Densification of Boron Carbide: Analysis by X-Ray Rietveld and SEM Cosentino1, P.A.S.L., Campos2, J.B., Avillez3, R.A

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Densification of Boron Carbide: Analysis by X-Ray Rietveld and SEM Cosentino1, P.A.S.L., Campos2, J.B., Avillez3, R.A. e Costa4, C.A (1) Centro Tecnológico do Exército – Laboratório de Materiais - Av das Américas, 28705 - Rio de Janeiro/RJ – Brasil – CEP: 23020 470 (2) Instituto Nacional de Tecnologia, RJ, Brasil (3) Pontifícia Universidade Católica, RJ, Brasil (4) Universidade Federal do Rio de Janeiro, RJ, Brasil [email protected]

Keywords: carbide, boron, x-ray diffraction and Rietveld.

Abstract: High energy milling was used to produce submicron boron carbide (B4C) particles to increase its final density. The milling process introduced certain quantity of ZrO2 nanoparticles from the milling balls. During sintering, the ZrO2 nanoparticles reacted with the B4C and other carbides used as sintering additives. The resulting phases were analyzed by Rietveld X-ray Diffraction and scanning electron microscope (EDS/SEM). The results clearly showed the contamination of the B4C powder by ZrO2 (from the milling balls) and iron (from the milling walls) and, for the sintered parts, many different phases were identified by the combination of both techniques.

Introduction:

Boron Carbide (B4C) is an important ceramic material, which has been used for structural and nuclear applications. It is the third hardest material with excellent abrasive resistance, in fact, only diamond and boron nitride possesses higher hardness (1). Its lower density together with a high elastic modulus makes it suitable for lightweight military armor too (2). However, its covalent nature and corresponding low diffusion mobility allied with high melting point (2427oC) make it one of most difficult material to sinter, namely, reach high density (3). The prevailing covalent character of the crystalline bonds of boron carbide determines its unique mechanical properties and low sinterability, which has been surrounded by the use of hot pressing, the most used method for obtaining high-dense boron carbide by the industry. Another approach to improve the final density is by powder activation, which can be performed by the combination of physical and chemical methods (1). The reactivity of a powder is generally associated with the particle size distribution, size itself and surface area. It is desired that the particles possesses submicron size, a distribution that is not too narrow and high surface area. One way to achieve this goal is using high energy milling, such as the planetary mill, which turns out to modify the powder surface conditions resulting in a non-equilibrium state (4). The additives used in B4C process are intended to decrease the sintering temperature (6) and pressure, and reported examples are : AlF3, Be2C, TiB2, W2B5, SiC, Al, Mg, Ni, Fe, Cu, Si and C. Carbon is considered to be the most effective of them and, consequently, is quite used by the industry, but it is used in a fenolic resin form. A new approach for additives is based on carbides (7). They have been observed to improve the B4C sintering, but inducing reactions that lead to second phase precipitation, which might affect fracture toughness, strength and hardness(8). One critical step when carbides are used is to identify the new phases formed during sintering. The present study evaluated the phases formed when B4C was fluxed with VC, Cr3C2 or amorphous carbon were hot pressed above 2000ºC. The phases were analyzed via the semi- quantitative x-ray Rietveld method and the results showed the method to be a powerful and simple tool to identify most of the phases presents.

1 Experimental Procedure

A mixture of B4C with C amorphous, VC or Cr3C2 additives, fluxed with 2 and 4 wt%, was high-energy milled (planetary mill) in IPA (isopropyl alcohol) and ZrO2 spheres during 2 hours. The vessel was coated with WC-Co by HVOF (high velocity oxygen fuel). The powder was dried to remove the IPA. The sintering process was performed by hot pressing at 1800°C and 20 MPa pressure, in argon. The next step was to cut and polish the samples with diamond pastes until they achieve a mirror aspect. The phases were then identified by X-ray diffraction (Pan-Analytical X`Pert PRO) o with a CuKα radiation, a scanning step of 0.05 and a collecting time of 5 seconds per step. The data was semi-analyzed by the Rietveld method using Bruker-AXS TOPAS, version 2.1, with a fundamental parameters approach (9). The microstructure characterization was performed with the Scanning Electron Microscopy (JEOL JSM-6460 LV) and the EDS (Energy Dispersive Spectroscopy).

Preliminary Results and Discussion

The Figures 1 and 2 shows Rietveld calculation and the measured diffraction patterns of the B4C powders homogenized and milled for 2 h, respectively. When both figures are compared, the effect of the milling process on the original powder diffraction can be observed. The Figures 3 and 4 shows Rietveld calculation and the measured diffraction patterns of the B4C powders homogenized and milled for 2 h, with 2% Cr3C2 of additive. When both figures are compared, the effect of the additive on the powder diffraction can be observed. The homogenized and the milled powders were hot pressed and the overall result, with all the addittives (Cr3C2, VC and C amorphous) can be seen in Figures 5 and 6. The homogenized and the milled powders were hot pressed with 2% Cr3C2 and the result can be observed in Figures 7 to 10. The Rietveld calculation and the measured diffraction patterns clearly demonstrated the excellent agreement between the experimental data (blue line) and the simulated pattern (red line). The new phases formed were identify by the Rietveld method and chemical analysis obtained by the SEM/EDS. Also, the technique quantified the phases presents - it was necessary to supply the software TOPAS with the chemical elements obtained by the SEM/EDS analysis, exemplified in Figure 11. It was observed that as powders were milled the diffraction peaks becomes smaller, which was a consequence of the particle size reduction to almost nanosize. The formation of new phases during the sintering were only observed when the powder was high energy milled; if the powder was homogenized, such new phases were not observed. It is then suggested that the high energy milling process used was able to modify the surface behavior of the particles to a stated far off from the homogenized one. The next step is study how the phases were formed and verify if they affect the mechanical properties of the material.

Conclusion

The Rietveld method revealed to be an excellent technique to identify and to quantify the finely dispersed phases (nano crystallites) in the boron carbide matrix, originated from the planetary milling process, when metallic carbides (VC and Cr3C2) and Camorphous additives were used.

Figure 1 – Difractogram (blue line) and Rietveld simulation (red line) of the original powder of B4C

Figure 2 - Difractogram (blue line) and Rietveld simulation (red line) of the B4C powder (2h milling)

Figure 3 - Difractogram (blue line) and Rietveld simulation (red line) of the B4C powder with 2% Cr3C2.

Figure 4 - Difractogram (blue line) and Rietveld simulation (red line) of the B4C powder (2h milling and 2% Cr3C2)

Figure 5 – Difractogram of B4C pellets – with original powder and with additives

Figure 6 – Difractogram of B4C pellets – with powder 2h milled and with additives

4 Figure 7 – Difractogram (blue line) and Rietveld simulation (red line) in the original pellet of B4C

Figure 8 - Difractogram (blue line) and Rietveld simulation (red line) in the pellet of B4C (2h milling)

Figure 9 - Difractogram (blue line) and Rietveld simulation (red line) in the pellet of B4C (2% Cr3C2)

Figure 10 - Difractogram (blue line) and Rietveld simulation (red line) in the pellet of B4C (2h milling and 2% Cr3C2)

Figure 11 – SEM/EDS of phases at the grain boundary, according with Rietveld simulation. Bibliography

1. Thévenot, F., “Boron Carbide – A Comprehensive Review”, J of the European Ceramic Society, vol. 6, pp.205-225, 1990. 2. Matchen, B. “Applications of Ceramic in Armor Products”, Key Engng Mat, vols. 122-124, pp 333-342, 1996. 3. Lipp, A. “Boron Carbide – Production, Properties, Application”, Technische Rundschau, Numbers 14, 28, 33 (1965) e 7 (1966), Elektroschmetzwerk Kempten Gmbh, München. 4. Suryanarayana, C., “Mechanical Alloying and Milling”, Progress in Materials Science 46, pp 1 – 184, 2001. 5. Barsoum, M., Fundamentals of Ceramics, McGraw Hill, 1997. 6. McCauley, J. et al. “Microstructural Characterization of Commercial Hot-Pressed Boron Carbide Ceramics”, J. Am. Ceram. Soc., 88 [7] 1935–1942, 2005. 7. Rogl, P. et Bittermann, H. “Ternary metal boron carbides”, Int. J. of Refractory Metals & Hard Materials, pp 27 – 32, 1999. 8. Krastic, V.D. e Skorokhod V. Jr., ‘‘High Strength–High Toughness B4C– TiB2 Composites,’’ J. Mater. Sci. Lett., 19, 237–9, 2000. 9. Cheary, r. W. e Coelho, A., “A fundamental parametrs approach to X-Ray line -profile fitting”, J. Appl. Cryst., 25, pp. 109-121, 1992.