Theoretical investigation and experimental confirmation of crystalline and amorphous Si, B, C, N–based hard materials

The newly discovered phase, cubic BC2N, has attracted great interest during the past two decades. This is because the hardness of the novel superhard phase can even reach up to ~ 76 GPa, only behind diamond (~ 100 GPa). It is ranked as the second hardest material ever known, even harder than cubic BN (~ 50 GPa). The fantastic property of superhardness immediately gained wide attention after the findings. However, the crystal structure and atomic configuration of the novel phase have still not been determined so far due to the similar atomic size of B, C, and N. Experimental results of the crystal are also inconsistent.
Theoretical calculations are employed to attempt to unveil the crystal structure of the phase by constructing diverse structural models. Due to the difference of the structural models, these calculated results are contradictory as well.
Here we propose a solid solution model to search possible structures of the crystalline phase, which are classified by the parameter of the degree of mixture. The powerful feature of our model lies in the aspect that it not only successfully illustrates the discrepancy observed in experiments, but also unifies the inconsistency shown in theoretical calculations. Meanwhile,
we extended our model to B–C–N compositions along the C–BN isoelectronic line and broader areas in the ternary B–C–N phase diagram besides BC2N, with the expectation of discovering potential superhard phases that can be comparable to or even harder than cubic BC2N. Indeed, there exist such areas in the ternary B–C–N phase diagram in which they are harder than BC2N. Our prediction of superhard phases can provide general guidance for experimental works to intentionally prepare such superhard phases by regulating correspondingly experimental parameters. In addition, we also extended our model to cubic
B–C crystals. The excellent agreement between experimental results and our theoretical ones clearly demonstrates the transferability of our model to other similar covalent crystalline materials.
To the best of our knowledge, we performed, for the first time, theoretical calculations of amorphous B–C–N materials at the atomic scale. Based on first–principles calculations, the relation among chemical composition, microstructure, and mechanical properties was established. This is also the first time to build the relation depending on so many chemical compositions of amorphous B–C–N films. The relation can give the general description of the
distribution of mechanical properties in the ternary B–C–N phase diagram. Thus it can also guide the experimental works to prepare those compositions with better mechanical properties. We also synthesized amorphous B–C–N films by changing diverse experimental parameters to verify our theoretical results. The obtained compositions in experiments are mostly located in the area that has lower formation energy according to our theoretical calculations, corresponding to those compositions that are easier to obtain. In other words, our experimental works can reproduce the theoretical works well with reasonable accuracy.
Besides B–C–N materials, Si–C–N materials were also prepared with the expectation of extracting the relations between mechanical parameters for covalent amorphous materials, such as the relation of hardness and Young§#39;s modulus. If some other mechanical quantities can be found to have good relations with hardness, then the hardness can be indirectly evaluated
by theoretical calculations. In addition, the substrate effects were also discussed for such covalent amorphous materials.