Electrical Conduction in Thermally Sprayed Thin Metallic Coatings
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0890-Y09-11.1
Electrical Conduction in Thermally Sprayed Thin Metallic Coatings Atin Sharma, Richard J. Gambino, Sanjay Sampath Department of Materials Science and Engineering, Center for Thermal Spray Research, State University of New York at Stony Brook, Stony Brook, NY 11794-2275 USA ABSTRACT 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 –Vc) n throughout the composition range (with n=1.72 and Vc=0.09). These results show that that the coatings behave as a three-dimensional percolation system [1] and also indicate the asymmetric shape of the metallic and insulator regions in the coatings [2]. INTRODUCTION Thermal spray processing is traditionally used to make protective coatings such as thermal barrier or wear resistant coatings. There has been recent interest in plasma sprayed thin (typically ~50 µm thick) coatings for possible novel applications as components for thick film electronic devices and sensors. For instance, metallic coatings can be suitable candidates for applications as resistors, interconnects, strain gauges and magnetic sensors. However, limited scientific literature exists [3, 4] that specifically focuses on the properties of thermal spray thin coatings relevant to such applications. In our preliminary work many plasma sprayed metallic coatings have shown thickness dependence of electrical resistivity. Thickness effects in the case of plasma sprayed copper and solid-state deposited silver coatings (thickness ~10-70 µm) were attributed to oxidation, surface roughness, measurement discrepancies and interface adhesion [3]. However, the results were not investigated in details. The objective of the present work was to carry out a systematic study of the thickness dependence of electrical resistivity in the low thickness regime. A set of molybdenum coatings plasma sprayed to different thicknesses under the same process conditions were chosen for this investigation. It was found that the apparent density (or the volume fraction of metallic co
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