TRUEMICRO- STRUCTURE The microstructure expresses a high number of wrote an article in Metal Progress(predecessor to AM&P) called “A new name for Metallography? properties of a material. For this reason Try Materialography.” materialographic — or the older, more common During mechanical preparation of any material terminology, metallographic — preparation is specimen, abrasive grains act as machine tools, re- moving material and at the same time creating ar- important for both production and research. tifacts. By applying the correct abrasives, polishing cloths, and fluids in the most appropriate way, the Kay Geels* true structure can be revealed (Fig. 1). This article Struers A/S defines the nature of the true structure, and Copenhagen, Denmark describes the effects of mechanical grinding and polishing.

n 1863, H.C. Sorby prepared the first “true Definition of the true structure structure” of steel, but for many years the im- The goal for all materialographic preparation portance of the preparation process was ne- must be the true structure. Based on Vilella and glected, resulting in metallographic structures Samuels, the true structure can be defined as Iwith many artifacts. In the 1930s, the American J.R. • No deformation Vilella studied the preparation of a “true structure,” • No scratches and in the period from the 1950s to the 1970s, the • No pull-outs Australian L.E. Samuels conclusively described • No false structures how to prepare a true metallographic structure by • No introduction of foreign elements mechanical methods. • No smearing Today, the word “materialography” may be a • No relief or rounded edges more appropriate term than “metallography,” be- •No thermal damage cause metallographers work not only with metals, With mechanical polishing, an approximate true but also with ceramics, plastics, and electronic parts. structure can be shown when the correct procedures In fact, as early as 1968 Crowther and Spanholtz are followed, even with very heterogeneous mate- *Member of ASM International rials, whereas electrolysis may create problems if

12 Undisturbed or insignificantly disturbed material

10

8 Significant deformation

6

SiC 600 grit 4

2 SiC 400 grit Fragmented layer

SiC 220 grit

This diagram shows how a correct preparation process re- veals the true microstructure of a specimen. Specifically, it shows the grit materials and sizes for annealed poly- crystalline 70:30 brass. The grain rolls between the specimen and the disk during lapping. 28 ADVANCED MATERIALS & PROCESSES/FEBRUARY 2001 more than one phase is present in the structure At 220 grit SiC paper, different angles and shapes during electrolytic polishing. Considering that most are evident, and the grain size varies widely. At 220 materials are heterogeneous (or nonconductive), me- grit, the average grain size is 59 microns and the chanical polishing is by far the most preferred largest allowed grain size is 74 microns. Because of method, and this will be the only one discussed. this large variation in rake angles, shape, and size, of MATERIALS Two effects should be considered during me- only 1 or 2% of the many visible grains actually re- chanical preparation: Removal rate, which is the move material by cutting a chip. About the same rate at which material is removed; and deforma- proportion produce scratches in the surface by tion, which is the nature and depth of the plasti- plowing. The remainder have no effect. During the cally deformed layer that is produced in the spec- process, the SiC grains are broken down, and after imen surface. In general, the removal rate should a period of time, most grains become relatively flat be as high as possible and the amount of deforma- from fracturing and wear, so that cutting a chip tion should be as low as possible. Success depends changes into plowing. on the interaction between the abrasive grain and When a diamond grinding disk is the grinding the specimen surface. surface, with the diamonds partially embedded in a bond, the process is somewhat different from the Removal rate — grinding grinding paper. The fracturing is lower, partly be- Grinding is the first stage in the preparation cause the diamond grains are not as brittle as the process. When preparing a specimen, both cutting SiC and partly because the grain is fixed in the and grinding involve abrasives. These are either fixed in a bond as on a cut-off wheel, or fixed to a Rake angle coated surface such as grinding paper. In the case of Specimen a composite disk, the abrasive grains are added to the surface in a free flow, but even in this case, the grain is fixed. Preferred abrasives are silicon car- bide (SiC), aluminum oxide (Al2O3), and diamond. Three modes of interaction are recognized: • Lapping:The grain rolls between the specimen and the preparation disk (Fig. 2). A corner of the grain digs into the specimen and turns, leaving a Cutting Plowing Ridges small cavity with strong deformation. The removal rate is very low, and lapping is not suited for met- allographic preparation. • Grinding:The grain is fixed and acts as a ma- chine tool, because the rake angle is correct for cut- ting a chip (Fig. 3). The rake angle can be positive, zero, or negative, and grinding takes place only when the angle is positive, zero, or to a certain de- Fig. 3 — These diagrams show rake angles, cutting, and plowing. gree negative. At a given negative angle, the crit- ical rake angle, the chip is not made anymore and the grain starts “plowing” instead of cutting. When grinding, the efficiency of material removal ap- proaches 100%. Specimen • Plowing:When plowing (Fig. 3), the rake angle is so negative that only a groove is made in the spec- imen surface. A standing wave bulge forms in front Silicon carbide paper when new of the grain, and material is displaced into a ridge, on each side of the groove. The removal rate ap- proaches zero. The most effective geometric shape of the abra- Specimen sive grain is a V-form (Fig. 4), creating an efficient chip, provided that the rake angle is correct. If the grain is flat, the cross-section of the chip is reduced, Silicon carbide paper in use and in the case of flat grains of a certain size, the specific pressure between grain and surface is re- Fig. 4 — The most effective geometric shape of the abrasive grain is the V-form duced and the grain does not cut, resulting in (top). After a period of time, most will become relatively flat from fracture and wear plowing or no action at all. (bottom), so that cutting is changed to plowing.

ADVANCED MATERIALS & PROCESSES/FEBRUARY 2001 29 bond. It appears that the diamond grains composed formed layer becomes important in metallography, of facets constitute effective cutting points, which when the plastic deformation changes the mi- The fluid give a higher removal rate. The diamond grinding crostructure of the specimen in a way that can be disk normally has a constant removal rate over a detected during microscopic examination. The layer added long period of time, as long as the surface is not is then an important potential source of false struc- during the clogged by ground-off material. This may be tures, or preparation artifacts. The avoidance of polishing avoided with an efficient flow of grinding fluid and these artifacts is one of the primary objectives of a a configuration of the surface so that the debris is preparation sequence. process channeled away from the surface. • Influence of grinding:During grinding, the abra- has an Composite disks (platens) are mostly for the fine sive grains act as machine tools set at different rake important steps in grinding. The surface consists of a resin re- angles. When a chip is separated from the surface, inforced with a metal powder, and the abrasive the shear strains are concentrated in the so-called influence (mostly diamond) is added during the process. It shear zone in front of the tool. A region adjacent to on the is very important that the surface be able to fix the this shear zone, and extending into the specimen removal abrasive grain, establishing a grinding process. If in advance of the tool, is also plastically deformed, many grains are rolling between the disk surface though to a lesser degree. rate. and the specimen surface, a lapping action is es- • Influence of polishing:The depth of the deformed tablished, causing a very low removal rate and layers is an order of magnitude smaller than that heavy deformation. Heating of the specimen sur- on surfaces ground with SiC paper. In principle, face might cause artifacts, and if debris is not re- even the finest abrasive will create a deformed layer. moved, the clogged grinding surface causes severe With very fine abrasives such as alumina and silica damage to the specimen surface. for the final polishing step, a very clean surface can be made. Silica has a grain size of a fraction of a mi- Removal rate — polishing cron, creating a combined mechanical and chem- Polishing is the last stage in mechanical prepa- ical material removal. ■ ration. Polishing cloths are mostly textiles, and range from very hard and plane, to very resilient with a nap. Polishing is very important for two rea- For more information:Kay Geels, Struers A/S,Copen- sons: the deformation from grinding must be re- hagen, Denmark; e-mail: [email protected]. In the moved (rough polishing); and the surface estab- United States: Struers Inc., 820 Sharon Drive, Westlake, lished after the last polishing step (final polishing), OH 44145-1598; tel: 440/871-0071; e-mail: kelly.castledine should represent the “true structure.” @struers.com; Web site: www.struers.com. For many years, alumina in different grain sizes and crystal structures was the most preferred abra- Bibliography sive for polishing. Very fine grain sizes, 0 to 1 mi- 1. Metal Progress,by D. S. Crowther and R. B. Spanholtz, cron and 0 to 0.1 micron after grinding, result in a Sept. 21, 1968. low removal rate, making it impossible to remove 2. Prac. Metallography, by G. Petzow and F. Mücklich, Vol. the deformed layer created by grinding. In the 33, 1996, p. 64-82. 1930s, Vilella pointed out the importance of rough 3. “The Metallurgical Work of Henry Clifton Sorby and polishing with 600 grit alumina on a cloth impreg- an Annotated Catalogue of his Extant Metallurgical Spec- nated with paraffin. With the introduction of dia- imens,” by R.G.I. Edyvean and C. Hammond: J. of His- mond as an abrasive, the removal rates were in- torical Metallurgy, 1998. creased considerably, while at the same time 5. Metallograhic Polishing by Mechanical Methods,by L. E. Samuels: American Society for Metals, Metals Park, Ohio, introducing less deformation and a better overall 1982. quality of the specimen. 6. Metallographic Technique for Steel,by J. R. Vilella: Amer- For rough polishing, hard cloths create a higher ican Society for Metals, Cleveland, Ohio, 1938. load on the grain, giving a larger chip. Softer, more 7. Metalog Guide, by Bjerregaard, Geels, Ottesen, and resilient cloths for the final steps ensure smaller Rückert: Struers A/S, Copenhagen, Denmark, 1996. scratches and less deformation of the surface. 8. Surface Preparation and Microscopy of Materials,by B. The fluid added during the polishing process has Bousfield: John Wiley & Sons Ltd., Chichester, U.K., 1992. an important influence on the removal rate. In most 9. Pract. Metallography,by M. J. Damgaard and Kay Geels, cases, fluids based on a hydrocarbon such as Vol. 35, p. 587-599, 1998. kerosene give the highest removal rate. A similar effect can result with an alcohol-based fluid. Fluids are also important for surface cooling.

Grinding and polishing deformation The separation of a chip during machining op- erations induces complex systems of plastic defor- mation in both the separating chip and the spec- imen material. An inevitable consequence is that a How useful did you find the information layer plastically deformed during machining, is left presented in this article? in the new surface. In general terms, the strains in Very useful, Circle 277 Of general interest, Circle 278 this layer are very large at the surface and decrease Not useful, Circle 279 more or less exponentially with depth. This de-

30 ADVANCED MATERIALS & PROCESSES/FEBRUARY 2001