1. Introduction
Braking function is a critical element in any vehicle. Whereas various braking technologies are in use, friction braking still remains the most prevalent. It is considered as the most effective and reliable, which is of particular interest for a component used in emergency situations. Particular attention has been paid over the years to the development of these braking systems, which realize this conversion of the vehicle’s kinetic energy into other forms, primarily heat, as the brake pads interact with the brake disc. However, the use of friction brakes still involves significant unresolved problems. Thus, friction instabilities, thermal fatigue [
1], wear [
2] and particles or noise emissions such as squealing [
3] remain as significant challenges to the improvement of these systems. The occurrence of brake squeal in vehicles is one of the most expensive issues in industrial applications (after-market) [
4], and significant efforts are made to mitigate this [
5]. It is a particularly problematic issue for people, due to its high frequency and acoustic pressure (above 1 kHz and starting at 70 dB). This challenge is all the more true for electric vehicles, which are even more sensitive to noise due to the absence of noise-covering combustion engine sounds. It is commonly accepted that squealing comes from self-excited vibrations of the brake system, which endure unstable dynamical behavior (like mode coupling) under sliding frictional contact conditions [
6]. But a complete understanding of its physics is still lacking, as it remains a complex phenomenon affected by multiple factors on multiple scales from the behavior of structural components to external excitations, as well as the dynamics of the friction interface during braking.
Over the years, to enhance our understanding of the mechanisms leading to squeal events, researchers have conducted experiments and numerical studies to investigate the behavior of the contact interface. Experimental studies shown that within a sliding contact, the evolution of imposed conditions [
7,
8] and macroscopic aspects [
9], such as changes in the friction material, surface’s geometry and tribolayer (thin film at the interface) are interconnected and have an influence on the squeal occurrence [
10]. Different rotation speeds of the rotating part, for example, lead to the evolution of vibration responses and contact behaviors. Relationships can be observed, such as a stronger vibration response occurring at lower disc rotation speed, like 200 rpm, leading to high sound pressure level [
11] and also severe localized wear and rough pad surface. Moreover, research works reported a strong correlation between wear and squeal occurrence in braking systems [
12]. From prepared parts [
13], successive contacts gradually modify the contact behavior with an elevation of the friction coefficient until it starts to plateau around a critical value. Squeal occurrence sharply increases once this contact behavior is reached, coinciding with increased use of parts. However, it has been observed that the friction coefficient and its variation are not enough to explain squeal occurrence, as many studies address the fact that mode coupling can take place under a constant friction coefficient. This coefficient should rather be viewed as a response of the tribological system. Finite element models employing complex eigenvalue analysis (CEA) have also demonstrated the sensitivity of components (dynamic modes) and system parameters (assembly with connections) on squeal propensity [
14]. The influence of thermomechanics and friction materials on the dynamic response has also been highlighted [
15,
16]. However, the models are still not predictive.
Additional studies focused on the evolution of friction material and tribology [
17], demonstrating the complexity of involved mechanisms, as the friction material undergoes modifications due to thermal, mechanical and tribological loading as far as the tribolayer. On both mesoscopic (contact geometry) and microscopic (particles flow) scales, it is known from the third-body approach proposed by Godet [
18] and Berthier [
19] that the flows and compaction of worn particles (which constitute the major part of what is called the third body) play a crucial role in the creation of the tribological surface layers [
20]. These changes at a microscopic scale such as evolution of the contact surface heterogeneity (due to material degradation and presence of a third body) significantly impact squeal occurrence. This is interconnected with the tribological circuit and its complex behavior [
21]. It has been shown that without changes to the interface (no wear of friction material, and hence no third body in the contact and constant macro contact surface), a tribological system remains silent and squeal only occurs upon the addition of a third body. Squeal occurrence is then highly dependent of the size and nature of particles flowing through the contact [
22], in addition to the rest of the contact conditions (friction coefficient, dissipated energy, etc.). Numerical works also show that the emergence of squeal is dependent on the evolution of the surface and material transformation beneath the surface of friction material [
23].
Most recent experimental studies addressing squeal issues and tribology in general tend to include the observation of the worn surface after contact [
24]. The added value of these observations is, however, often limited, as after opening the contact, a significant amount of critical information is lost, particularly if the observation requires a lot of manipulation of the parts. The distribution of contact plateaus in the tribolayer, which are formed by hard ingredients and wear debris compaction around them, have been shown to have an influence on squealing events [
25]. The impact of the changes in the actual contact localization at macro and meso scales on the squealing frequencies of a pin-on-disc system have also been highlighted [
26]. This research work emphasizes how the evolution of the contact state during dry friction significantly influences the squeal occurrence. However, the contact is not accessible during contact, which makes characterizing its state only feasible before and after the tests. Therefore, it would be very useful to have an indicator to identify the conditions in which squeals emerge, in order to anticipate how to avoid them. In this work, we propose to explore indicators other than global ones, such as friction coefficient or disc temperature, which are often linked, but only partially correlated with squeal noise. To this end, the indicator must be related to the phenomena described above, hence the idea of a carefully chosen intrusive measurement. Thermomechanical constraints on the friction material are monitored during tests with embedded thermocouples and Foucault’s displacement sensors, while a microphone records the noise. A discrete tracking of the friction material surface with a remote head profilometer is introduced, allowing for optical observations as well as profilometry without disassembling the parts.
4. Discussion
Figure 12 presents an overview of all frequency classes, regarding the max temperature of the pin (maximum of the two thermocouples) in the last test performed just before the profilometry acquisition. The surface class is also indicated to compare with squeal classification. Class
A for initial state has been removed here, and points circled in the gray tinted area correspond to observations realized before the transformation cycle with high thermal solicitation (test 0 to 215). As seen before, surfaces with topography of class
B are only obtained if the pin temperature previously reached more than 140 °C. And those tests only correspond to squeal class
2.1 kHz Squeal during the running-in phase, or
5.3 kHz Squeal for the rest of the campaign. Below 140 °C, behavior appears more complex. Surface class
D seems to occur most of the time for the lower temperature, but there is an overlap in term of temperature with the surface
C, which is also present around 110/120 °C. A connection is highlighted between maximum pin temperature, squeal classes and surface classes.
Relationships have been established here between surface and squeal classes regarding the temperature solicitation, but only considering the maximum pin temperature of the test before profilometry. Yet, the profilometry observations are relatively scarce in comparison to the number of tests performed and the temperature level reached alone does not suffice to indicate squeal frequencies at lower temperatures. As mentioned before, temperature variations and heat kinetics on the surface area must also be considered to differentiate noise emissions. The impact of a test cycle reaching temperatures sufficient to potentially alter the material of the pin must also be acknowledged. The strong correlation between maximum temperatures and the pin surface state take places after this intense thermal solicitation.
A more in-depth analysis of the thermal solicitation on squeal for tests after the transformation cycle (tests 240 to 526) is presented in
Figure 13. The maximum difference of temperature between the two thermocouples is displayed (trailing edge minus leading edge thermocouple) regarding the maximum reached temperature. All resulting points are connected to the centroïd (center of mass represented by a triangle) of their respective point clouds, which have the convex hull of their area colored for a better visualization. For this representation, squeal class
2.1 kHz Squeal is excluded due to its limited number of data points. The number of displayed tests is still more substantial than the number of profilometry observations, allowing for the verification of previously observed hypotheses across a larger sample group.
As previously noted, the maximum pin temperature remains a distinguishing factor for class 5.3 kHz Squeal, with only a few tests of class 3.3 kHz Squeal falling within the same temperature range. Some of these few tests of class 3.3 kHz Squeal showing a maximum pin temperature above 150 °C correspond to tests 417 to 419, which takes place in a high-temperature test series with potential material transformations being able to impact squeal frequency. The rest of these tests correspond to tests 496 and 497, where a squeal at 3.3 kHz occurs only at the beginning of the test and disappears when the temperature rises. So those points should in reality be linked with a lower temperature than the maximum pin temperature reached during the whole test. A more precise classification of squeal frequency regarding pin temperature would allow for an even more clear discrimination of temperature domains of each squealing frequency.
At lower temperatures, although classes No Squeal, 3.3 kHz Squeal and 18 kHz Squeal share a similar range of maximum pin temperature, clear differences emerge in terms of the temperature gap between the two thermocouples. Class 18 kHz Squeal in particular is clearly delineated on this graph, appearing from 100 °C and only for low or negative temperature differences. While, in the majority of tests, the thermocouple at the trailing edge reaches higher temperatures, for some tests, the one at the leading edge of the contact exhibits higher temperature, triggering this particular frequency. This variation in temperature differences can be associated with an uneven distribution of contact pressure, which indicates a change in the load bearing. This change in bearing capacity may itself be linked to thermomechanical effects (e.g., dilatation) or interface variations (tribolayer or material detachment).
In contrast, the absence of squealing (class No Squeal) is noted between 50 °C and 150 °C, but only when the thermocouple at the trailing edge records significantly higher temperatures than its counterpart. Class 3.3 kHz Squeal displays the widest range in both maximum pin temperature and temperature differences. However, in most tests where this frequency is triggered, the temperature rise at the trailing edge exceeds that at the leading edge by a margin of around 10 degrees. In this representation, classes No Squeal and 3.3 kHz Squeal still overlap, yet their centroids suggest that a load-bearing area predominantly located on one side of the surface (specifically, at the trailing edge of the contact) tends to favor the absence of squealing. In this configuration, the exit of the contact is more confined, and the internal flow of particles through the contact might be limited if they are mostly emitted near the exit of the contact.
The present study has highlighted a significant correlation between pin temperature and squeal occurrence during dry sliding contacts. This finding aligns with previous research emphasizing the crucial role of temperature in tribological phenomena associated with these events. Investigation of the pin’s profilometry has shown a robust correlation between the maximum pin temperature and the resulting surface condition. This suggests that the temperature seen by the material is a main contributing factor to the surface state of the pin. Through the imagery of the profilometry, it appears that the behavior of the third body through the contact and its ability to resorb asperities of the surface, hence lowering its macroscopic roughness, is significantly impacted by the temperature. These results have been confirmed by other experiments on the same device. An extension to other systems would require further investigation, similar to those proposed here, i.e., extended instrumentation for temperature measurement to obtain information on contact localizations. These localizations depend on the rigidity of the system, as does the vibratory response.
Lower-temperatures solicitations allow for a more powdery third body to be trapped in asperities, reducing surface roughness parameters on a scale of several millimeters. It might result in more opportunities for the load-bearing area to evolve as powdery material accumulates or is compacted, thereby triggering a greater variety of squealing frequencies. Conversely, higher thermal solicitations lead to the clearing of asperities and porosities, amplifying the role of millimetric patterns on the surface in contact localization and resulting in more specific and stable squealing frequency. Monitoring the pin’s temperature during tests could potentially allow some squealing frequencies to be entirely avoided.
5. Conclusions
The complexity of the phenomena involved in squeal noise explains why, despite the huge amount of work dedicated to this issue, predicting them remains a challenge. It was proposed in this work to identify whether a physical indicator could be obtained to predict squeal situations. It was shown that an operando thermal measurement could enable this prediction. The relevance of this measurement is explained by the links with the physical phenomena influencing squeal, i.e., macroscopic and microscopic contact in relation to thermomechanics and tribology. It appears that variations in these phenomena can be captured by subsurface thermal measurement. However, the complexity of these phenomena can make measurements tricky because it is both far from the surface and relatively simple. It has been shown that a prediction of squeal is achievable by processing both the temperature level and the temperature difference between two measurements located in front of and behind the direction of sliding, provided that the loading history is considered, in particular the high-temperature events which transform the pad material. Discrete surface analysis has shown that the temperature level affects the tribolayer and the difference between the measurements affects the macroscopic load-bearing variation. This type of indicator seems to be able to reflect the intricate mechanisms governing brake system behavior.
Different improvements can be suggested as future work. In-depth processing of squealing events and frequency classification could explain at least in part the overlaps of the fields observed in
Figure 13. The number of sensors could be increased, particularly thermal, to access finer location information. Data processing is an important area of improvement, whether by adding information from sensors other than thermal or by using more systematic processing such as machine learning methods, or even by extended processing of measurements to go back to surface information (inverse models). Finally, while, in this study, the focus has been placed on the pin, extensive observation of the disc’s surface would be necessary to have a more complete comprehension of the contact behavior.
Various applications could be found for this work. The insights derived from this study could inform the development of strategies to mitigate squeal occurrences by addressing factors such as temperature mitigation. Conditions of brake use that are conducive to squeal could be defined and avoided, for example, with the design of the brake or with the combination of other brakes, such as regenerative brakes.