Pressure- and Additive-Mediated Sintering of B 4 C at Relatively Low Temperatures

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RCH 2017—1231

Fig. 1—Vickers hardness under different conditions of sintering are given 1 through 5 with conditions, such as 1: sintered at 1273 K (1000 C) with Ni additive and without pressure, 2: sintered of the as-received powder at 1273 K (1000 C) and pressure of 2 GPa without Ni additive, 3: sintered of the ball-milled (milled for 60 min) B4C and Ni powder at 1273 K (1000 C) and a pressure of 2 GPa, 4: sintered of the ball-milled (milled for 180 min) B4C and Ni powder at 1273 K (1000 C) and a pressure of 2 GPa and 5: hardness of pure monolithic B4C without any additive.

Fig. 2—Nanoindentation hardness of sintered B4C with and without additive.

large B4C particles are 36 ± 1.5 and 327 ± 15.0 GPa, respectively (see Figure 2). The microstructure of the sintered (2 GPa and 1273 K (1000 C)) as-received powder without additive is shown in Figures 3(a) and (b). The porosity observed on the surface of the samples is quite low. However, the sintered sample without the additive showed that the fracture surface due to indentation, Figure 3(b), is intergranular, suggesting sintering at 2 GPa and 1273 K (1000 C) is not effective in bonding between B4C particles. On the other hand, the sintering with additive at 2 GPa and 1273 K (1000 C) has significantly improved the bonding. The microstructure of the sintered sample (180 minutes milled) with Ni additive is shown in Figure 4(a). In this case, the fracture surface due to indentation is not predominantly intergranular (Figure 4(b)). The mechanism of sintering mostly involves surface diffusion, lattice diffusion, vapor transport, grain boundary diffusion, and[28] plastic flow. The sintering rate (de/dt) can be given by : 1232—VOLUME 48A, MARCH 2017

Fig. 3—(a) A SEM image after sintering as-received B4C at high pressure without additive. (b) A SEM image of the part of the microindent of high pressure-sintered B4C showing the intergranular fracture.

e_ ¼

dq AD/n X ¼ m ðpa þ RÞ; qdt G kT

½1

where q is the density, ‘A’ is the numerical constant, D is the diffusivity, G is the grain size, T is the temperature, t is the time, k is the Boltzmann constant, X is the atomic volume, R is the sintering stress, / is the stress intensification factor, m and n are exponents, and pa is the applied pressure. As the lattice, surface and grain boundary diffusion for B4C below 0.5Tm are small, and the sintering rate (de/dt) can be enhanced by the application of pressure and decreasing the grain size using Eq. [1]. High external pressures drive up the stress at particle–particle interface such that the energy at the particle–particle interface is much higher than the surface energy of the free surface (i.e., void-particle) and the relative rates of densification-related mecha[29] nisms are much faster than coarsening mechanisms. The sintering temperature is well below the melting temperature of Ni (1455 C, 1773 K). In addition, unlike Bi and ice, the melting temperature of Ni increases with the compressive pressure. Note the METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 4—(a) SEM image o