hardness impressions are made along a line from a surface or a specific point in the sample to systematically measure the hardness variation within a sample. Thin Coatings - The hardness of coatings as thin as a few microns can be determined by measuring directly on the coated surface of a sample. Coating thickness must be known to assess accuracy of these measurements. Knoop Microindentation Hardness Survey for Chromium-Plated, Case-Hardened Steel Knoop Indenter Vickers Indenter Handbook of Analytical Methods for Materials Copyright © 2001 by Materials Evaluation and Engineering, Inc. 26 TYPICAL APPLICATIONS • Bulk hardness of small or thin samples • Heat treated steel case depth evaluation • Decarburization in steels • Evaluation of welds • Hardness of thin coatings • Evaluation of machinability SAMPLE REQUIREMENTS Most microindentation hardness testing is performed on samples that have been metallographically mounted and polished. These samples are usually no larger than about 1 in. (25 mm) by 1 in. (25 mm) by 1/2 in. (12 mm) thick. Larger samples can be tested with special fixturing. Thin, flat samples, such as sheet material, can be tested without mounting or preparation if the surface finish is suitable. The ideal surface finish is a high-quality metallographic polish. Where polishing is not feasible, the surface finish must be sufficiently smooth and reflective to clearly resolve the microscopic hardness impression with the measuring microscope. The specific finish requirement depends on the material and test load. MICROINDENTATION HARDNESS TESTING Microhardness Tester Handbook of Analytical Methods for Materials Copyright © 2001 by Materials Evaluation and Engineering, Inc. 27 NANOINDENTATION HARDNESS TESTING DESCRIPTION OF TECHNIQUE Nanoindenting is a new method to characterize material mechanical properties on a very small scale. Features less than 100 nm across, as well as thin films less than 5 nm thick, can be evaluated. Test methods include indentation for comparative and quantitative hardness determination and scratching for evaluation of wear resistance and thin film adhesion. Nanoindenting is performed in conjunction with atomic force microscopy (AFM). The area for testing is located by AFM imaging, and indentations and scratching marks are imaged by AFM after testing. A three-sided, pyramid-shaped diamond probe tip is typically used to indent, scratch and image the sample. For indentation, the probe is forced into the surface at a selected rate and to a selected maximum force. In scratching, the probe is dragged across the sample surface. The force, rate, length and angle of the scratch is controlled. Imaging is performed in situ using the probe in intermit- tent contact (tapping mode) AFM. The depth of the indentation is measured from the AFM image to evaluate hardness. A force-displacement curve obtained during indentation also provides indications of the sample material’s mechanical and physical properties. ANALYTICAL INFORMATION Nanoindentation - Indentation forces ranging from 1µN to 100 mN can be made to measure material hardness. Indentation depth or area is inversely proportional to hardness. Force displace- ment curves obtained during the indentation process indicate hardness and elastic modulus properties. Scratching - Patterns scribed in the sample surface show the potential for spalling or delamination of thin films. Wear Testing - The diamond probe tip is repeatedly scanned over the same sample surface area at a selected force. Wear durability is measured by the material lost from the tested surface. Depth of material loss is measured by AFM imaging after testing. AFM Images of Two Impressions in a Polymer Handbook of Analytical Methods for Materials Copyright © 2001 by Materials Evaluation and Engineering, Inc. 28 TYPICAL APPLICATIONS • Hardness measurements for submicron-size features • Thin film adhesion evaluation • Coating wear durability evaluation • Elastic modulus comparisons for thin films SAMPLE REQUIREMENTS Samples up to 8 in. (200 mm) across and 1 in. (25 mm) thick can be fixtured and tested in some instruments. Larger samples can be fixtured for access to limited surface areas. The sample surface must have a smooth finish for uniform indentation and to allow AFM imaging for indentation depth. The required finish depends on the material and the test force. NANOIDENTATION HARDNESS TESTING AFM Section Analysis for Nanoindentation Hardness Handbook of Analytical Methods for Materials Copyright © 2001 by Materials Evaluation and Engineering, Inc. 29 QUANTITATIVE CHEMICAL ANALYSIS DESCRIPTION OF TECHNIQUES Quantitative chemical analysis is performed to accurately determine the concentration of elements in the material comprising a given sample. A variety of analysis techniques are used for metals and alloys to determine the alloy composition of raw materials to verify conformance to a specification or to identify the alloy used to make a specific component. Quantitative analysis methods are also used occasionally for evaluation of foreign material contaminants in special cases for failure analysis or investigation of product manufacturing or handling problems. Quantitative chemical analysis may be performed by one or more complimentary techniques, com- monly including spark optical emission spectroscopy (Spark-OES), inductively-coupled plasma spectroscopy optical emission spectroscopy (ICP-OES), x-ray fluorescence spectroscopy (XRF), wet chemical analyses, combustion methods, and inert gas fusion(IG).The specific technique chosen will depend on the type of sample, quantity of material available for analysis, desired result, and cost constraints. In most cases, the applicable analysis techniques can detect parts-per-million concentra- tions or better. Most of these techniques are destructive to the original sample. XRF can be performed nondestruc- tively and Spark-OES can be performed with only minimal surface damage if the specimen size configuration allow the part to fit into the instrument without cutting. For the remaining methods discussed here, a small specimen is removed from the sample and is consumed in the analysis. Prior to the widespread availability of analytical instruments, chemical analyses were performed by dissolving the sample and performing a specific chemical reaction with a standardized reagent for each element of interest. These ‘wet chemistry’ techniques are typically labor intensive and time consuming, and sometimes less accurate than the current instrumental methods. Wet Chemistry - These methods include gravimetric and titrimetric techniques. An example of a gravimetric technique is the precipitation of chloride ion with silver to form a silver chloride precipitate which is dried and weighed to determine the chloride concentration in the original sample solution. Titrimetric procedures are typically based on acid-base reactions or complexing agents for metal ions. Since wet chemical analyses are now less common for the analysis of metals and similar inorganic materials, the remainder of this section will focus on the instrumental methods of analysis. Spark-OES - Spark optical emission spectroscopy is a technique used for direct analysis of solid metal samples. The specimen is prepared by grinding to obtain a uniform, clean, flat area about 1 to 2 cm across. The prepared sample is placed in the spark-OES instrument and flooded with argon. A rapid series of high energy sparks are created across the argon-filled gap between an electrode (cathode) and the prepared sample’s surface (acting as the anode). The sparks first ionize the argon, creating a conductive plasma. Secondly, the sparks melt, evaporate, and excite the sample elements at the spark point of impact. When the excited atoms in the plasma relax (de-excite) to a lower energy state, they emit light at characteristic wavelengths for each element. The intensities of these Handbook of Analytical Methods for Materials Copyright © 2001 by Materials Evaluation and Engineering, Inc. 30 emissions at the characteristic wavelengths are detected, measured, and compared to intensities for known standards to provide quantitative results. The total duration of the sparking is only a few milliseconds. Prior to actual measurements, the sample surface may be subjected to high power discharges to melt the surface and create a more homogeneous material. XRF - X-ray fluorescence spectroscopy is a technique that can be used for direct analysis of solid metal samples, thin metal films, petroleum products, cement, coal, and various other materials. XRF is a fast technique and is non-destructive to the sample. It is frequently used for analyses performed in the field and for industrial quality control. An x-ray tube is used to irradiate the sample with a primary beam of x-rays. Some of the impinging primary x-rays are absorbed by the sample elements in a process known as the photoelectric effect. The photoelectric effect occurs when all the energy of a primary x-ray is absorbed by an electron in an atom’s innermost electron shell. This causes excitation and ejection of the absorbing electron (photoejection). The electron vacancies caused by the photoelectric effect are filled by electrons from higher energy states, and x-rays are emitted (fluorescence) to balance the energy difference between the electron states. The x-ray energy is characteristic of the element from which it was emitted. The fluorescence x-rays are collimated and directed to an x-ray detector. The energy of each x-ray and number of x-rays at each energy are recorded. The x-ray intensities (counts) at each energy are compared to values for known standards for quantitatively analysis of the unknown specimen. ICP-OES - Inductively coupled plasma-optical emission spectroscopy is a technique for analyzing the concentration of metallic elements in solid and liquid samples. Like spark-OES, ICP-OES uses the optical emission principles of exited atoms to determine the elemental concentration. However, for ICP-OES, solid samples are dissolved (digested) in an appropriate solvent (typically acid) to pro- duce a solution for analysis. The resulting sample solution (or an original liquid solution for analysis) is often diluted in water to obtain a final specimen suitable for analysis. The ICP-OES instrument uses argon gas flowing through a torch consisting of three concentric quartz tubes. A copper coil circumscribing the top of the torch is connected to a radio frequency (RF) generator. The use of the copper coil with the RF power is called inductive coupling. When the RF power is applied in the copper coil, an alternating current occurs within the coil. The oscillation of the alternating current causes electric and magnetic fields at the end of the torch. A spark applied to the argon gas causes some electrons to be stripped from the argon atoms. The electrons are caught and accelerated by the RF generated electric/magnetic field. The high energy free electrons collide with other atoms, stripping off more electrons in a chain reaction, resulting in a plasma of electrons, ions, and atoms. This is known as an inductively coupled plasma (ICP) discharge. This ICP discharge is maintained as the RF energy is continually transferred to the plasma by the copper coil. The liquid samples are nebulized into an aerosol and introduced into the center of the plasma. The plasma excites the sample atoms, which subsequently relax to a lower energy state by emitting light at elementally characteristic wavelengths. The intensities of these characteristic wavelengths are de- tected, measured, and compared to intensities for known standards to provide quantitative results. QUANTITATIVE CHEMICAL ANALYSIS . Handbook of Analytical Methods for Materials Copyright © 2001 by Materials Evaluation and Engineering, Inc. 21 LIGHT MICROSCOPY DESCRIPTION OF TECHNIQUE Light microscopy in materials. morphology of corrosion or cracks can be characteristic of the failure mode. Stress Corrosion Cracking at SS Weld Microstructure of Welded Titanium Handbook of Analytical Methods for Materials. surface. Depth of material loss is measured by AFM imaging after testing. AFM Images of Two Impressions in a Polymer Handbook of Analytical Methods for Materials Copyright © 2001 by Materials Evaluation