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Non-Asbestos Organic (NAO) disc pads and Low Steel Lomet disc pads were subjected to high and low humidity conditions to discover how humidity affects these two classes of formulations for physical properties, friction, wear and noise characteristics. The 2 classes of formulations show similarities and differences in response to increasing humidity. The humidity effect on deformation of the surface microstructure of the gray cast iron disc is also investigated. Humidity implications for pad quality control and brake testing are discussed.

Passenger car NAO(Non-Asbestos Organic) disc pads were subjected to low and high humidity conditions. Humidity is found to measurably affect pad dimensions, pad hardness, compressibility, friction, pad wear, disc wear, disc roughness, DTV(Disc Thickness Variation) and brake noise. Also the friction film is found to absorb a significant amount of moisture. It is essential to have a tight control of temperature and humidity for brake testing and quality control if meaningful data are to be generated with minimum variability. Seasonal changes must be considered for brake testing on the road.

Copper-free disc pads of 9 different compositions were made using a traditional hot molding process and tested to study frictional behavior. It is found that the friction coefficient consists primarily of two parts; one part controlled by the plastic deformation of the friction surface region of the disc and pad, and the second part controlled by the total wear of the disc and pads. As the plastic deformation and the wear are non-linear with respect to the load and sliding speed, the friction coefficient becomes a non-linear function of the load and speed. Under moderate braking conditions, the plastic deformation part is more significant in determining the friction coefficient while under more severe braking conditions, the wear contribution becomes more significant. The frictional behavior of a fade cycle is explained, and the correlation between brake squeal and disc wear is confirmed.

When two identical brakes are simultaneously tested on a vehicle chassis dynamometer, very often the left hand brake is found to squeal more or less than the right hand brake, all at different frequencies. This study was performed to develop some understanding of this puzzling phenomenon. It is found that as the wear rate difference between the inner pad and the outer pad increases, low frequency (caliper and knuckle) squeals occur more and more, and as the differential wear becomes larger and larger, high frequency (disc) squeals occur less and less, finally disappearing all together. Discs and calipers are found to affect the differential pad wear, in turn affecting brake squeal generation.

The moisture sorption-desorption kinetics of copper-free brake pads was studied in detail. The sorption-desorption behavior is dependent on the environmental temperature and humidity. At 24 °C under 54% RH, the sorption increases rapidly for a week or so identified as the first stage of sorption, enters the second stage of negligible weight gain for a month and then the third stage of rapid sorption again. With increasing moisture sorption, the pad thickness increases through the 3 stages and the dynamic modulus also increases through the 3 stages. Friction materials lose moisture rapidly at 130°C and behave like desiccants. The sorption-desorption phenomenon significantly influences the friction coefficient -- a higher moisture content leading to lower friction coefficients. It is demonstrated that the rising friction coefficient for the half a dozen braking stops at the beginning of every brake testing in general is due to moisture desorption caused by rising pad temperatures.

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.

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.

Earlier publications have demonstrated that pad and disc properties change during storage and also during the SAE J2522 Brake Effectiveness Test Procedure. The current investigation was undertaken to find out how the properties change under milder braking conditions, using the SAE J2707 Wear Test Procedure. A copper-free formulation was selected for the investigation and tested on an inertia dynamometer using a front caliper designed for a passenger car. The pad dynamic modulus changed up or down throughout the test, depending on the test conditions. The pad dynamic modulus, the pad natural frequencies and the disc natural frequencies all decreased by the end of the test. Under high-speed, high-deceleration and high-temperature braking conditions, the pad surface region permanently expands, which results in reduced dynamic modulus and also leads to reduced pad thickness loss as compared with pad weight loss.

Commercial heavy truck drum linings of 4 different compositions were tested using the Chase tester under constant loads and temperatures at a constant speed in order to find out how lining wear might affect the friction coefficient. When the lining wear increases, the friction coefficient increases linearly under a condition of constant load, speed and temperature. However, when the lining wear approaches zero, the friction coefficient still remains relatively high, indicating other factors are also involved in controlling friction such as interface deformation and others. As the temperature increases or the load increases, the wear contribution to the friction becomes less and less effective. All these observations are discussed and explained in terms of wear particle formation and friction film behavior.

Two sets of OE quality brake discs were evaluated for their equivalence in friction and wear under a humidity controlled condition in order to avoid the influence of humidity on friction and wear. These discs were received from two different suppliers located in two different countries. Small differences were found in disc chemistry and microstructure, which resulted in differences in disc properties, and friction and wear characteristics. It is recommended that extreme care must be exercised in determining the performance equivalence of one disc from one supplier against another disc from a second supplier.

The moisture adsorption kinetics of copper-free brake pads was studied to confirm an earlier finding that the adsorption weight gain follows a logarithmic relationship with respect to the square root of humidity exposure time and the relationship is linear in the beginning. When the pad cure temperature was raised from 120 to 180 and 240 °C, the adsorption rate increased. The 180 °C cure produced the highest pad modulus. With increasing moisture adsorption, the pad compression modulus increased just like the pad dynamic modulus, meaning decreasing compression/compressibility while the ISO ‘compressibility’ determined after 3 compressions under 160 bars increased in contradiction. It is concluded that the ISO ‘compressibility’ is a destructive hardness measurement like the Gogan or Rockwell hardness: the key difference is the indenter covers the entire surface of the pad. The true compressibility must be determined as an inverse function of bulk modulus.

A previous investigation showed that minor variations in alloying elements in gray cast iron disc contributed to measurable differences in friction and disc wear. This investigation was undertaken to find out if and how the increased friction and disc wear might affect brake squeal. The SAE J2522 and J2521 dynamometer procedures as well as an OEM noise dynamometer procedure and a chassis dynamometer noise procedure were used to find out if a correlation between disc wear and brake squeal could be discovered. In all cases, as the wear rate of a disc increases under a given set of test conditions, disc material transfer to the pad surface increases, which results in increased friction and brake squeal. Also a good method to detect disc variability (disc to disc, within a disc) is discussed.

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.

Earlier publications show that brake pad physical properties such as hardness, modulus and natural frequencies continue to increase at room temperature over a period of 12 months and that the changes are faster during the first 3 - 6 months. The current investigation was undertaken to see how the properties might change during testing for the pads as well as for the discs. Low-copper and copper-free formulations were tested on pickup truck and passenger car brakes. In all cases, the dynamic modulus and natural frequencies are found to decrease (not increase) after the SAE J2522 performance testing, indicating that the stiffness of the pad and that of the disc decrease faster than the mass loss due to wear. Also, the inboard pad and the outboard pad change at two different rates.

Rear disc brake squeal test results confirm the disc wear - brake squeal correlation reported earlier on front disc brakes. A significant amount of Fe transferred from the disc to the NAO pad surface is detected and the distribution of the transferred Fe is very non-uniform on the pad surface. The pad surface formulation reaches that of Low-Steel Lomets. Disc pads from a noisier brake retain more transferred particles than from a less noisy brake. The pad surface retains more transferred Fe after noise test procedures than after performance test procedures. The transferred Fe particles are either barely visible or invisible. During brake noise test procedures, discs wear in weight as much as disc pads. No correlation is found between average in-stop Mu, maximum in-stop Mu or in-stop delta Mu and brake squeal.

It is widely believed or speculated that higher pad compressibility leads to reduced brake squeal and that caliper design can affect brake squeal. After encountering anecdotal contradictory cases, this investigation was undertaken to systematically generate basic data and clarify the beliefs or speculations. In order to adjust pad compressibility, it is common to modify pad molding temperatures, pressures and times, which in addition to changing the compressibility, changes friction coefficient and physical properties of the pad at the same time. In order to separate these two effects, NAO disc pads were prepared under the same molding conditions while using different thicknesses of the underlayer to achieve different compressibilities, thus changing the compressibility only without changing the friction coefficient and physical properties of the pad.

Heavy truck brake blocks are found to swell (or expand) permanently during testing or usage, especially so at high temperatures, thus leading to longer durability as measured by thickness loss, similar to light vehicle disc pads. This swelling phenomenon occurs continuously in the layer adjacent to the friction surface during testing or usage; not a one time event. The thickness loss estimated from the weight loss is always greater than measured thickness loss. Brake block wear does not increase linearly with increasing normal load, and the load-sensitivity of block wear is very much dependent on the products. A new test procedure has been developed for generating friction-vs.-temperature and wear-vs.-temperature data at a constant temperature employing intermittent braking on the Chase Brake Lining Quality Tester (SAE J661) and friction material wear can be compared on equivalent-work basis.

The current investigation was undertaken to find out if lighter-weight passenger car disc pads would exhibit wear behaviors similar to pickup truck pads and commercial heavy truck drum linings in terms of the permanent volume expansion of the friction material contact surface region. 2 high-copper NonAsbestos Organic formulations and 3 copper-free LowMet formulations were tested according to the SAE J2522 test procedure. In all cases, the measured pad thickness loss was found to be less than the thickness loss calculated from the weight loss, indicating pad volume expansion in the pad surface region, in full agreement with the results from the pickup truck and heavy trucks. The heataffected swollen/expanded layer ranges from 0.27 to 0.61 mm in thickness depending on the formula and test conditions. Due to the expansion, pad durability projections made from test results based on high temperature city traffic tests can result in underestimating the actual durability.

Copper-free NAO disc pads of passenger cars were investigated for a combination of prior braking conditions and moisture adsorption influencing in-stop friction and noise during low-speed stops, and in-stop-friction during moderate-speed stops. Prior braking conditions and moisture adsorption strongly influence subsequent in-stop friction behavior and noise at room temperature. The low-speed in-stop friction behavior looks totally different from that of moderate-speed stops. The low-speed in-stop friction increasingly oscillates with increasing moisture adsorption and goes down towards the end of a stop, which is accompanied by increasing low-frequency noise. The moisture content needs to be quantified/specified to obtain repeatable/reproducible brake test results as the moisture is an unintended and uncontrolled ingredient of a friction material. As the disc surface roughness increases due to prior braking conditions, the friction coefficient of low-speed stops is found to decrease.

The dynamics and, in particular, the NVH phenomena in brakes are still in the focus of research. Recent investigations of for example Rhee et al. show two principal vibrational forms of the linings on the rotor [1]. The first form is characterized by vibrations where both linings are in-phase (minimal differential torque between the inner pad and the outer pad). This produces in-plane vibrations of the rotor and results in high-frequency squealing events in the brake. The second form is an antiphase vibration of the brake linings with respect to each other (increased differential torque between the inner pad and the outer pad). This produce directly out-of-plane vibrational modes of the disc, which results in lower-frequency caliper and rotor oscillations. One hypothesis is that different wear densities of the linings essentially characterize the two vibrational modes. The wear behavior is not taken into consideration of this paper as it will be discussed in further publications.