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.
Highlights
Cu-based metallic brake pads are commonly used for trains running over 250 km/h
Based on the contact angle data (Table 3), it is observed that both abrasives exhibit poor wettability by Cu [37, 38]; titanium carbide (TiC) shows better wettability by Fe than Al2O3 [37, 39] because there is a reciprocal transport of atoms between the molten Fe and TiC, which is not the case for molten Fe and Al2O3 [37]
The effect of the abrasives on the friction and wear performances was affected by the combination of the type, volume fraction, and initial braking speeds (IBS)
Summary
Cu-based metallic brake pads are 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]. The selection of abrasives is dependent on their hardness, fracture toughness, size, shape, content, and aggressiveness against the mated disks [2, 3]. According to Hu et al [4], the effect of abrasives on the friction performance depends on intrinsic factors such as mechanical properties, and on external factors such as the shape and size of the abrasive particles as well as the operating conditions
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