Esatest Principle

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ESATEST® Hardness tester Working principle

Traditionally hardness is measured by pressing an hard indenter of known size and form with a known load on the surface of the material to be checked, and by determining afterwards the dimensions of the indentation either by optical or by mechanical means.

The ELECTRICAL MEASUREMENTS seemed till now excluded, due to the fact that the normally used diamond indenters are electrical insulators. A Covering Method worldwide patented by ERNST and developed inside our organization allows getting a conductive surface on diamonds. If such diamonds are used as indenters, they can sense the electrical resistance during the diamond indention thus knowing at any moment the depth of penetration.

Figure 1

Figure 1 shows a schematically view of an indenter with a conducting surface. This conducting surface forms an electrical bridge between the work piece and the metallic mounting of the diamond indenter. By measuring the electrical resistance between the mounting and the work piece, the instrument indicates the position of the diamond referred to the specimen surface. It is clear that higher will be the load applied and higher will be the penetration in the material and lower will be the measured resistance. We can describe the working principle of figure 1 were diamond indenter works as a potentiometer. The resistance is represented by the conducting layer on the surface of the diamond indenter. Contact point of the surface of the work piece with the diamond represents the slider.

ESATEST hardness testers are using the following MEASURING PRINCIPLE

Figure 2

During the loading for each indentation the tester is continuously recording the values for the electrical resistivityand for the load. In figure 2 you can find schematic examples of such load-resistivity curves. From these curves the instrument has to determine the hardness of the work piece. For simplicity the instrument works as acomparator. That means, in a first step it records a load-resistivity curve obtained from an indentation on a calibrated reference block. This curve is stored in the memory of the instrument. All other curves produced on metallic specimen are compared to this “REFERENCE” CURVE. In order to calculate the hardness of the work piece, the instruments selects on the two curves - the reference curve and the MEASURED CURVE - those points, which have the same electrical resistivity. The same electrical resistivity means, that the penetration forthe two points on the two curves has been the same. Due to the definition of the hardness, which reads H = Load / Contact Surface, the ratio of the loads of the two points selected in the described way, is equal to the ratioof the hardness of the two materials. Knowing the loads of the two points and the hardness of the reference specimen it is easy to calculate the hardness of the work piece. This calculation can be done along the whole load-resistivity curve. Therefore it gets in one single indentation the hardness for all loads between zero

and the maximum applied load.

In the following example the test is carried out on homogeneous materials and the output hardness curve is an horizontal line (figure 3). For materials with harder or softer layers on the surface, the curve deviates at low loads from the horizontal line towards higher or lower values.

Figure 3

ESATEST ADVANTAGES AND LIMITATIONS

ADVANTAGES

•Using the indenter also as Depth Sensor there is no need for complicated measuring structures.

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•There is no need to have a direct access to the surface. Installing the indenter on a lever as it is done in the ESATEST, it is possible to measure the hardness in places where till now the hardness could only be determined after destruction of the piece, as for example in holes and cavities, or on the gear tooth. The instrument is not sensitive to elastic deformations of the load train, of the work piece or of the support.

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•The portable version allows measuring on vertical surfaces or even upside down.

LIMITATIONS:

•The instrument can only work on metallic surfaces.

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•It has been observed that measurements or on titanium spoil the surface, adding additional conductivity: for this reason the instrument can be used on these metals only by polishing the diamond surface quite often.

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•The conductive layer produced by the covering process is rather thin and is slowly eroded by the measurements. Its lifetime is limited to about 20.000/25.000 measurements on normal steels.

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•Sintered hard metals as WIDIA erode rapidly the conducting layer.

RANGE OF ESATEST INSTRUMENTS :

Handy MTR

HTD 900