Hardness Test - Indentation hardness (plastic hardness)





Principle

 

Continuous recording of load and depth during the indentation of three or four sided diamond pyramids. Calculation of the contact area under load by deducting the elastic deformation components and by considering shape deviations of the specimen.
The relation to plastic deformation components only is what distinguishes indentation hardness from Martens hardness.

A conversion to Vickers hardness is possible by using the formula HV= 92.62 * HIT.

Formulas

 

HIT = Fmax / Ap(hc) ... Indentation hardness

Fmax ... Maximum applied test force
Ap(hc) ... Projected contact area between indenter and sample
Ap = 24.5 ⋅ hc2 ... for an ideal Vickers indenter
hc ... Contact depth under test force, calculated according hc = hmax - ε* Fmax/S
S ... Contact stiffness
ε = m * hs /(hmax - hr)
hr - Intersection point of the tangent to the unloading curve with the indentation depth axis
Er = 0.5 ⋅ √ π ⋅ S / √ Ap ... reduced modulus

Designation of Indentation hardness HIT

Example: HIT 0.5 / 10 / 20 / 30 = 11300 N/m2
0.5 ... test force in N
10 ... Application time of test force in s
20 ... Duration time of test force in s
30 ... Unloading time in s
11300 ... Hardness number




Conditions
  • Differentiation between nano- (h < 200 nm), micro- (F < 2 N) and makro- (F < 30000 N) range
  • The sample has to be either 10 times thicker than the indentation depth or 3 times thicker than the indentation diameter.
  • Realization of the test at stable temperature conditions. External temperature influences have to be controlled and the temperature has to be kept stable; otherwise a drift correction has to be carried out.
  • The zero point of the measurement (sample surface) has to be determined with an accuracy < 1%.
  • Vertical and shock-free appliance of the test load.
  • Low approach speeds need to be chosen (Nano range 10 - 20 nm/s)
  • The exact measuring cycle is to be indicated.
Typical case:
  • 30 s loading
  • > 10 s hold period at maximum load
  • 10s unloading to 10-20 % of the maximum load
  • > 30 s (better 60 s) hold period for temperature drift determination
  • final unloading
History

 

The first registered hardness measurements in the micro range where load and indentation depth have been recorded accurately and continuously can be dated back to mid 70 s. The first activities in this field hav been done independentely from each other by one Russian and one German research group:
[1] S. I. Bulychev, V. P. Alekhin, M. Kh. Shoroshow, A. P. Ter-novskij, G. D. Shynrev, Zavodskaya Laboratoriya 41 (1975) 1137
[2] F. Fröhlich, P. Grau, Anordnung einer registrierenden Härteprüfung unter Last, DDR Patent 121 386 (25.09.1975)
[3] F. Fröhlich, P. Grau, W. Grellmann, Performancs and Analysis of Recording Microhardness Tests, phys. Stat. Sol. (a) 42 (1977) 79-89.

The indentation hardness relates, in contrast to the Martens hardness), to plastic deformation components only and can therefore be compared with the Vickers hardness directly. Recognized theories for the calculation (and therefore the deduction) of the elastic deformation components have been first and foremost developed by
[4] J. L. Loubet, J. M. Georges, G. Meille, ASTM STP 889 (1986) 72, Ed. P. J. Blau, B. R. Lawn
as well as by
[3] M. F. Doerner, W. D. Nix, J. Mater. Res. 1 (1986) 601. entwickelt.

Finally, the method developed by Oliver and Pharr in 1992 has been proved the most accurate. The evaluation principle developed by Oliver and Pharr has been adopted as the new international standard ISO 14577
[4] W. C. Oliver, G. M. Pharr, J. Mater. Res. 7 (1992) 1564.

Valid standard

 

DIN ISO 14577 1 - 4 Last revision 2015 part 1 - 3
Metallic materials - Instrumented indentation test for hardness and materials parameters
Part 1: Test method
Part 2: Verification and calibration of testing machines
Part 3: Calibration of reference blocks
Part 4: Test method for metallic and non-metallic coatings

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