Unprecedented B solubility in cubic (Hf,Ta,Ti,V,Zr)B-C-N coatings

The impact of compositional complexity in Ti-B-C-N based coatings was investigated by depositing Ti-rich (Hf,Ta,Ti,V,Zr)B-C-N with varying B/C ratios (from 0/21 to 32/0 at%). Despite a high B content of up to 32 at%, all coatings exhibit a fine-grained columnar single-phase face-centered-cubic (fcc) solid solution without any boride phase. All coatings show similar hardness values of 37 − 38 GPa , but increasing the B content reduces the indentation modulus. Ab initio calculations support this trend in elasticity and confirm the stability of these solid solutions. Despite increasing the compositional complexity, the addition of B and C has little influence on the lattice distortion. Among the coatings, offers the best combination of high hardness (37.7 ± 1.0 GPa ) and fracture toughness ( K I C = 4 . 0 ± 0.5 MPa*m (1/2) ). This compositionally complex, single-phase fcc-structured coating retains its hardness – even slightly increasing to 38.3 ± 1.3 GPa – upon vacuum annealing to 1200 °C. X-ray diffraction and atom probe tomography confirm its high-temperature phase stability, with phase transformation occurring only above 1200 °C. These results demonstrate the advantages of compositionally complex coatings, which outperform simpler Ti-B-N or Ti-C-N coatings with similar structure, and that the solubility limits can be extended beyond known boundaries by advanced materials science.