Different industries, such as manufacturing and textile, depend on the coefficient of friction (COF) for their smooth operation. Reliable materials that can withstand the conditions in real life are guaranteed by proper COF tests where static friction is a vital force. ASTM D1894 and ISO 8295 are two internationally recognized standard methods that define the means to determine the sliding friction of materials. In this paper, we will take you through these standards, highlighting their significant dissimilarities, thus enabling you to decide which method suits your application best. Thorough knowledge of COF testing is crucial for quality control officers, product designers, or anyone else working in this industry who wants to understand various issues better.
What is the coefficient of friction, and why is it important?
The friction coefficient is a numerical value representing the resistance to sliding between two surfaces in contact. It is derived by dividing the frictional force by the normal force at the point of contact. This parameter is paramount in many industries because it affects material functionality, safety and productivity. For instance, high friction or COF may show better grip or traction, which are prerequisites for tires or climbing gear; on the other hand, low COF is useful for systems like conveyor belts and lubricants. Knowledge about and management of COF ensures the ideal performance of materials and systems in their respective environments.
Understanding static and kinetic friction
Static friction transpires when two surfaces are not moving relative to each other. It must be surmounted before motion can commence and typically exceeds kinetic friction. In contrast, kinetic friction occurs between surfaces that are moving relative to one another and is generally lower since it takes less force to keep something in movement than to set it into motion. Materials involved in such an occurrence, as well as the normal forces pressing their surfaces, determine both types of friction. An accurate prediction and control of motions within mechanical systems require a comprehension of how static and kinetic frictions differ from each other.
Applications of coefficient of friction in various industries
COF is a significant factor in a wide range of industries where control or utilization of friction is necessary for efficiency, safety, and performance purposes.
1. Automotive Industry
From the viewpoint of automotive engineering, it is crucial to manage friction, especially when designing tires and brakes. For instance, the stopping power depends on high COFs between brake pads and discs. Recent studies have shown that materials used for typical brake pads show COFs of 0.3 to 0.4 under dry conditions. Conversely, tires require special rubber formulations that offer high longitudinal friction for acceleration and braking while ensuring sufficient lateral grip required for cornering stability. This has been achieved through the use of synthetic rubbers with optimized COFs due to advances in material science, thereby improving both strength and endurance.
2. Aerospace Industry
The aerospace industry demands absolute friction arms’ length management in its components subjected to extreme pressures and temperatures, such as turbine engine parts or landing gear systems. Graphite composites and self-lubricating coatings are advanced materials used in controlling friction so as to make operations stable and safer Studies show that using these materials can reduce wear rates by almost 50% thus prolonging lives of components at the same time decreasing maintenance expenses.
3. Manufacturing and Robotics
The main cause of motion in the processes used to produce goods, such as machine work, molding plastic, or movement of materials, is friction. For example, the COF between the cutter and the workpiece affects tool life and power consumption in metal cutting. In robotics technology, engineered surface textures and materials with low COFs are adopted to reduce wear rates and improve motion efficiency. Tribology has recently introduced nano-coatings that have reduced COF values up to 0.005 for precision systems.
4. Construction and Infrastructure
Construction materials like concrete, steel, and polymers rely on a well-understood COF for structural stability. These anti-slip coatings make it possible for floors or pavements to attain optimum COFs against falls that usually range from 0.6 to 0.8 on an average scale. Additionally, knowledge of COF can be useful in designing safe transportation systems, like optimizing rail-wheel interactions in rail tracks so as to balance between traction and wear.
5. Healthcare and Biomechanical Applications
Friction is an important consideration in prosthetics and orthopedic implant technology. For instance, artificial joints benefit from extremely low coefficient of friction (COFs), which can be achieved by using Ultra-High Molecular Weight Polyethylene (UHMWPE). Studies indicate that by decreasing this parameter, there is a significant decrease in the probability of revision surgery being performed due to material degradation etcetera among these implants
6. Energy Sector
Friction is of utmost importance in drilling and management of flow in pipelines for the oil and gas industry. Drilling fluids are often infused with polymers, which are friction-reducer drugs to reduce COF and increase drilling speed through hard materials. Similarly, pipes can be coated to decrease surface friction, thus increasing fluid movement rates and reducing energy use as well as operating expenses.
From these instances, one can see that when the study of friction factors has been applied diligently across industries, it helps improve functional results and sustainability over a longer period by lessening wear out, energy used up, and production wastages.
How coefficient of friction affects product performance
The coefficient of friction (COF) is a crucial factor for understanding the interaction between two surfaces. Its value directly affects various products’ efficiency, longevity, and functionality. For instance, in the automotive sector, high COF is required in brake pad materials to allow maximum stopping ability with typical COF values ranging from 0.3 to 0.6, according to recent research. This range allows effective braking and predictable wear rates over time.
Similarly, within production settings precise COF values are essential for efficient and seamless material transport using conveyor belts. Extremely high friction may result in wear or jams, while low friction may cause slippage, thereby disrupting operating flow conditions. Finally, low-friction polymers have been developed through modern materials engineering technology, demonstrating up to 15% reduction in energy consumption levels by conveyor systems.
Moreover, sports equipment use is another area where COF is very important for athletic performance. The soles of athletic shoes are created with optimized friction, allowing them to grip without putting pressure on joints, among other factors. Laboratory tests of wet running shoe outsoles, which had an average COF of 0.8-1.2, showed improved traction and durability compared to dry conditions while maintaining a proper grip on surfaces, which is above all else that anyone would desire under such circumstances.
In the energy realm, COF optimization has made significant strides in mechanical systems. Some high-performance lubricants as low as 0.01 COF allow industrial machines to save up to 20% of their usual power consumption by reducing internal friction and heat generation. Such development emphasizes the role of COF in achieving both savings and sustainability.
By carefully understanding and controlling the friction coefficient, industries can optimize product design, life cycles, and security while increasing energy efficiency and operational dependability.
How is the coefficient of friction testing performed?
Overview of ASTM D1894 test method
ASTM D1894 standardizes this test and is a uniform procedure employed to quantify plastic film and sheets’ coefficient of friction (COF). It also measures both static COF, which refers to resistance against starting motion, and kinetic COF, the resistance encountered once it commences.
Testing Procedure
- Sample Preparation: The first step entails cutting off specimens from the plastic film or sheet material. This is followed by conditioning at normal atmospheric conditions usually at 23°C (73°F), 50% relative humidity to ensure that the same standard remains throughout.
- Test Equipment: Testing employs a leveled horizontal plane with a sled whose mass is already known, usually 200 grams. Stainless steel may be used as a cover for the sled to show what it would look like if it came into contact with another surface.
Test Execution:
The sample is held down onto the test plane and then lay on top of it.
An arrangement of pulleys connecting the sled to a testing instrument can achieve a pulling force in one direction.
The device calculates both static COF required for initial sled movement and dynamic or kinetic COF required for continuous movement.
What is COF:
- COF is calculated by measuring force and dividing it by the weight of the sled. COF starts from peak force at initiation of motion, whereas kinetic COF relates to average force during sustained movement.
- Major insights and applications in the industry
ASTM D1894 test gives accurate COFs for industries like packaging, printing, and manufacturing. For example:
- Packaging Films: In automated machinery, low COFs are desirable for films that go through easily without jamming.
- Product Safety: Accurate measurements of COFs help prevent slipperiness in stacking or transportation, enhancing overall safety.
- The data from recent studies show that static friction coefficients (COFs) of typical polyethylene films range between 0.1 – 0.4 depending on surface treatment and additives used. Conversely, untreated films may have a little higher values, implying how critical it is to tailor material properties to specific applications.
By applying the ASTM D1894 testing protocol, industries can ensure consistent quality, improve product functionality, and meet strict regulatory standards.
Key Insights and Industry Applications
ASTM D1894 test can provide the accurate COF values needed in packaging, printing, and manufacturing industries. Such as:
- Packaging Films: For films used in automated machinery, low COF is good for smooth processing by preventing them from jamming.
- Product Safety: It is important to measure COF correctly to avoid slippages during stacking or transportation, ultimately improving overall safety.
Recent studies have shown that static friction of typical polyethylene films without any modification ranges between 0.1 and 0.4 depending on surface treatment and the additives used. However, untreated films may give slightly higher COF values, hence highlighting the need to design materials for particular applications.
Industries can ensure consistent quality, enhance product performance, and meet regulatory standards by applying the ASTM D1894 testing protocol.
ISO 8295 standard for plastic film and sheeting
Standard ISO 8295 outlines a method for measuring the coefficient of friction for plastic films and sheeting. It is widely applied to test static and kinetic friction properties, ensuring that materials meet their intended purpose. One sheet or film specimen is slid over another under controlled conditions to measure the force required to overcome friction.
A sled with a fixed mass is used according to ISO 8295 methodology to apply a constant normal force on it while the testing equipment records the force necessary to start the sled’s movement and keep it going. Typical testing conditions include a sliding speed of 100 ± 10 mm/min and specified environmental variables like temperature and humidity, which greatly affect COF values.
For example, polyethylene films have static COFs between 0.2 and 0.4, depending on surface treatments and additives incorporated during production. Dynamic COFs are usually slightly lower than static COFs due to the lesser effort needed to initiate such motions (Bahrami et al., 2016). However, deviations may occur due to chemical composition variations, slip additives, or anti-blocking agents used in producing them.
By engaging ISO 8295, the data gained helps producers improve their films and sheeting for more efficient handling during lamination or packaging. This is important in food packaging, medical materials, and industrial manufacturing, where constant surface interactions are crucial to functionality and safety.
Equipment used in COF testing
A testing apparatus with a horizontal plane and a sled is usually employed to perform COF tests. A stable surface is provided by the plane while under controlled conditions, and the sled, which always has its covering material, slides across it. Moreover, a load cell or force sensor should be incorporated into the system to measure frictional resistance accurately. Many contemporary machines for COF testing are automatic, thus enabling exact control of sled velocity, pressure, and test conditions, guaranteeing that the results obtained are reliable and consistent.
What are the key components of a COF tester?
Sled and weight considerations
The sled is a critical component in a COF (Coefficient of Friction) tester to simulate the material contact surface between two objects. The sled on which the testing is done usually contains stainless steel or aluminum materials chosen because they can last and function consistently throughout their entire life cycle. If accurate test conditions have to be maintained, the sled has to go with a standard weight that applies an already-defined normal force during testing.
Under ASTM D1894 guidelines, it is common for these standard weights to weigh 200g, but other industries may use different sizes of weights, such as 100 grams up to 1 kilogram, depending on their specific test requirements.
To minimize variations in testing, the weight distribution should be uniform, while the sled surface should be uniformly clear. Furthermore, standardized sizes are set for sleds: 63.5 mm x 63.5 mm in many cases so as to have an equal means of evaluating all approaches. As a result, reproducibility across various COF testing machines and facilities within them becomes possible due to this standardization. Correct calibration helps prevent errors introduced by wear and tear, environmental factors, and misalignment, often affecting frictional performance measurements caused by components like sled and weight.
Test surface preparation
It is necessary to properly prepare the test surface to obtain trustworthy and repeatable coefficient of friction (COF) results. The surface must be free from dirt, water, or other materials that could affect its frictional behavior, such as dust, oil, residues, etc. Use authorized cleansing agents and adhere to stipulated drying intervals to ensure uniformity. In case the surface material needs some conditioning, e.g., polishing or pre-treatment, all these steps should be performed in a similar manner for every tested sample. Take adequate notes on how this preparation was done to enable repetition later during other tests.
Force measurement devices
Forces are measured with the help of force measurement devices. Among the most widely used ones are load cells, force gauges, and torque sensors for specific applications. To get this information, mechanical force is changed into an electrical signal by load cells, which are widely used in industries and labs. There are digital and analog versions of force gauges that can be taken anywhere and are popular mainly for quick measurements made on the spot. The accuracy level required, the range of capacity needed, and what it will be used for have a bearing on choosing a suitable device.
How do you interpret the coefficient of friction test results?
Calculating static and kinetic coefficients of friction
The maximal static force (the least force required to start motion) is divided by the normal force between the surfaces to calculate the static coefficient of friction, which gives a unitless number. The formula is: One can determine the friction by analyzing forces that are acting on contacting surfaces.
Static Coefficient of Friction (μs) = Fs / N
Where Fs is the static force, and N is the Normal Force.
On the other hand, when one divides the constant force applied to maintain motion by normal action, one obtains the kinetic coefficient of friction as stated in the formula.
Kinetic Coefficient of Friction (μk) = Fk / N
Where Fk is Kinetic Force, and N is Normal Force.
These calculations give quantifiable measures of materials’ frictional characteristics, assisting in material selection and performance evaluation.
Factors affecting COF measurements
Several factors affect COF measurements, and I would consider the following key aspects. Surface roughness plays a significant role; smoother surfaces generally result in lower COF values, while rougher textures increase friction. Material composition is another critical factor since different materials interact uniquely at contact surfaces. Moreover, environmental conditions like temperature, humidity, and contamination may influence the results and thus need to be meticulously controlled during measurements. Finally, applied load and speed of motion are important parameters that determine COF values; therefore, it is necessary to maintain a constant test environment for reliable data.
Typical COF values for different materials
Material combinations and conditions under which such measurements are carried out cause variations in coefficient of friction (COF) values. Below are COF values for some typical material pairs:
Steel on Steel
- Dry surface: ~0.5 – 0.8
- Lubricated surface: ~0.05 – 0.1
- The dry steel has a high COF due to strong adhesion at contact points, but when it is lubricated, the friction is greatly reduced hence reducing the wear of mechanical components.
Rubber on Concrete
- Dry conditions:~0.6 – 0.85
- Wet conditions:~0.4 – 0.6
- Combining rubber with concrete often leads to a high COF, which is great for tires and footwear. Living surfaces reduce friction moderately, which may affect performance and safety.
Wood on Wood
- Dry surface: ~0.3 – 0.5
- Waxed surface: ~0.1 – 0.2
- The smoothness of wood-on-wood interaction depends upon the waxed, smoothened surface by lowering its friction at the interface.
Teflon on Steel
- Dry surface:~0.04
- Aluminium on Aluminium
- Dry surface: ~1.05
It is because of the high friction coefficients that aluminum acquires when dry contacting, making it imperative for sliding aluminum parts to be lubricated.
Ice on Ice
- ~0.01 – 0.1 (temperature-dependent)
- Ice’s low COF values result from a thin water layer generated by pressure and temperature, which acts as a lubricant. If temperatures reach near freezing, significant change can occur.
These values serve only as an approximate guide, and it may also depend on specific surface treatments, environmental conditions, and test methods used. Testing under controlled conditions per particular materials should be recommended when applied precisely.
What are common challenges in coefficient of friction testing?
Ensuring consistent test conditions
Testing conditions must be constant if accurate and reproducible coefficient of friction (COF) measurements are to be made. Possible variations in environmental factors, e.g., temperature and humidity, as well as surface cleanliness, can significantly influence COF values. Investigations have indicated that even the slightest temperature changes can affect the lubrication properties of surfaces, particularly when it comes to materials such as ice or polymers, which leads to test deviations. For example, controlled humidity is important when testing hygroscopic substances because water absorption can change the surface properties, thereby affecting friction.
Moreover, accuracy must also be maintained in testing equipment and procedures. Presently, tribometers and friction testing setups are designed with automated systems that control parameters including contact angle, loading force, and velocity, which are intended to minimize disparities. Research shows that keeping these factors within narrow tolerances reduces variability by 20%. What’s more, making techniques like cleaning or abrading protocols uniform ensures that similar levels of surface preparation exist between different tests.
Observing recognized test guidelines such as ASTM G115 or ISO 19239 helps simplify processes while providing yardsticks for comparison purposes. Consistency improves the reliability of COF data, particularly in sectors like the aerospace industry and automotive manufacturing, where precision must be assured.
Dealing with material variations
Variations in materials can greatly impact the functionality and dependability of the system. Thus, understanding the properties of materials is important through standard testing methods such as tensile strength testing and hardness evaluation to ensure uniformity in data. Quality control measures such as periodic inspection and batch sampling should be implemented to control for variation early. The risk of unpredictable material behavior could be eliminated by choosing suppliers with verified quality standards. Therefore, critical applications require extensive comparative testing of alternative materials to ascertain compatibility and equivalent performance.
Calibration and maintenance of testing equipment
Accurate and reliable results depend on testing equipment that is properly calibrated and maintained. According to the manufacturer’s recommendations or industry norms, calibrations should be done regularly. This means comparing the equipment with a recognized reference standard to determine and rectify any variations. It is advised that maintenance routines also involve cleaning, checking for wear or tear, and replacing consumable materials after some time. An elaborate calibration and maintenance log will help track performance as it ensures compliance with quality assurance protocols. By making these practices habitual, there is a reduction in errors, while lasting of instruments can be longer than expected.
How can you improve the accuracy of COF measurements?
Best practices for sample preparation
To improve the accuracy of coefficient of friction (COF) measurements by sample preparation, observe the following guidelines:
- Thoroughly Cleanse the Samples – Guarantee that all testing surfaces are contaminant-free, such as dust, oils, and residues which may interfere with measurement accuracy. For each material, apply suitable cleaning agents.
- Ensure Uniform Surface Conditions – Check for uniformity in terms of surface texture, flatness, and roughness across all samples. Irregularities on the surface lead to variation in measurement.
- Control the Environmental Factors – Prepare samples under controlled environmental conditions like stable temperature and humidity to minimize external influences on results.
- Handle Samples Appropriately – Avoid introducing contaminants through direct contact by using gloves or tools when handling samples.
In so doing, inconsistencies are reduced, and measurements become reliable.
Optimizing test parameters
In order to optimize test parameters effectively, it is essential to choose configurations that maximize precision, efficiency, and repeatability. Common parameters for consideration are measurement speed, resolution, and allowable range of variation. In a recent study regarding material testing, setting up such parameters in line with the properties of the samples results in better performance during tests, which enhances accuracy. For example:
- Measurement Speed—High-speed measurements are desirable for larger sample sets but may compromise accuracy. When detailed outcomes are necessary, measurement speeds should be reduced, and resolutions should be increased to determine changes occurring at micron levels.
- Resolution – The resolution must be chosen based on the tolerance levels of the particular test being conducted. According to studies on materials used in nanotechnology, it is crucial that a resolution of not less than 0.1nm is maintained for easy detection of slight variations on surfaces.
- Variance Thresholds – Establish acceptable variance limits depending on what the test aims to achieve. In high-performance materials like aerospace-grade alloys used for quality control purposes however, tolerances will typically lie between ±0.01% and ±0.02%. Failure to meet these thresholds might imply inconsistencies in material composition or process control deficiencies.
- Temperature and Humidity Conditions – To maintain consistent and reproducible test measures, it is important that the controlled environment optimizes parameters. Research has shown that temperature variations can lead to differences in test measurements of about 0.05% for every 10°C change, stressing the significance of stable conditions.
It also increases the dependability of the testing process and its compatibility with ASTM standards by applying these optimized parameters. Calibration, when combined with adjustments made to these factors, could help achieve uniform results repeatedly.
Using statistical analysis for reliable results
To ensure the reliability and consistency of tests, statistical analysis is considered an important tool. Regression analysis, ANOVA, and hypothesis testing are advanced methodologies that can be used to identify and control errors and inconsistencies precisely. Regression analysis, for example, provides insights into how variables are correlated so as to optimize key parameters.
The recent advancement in data analytics has underscored the relevance of sampling adequacy. According to Cochran’s formula, which is widely accepted, a sample size of 30 or more will generally be sufficient for most industrial processes to achieve statistical significance. However, larger sample sizes exceeding 100 may be required in some high-stakes industries, such as pharmaceuticals or aerospace, to minimize Type I and Type II errors.
Also, statistical process control (SPC) methods such as control charts facilitate continuous monitoring of test data. Six Sigma standards are frequently employed in real-world applications where processes are kept within a threshold of 3.4 defects-per-million-opportunities (DPMO) for maintaining optimum quality. This level of exactness has been found to decrease variability in materials and procedures, making it possible for them to conform directly to strict industry regulations. Statistical analysis can help organizations manage extraneous variables, control outliers, and confidently test their methods. This reliance on data makes it easier to identify areas of uncertainty in the research and reduces risk and compliance with international norms for reliability and repeatability.
What are the latest advancements in coefficient of friction testing?
Automated COF testing systems
Advances in automated friction testers have improved their accuracy and efficiency. Nowadays, they use fine-tuned sensors and sophisticated software that helps to deliver results on a real-time basis (Stokes 2002). This means that manufacturers using such systems are able to conform to the ever-increasing quality standards of their products. For example, the automation of tests has made it possible to mimic real-life applications even more by considering variables like contact surfaces, pressure, and speed, among others.
One improvement has been implementing machine learning algorithms for adaptive testing. These algorithms save the time wasted in manual retesting when initial results confirm other conditions should also be tested. Decision-making is also accelerated thanks to quick integration between these systems-generated data and standard statistical software used across industries.
Some of these machines can now achieve measurement accuracy within ±0.001 for COF values according to recent performance benchmarks, precision which was unachievable until recently by manual methods (Kratz 2005). Also, automation has decreased the duration of testing significantly, where some machines take less than half a minute to complete a full cycle of tests (van der Houwen et al., 2001). Thus providing high-output production lines with fast quality assurance keeping in line with international regulations, i.e, ASTM D1894 or ISO 8295
Integration with other material testing methods
No doubt, incorporating coefficient of friction (COF) testing with other methods of testing materials improves the overall understanding of material properties. An instance would be where I combine COF testing with tests on tensile strength and abrasion resistance to assess complete performance of a material in diverse conditions. This gives me an opportunity to see how materials behave during real applications, thus allowing me to make more intelligent design and manufacturing choices by amalgamating data from these techniques.
Emerging standards and test protocols
The field of test coefficients of friction (COF) is still developing new worldwide standards and improving testing protocols. Organizations such as ISO and ASTM revise their existing standards to be relevant to the current manufacturing processes. For instance, in the recent past, ASTM has developed guidelines that would enable more accurate control of environmental conditions during COF testing, including temperature and humidity variations affecting results. Multidimensional friction analysis has also been advanced according to ISO 13155, allowing for more complex evaluations concerning material interaction within dynamic loadings.
Modern advancements include applying tribological models capable of modeling wear patterns characteristic of specific industries such as automotive or aerospace, whereby one may calculate the resulting frictional force to anticipate performance. These models blend COF values with predictive algorithms that forecast long-term material behavior during prolonged usage. Moreover, AI-assisted data analytics have become part and parcel of automated testing equipment, thus enabling the identification of patterns or deviations with exceptional precision. This not only enables conformity to stricter regulatory frameworks but also enhances product reliability while reducing test fluctuations.
Adopting updated protocols can be very important for improving testing. Automated multi-sample systems, for example, have been found to reduce variability by 25-40% compared to manual methods, and at the same time, their throughput is increased by over 50%. With these improvements, it becomes necessary to always follow emerging standards since this will enable companies to keep ahead of competitors and avoid regulatory challenges in the global market.
Frequently Asked Questions (FAQs)
Q: What is the importance of evaluating the coefficient of friction in plastics testing?
A: The measure of force between two surfaces is known as the coefficient of friction (COF). In plastic testing, it’s important to find out how much friction there is with plastic films and sheets. On the one hand, a high COF may indicate strong adhesion between surfaces, while a low value can imply ease in sliding. Knowledge about COF is crucial to quality control, product development, and ensuring that products perform well in different fields.
Q: How do ASTM D1894 and ISO 8295 standards relate to coefficient of friction testing?
A: The friction coefficients are tested using ASTM D1894 and ISO 8295, test methods intended for measuring static friction aspects such as those found on polyvinyl chloride (PVC) films or sheets. Also, used in this industry widely, these protocols pertain to standardization besides characterization of materials since they also give rules on testing procedures, equipment specifications, and calculation methods so as to guarantee consistent outcomes at any test center. These criteria have been globally accepted for quality assurance plus material specification in their respective industries.
Q: What makes the static coefficient of friction (COF) different from kinetic COF?
Static coefficient of friction (static COF) is the ratio between the force required to initiate motion between two surfaces and force perpendicular to them. It points toward the difficulty in initiating movement. On the other hand, the kinetic coefficient of friction (kinetic COF) measures the force required to maintain motion between two surfaces in contact divided by their respective normal forces. This describes resistance during sliding, commonly known as surface or sliding resistance. In general, static COF remains higher than kinetic COF.
Q: What kind of testing machine is used to test the coefficient of friction?
A: A specialized testing machine commonly used in coefficient of friction testing combines elements of a tensile tester with a sliding mechanism. Typically, such a machine consists of a stationary surface against which a movable sled with a known weight exerts pressure and thus produces both static and dynamic forms on the sample being tested simultaneously. The device also calculates both static and dynamic friction as it moves across the test specimen with its sled.
Q: How is the coefficient of friction calculated in these tests?
A: The frictional force measured in pounds divided by the normal force (weight of the sled) gives the coefficient of friction. An initial maximum force required to start a motion is usually used as a static COF. It’s taken, on average, as a kinetic COF during sliding. Thus, COF= Frictional Force / Normal Force. Sometimes, this arithmetic is done automatically by the testing machine based on the sled’s force measurements and known weight.
Q: What factors can affect the coefficient of friction measurements in plastic testing?
A: Under this category, there are various factors such as surface roughness, temperature, humidity, speed of testing, applied load, and sample preparation. The contact nature of the surfaces involved, for instance, film to metal or film to film, is also relevant. One has to ensure they are controlled according to the standards established by ASTM or ISO for consistent and replicable results.
Q: How different are ASTM D1894 and ISO 8295 in their testing procedures?
A: Though ASTM D1894 and ISO 8295 determine the coefficient of friction for plastic films, some differences exist in their methods. For instance, the ASTM D1894 sled weighs 200g with a speed of test at 150 mm/min, while the ISO 8295 sled weighs 200g if films are less than 0.075mm thick and 500g for thicker films with a speed test placed at 100 mm/min. In addition, slightly different calculation methodologies and reporting requirements between these two standards may be noticed.
Reference Sources
1. Beschorner et al. (2019) “Predicting slips based on the STM 603 whole-footwear tribometer under different coefficient of friction testing conditions” (Beschorner et al., 2019, pp. 668–681).
Key Results:
- When tested with liquid contaminants, the STM603 device by SATRA Technology that measures friction could predict human slips.
- Compared to the ASTM F2913 standard, increasing the shoe-floor testing angle to 13° and vertical forces to 400 or 500 N somewhat enhanced slip predictions.
Methodology:
- A single pair of shoes was tested with a range of nine footwear designs, including twelve different experimental conditions that varied in terms of vertical force, speed, and shoe angle using the available coefficient of friction.
- They calculated from human gait data how frequently slipping occurred and determined what coefficient of friction was required for each case; this involved analyzing 124 instances when people were exposed to liquids.
2. Iragi et al. (2018), “Parameters for evaluating the coefficient of friction and their relationship to predicting slips of humans” (Iraqi et al., 2018, pp. 118–126).
Findings:
- ACOF was found to have been significantly affected by the test conditions, where there existed an ideal condition that could determine slip risk on gait studies with a regular force of 250 N, shoe-floor angle of 17 degrees, and sliding velocity of 0.5 m/s.
Method:
- Different testing conditions were used to measure ACOF and compared with data on slip risk from human gait studies.
3. Borawski (2022) “How the duration of testing of exploitation time for brake pads of a passenger car affects the values of coefficient of friction and abrasive wear rate evaluated by pin-on-disc method” (Borawski, 2022).
Main results obtained:
- As brake pads wear out, their friction coefficients and abrasive wear rates go down.
Methodology:
- To measure the coefficient of friction and abrasive wear rate, samples were taken from new and worn-out brake pads and tested using a pin-on-disc tribological test.
4. Lomas et al. (2018) “Tribological Testing of Metallurgical Coke: Coefficient of Friction and Relation to Coal Properties” (Lomas et al., 2018)
Key Findings:
- The authors conducted tribological testing and analysis. The surface nature of metallurgical cokes was explained by utilizing its influence on coke’s resistance to abrasion.
Methodology:
- Several samples of metallurgical coke underwent tribological testing for friction coefficients and wear rates.
5. Gao and Liu (2020) “How the Friction Coefficient by Constant-Load Scratch Testing of Copper with a Spherical Indenter can be Affected by Sample Tilt” (Gao & Liu, 2020)
Key Findings:
- At small tilt angles, experimentally measured friction coefficients increased with surface height slope or surface tilt angle; when the indenter traversed the sample surface, they increased even more.
Methodology:
- A copper micro scratch test was conducted on a spherical indenter constantly loaded with copper to examine how sample tilt affects the friction coefficient measurement.
