Interaction between Copper and Modified Carbon Surfaces

Research project P 14534 Interaction between Copper and Modified Carbon Surfaces Christoph EISENMENGER- SITTNER 09.10.2000 The improvement of interface properties in Metal Matrix Composites (MMCs) is of considerable scientific and technological interest. The aim of this project is the enhancement of the adhesion of copper coatings on carbon fibers which are to be used for the preparation of MMCs. It will be realized by using Physical Vapor Deposition (PVD) and by applying a plasma treatment to modify the surface of carbon/graphite substrates. The surface treatment can be followed by an additional deposition of adhesion enhancing intermediate layers. The influence of different treatments on adhesion will be measured by the interaction between copper coated AFM- tips and the modified surfaces. Force-distance-diagrams determined in an HV-environment avoid the contact of the modified surfaces with the ambient atmosphere. The interface between the coatings and the C-substrate will be characterized by TEM in respect to structure, morphology and interdiffusion layers. The chemical state of the surface after the plasma treatment is analysed by X-Ray Photoelectron Spectroscopy (XPS), Secondary Ion Mass Spectroscopy (SIMS) and Auger Electron Spectroscopy (AES). Destructive and non destructive testing methods will characterize the adhesion and the thermal contact resistance between copper and the carbon/graphite substrate. The adhesion of copper coatings on carbon fibers will be determined by single fiber tests (pull out and push out tests).

Interfaces influence a large variety of physical and chemical properties of composite materials such as mechanical and thermocycling durability, thermal conductivity and corrosion resistance. Carbon-fiber reinforced copper, which is a candidate for heat sinks in high performance electronic components suffers from a particularly weak interfacial adhesion between the Carbon-fiber and the copper matrix due to the total immiscibility of the two materials. Within the present project several routes to improve the mechanical adhesion and to reduce the thermal contact resistance between copper and carbon were investigated. The key point of the project was the assumption that the system of carbon fiber and metal matrix can be modelled by plane substrates consisting of disordered carbon coated with copper. This allowed the detailed characterization of the interface between copper and carbon by using several surface analytical tools such as Scanning Force Microscopy, Transmission Electron Microscopy and Scanning Auger Microscopy. For the preparation of the model samples Physical Vapor Deposition Methods were chosen because they allow both, the pre treatment of the carbon substrate by innovative plasma based methods and the preparation of metallic interlayers in the nanometer range which act as adhesion promoters. Within the project it could be shown that the results obtained from the model samples can be transferred to a real fiber reinforced composite. A significant improvement in the thermal conductivity of the composite material was achieved and could be directly correlated to adhesion data and thermal properties investigated on sample models. In addition the above mentioned combination of complementary surface analytical techniques yielded important results regarding the modification of the wettability of carbon by copper by metallic interlayers which are the basis of ongoing research within an integrated project of the 6th EU framework program.