Hardness measurement technique on the transversal section required particular attention in the specimen preparation; hardness tests were performed by using a Knoop indenter, according to the standard ISO 6441-1.

From theoretical point of view hardness physically is not well defined for polymers, since in most cases polymers show viscoelastic flow, i.e. the "hardness" is not constant over measuring time. - But if one need to have nevertheless an information on something like "hardness", I would recommend to use a micro hardness tester, if it allows registration of force-distance curves. From force-distance curves different hardness values can be determined, e.g. martens hardness, indentation hardness, universal hardness, Vickers hardness, etc. and one gets additional information on the Youngs modulus.

For polymers the viscoelastic flow of the material under load may falsify the hardness measurement. From practical point of view one can eihter make fast and short measurments (e.g. 1 sec load and unload) to minimize the effect of flow and give the "hardness" of the film, or one can make a long hold time under load (e.g. 120sec) to characterize the viscoelastic or drift properties of the material.

Best regards

Kirsten

Mr. Kirsten, Got what you explained. Suppose a polymer is brittle in tension and and ductile in compression. Does conducting hardness test below glass transition temperature induce plastic deformation ? Loading is at a faster rate , so no question of transition from glassy state to rubbery state (even temperature less than glass transition temperature). Plastic deformation during hardness test is dependent on compression curve , right ? Indentation test is nothing but resistance to plastic deformation when compressive load is applied , is it not ?

In Indentation e.g. with a Vikers or Berkovich indentor you have a complex 3 dimensional stress state. Probably the main will be compressive (vertical and laterally). But there may also be lateral tension. I believe that the ductile compressive components should lead to a plastic deformation and allow a meaning full hardness measurement. - By the way: an interessting material which is brittle in tension and ductile in compression.

Thanks Mr. Kirsten