Microstructure and mechanical properties of multilayer $\alpha$-AlN/$\alpha$-BCN coating as functions of the current density during sputtering of a B$_4$C target

Multilayer AlN/BCN coating of nanometer scale have been prepared by magnetron sputtering of Al and B$_4$C targets in an argon–nitrogen atmosphere during deposition on a Si substrate. These coating have an X-ray amorphous structure and the maximum Knoop hardness of 27 GPa (at the current density 100 mA). The first-principle molecular dynamics calculations show that the B4–BN layer is dynamically unstable; thus, it will not be epitaxial and will be amorphous or have a structure different from the B4–BN structure. The thermal vacuum annealing from 600 to 800$^{\circ}$C of samples with multilayer nanosized coating leads to the decrease in the Knoop hardness to 18 GPa; however, the coating structure is retained X-ray amorphous.