Application Note | Static Friction
Static Coefficient of Friction Measurement Using Tribometer
Capturing the Friction Peak Before Motion with an Ultra-Low-Speed Ramp
Research & Experimental Testing
Duanjie Li
Visual Design & Editorial
Andrew Shore
Introduction
The measurement of friction force and the calculation of the coefficient of friction, COF, are critical for many industrial applications, e.g. brakes, valves, clutches and similar power- and motion-control mechanisms. High friction during machine operation not only generates frictional heating that might cause failure of a machine part, but also consumes a large amount of energy. It is estimated that one-third to one-half of the world’s energy production is wasted through friction1.
Importance of Static Friction Measurement
The static COF is defined as the COF corresponding to the maximum force that must be overcome to initiate macroscopic motion between two bodies2. To simulate different application conditions, a variety of ASTM compliant experimental setups have been developed to evaluate the static COF, such as ASTM D1894, D2047 and D3247. However, a reliable measurement of static COF remains challenging due to the influence of numerous factors, such as reactions between surfaces, interlocking surface features, or other more subtle phenomena such as van der Waals forces, cohesion of surface films, and even microscopic solid static junctions between surfaces.
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Measurement Objective
In this study, we would like to showcase the capacity of the NANOVEA Tribometer for measuring the static COF. The COF of an aluminum block against glass and Teflon samples was measured and analyzed as examples.
NANOVEA T2000 High Load
Pneumatic Tribometer
Test Procedure
The coefficient of friction, COF, of an aluminum block on glass and Teflon surfaces was evaluated by the NANOVEA Tribometer. The speed increased at an exponential rate in a step-less fashion from 0.001 to 0.6 rpm (see Fig. 2), in order to maintain extremely low speeds for static COF measurement at the beginning of the test while finishing the total experiment in a reasonably short time. With the 10 mm radius, these two rotational speeds correspond to sliding speeds of 1.05 × 10−6 m/s and 6.3 × 10−4 m/s, respectively. The COF was recorded in situ with a time interval of 0.1 s. The test parameters are summarized in Table 1.
| Parameter | Glass | Teflon |
|---|---|---|
| Normal force | 50 N | 5 N |
| Speed | 0.001 to 0.6 rpm, continuous, exponential | 0.001 to 0.6 rpm, continuous, exponential |
| Duration of test | 5 min | 5 min |
Table 1: Test parameters of the COF measurement.
Results and Discussion
The evolution of the COF of the Al block against the glass and Teflon samples is shown in Fig. 3, and the static and average dynamic COFs are summarized and compared in Table 2. The precise control of the motor at extremely low speed starting at 0.001 rpm allows us to observe a progressive friction force buildup at the beginning of the tests for both the glass and Teflon samples. When this friction reaches a certain threshold, relative motion at the interface of the tested sample and the Al block takes place, and the subsequent measured COF decreases. This initial spike of the force at the beginning of the COF test arises from static friction, defined as the COF corresponding to the maximum force that must be overcome to initiate macroscopic motion between two bodies. It may result from several sources, such as reactions between surfaces, interlocking surface features, or other more subtle phenomena such as van der Waals forces, cohesion of surface films, and even microscopic solid static junctions between surfaces.
Due to the complication of the above factors, variations of the measured static COF are observed in this study. Several precautions should be taken to ensure a repeatable measurement2–4:
- Surface finish is critical: the surface roughness has a significant impact on the COF measurement, including directional machine marks, inhomogeneous asperities, etc.
- Properly clean and dry the specimens immediately prior to testing.
- Carefully adhere to the specified procedure.
- Ensure that the tested sample surface is horizontal.
- Sliding velocity and normal force may affect the frictional characteristics of many material couples; verifying the system’s sensitivity to these factors is recommended.
Following the static COF measurement, the evolution of the dynamic COF is measured after macroscopic relative motion between the Al block and the tested sample surface takes place.
| Sample | Static COF | Dynamic COF |
|---|---|---|
| Glass | 0.145 | 0.097 |
| Teflon | 0.182 | 0.064 |
Table 2: Static and dynamic COFs of the Al block against glass and Teflon.
Conclusion
In this study, we show that the NANOVEA Tribometer measures the static coefficient of friction of different material couples in a controlled and monitored manner, thanks to its high-precision speed control motor capable of moving the rotational stage at extremely low speeds down to 0.001 rpm. This static friction measurement allows users to improve their fundamental understanding of the static friction mechanism of materials and select the best material couple for the intended tribological applications.
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.
References
1. Szeri, A. Z., Tribology: Friction, Lubrication and Wear, Hemisphere Publishing.
2. Mang, T.; Bobzin, K.; Bartels, T., Industrial Tribology: Tribosystems, Friction, Wear and Surface Engineering, Lubrication, Wiley-VCH, Weinheim, 2011.
3. Malhotra, R. C.; Sharma, J. P., Proceedings of the First World Conference in Industrial Tribology, New Delhi, 1972 (1973).
4. ASTM G133, Standard Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear.
Frequently Asked Questions About Static Friction Measurement
What is the static coefficient of friction?
The COF corresponding to the maximum force that must be overcome to initiate macroscopic motion between two bodies.
Why is static friction hard to measure?
It is influenced by reactions between surfaces, interlocking features, van der Waals forces, cohesion of surface films and microscopic solid junctions, so results vary unless conditions are tightly controlled.
How was the static COF captured?
The stage speed rose exponentially and continuously from 0.001 to 0.6 rpm on a 10 mm radius (1.05 × 10⁻⁶ to 6.3 × 10⁻⁴ m/s), keeping speeds extremely low at the start, with COF recorded every 0.1 s.
What were the results?
Aluminum on glass (50 N): static COF 0.145, dynamic 0.097. Aluminum on Teflon (5 N): static 0.182, dynamic 0.064.
What precautions improve repeatability?
Control surface finish, clean and dry specimens just before testing, follow the procedure carefully, keep the sample surface horizontal, and check sensitivity to sliding velocity and normal force.
Which standards cover static friction?
Setups such as ASTM D1894, D2047 and D3247 address static COF for different application conditions.
