Micro Hardness Testing

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.

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.

Vickers indenter

• Vicker indenter.• Highly polished• pointed square base• pyramidal diamond• with face angles of 136

Vicker Indenter

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.

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.

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.

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.

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.

Steel:Su = 3.45 HB (MPa)

Cast Iron:Su = 5.35HB -500 (MPa)

Brass:Su = 2.9HB + 50 (MPa)

Strength from Brinell Hardness