Processing and study of the wear and friction behaviour of discrete graded Cu hybrid composites

Discrete functionally graded composites are the novel composites which have high potential in the brake friction material applications. In this paper, we have prepared discrete functional graded Cu/10%SiC/ 20%graphite(Gr)/10%boron nitride (h-BN) hybrid composites by the layer stacking compaction and pressure sintering techniques. We have considered two types of composites based on h-BN particle sizes. The size ranges of h-BN used were 140–180 and 3–25 µm. The friction and wear properties of the composites were evaluated in a laboratory scale brake inertial dynamometer at low (5, 10 m s-1) and high sliding speeds (30, 35 m s-1) and, high braking load (2000 N) conditions. In addition, we have performed microstructure characterization, density, hardness and flexural strength measurements. Wear surface morphology studies were also carried out using stereoscope and scanning electron microscope. Our experiments lead to the following important results: (1) the large size h-BN particle improves the densification of the hybridized composite layer and provides higher wear resistance and better braking performance at all sliding speeds, (2) the wear loss (by mass) and the stopping distance/time increase with sliding speeds due to the increase in the braking energy, (3) at low sliding speeds (5, 10 m s-1), abrasive wear is the main wear mechanism, whereas many different wear mechanisms (delamination, oxidation, abrasive) are cooccuring at higher sliding speeds (30, 35 m s-1), (4) the mechanical properties (flexural strength and surface hardness) of composites are not affected by the h-BN particle size, (5) the incorporation of copper layer in the discrete layer structure deflects and arrests the crack at the copper/composite layer interface, thus improving the fracture resistance in addition to improving the bulk thermal conductivity.