Disc brakes emit ultrafine, fine, and coarse particles into the ambient air. According to estimations, 21% of total traffic-related PM10
emissions in urban environments originate from airborne brake wear particles [1
] and particle number emission factors are in the magnitude of 1010
during real-world driving conditions [2
]. While PM10
relevant fine and coarse particles are generated due to mechanical friction processes, ultrafine particles in the magnitude of 10 nm are generated by nucleation [3
]. Nucleation occurs during braking events above a critical temperature, probably when organic materials start decomposition and gases evaporate from the brake pad, condensate in the cooling airflow, and agglomerate to nanometer-sized particles. A typical time series of a nucleation event is shown in Figure 1
. Nucleation caused by braking events above a critical temperature is a well-known phenomenon and was described by several authors on pin-on-disc tribometers as well as on full-scale brake dynamometers while temperature’s influence on PM10
emissions is only known on a tribometer or rather not independently from other variables. The current state of research regarding the temperature’s influence on PM10
and particle number emissions is summarized below.
Ramousse et al. [5
] performed a differential thermal analysis of brake pad materials in combination with a mass spectrometer analysis. Their investigation revealed that decomposition of the pad’s binder starts at 250 °C, coal decomposes in two stages between 300 and 700 °C, iron between 500 and 800 °C and graphite between 600 and 800 °C.
Nosko et al. performed measurements on a pin-on-disc tribometer with sliding speeds of up to 2.6 m/s [6
]. The temperature was varied by slowly increasing sliding speed. He found a critical temperature for ultrafine particle generation of approximately 170 °C. Coarse and fine particle emissions between electrical mobility diameters or optical diameters of 100 nm and 10 µm increased and were described with emission coefficients normalized on the friction energy.
Alemani et al.’s [9
] investigations on a pin-on-disc test rig showed a strong temperature influence on the steady-state number concentration, which exceeds the sensitivities of surface pressure, friction velocity, and friction power. His measurements also showed a sudden increase of 4 orders for the number concentration in the range of 170–190 °C, which is in accordance with the investigations of other researchers. He assumes a decomposition process of phenolic binders in the friction lining as a possible cause for this increase.
Experiments on a pin-on-disc test rig by Pericone et al. [10
] match with the observations described above where an increase in the number concentration of ultrafine particles by 4 to 6 orders of magnitude occurs when a transition temperature of 165–190 °C is exceeded.
In measurements carried out by Sachse et al. [11
] on a dynamometer without enclosure, a shift of the maximum in particle’s number size distribution from 100 nm to 20 nm was found at different starting temperatures of the brake event. Sachse mentions a critical temperature of 180 °C, from which the number concentration increases by at least one order of magnitude.
Agudelo et al. [12
] investigated the influence of the average initial rotor temperature on the particle number emission rate during various standardized driving cycles on an enclosed dynamometer test bench. The highest average emission rates occurred at the highest average temperatures. There is an obvious trend of high emission rates at high temperatures, but the temperature influence was not varied independently.
Kukutschová et al. [13
] (p. 1005) measured nucleation events in dynamometer tests at average rotor temperatures of 300 °C, which is a significantly higher temperature than in other publications. Based on a differential thermal analysis Kukutschová found a heterogeneous ignition process of binder (300 °C), charcoal (400 °C), graphite (550 °C), and Iron (750 °C).
Investigations on an enclosed dynamometer by Mathissen and Farwick zum Hagen et al. [2
] also found critical temperatures for the generation of nucleation particles at approximately
150 °C Experiments with used parts revealed that the nucleation events were not reproducible after a mileage of 6000 km [15
]. During the LACT (Los Angeles City Traffic) driving cycle no correlation between PM10
and temperature was found [2
As literature describes the decomposition of pad materials and an increase of particle number emissions and PM10 (only tribometer study) due to increasing temperatures, the following five research questions are derived from the current state of research to generate potential reduction approaches:
Ultrafine particle emission:
Is it possible to prevent nucleation processes or to increase the critical temperature by replacing organic compounds in the brake pad e.g., organic binder materials?
How does thermal decomposition processes of pad materials influence the reproducibility of nucleation events?
Fine and coarse particle emission (PM10):
How can the influence of disc temperature on PM10 emissions be quantified on a full-scale dynamometer setup?
How does thermal decomposition processes of pad materials affect the temperature dependency of PM10 emissions?
How does thermal decomposition processes of pad materials influence the reproducibility of the measurement of emission maps for PM10 depending on temperature?
The investigation of those research questions is subject of this paper and the methodology used to answer these questions as well as the experimental setup is described in the next section.
In accordance with the current state of research nucleation events were generated above a temperature of 180 °C. This temperature is in accordance with the range of several authors in the literature, except for Kukutschová [13
] (p. 1005) who found nucleation above 300 °C disc temperature. 180 °C is lower than the decomposition temperature of the raw binder material [5
], which has the lowest decomposition temperature (250–300 °C) of organic pad compounds. Because the local temperature in the friction surface is expected to be higher than the integral disc temperature, this discrepancy between decomposition temperature and critical nucleation temperature was to be expected. Possibly decomposition in the friction zone already takes place at lower disc temperatures but the emitted concentration of nucleation particles is not sufficient to agglomerate to a detectable diameter as it happened during the first triangle of the test with the inorganic binder pad (Appendix B
) when nucleation was only detected by CPC but not by the FMPS.
The generation of nucleation particles by decomposition of organic pad materials was not repeatable in that manner that the amount of nucleation particles decreased for identical sequential brake events, so the first nucleation event at one temperature level emits the biggest amount of nucleation particles. This phenomenon is in accordance with the explanation that nucleation is caused by decomposition because the material which is “available for decomposition” next to the friction surface is limited and is consumed with ongoing heat input. The observation from Farwick zum Hagen et al. [15
] that pad materials with a mileage of 6000 km did not produce nucleation particles up to 215 °C could be also explained by the “consumption” of the organic binder by decomposition during previous hypothetical high-temperature brake events.
The tested pads with an inorganic binder material, which is thermally stable up to 800 °C, have a higher critical temperature than the tested pads with an organic binder material. The observed critical temperature shifts about 60 °C with the inorganic binder. As known from Ramousse [5
] and Kukutschovà [13
] besides phenolic binder also other organic materials, which are present in brake pads, are decomposed at high temperatures, e.g., coal at 300–400 °C. Analogous to the decomposition of the binder material the decomposition temperature of coal and disc temperature for the onset of nucleation are not equal due to the thermocouple’s position 0.5 mm below the friction surface.
According to Figure 8
, the hysteresis behavior of PM10
correlates with the critical decomposition temperature of the phenolic binder as PM10
start to decrease at the same temperature as the nucleation occurs. This suggests that the decomposition is responsible for this decreasing behavior. This observation also suggests that PM10
correlate with each other, as it can be seen during the second regime in Figure 8
. A correlation between PM10
is plausible as abrasive friction is based on the plastic deformation of friction partners causing wear particles, which are emitted into the ambient air as particulate matter, which consists partially of PM10,
but also out of bigger particles.
The hypotheses that decomposition of phenolic binder leads to the observed hysteresis behavior is substantiated by the fact that the absence of an organic binder leads to an emission and friction behavior without temperature hysteresis, as it can be seen from the experiments with an inorganic binder (Figure 9
For the pads with an organic binder, a behavior without hysteresis was only achieved below the critical temperature (Figure 10
and Figure 11
). Below the critical temperature, a repeatable temperature influence of PM10
was observed. Nevertheless, it is not certain if this repeatability below the critical temperature is achieved by the presence of an intact, non-oxidized binder or by other unknown processes. However, the correlation of critical temperature for ultrafine particles and the change in PM10
emission behavior indicate a causal relation.
With both organic and inorganic binder material an adaption of µ
can be observed during the first brake events of the test. Such adaption processes are well known from the literature, e.g., from tribometer investigations of Ostermeyer and Eriksson [18
Based on the described results and the subsequent discussion, the five research questions mentioned in the introduction section are answered below to summarize the knowledge gained during this investigation:
5.1. Ultrafine Particles
Research question 1: Is it possible to prevent nucleation processes or to increase the critical temperature by replacing organic compounds in the brake pad e.g., organic binder materials?
In accordance with the current state of research, nucleation events were generated and the critical temperature for the conventional pad material with an organic binder is in the well-known range of 180 °C. By means of a systematic temperature variation (triangular temperature test signal), the investigation independently from other influencing factors as velocity, pressure, and friction history was enabled. For the purpose of a more detailed visualization of nucleation, a graphical presentation was chosen. This allows plotting temperature dependency of the size distribution as well as the increase of particle emission in one diagram.
The hypothesis, known from the literature, that phenolic binder material’s decomposition processes are responsible for nucleation events, stays valid as the critical temperature was increased by the usage of prototypical pad material without phenolic resin binder. The critical temperature shifts about 60 °C to a higher level.
Regarding a possible legislative regulation of particle number emissions, this shift of 60 °C is relevant, because the test procedure, planned PMP, allows brake temperatures of up to 170 °C at the front axle and 180 °C at the rear axle [21
]. Both temperatures are in the range of known critical temperatures and nucleation cannot be precluded during such a test.
Research question 2: How does thermal decomposition processes of pad materials influence the reproducibility of nucleation events?
The amount of generated nucleation particles decreases with ongoing brake events at one temperature level. This indicates a thermal aging effect of the pad material caused by decomposition. It is unknown, up to which thermal load the pad material can recover nor the penetration depth of decomposition processes for a specific thermal load. This question could be the subject of further investigation as the question rises, if nucleation processes are a real-world phenomenon or only for new pads under laboratory conditions.
5.2. Fine and Coarse Particles/PM10 Emission
Research question 3: How can the influence of disc temperature on PM10 emissions be quantified on a full-scale dynamometer setup?
This investigation revealed that single and advanced temperature triangles are a suitable method to quantify the influence of disc temperature on PM10
emissions of a full-scale disc brake system on an enclosed dynamometer. According to Farwick zum Hagen and Mathissen [2
] (p. 5148) a correlation between temperature and PM10
could not be found during real-world driving cycles like the LACT. This indicates that other influencing parameters are stronger than temperature, corresponding with other previous studies [22
]. In general, PM10
increases approximately by factor 2 within the temperature range of 80 to 160 °C.
Research question 4: How does thermal decomposition processes of pad materials affect the temperature dependency of PM10 emissions?
If the critical temperature is exceeded the PM10(T) and µ(T) behavior changes simultaneously from increasing to decreasing trend with an additional hysteresis shape. Due to this correlation, it is assumed that the decomposition of organic binder leads to a decrease of PM10(T). This assumption is reinforced by the observation that the pads with an inorganic binder do not change their behavior in a similar way. All tests revealed a correlation between PM10 and µ.
Research question 5: How does thermal decomposition processes of pad materials influence the reproducibility of the measurement of emission maps for PM10 depending on temperature?
Below the critical temperature, a repeatable temperature influence could be measured due to the usage of an advanced temperature triangle experiment, which consists of multiple triangles with increasing and decreasing number of temperature levels.
In addition to previous studies on the topic of influencing parameters on brake wear particle emission [17
], this work contributes to the quantification of the influencing parameters on the emission behavior of wear particles by passenger car disc brakes. Furthermore, a reduction approach for particle number emissions was presented and assessed by using inorganic binder materials for brake pads.