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The Normal-Load and Sliding-Speed Dependence of the Coefficient of Friction, and Wear Particle Generation Contributing to Friction: High-Copper and Copper-Free Formulations

Compact International (1994) Co., Ltd.-Meechai Sriwiboon, Nipon Tiempan, Kritsana Kaewlob
SKR Consulting Inc.-Seong K. Rhee
Published 2019-09-15 by SAE International in United States
Automotive brakes operate under varying conditions of speed and deceleration. In other words, the friction material is subjected to a wide range of normal loads and sliding speeds. One widely accepted test procedure to evaluate, compare and screen friction materials is the SAE J2522 Brake Effectiveness test, which requires full-size production brakes to be tested on an inertia brake dynamometer. For the current investigation, disc pads of two types of 10 different formulations (5 high-copper and 5 copper-free formulations) were prepared for testing on a front disc brake suitable for a pickup truck of GVW 3,200 kg. Each pad had 2 vertical slots, and one chamfer on the leading edge and also on the trailing edge of the pad. One segment of the test procedure looks at the coefficient of friction (Mu) under different brake line pressures and different sliding speeds to determine its stability or variability. In all cases, the Mu is found to be dependent on the normal load and sliding speed, contrary to the commonly called “Amontons-Coulomb’s Laws of Friction”. According to…
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Aging Effect on Disc Pad Properties

Compact International (1994) Co., Ltd.-Meechai Sriwiboon, Kritsana Kaewlob, Nipon Tiempan
SKR Consulting Inc.-Seong K. Rhee
Published 2019-09-15 by SAE International in United States
One low-copper formulation and one copper-free formulation were made into disc pads, and both of them were cured under 4 different conditions. These pads had no backing layer and no scorched layer. Pad thickness, dynamic modulus and natural frequencies were continuously monitored over a period of 12 months. After 12 months at room temperature, pad thickness, dynamic modulus and natural frequencies all increased to higher values. The low-copper formulation increased relatively rapidly during the first 60 days and the copper-free formulation increased relatively rapidly for the first 90 days, and then slowly thereafter. Two competing processes are found to be taking place simultaneously; internal stress relief leading to pad expansion and cross-linking of the resin leading to pad shrinkage. As the pad properties are changing continuously, the timing of property measurement becomes an important issue for quality assurance. Implications of these changing properties are discussed for friction, wear, brake squeal and squeal modeling/simulation, and simple non-destructive test methods are recommended for checking pad quality consistency.
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Non-Asbestos Organic (NAO) Disc Pad Wear Behavior: Divergence of Thickness Loss and Weight Loss

Compact International (1994) Co. Ltd.-Meechai Sriwiboon, Nipon Tiempan, Kritsana Kaewlob
SKR Consulting LLC-Seong Rhee
Published 2018-10-05 by SAE International in United States
There is anecdotal evidence that disc pad wear numbers measured in thickness loss and disc pad wear numbers measured in weight loss do not show the same wear trends after wear or performance testing. However, research papers on this topic are difficult to find. Therefore, this investigation was undertaken to study and document this behavior in detail on high-copper, low-copper and no-copper (or copper-free) NAO pads. In all cases, thickness loss measurements are found to be substantially lower than expected from the weight loss data according to the SAE J2522 test schedule. This divergence is caused by pad swelling in the pad layer adjacent to the friction contact surface during brake testing at high temperatures. In addition to formulation changes, disc pad processing conditions such as mixing time and hot molding pressure are found to affect pad swelling. As pad physical properties, especially in the layer adjacent to the friction contact surface, are expected to dynamically change during braking due to the pad swelling, one has to seriously question any attempt to correlate physical properties…
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Disc Pad Physical Properties vs. Porosity: The Question of Compressibility as an Intrinsic Physical Property

SAE International Journal of Materials and Manufacturing

Compact International (1994) Co., Ltd.-Meechai Sriwiboon, Nipon Tiempan, Kritsana Kaewlob
Industrial Measurement Systems Inc.-Donald Yuhas
  • Journal Article
  • 05-11-01-0007
Published 2017-09-17 by SAE International in United States
Disc pad physical properties are believed to be important in controlling brake friction, wear and squeal. Thus these properties are carefully measured during and after manufacturing for quality assurance. For a given formulation, disc pad porosity is reported to affect friction, wear and squeal. This investigation was undertaken to find out how porosity changes affect pad natural frequencies, dynamic modulus, hardness and compressibility for a low-copper formulation and a copper-free formulation, both without underlayer, without scorching and without noise shims. Pad natural frequencies, modulus and hardness all continuously decrease with increasing porosity. When pad compressibility is measured by compressing several times as recommended and practiced, the pad surface hardness is found to increase while pad natural frequencies and modulus remain essentially unchanged. However, there is no consistent pattern in compressibility change with increasing porosity, and thus a question arises on the validity of compressibility measurement as an intrinsic physical property measurement. Also after 12-months of ageing at room temperature, all the properties are found to change significantly, but property change trends with increasing porosity remain…
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A Study of Pad Properties vs. Friction, Wear and Brake Squeal: Processing/Porosity Effects

SAE International Journal of Materials and Manufacturing

Compact International Co. Ltd.-Kritsana Kaewlob, Nipon Tiempan, Rungrod Samankitesakul
Compact International Co., Ltd.-Meechai Sriwiboon
  • Journal Article
  • 2016-01-1915
Published 2016-09-18 by SAE International in United States
As some brake engineers believe that brake squeal can be related to pad hardness, friction coefficient or compressibility while others disagree, a study has been undertaken to develop further insights. Two commercial formulas, one low-copper NAO and the other copper-free NAO, were made into disc pads of varying porosity without an underlayer and they were checked for specific gravity, porosity, hardness (HRS and HRR), natural frequencies, compressibility, friction, wear and squeal. With increasing porosity, the hardness and natural frequencies continue to decrease. The compressibility definitely does not increase, but rather slightly decrease or stays the same. The coefficient of friction decreases for the low-copper along with pad and disc wear reduction, and increases for the copper-free along with pad wear increase with no change in disc wear. No obvious correlation emerges between brake squeal and pad hardness, friction coefficient or compressibility. After the pads were compressed under 30, 100 and 160 bars, the hardness of the pad surface is found to increase, suggesting permanent compaction under the pressures while the interior of the pad, 5…
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Brake Squeal and Wheel Dust vs. Disc Wear: No-Copper, Low-Copper and High-Copper NAOs

SAE International Journal of Materials and Manufacturing

Compact International Co., Ltd.-Meechai Sriwiboon, Nipon Tiempan, Kritsana Kaewlob
SKR Consulting LLC-Seong Rhee
  • Journal Article
  • 2015-01-2660
Published 2015-09-27 by SAE International in United States
In a previous investigation, brake squeal was found to be related to disc wear, but not to pad wear or in-stop average coefficient of friction as tested according to the SAE J2522 performance procedure, using Low-Copper NAOs. To further validate the disc wear - squeal correlation, a variety of formulations of No-Copper, Low-Copper and High-Copper NAO disc pads were made and tested to investigate friction, pad wear, disc wear, brake squeal and wheel dust formation. It is found that disc weight loss measured at the end of the burnish cycle of the SAE J2522 (AK Master) is closely related to dynamometer/vehicle brake squeal and vehicle wheel dust formation, and that there is a critical disc wear rate of approximately 1.0 grams for the current brake system, below which brake squeal and wheel dust are minimal. It is also found that there is no correlation between brake squeal and in-stop average friction coefficient, in-stop maximum friction coefficient or in-stop friction coefficient variations during the burnish cycle of SAE J2522 (AK Master).
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Influence of Formulation and Process Modifications on Brake Friction, Wear and Squeal: Low-Copper NAOs and Importance of Disc Wear

Compact International Co., Ltd.-Meechai Sriwiboon, Nipon Tiempan, Kritsana Kaewlob
SKR Consulting LLC-Seong Kwan Rhee
Published 2014-09-28 by SAE International in United States
The influence of processing conditions on Low-Copper NAO disc pads were investigated as part of an effort to develop Low-Copper disc pad formulations as this kind of information is not readily available in open literature. Processing conditions as well as formulation modifications are found to influence friction, pad wear, disc wear and brake squeal. Low-Copper disc pads for pick-up trucks, equivalent to an OE pad, are developed. It is also found that brake squeal measured during the SAE J2522 (AK Master) Performance testing is related to the combined total wear rate of the disc plus the inner/outer pads or the disc wear rate alone, and that there is a threshold wear rate, above which brake squeal increases rapidly.
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