Department of Physics & Materials Science
City University of Hong Kong
Contents Table of Contents ………………………………………….…………… 1
Project 1: Scanning Electron Microscopy (SEM) …………….……… 2
Project 2: Microscopic Sample Preparation and Examination …...… 5
Project 3: Electropolishing ………………………….…………………. 7
Cover of Laboratory Report ……………………….…………………. 10
Project 1: Scanning Electron Microscopy Objectives 1. To learn sample preparation and how to use a scanning electron microscope (SEM) for microscopic analysis 2. To learn the basic techniques and necessary conditions to acquire images by SEM
Apparatus Sample preparation equipment and scanning electron microscope
Background In 1937, Manfred von Ardene invented a microscope with high magnification by scanning a focused electron beam and in the early 1940s, Hillier of the Radio Corporation of America conceived the idea of using a focused electron beam to perform localized X-ray spectroscopic analysis. Several years later, the first practical electron-probe X-ray spectrometer was designed by Castang at the University of Paris. This micro-analyzer proved to be a major breakthrough, since the instrument made possible nondestructive analysis of micrometersized volumes. Until the late 1960s, commercial electron probe microanalysis (EPMA) instruments comprised four major components: (i) an electron gun to produce a stable electron beam with a diameter of 1 µm or smaller, (ii) optics to view the sampled area, (iii) a precision stage for accurately locating and translating the sample under the electron beam, and (iv) one or more X-ray spectrometers to measure characteristic X-rays. Recently, EPMA has been augmented with scanning electron microscopy. In SEM, the image is obtained by monitoring either the backscattered or transmitted electrons and the elemental composition is determined by energy-dispersive X-ray analysis (EDS). SEM reveals an apparent 3-D view of the object. The contrast, or shading, in an SEM image depends on the surface topography and the better sharpness compared to optical microscopy is the result of the larger depth of focus. Nevertheless, it is important to keep in mind that the SEM image may contain artifacts that may or may not reflect the reality of the sample and prior knowledge about the object may be necessary. An SEM picture often looks like something we recognize in the real world thereby causing possible misinterpretation about the actual object. Without prior knowledge of the sample, the operator must prepare the specimen in such a manner that the resulting image is meaningful. A practical solution is to obtain “stereo” images from the object by rapidly changing the viewing angle or rotating the sample. Many types of samples can be analyzed by SEM. Ideally, the sample should be a good electrical conductor to avoid charge build-up during electron bombardment. In order to
analyze insulating samples such as ceramics, glass, or biological specimens, a thin conducting film of gold or carbon can be deposited prior to conducting the analysis. The general materials requirements for SEM are shown in the following: 1. The sample must be small enough to fit on the specimen stage while allowing necessary motions such as sample rotation and tilting. 2. The sample must be compatible with a vacuum environment which is usually about 1× 10-5 Torr. 3. Some materials have a high vapor pressure that may negatively affect the vacuum and the structure may also change at low pressure. Precaution must be exercised. 4. The sample must be reasonably electrically conducting and properly connected to the stage to avoid charging. 5. The sample must have the ability to emit secondary electrons in a fairly large quantity or can be coated with a metal with a large coefficient of secondary electron emission.
Cleaning: The specimen, especially the surface, must be as clean as possible. Grease and oil will degrade the image, and small surface articles may charge up and degrade the contrast in nearby areas. If a specimen is to be coated, it must be cleaned prior to deposition as well. The general guideline is to use a solvent or solution that will dislodge, dissolve, or remove the unwanted material. If multiple cleaning solutions are necessary, the subsequent one should have high cleaning power than the preceding one.
Sample Mounting: The requirements for glues are that they must be fast drying, strong enough to hold the specimen, and yet not so strong that the specimen cannot be removed after drying. They should show little out-gassing after a reasonably short drying time, particularly in a vacuum. Common glues are silver paint, silver paste, white glue, double-stick scotch tape, etc.
Sample Preparation: Often nothing needs to be done to the specimen other than mounting. The charging effect can sometimes be exploited to image oxidized products on metals, distinguish between conductors and non-conductors, and investigate the spatial relationship in graphite-epoxy composite materials, for example.
Polishing: Polishing is sometimes necessary for cross-sectioned samples, particularly those composed of different materials. Polishing is the best way to prepare a sample when the backscattered mode is used to delineate different materials from a cross-section such as a brazed joint.
Etching: Polished metal surfaces can be etched in the same manner as in standard metallographic analysis prior to examination by SEM.
Warning No students are allowed to operate any part of the SEM without the permission of the laboratory staff.
Laboratory Report Your report must include the following aspects: 1. Functions of the SEM 2. Procedures to obtain an image (Make your list as detailed as possible) 3. Effects of the spot size on image resolution (high magnification and low magnification) 4. Effects of the sample tilt angle on image quality 5. Effects of different electron acceleration voltages on image quality 6. Effects of the coating on image quality 7. Differences in the images between the secondary electron mode and back-scattered electron mode In addition to the data and analysis, your report should include a short essay (400-600 words) describing the working principle and instrumentation of SEM.
Project 2: Microscopic Sample Preparation and Examination Objectives 1. Preparation of specimens for metallographic examination 2. Microstructure observation by optical microscopy
Apparatus Sample preparation equipment and optical microscope
Background Microstructural investigation can impart information about the morphology and distribution of different phases as well as certain crystal imperfections. Optical microscopy is the most common technique since the equipment is quite inexpensive and the images can be obtained and interpreted relatively easily. The two-dimensional surface of a polished and etched specimen exhibits features indicative of the three-dimensional microstructure. Etching not only delineates grain boundaries, but also allows the different phases to be discerned based on the differences in brightness, shape, and color of the grains. Differences in contrast may result from light absorption by different phases. Etching results from preferential electrochemical attack or staining of the surface. Grain boundaries are often anodic relative to the bulk metal in the interior of the grain and so they are etched preferentially. Staining proceeds by deposition of the solid etching product on the specimen surface via chemical reactions between the etchant and specimen. Under favorable conditions, the use of the proper etchant enables identification of the constituents. Since the nature of the fractured surfaces and sites of the various constituents phases can also be monitored, optical microscopy is often used in the industry to investigate failure of materials and components. Microstructural examination can provide quantitative information about the following materials properties: (i) Grain size (ii) Interfacial area per unit volume (iii) Dimensions of different phases (iv) Amount and distribution of different phases
In preparing a specimen for microscopic examination, it is first necessary to produce a surface which appears perfectly flat and scratch-free when viewed by a microscope. Following are the typical procedures: 1. Grind the specimen sequentially with different grades of SiC papers, e.g., 400, 800, and 1200. 2. Polish the specimen to remove the marks left by grinding. 3. Etch the polished specimen with a suitable reagent. The polishing process may produce a thin layer of amorphous metal that masks the crystal structure. Hence, in order to reveal the crystal structure accurately, the specimen needs to be etched in a suitable reagent. 4. Observe and describe the microstructural features. Before placing the specimen on the microscope stage, fix the specimen on a glass slide with plasticine and level the etched surface with the aid of a leveling device. Compare your sample with the pictures on the lab wall and identify which type of steels your sample belongs. 5. Find the teaching staff to assess the quality of your product.
Laboratory Report You are not required to submit a report for this project. Some questions may be asked by the teaching staff in the session and your performance in the laboratory will contribute to the grade of this project.
Project 3: Electropolishing Objective To determine the optimal conditions for electropolishing of a given metal/alloy
Apparatus Metal plates, abrasive paper, power supply, electrolyte, plate, ice, beakers, brackets, connecting wires, timer, etc.
Background Electropolishing is used to smooth a metal surface anodically in an acidic or alkaline medium. In the process, the products from anodic metal dissolution react with the electrolyte to form a film on the metal surface. Two types of films have been observed: (a) a viscous liquid that is nearly saturated or supersaturated with the dissolution products and (b) anodically discharged gas (usually oxygen). Both types of films exist simultaneously in most commercial electropolishing solutions and the gas appears as a blanket on the viscous film. Which type of films depends on the metal and electrolyte. For example, when brass is electropolished in orthophosphoric acid, a gaseous film forms, whereas in chromic acid, a viscous liquid film forms. Neither film conforms closely to the micro-roughness of a metal surface but rather the macrocontour. Thus, the film is effectively thinner on micro-projections and thicker in microdepressions. Resistance to current flow is smaller at micro-projections and hence, more current can flow there than at micro-depressions. This situation is augmented by relatively shorter diffusion paths for acceptor anions to the projections and by-products dissolved from the projections. The result is more rapid dissolution of the projections to cause microleveling of the surface. Hence, a metal surface with little light scattering and a shiny appearance is produced. Nonuniform anodic films are produced due to local variations in the metal dissolution rate and this effect can be minimized or prevented by moving the specimen during electropolishing. Electrolytic polishing is suitable for homogeneous materials such as aluminum and copper. In general, soft single-phase alloys are suitable along with stainless steels and titanium alloys. Electrolytic polishing is preferred in order to eliminate residual damage or stress from the surface. Figure 1 shows a typical current density curve during etching and polishing. This curve varies with materials and electrolytes and the factors affecting the results are described in the following.
Figure 1: Current density curve.
Procedures 1. Grinding The surface should be flat and preferably ground using grid 800/1200 SiC paper. Before polishing, make sure the surface is free of grease or contaminants.
2. Voltage A high voltage usually leads to preferential removal of materials due to variations in the anodic film thickness which depends on the surface topography. Lowering the voltage may give rise to undesirable pitting. The optimal current density depends on the electrolyte, surface area, and materials.
3. Temperature The temperature of the electrolyte must be kept well below the flash point. It is advantageous to keep the temperature at about 15° C and pitting is related to the temperature.
4. Time Many artifacts can be produced if polishing is excessive. In mechanical polishing, a time duration that is not optimal may degrade the sample quality.
5. Electrolyte and Flow The balance between pitting and irregular polishing can often be resolved by selecting the proper electrolyte. General-purpose electrolytes cover a wide variety of materials but specific electrolytes may be more suitable in some instances. The electrolyte flow can affect the thickness of the anodic layer, although the effect is not as substantial as that arising from a voltage change. The flow must be constant and uniform to accomplish a balanced metallic 8
ion distribution. In the experiment, phosphoric acid is used as the electrolyte and the flow of the electrolyte is set to be constant due to the limited time.
6. Experimental Procedures 1. Record the information about the metal and electrolyte plate. 2. Grind and polish the specimen mechanically. 3. Set up the apparatus for electropolishing as shown in Figure 2. Immerse the beaker containing the electrolyte in a water bath at the desired temperature. 4. Acquire the current density curves at room temperature. 5. Choose the proper voltage for electropolishing. 6. Conduct electropolishing at the proper voltage for different time durations. Afterwards, the electropolished specimen should be flushed with water, rinsed with alcohol, and dried. Examine the specimen surface on an optical microscope to check the quality. Pictures should be taken from the specimen surface and included in the laboratory report. 7. Repeat steps 2, 3, 4, 5, and 6 above at 0 °C. 8. Confirm the appropriate temperature, voltage, and time for the given specimen.
Figure 2: Schematic diagram of electropolishing.
Laboratory Report 1. Plot the current density curves and include the pictures of the specimen surface. 2. Report the appropriate electropolishing conditions for the given metal/alloy and electrolyte. Discussion and explanations should also be provided.
City University of Hong Kong Department of Physics and Materials Science
Course Code: Student Name: Student Number: Lab Date and Time: