Achieving ultrastrong-tough CrAlN coatings with low friction coefficient by Y incorporating

Despite having otherwise outstanding hardness and oxidation resistance, CrAlN-based hard coatings are limited by poor fracture toughness and high friction. Although incorporating alloying elements such as C and Magnéli phase-forming elements like V, W, Nb, etc, offers one opportunity for additional toughness and low friction, it requires significant and undesirable compromises to high-temperature performance. Therefore, this work explores an alternative, facile route to achieve ultrastrong-tough CrAlN-based coatings with low friction by incorporating Y. The results indicated that the doped Y elements primarily dissolved in the CrAlN crystalline lattice. Increasing the Y content from 0 to 2 at.% resulted in a change in preferred orientation from (111) to (200), accompanied by a crystal transition from typical columnar grains to unusual equiaxed grains. Notably, when the Y content was about 1.2 at.%, the coating exhibited the maximum toughness of 9.65 MPa·m1/2, almost 2 times larger than that of pure CrAlN coating. Additionally, this coating demonstrated a high hardness of 36.7 GPa and a reduced coefficient of friction (COF) of 0.32. These improvements were attributed to the synergistic contribution of the ultrastrong-tough capability and the formation of Cr2O3 lubricating phases, facilitated by the small amount of Y doping.