Principally, ultra-hard materials – materials whose hardness is attributable to covalent bonding – can be represented using the “composition cycle” shown in . This cycle involves the four elements carbon (C), boron (B), silicon (Si) and nitrogen (N). The combination of any two chemical species from this composition cycle produces a compound exhibiting ultra-high hardness, e.g., CBN, SiC, Si3N4, B4C and the recently recognized C3N4.
Figure 1 Composition cycle of ultra-hard materials (C–N–B–Si) .
Under ambient conditions – while nitrogen reside in a gaseous form as individual chemical species – boron, silicon and diamond are known to prefer a solid state. Due to its abundance and its capability to form better oxides, silicon dominated the electronic consumer market [12
] while the ubiquitous use of diamond originates from its unique features such as high thermal conductivity, high wear resistance and its ability to form extremely sharp cutting edges [13
]. Moreover, both diamond and silicon are known to be hard and brittle [14
] in their sp3
-bonded state. Two commercially available materials from this composition cycle, diamond (C) and cubic boron nitride (CBN), possess ultra-high hardness (attributed both to sp3
-bonding and relatively short bond lengths) and, for this reason, they are frequently used to manufacture cutting tips. While diamond resides in a cubic lattice structure, CBN possesses a zinc-blende structure having boron atoms at the crystal site (0,0,0) and nitrogen atoms in the respective centers of the boron tetrahedra. Although, a chemical bond between the carbon atoms in diamond is stronger than the corresponding isoelectronic bond between nitrogen and boron atoms, we propose that cubic boron nitride “CBN” could be an alternative appropriate choice for high-temperature nanotribology applications because of its superior thermal and chemical stability compared to that of diamond. Even though diamond and CBN have similar lattice structures, their surface chemistry is different. In a CBN lattice, boron atoms have only three valence electrons on the surface while nitrogen atoms have five. However, two of these five electrons of nitrogen can form a stable pair, leaving three valence electrons to bond with boron. On the contrary, a diamond lattice has four valence electrons; therefore, only a maximum of three electrons on the surface can have stable bonding between them. Consequently, this leads to the possibility that the remaining one or two electrons of each surface atom in diamond react readily with other materials like iron, nickel and even silicon [16
] in a tribological environment. This seems to be a plausible reason why CBN was found as an efficient cutting tip to machine ferrous alloys [17
] and silicon [18
] in comparison to a diamond tip. Hence, in contrast to diamond, CBN has fewer dangling bonds on the surface which makes it more inert.