Electrical Conduction in Thermally Sprayed Thin Metallic Coatings

摘要

The in-plane electrical conductivity and the apparent density of air plasma sprayed (APS) molybdenum coatings having thicknesses in the range 30μm to 450μm were measured as a function of coating thickness. Both conductivity and the apparent density were found to increase with thickness until a saturation density (～ 80% of the bulk density of molybdenum metal) was achieved. Attributing this increase in the apparent density to the increased volume fraction of the metallic component (or decreased porosity volume) in the coatings allows for the treatment of the problem in a framework of percolation theories. In order to eliminate the contribution of surface roughness the results were further analyzed using a two-layer parallel resistor model of the coating. In this model the top layer, which was composed of coating roughness, was assumed to have fixed thickness and conductivity whereas the conductivity of the bottom layer was assumed to vary with thickness (density). A fit to the data obtained from the above model showed that the conductivity of the bottom layer obeys a power law relationship of the type σ α (V-V_c)~n throughout the composition range (with n=1.72 and V_c=0.09). These results show that that the coatings behave as a three-dimensional percolation system and also indicate the asymmetric shape of the metallic and insulator regions in the coatings.