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Chapter VIII Micro-hardness studies… 3. Grain boundaries (grain boundary hardening) 4. The chemical forces in the lattice of the material (intrinsic hardness). Thus, the hardness of a pure (free from impurities) and well-annealed (free from excessive number of dislocations and grain boundaries) crystal represents the intrinsic hardness, which reflects the bond strength. Creation of hardness indentation and obstacles are shown in the form of schematic diagram in Figure (8.1). The low-load indentation method of hardness testing is popular because it offers several advantages viz.: 1. It is simple and does not consume much time 2. It can be used on samples with small areas, e.g. small crystals 3. It can be used on brittle materials which cannot withstand large loads, e.g. glasses. Figure:8.1 Schematic diagram of resistance to motion of dislocation [13]. To cover a broad range of materials with a wide range of testing conditions, the static indentation tests are the most favored ones and give much insight in studying the properties of a given material. The four main tests in this category are as follows: 287
Chapter VIII Micro-hardness studies… 1. Brinell Test It is perhaps the first scientific technique to measure hardness of materials. In a standard Brinell test [16] a hard steel ball of about 1 cm. diameter is pressed normally on the surface of metal under examination for a standard period of 30 sec. After the removal of the load, the indentation impression is measured. The impression may not be circular due to some reasons necessitating the measurement of two diagonals at right angles and using the mean value for estimating the hardness number. To reduce various errors, usually, the geometrical similarity of indentation marks should be preserved and hence the diameter of the impression mark should be kept within the range of 0.25 to 0.50 times the diameter of the ball. The Brinell hardness number, or simply the Brinell number, can be obtained by dividing the load used, in kilograms, by the actual surface area of the indentation, in square millimeters. The result is a pressure measurement, but the units are rarely stated. The Brinell hardness test uses a desk top machine to press a 10 mm diameter, hardened steel ball impressed onto the surface of the test specimen. The machine applies a load of 500 kilogram for soft metals such as copper and brass. A 1500 kilogram load is used for aluminum castings and a 3000 kilogram load is used for materials such as iron and steel. The load is usually applied for 10 to 15 second. After the impression is made, a measurement of the diameter of the resulting round impression is taken. The hardness is calculated by dividing the load by the area of the curved surface of the indention. Among the three hardness tests, the Brinell ball makes the deepest and the widest indentations, so the test averages hardness over a wider amount of material, which is more accurate 288
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Saurashtra University Re - Accredit
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Statements Under O. Ph.D.7 of Saura
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ACKNOWLEDGEMENT The author expresse
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in-law, Mother- in -law and all oth
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of NLO materials in rapidly develop
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doped samples in comparison to the
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micro-hardness decreases. Also, as
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Papers Published in National and In
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Seminar on Recent Advances in Conde
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