Properties and microstructure of nickel-coated graphite flakes/copper composites fabricated by spark plasma sintering

Copper (Cu) matrix composites reinforced with graphite flakes (GFs) were fabricated by spark plasma sintering, and electroless nickel plating was introduced to improve the interfacial bonding between GFs and Cu matrix. The microstructures and morphology of the composites were characterized by scanning electron microscopy and X-ray diffraction, and three-point bending test was performed to obtain the bending strength of the composites. The results showed obvious improvement in the bending properties and coefficient of thermal expansion (CTE) because of the introduction of Ni-P transition layer, and an ultralow negative CTE of -3.85 ppm K-1 for metal matrix composites was obtained for the first time. The volume shrinkage of GFs along c-axis direction under uniaxial compression stress as well as good interfacial bonding were considered to be the key reasons for the counterintuitive CTE of metal matrix composite with graphite inclusion. Furthermore, the obtained CTEs are in excellent agreement with the values reported by using the theoretical models. The ultralow CTE in Z direction, together with good mechanical properties make the composite fabricated in this study an appropriate candidate for electronic packaging material with two-dimensional heat dissipation function. (C) 2017 Elsevier Ltd. All rights reserved.