TiC-based nanocomposite coatings as electrical contacts

This Thesis concerns the advanced surface engineering of novel TiC-based nanocomposite and AgI electrical contact materials. The objective is to make industrially applicable coatings that are electrically conductive and wear-resistant, and have a low coefficient of friction. I have studied electrical contact systems consisting of a Cu substrate with a Ni diffusion barrier and loading support, and a conductive top coating. The contact systems were characterized by x-ray diffraction and photoelectron spectroscopy, analytical electron microscopy, ion beam analysis, nanoindentation, resistivity, and contact resistance measurements. Nc-TiC/a-C/SiC nanocomposite coatings consisting of nanocrystalline (nc) TiC embedded in an amorphous (a) matrix of C/SiC were deposited by magnetron sputtering with rates as high as 16 μm/h. These coatings have a contact resistance comparable with Ag at high loads (~800 N) and a resistivity of 160-770 μΩcm. The electrical properties of the contact can be improved by adding Ag to make nc-Ag/nc-TiC/a-SiC nanocomposites. It is possible to tailor the size and distribution of the Ag grains by varying the fraction of amorphous matrix, so as to achieve good conductivity in all directions in the coatings. Ti-Si-C-Ag coatings have a contact resistance that is one magnitude larger than Ag at lower loads (~1 N), and a resistivity of 77-142 μΩcm. The conductivity of the matrix phase can be increased by substituting Ge, Sn or Cu for Si, which also reduces the Ag grain growth. This yields coatings with a contact resistance twice as high as Ag at loads of 1 N, and a resistivity 274-1013 μΩcm. The application of a conductive top layer of Ag-Pd upon a Ti-Si-C-Ag:Pd coating can further reduce the contact resistance. For barrier materials against Cu interdiffusion, it is shown that conventional electroplating of Ni can be replaced with sputtering of Ni or Ti layers. This is an advantage since both contact and barrier layers can now be deposited in and by the same deposition process. For Ti-B-C coatings deposited by magnetron sputtering, I demonstrate promising electrical properties in a materials system otherwise known for its good mechanical properties. In coatings of low B concentration, the B is incorporated into the TiC phase, probably by enrichment on the TiC{111} planes. The corresponding disturbance of the cubic symmetry results in a rhombohedral TiC:B structure. Finally, it is shown that AgI coatings consisting of weakly agglomerated AgI grains function as solid lubricant on Ag contacts. In an Ag sliding electrical contact, AgI decreases the friction coefficient from ~1.2 to ~0.4. After a few hundred operations, AgI grains have deagglomerated and Ag from the underlying layer is exposed on the surface and the contact resistance decreases to < 100 μΩ.