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Micro hardness testing
- 2. Micro Hardness Testing • Forcing a diamond indenter of specific geometry into the surface of the test piece with a load (1 to 50,000 gf). • Three type of test: i) Knoop ii) vicker iii)ultrasonic. • IN knoop and vicker, size of resulting un-recovered indentation by using a microscope. • Measured change in the frequency of a vibrating diamond-tipped rod (no microscope). • All these testing methods produce an indentation depth of less than 19 micron.
- 3. Knoop indenter Knoop indenter. Highly polished, rhombic-based pyramidal diamond that produces a diamond shaped indentation Ratio between long and short diagonals of about 7-1.The pyramid shape has an angle, (longitudinal) 172-30, transverse angle of 130-0.
- 4. Vickers indenter • Vicker indenter. • Highly polished • pointed square base • pyramidal diamond • with face angles of 136
- 6. Interesting information • For same load the vicker indenter penetrates about twice as far into the specimen as the Knoop indenter. • Therefore the vicker test is less sensitive to minute differences in surface condition than knoop test.
- 7. Difference in Knoop and vicker The basic differences are: • i) diamond indenter shape ii) Load • Knoop indenter. Highly polished, rhombic-based pyramidal diamond that produces a diamond shaped indentation. • Ratio between long and short diagonals of about 7-1.The pyramid shape has an angle, (longitudinal) 172- 30, transvers angle of 130-0. • Vicker indenter. Highly polished pointed square base pyramidal diamond with face angles of 136.
- 8. Measuring the Indentation A microscope in conjunction with the hardness tester is used to determine the size of indentation. Proper magnification is important for accurate measurements. max---min Indentation length (micron) Mag.( max---min) Less then 76 measurement. 400 • 76-----------125 800---------300 • More then 125 600---------200 • The ends of indentation diagonals must be brought into sharp focus.
- 9. Testing equipment • Tester vary primarily in load range capabilities. (micro- hardness to mac-rohardness) • Most of them accommodate either the knoopor or vicker indenters. • Application of load by dead weight or by weights and a lever. • Load from 1-10 gf are directly applied to the top of the indenter. • Application. • Research and development, micro-constituents, small parts, foil and wire ,hardened parts.
- 10. Specific Application • Measuring hardness of too small work pieces. • Measuring hardness close to edges • Foil or wire too thin or too small • Monitoring of carburising or nitriding operations • Hardness of individual constituents (micro) • Measuring hardness of surface layers such as plating etc.
- 11. Strength from Brinell Hardness Steel: Su = 3.45 HB (MPa) Cast Iron: Su = 5.35HB -500 (MPa) Brass: Su = 2.9HB + 50 (MPa)