Structure, mechanical properties, thermal stability and oxidation resistance of arc evaporated CrAlBN coatings

Abstract CrAlN coatings are recognized as outstanding protective layers for machining tools owing to their high hardness and exceptional oxidation resistance. However, the thermal stability of CrAlN coatings is relatively inferior. For tailoring the thermal properties of CrAlN coatings and enabling them to serve for severe working conditions, the element of B was introduced into CrAlN coatings in this study. Cr0.42Al0.58N, Cr0.39Al0.58B0.03N and Cr0.35Al0.59B0.06N coatings were deposited by cathodic arc evaporation, and the influence of B-incorporation on the structure, mechanical and thermal properties of CrAlN coatings were studied in detail. X-ray diffraction determinations manifest that the three coatings reveal a face-centered cubic structure. The hardness of Cr0.48Al0.52N is increased from 30.2 ± 0.7 GPa to 35.0 ± 1.3 GPa (Cr0.39Al0.58B0.03N) and 36.4 ± 1.2 GPa (Cr0.35Al0.59B0.06N), respectively, due to solid solution strengthening and grain refinement. Adding B into CrAlN coatings postpones the formation of wurtzite (w-) AlN and hexagonal (h-) Cr2N, thus improving their thermal stability. Additionally, the oxidation resistance of CrAlN coatings is further optimized by B-addition owing to the promoted formation of alpha (α-) Cr2O3. The isothermal oxidation experiment at 1100 °C for 15 h reveals that the oxide thicknesses of Cr0.39Al0.58B0.03N and Cr0.35Al0.59B0.06N coatings are dramatically decreased to ~1.6 and 1.8 μm, compared with the ~2.7 μm of Cr0.42Al0.58N coating.