Research progress on friction and wear properties of powder metallurgy brake pad

Abstract

The terrain of China is relatively complex, which leads to the complex and changeable working conditions of high-speed trains during braking. Studying the influence under different braking conditions on the performance of train brake pads is a problem worthy of in-depth discussion. In order to explore the performance changes of powder metallurgy brake pads during train braking, this paper summarizes the friction and wear properties and wear mechanism of powder metallurgy brake pads under different braking conditions; combined with the research status of powder metallurgy brake pad materials, it provides directions for the research of new powder metallurgy materials. Provide reference for the improvement of brake pads of high-speed train in the future.

Keywords

High-speed train braking condition friction wear powder metallurgy brake pad

Introduction

The operation of the Shinkansen in Japan in the 1960s marked the dawn of the high-speed era. China’s high-speed railway started late, but it has developed rapidly. From the “Four Vertical and Four Horizontal” in 2008 to “Eight Vertical and Eight Horizontal” today, China has built a relatively complete railway network and high-speed railway network.^1,2 By 2020, China’s high-speed railway has operated for nearly 38,000 km. With the development of high-speed railways, the speed of trains has gradually increased, and the operating speed of trains has become one of the important indicators to measure the development level of a country’s railways.³ As the speed of the train increases, the braking performance of the train has become more important. The brake pad is an important component of a high-speed train braking system, its friction and wear properties and wear mechanism has become the focus of scholars.

The performance of the brake pad relates to the protection and comfort of train operation, and train braking will occur in various situations, such high speed, high pressure, different weather, etc. which requires the brake pads to own a stable coefficient of friction, low wear rate and other properties.⁴ At this stage, most of the operating high-speed trains use copper-based powder metallurgy brake pads, and a few used iron-based powder metallurgy brake pads.⁵ In addition, the application fields of powder metallurgy brake pads continue to expand, such as wind turbines, aircraft, automobiles, etc. And the application prospects are broad.

Based on the higher than, the friction and wear properties and wear mechanism of metallurgy brake pads studied during this paper. This paper expounds the development status of powder metallurgy brake pad materials, and summarizes the influence of initial braking speed, braking pressure, rain and snow weather on the performance of the powder metallurgy brake pads, and makes a prospect for the research of powder metallurgy brake pads. It will provide references for the development and performance improvement of powder metallurgy brake pad in the future.

Research status of powder metallurgy brake pad materials

Powder metallurgy brake pads mainly include copper-based powder metallurgy brake pads and iron-based powder metallurgy brake pads. The mature iron-based powder metallurgy brake pad has the advantages of high friction coefficient, low wear rate, good heat resistance, and strong oxidation resistance.⁶ Copper-based powder metallurgy brake pads have the characteristics of excellent wear resistance and environmental adaptability, stable friction coefficient, and under high temperature conditions, the matrix strength of copper-based powder metallurgy brake pads is high, so the damage to the brake disk is small, and it is most used on high-speed trains.^7,37 However, due to the different selection of copper, there will be a big difference in its performance. For example, copper and brass are rubbed with steel disks respectively, and brass shows a more stable friction coefficient.⁸

Research status of iron-based brake pad materials

Conventional Fe-C-based brake pads have many problems such as poor high temperature oxidation resistance, low coefficient of friction and high wear rate.⁹ In order to solve these problems, Shi et al.^10,11 added Ni element in the matrix group of Fe-C-based brake pads, which improved the wear resistance, oxidation resistance and matrix strength of the brake pad. Xiang¹² added the mullite powder to the Fe-C-Ni-based powder metallurgy brake pads, which improved heat resistance, friction coefficient, and wear resistance of the iron-based brake pad.¹³ However, due to the affinity of iron-based brake pad with their counterparts, it is easy to adhere to the brake disk and cause great damage to the brake disk. Therefore, iron-based brake pads are rarely used on the high-speed trains.^14,15

Research status of copper-based gate materials

Copper-based powder metallurgy brake pad is made by pressing and sintering Cu as the matrix, Fe and Sn as alloy elements, SiO₂ as friction components, graphite and Pb as lubricating components, supplemented by other additives.¹⁶ To improve the performance of the copper-based brake pad, Li¹⁷ added Al to the matrix material and Al₂O₃ to the friction components to obtain the aluminum bronze-based powder metallurgy brake pad, which was similar to ordinary copper-based brake pads. Compared with brake pads, matrix strength, corrosion resistance and heat resistance are greatly improved. However, the addition of Al₂O₃ has a great influence on the friction performance of the brake pad, which leads to an increase of the wear amount and a sudden change of the friction coefficient. In order to reduce the wear rate and improve the friction coefficient, some scholars add a certain amount of ZrO₂ to the friction components of copper-based powder metallurgy brake pads.¹⁸ Some scholars added the copper-clad iron powder to the brake pad,¹⁹ while reducing the wear rate, the porosity was reduced and the strength of the brake pad matrix was increased. Hao et al.²⁰ added different ceramic fiber materials to copper-based powder metallurgy brake pads, and found that wollastonite fibers can improve the physical performance of the brake pad and at the same time have the effect of increasing friction; the addition of aluminum silicate fiber mainly make the brake pad improves the wear resistance; however, the fiber material will reduce the density and hardness of the brake pad, and will reduce the friction coefficient of the brake pad, thereby the brake pad braking performance was reduced. Zhou et al.²¹ explored the friction and wear properties of copper-based powder metallurgy materials by changing the content of SiC, and found that with the increase of SiC content, the friction coefficient first decreased and then increased, added excess SiC can improve friction and wear performance under heavy load. However, when the SiC content in copper-based powder metallurgy materials is too high, the porosity of the material will increase, causing some hard particles to fall off, and causing serious damage to the brake disk. In addition, Zhang et al.²² added Ti₃SiC₂ as a lubricating material to the brake pad, the study found that the strength of the copper-based powder metallurgy gate prepared by using Ti₃SiC₂ is better than that of traditional materials, and it has good high temperature oxidation resistance, but it is necessary to continue to study the ratio of Ti₃SiC₂ to friction components and other lubricating components, so as to optimize the performance of the brake pads.

At present, due to less use of iron-based powder metallurgy brake pads, the subsequent chapters will focus on the friction and wear performance of copper-based powder metallurgy brake pads under different braking conditions.

Influence of braking conditions on brake pad performance

The braking conditions such as initial braking speed, braking pressure and climate are the factors that affect the braking performance of copper-based powder metallurgy pad. Among them, the changing trend of friction coefficient and wear rate of brake pad is affected by initial braking speed; the braking pressure mainly affects the real friction area of the brake pad, thereby changing the wear mechanism of the brake pad; different climatic conditions affect the friction temperature of the brake pad and the wear of brake disks.

By reason of the long service period of the brake pad, it is difficult to carry out the train tracking test. Therefore, to study the friction and wear properties of the powder metallurgy brake pad usually used test equipment such as a 1:1 brake test bench, and friction and wear test machine. Using this kind of test equipment, the friction and wear performance of the brake pad under different braking conditions can be explored.^23,24

Influence of initial braking speed on brake pad performance

The initial braking speed is the main element influencing the properties of powder metallurgy brake pad.²⁵ When the train is braking, the size of the initial speed affects the change of the wear rate and the friction coefficient of the brake pad, thus influencing the braking properties of the brake pad.

Gao et al.²⁶ and Li et al.²⁷ studied the friction and wear and heat dissipation performance of copper-based powder metallurgy brake pads with different initial braking speed by using a 1:1 braking test bench. Gao et al.²⁶ made a copper-based powder metallurgy brake pad, they found that the self-made brake has good thermal conductivity, low noise and low vibration, and the change of friction coefficient was in line with the standard of the International Union of Railways at that time. However, due to the backward preparation technology at that time, the wear of the brake pad developed was too large. To reduce the amount of wear, Li et al.²⁷ adjusted the ratio of abrasive to graphite, which not only reduced the wear of the brake pads, but also reduced the damage to the brake disk, and in the friction test within the initial braking speed of 200–350 km/h, they found that the friction coefficient curve of the brake pad is stable, basically between 0.32 and 0.42; and in the 350 km/h high-speed braking test, the maximum temperature of the brake disk was 520°C, during the whole test process, the brake disk did not produce hot spots, and no obvious scratches, grooves and other damage were found. It is shown that the brake pad had great friction and wear performance, and the damage to the pair was small.

Under the condition of constant braking pressure, with the braking speed increased the wear amount of copper-based powder metallurgy brake pad increased, and the friction coefficient decreased slowly, the lower of the speed, the greater and more unstable the friction coefficient of the brake pad, and the higher the speed, the smaller of friction coefficient and the more stable.^28,29 The reason for this change was: at low speed, the temperature of the brake pad surface was low, matrix strength is basically unchanged, and the friction coefficient shows a high value; at high speed, the copper matrix softens at high temperature, which increased the flow of the friction film and reduced the friction coefficient.³⁰

During braking, the wear mechanism of copper-based powder metallurgy brake pad under different initial braking speed is also the focus of scholars. Dong³¹ explored the wear mechanism of copper-based powder metallurgy brake pad in the speed range of 120–190 km/h through a friction and wear testing machine. They found that at low speed it was mainly to overcome the meshing, and at high speed it was mainly to destroy and regenerate the oxide film. To explore the wear mechanism of the brake pad in the higher speed range, Wang and Ru³² used a friction and wear tester to explore the wear mechanism of the brake pad in the range of 60–380 km/h. They found that the brake pad wear was mainly caused by the combined effect of oxidation wear, abrasive wear and fatigue wear. Jayashree et al.³³ conducted tests at speeds of 1.57 and 7 m/s respectively, and the study was given that at 1.57 m/s, the abrasive wear which mainly wore of the brake pad; at 7 m/s, the oxidative wear which mainly wore of the brake pad, however, there were only two speeds in the study, and the continuous change process of the wear mechanism with the speed was not obvious. Wang et al.³⁴ studied the continuous change process of the wear mechanism of copper-based powder metallurgy brake pads in the range of 120–200 km/h by using a friction and wear testing machine. They found that at low speed, the wear of the brake pads is fatigue wear; with the braking speed increased, the wear of the brake pad turns into adhesive wear accompanied by abrasive wear; as the speed further increased, the brake pad wear was oxidation wear. Zhang and Deng³⁵ studied the wear mechanism of the brake pad changed with the speed that was given on the friction and wear tester. At low speed, the actual friction area was small, and the brake pad surface was rough. The wear mechanism was mainly abrasive wear. As the speed increased, the temperature of the friction surface increased, and the matrix softened. The wear mechanism was mainly adhesive grinding and accompanied by oxidative wear. The wear mechanisms they give vary at low speeds, this was due to the different low-speed speeds of the test and the different materials and manufacturing processes of the brake pads; the wear mechanism at high speed was oxidative wear. This was because, at high speed, the temperature of the brake pad rose, which oxidizes the brake pad surface, resulting in oxidative wear.

When the train brakes, not only the brake pad is required to have a higher friction coefficient, but also the friction coefficient is required to have high stability, therefore, the stability coefficient of the friction is used to describe the stability of the friction coefficient. The stability coefficient of friction coefficient can be calculated by formula (1)¹⁶:

α = \bar{μ} / μ_{\max}

(1)

In the formula $α$ is the stability coefficient of friction coefficient; $\bar{μ}$ and $μ_{\max}$ are the average friction coefficient and the maximum friction coefficient under a certain initial braking speed respectively.

Zhu et al.³⁶ explored the changes of wear rate and friction coefficient of copper-based powder metallurgy brake pad under the same braking pressure and different initial braking speed through a friction and wear testing machine. They found with the initial braking speed increased that the wear rate increased first and then decreased; the change of friction coefficient and friction stability coefficient of the brake pad was shown in Table 1. From Table 1 the friction coefficient showed a trend of increasing first and then decreasing, but the stability coefficient was above 90%. It was shown that the brake pad friction coefficient had high stability.

Table 1.

Friction stability coefficient of brake pad under different braking speed.³⁶

Item	Braking speed (km/h)
	100	150	200	250	300
$\bar{μ}$	0.26	0.30	0.33	0.31	0.31
$μ \max$	0.28	0.32	0.34	0.32	0.33
$α$ (%)	92.9	93.8	97.1	96.9	93.9

The dual friction pairs of diverse materials have a great impact on the performance of the powder metallurgy brake pad. Wang et al.²⁵ conducted a friction test with copper-based powder metallurgy materials using aluminum balls, steel balls and ceramic balls under the same pressure conditions. The study found that with the initial braking speed increased, the steel ball and the ceramic ball instantaneous friction coefficient were changed gradually. Their average friction coefficients all showed a downward trend, and the wear amount of copper-based powder metallurgy materials showed an upward trend, and when ceramic balls were matched with copper-based powder metallurgy materials, the wear amount of copper-based materials was greater than that of steel balls. The instantaneous friction coefficient of the aluminum ball fluctuated obviously, and after friction, large-scale furrow like wear marks appear on the surface of the aluminum ball, and the wear of the aluminum ball is large. This was shown that aluminum brake disks were not suitable for high-speed trains and other facilities. The ceramic brake disks and cast steel brake disks have excellent performance and can be used in high-speed trains, airplanes, high-performance sports cars and other equipment. However, after friction, the ceramic brake disk surface will quickly accumulate the heat, and the heat dissipation will be slow, which will cause the copper-based powder metallurgy brake pad to soften in advance. Therefore, the heat dissipation of the ceramic brake disk was an urgent problem to be solved.

Influence of brake pressure on the performance of brake pads

In the process of train braking, the braking pressure was a significant impact on the brake pad performance. In order to study the brake pressure how effect the performance of the brake pad, under the condition of constant initial braking speed, Zeng and Li³⁷ used a friction and wear tester to research the friction and wear properties of copper-based powder metallurgy brake pad within the braking pressure of 0.2–1.0 MPa. The result was shown in Figure 1, as the braking pressure increased, the brake pad friction coefficient increased first and then decreased, and the wear amount increased continuously; this was since with the braking pressure increased, the micro-protrusions on the surface of the brake pads were destroyed, resulting in increased friction coefficient and wear; as the braking pressure increased again, the friction surface temperature increased, which lead to the softening of the brake pad matrix, which slowed the increase in the wear amount and reduced the friction coefficient.³⁸ However, the research process was limited to the comparison of different braking pressures between the same braking speed. Zhang et al.³⁹ tested the friction and wear properties of brake pads under different pressures at speed of 28 and 69 m/s, respectively. The results showed that at 28 m/s, with the increase of braking pressure the brake pad friction coefficient increased, this was because at low speed, with the increase of braking pressure, the actual contact friction area increased, which increased the wear rate and the friction coefficient; at 69 m/s, the brake pad friction coefficient increased first and then decreased, but the wear rate decreased, this was because at high speed with the braking pressure increased, the softening of the copper matrix reduced the friction coefficient and thus the wear rate. To study further increase the braking pressure, and the performance change of the brake pads. Yang et al.⁴⁰ verified that the friction coefficient and temperature of the brake pads have a monotonically increasing relationship with the braking pressure in the range of the braking pressure of 1.0–10.0 MPa through the friction and wear test machine, this may be due to the increase of the test pressure, so that the oxide film on the brake pad surface has been in a state of destruction and regeneration, lead to an increase of the friction coefficient and wear rate of the brake pad. Ma et al.⁴¹ measured the friction performance of copper-based brake pads in the high braking pressure range (10.0–56.0 MPa). The verification has shown that the brake pad friction coefficient decreased with the increase of the braking pressure, this was because as the braking pressure increased, the lubricating components of the brake pad will form a smooth and complete friction film, thereby reducing the friction coefficient.

Figure 1.

Variation curve of friction coefficient and wear amount of brake pad under different pressure.³⁷

It is also a hot spot to study the wear mechanism of copper-based powder metallurgy brake pads under different braking pressures. Gao et al.²⁶ used a constant speed friction tester to conduct intermittent and continuous braking tests on copper-based powder metallurgy brake pads within the braking pressure range of 0.45–0.9 MPa. They found that at low pressure, due to the small load, the heat generated by friction was limited, the strength of the matrix was basically unchanged, and the friction coefficient showed a higher value; at medium pressure, the temperature rose, causing the matrix to soften and the friction coefficient to decrease; at high pressure, the continuous rise of temperature causes the friction surface to form a third body,^42,43 in addition, the further oxidation of the third body increased the hard oxide, thereby increased the friction coefficient.

Zhu et al.^36,44 used a friction and wear tester to test and analyze the wear mechanism of the brake pads at a speed of 200 km/h and a braking pressure of 0.2–1.0 MPa. The study believed that when the pressure was low, some hard particles were mainly combined with the matrix, which led to an increase in the wear rate of the brake pad; when the pressure was high, due to the increased temperature, the fluidity of the third body generated by friction increase, so that the wear rate of the brake pad tended to be stable. Zhang et al.³⁹ showed that at low speed and low pressure, the heat generated by friction was less, and a continuous oxide film cannot be formed on the friction surface, and adhesive wear dominates, which led to an increase in friction coefficient and wear rate; Under high speed and high pressure, more heat was generated by friction, continuous oxide film was formed on the friction surface, and oxidative wear dominates, thereby reduced the friction coefficient and wear rate of the brake pad. Since the friction tested in Gao et al.²⁶ was carried out from high speed to low speed, at high speed and high pressure, the friction surface had formed a dense third body, so its friction coefficient showed a high value; In other kinds of literature, the load was applied gradually, so the friction coefficient decreased under high pressure.

Influence of weather conditions on brake performance

During the actual operation of the train, various weather conditions will be encountered. In order to study the influence of rainy days on the performance of powder metallurgy brake pads, Han et al.⁴⁵ studied the friction and wear properties of the brake pads which work under wet conditions by using a pin-on-disk friction machine. They found that the temperature of the friction surface was lower in low-speed areas, and the water film formed on the friction surface played the role of isolation and lubrication, and reduced the friction coefficient and wear rate; the temperature generated by friction was high in high-speed areas, which was disadvantageous to the water film formation, but the moisture reduced the softening degree of the matrix, so that the wet friction coefficient was large. In order to further explored the changes of the brake pad surface temperature and friction coefficient under wet and dry conditions, Fu et al.⁴⁶ carried out the test by constant speed friction machine, they found that under wet conditions, it could not only reduce the temperature of friction surface of copper-based powder metallurgy brake pad, but also reduced the wear rate of brake pad under high pressure. This may be due to the formation of a water film on the friction surface, which acted to isolate the micro-protrusions on the brake pad surface, thereby reducing the wear rate. In order to further explore the changes of the brake pad surface temperature and friction coefficient under wet and dry conditions, Wang et al.⁴⁷ used a 1:1 braking dynamic test bench to conduct braking tests on copper-based powder metallurgy brake pads under dry and wet conditions, the study found that under the same initial speed, regardless of dry or wet conditions, as braking pressure increases, the average friction coefficient decreased for copper-based powder metallurgy brake pads, and the surface temperature of brake disk increased; under wet conditions, the temperature and friction coefficient of the brake pad was lower than those dry conditions, but at high pressure and high speed, the friction coefficient was very similar. This was because, under high pressure, the friction surface temperature was too high, so that the friction surface lost the water film lubricating effect, so the average coefficient of friction was very similar.

In northern cities, there will have alpine and snowy weather in winter. In order to study the influence of alpine and snow weather on the performance of powder metallurgy brake pads, Qian et al.⁴⁸ conducted an experimental study on the friction coefficient of copper-based powder metallurgy brake pads under alpine conditions using a 1:1 brake test bench. They found that the friction coefficient increased at low speed, and the change was not obvious at high speed; when the brake pads were covered with ice and snow, the damage to the brake disk was greater. This was because the hard substances in the ice and snow enter the friction surface, resulting in scratches on the brake disk.

Wu et al.⁴⁹ found that under the low temperature snowmaking environment of −15°C, the average friction coefficient of the brake pad increased with the increase of the braking pressure, but when there was residual sand or debris on the friction surface, the friction coefficient decreased at a certain speed, will decline due to changes in the “third body.” Zhang et al.⁵⁰ further studied the change of properties of copper-based powder metallurgy brake pads at −20°C, and found that at −20°C, the friction coefficient of the brake pads increased firstly, with the increase of braking pressure, they decreased; when the dynamic speed increased, the friction coefficient first decreased and then increased; and the wear rate was slightly higher than that at 20°C.

In order to adapt to this extreme weather, Li et al.⁵¹ designed copper-based powder metallurgy brake pads in alpine conditions, and verified the pure air emergency braking and the emergency braking on ice and snow weather through the 1:1 braking test. The test showed that the average friction coefficient could be stabilized between 0.35 and 0.42 in pure air or in ice and snow weather, and the surfaces of the brake pad and disk were in good condition without hot spots, cracks and block drops. Wu et al.⁵² established a brake disk wear model in ice and snow weather, and further explained the wear mechanism, as shown in Figure 2. Before braking, the surface of the brake pad was covered with ice involved plenty of hard particles (Figure 2(a)); at the beginning of the braking, the ice melting on the brake pad surface, and the friction surface is scratched by the hard particles in the ice and damaged as a result. The chips were generated, and as the brake pressure and temperature increased, the chips joined together (Figure 2(b)); the newly generated chips stick to the pre-existing chips and were sandwiched between the pads and the brake disk (Figure 2(c)); the large chips scratch the brake disk, and the accumulation became thicker and thicker (Figure 2(d)), which led to serious wear of the brake disk. When the train brakes in high-cold, ice-snow weather, the hard substances in the ice and snow will damage the brake disk greatly, and the low temperature environment will also affect the wear rate. Therefore, the development of brake pads and brake disks suitable for high-cold and ice-weather can reduce loss and improve braking capacity.

Figure 2.

Model of the severe wear process of the brake disk⁵²: (a) before braking, (b) hard particles scratch the brake disc and the resulting chips, (c) the chips that join together, and (d) the chips that join together form waste.

Conclusion

Powder metallurgy brake pads have the advantages of good thermal conductivity, wear resistance and stable friction coefficient. They define a wide variety of high-speed railways, automobiles, aircraft, and other fields. Although scholars have made phased progress in the powder metallurgy brake pads, there are still deficiencies for brake pads in the research of new materials and friction and wear properties. At present, the research results on the influence of powder metallurgy on the friction and wear performance of brake pads and the work required to improve the performance of brake pads in the future are as follows:

For powder metallurgy brake pads, the research on friction materials at this stage has been applied to the brake pads of high-speed trains whose speeds are up to 350 km/h or even higher, such as graphite, MoS₂, Ti₃SiC₂, etc. as lubricating components, Al₂O₃, ZrO₂, etc. as friction components; In the future development, it is necessary to continuously study new materials with excellent performance and economy applicative, and develop new material brake pads suitable for trains with higher speeds. For example, metal fiber materials can improve the strength of the matrix and prevent cracks in the friction materials; nano-friction materials can effectively reduce the porosity and improve the wear resistance of the friction materials.

The braking test of powder metallurgy brake pad can show the performance of the brake pad in an all-around way. At this stage, the research on the friction and wear performance of the brake pad mainly uses test equipment such as a brake bench, but this equipment still has a big gap with the actual operation of the train, such as the coupling of braking, vibration, and impact in curve. In the future, we need to consider the combination of various conditions in the brake performance test, and supplemented by numerical analysis, so as to achieve the effect of simulating the actual situation and make the test results more convincing.

Footnotes

Acknowledgements

The authors thank the Institute for Complex Networks and Visualization for their support.

Handling Editor: Chenhui Liang

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors received National Natural Science Foundation of China (79970045, 61179023) and Digital Twins Mega-Science Center of High Platform Construction “112 Plan,” Qingdao University of Technology (10606029) for the research of this article.

ORCID iD

Jiawei Chen

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