Boron phosphide BP and boron subphosphide B₁₂P₂ are usually considered as two defined compounds of the boron-phosphorus system. These phases exhibit promising mechanical properties, including Vickers hardness values in the order of 30-35 GPa [1][2] and low densities of 2.94 and 2.47 g.cm^-3, respectively [3][4]. These phases remain largely unknown in the literature, mainly due to the time and difficulty of developing synthesis methods, and consequently are not widely used in industry. Recently, in order to allow large scale production while maintaining high purity, synthesis methods such as Self-propagating High-temperature Synthesis (SHS) and mechanochemical processes [5][6] have been adopted.

SHS applied to BP powders leads to temperatures above 1100°C and induce partial decomposition of BP to B₁₂P₂ [7]. To elucidate the corresponding mechanism, a deep investigation of the thermal stability of BP has been carried out. By focusing on the morphological and structural characterization by means of SEM, HR-TEM and electron diffraction of powders heat treated at different temperatures, it was possible to show that the growth of B₁₂P₂ crystallites resulted from a mechanism supported by the formation of a gaseous phase. These characterizations have led to a proposal of the decomposition mechanism.

Furthermore, the densification behavior of BP and B₁₂P₂ powders by spark plasma sintering (SPS) has been also studied. More precisely, the competition between the densification and thermal decomposition has been studied. This study took into account the thermomechanical cycle applied during SPS, the chemical purity and the particle size of the boron phosphide powder. Some promising mechanical properties of the sintered B₁₂P₂ sample have been obtained such as high Vickers hardness (> 35 GPa) and Young's modulus (≈ 435 GPa).

References:
[1]           V. L. Solozhenko and V. Bushlya, ‘Mechanical Properties of Boron Phosphides’, J. Superhard Mater, 2019
[2]           R. Gui and al., ‘Strain stiffening, high load-invariant hardness, and electronic anomalies of boron phosphide under pressure’, Phys. Rev. B, 2020
[3]           U. Nwagwu, ‘Flux growth and characteristics of cubic boron phosphide’, 2013
[4]           P. Yang and T. L. Aselage, ‘Synthesis and cell refinement for icosahedral boron phosphide B12P2’, Powder Diffr, 1995
[5]           Y. L. Godec, V. Mukhanov, V. Solozhenko, and P. Sokolov, ‘Production of boron phosphide by reduction of boron phosphate with an alkaline metal’, WO2015097244A1, 2015
[6]           Y. L. Godec, V. Mukhanov, V. Solozhenko, P. S. Sokolov, and D. Vrel, ‘Mechanochemical process for the production of BP, B12P2 and mixtures thereof, in particular as nanopowders’, US10519039B2, 2019
[7]           V. A. Mukhanov, P. S. Sokolov, Y. L. Godec, and V. L. Solozhenko, ‘Self-propagating high-temperature synthesis of boron phosphide’, J. Superhard Mater, 2013