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Category: High Temperature Tribology

 

In-Situ Wear Measurement at High Temperature

In-Situ Wear Measurement at High Temperature

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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-desk 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 mechanical action of the opposite surface1. 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 towards specific wear mechanisms or processes, such as adhesive wear, abrasive wear, surface fatigue, fretting wear and erosive wear2. 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 setups34for 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 conrm 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.

Tribology on Piston Operations

Friction loss accounts for approximately 10% of total energy in fuel for a diesel engine [1]. 40-55% of the
friction loss comes from the power cylinder system. The loss of energy from friction can be diminished
with better understanding of the tribological interactions occurring in the power cylinder system.

A significant portion of the friction loss in the power cylinder system stems from the contact between
the piston skirt and the cylinder liner. The interaction between the piston skirt, lubricant, and cylinder
interfaces is quite complex due to the constant changes in force, temperature, and speed in a real life
engine. Optimizing each factor is key to obtaining optimal engine performance. This study will focus on
replicating the mechanisms causing friction forces and wear at the piston skirt-lubricant-cylinder liner
(P-L-C) interfaces.

Click to Learn about Piston Tribology!

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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