Hardness Testing Section 6.10

Hardness “A measure of a material’s resistance to localized plastic deformation.” Early arbitrary hardness indexing ◦ What scratches what? Which is “softer?” ◦ Mohs Scale

Hardness

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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 alloys and some plastics Normal or Superficial Rockwell Hardness

Depend on normal or superficial Normal ◦ 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)

10.00 mm ball Loads ◦ 500 and 3000 kg Loaded for ~10 s HB scale Brinell Hardness

Small diamond shape 1 to 1000 g loads Microindentation ◦ Requires microscope HK and HV scales Knoop – ceramics Knoop and Vickers

Diamond tipped hammer Measures rebound from a fixed height Scleroscope

Shore Durometer Tests polymers, elastomers, and rubber

Conversions Between scales ◦ Figure 6.18 ◦ Charts in the lab Between Hardness and Tensile ◦ TS(MPa) = 3.45 x HB  Steel alloys only