Using a Trobometer for Scratch Hardness Measurement

The resistance of materials to permanent or plastic deformation is measured by hardness. There are three types of hardness measurements: indentation hardness, rebound hardness and scratch hardness.

A scratch hardness test measures the hardness of a material to abrasion and scratches caused by friction from a sharp objecti. The German mineralogist Friedrich Mohs developed the idea in 1820, allocating talc a value of 1 and diamond a value of 10, and it is still widely used to rank the physical property of mineralsii.

The scale developed by Mohs was not linear and so a more accurate qualitative scratch hardness measurement was developed and is described in ASTM standard G171-03iii. In this scratch hardness measurement, the surface of the tested material is scratched along a path under constant normal force with constant normal speed by a diamond stylus of specified geometry.

The scratch hardness number (HSp) is calculated using the average width of the scratch. This technique provides a simple solution of scaling the hardness of different materials, including: ceramics, metals, coated surfaces and polymers.

Measurement Objective

The Nanovea Tribometer is used in this study to measure the scratch hardness of different metals. It has the capacity to perform scratch hardness measurement with high precision and reproducibility and this makes it a more complete system for mechanical and tribological evaluations.

Scratch stylus on the test specimen.

Figure 1. Scratch stylus on the test specimen.

Test Conditions

The Nanovea Tribometer was used to perform scratch hardness tests on three polished metals (Cu110, SS304 and AI6061). A conical diamond stylus with tip radius of 200 mm and apex angle 120° was used. In order to ensure local linearity of the scratch track for accurate track width measurement, the scratch test was performed on the sample fixed on the rotative sample stage with a large distance of 3 cm to the stage center.

To ensure reproducibility of the results, each sample was tested for three scratches under the same parameters. The integrated non-contact optical profilometer was used to perform the scratch track profile scan after the scratch test.

Table 1. Test parameters of the scratch hardness measurement.

Test parameters Value
Normal force 10 N
Sliding speed 20 mm/min
Sliding distance 10 mm
Atmosphere Air
Temperature 24 °C (room)

Results and Discussion

Figure 2 shows the scratch profiles of three metals (Cu110, SS304 and AI6061) measured by the integrated non-contract optical profilometer. This enables comparison of the scratch hardness of different materials. As the stylus travels at a constant load of 10 N and ploughs into the metal, pile-ups form at the sides of the scratch tracks.

This is because the metal in the scratch tracks is pushed and deformed to the side. Using the peaks of the two pile-ups, the track width can be determined and then used for the calculation of scratch hardness.

Figure 3 shows the images of the scratch tracks after the test by Tribometer, examined under the optical microscope. The scratch tracks made using the Nanovea Mechanical Tester are shown for comparison. Table 2 summarizes and compares the measured scratch track width and calculated scratch hardness number (HSp).

The metals show different wear tracks – 228, 92 and 170 mm for Cu110, SS304 and AI6061 respectively, resulting in calculated HSp of 0.49, 3.01 and 0.88 GPa. The measured scratch track by the mechanical tester equipped on the Tribometer is in agreement with the scratch track measured by microscope. The scratch tracks created by the mechanical tester and the tribometer show comparable width, and this in turn results in calculated scratch hardness in close HSp values.

As well as the scratch hardness computed from the scratch track width, the evolution of coefficient of friction (COF) was recorded in situ during the scratch hardness test. This is shown in Figure 4. This information provides insight into mechanical failures that may take place during scratching and this enables users to detect mechanical defects and investigate further the scratch behavior of the material tested.

Using the Nanovea Tribometer equipped with the integrated profilometer, the scratch hardness test can be completed within a couple of minutes and exhibits good precision and repeatability. The scratch hardness test in this study gives a good alternative solution for hardness measurement and makes Nanovea tribometer a more complete system for comprehensive tribo-mechanical evaluation.

Scratch track profiles after the scratch hardness tests.

Figure 2. Scratch track profiles after the scratch hardness tests.

Scratch tracks under the microscope after the measurements (100X).

Figure 3. Scratch tracks under the microscope after the measurements (100X).

Table 2. Summary of scratch track width and scratch hardness number.

Scratch hardness by Tribometer Scratch hardness by Mechanical Tester
Scratch track width (µm) HSP (GPa) Scratch track width (µm) HSP (GPa)
Al6061 170±6 0.88 174±11 0.84
Cu110 228±8 0.49 220±1 0.52
SS304 92±4 3.01 89±5 3.20

The evolution of coefficient of friction during the scratch hardness tests.

Figure 4. The evolution of coefficient of friction during the scratch hardness tests.

Conclusion

This study showcases the capacity of the Nanovea Tribometer in performing scratch hardness tests that comply with ASTM G171-03. The scratch hardness test at a constant load is a simple alternative solution for comparing the hardness of materials using the tribometer equipped with a profilometer. Compared to conventional Mohs scratch hardness tests, the Nanovea Tribometer accurately measures the quantitative hardness value and monitors the evolution of coefficient of friction in situ.

Additionally, the Nanovea Tribometer offers repeatable and precise wear and friction testing using ISO and ASTM complicate liner and rotative modes, with optional lubrication, high-temperature wear and tribo-corrosion modules available in one integrated system. The optional 3D non-contact profiler is available for high resolution 3D imaging of wear track as well as other surface measurements such as roughness. Learn more about the Nanovea Tribometer and Lab Services.

Measurement Principles

Scratch hardness of materials using a diamond stylus is based on the standard ASTM G171-03iii. The test sample is prepared either to represent the application of interest or to enable scratch width measurement. As shown in Figure 1, the stylus applies a constant normal force to the test sample and the scratch track is formed by the relative sliding movements of the stylus against the sample surface.

The average width of the scratch track is calculated and then used in the following formula to determine the scratch hardness number (HSp):

Schematic of scratch hardness measurement.

Figure 1. Schematic of scratch hardness measurement.

Where P is the normal force, w is the scratch width and HSp is the scratch hardness number.

Other Possible Measurements by Nanovea Tribometer

Linear (reciprocating wear) wear, rotative (pin-on-disc) wear, wear in different environments including corrosive, lubrication and high temperature, etc.

References

  1. Wredenberg, Fredrik; PL Larsson (2009). "Scratch testing of metals and polymers: Experiments and numerics". Wear 266 (1–2): 76.
  2. Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 22 Feb. 2009 "Mohs hardness."
  3. ASTM G171 - 03(2009), "Standard Test Method for Scratch Hardness of Materials Using a Diamond Stylus."

Nanovea.

This information has been sourced, reviewed and adapted from materials provided by Nanovea.

For more information on this source, please visit Nanovea.

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