Impact of gradient nanocomposite coating design on the surface mechanical properties of soft composite substrate

This study examines the influence of gradient nanocomposite coatings on the surface mechanical properties of soft composite substrates (modulus: 150 MPa). Polyvinylpyrrolidone reinforced with halloysite nanotubes was used to fabricate gradient (e.g. 5/15/30 wt%) and reverse gradient (e.g. 30/15/5 wt%) coatings for comparative analysis. Reverse gradient coatings consistently exhibited superior surface properties, particularly in thicker (30 mu m) systems, where stiffer sublayers effectively transferred stress to the surface. The 30/15/5 reverse gradient coating achieved a modulus of 2.92 GPa, significantly outperforming its gradient counterpart (0.33 GPa) at similar indentation depths. In contrast, thinner (15 mu m) gradient coatings with smaller concentration variations (5/10/15 wt%) facilitated a significantly smoother mechanical transition and improved stress distribution, while larger concentration differences (5/15/30 wt%) amplified substrate effects, increasing indentation depth. These findings highlight the trade-offs in coating design: gradient coatings optimize mechanical transitions, making them ideal for functional interfaces and load-distributing layers, while reverse gradient coatings ensure greater mechanical consistency, making them suitable for protective and structural applications requiring enhanced surface durability.