Graphitization Of Nanodiamond And Properties Of Nanodiamond/Cu Composite

Nanodiamond (ND) powders synthesized by explosive detonation possess not only excellent characteristics of bulk diamond such as superhardness, chemical stability and high thermal conductivity, but also special properties of nanomaterials. Therefore, ND powders as a kind of promising reinforcement have great application potentials in the field of copper matrix composite. In the paper, ND reinforced copper matrix composite (ND/Cu) was prepared by powder metallurgy technique. Graphitization of ND and the composite properties ( tensile strength, softening temperature, wear resistance and corresponding strengthening mechanism ) were mainly investigated.The microstructural transformation and graphitized mechanism of ND were discussed after the nanoparticles were annealed at 900～1400°C. The onion-like carbons began to form in the range of 1100～1200°C and the graphitized temperature of diamond nanoparticles changed with the crystalline degree corresponding to nanoparticle size. Annealed at 1400°C for 60min all the diamond nanoparticles could transform into onion-like carbons, and there existed an intermediary phase–Bucky diamond with a diamond core encased in fullerene-like shells. The transformation process of onion-like carbons from ND included: formation of graphite fragments with hexagonal carbon rings, connection and curvature of graphite sheets around the surface of nanoparticles, closure of graphite layers from the particle surface towards the center. Composite powders (ND+Cu) after ball milling were cold pressed and sintered. We have obtained the optimized parameters of powder metallurgy by analyzing the effect of initial pressure, sintering temperature and time on composite microstructure and properties. ND/Cu composite was prepared by the optimized process and the effect of ND content on composite properties was discussed. The results showed that low ND content (less than 1.0wt.%) could obviously improve the tensile strength, thermal stability and wear resistance of composite. When the ND content exceeded 1.0wt.%, the tensile strength of composite decreased owing to the aggregating of ND.The effect of surface modification on ND dispersion and composite properties was investigated. After annealed at 1000°C for 60min, ND aggregates were diminished due to the modification of surface structure and surface chemistry. The electrical conductivity and strength of composite were increased by surface modification of ND. The 1.0wt.% ND/Cu composite possessed preferable properties of electrical conductivity>80%IACS, hardness>HV120, tensile strength>200MPa and softening temperature>800°C.Tribological behavior of ND/Cu composite and the mechanism of ND on reducing the friction were studied by ring-on-block wear method. The results indicated that the wear process of composite was comprehensive effect of adhesion wear, oxidation wear, abrasive wear and fatigue wear. On improving the wear resistance of ND/Cu composite, ND particles exhibited self-lubrication characteristic by reducing friction coefficient and increasing the matrix strength.The microstructure and interface characterization were carried out by high- resolution transmission electron microscope and scanning electron microscope. The results demonstrated that the ND particles were homogeneously dispersed in the copper matrix. Dispersion strengthening of Orowan mechanism was the main factor of ND reinforced copper matrix composite. ND and copper matrix combined closely to form the directly integrating interface and there was no dissolution and diffusion in the interfacial area.