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In article <Bs5s0s.843@world.std.com> fhapgood@world.std.com (Fred Hapgood) writes:
>I have been reading in a geology text about something called a
>'Moh scale' that quantifies degrees of mineral hardness.  Apparently
>some gentleman by that name placed ten minerals in order of hardness
>and assigned them the digits 1-10.  This seems primitive.  Is there
>a more comprehensive and precise measure of hardness in use?

The Mohs scale is a measure of the scratch hardness.  Minerals are arranged
in order such that a mineral will scratch another mineral with a lower
number.  Another method of measuring scratch hardness is with jeweled
scratching points.  These use either a set load and measure the width of the
scratch or else they are loaded until a given width of the scratch is
obtained and the load is measured.

Another method of measuring the hardness is the Brinell hardness number where
a hardened sphere is forced into the material under a given load and the
diameter of the indentation is measured.  The Brinell number is

BHN=P/(pi*D/2)*(D-sqrt(D^2-d^2))

where P is the load (kg), D is the diameter of the spherical indenter (mm),
and d is the diameter of the impression (mm).

There are other methods such as Rockwell, and Vickers hardness number.  All
of these should be in any Mechanical Engineering Handbook.


reference: Marks' Standard Handbook for Mechanical Engineers; Baumeister,
Avallone, Baumeister; McGraw-Hill Book Company


Philip Peregino

..what was omitted is that all these different hardness scales give
different hardness numbers (even if they are all converted to the same
units, e.g GPa or kgf.mm^-2). It is even possible for two materials
which have the same hardness on one scale to be different ahrdnesses on
another - this is because each test actually involves a bundle of more
fundamental properties: yield stress, elasticity, strain-haredning 
ciefficient just for starters.

Most research on indentation uses cones or pyramids because the stress
analysis is more straight forward, i.e. Vickers or Knoop indenters.

For just ordinary quality assourance one of the Rockwell series of tests
is easiest because it measures the difference in indentation depth
between a minor and an major load, so is more robust to errors in setup.
--Philip
ref: "Factors affecting the microhardness of solids", PhD thesis, Univ. of
Cambridge, UK., 1979, P.M.Sargent
--
-Philip Sargent
Cambridge University Engineering Dept., Design Centre & Materials Group