(179i) Atomic Layer Deposition of Sintering Aids Accelerates Tungsten Powder Metallurgy

Tungsten is a dense, highly refractory metal used in a wide range of engineering applications where high temperatures are present. Due to its high melting point, high hardness, and poor ductility under normal conditions, there is interest in producing near-net shape tungsten parts using powder metallurgy to reduce or eliminate difficult machining operations. During this process, tungsten powder is pressed into a desirable shape and sintered at high temperature to bond the particles and remove porosity. Trace amounts of transition metal sintering aids such as Pd or Ni can greatly accelerate this process by forming a thin intergranular film that provides a fast diffusion pathway for tungsten atoms below the eutectic point. This work deposits small amounts of Pd onto tungsten powder using particle atomic layer deposition (ALD), which ensures uniform coverage of the primary tungsten particles and minimal effects on power properties relevant for downstream powder metallurgy processing. The morphology of the ALD Pd coating is shown to enhance the performance of the sintering aid by lowering the temperature and which sintering occurs.

This work is the first use of particle ALD to coat a dense, cohesive refractory metal powder. Pd was deposited using alternating doses of Pd(hfac)₂ and formalin vapors through a heated fluidized bed of 1-5 μm tungsten powder. A composition of approximately 0.1wt% Pd was achieved with 25 ALD cycles. For comparison, a sample with matching composition was prepared by immersing the W powder in a Pd salt solution and drying. The powder compositions were characterized by ICP-OES and LECO oxygen analysis, and the morphology will be characterized by surface area analysis, SEM, and TEM. The sintering kinetics of the powders were characterized using dilatometry up to 2000°C. Analysis of the sintering rate profiles suggests that the lower-temperature sintering of the ALD Pd-W powder is not due to a change in mechanism. Instead, the morphology of the ALD Pd coating likely assists the formation of the Pd-rich intergranular film responsible for accelerated tungsten diffusion. This is supported by further analysis of fully sintered samples using EDS, which reveals a more uniform distribution of Pd in the final tungsten microstructure when using the ALD Pd-W powder.

These findings will help improve near-net shape tungsten manufacturing by lowering the required sintering temperatures and times to achieve a dense part. Lowering the cost of tungsten parts will allow more of their unique properties to be fully utilized by the engineering community and accelerate development of high-temperature components.