Tribological and mechanical behavior of multilayer Cu/SiC + Gr hybrid composites for brake friction material applications
Introduction
The potential use of Cu/SiC composites in frictional applications such as brakes and clutches has not yet been widely explored in spite of their wide use in thermal and electrical applications [1], [2], [3]. Good friction materials generally possess a high friction coefficient, high wear resistance, high thermal conductivity and thermo-mechanical stability, high toughness and a low coefficient of thermal expansion (CTE). These properties can be obtained in Cu/SiC metal matrix composite as noted by [4] at low load (15 N) and speed (1 m/s) conditions. Similarly, other studies on Cu/SiC or Cu/SiC + graphite(Gr) composites at very low speed () and load () conditions have also noted good wear resistance [2], [5], [6]. At higher sliding speeds and loads of more than 4 m/s and 100 N respectively, different wear mechanisms may be predominant due to complex interactions of thermal loading and deformation. Importantly, the localization of frictional heat on the sliding surface may cause thermally induced micro-cracking and a concomitant reduction in the wear resistance and fracture toughness [7]. To withstand the high interface temperatures, brake materials should possess high thermal stability and thermal conductivity to effectively dissipate the generated heat. Further, the crack resistance of the composites should be high to avoid catastrophic failure during emergency braking conditions. While the presence of ceramic particles improves the wear resistance, it proves detrimental to the overall thermal conductivity and crack resistance.
Traditionally, metal matrix composite brake pads comprise of a uniform distribution of ceramic particles in a metal matrix. Noting that the contrasting requirements of high wear resistance and thermal conductivity are separated spatially, Prabhu et al. [8] have studied the wear properties of layered Fe/SiC composites. In this approach, the layers of different physical, mechanical, and thermal properties are built into the composite to optimize the properties. These composites have been termed multi-layer composites (MLC) [9], [10], and are a subclass of functional graded materials. MLCs possess a sharp change in microstructure and/or composition along a given direction. MLCs have been found to be promising candidates in many applications such as in dental implants, thermal barrier coating, cutting tools, solid oxide fuel cells, semi-conductor devices, bullet-proof vests and armor plates [11]. Hunt et al. [12] and Erdogan [13] suggested that the presence of reinforced and unreinforced regions in the MLC reduced the driving force for crack propagation and thereby improve toughness. Arslana et al. [7] processed a three-layer alumina based composite by powder metallurgy and showed improved fracture toughness, damage resistance and thermal conductivity compared to conventional single layer composites. They also suggested that the layer structure may present potential in improving the wear resistance of the composites. The numerical model by Leon [14] showed that a discrete graded structure may improve damage resistance and toughness. We note that most of these studies on graded composites have been focused on the fracture mechanics aspects [13], [15]. A study of the wear resistance by Prabhu et al. [8] of multilayer Fe/SiC composites showed that the layer structure provides promising wear and friction properties compared to uniform Fe/SiC composites. However, to our knowledge, there have been no studies of the tribological properties of multilayer Cu matrix hybrid composites at a range of sliding speeds (5–35 m/s) and high load (2000 N) conditions.
In this work, we have studied layered composites with particle size gradients as well as composition gradient by varying the size and volume fraction of the SiC particles. The layered composites have an outer layer consisting of a composite to provide strength, thermal stability, wear resistance and an inner metallic layer to improve bulk thermal conductivity, toughness and damage resistance. In addition, the composite layers were hybridized with a graphite solid lubricant to stabilize the friction at high temperature [16]. Also, the addition of graphite in the Cu/SiC composites improves thermal conductivity, machinability, damping and anti-seizure properties [5], [6]. These layered composites were studied at a range of sliding speed conditions (5–35 m/s) to understand their dry sliding wear and friction behavior. In addition, the mechanical and microstructural characterization of the composites has been carried out. The potential wear mechanisms and elements/compounds present in the wear surface as well as the fracture surfaces were studied.
Section snippets
Materials processing
For the preparation of the composites, the raw material powders used were commercially pure powders of copper, tin, SiC, barium sulphate, graphite and zinc stearate. The mean size, purity, grade, apparent density and manufacturer of the powders are given in Table 1. We have processed two types of multilayer hybrid composites (MLCs) using powder metallurgy. In addition, single layer composites (SLCs) of two types were also prepared for the purpose of comparison. The sequence of the layering and
Density and porosity
The density and % porosity of the composites are shown in Table 3. The MLCs possess higher density than the SLCs due to the presence of high density Cu layer in the MLCs. However, it is interesting to see that the Cu layer in the MLC has a low porosity as seen in Fig. 5 primarily due to the higher diffusivity of Cu atoms. The higher density of MLCs shows that the sintering cycle chosen is appropriate for the fabrication of MLCs.
Characterization of layer interface
The layer structure of the representative MLC (CLS composite) and
Conclusions
We have studied wear behavior of Cu/SiC + Gr hybrid composites in three multi-layer and two single layer configurations. The tribological and mechanical behavior of the composites were also evaluated and compared. The important findings are the following:
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The wear resistance and braking performance of the composites are increased by the presence of a layer structure in the composite. Small size particles are better in providing better braking behavior and low wear rate. A layered structure is
Acknowledgments
We greatly acknowledge the materials and testing support rendered by HAL, Bangalore.
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