Processability of Nickel-Boron Nanolayer Coated Boron Carbide

Abstract

This dissertation work focuses on the processability improvement of B4C, especially the compaction and sintering improvement of B4C by applying a Ni-B nanolayer coating on individual B4C particles. A modified electroless coating procedure was proposed and employed to coat nanometer Ni-B layer onto micron-sized B4C particles. The thickness was able to be tuned and controlled below 100 nm. Key parameters, including the amount of nickel source, the amount of the surface activation agent (PdCl2), the amount of the complexing agent (C2H8N2), and the addition rate of the reducing agent (NaBH4) were studied. When the targeted thickness was 5 nm, a continuous and uniform nanolayer coating was obtained with the optimal condition of individual parameter combined. Reduction of the as-coated B4C powder in a H2-Ar atmosphere was studied between 400-900ºC to reduce the surface oxidesâ Ni2O3 and B2O3. Reduction at 800ºC in hydrogen atmosphere was found to be the most effective condition to remove oxygen in the coating layer, with Ni2B as the reduction product. Compaction of the as-received, separated and uncoated, and separated with Ni-B coating B4C powders using uniaxial die compaction and combustion driven compaction (CDC) techniques was studied. CDC technique showed the advantage over the traditional uniaxial die compaction by yielding much higher green density and green strength (73% vs. 53.8% green density for the Ni-B coated B4C). Among compacts obtained from the same technique, Ni-B coated B4C compact yielded the densest packing with crack-free compact surface and the highest strength, demonstrating more bonding between B4C particles provided by Ni-B surface coating. Sintering of the Ni-B coated B4C in an Ar atmosphere between 1150 - 1600ºC with soaking time of 2 hrs and 10 hrs was studied. Liquid phase was found to form during the sintering process. Density measurement showed that the liquid phase Ni-B formed greatly facilitated B4C densification. Considerable density increase and inter-granular connection was achieved when sintered at 1600ºC for 10 hrs. The density enhancement by Ni-B coating was supported by transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS) examination which showed that there was B4C species diffusion into liquid Ni-B phase. This liquid phase enhanced the diffusion of B4C species and formed strong bonding between B4C grains by dissolving small B4C particles and sharp edge and corners of B4C particles. Strength test demonstrated that the Ni-B coating dramatically improved the strength of B4C compacts by yielding a much higher strength of the Ni-B coated samples than the uncoated samples (13.97 vs. 1.79 MPa for the uinaxial die compacted samples, 27.03 vs. 2.21 MPa for the CDC samples). Electrical conductivity Ni-B coated B4C samples was also shown to be improved with the electrical resistivity being reduced from infinite for pure B4C samples to 1.8à 10-3 Ω⠢m for the Ni-B coated samples. This research work has shown that with the Ni-B coating, B4C densification can start at a temperature as low as 1600ºC via a liquid phase sintering process.