Abstract
This work concerns boron carbide matrix composites with the addition of hexagonal boron nitride particles (h-BN) as a solid lubricate. The composite materials were hot-pressed and analysed in terms of phase, structure, and microstructure changes in relation to the h-BN content. The uniaxial pressure applied during the manufacturing process allowed the orientation of single h-BN particles and its agglomerates in perpendicular direction to the pressing axis. The anisotropy of heat transfer and thermal expansion coefficient (CTE) and density changes in relation to temperature are discussed. Thermal diffusivity and conductivity were measured in relation to the material direction by the laser flash analysis method (LFA). In this paper, understanding the heat flow and CTE changes allowed explaining the results of investigated subtractive laser processes of the manufactured composites. The laser ablation process was conducted on B4C/h-BN composites in parallel and perpendicular direction to each other. It was done in a continuous work (CW) mode at 50 W with a 40 µm spot and 3 mm/s beam travel speed. The influence of h-BN particles and their orientation on thermal properties is discussed. The effect of laser processing on B4C/h-BN composites was also discussed in relation to the material surface roughness measured with a confocal microscope, microstructure observations, density, and thermal properties changes in relation to the material direction.
Highlights
The boron carbide is a low density material of 2.51 g/cm3 with rhombohedral crystallographic structure including a high melting point of about 2400 ◦ C, very high hardness of 38 GPa, a good elastic, excellent wear and mechanical properties, and high neutron absorption cross section [1,2,3,4]
There is a decrease of this amount of hexagonal boron nitride particles (h-BN) allows obtaining slightly about 7% higher thermal properties in parallel direction to property possibly due to phonon-phonon scattering (Figures 20 and 21)
The higher amount of h-BN allows obtaining slightly about 7% higher thermal properties in parallel direction to decrease in thermal properties of this material with a temperature rise is visible for composites the pressing axis than for the reference sample of 29.6 W/(m∙K)
Summary
The boron carbide is a low density material of 2.51 g/cm with rhombohedral crystallographic structure including a high melting point of about 2400 ◦ C, very high hardness of 38 GPa, a good elastic, excellent wear and mechanical properties, and high neutron absorption cross section [1,2,3,4]. The hexagonal boron nitride, called white graphite, was very often investigated by researchers due to the possibilities to decrease the friction coefficient for cutting tools and bearing sliding applications [10,11]. This phase has been inserted into various oxides, nitrides, and carbides of advanced ceramic materials, for example: Si3 N4 , SiC, Al2 O3 , B4 C, SiAlON. The correlation between thermal properties and laser ablation of boron carbide/h-BN composites provides new information crucial for understanding the rapid heat processing of this composite system and gives new data in the literature concerning the material behavior during laser processing
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