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Expansion of the scope of services for the characterization of small indenters for hardness measurement technology

Categories:
  • Division 5
  • Metrology for Economy
22.12.2020

Instrumented indentation test or micro-and nano-indentation is a powerful method for characterizing the mechanical properties of materials. The geometry of the indenter used in the instrumented indentation test has a great influence on the measured hardness value. On the one hand, the surface function of the indenter, its projected surface, must be characterized as as accurately as possible as a function of the indentation depth, is directly incorporated into the calculation of the mechanical properties of the specimen. On the other hand the DIN EN ISO 14577 standard places requirements for the tip radius and the opening angle of the mostly pyramidal indenting bodies, the compliance of which must be verified by appropriate measurements. Diamond indenters cannot be perfectly created and deviate from the ideal shape in small penetration depths. For the calibration of the indenter geometry, a retraceteds scanning force microscope is recommended normatively at small penetration depths, to achieve a low uncertainty of the measured surface function.

A Berkovich indenter used by WG 5.12 for the instrumented indentation test (Figure 1 a)) was measured with the metrological scanning force microscope of WG 5.23. After extensive cleaning of the indenter the measurements was made with a lateral measuring range of 1 µm x 1 μm up to 8 µm x 8 μm. To determination of the tip radius, the opening angle as well as the surface function of the indenter from the obtained measurement data was developed its own evaluation software. This determines, for the contact area, the area between the individual scan points and adds them starting from the tip of the indenter. For the projected surface, the scan points that are equal to the tip of the indenting body are determined and after that the area is stretched by these points (Figure 1 b)).

From the surface function, the tip radius and the opening angle of the indenter could be determined and the requirements of DIN EN ISO 14577 could be checked.

An initial analysis of the measurement uncertainty of the determined surface function was carried out using the specially developed evaluated software with a simulative approach. Each time, a geometric parameter is varied, while keeping the other parameters constant and determines how the surface function changes. The results of the study suggests that a relative uncertainty (k=2) of the area functiondetermination of about 2 % can be achieved. This uncertainty is dominated by the mounting tilt angle of the indenter to the axis of the indenter holder.

Due to the successful introduction of the area function for indenters for the instrumented indentation test, the process can now also be offered as a service for customers.


Figure 1: a): Geometry measured with the metrological scanning force microscope of a Berkovich indenter.
(b): Area function of the indenter determined from the measured data

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