Role of titanium carbide and alumina on the friction increment for Cu-based metallic brake pads under different initial
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ISSN 2223-7690 CN 10-1237/TH
RESEARCH ARTICLE
Role of titanium carbide and alumina on the friction increment for Cu-based metallic brake pads under different initial braking speeds Tao PENG, Qingzhi YAN*, Xiaolu ZHANG, Yan ZHUANG Laboratory of Special Ceramics and Powder Metallurgy, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Received: 10 August 2019 / Revised: 25 November 2019 / Accepted: 03 August 2020
© The author(s) 2020. Abstract: To understand the effect of abrasives on increasing friction in Cu-based metallic pads under different braking speeds, pad materials with two typical abrasives, titanium carbide (TiC) and alumina (Al2O3), were produced and tested using a scale dynamometer under various initial braking speeds (IBS). The results showed that at IBS lower than 250 km/h, both TiC and Al2O3 particles acted as hard points and exhibited similar friction-increasing behavior, where the increase in friction was not only enhanced as IBS increased, but also enhanced by increasing the volume fraction of the abrasives. However, at higher IBS, the friction increase was limited by the bonding behavior between the matrix and abrasives. Under these conditions, the composite containing TiC showed a better friction-increasing effect and wear resistance than the composite containing Al2O3 because of its superior particle-matrix bonding and coefficient of thermal expansion (CTE) compatibility. Because of the poor interface bonding between the matrix and Al2O3, a transition phenomenon exists in the Al2O3-reinforced composite, in which the friction-increasing effect diminished when IBS exceeded a certain value. Keywords: high-speed train; friction mechanisms; brake pads; abrasives
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Introduction
Cu-based metallic brake pads are now commonly used for trains running over 250 km/h. They are designed to stop the train at a safe distance. To meet this goal, pad manufacturers usually add some abrasives to the materials because they believe that abrasives can prevent the buildup of friction films on brake surfaces, enhance bite or engagement, and improve brake efficiency [1]. Generally, the selection of abrasives is dependent on their hardness, fracture toughness, size, shape, content, and aggressiveness against the mated disks [2, 3]. However, the principles behind the selection of abrasives for brake pad materials are poorly understood owing to the difficulty in
determining the exact role of abrasives on the contact surfaces of multiphase inhomogeneous materials. According to Hu et al. [4], the effect of abrasives on the friction performance not only depends on intrinsic factors such as mechanical properties, but also on external factors such as the shape and size of the abrasive particles as well as the operating conditions. In the past, many studies have focused on hardness, fracture toughness, stiffness [3, 5–7], size, shape, and volume fraction of the abrasive [5, 8–11], as well as the operating conditions [12–16]. Among these studies, the abrasive wa
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