Thermal stability of superhard Ti–B–N coatings

Ternary transition-metal boron nitride Ti–B–N offers outstanding hardness and thermal stability, which are increasingly required for wear resistant applications, as the protective coatings are subjected to high temperature, causing thermal fatigue. Ti–B–N coatings with chemical compositions close to the quasibinary TiN–TiB2 tie line and boron contents below ∼ 18 at.% contain a crystalline supersaturated NaCl structure phase, where B substitutes for N. Annealing above the deposition temperature causes precipitation of TiB2, which influence dislocation mobility and hence the hardness of TiB0.40N0.83 remains at a very high level of ∼ 43 GPa with annealing temperature Ta up to 900 °C. Growth of Ti–B–N coatings with B contents above ∼ 18 at.% results in the formation of nm sized TiN and TiB2 crystallites embedded in a high volume fraction of disordered boundary layer. The compaction of this disordered phase during annealing results in a hardness increase of TiB0.80N0.83 coatings from the as-deposited value of ∼ 37 GPa to ∼ 42 GPa at Ta = 800 °C. Excess B during growth of TiB2.4 coatings causes the formation of bundles of ∼ 5 nm wide TiB2 subcolumns encapsulated in a B-rich tissue phase. This nanocolumnar structure is thermally stable up to temperatures of ∼ 900 °C, and consequently the hardness remains at the very high level of ~ 48 GPa, as nucleation and growth of dislocations is inhibited by the nm sized columns. Furthermore, the high cohesive strength of the B-rich tissue phase prevents grain boundary sliding.