Carbon uptake during Spark Plasma Sintering: investigation through the analysis of the carbide footprint in a Ni-W alloy

In the majority studies of materials produced by Spark Plasma Sintering (SPS), the powders are consolidated in graphite dies using graphite punches and protective graphite foil. As a result, carbon uptake by the sintered material can occur. In this study, a Ni-15 at% W alloy was studied in this context for the first time and was chosen as a suitable model system, in which tungsten forms stable carbides whereas nickel does not, but offers a medium for carbon diffusion into the interior of the compact. In a disk-shaped 3 mm-thick compact Spark Plasma Sintered at 900 degrees C, carbon uptake resulted in the formation of tungsten carbide WC particles ranging from 0.2 to 2 mu m in the subsurface layers of the compact (within distances 50-100 mu m from the interface with the foil). The size of the WC particles varied with distance, smaller particles forming in the vicinity of the interface-in the area in which the nucleation was favoured at high carbon concentrations. However, it was not only the subsurface layer that was affected by the presence of carbon: particles of Ni2W4C were found at depths greater than 100 mu m from the interface and throughout the volume of the compact. The distribution of the submicron WC particles and particles of Ni2W4C corresponded to a network of boundaries between the agglomerates of the Ni-W powder that was consolidated into a compact. These boundaries offered paths for faster diffusion of carbon from the foil when compared with the volume of the agglomerates. The carbide subsurface layer dramatically changed the interdiffusion behaviour of the sintered material in a pair with aluminum due to a significantly reduced concentration of tungsten capable of diffusing within a metallic phase.