Elsevier

Applied Surface Science

Volume 258, Issue 5, 15 December 2011, Pages 1862-1868
Applied Surface Science

Possible stibnite transformation at the friction surface of the semi-metallic friction composites designed for car brake linings

https://doi.org/10.1016/j.apsusc.2011.10.063Get rights and content

Abstract

After a friction process several changes in phase composition of friction composites are often registered. High temperature, accompanied by high pressure induced during braking can cause initiation of chemical reactions which do not run at room or elevated temperatures under the atmospheric pressure. Most of the studies in the field of tribochemistry at friction surfaces of automotive semi-metallic brake linings deal with phenolic resin degradation and corrosion of metallic components. The paper addresses the formation of elemental antimony as well as the alloying process of iron with antimony observed on the surface of laboratory prepared semi-metallic friction composites containing stibnite. The role of alumina abrasives in the process of stibnite transformation is also discussed and mechanism of stibnite transformation was outlined.

Highlights

► Transformation of Sb2S3 in semi-metallic fiction composites for car brake lining. ► Antimony formation during friction process of brake linings. ► Alloying of iron from steel wool by antimony and ɛ-FeSb and FeSb2 formation

Introduction

Semi-metallic friction composites are commonly used in the manufacturing of car brake linings. There are several reports providing comprehensive review on the formulation and the preparation of the friction composites suitable for car brake lining manufacturing [1], [2], [3], [4]. The components used in formulations of friction composites can be divided into four basic groups: (i) binders (e.g. phenolic resin), (ii) abrasives (zircon, alumina, etc.), (iii) fillers (barite, calcite, different nut shells, etc.) and iv) functional fillers (e.g. solid lubricants – metal sulfides, graphite). The required functionality of the given friction composite is closely related to the proper selection of components from these four groups. The metallic ingredients increase wear resistance, hardness of composites and improve their thermal diffusivity, and also play an important role in the formation of primary contact plateaus. These plateaus are in direct contact with the rotating counterpart (typically cast iron disc) and transfer the pressure (typically 1.2 MPa during soft braking and 10 MPa in extreme situations) which originates during the braking. The pad covers approximately 10% of the corresponding rubbing surface, and contact plateaus cover typically 15–20% of a brake lining surface [5]. The function of the braking counterparts is to decelerate the velocity of a vehicle, whereas the kinetic energy of rotor is transferred into thermal energy what is reflected in the increasing of the temperature of both braking counterparts. The rotating disc is intermittently in contact with the lining during braking and its temperature rarely reaches values higher than 400 °C [6]. On the other hand the brake pad is in contact with the rotating disc during the braking, and the local overheating of the friction surface can rise to a considerably higher temperature than is the mean temperature of a cast iron rotor.

The high temperature on the friction surface, together with the direct, sliding contact of the brake lining with the rotor are responsible for mechanochemical reactions, in this case the tribochemical reactions, which occur during braking. Due to the complex composition of brake linings (common friction composites contain typically 7–20 ingredients) it is difficult to describe all tribochemical reactions in detail. A typical phenomenon occurring at lower temperatures is a degradation of phenolic resin. The onset temperature of phenolic resin degradation is dependent on its nature, modification, and presence of metals which act as catalysts for its degradation [7]. Other reported tribochemical reactions comprise oxidation of metallic components, e.g. iron or copper [8].

Stibnite (Sb2S3) and molybdenite (MoS2) are the most common solid lubricants used in friction composites designed for car brake lining applications, and their effect on friction-wear performance is described in a number of research works, e.g. in [9], [10], [11]. The melting point of Sb2S3 is 550 °C and for MoS2 is 1185 °C. In the presence of oxygen Sb2S3 undergoes an oxidation process, whereas at lower oxygen concentration Sb2O3 originates while in an oxygen rich environment Sb2O4 is formed. In addition to the oxygen concentration, temperature also plays a significant role in the given antimony oxide formation. The oxidation of Sb2S3 starts to occur in a temperature range between 290 and 340 °C when the product of the reaction is Sb2O3, at higher temperatures approx. 500 °C Sb2O4 begins to form. Chemical equations related to both oxides formation are described by Eq. (1a) and (1b), respectively [12].2Sb2S3 + 9O2 = 2Sb2O3 + 6SO2Sb2S3 + 5O2 = Sb2O4 + 3SO2

Chemical and phase changes induced by the friction process of the friction composites containing Sb2S3 and MoS2 as the solid lubricants were studied by Filip et al. [13]. The authors did not observe any oxidation of stibnite during the friction process even if the temperature reached 700 °C. Kim et al. [14] suggested that formation of antimony oxides at elevated temperatures protect the occurring of the fade phenomenon. If the Sb2O3 is formed, its reaction with Sb2S3 can occur (Eq. (2)) to form the elemental antimony [12].2Sb2O3 + Sb2S3 = 6Sb + 3SO2

Different forms of carbon, mainly graphite, represent common components of friction composites. The content of carbon increases in the friction layer as a result of the organic matter degradation. In the presence of carbon antimony oxides undergo reduction process and metallic antimony can originate; see the reaction (3) for Sb2O4 [15]:Sb2O4 + 4C = 2Sb + 4CO

In semi-metallic friction composites the presence of metals in different forms (chips, wool, and powder) can rise to 50 vol.%. The iron chips or the steel wool are among the most common metallic ingredients. In the presence of iron, and at elevated temperatures, Sb2S3 undergoes reaction according to the following reaction scheme (4) [15].Sb2S3 + 3Fe = 3FeS + 2Sb

Baláž et al. [16] used the reaction (4) for nanosized antimony preparation using high energy ball milling of the mixture of Sb2S3 and Fe powder. Using the XRD method, and the measurement of magnetization data, the authors approved completion of reaction (4) after 120 min long milling.

Recently some authors have mentioned brake pads as a significant source of antimony entering the living environment [17]. Lijima et al. [18] estimated the average emission of antimony in the form of brake dust with particle size characterized as PM2.5 to reach 6.3 ton/yr in Japan. Sb2O3, as one of the possible antimony compounds entering the living environment, is classified as a compound with possible carcinogenic effect to humans (Group 2B) [19], but there still exists more the hypothesis about its presence in brake dusts than scientific confirmation.

The aim of this work is to study the stibnite transformation on the surface of the semi-metallic friction composites during the friction process, as well as to discuss the effect of alumina abrasives on its transformation.

Section snippets

Sample preparation

The designed formulations of the semi-metallic friction composite without abrasives assigned as SM_0, and the composite with alumina abrasives assigned as SM_Al, are listed in Table 1.

All the raw materials were mixed using a blender (Electrolux EBR100) for 2 min. After the homogenization the prepared mixture was pressed by the JFY60 molder made by Jilin Wanda Mechanical Co., Ltd. for 6 min at 165 °C and 25 MPa. The friction composites were post-cured for 60 min at 120 °C, next for 60 min at 150 °C, and

Friction performance and SEM study of the friction layer

The graphical presentation of the temperature dependency of coefficient of friction (COF) on the rotor temperature is depicted in Fig. 2. Although the addition of the Al2O3 abrasive increases the value of COF, the trend of COF temperature dependency is very similar for both composites. Decrease in the COF value at a temperature higher than 250 °C is connected with degradation of phenolic resin which serves as a binder. Expected stabilization of COF due to the formation of antimony oxides as

Conclusions

Using the data obtained with a combination of SEM-EDX, DTA, and XRD the following conclusions can be proposed:

  • 1.

    The character of the friction surface significantly differs for the composite with and without the alumina abrasives. On the sample without the abrasive (SM_0), the primary contact plateaus presented by the steel wool cover the prevailing part of the friction surface. On the friction surface of the sample with alumina (SM_Al), the iron primary contact plateaus are extensively covered

Acknowledgement

Financial supports from the Ministry of Education of the Czech Republic (projects ME10121, CZ.1.05/2.1.00/01.0040), National Natural Science Foundation of China 50673012 and Sino-Czech Cooperation Project (39-9) are gratefully acknowledged. The authors would like to thank Daniel Casten for language corrections.

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