Microhardness
Microhardness turns up in 2 fields: heat treating, thermal spray. Here is what it means to the people in each one.
Microhardness in heat treating
Hardness measured with very small loads under a microscope, used for thin cases and individual phases.
Indentation testing with loads from a few grams up to 1 kgf, read optically on the Vickers (HV) or Knoop (HK) scale. On case-hardened parts, a microhardness traverse places indents at small steps inward from the surface of a polished cross section, and the curve gives effective and total case depth. Knoop's long, shallow indent suits readings very close to the edge.
"Run a microhardness traverse at 0.002 steps and plot it, I want to see where it crosses 513."
Microhardness in thermal spray
Hardness of a coating measured with tiny diamond indents on a polished cross section.
Vickers or Knoop indentation with small loads, commonly 300 grams (HV0.3), placed in the middle of the coating thickness on a mount, usually averaged over about ten readings. Coatings are too thin and too porous for normal Rockwell testing, so microhardness is the standard check. Typical values are around 1,000 to 1,300 HV for HVOF tungsten carbide and lower for chromium carbide and metallic coatings.
"Microhardness averaged 1,150 HV0.3 on the carbide coupon, scatter's tight, good to go."