Hardness Test - Martens hardness





Principle

 

Continuous recording of load, depth and time during the indentation of three or four sided diamond pyramids. Calculation of the contact area under load from the maximum indentation depth hmax and the known indenter shape. Elastic and plastic parts of the deformation are not distinguished or separated. Therefore this hardness definition cannot be well compared with other hardness definitions.

Formulas

 

HM = F / As(h) ... Martens hardness
Fmax ... Maximum applied test force
h ... Indentation depth
As(h) ... Surface area of the indenter for maximum indentation depth
As = 26.43 ⋅ h2 ... for an ideal Vickers indenter (without tip rounding)
As = 26.98 ⋅ h2 ... for an ideal modified Berkovich indenter (without tip rounding)

Differential Martens hardness

HM
diff
=
1
(
∂
 
h
∂
 
√
F
)
2
 
A
s
h
2


Designation of Martens hardness HM

Example: HM 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
  • Division into nano- (h < 200 nm), micro- (F < 2 N) and macro- (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 70s. 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. Ternovskij, 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 development of the former universal hardness (now: Martens hardness) has been undertaken, especially in Germany, in connection with the development of the measuring device Fischerscope H100. It dates back to work of W. Weiler of the "Physikalisch-Technischen Bundesanstalt" as well as to D. Dengel and H.-H. Behnke.
[4] W. Weiler, Zur Definition einer neuen Härteskala bei der Ermittlung des Härtewertes unter Prüfkraft, Materialprüfung 28 (1986) 217-220.

Through relating to the maximum indentation depth the hardness value can be deduced and measured easily and without taking a model as a basis. However, it gets bought through mixing elastic and plastic characteristics and due to the bad comparability with existing hardness definitions especially with very elastic materials.

Valid standard

 

DIN ISO 14577 1 - 3
Metallic materials - Instrumented indentation test for hardness and materials parameters

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