Design principle of gradient elastic modulus transition layer via substrate mechanical property

The differences in properties between ceramic coatings and their alloy substrates are the main factor that affects the adhesion between the coating and the substrate. Therefore, it is necessary to design a transition layer between them. Gradient elastic modulus CrN (G-CrN) coatings with various thicknesses of the gradient elastic modulus transition layers (G-layer) were prepared on Ti6Al4 V titanium alloy (TC4) and W6Mo5Cr4V2 high-speed steel (W6) substrates by varying the reactive flows using the hot-wire plasma-enhanced magnetron sputtering technique. The results showed that the elastic modulus difference (ΔE) between W6 and the G-CrN coatings was smaller than that between G-CrN and TC4. The large ΔE resulted in an asynchrony of TC4’s plastic deformation and G-CrN’s elastic deformation, leading to a large interfacial tensile stress and surface strain under an impact load, and therefore poor G-CrN/TC4 adhesion. A thick G-layer, up to 0.9 μm, effectively compensated for the deformation asynchrony, and the G-CrN/TC4 adhesion improved to 80 N. The CrN/W6 adhesion remained excellent, beyond 100 N for G-CrN/W6 with a 0.2−0.9 μm thickness of the G-layers, because its ΔE was small and the deformation asynchrony was reduced. The mechanism of the poor coating-substrate adhesion was attributed to the deformation asynchrony caused by the large coating-substrate ΔE, which can be improved by a reasonably thick G-layer.