Influence of chemical composition on mechanical properties of spark plasma sintered boron carbide monoliths

Abstract

Fully dense boron carbide monoliths exhibiting fine microstructure (i.e., submicrometric grain size) are sintered by Spark Plasma Sintering. Two different commercial powder batches, exhibiting different stoichiometries (i.e., B/C ratio and oxygen content) and various amounts of secondary phases (i.e., boric acid and free carbon), are used. Their chemical composition is well‐defined by coupling different methods (Transmission Electron Microscopy associated with XRD analyses, and Instrumental Gas Analysis), and are correlated with their mechanical properties, characterized from meso‐ to macro‐scopic scales by nano‐indentation and ultrasonic pulse echography. The presence of secondary phases (graphite and boric acid) is evidenced in various proportions in each powder batch. If the boric acid disappears during sintering, the graphite remains. However, for the considered amounts of graphite (lower than 1 wt%), the low variations in graphite content have no significant effect on hardness and elasticity values. At the opposite, the presence of oxygen in boron carbide lattice, leading to a boron oxycarbide phase, induces a decrease in both hardness and elasticity properties.

Mechanical properties of submicronic and nanometric boron carbides obtained by Spark Plasma Sintering: Influence of B/C ratio and oxygen content

“…The lower apparent hardness and elasticity of TK material is a consequence of the lower intrinsic properties of the oxycarbide phase. This phenomenon has already been reported by Réjasse et al [22] where a decrease of the Young modulus was found when increasing the oxygen amount in fully dense boron carbide specimens having similar grain sizes. Moreover, as depicted in Figure 7.b, it was also observed a decrease in the intrinsic hardness, which is in agreement with the values of this work.…”

“…Indeed, as previously shown by Niihara et al [39], a difference in the B/C ratio can affect the hardness. The measured B/C ratios of HD and TK materials are reported on this abacus, displayed in Figure 7.a, as well as a reference material MT [22]. The hardness of HD and MT samples seems to be in accordance with the evolution of hardness with the B/C ratio.…”

“…Fully dense boron carbide ceramics with relative densities higher than 98 % associated with micrometric or submicrometric grain size distributions have been manufactured without sintering aids using Spark Plasma Sintering (SPS) [15][16][17][18][19][20][21][22].…”

Novel Pathway for the Combustion Synthesis and Consolidation of Boron Carbide

“…B 4 C obtained by a carbothermic reduction of boron oxide/boric acid allowed achieving a similar densification value when sintered at higher temperature (2100 °C), as it comprised particles of up to 5 μm [ 61 ]. Commercially available B 4 C nanopowder (591 ± 26 nm) was possible to sinter at a comparatively lower temperature but at higher pressure [ 62 ]; however, it was not superior to the SHS-produced B 4 C in terms of mechanical performance. Microwave-synthesized B 4 C comprising 50−300 nm sized particles after the SPS densification at similar conditions demonstrated a similar relative density value but higher microhardness [ 49 ].…”

Effect of powder purification by heat-treatment on the creep behaviour of dense boron carbide monoliths

“…According to literature, the mechanical properties of full dense monoliths depend on variation of chemical composition, i.e. carbide stoichiometry and presence of secondary phases such as free carbon [11][12][13]. Niihara et al [11] emphasized that, for a B/C ratio higher than 4, increasing B content in boron carbide plates obtained by chemical vapor deposition resulted in reduction of hardness and toughness, which was attributed to reduction of bond strength in the boron carbide structure.…”