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Hardness“A measure of a material’s resistance
to localized plastic deformation.”
Early arbitrary hardness indexing◦What scratches what? Which is “softer?”◦Mohs Scale
Diamonds are “hard”Talc is “soft”
◦You can scratch talc with a diamond◦Cannot scratch a diamond with talc
(Frypans and Metal Spatulas) (Sharpening tools) (Whet stones)
Hardness
Hardness test performed more frequently than any other mechanical test.◦Simple and inexpensive
Little preparation for specimens Machinery not quite as expensive as others
◦Little deformation Small bump or indentation, not large fracture
◦Can sometimes estimate tensile strength
Hardness
Most common◦Simple◦Little skill required
Uses◦Indentors and weights
Allow testing of alloysand some plastics
Normal or Superficial
Rockwell Hardness
Depend on normal or superficialNormal
◦B – 1/16” tungsten carbide ball, 100 kg◦C – “Brale” conical diamond, 150 kg
Superficial◦15N – “Brale”, 15 kg◦15T – 1/16” ball, 15 kg
Brale scales used for hardest materials
Rockwell (Indenters)
Small diamond shape1 to 1000 g loadsMicroindentation
◦Requires microscopeHK and HV scalesKnoop – ceramics
Knoop and Vickers