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HIGH TEMPERATURE SCRATCH HARDNESS

USING TRIBOMETER

Prepared by

Duanjie Li, PhD

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INTRODUCTION Hardness measures the resistance of materials to permanent or plastic deformation. Originally developed by German mineralogist Friedrich Mohs in 1820, scratch hardness test determines the hardness of a material to scratches and abrasion due to friction from a sharp objecti. The Mohs' scale is a comparative index rather than a linear scale, therefore a more accurate and qualitative scratch hardness measurement was developed as described in ASTM standard G171-03ii. It measures the average width of the scratch created by a diamond stylus and calculates the scratch hardness number (HSP). IMPORTANCE OF HARDNESS MEASUREMENT AT HIGH TEMPERATURES Materials are selected based on the service requirements. For applications involving significant temperature changes and thermal gradients, it is critical to investigate the mechanical properties of materials at high temperatures to be fully aware of the mechanical limits. Materials, especially polymers, usually soften at high temperatures. A lot of mechanical failures are caused by creep deformation and thermal fatigue taking place only at elevated temperatures. Therefore, a reliable technique for measuring hardness at high temperatures is in need to ensure proper selection of the materials for high temperature applications. MEASUREMENT OBJECTIVE In this study, the Nanovea Tribometer measures the scratch hardness of a Teflon sample at different temperatures from room temperature to 300 oC. The capacity of performing high temperature scratch hardness measurement makes Nanovea Tribometer a versatile system for tribological and mechanical evaluations of materials for high temperature applications.

Fig. 1: Scratch hardness test performed in the oven.

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TEST CONDITIONS The Nanovea Tribometer was used to perform the scratch hardness tests on a Teflon sample at temperatures ranging from room temperature (RT) to 300 oC. Teflon has a melting point of 326.8 oC. A conical diamond stylus of apex angle 120° with tip radius of 200 µm was used. The Teflon sample was fixed on the rotative sample stage with a distance of 10 mm to the stage center. The sample was heated up by an oven and tested at temperatures of RT, 50 oC, 100 oC, 150 oC, 200 oC, 250 oC and 300 oC. The test parameters are summarized in Table 1.

Test parameters Value Normal force 2 N Sliding speed 1 mm/s Sliding distance 8 mm per temperature Atmosphere Air Temperature RT, 50 oC, 100 oC, 150 oC,

200 oC, 250 oC and 300 oC Table 1: Test parameters of the scratch hardness measurement.

RESULTS AND DISCUSSION The scratch track profiles of the Teflon sample at different temperatures is shown in Fig. 2 in order to compare the scratch hardness at different elevated temperatures. The pile-ups at the sides of the scratch tracks form as the stylus travels at a constant load of 2 N and ploughs into the Teflon sample, pushing and deforming the material in the scratch track to the side. The scratch tracks were examined under the optical microscope as shown in Fig. 3. The measured scratch track width and calculated scratch hardness number (HSP) are summarized and compared in Fig. 4. The scratch track width measured by the microscope is in agreement with that measured using profilometer – the Teflon sample exhibits a larger scratch at higher temperatures. Its scratch track width increases from 281 to 539 µm as the temperature elevates from RT to 300oC, resulting in decreased HSP from 65 to 18 MPa. The scratch hardness at elevated temperatures can be measured with high precision and repeatability using the single Nanovea Tribometer. It provides an alternative solution for hardness measurement, and makes Nanovea tribometer a more complete system for comprehensive high-temperature tribo-mechanical evaluation.

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Fig. 2: Scratch track profiles after the scratch hardness tests at different temperatures.

Scratch at RT: Scratch at 50 oC:

302 µm 280 µm

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Scratch at 100 oC: Scratch at 150 oC:

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Fig. 3: Scratch tracks under the microscope after the measurements at different

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Fig. 4: Evolution of the scratch track width and scratch hardness vs. the temperature.

CONCLUSION In this study, we showcases that the Nanovea Tribometer measures the scratch hardness at elevated temperatures in compliance to ASTM G171-03. The scratch hardness test at a constant load provides an alternative simple solution for comparing the hardness of materials using the tribometer. The capacity of performing scratch hardness measurement at elevated temperatures makes Nanovea Tribometer an ideal tool for evaluating the high temperature tribo-mechanical properties of materials. Nanovea Tribometer also offers precise and repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional high temperature wear, lubrication and tribo-corrosion modules available in one pre-integrated system. Optional 3D non-contact profiler is available for high resolution 3D imaging of wear track in addition to other surface measurements such as roughness. Learn More about the Nanovea Tribometer and Lab Services i Wredenberg, Fredrik; PL Larsson (2009). "Scratch testing of metals and polymers: Experiments and numerics". Wear 266 (1–2): 76.

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ii ASTM G171 - 03(2009), "Standard Test Method for Scratch Hardness of Materials Using a Diamond Stylus."

APPENDIX: MEASUREMENT PRINCIPLE Scratch hardness of materials using a diamond stylus is based on the standard ASTM G171-03iii. The test sample is prepared to represent the application of interest or polished to facilitate scratch width measurement. A constant normal force is applied by the stylus to the test sample and the scratch track is formed by the relative sliding movement of the stylus against the sample surface (see Fig. 5).

Fig. 5: Schematic of scratch hardness measurement.

The average width of the scratch track is determined, and the scratch hardness number, HSP is determined using the following formula:

𝐻𝐻𝐻𝐻𝑃𝑃 =8𝑃𝑃𝜋𝜋𝑤𝑤2

where HSP is the scratch hardness number, P is the normal force, and w is the scratch width. Other possible measurements by Nanovea Tribometer: Rotative (pin-on-disc) wear, linear (reciprocating wear), wear in different environment, including high temperature, corrosive and lubrication, etc.