Coating of HfC on Tungsten Powder by Fluidized Bed CVD

For the fabrication of sintered W-Hf-C alloy, coating of HfC on tungsten powder was attempted by a fluidized bed chemical vapor deposition (CVD). A commercial tungsten powder 3∼5 μm in diameter was used. Argon gas was employed as a fluidized bed former and hafnium chlorides, methane and hydrogen were utilized as raw materials of CVD. Hafnium chlorides were made by internal chlorination. At the preliminary investigation on the formation of fluidized bed, a granular ceramic filter was revealed to have a good performance as a distributor for fine tungsten powder. A foam filter was not suitable for a distributor which was choked in a short operation time. Powder feeding of about 10 g/cm2 associated with argon velocity of 0.2∼0.5 m/s was suitable for forming a stable fluidized bed. W powder showed good fluidity at room temperature due to the existence of WO3 on the particle surface, but at high temperature the fluidity disappeared because the sublimation of the oxide occurred at 1100 K. In order to make stable fluidized bed at high temperature, pre-coating at 900 K and a forced mixing were necessary. Change of bulk density seemed to be useful to evaluate a fluidity of the powder. Auger analysis indicated that Hf containing coated layer of 9 nm was obtained by fluidized bed CVD at 1553 K. EPMA and GDMS showed the coated powder containing 0.35∼0.4 mass% Hf. The relative density of sintered compact which was obtained by sintering of the coated powder at 2573 K for 3.6 ks in vacuum was 92.7%. The sintered specimen contained large amount of W2C and Hf coexisted with W2C.