Decomposition and segregation phenomena in PVD TiBN coatings

TiBN coatings synthesized via physical vapor deposition (PVD) with a B content of 8 at.% and 25 at.% were investigated in terms of their phase and thermal stability, nanoscale composition and mechanical properties, using both experimental and theoretical methods. Experimentally, various spectroscopic and diffraction methods were used to determine their phase composition, which were then compared to the thermodynamically expected phase composition, calculated using CALPHAD methods. Additional annealing was applied to test the coatings thermal stability and phase evolution. We find that the coating with B content of 8 at.% remains in metastable single-phase state due to a small driving force for decomposition. In contrast, the 25 at.% coating has a stronger tendency to decompose into the face-centered cubic (fcc) TiN and hexagonal (h) TiB2, as observed from the experiment. Thermodynamic calculations, built on ab-initio determined segregation energies reveal that segregation of B to grain boundaries (GBs) is expected to occur for the metastable thermodynamic conditions of the coatings, but not for the equilibrium conditions. Atom probe tomography (APT) measurements do not reveal any clear signature of B segregation to the GBs, but rather indicate decomposition into fcc-TiN and h-TiB2.