LOW-TEMPERATURE CARBURIZATION OF HIGH-SURFACE-AREA TUNGSTEN POWDERS

Reductive decomposition of WO3 precursor powder gives high surface area alpha-W at temperatures above 650 degrees C, beta-W at temperatures below 575 degrees C, and mixtures of alpha-W and beta-W (in various ratios) at intermediate temperatures. The carburization behavior of these powders in flowing CO over the temperature range 350 degrees-700 degrees C is sensitive to the initial crystalline stare of the powder. The alpha-W powder can be carburized directly to WC phase in an appropriately high carbon activity environment, whereas the carburized beta-W powder becomes trapped in a metastable W2C state. A simple model is presented to account for this difference in carburization behavior of alpha-W and beta-W phases.