Grain size refinement of hard nitride coating to mitigate fatigue performance degradation in ductile metal substrate

This paper proposes an effective way to relieve the side effect of hard nitride coating on tough metal substrate by reducing coating grain size and revealing its inner mechanism. Utilizing the thickness-dependent effect of physical vapor deposited hard coating grain size, TiN coatings with grain sizes of 9.6 nm, 16.5 nm, and 23.4 nm were prepared on the surface of 2A70 aluminum alloy fatigue specimen by filtered cathodic vacuum arc deposition. Rotating bending fatigue tests revealed that the median fatigue limits of 2A70, 2A70 with TiN coating at grain sizes of 9.6 nm, 16.5 nm, and 23.4 nm are 165.83 MPa, 127.5 MPa, 113.75 MPa, and 112.5 MPa, respectively. Under high-cycle fatigue loading, the TiN coating with a grain size of 9.6 nm exhibits the least damage to the substrate’s fatigue performance. And the fatigue resistance of TiN determines its damage extent to metal substrate’s fatigue behavior. Molecular dynamics analysis shows that the maximum stress experienced by TiN coatings increases with grain size after undergoing various fatigue loading cycles. Under alternating loads, smaller grains with more grain boundaries lead to predominant intergranular sliding in the TiN coating.