Improving the mechanical properties and wear resistance of Mo-doped AlCrN coating via implementing a gradient structural design

The gradient-structured alloy designs for nitride coatings have gained significant attention, as persistent challenges related to low toughness and insufficient hardness in the coating have considerably hindered their effectiveness in numerous applications. Four different variants of AlCrMoN gradient coatings (Mo-1, Mo-2, Mo-3, and Mo-4), featuring different Mo gradient concentration were prepared by using magnetron sputtering. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis revealed that all deposited gradient coatings exhibited a face-centered cubic (fcc) solid solution phase of (Al, Cr)N, featuring a compact columnar grain morphology. As the Mo gradient concentration increased, the columnar structure of Mo-1 transformed into a dense columnar structure in Mo-4, accompanied by a reduction in the average grain size from 17 nm in Mo-1 to 10 nm in Mo-4. The Mo-4 gradient coating design has demonstrated the superior mechanical properties, where the hardness (H) and elastic modulus (E) identified at ∼11 GPa and ∼210 GPa in Mo-1 were escalated to ∼24 GPa and ∼231 GPa in Mo-4. Consequently, a high H/E ratio (∼0.10 GPa) and H³/E² (∼0.26 GPa) were obtained in Mo-4 coating. Room temperature tribological assessment revealed that the Mo-4 coating also has lowest average coefficient of friction of 0.38 with minimal wear rate of 0.5 × 10⁻⁷ mm³/N·m. The Mo-4 coating also retains its superior tribological performance upon annealing at 700 °C and 800 °C, signifying the stability of the coating. This outcome can be ascribed to solid solution strengthening, a combination of favorable mechanical properties, optimized residual stress, and the substantial development of a lubricant tribo-film (such as MoO₃), which is facilitated by the enrichment of a molybdenum gradient concentration within coating.