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Category: High Temperature Tribology

 

High Temperature Scratch Hardness using a Tribometer

HIGH TEMPERATURE SCRATCH HARDNESS

USING A TRIBOMETER

Prepared by

DUANJIE, PhD

INTRODUCTION

Hardness measures the resistance of materials to permanent or plastic deformation. Originally developed by a 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 object1. 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 G171-032. It measures the average width of the scratch created by a diamond stylus and calculates the scratch hardness number (HSP).

IMPORTANCE OF SCRATCH 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 T50 Tribometer measures scratch hardness of a Teflon sample at different temperatures from room temperature to 300ºC. The capability of performing high temperature scratch hardness measurement makes the NANOVEA Tribometer a versatile system for tribological and mechanical evaluations of materials for high temperature applications.

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T50

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TEST CONDITIONS

The NANOVEA T50 Free Weight Standard Tribometer was used to perform the scratch hardness tests on a Teflon sample at temperatures ranging from room temperature (RT) to 300°C. Teflon has a melting point of 326.8°C. 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°C, 100°C, 150°C, 200°C, 250°C and 300°C.

TEST PARAMETERS

of the high temperature scratch hardness measurement

NORMAL FORCE 2 N
SLIDING SPEED 1 mm/s
SLIDING DISTANCE 8mm per temp
ATMOSPHERE Air
TEMPERATURE RT, 50°C, 100°C, 150°C, 200°C, 250°C, 300°C.

RESULTS & DISCUSSION

The scratch track profiles of the Teflon sample at different temperatures are shown in FIGURE 1 in order to compare the scratch hardness at different elevated temperatures. The material pile-up on the scratch track edges forms 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 FIGURE 2. The measured scratch track widths and calculated scratch hardness numbers (HSP) are summarized and compared in FIGURE 3. The scratch track width measured by the microscope is in agreement with that measured using the NANOVEA Profiler – the Teflon sample exhibits a wider scratch width 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 NANOVEA T50 Tribometer. It provides an alternative solution from other hardness measurements and makes NANOVEA Tribometers a more complete system for comprehensive high-temperature tribo-mechanical evaluations.

FIGURE 1: Scratch track profiles after the scratch hardness tests at different temperatures.

FIGURE 2: Scratch tracks under the microscope after the measurements at different temperatures.

FIGURE 3: Evolution of the scratch track width and scratch hardness vs. the temperature.

CONCLUSION

In this study, we showcase how 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 measurements at elevated temperatures makes the NANOVEA Tribometer an ideal tool for evaluating the high temperature tribo-mechanical properties of materials.

The 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 tracks in addition to other surface measurements such as roughness.

1 Wredenberg, Fredrik; PL Larsson (2009). “Scratch testing of metals and polymers: Experiments and numerics”. Wear 266 (1–2): 76
2 ASTM G171-03 (2009), “Standard Test Method for Scratch Hardness of Materials Using a Diamond Stylus”

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In Situ Wear Measurement at High Temperature

IN SITU WEAR MEASUREMENT AT HIGH TEMPERATURE

USING TRIBOMETER

IN-SITU WEAR MEASUREMENT Aerospace Tribometer

Prepared by

Duanjie Li, PhD

INTRODUCTION

The Linear Variable Differential Transformer (LVDT) is a type of robust electrical transformer used to measure linear displacement. It has been widely used in a variety of industrial applications, including power turbines, hydraulics, automation, aircraft, satellites, nuclear reactors, and many others.

In this study, we feature the add-ons of LVDT and high temperature modules of the NANOVEA Tribometer which allow the change of wear track depth of the tested sample to be measured during the wear process at elevated temperatures. This enables users to correlate different stages of wear process with the evolution of COF, which is critical in improving fundamental understanding of the wear mechanism and tribological characteristics of the materials for high temperature applications.

MEASUREMENT OBJECTIVE

In this study. we would like to showcase the capacity of NANOVEA T50 Tribometer for in situ monitoring the evolution of the wear process of materials at elevated temperatures.

The wear process of the alumina silicate ceramic at different temperatures is simulated in a controlled and monitored manner.

NANOVEA

T50

TEST PROCEDURE

The tribological behavior, e. g. coefficient of friction, COF, and wear resistance of alumina silicate ceramic plates was evaluated by the NANOVEA Tribometer. The alumina silicate ceramic plate was heated up by a furnace from room temperature, RT, to elevated temperatures (400°C and 800°C), followed by the wear tests at such temperatures. 

For comparison, the wear tests were carried out when the sample cooled down from 800°C to 400°C and then to room temperature. An AI2O3 ball tip (6mm dia., Grade 100) was applied against the tested samples. The COF, wear depth and temperature were monitored in situ.

TEST PARAMETERS

of the pin-on-disk measurement

Tribometer LVDT Sample

The wear rate, K, was evaluated using the formula K=V/(Fxs)=A/(Fxn), where V is the worn volume, F is the normal load, s is the sliding distance, A is the cross-sectional area of the wear track, and n is the number of revolution. Surface roughness and wear track profiles were evaluated by the NANOVEA Optical Profiler, and the wear track morphology was examined using an optical microscope.

RESULTS & DISCUSSION

The COF and wear track depth recorded in situ are shown in FIGURE 1 and FIGURE 2, respectively. In FIGURE 1, “-I” indicates the test performed when the temperature was increased from RT to an elevated temperature. “-D” represents the temperature decreased from a higher temperature of 800°C.

As shown in FIGURE 1, the samples tested at different temperatures exhibit a comparable COF of ~0.6 throughout the measurements. Such a high COF leads to an accelerated wear process which creates a substantial amount of debris. The wear track depth was monitored during the wear tests by LVDT as shown in FIGURE 2. The tests performed at room temperature before sample heating up and after sample cooling down show that the alumina silicate ceramic plate exhibits a progressive wear process at RT, the wear track depth gradually increases throughout the wear test to ~170 and ~150 μm, respectively. 

In comparison, the wear tests at elevated temperatures (400°C and 800°C) exhibit a different wear behavior – the wear track depth increases promptly at the beginning of the wear process, and it slows down as the test continues. The wear track depths for tests performed at temperatures 400°C-I, 800°C and 400°C-D are ~140, ~350 and ~210 μm, respectively.

COF during pin-on-desk Tests at different temperatures

FIGURE 1. Coefficient of Friction during pin-on-disk tests at different temperatures

Wear track depth of the alumina silicate ceramic plate at different temperatures

FIGURE 2. Evolution of wear track depth of the alumina silicate ceramic plate at different temperatures

The average wear rate and wear track depth of the alumina silicate ceramic plates at different temperatures were measured using NANOVEA Optical Profiler as summarized in FIGURE 3. The wear track depth is in agreement with that recorded using LVDT. The alumina silicate ceramic plate shows a substantially increased wear rate of ~0.5 mm3/Nm at 800°C, compared to the wear rates below 0.2mm3/N at temperatures below 400°C. The alumina silicate ceramic plate does not exhibit significantly enhanced mechanical/tribological properties after the short heating process, possessing a comparable wear rate before and after the heat treatment.

Alumina silicate ceramic, also knows as lava and wonderstone, is soft and machinable before heating treatment. A long process of firing at elevated temperatures up to 1093°C can substantially enhance its hardness and strength, after which diamond machining is required. Such a unique characteristic makes alumina silicate ceramic an ideal material for sculpture.

In this study, we show that heat treatment at a lower temperature that the one required for firing (800°C vs 1093°C) in a short time does not improve the mechanical and tribological characteristics of alumina silicate ceramic, making proper firing an essential process for this material before its usage in the real applications.

 
Wear rate and wear track depth of the sample at different temperatures 1

FIGURE 3. Wear rate and wear track depth of the sample at different temperatures

CONCLUSION

Based on the comprehensive tribological analysis in this study, we show that the alumina silicate ceramic plate exhibits comparable coefficient of friction at different temperatures from room temperature to 800°C. However, it shows a substantially increased wear rate of ~0.5 mm3/Nm at 800°C, demonstrating the importance of proper heat treatment of this ceramic.

NANOVEA Tribometers are capable of evaluating the tribological properties of materials for applications at high temperatures up to 1000°C. The function of in situ COF and wear track depth measurements allows users to correlate different stages of wear process with the evolution of COF, which is critical in improving fundamental understanding of the wear mechanism and tribological characteristics of the materials used at elevated temperatures.

NANOVEA Tribometers offer 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. NANOVEA’s unmatched range is an ideal solution for determining the full range of tribological properties of thin or thick, soft or hard coatings, films and substrates.

Optional 3D Non-Contact Profilers are available for high resolution 3D imaging of wear tracks in addition to other surface measurements such as roughness.

IN-SITU WEAR MEASUREMENT

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Rotative or Linear Wear & COF? (A Comprehensive Study Using the Nanovea Tribometer)

Wear is the process of removal and deformation of material on a surface as a result of the mechanical action of the opposite surface. It is influenced by a variety of factors, including unidirectional sliding, rolling, speed, temperature, and many others. The study of wear, tribology, spans many disciplines, from physics and chemistry to mechanical engineering and material science. The complex nature of wear requires isolated studies toward specific wear mechanisms or processes, such as adhesive wear, abrasive wear, surface fatigue, fretting wear, and erosive wear. However, “Industrial Wear” commonly involves multiple wear mechanisms occurring in synergy.

Linear reciprocating and Rotative (Pin on Disk) wear tests are two widely used ASTM-compliant setups for measuring sliding wear behaviors of materials. Since the wear rate value of any wear test method is often used to predict the relative ranking of material combinations, it is extremely important to confirm the repeatability of the wear rate measured using different test setups. This enables users to carefully consider the wear rate value reported in the literature, which is critical in understanding the tribological characteristics of materials.

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Evaluating Brake Pads with Tribology


Importance of Evaluating Break Pad Performance

Brake pads are composites., a material made up of multiple ingredients, that must be able to satisfy a large number of safety requirements. Ideal brake pads have high coefficient of friction (COF), low wear rate, minimal noise, and remain reliable under varying environments. To ensure the quality of brake pads are able to satisfy their requirements, tribology testing can be used to identify critical specifications.


The importance of the reliability of brake pads is placed very high; the safety of passengers should never be neglected. Therefore, it is key to replicate operating conditions and identify possible points of failure.
With the Nanovea Tribometer, a constant load is applied between a pin, ball, or flat and a constantly moving counter material. The friction between the two material is collected with a stiff load cell, allowing the collection of material properties at different loads and speeds and tested in high temperature, corrosive, or liquid environments.



Measurement Objective

In this study, the coefficient of friction of the brake pads were studied under a continuously increasing temperature environment from room temperature to 700°C. The environmental temperature was raised in-situ until noticeable failure of the brake pad was observed. A thermocouple was attached to the backside of the pin to measure the temperature near the sliding interface.



Test Procedure and Procedures




Results and Discussion

This study focuses mainly on the temperature at which brake pads start to fail. The COF obtained do not represent real-life values; the pin material is not the same as brake rotors. It should also be noted that the temperature data collected is the temperature of the pin and not the sliding interface temperature

 








At the start of the test (room temperature), the COF between the SS440C pin and brake pad gave a consistent value of approximately 0.2. As the temperature increased, the COF steadily increased and peaked at a value of 0.26 near 350°C. Past 390°C, the COF quickly starts decreasing. The COF began to increase back to 0.2 at 450°C but starts decreasing to a value of 0.05 shortly after.


The temperature at which the brake pads consistently failed is identified at temperatures above 500°C. Past this temperature, the COF was no longer able to retain the starting COF of 0.2.



Conclusion




The brake pads have shown consistent failure at a temperature past 500°C. Its COF of 0.2 slowly rises to a value of 0.26 before dropping down to 0.05 at the end of the test (580°C). The difference between 0.05 and 0.2 is a factor of 4. This means that the normal force at 580°C must be four times greater than at room temperature to achieve the same stopping force!


While not included in this study, the Nanovea Tribometer is also able to conduct testing to observe another important property of brake pads: wear rate. By utilizing our 3D non-contact profilometers, the volume of the wear track can be obtained to calculate how quickly samples wear. Wear testing can be conducted with the Nanovea Tribometer under different test conditions and environments to best simulate operating conditions.

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High Temperature Tribology

High Temperature Scratch Hardness Using Tribometer

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 high temperature scratch hardness is in need to ensure proper selection of the materials for high temperature applications.

High Temperature Scratch Hardness Using Tribometer

 

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