Conventional Micro Hardness versus Instrumented Indentation

Many people rely on “conventional” micro hardness measurement technique to obtain hardness values, in which case, hardness is computed using the diagonals of an imprint. While the technique works well for bulk materials and thick coatings, it is not appropriate for small features, thin coatings and materials such as glass and ceramics that crack under too much load. Also, the smaller the indent, the greater the error on the measurement.

Instrumented indentation is based on the analysis of the load-penetration curve measured while doing an imprint. It is computer-controlled and it is not influenced by the operator’s judgment of the imprint’ shape. Properties such as hardness and elastic modulus can be determined with a single indent. Many are mistakenly using conventional measurement techniques when they should use instrumented indentation.

When time comes to choose a proper technique to measure the hardness, keep the following in mind:
• A hardness value is invalid if an indent shows sign of cracking or pile up.
• Ideally an indent should not be deeper than 1/7 to 1/10 of the coating thickness.
• When measuring the hardness of a coating using a cross section, the distance between the edge of coating (surface/interface) and the indent itself should be at least 3 times the size of the diagonals.
•The hardness of most samples can be measured using instrumented indentation without the need to have a polished cross-section.