Percolation Study in Thermally Sprayed Al2O3-Ni Reinforced Composites

Property percolation threshold of two phase composites is strongly related to the morphology of the phases and anisotropy in the microstructure.  In typical spherical particle based two phase systems the percolation threshold is roughly 20 volume percent of the secondary phase but results have shown the numbers to be much less than 10% for thermal sprayed coatings and is dependent on particle size and morphology.
In this study alumina nickel composite coatings of different compositions were prepared and percolation threshold analyzed for electrical conductivity.  Percolation threshold depends on electrical conductivity of the metal and ceramic phases and is greatly influenced by the aspect ratio of the particles and the orientation of the lamellae. Al2O3-Ni composite coatings with composition varying from 1.5 to 9 volume percent of Nickel were made using atmospheric plasma spraying (APS) and high velocity oxy-fuel (HVOF) spraying. Using a four point contact technique, electrical resistivity was measured in both the in-plane direction and through-thickness direction. The electrical resistivity of HVOF composites shows an insulator-conductor transition within a narrow composition range, in both the in-plane direction and through-thickness direction.