In this work, scanning force microscopy (SFM) is used to measure tribological characteristics of a novel, compound film. Hard coatings are often applied to engineering surfaces for reduction of friction and wear. Here, we show that a soft, flexible, intermediate layer placed between substrate and hard outer coating provides considerable enhancement of the wear protection. Previously, we demonstrated that such compound films provide a means to controllably tune surface stiffness, thus opening interesting nanomechanical applications.¹ Titania films of several nm thickness are coated onto substrates of silicon, kapton, polycarbonate (PC), and polydimethylsiloxane (PDMS) and the scratch resistance is measured by SFM. When PDMS is applied as an intermediate layer between any stiffer substrate and the thin titania outer layer, marked improvement in the scratch resistance is achieved. This is shown by quantitative wear tests on silicon and kapton substrates coated with PDMS which is subsequently capped by a titania layer with thickness ranging from several nm to several tens of nm. In addition to the improvement in scratch/wear resistance, nano-friction studies performed in the SFM showed that the PDMS cushion layer reduced the friction coefficient of the titania coating by more than a factor of two.

To demonstrate a technological application of such coatings, they were applied to the common lens material PC. Here, 40 nm or titania was deposited by liquid phase deposition² on a ten micrometer-thick PDMS “cushion” on top of the PC. In scratch tests, load to failure was increased by 5x relative to untreated PC and more than doubled relative to titania on PC without the cushion layer. These thin coatings had no detrimental effect on the optical properties of the PC. This work thus demonstrates, a simple, robust, and practical means of improving tribological properties in practical applications.

The physical basis of this effect is explored by means of Finite Element Analysis, and we suggest a model for friction reduction based on the "cushioning effect” of a soft intermediate layer.